Bryan E. Anderson, MD

The Diagnosis: Primary Cutaneous γδ T-cell Lymphoma

Primary cutaneous γδ T-cell lymphoma (PCGDTL) is a distinct entity that can be confused with other types of cutaneous T-cell lymphomas. Often rapidly fatal, PCGDTL has a broad clinical spectrum that may include indolent variants—subcutaneous, epidermotropic, and dermal.¹ Primary cutaneous γδ T-cell lymphoma represents less than 1% of all cutaneous T-cell lymphomas.² Diagnosis and treatment remain challenging. Patients typically present with nodular lesions that progress to ulceration and necrosis. Early lesions can be confused with erythema nodosum, mycosis fungoides, or infection on clinical examination; biopsy establishes the diagnosis. Typical findings include a cytotoxic phenotype, variable epidermotropism, dermal and subcutaneous involvement, and loss of CD4 and often CD8 expression. Testing for Epstein-Barr virus expression yields negative results. The neoplastic lymphocytes in dermal and subcutaneous PCGDTL typically are T-cell intracellular antigen-1 (TIA-1) and granzyme positive.¹

Immunohistochemistry failed to reveal CD8, CD56, granzyme, or T-cell intracellular antigen-1 staining of neoplastic cells in our patient but stained diffusely positive with CD3 and CD4. A CD20 stain decorated only a few dermal cells. The patient’s skin lesions continued to enlarge, and the massive lymphadenopathy made breathing difficult. Computed tomography revealed diffuse systemic involvement. An axillary lymph node biopsy revealed sinusoids with complete diffuse effacement of architecture as well as frequent mitotic figures and karyorrhectic debris (Figure 1A). Negative staining for T-cell receptor beta-F1 of the axillary lymph node biopsy and clonal rearrangement of the T-cell receptor gamma chain supported the diagnosis of PCGDTL. Nuclear staining for Epstein-Barr virus–encoded RNA was negative. Human T-cell leukemia virus type 1 antibodies and polymerase chain reaction also were negative. Flow cytometry demonstrated an atypical population of CD3⁺, CD4⁺, and CD7− γδ T lymphocytes, further supporting the diagnosis of lymphoma.

The median life expectancy for patients with dermal or subcutaneous PCGDTL is 10 to 15 months after diagnosis.³ The 5-year life expectancy for PCGDTL is approximately 11%.² Limited treatment options contribute to the poor outcome. Chemotherapy regimens such as CHOP (cyclophosphamide, doxorubicin, vincristine, prednisolone) and EPOCH (etoposide phosphate, prednisone, vincristine sulfate, cyclophosphamide, doxorubicin hydrochloride) have yielded inconsistent results. Stem cell transplant has been tried in progressive disease and also has yielded mixed results.² Brentuximab is indicated for individuals whose tumors express CD30.⁴ Associated hemophagic lymphohistiocytosis portends a poor prognosis.⁵

Despite treatment with etoposide, vincristine, doxorubicin, and high-dose oral steroids, our patient developed progressive difficulty breathing, stridor, kidney injury, and anemia. Our patient died less than 1 month after diagnosis—after only 1 round of chemotherapy—secondary to progressive disease and an uncontrollable gastrointestinal tract bleed. The leonine facies (Figure 1B) encountered in our patient can raise a differential diagnosis that includes infectious as well as neoplastic etiologies; however, most infectious etiologies associated with leonine facies manifest in a chronic fashion rather than with a sudden eruption, as noted in our patient.

Leprosy is caused by Mycobacterium leprae, a grampositive bacillus. The condition manifests across a spectrum, with the poles being tuberculoid and lepromatous, and borderline variants in between.^6-8 Lepromatous leprosy arises in individuals who are unable to mount cellular immunity against M leprae secondary to anergy.⁶ Lepromatous leprosy often presents with numerous papules and nodules. Aside from cutaneous manifestations, lepromatous leprosy has a predilection for peripheral nerves and specifically Schwann cells. Histologically, biopsy reveals a flat epidermis and a cell-free subepidermal grenz zone. Within the dermis, there is a diffuse histiocytic infiltrate that typically is not centered around nerves (Figure 2).^6,7 Mycobacterium leprae can appear scattered throughout or clustered in globi. Mycobacterium leprae stains red with Ziehl-Neelsen or Wade-Fite stains.^6,7 Immunohistochemistry reveals a CD4⁺ helper T cell (TH2) predominance, supported by the increased expression of type 2 reaction cytokines such as IL-4, IL-5, IL-10, and IL-13.⁸

Diffuse large B-cell lymphoma (DLBCL) embodies 10% to 20% of all primary cutaneous lymphomas; it is more prevalent in older adults (age range, 70–82 years) and women. Clinically, DLBCL presents as either single or multiple rapidly progressing nodules or plaques, usually violaceous or blue-red in color.^9,10 The most common area of presentation is on the legs, though it also can surface at other sites.⁹ On histology, DLBCL has clearly malignant features including frequent mitotic figures, large immunoblasts, and involvement throughout the dermis as well as perivascularly (Figure 3). Spindle-shaped cells and anaplastic features can be present. Immunohistochemically, DLBCL stains strongly positive for CD20 and B-cell lymphoma 2 (Bcl-2) along with other pan–B-cell markers.^9-11 The aggressive leg type of DLBCL stains positively for multiple myeloma oncogene 1 (MUM-1).^9,11

Cutaneous metastatic adenocarcinoma from internal malignancies occurs in approximately 5% of cancer patients with metastatic spread.¹² Most of these cutaneous lesions develop in close proximity to the primary tumor such as on the trunk, head, or neck. All cutaneous metastases carry a poor prognosis. Clinical presentation can vary greatly, ranging from painless, firm, or elastic nodules to lesions that mimic inflammatory skin conditions such as erysipelas or scleroderma. The majority of these metastases develop as painless firm nodules that are flesh colored, pink, red-brown, or purple.^12,13 The histopathology of metastatic adenocarcinoma demonstrates an infiltrative nodular appearance, though there rarely are well-circumscribed nodules found.¹³ The lesion originates in the dermis or subcutaneous tissue. It is a glandulartype lesion that may reflect the tissue of the primary tumor (Figure 4).^12,14 Immunohistochemical stains likely will remain consistent with those of the primary tumor, which is not always the case.¹⁴

