Reply to Hall et al.: Hypersensitivity Pneumonitis Mortality by Industry and Occupation.

From the Authors:

We welcome the comments by Dr. Hall and colleagues on our identified long-term trends in hypersensitivity pneumonitis (HP) mortality in the United States (1) and share the goal of improving our understanding of the burden of HP and increasing our awareness of this disease to improve diagnosis and treatment.

HP is assuredly a complex, multifaceted lung disease, involving both genetic and environmental risk factors. We agree that the racial differences observed in our study are not solely due to genetic factors. As noted in our Discussion, the contribution of specific exposures is an important but unmeasured variable in our study, and region-specific variations in HP mortality are almost certainly affected by differences in occupational exposure across states. However, regardless of ethnicity/race confounding bias by workplace exposure, genetic variation (which can be related to and differ by race) is likely one of the many risk factors that act together to cause disease and should be considered when interpreting epidemiological data. For example, significant ethnic/racial differences have been described in idiopathic pulmonary fibrosis that may be related to genetic differences (2), and differences in specific gene polymorphisms associated with susceptibility to HP have been observed both between groups with different ethnic backgrounds and within ethnic/racial groups (3–5). Further work is needed to fully understand the role of genetic variation in the molecular and cellular processes, clinical characteristics, and outcomes of HP.

We certainly agree that occupational exposures are an important contributor to the overall burden of HP in the United States and that they should always be considered in both individual and population studies. Since the 1980s, various agricultural activities, including crop and animal farming, have been the most frequently recorded industries and occupations on death certificates for individuals with HP. However, when studying the role of occupation, an important limitation of the National Occupational Mortality Surveillance database is that less than half of the states provide data on the usual industry and occupation of decedents (limiting generalizability). Importantly, in contrast to work-related HP, nonoccupational HP cases are not reportable and therefore are likely undercounted and underrepresented in assessments of the relative contribution of occupational and nonoccupational exposures responsible for HP cases nationwide (6).

We hope that the insight gained from our study and the complementary data of Hall and colleagues will serve as a call to state health department officials and federal agencies to revise key policies regarding disease surveillance programs, risk management, and enforcement of exposure limits, particularly in the agricultural sectors, in an effort to reduce the impact of HP in the country.