Studies of tuberculosis in man in relation to infection in cattle.

A study has been made in Sweden to investigate whether the risk of tuberculous infection and its trend with time in man in different areas were related to varying prevalences of tuberculous infection in cattle. It was found that the level of the infection risk in man was related to the prevalence of tuberculous infection in cattle, varying from 3-4 per cent in 1935 at age 15 in counties with less than 2 per cent infected cattle to 5-8 per cent in counties with 20 per cent infected cattle or more. However, the downward trend to the risk of infection with time was not to found be associated with the prevalence of tuberculous infection in cattle. The correlation between the risk of infection in man and the prevalence of infection in cattle in the 24 Swedish counties was positive and highly significant (+0-79). The relationship between cattle tuberculosis and tuberculosis in man was further studied by correlating infection in cattle with tuberculosis mortality and morbidity in man. The correlation with mortality was negative and highly significant (-0-77), i.e. counties with little cattle tuberculosis had a high tuberculosis mortality in man, and vice versa. There were similar large negative correlations with measures of tuberculosis incidence. When variations between the counties in relevant environmental factors, namely capital, urbanization and overcrowding, were taken into account, a strong positive association remains between the prevalence of infected cattle and the risk of tuberculous infection in man, but the associations with tuberculous mortality and morbidity, though they remain negative, become weaker. It is suggested that the probable explanation of these findings is the long-term protection against adult infection with human tubercle bacilli conferred by bovine infection in childhood.