Merkel cell carcinoma (MCC) is an aggressive cutaneous malignancy of epithelial and neuroendocrine origin, first described as trabecular carcinoma due to the arrangement of tumor resembling cancellous bone.^15,16 Merkel cells are mechanoreceptors found near nerve terminals.¹⁷ Approximately 80% of MCCs are associated with Merkel cell polyomavirus, which is a small, double-stranded DNA virus with an icosahedral capsid.^17,18 Merkel cell polyomavirus–positive cases of MCC tend to have a better prognosis. In Merkel cell polyomavirus–negative MCC, there is an association with UV damage and increased chromosomal aberrations.¹⁸ Merkel cell carcinoma is known for its high rate of recurrence as well as local and distant metastasis. Nodal involvement is the most important prognostic indicator.¹⁵ Clinically, MCC is associated with the AEIOU mnemonic (asymptomatic, expanding rapidly, immunosuppression, older than 50 years, UV exposed/fair skin).^15-17 Lesions appear as red-blue papules on sun-exposed skin and usually are smaller than 2 cm by their greatest dimension. On histopathology, MCC demonstrates small, round, blue cells arranged in sheets or nests originating in the dermis and occasionally can infiltrate the subcutis and lymphovascular surroundings (Figure 5).^16-19 Cells have scant eosinophilic cytoplasm and may have fine granular chromatin. Numerous mitotic figures and apoptotic cells also are present. On immunohistochemistry, these cells will stain positive for cytokeratin AE1/AE3, anticytokeratin (CAM 5.2), CK20, and CD56. Due to their neuroendocrine derivation, they also are commonly synaptophysin, neuron-specific enolase, and chromogranin A positive. Notably, MCC will stain negative for leukocyte common antigen, CD20, CD3, CD34, and thyroid transcription factor 1 (TTF-1).^16,17

Primary cutaneous γδ T-cell lymphoma can be difficult to diagnose and requires urgent treatment. Clinicians and dermatopathologists need to work together to establish the diagnosis. There is a high mortality rate associated with PCGDTL, making prompt recognition and timely treatment critical. Acknowledgments—Thank you to our colleagues with the Penn State Health Hematology/Oncology Department (Hershey, Pennsylvania) for comanagement of this patient.

Acknowledgments
Thank you to our colleagues with the Penn State Health Hematology/Oncology Department (Hershey, Pennsylvania) for comanagement of this patient.

The Diagnosis: Primary Cutaneous γδ T-cell Lymphoma

Primary cutaneous γδ T-cell lymphoma (PCGDTL) is a distinct entity that can be confused with other types of cutaneous T-cell lymphomas. Often rapidly fatal, PCGDTL has a broad clinical spectrum that may include indolent variants—subcutaneous, epidermotropic, and dermal.¹ Primary cutaneous γδ T-cell lymphoma represents less than 1% of all cutaneous T-cell lymphomas.² Diagnosis and treatment remain challenging. Patients typically present with nodular lesions that progress to ulceration and necrosis. Early lesions can be confused with erythema nodosum, mycosis fungoides, or infection on clinical examination; biopsy establishes the diagnosis. Typical findings include a cytotoxic phenotype, variable epidermotropism, dermal and subcutaneous involvement, and loss of CD4 and often CD8 expression. Testing for Epstein-Barr virus expression yields negative results. The neoplastic lymphocytes in dermal and subcutaneous PCGDTL typically are T-cell intracellular antigen-1 (TIA-1) and granzyme positive.¹

Immunohistochemistry failed to reveal CD8, CD56, granzyme, or T-cell intracellular antigen-1 staining of neoplastic cells in our patient but stained diffusely positive with CD3 and CD4. A CD20 stain decorated only a few dermal cells. The patient’s skin lesions continued to enlarge, and the massive lymphadenopathy made breathing difficult. Computed tomography revealed diffuse systemic involvement. An axillary lymph node biopsy revealed sinusoids with complete diffuse effacement of architecture as well as frequent mitotic figures and karyorrhectic debris (Figure 1A). Negative staining for T-cell receptor beta-F1 of the axillary lymph node biopsy and clonal rearrangement of the T-cell receptor gamma chain supported the diagnosis of PCGDTL. Nuclear staining for Epstein-Barr virus–encoded RNA was negative. Human T-cell leukemia virus type 1 antibodies and polymerase chain reaction also were negative. Flow cytometry demonstrated an atypical population of CD3⁺, CD4⁺, and CD7− γδ T lymphocytes, further supporting the diagnosis of lymphoma.

The median life expectancy for patients with dermal or subcutaneous PCGDTL is 10 to 15 months after diagnosis.³ The 5-year life expectancy for PCGDTL is approximately 11%.² Limited treatment options contribute to the poor outcome. Chemotherapy regimens such as CHOP (cyclophosphamide, doxorubicin, vincristine, prednisolone) and EPOCH (etoposide phosphate, prednisone, vincristine sulfate, cyclophosphamide, doxorubicin hydrochloride) have yielded inconsistent results. Stem cell transplant has been tried in progressive disease and also has yielded mixed results.² Brentuximab is indicated for individuals whose tumors express CD30.⁴ Associated hemophagic lymphohistiocytosis portends a poor prognosis.⁵

Despite treatment with etoposide, vincristine, doxorubicin, and high-dose oral steroids, our patient developed progressive difficulty breathing, stridor, kidney injury, and anemia. Our patient died less than 1 month after diagnosis—after only 1 round of chemotherapy—secondary to progressive disease and an uncontrollable gastrointestinal tract bleed. The leonine facies (Figure 1B) encountered in our patient can raise a differential diagnosis that includes infectious as well as neoplastic etiologies; however, most infectious etiologies associated with leonine facies manifest in a chronic fashion rather than with a sudden eruption, as noted in our patient.

Leprosy is caused by Mycobacterium leprae, a grampositive bacillus. The condition manifests across a spectrum, with the poles being tuberculoid and lepromatous, and borderline variants in between.^6-8 Lepromatous leprosy arises in individuals who are unable to mount cellular immunity against M leprae secondary to anergy.⁶ Lepromatous leprosy often presents with numerous papules and nodules. Aside from cutaneous manifestations, lepromatous leprosy has a predilection for peripheral nerves and specifically Schwann cells. Histologically, biopsy reveals a flat epidermis and a cell-free subepidermal grenz zone. Within the dermis, there is a diffuse histiocytic infiltrate that typically is not centered around nerves (Figure 2).^6,7 Mycobacterium leprae can appear scattered throughout or clustered in globi. Mycobacterium leprae stains red with Ziehl-Neelsen or Wade-Fite stains.^6,7 Immunohistochemistry reveals a CD4⁺ helper T cell (TH2) predominance, supported by the increased expression of type 2 reaction cytokines such as IL-4, IL-5, IL-10, and IL-13.⁸

Diffuse large B-cell lymphoma (DLBCL) embodies 10% to 20% of all primary cutaneous lymphomas; it is more prevalent in older adults (age range, 70–82 years) and women. Clinically, DLBCL presents as either single or multiple rapidly progressing nodules or plaques, usually violaceous or blue-red in color.^9,10 The most common area of presentation is on the legs, though it also can surface at other sites.⁹ On histology, DLBCL has clearly malignant features including frequent mitotic figures, large immunoblasts, and involvement throughout the dermis as well as perivascularly (Figure 3). Spindle-shaped cells and anaplastic features can be present. Immunohistochemically, DLBCL stains strongly positive for CD20 and B-cell lymphoma 2 (Bcl-2) along with other pan–B-cell markers.^9-11 The aggressive leg type of DLBCL stains positively for multiple myeloma oncogene 1 (MUM-1).^9,11

