Johne’s disease is a chronic, progressive intestinal disease caused by infection with Mycobacterium avium subspecies paratuberculosis (MAP).
Animals are usually infected as calves with approximately 80% of infections occurring within the first month of life. The calf may be infected while in the womb, by drinking infected colostrum and milk, or by ingesting faeces with the latter being by far the most important. MAP may last for a year in slurry or on pastures. An infected cow can shed billions of MAP bacteria into the environment for years prior to showing any clinical signs of the disease. Only 1-5% of infected cows in a herd will show clinical signs of the disease. The rest of the infected animals will appear healthy, highlighting the need for testing. Infection is almost always introduced to a herd by purchasing infected replacement breeding stock including bulls.
Johne’s disease has a significant impact on the technical performance of the cow, affecting productivity and fertility. Research has demonstrated that cows infected with Johne’s suffer from impaired production, reduced fertility and higher instances of mastitis and lameness.