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Abstract
Mycobacterium avium subsp. paratuberculosis (Mptb) causes Johne's disease in ruminants.
Disease control programmes aim to break the faecal-oral cow-calf transmission cycle
through hygienic calf rearing and removal of affected cows from the herd, but these
programmes do not take account of the potential for congenital infection. The aims
of this study were to critically review research on in utero infection, determine
the prevalence of fetal infection in cattle through meta-analysis and estimate the
incidence of calves infected via the in utero route. About 9% (95% confidence limits
6-14%) of fetuses from subclinically infected cows and 39% (20-60%) from clinically
affected cows were infected with Mptb (P<0.001). These are underestimates for methodological
reasons. The estimated incidence of calf infection derived via the in utero route
depends on within-herd prevalence and the ratio of sub-clinical to clinical cases
among infected cows. Assuming 80:20 for the latter, estimates of incidence were in
the range 0.44-1.2 infected calves per 100 cows per annum in herds with within-herd
prevalence of 5%, and 3.5-9.3 calves in herds with 40% prevalence. These estimates
were not markedly sensitive to the value chosen for the proportion of clinical cases.
In utero transmission of Mptb could retard the success of disease control programmes
if the opportunities for post natal transmission via colostrum/milk and environmental
contamination were able to be controlled. The consequences of fetal infection for
the calves so infected are discussed in the context of diagnosis and vaccination together
with recommendations for future research.