Cutaneous metastatic adenocarcinoma from internal malignancies occurs in approximately 5% of cancer patients with metastatic spread.¹² Most of these cutaneous lesions develop in close proximity to the primary tumor such as on the trunk, head, or neck. All cutaneous metastases carry a poor prognosis. Clinical presentation can vary greatly, ranging from painless, firm, or elastic nodules to lesions that mimic inflammatory skin conditions such as erysipelas or scleroderma. The majority of these metastases develop as painless firm nodules that are flesh colored, pink, red-brown, or purple.^12,13 The histopathology of metastatic adenocarcinoma demonstrates an infiltrative nodular appearance, though there rarely are well-circumscribed nodules found.¹³ The lesion originates in the dermis or subcutaneous tissue. It is a glandulartype lesion that may reflect the tissue of the primary tumor (Figure 4).^12,14 Immunohistochemical stains likely will remain consistent with those of the primary tumor, which is not always the case.¹⁴

Merkel cell carcinoma (MCC) is an aggressive cutaneous malignancy of epithelial and neuroendocrine origin, first described as trabecular carcinoma due to the arrangement of tumor resembling cancellous bone.^15,16 Merkel cells are mechanoreceptors found near nerve terminals.¹⁷ Approximately 80% of MCCs are associated with Merkel cell polyomavirus, which is a small, double-stranded DNA virus with an icosahedral capsid.^17,18 Merkel cell polyomavirus–positive cases of MCC tend to have a better prognosis. In Merkel cell polyomavirus–negative MCC, there is an association with UV damage and increased chromosomal aberrations.¹⁸ Merkel cell carcinoma is known for its high rate of recurrence as well as local and distant metastasis. Nodal involvement is the most important prognostic indicator.¹⁵ Clinically, MCC is associated with the AEIOU mnemonic (asymptomatic, expanding rapidly, immunosuppression, older than 50 years, UV exposed/fair skin).^15-17 Lesions appear as red-blue papules on sun-exposed skin and usually are smaller than 2 cm by their greatest dimension. On histopathology, MCC demonstrates small, round, blue cells arranged in sheets or nests originating in the dermis and occasionally can infiltrate the subcutis and lymphovascular surroundings (Figure 5).^16-19 Cells have scant eosinophilic cytoplasm and may have fine granular chromatin. Numerous mitotic figures and apoptotic cells also are present. On immunohistochemistry, these cells will stain positive for cytokeratin AE1/AE3, anticytokeratin (CAM 5.2), CK20, and CD56. Due to their neuroendocrine derivation, they also are commonly synaptophysin, neuron-specific enolase, and chromogranin A positive. Notably, MCC will stain negative for leukocyte common antigen, CD20, CD3, CD34, and thyroid transcription factor 1 (TTF-1).^16,17

Primary cutaneous γδ T-cell lymphoma can be difficult to diagnose and requires urgent treatment. Clinicians and dermatopathologists need to work together to establish the diagnosis. There is a high mortality rate associated with PCGDTL, making prompt recognition and timely treatment critical. Acknowledgments—Thank you to our colleagues with the Penn State Health Hematology/Oncology Department (Hershey, Pennsylvania) for comanagement of this patient.

Acknowledgments
Thank you to our colleagues with the Penn State Health Hematology/Oncology Department (Hershey, Pennsylvania) for comanagement of this patient.

The Diagnosis: Primary Cutaneous γδ T-cell Lymphoma

Primary cutaneous γδ T-cell lymphoma (PCGDTL) is a distinct entity that can be confused with other types of cutaneous T-cell lymphomas. Often rapidly fatal, PCGDTL has a broad clinical spectrum that may include indolent variants—subcutaneous, epidermotropic, and dermal.¹ Primary cutaneous γδ T-cell lymphoma represents less than 1% of all cutaneous T-cell lymphomas.² Diagnosis and treatment remain challenging. Patients typically present with nodular lesions that progress to ulceration and necrosis. Early lesions can be confused with erythema nodosum, mycosis fungoides, or infection on clinical examination; biopsy establishes the diagnosis. Typical findings include a cytotoxic phenotype, variable epidermotropism, dermal and subcutaneous involvement, and loss of CD4 and often CD8 expression. Testing for Epstein-Barr virus expression yields negative results. The neoplastic lymphocytes in dermal and subcutaneous PCGDTL typically are T-cell intracellular antigen-1 (TIA-1) and granzyme positive.¹

Immunohistochemistry failed to reveal CD8, CD56, granzyme, or T-cell intracellular antigen-1 staining of neoplastic cells in our patient but stained diffusely positive with CD3 and CD4. A CD20 stain decorated only a few dermal cells. The patient’s skin lesions continued to enlarge, and the massive lymphadenopathy made breathing difficult. Computed tomography revealed diffuse systemic involvement. An axillary lymph node biopsy revealed sinusoids with complete diffuse effacement of architecture as well as frequent mitotic figures and karyorrhectic debris (Figure 1A). Negative staining for T-cell receptor beta-F1 of the axillary lymph node biopsy and clonal rearrangement of the T-cell receptor gamma chain supported the diagnosis of PCGDTL. Nuclear staining for Epstein-Barr virus–encoded RNA was negative. Human T-cell leukemia virus type 1 antibodies and polymerase chain reaction also were negative. Flow cytometry demonstrated an atypical population of CD3⁺, CD4⁺, and CD7− γδ T lymphocytes, further supporting the diagnosis of lymphoma.

The median life expectancy for patients with dermal or subcutaneous PCGDTL is 10 to 15 months after diagnosis.³ The 5-year life expectancy for PCGDTL is approximately 11%.² Limited treatment options contribute to the poor outcome. Chemotherapy regimens such as CHOP (cyclophosphamide, doxorubicin, vincristine, prednisolone) and EPOCH (etoposide phosphate, prednisone, vincristine sulfate, cyclophosphamide, doxorubicin hydrochloride) have yielded inconsistent results. Stem cell transplant has been tried in progressive disease and also has yielded mixed results.² Brentuximab is indicated for individuals whose tumors express CD30.⁴ Associated hemophagic lymphohistiocytosis portends a poor prognosis.⁵

Despite treatment with etoposide, vincristine, doxorubicin, and high-dose oral steroids, our patient developed progressive difficulty breathing, stridor, kidney injury, and anemia. Our patient died less than 1 month after diagnosis—after only 1 round of chemotherapy—secondary to progressive disease and an uncontrollable gastrointestinal tract bleed. The leonine facies (Figure 1B) encountered in our patient can raise a differential diagnosis that includes infectious as well as neoplastic etiologies; however, most infectious etiologies associated with leonine facies manifest in a chronic fashion rather than with a sudden eruption, as noted in our patient.

