VAIL, COLO. — West Nile virus has very quickly become the most important cause of arbovirus encephalitis in the United States.
West Nile virus (WNV) first appeared in the Western Hemisphere in New York City in 1999. Yet today the overall frequency of West Nile encephalitis in the United States approaches that of herpes simplex encephalitis, Dr. Kenneth L. Tyler observed at a conference on pediatric infectious diseases sponsored by the Children's Hospital, Denver.
“If you account for the fact that most West Nile cases are concentrated in the summer months and in certain geographic areas, whereas herpes of course is year-round and not with any geographic predilection, in some of those areas the relative incidence of West Nile encephalitis over herpes simplex encephalitis is actually quite large,” said Dr. Tyler, professor of neurology, medicine, and microbiology at the University of Colorado, Denver.
The often-cited figure for the U.S. incidence of herpes simplex encephalitis is four cases per 1 million population per year. That means in the central states, where WNV is concentrated, the incidence of WNV encephalitis is actually at least 25-fold greater than herpes encephalitis, he added.
Neurologic disease caused by WNV takes three forms: encephalitis, accounting for 55%-60% of cases; meningitis, accounting for 25%-40%; and the often-devastating acute flaccid paralysis, comprising 5%-10%.
Patients with meningitis or encephalitis typically develop fever, headache, nausea and vomiting, and myalgia. Rash, diarrhea, and arthralgia are also common.
Hallmarks of encephalitis, in which the infection involves brain tissue, are a change in mental status and a variety of movement disorders, including postural or kinetic tremor and parkinsonism. Weakness is often impressive. Ataxia and myoclonus are common. Seizures are not.
The cranial nerves most often involved are the second, manifested in blurring and other visual changes, and the seventh, with a uni- or bilateral facial paralysis in a Bell's palsy-like pattern, the neurologist continued.
Unlike in herpes simplex encephalitis, where well over 90% of biopsy-proven affected patients display intracranial neuroimaging abnormalities, that's true for only about half of patients with WNV encephalitis, and even then special MRI techniques such as flare or fast-spin often are required.
The cerebrospinal fluid (CSF) can provide a clue to the diagnosis of WNV meningitis and encephalitis. Unlike in the classic picture of CNS viral infections, where the CSF shows lymphocytic pleocytosis, Dr. Tyler demonstrated in a study of 250 patients with WNV meningitis or encephalitis that 37% of encephalitis patients and 45% of meningitis patients had polymorphonuclear leukocytes as the predominant cell type in their CSF, a pattern that often persisted for a week or more (Neurology 2006;66:361-5). The CSF contained normal glucose levels, elevated protein, and a mean of about 226 cells/mm
Another diagnostic clue is that the CSF of a patient with WNV CNS disease often appears highly reactive, “almost like a cancer,” according to Dr. Tyler.
Studies within the last year indicate the recovery phase following symptomatic WNV infection may be more prolonged than originally thought, even in patients with West Nile fever rather than neurologic disease. In a not-yet-published 108-patient Texas study, 60% of patients reported persistent symptoms of weakness, fatigue, memory deficits, personality change, new-onset depression, or walking difficulties after 1 year; 42% had continued complaints after 5 years.
However, in a more rigorous longitudinal study using validated test instruments, Dr. Mark Loeb of McMaster University, Hamilton, Ont., and his coinvestigators found that recovery from the physical, mental, mood, and fatigue symptoms of WNV took about 1 year on average, with patients having neuroinvasive disease taking slightly longer than those with West Nile fever (Ann. Intern. Med. 2008;149:232-41).
Acute flaccid paralysis is a poliomyelitislike disorder that is marked by the acute onset of rapidly progressive asymmetric flaccid weakness. Nearly 90% of affected patients reach their maximum degree of involvement in less than 24 hours.
“I've seen patients who were fine in the morning and quadriplegic in the evening. That's how rapidly it can progress,” Dr. Tyler said.
Acute flaccid paralysis is predominantly a lower motor neuron phenomenon. The paralyzed limbs typically have decreased or absent reflexes but preserved sensation. Cranial nerve involvement is a feature in 70% of cases.
Among the more common manifestations are dysphagia, extraocular eye movements, facial paralysis, and vocal cord paralysis. Respiratory failure occurs in 40% of cases. The mortality in patients with respiratory impairment is 50%, Dr. Tyler said.
Survivors of acute flaccid paralysis show variable improvement, coming mostly in the first 4 months. Persistent weakness is common among survivors who have respiratory involvement.
