The incidence of autoimmune hepatitis (AIH) may be rising, according to a prospective population-based study conducted in New Zealand.
From 2008 to 2016, the rising incidence of AIH led to a 40% increase in point prevalence, reported lead author Mehul Lamba, MD, of Christchurch (New Zealand) Hospital and colleagues.
The present study, which also assessed rates of primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC), adds data to an area of inquiry historically characterized by limited and inconsistent results, the investigators wrote in Clinical Gastroenterology and Hepatology. They suggested that mixed findings from previous studies may be because of differences in population and environmental factors, but also varying diagnostic criteria.
“The epidemiological trends of these autoimmune liver diseases therefore remain incompletely understood,” wrote Dr. Lamba and colleagues.
Their study evaluated trends in autoimmune liver diseases over a 9-year time frame in Canterbury, New Zealand. According to the investigators, this region is well suited to an epidemiological investigation because it is a clearly defined geographic area with approximately 600,000 people, most of whom rely on one tertiary care center: Christchurch Hospital. The bulk of the data therefore came from this center, while a minority of cases were gathered from local private gastroenterology practices, “making complete case ascertainment possible.”
Incidence of AIH, PBC, and PSC was assessed at three time points: 2008-2010, 2011-2013, and 2014-2016. AIH had the highest overall incidence, at 1.93 cases per 100,000 people, followed by PSC (0.92) and PBC (0.51).
While the rates of PBC and PSC did not change significantly over time, the incidence of AIH rose from 1.37 cases per 100,000 people in the period from 2008-2010 to 2.39 per 100,000 in 2014-2016 (P = .04), which computes to an incidence rate ratio of 1.69 (95% confidence interval, 1.02-2.84). Point prevalence was also significantly higher in 2016, compared with 2008, at 27.5 per 100,000 versus 19.7 per 100,000 (P < .01). The investigators described a bimodal age of presentation, with the first peak among patients younger than 20 years, and a second, larger peak among individuals aged 50-69 years.
According to the investigators, these findings “are concordant with the results observed in the European cohort,” citing a Danish study spanning 1994-2012 and a Dutch study spanning 2000-2010. They noted that the Danish study also reported a bimodal distribution of age incidence, as did a Swedish study, and another study from New Zealand. The stable levels of PBC and PSC align with two recent retrospective studies conducted in the United States and, they added.
“We believe that the observed differential trends in the incidence of these autoimmune liver diseases truly reflects their contemporary epidemiology,” the investigators wrote. They went on to suggest that the findings did not stem from an increase in diagnostic scrutiny because the study period did not include any significant changes in gastroenterology service, coding, or diagnostic criteria in the region studied.
“The increased incidence of AIH parallels rising incidence and prevalence of other autoimmune disorders such as [inflammatory bowel disease], type 1 diabetes, and multiple sclerosis in New Zealand, and it is unclear whether these autoimmune conditions share a common local environmental trigger,” they wrote. “Environmental factors likely play a central role augmenting phenotypic expression in genetically predisposed individuals.”
While Dr. Lamba and colleagues proposed several possible factors, such as increased exposure to pharmaceuticals, definitive factors remain elusive, which the authors cited as one limitation of their study. Another limitation they cited is the possibility that other etiologies were mistakenly classified as “probable” AIH; however, the chances of that are small, and the proportion of probable versus definitive AIH noted in this study do reflect those seen in other epidemiological studies.
“The reason for observed differential change in incidence of these autoimmune liver diseases is unclear,” they wrote, “and future collaborative prospective epidemiological study would be required to assess this further.”
The investigators reported no conflicts of interest.