Leprosy is caused by Mycobacterium leprae, a grampositive bacillus. The condition manifests across a spectrum, with the poles being tuberculoid and lepromatous, and borderline variants in between.^6-8 Lepromatous leprosy arises in individuals who are unable to mount cellular immunity against M leprae secondary to anergy.⁶ Lepromatous leprosy often presents with numerous papules and nodules. Aside from cutaneous manifestations, lepromatous leprosy has a predilection for peripheral nerves and specifically Schwann cells. Histologically, biopsy reveals a flat epidermis and a cell-free subepidermal grenz zone. Within the dermis, there is a diffuse histiocytic infiltrate that typically is not centered around nerves (Figure 2).^6,7 Mycobacterium leprae can appear scattered throughout or clustered in globi. Mycobacterium leprae stains red with Ziehl-Neelsen or Wade-Fite stains.^6,7 Immunohistochemistry reveals a CD4⁺ helper T cell (TH2) predominance, supported by the increased expression of type 2 reaction cytokines such as IL-4, IL-5, IL-10, and IL-13.⁸

Diffuse large B-cell lymphoma (DLBCL) embodies 10% to 20% of all primary cutaneous lymphomas; it is more prevalent in older adults (age range, 70–82 years) and women. Clinically, DLBCL presents as either single or multiple rapidly progressing nodules or plaques, usually violaceous or blue-red in color.^9,10 The most common area of presentation is on the legs, though it also can surface at other sites.⁹ On histology, DLBCL has clearly malignant features including frequent mitotic figures, large immunoblasts, and involvement throughout the dermis as well as perivascularly (Figure 3). Spindle-shaped cells and anaplastic features can be present. Immunohistochemically, DLBCL stains strongly positive for CD20 and B-cell lymphoma 2 (Bcl-2) along with other pan–B-cell markers.^9-11 The aggressive leg type of DLBCL stains positively for multiple myeloma oncogene 1 (MUM-1).^9,11

Cutaneous metastatic adenocarcinoma from internal malignancies occurs in approximately 5% of cancer patients with metastatic spread.¹² Most of these cutaneous lesions develop in close proximity to the primary tumor such as on the trunk, head, or neck. All cutaneous metastases carry a poor prognosis. Clinical presentation can vary greatly, ranging from painless, firm, or elastic nodules to lesions that mimic inflammatory skin conditions such as erysipelas or scleroderma. The majority of these metastases develop as painless firm nodules that are flesh colored, pink, red-brown, or purple.^12,13 The histopathology of metastatic adenocarcinoma demonstrates an infiltrative nodular appearance, though there rarely are well-circumscribed nodules found.¹³ The lesion originates in the dermis or subcutaneous tissue. It is a glandulartype lesion that may reflect the tissue of the primary tumor (Figure 4).^12,14 Immunohistochemical stains likely will remain consistent with those of the primary tumor, which is not always the case.¹⁴

Merkel cell carcinoma (MCC) is an aggressive cutaneous malignancy of epithelial and neuroendocrine origin, first described as trabecular carcinoma due to the arrangement of tumor resembling cancellous bone.^15,16 Merkel cells are mechanoreceptors found near nerve terminals.¹⁷ Approximately 80% of MCCs are associated with Merkel cell polyomavirus, which is a small, double-stranded DNA virus with an icosahedral capsid.^17,18 Merkel cell polyomavirus–positive cases of MCC tend to have a better prognosis. In Merkel cell polyomavirus–negative MCC, there is an association with UV damage and increased chromosomal aberrations.¹⁸ Merkel cell carcinoma is known for its high rate of recurrence as well as local and distant metastasis. Nodal involvement is the most important prognostic indicator.¹⁵ Clinically, MCC is associated with the AEIOU mnemonic (asymptomatic, expanding rapidly, immunosuppression, older than 50 years, UV exposed/fair skin).^15-17 Lesions appear as red-blue papules on sun-exposed skin and usually are smaller than 2 cm by their greatest dimension. On histopathology, MCC demonstrates small, round, blue cells arranged in sheets or nests originating in the dermis and occasionally can infiltrate the subcutis and lymphovascular surroundings (Figure 5).^16-19 Cells have scant eosinophilic cytoplasm and may have fine granular chromatin. Numerous mitotic figures and apoptotic cells also are present. On immunohistochemistry, these cells will stain positive for cytokeratin AE1/AE3, anticytokeratin (CAM 5.2), CK20, and CD56. Due to their neuroendocrine derivation, they also are commonly synaptophysin, neuron-specific enolase, and chromogranin A positive. Notably, MCC will stain negative for leukocyte common antigen, CD20, CD3, CD34, and thyroid transcription factor 1 (TTF-1).^16,17

Primary cutaneous γδ T-cell lymphoma can be difficult to diagnose and requires urgent treatment. Clinicians and dermatopathologists need to work together to establish the diagnosis. There is a high mortality rate associated with PCGDTL, making prompt recognition and timely treatment critical. Acknowledgments—Thank you to our colleagues with the Penn State Health Hematology/Oncology Department (Hershey, Pennsylvania) for comanagement of this patient.

Acknowledgments
Thank you to our colleagues with the Penn State Health Hematology/Oncology Department (Hershey, Pennsylvania) for comanagement of this patient.

The Diagnosis: Lichen Planus Pemphigoides

A skin biopsy from the right thigh demonstrated subepidermal blisters containing neutrophils (Figure 1). Direct immunofluorescence revealed linear basement membrane zone staining with C3 and trace staining with IgG (Figure 2), supporting a diagnosis of lichen planus pemphigoides (LPP). Oral prednisone—starting at 60 mg daily and tapered to 40 mg for a week, 20 mg for a week, then 10 mg for a month—along with triamcinolone ointment 0.1% to affected areas led to improvement. Hydrochlorothiazide and UV light therapy were discontinued. Doxycycline 100 mg twice daily and nicotinamide 500 mg twice daily prescribed as adjunctive therapy also led to improvement. The patient achieved remission with doxycycline and was doing well without prednisone; however, he experienced a flare of his disease about 6 months later and was started on mycophenolate mofetil 1 g twice daily after clearance from his gastroenterologist, given his history of hepatitis B. He has been doing well since starting mycophenolate mofetil.

