Severity of macroscopic lesions in 56 calves experimentally infected with Mycobacterium avium spp. paratuberculosis in relation to age at infection and infectiondose

Johne’s disease is a widely spread chronic wasting disease among cattle. Johne’s disease or paratuberculosis is caused by Mycobacterium avium subsp. paratuberculosis (MAP) and cows get infected at a young age. However, symptoms such as diarrhea and decreased milk production are usually developed after two to ten years. These symptoms are caused by a chronic granulomatous enteritis with regional lymphangitis and lymphadenitis. The aim of this study was to investigate the severity of macroscopic pathological changes in experimentally infected calves in relation to age at infection and infection dose. Therefore 56 calves were given an inoculum with a high dose or a low dose of MAP at either two weeks, 3, 6, 9 or 12 months of age. After 17 months necropsies on all the animals were performed. Macroscopic changes in the calves were ranked in different categories and a statistical analysis was performed on the outcome. Furthermore a large number of samples was collected and different screening tests were performed to confirm infection. Our results show a significant relation between infection dose and severity of macroscopical changes. Also a trend was seen in the relation between age of infection and severity of macroscopical changes