The prevalence of farmer's lung was studied in five districts of the French Doubs province (Rougemont, Clerval, Levier, Montbenoît, Mouthe) on the basis of a medical and occupational questionnaire and serological studies for precipitins. Of the 1763 active farmers (response rate 69%) who participated in the study, 270 were suspected of having farmer's lung and divided, on the basis of clinical and serological criteria, into probable and possible farmer's lung groups. This subgroup of 270 patients was examined in more detail with physical examination, chest radiographs, and pulmonary function tests. Probable farmer's lung was diagnosed in 77 cases (53 men and 24 women, mean age 51.7 years), a prevalence of 43.7/1000. Possible farmer's lung was present in a further 193 subjects. Fever was the most specific symptom. In the group with probable farmer's lung the frequency of crepitations, restrictive ventilatory defect, and radiological interstitial abnormalities was 21.5%, 9.5%, and 9.2% at the time of the study. Farmer's lung was more common in the middle tablelands than in the plain or mountain areas (p less than 0.01). The difference in prevalence of farmer's lung between the Mouthe mountain and Levier middle tableland districts (25.2/1000 and 60.8/1000) raises the possibility that cold may protect against the disease. Farmer's lung was more frequent in men and in the age group 51-55 years. Among possible occupational factors, only regular work connected with feeding animals was associated with a significant risk (odds ratio 2.55 with a 95% confidence interval of 1.09-5.93). The frequency of chronic bronchitis in the population studied was 14.8%. There was a close link between farmer's lung and chronic bronchitis, 50.6% of patients with probable farmer's lung having it but only 8.6% of patients without farmer's lung (p less than 0.001). In patients with probable farmer's lung chronic bronchitis was independent of smoking and age. There were fewer smokers in the probable farmer's lung than in the non-farmer's lung group (p less than 0.05).
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