Lichen planus pemphigoides is a rare autoimmune bullous dermatosis with features of both lichen planus and bullous pemphigoid.¹ Violaceous papules and tense bullae may be superimposed or arise independently. The chest, abdomen, back, and upper and lower extremities typically are involved.² Oral mucosal involvement with white reticular streaks or erosions and nail involvement have been reported.² Histopathologic and immunologic findings establish the diagnosis. Lichen planus pemphigoides is associated with subepidermal bullae and linear deposits of IgG and C3 on the basement membrane zone.¹ Autoantibodies to bullous pemphigoid (BP) antigens BP180 and BP230 are associated with LPP.³ The pathogenesis of LPP remains unclear, but there are associations with chronic diseases, medications, and certain therapies.^1,4-6 Several case reports have linked LPP to chronic viral hepatitis infections, as well as malignant tumors of the skin, mucosa, and gastrointestinal tract.² Lichen planus pemphigoides has been reported in a patient on entecavir for hepatitis B as well as in a patient treated for hepatitis C with interferon and ribavirin.¹ Lichen planus pemphigoides has been described in patients treated with the angiotensin-converting enzyme inhibitors enalapril, captopril, and ramipril.^4,5,7 UV phototherapy also has been associated with the development of LPP.⁶ Hydrochlorothiazide previously has been reported as a cause of drug-induced lichen planus.⁸ A PubMed search of articles indexed for MEDLINE using the terms lichen planus pemphigoides and hydrochlorothiazide revealed no reports of hydrochlorothiazide-induced LPP.

Lichen planus pemphigoides demonstrates overlap with other blistering dermatoses, such as BP, bullous lupus erythematosus, and bullous lichen planus. Although histologically and immunologically similar to BP, LPP can be differentiated clinically by the presence of violaceous papules or plaques typical of lichen planus.⁹ Bullous pemphigoid is more common in individuals older than 70 years, whereas LPP tends to occur in middle-aged adults.² Bullous lupus erythematosus usually is associated with manifestations of systemic lupus erythematosus and autoantibodies to collagen type VII.¹⁰ Salt-split skin studies demonstrate immunofluorescence on the dermal side of the split. Individuals affected by bullous lupus erythematosus typically have a history of photosensitivity.¹⁰ Blisters in LPP may form de novo from unaffected skin, whereas the bullae in bullous lichen planus are limited to existing lichenoid papules.⁹ The autoantibodies typical of LPP are absent in bullous lichen planus. Lichen planus actinicus is a variant of lichen planus that presents with annular, dyschromic, or violaceous plaques in a photodistributed pattern without bullous lesions.⁹

Lichen planus pemphigoides most commonly is treated with systemic corticosteroids. Topical steroids, dapsone, erythromycin, tetracycline and nicotinamide, azathioprine, and mycophenolate mofetil have been reported as adjuncts to systemic steroid therapy.^2,11 Most reports describe treatment success with resolution of blistering lesions.

The Diagnosis: Lichen Planus Pemphigoides

A skin biopsy from the right thigh demonstrated subepidermal blisters containing neutrophils (Figure 1). Direct immunofluorescence revealed linear basement membrane zone staining with C3 and trace staining with IgG (Figure 2), supporting a diagnosis of lichen planus pemphigoides (LPP). Oral prednisone—starting at 60 mg daily and tapered to 40 mg for a week, 20 mg for a week, then 10 mg for a month—along with triamcinolone ointment 0.1% to affected areas led to improvement. Hydrochlorothiazide and UV light therapy were discontinued. Doxycycline 100 mg twice daily and nicotinamide 500 mg twice daily prescribed as adjunctive therapy also led to improvement. The patient achieved remission with doxycycline and was doing well without prednisone; however, he experienced a flare of his disease about 6 months later and was started on mycophenolate mofetil 1 g twice daily after clearance from his gastroenterologist, given his history of hepatitis B. He has been doing well since starting mycophenolate mofetil.

Lichen planus pemphigoides is a rare autoimmune bullous dermatosis with features of both lichen planus and bullous pemphigoid.¹ Violaceous papules and tense bullae may be superimposed or arise independently. The chest, abdomen, back, and upper and lower extremities typically are involved.² Oral mucosal involvement with white reticular streaks or erosions and nail involvement have been reported.² Histopathologic and immunologic findings establish the diagnosis. Lichen planus pemphigoides is associated with subepidermal bullae and linear deposits of IgG and C3 on the basement membrane zone.¹ Autoantibodies to bullous pemphigoid (BP) antigens BP180 and BP230 are associated with LPP.³ The pathogenesis of LPP remains unclear, but there are associations with chronic diseases, medications, and certain therapies.^1,4-6 Several case reports have linked LPP to chronic viral hepatitis infections, as well as malignant tumors of the skin, mucosa, and gastrointestinal tract.² Lichen planus pemphigoides has been reported in a patient on entecavir for hepatitis B as well as in a patient treated for hepatitis C with interferon and ribavirin.¹ Lichen planus pemphigoides has been described in patients treated with the angiotensin-converting enzyme inhibitors enalapril, captopril, and ramipril.^4,5,7 UV phototherapy also has been associated with the development of LPP.⁶ Hydrochlorothiazide previously has been reported as a cause of drug-induced lichen planus.⁸ A PubMed search of articles indexed for MEDLINE using the terms lichen planus pemphigoides and hydrochlorothiazide revealed no reports of hydrochlorothiazide-induced LPP.

Lichen planus pemphigoides demonstrates overlap with other blistering dermatoses, such as BP, bullous lupus erythematosus, and bullous lichen planus. Although histologically and immunologically similar to BP, LPP can be differentiated clinically by the presence of violaceous papules or plaques typical of lichen planus.⁹ Bullous pemphigoid is more common in individuals older than 70 years, whereas LPP tends to occur in middle-aged adults.² Bullous lupus erythematosus usually is associated with manifestations of systemic lupus erythematosus and autoantibodies to collagen type VII.¹⁰ Salt-split skin studies demonstrate immunofluorescence on the dermal side of the split. Individuals affected by bullous lupus erythematosus typically have a history of photosensitivity.¹⁰ Blisters in LPP may form de novo from unaffected skin, whereas the bullae in bullous lichen planus are limited to existing lichenoid papules.⁹ The autoantibodies typical of LPP are absent in bullous lichen planus. Lichen planus actinicus is a variant of lichen planus that presents with annular, dyschromic, or violaceous plaques in a photodistributed pattern without bullous lesions.⁹

Lichen planus pemphigoides most commonly is treated with systemic corticosteroids. Topical steroids, dapsone, erythromycin, tetracycline and nicotinamide, azathioprine, and mycophenolate mofetil have been reported as adjuncts to systemic steroid therapy.^2,11 Most reports describe treatment success with resolution of blistering lesions.

The Diagnosis: Lichen Planus Pemphigoides

A skin biopsy from the right thigh demonstrated subepidermal blisters containing neutrophils (Figure 1). Direct immunofluorescence revealed linear basement membrane zone staining with C3 and trace staining with IgG (Figure 2), supporting a diagnosis of lichen planus pemphigoides (LPP). Oral prednisone—starting at 60 mg daily and tapered to 40 mg for a week, 20 mg for a week, then 10 mg for a month—along with triamcinolone ointment 0.1% to affected areas led to improvement. Hydrochlorothiazide and UV light therapy were discontinued. Doxycycline 100 mg twice daily and nicotinamide 500 mg twice daily prescribed as adjunctive therapy also led to improvement. The patient achieved remission with doxycycline and was doing well without prednisone; however, he experienced a flare of his disease about 6 months later and was started on mycophenolate mofetil 1 g twice daily after clearance from his gastroenterologist, given his history of hepatitis B. He has been doing well since starting mycophenolate mofetil.

