Extrinsic allergic alveolitis: incidence and mortality in the general population

Background: Extrinsic allergic alveolitis (EAA) is an important clinical entity, but its incidence and significance in the general population are uncertain. Aim: To estimate the incidence of EAA, and resulting mortality, in the UK. Design: General-population-based cohort study in a UK primary care database (THIN). Methods: THIN patients with an incident diagnosis of EAA were compared with a general population cohort whose members were 4:1 matched with EAA patients by age, sex and GP practice. Follow-up started at the first diagnosis of EAA (and at the same date in the matched controls) and ended at death or end of follow-up, whichever came first. Poisson, logistic, and Cox proportional hazard regression models were used; mortality rate, odd ratios, and hazard ratios were calculated. Results: We identified 271 incident cases of EAA (mean age at diagnosis 57 years, 51% male). Between 1991 and 2003, the incident rate for EAA was stable at ∼0.9 cases per 100 000 person-years. In comparison to the 1084 general population controls, patients with EAA were less likely to smoke (odds ratio 0.56, 95%CI 0.39-0.81), but had a marked increase in the risk of death (hazard ratio 2.98, 95%CI 2.05-4.33). Discussion: The incidence of EAA in the UK population appears to be stable overtime, and suggests about 600 new cases of EAA each year. People with EAA are less likely to smoke than the general population, but have a markedly increased mortality rate.