Evidence for age susceptibility of cattle to Johne's disease.

Calf rearing programs for the control of bovine Johne's disease (BJD) in dairy farms have been widely adopted globally and are based on evidence that the most significant risk factor for developing the disease is exposure of young calves to infectious doses of the causative organism Mycobacterium avium subsp. paratuberculosis (Mptb). Hygienic calf rearing practices aim to break the transmission cycle of Mptb by removing neonatal calves from their dams within 12h of birth and segregating replacement heifers from the herd until they are 12 months of age. But compliance with these interventions is difficult for many producers and delaying the removal of calves from their dams and earlier return of heifers to the herd are common practices. However, would changing these practices increase the risk of animals contracting BJD? Evidence for age susceptibility of calves and young adults to Mptb is reviewed. The experimental studies selected for inclusion in an analysis of the evidence were those designed specifically to address the issue and were confined to examination of 140 cattle in experiments conducted by eight groups of workers between the years 1938 and 2006. Approximately 75% of calves <6 months of age, 50% of those aged between 6 and 12 months, and just less than 20% of cattle >12 months old developed lesions indicative of BJD infection when exposed to any of the tested routes of Mptb infection. No direct evidence was found to support the commonly held view that calf removal from the dam for a maximum period of 12h is preferable to 24h. However the studies did show that if exposure to infection occurs at birth, then the risk of infection progressing to BJD is high, particularly in a highly contaminated environment or if the dam is infected.