Lichen planus pemphigoides is a rare autoimmune bullous dermatosis with features of both lichen planus and bullous pemphigoid.¹ Violaceous papules and tense bullae may be superimposed or arise independently. The chest, abdomen, back, and upper and lower extremities typically are involved.² Oral mucosal involvement with white reticular streaks or erosions and nail involvement have been reported.² Histopathologic and immunologic findings establish the diagnosis. Lichen planus pemphigoides is associated with subepidermal bullae and linear deposits of IgG and C3 on the basement membrane zone.¹ Autoantibodies to bullous pemphigoid (BP) antigens BP180 and BP230 are associated with LPP.³ The pathogenesis of LPP remains unclear, but there are associations with chronic diseases, medications, and certain therapies.^1,4-6 Several case reports have linked LPP to chronic viral hepatitis infections, as well as malignant tumors of the skin, mucosa, and gastrointestinal tract.² Lichen planus pemphigoides has been reported in a patient on entecavir for hepatitis B as well as in a patient treated for hepatitis C with interferon and ribavirin.¹ Lichen planus pemphigoides has been described in patients treated with the angiotensin-converting enzyme inhibitors enalapril, captopril, and ramipril.^4,5,7 UV phototherapy also has been associated with the development of LPP.⁶ Hydrochlorothiazide previously has been reported as a cause of drug-induced lichen planus.⁸ A PubMed search of articles indexed for MEDLINE using the terms lichen planus pemphigoides and hydrochlorothiazide revealed no reports of hydrochlorothiazide-induced LPP.

Lichen planus pemphigoides demonstrates overlap with other blistering dermatoses, such as BP, bullous lupus erythematosus, and bullous lichen planus. Although histologically and immunologically similar to BP, LPP can be differentiated clinically by the presence of violaceous papules or plaques typical of lichen planus.⁹ Bullous pemphigoid is more common in individuals older than 70 years, whereas LPP tends to occur in middle-aged adults.² Bullous lupus erythematosus usually is associated with manifestations of systemic lupus erythematosus and autoantibodies to collagen type VII.¹⁰ Salt-split skin studies demonstrate immunofluorescence on the dermal side of the split. Individuals affected by bullous lupus erythematosus typically have a history of photosensitivity.¹⁰ Blisters in LPP may form de novo from unaffected skin, whereas the bullae in bullous lichen planus are limited to existing lichenoid papules.⁹ The autoantibodies typical of LPP are absent in bullous lichen planus. Lichen planus actinicus is a variant of lichen planus that presents with annular, dyschromic, or violaceous plaques in a photodistributed pattern without bullous lesions.⁹

Lichen planus pemphigoides most commonly is treated with systemic corticosteroids. Topical steroids, dapsone, erythromycin, tetracycline and nicotinamide, azathioprine, and mycophenolate mofetil have been reported as adjuncts to systemic steroid therapy.^2,11 Most reports describe treatment success with resolution of blistering lesions.

To the Editor:

There are a wide variety of zoonotic diseases that can be transmitted from birds to humans. Pigeons, chickens, starlings, canaries, and parakeets are known reservoirs of one particular zoonotic infection caused by the parasitic arthropod Dermanyssus gallinae.¹ Dermanyssus gallinae (chicken mite) and Ornithonyssus sylviarum (northern fowl mite) are collectively referred to as bird mites. When these mites are unable to take blood meals from birds, they search out alternative hosts²; in humans, this often leads to the development of pruritic dermatitis.³

A 30-year-old woman presented to our clinic for evaluation of severe generalized pruritus accompanied by a sensation of “bugs on the skin” of 2 weeks’ duration. She noted the pruritus worsened when she was sitting outside on her porch. A few days prior to presentation, she noticed a small, “pinpoint-sized bug” on her arm (<1 mm in size), which she brought in for identification (Figure).

The bug was identified as a bird mite (Dermanyssus gallinae) on light microscopy, which was later confirmed by a medical entomologist. After the diagnosis of bird mite dermatitis was made, the patient noted there was a nest of starlings above the light on her porch. When she later investigated the nest following the current presentation, she noted many small mites crawling around the nest. The nest was removed and her symptoms resolved completely within 2 weeks without treatment.

Bird mites belong to the Arachnida class, under the order Acari. In 1958, Williams⁴ noted D gallinae’s ability to feed on human blood. Bird mites have 5 stages of development: egg, larva, protonymph, deutonymph, and adult. Protonymphs, deutonymphs, and adults can bite humans for a blood meal.⁵ Bird mites range from 0.3 to 1 mm in length and have nonsegmented, egg-shaped bodies with 4 pairs of legs. Before taking a blood meal, bird mites generally are a translucent brown color, and appear red when engorged with blood.² Their small size makes them barely visible to the unaided eye. Of note, D gallinae and O sylviarum can be distinguished from each other based on subtle differences in morphology; for instance, the posterior genitoventral shield of O sylviarum is narrowly rounded, whereas it is broadly rounded in D gallinae. The dorsal shield of O sylviarum abruptly narrows posteriorly but is more smoothly narrowed in D gallinae.⁶ Additionally, O sylviarum tends to cause more irritating dermatitis in humans than D gallinae.³

Although they can be found worldwide, D gallinae and O sylviarum undergo optimal development at 20°C to 25°C and 70% humidity.^3,5,7 Bird mites generally develop over the course of 5 to 12 days; thus, the population of bird mites in a single nest may grow to the tens of thousands before young birds permanently leave. Dermanyssus gallinae can survive for months in abandoned nests without a blood meal, while O sylviarum can survive for several weeks.⁸ It is important to note that humans are not ideal hosts for bird mites, as they are unable to survive for extended periods of time or reproduce on human hosts.⁹

When bird mites are no longer able to obtain blood meals from nesting birds, they begin their nocturnal migration to find suitable hosts. Bird nests generally are abandoned in late spring; thus, most patients with bird mite dermatitis present to clinics with bird mite dermatitis in late spring and early summer.¹⁰ Mites often travel through cracks in doors, floors, walls, and ceilings but also can gain access to living areas through ventilation ducts and air conditioning units.¹ The mite’s bite and crawling on the skin is sometimes noticed by the patient. In general, however, intense itching is not observed until about 1 to 3 days after the mite makes contact with the skin. Patients often report that pruritus is worst at night.⁹ Papules and vesicles (bite reactions) may accompany the pruritus, and physicians commonly find bloody crust and excoriations in particularly pruritic areas.⁵ Urticarial plaques and diffuse erythema occasionally also may be present.⁹ Bird mites sometimes can be scraped from the skin and observed under light microscopy.¹¹ Blood eosinophilia is not found in bird mite dermatitis. On histologic examination, perivascular eosinophilic infiltration can be seen in the upper part of the dermis.¹²

The differential diagnosis in patients with pruritic dermatitis of unknown origin generally includes scabies, pediculosis, and dermatitis caused by other types of infestation. However, unlike scabies, bird mites do not cause burrows to form on the skin.⁹ The presence of a bird’s nest near the area where the patient lives places bird mite dermatitis higher in the differential.

