Are Farming and Animal Exposure Risk Factors for the Development of Granulomatosis With Polyangiitis? Environmental Risk Factors Revisited: A Case-control Study.

OBJECTIVE: To investigate the possible association between animal exposure and risk for granulomatosis with polyangiitis (GPA). METHODS: Patients with GPA at the Department of Rheumatology, Uppsala University Hospital, between January 1, 2011, and December 31, 2018, were consecutively included. All patients filled in a questionnaire on possible environmental exposures: occupation, hobbies, and animal contact. As controls we included 128 patients with rheumatoid arthritis (RA) and 248 population controls collected from the Epidemiological Investigation of Rheumatoid Arthritis (EIRA) study, matched for age, sex, and geographical area of residence. The controls filled out a questionnaire on current and past contact with farming and animals, at the time of the RA patient's diagnosis. RESULTS: A total of 62 patients with GPA, 128 patients with RA, and 248 population controls were included in the study. GPA was significantly associated with horse exposure, with a 2- to 3-fold increased risk compared with RA (OR 3.08, 95% CI 1.34-7.08) and population controls (OR 2.61, 95% CI 1.29-5.29). Borderline increased risks were found for any animal contact, but no association was found when analyzing contact with cats/dogs only. A significant association was found between GPA and farming compared to the population controls (OR 7.60, 95% CI 3.21-17.93). CONCLUSION: This study has identified for the first time, to our knowledge, a significant association between exposure to specific animals, namely horses, and the development of GPA. The results also support previous studies reporting an association between farming and GPA, underscoring the possibility of exogenous factors as initiators in the development of GPA.