Dermanyssus gallinae is a known vector of bacteria (eg, Salmonella, Shigella, Staphylococcus, Spirochaete, Rickettsia, Pasteurella, Chlamydia psittaci, Erysipelothrix rhusiopathiae) as well as the viruses that cause Eastern and Western equine encephalitis and St. Louis encephalitis. Transmission of these bacteria and viruses is known in birds, but transmission to humans has not been reported.^2,5,9,13

The management of bird mite dermatitis is straightforward. Usually mites can be successfully removed from the skin simply by bathing. Symptomatic treatment for bites with antihistamines and topical corticosteroids is sometimes but not always necessary.² Unlike scabies or lice, there is no need for treatment with lindane.¹ In terms of the prevention of additional bites, any bird nests located near living areas should be removed. Because bird mites often retreat back to nests between blood meals, insecticide sprays generally are unnecessary in interior spaces. Synthetic pyrethroids (eg, bifenthrin, cyfluthrin, cypermethrin, deltamethrin, cyhalothrin) can be used outside and in attics where nests may be located.^2,14,15 However, the ability of bird mites to develop resistance to repeated chemical control could become a future concern.¹⁶

Research regarding the true incidence of bird mite dermatitis is lacking. Some researchers believe that the condition is underreported, possibly due to its uncommon environmental origin.³ Reports of bird mite dermatitis in the literature also are scarce. Our case demonstrates the importance of taking a thorough patient history to rule out exposure to bird mites. All patients with pruritic dermatitis of unknown origin should be questioned about possible contact or proximity to bird nests. These simple questions can lead to the correct diagnosis and a treatment plan that will quickly and effectively resolve the pruritic skin eruption.

To the Editor:

There are a wide variety of zoonotic diseases that can be transmitted from birds to humans. Pigeons, chickens, starlings, canaries, and parakeets are known reservoirs of one particular zoonotic infection caused by the parasitic arthropod Dermanyssus gallinae.¹ Dermanyssus gallinae (chicken mite) and Ornithonyssus sylviarum (northern fowl mite) are collectively referred to as bird mites. When these mites are unable to take blood meals from birds, they search out alternative hosts²; in humans, this often leads to the development of pruritic dermatitis.³

A 30-year-old woman presented to our clinic for evaluation of severe generalized pruritus accompanied by a sensation of “bugs on the skin” of 2 weeks’ duration. She noted the pruritus worsened when she was sitting outside on her porch. A few days prior to presentation, she noticed a small, “pinpoint-sized bug” on her arm (<1 mm in size), which she brought in for identification (Figure).

The bug was identified as a bird mite (Dermanyssus gallinae) on light microscopy, which was later confirmed by a medical entomologist. After the diagnosis of bird mite dermatitis was made, the patient noted there was a nest of starlings above the light on her porch. When she later investigated the nest following the current presentation, she noted many small mites crawling around the nest. The nest was removed and her symptoms resolved completely within 2 weeks without treatment.

Bird mites belong to the Arachnida class, under the order Acari. In 1958, Williams⁴ noted D gallinae’s ability to feed on human blood. Bird mites have 5 stages of development: egg, larva, protonymph, deutonymph, and adult. Protonymphs, deutonymphs, and adults can bite humans for a blood meal.⁵ Bird mites range from 0.3 to 1 mm in length and have nonsegmented, egg-shaped bodies with 4 pairs of legs. Before taking a blood meal, bird mites generally are a translucent brown color, and appear red when engorged with blood.² Their small size makes them barely visible to the unaided eye. Of note, D gallinae and O sylviarum can be distinguished from each other based on subtle differences in morphology; for instance, the posterior genitoventral shield of O sylviarum is narrowly rounded, whereas it is broadly rounded in D gallinae. The dorsal shield of O sylviarum abruptly narrows posteriorly but is more smoothly narrowed in D gallinae.⁶ Additionally, O sylviarum tends to cause more irritating dermatitis in humans than D gallinae.³

Although they can be found worldwide, D gallinae and O sylviarum undergo optimal development at 20°C to 25°C and 70% humidity.^3,5,7 Bird mites generally develop over the course of 5 to 12 days; thus, the population of bird mites in a single nest may grow to the tens of thousands before young birds permanently leave. Dermanyssus gallinae can survive for months in abandoned nests without a blood meal, while O sylviarum can survive for several weeks.⁸ It is important to note that humans are not ideal hosts for bird mites, as they are unable to survive for extended periods of time or reproduce on human hosts.⁹

When bird mites are no longer able to obtain blood meals from nesting birds, they begin their nocturnal migration to find suitable hosts. Bird nests generally are abandoned in late spring; thus, most patients with bird mite dermatitis present to clinics with bird mite dermatitis in late spring and early summer.¹⁰ Mites often travel through cracks in doors, floors, walls, and ceilings but also can gain access to living areas through ventilation ducts and air conditioning units.¹ The mite’s bite and crawling on the skin is sometimes noticed by the patient. In general, however, intense itching is not observed until about 1 to 3 days after the mite makes contact with the skin. Patients often report that pruritus is worst at night.⁹ Papules and vesicles (bite reactions) may accompany the pruritus, and physicians commonly find bloody crust and excoriations in particularly pruritic areas.⁵ Urticarial plaques and diffuse erythema occasionally also may be present.⁹ Bird mites sometimes can be scraped from the skin and observed under light microscopy.¹¹ Blood eosinophilia is not found in bird mite dermatitis. On histologic examination, perivascular eosinophilic infiltration can be seen in the upper part of the dermis.¹²

The differential diagnosis in patients with pruritic dermatitis of unknown origin generally includes scabies, pediculosis, and dermatitis caused by other types of infestation. However, unlike scabies, bird mites do not cause burrows to form on the skin.⁹ The presence of a bird’s nest near the area where the patient lives places bird mite dermatitis higher in the differential.

Dermanyssus gallinae is a known vector of bacteria (eg, Salmonella, Shigella, Staphylococcus, Spirochaete, Rickettsia, Pasteurella, Chlamydia psittaci, Erysipelothrix rhusiopathiae) as well as the viruses that cause Eastern and Western equine encephalitis and St. Louis encephalitis. Transmission of these bacteria and viruses is known in birds, but transmission to humans has not been reported.^2,5,9,13

The management of bird mite dermatitis is straightforward. Usually mites can be successfully removed from the skin simply by bathing. Symptomatic treatment for bites with antihistamines and topical corticosteroids is sometimes but not always necessary.² Unlike scabies or lice, there is no need for treatment with lindane.¹ In terms of the prevention of additional bites, any bird nests located near living areas should be removed. Because bird mites often retreat back to nests between blood meals, insecticide sprays generally are unnecessary in interior spaces. Synthetic pyrethroids (eg, bifenthrin, cyfluthrin, cypermethrin, deltamethrin, cyhalothrin) can be used outside and in attics where nests may be located.^2,14,15 However, the ability of bird mites to develop resistance to repeated chemical control could become a future concern.¹⁶

Research regarding the true incidence of bird mite dermatitis is lacking. Some researchers believe that the condition is underreported, possibly due to its uncommon environmental origin.³ Reports of bird mite dermatitis in the literature also are scarce. Our case demonstrates the importance of taking a thorough patient history to rule out exposure to bird mites. All patients with pruritic dermatitis of unknown origin should be questioned about possible contact or proximity to bird nests. These simple questions can lead to the correct diagnosis and a treatment plan that will quickly and effectively resolve the pruritic skin eruption.

To the Editor:

There are a wide variety of zoonotic diseases that can be transmitted from birds to humans. Pigeons, chickens, starlings, canaries, and parakeets are known reservoirs of one particular zoonotic infection caused by the parasitic arthropod Dermanyssus gallinae.¹ Dermanyssus gallinae (chicken mite) and Ornithonyssus sylviarum (northern fowl mite) are collectively referred to as bird mites. When these mites are unable to take blood meals from birds, they search out alternative hosts²; in humans, this often leads to the development of pruritic dermatitis.³

A 30-year-old woman presented to our clinic for evaluation of severe generalized pruritus accompanied by a sensation of “bugs on the skin” of 2 weeks’ duration. She noted the pruritus worsened when she was sitting outside on her porch. A few days prior to presentation, she noticed a small, “pinpoint-sized bug” on her arm (<1 mm in size), which she brought in for identification (Figure).

The bug was identified as a bird mite (Dermanyssus gallinae) on light microscopy, which was later confirmed by a medical entomologist. After the diagnosis of bird mite dermatitis was made, the patient noted there was a nest of starlings above the light on her porch. When she later investigated the nest following the current presentation, she noted many small mites crawling around the nest. The nest was removed and her symptoms resolved completely within 2 weeks without treatment.

Bird mites belong to the Arachnida class, under the order Acari. In 1958, Williams⁴ noted D gallinae’s ability to feed on human blood. Bird mites have 5 stages of development: egg, larva, protonymph, deutonymph, and adult. Protonymphs, deutonymphs, and adults can bite humans for a blood meal.⁵ Bird mites range from 0.3 to 1 mm in length and have nonsegmented, egg-shaped bodies with 4 pairs of legs. Before taking a blood meal, bird mites generally are a translucent brown color, and appear red when engorged with blood.² Their small size makes them barely visible to the unaided eye. Of note, D gallinae and O sylviarum can be distinguished from each other based on subtle differences in morphology; for instance, the posterior genitoventral shield of O sylviarum is narrowly rounded, whereas it is broadly rounded in D gallinae. The dorsal shield of O sylviarum abruptly narrows posteriorly but is more smoothly narrowed in D gallinae.⁶ Additionally, O sylviarum tends to cause more irritating dermatitis in humans than D gallinae.³

Although they can be found worldwide, D gallinae and O sylviarum undergo optimal development at 20°C to 25°C and 70% humidity.^3,5,7 Bird mites generally develop over the course of 5 to 12 days; thus, the population of bird mites in a single nest may grow to the tens of thousands before young birds permanently leave. Dermanyssus gallinae can survive for months in abandoned nests without a blood meal, while O sylviarum can survive for several weeks.⁸ It is important to note that humans are not ideal hosts for bird mites, as they are unable to survive for extended periods of time or reproduce on human hosts.⁹

When bird mites are no longer able to obtain blood meals from nesting birds, they begin their nocturnal migration to find suitable hosts. Bird nests generally are abandoned in late spring; thus, most patients with bird mite dermatitis present to clinics with bird mite dermatitis in late spring and early summer.¹⁰ Mites often travel through cracks in doors, floors, walls, and ceilings but also can gain access to living areas through ventilation ducts and air conditioning units.¹ The mite’s bite and crawling on the skin is sometimes noticed by the patient. In general, however, intense itching is not observed until about 1 to 3 days after the mite makes contact with the skin. Patients often report that pruritus is worst at night.⁹ Papules and vesicles (bite reactions) may accompany the pruritus, and physicians commonly find bloody crust and excoriations in particularly pruritic areas.⁵ Urticarial plaques and diffuse erythema occasionally also may be present.⁹ Bird mites sometimes can be scraped from the skin and observed under light microscopy.¹¹ Blood eosinophilia is not found in bird mite dermatitis. On histologic examination, perivascular eosinophilic infiltration can be seen in the upper part of the dermis.¹²

The differential diagnosis in patients with pruritic dermatitis of unknown origin generally includes scabies, pediculosis, and dermatitis caused by other types of infestation. However, unlike scabies, bird mites do not cause burrows to form on the skin.⁹ The presence of a bird’s nest near the area where the patient lives places bird mite dermatitis higher in the differential.

Dermanyssus gallinae is a known vector of bacteria (eg, Salmonella, Shigella, Staphylococcus, Spirochaete, Rickettsia, Pasteurella, Chlamydia psittaci, Erysipelothrix rhusiopathiae) as well as the viruses that cause Eastern and Western equine encephalitis and St. Louis encephalitis. Transmission of these bacteria and viruses is known in birds, but transmission to humans has not been reported.^2,5,9,13

The management of bird mite dermatitis is straightforward. Usually mites can be successfully removed from the skin simply by bathing. Symptomatic treatment for bites with antihistamines and topical corticosteroids is sometimes but not always necessary.² Unlike scabies or lice, there is no need for treatment with lindane.¹ In terms of the prevention of additional bites, any bird nests located near living areas should be removed. Because bird mites often retreat back to nests between blood meals, insecticide sprays generally are unnecessary in interior spaces. Synthetic pyrethroids (eg, bifenthrin, cyfluthrin, cypermethrin, deltamethrin, cyhalothrin) can be used outside and in attics where nests may be located.^2,14,15 However, the ability of bird mites to develop resistance to repeated chemical control could become a future concern.¹⁶

Research regarding the true incidence of bird mite dermatitis is lacking. Some researchers believe that the condition is underreported, possibly due to its uncommon environmental origin.³ Reports of bird mite dermatitis in the literature also are scarce. Our case demonstrates the importance of taking a thorough patient history to rule out exposure to bird mites. All patients with pruritic dermatitis of unknown origin should be questioned about possible contact or proximity to bird nests. These simple questions can lead to the correct diagnosis and a treatment plan that will quickly and effectively resolve the pruritic skin eruption.