Johne’s disease is an invariably fatal disease of cattle, goats, deer, camelids and sheep.
Johne’s disease is caused by the bacteria Mycobacterium paratuberculosis. The bacteria grows in the intestine wall which becomes thickened and less effective at absorbing nutrients from the food.
The bacteria which causes JD is very persistent, with the bacteria able to live for a period of up to 12 months on the ground. Contamination of the ground must therefore be considered in any control program.
In cattle, calves up to 30 days are very susceptible and readily infected if exposed to the bacteria. They are still susceptible up to 12 months — they do not often become infected after the first year. In most cases, cattle are infected on their property of birth.
The most common method of animal-to-animal spread is to the calf from its dam via germs on the udder. Calves can also be infected by cross-suckling. Animals up to 12 months of age may pick up infection from contaminated ground. Some calves may also be born infected.
Because of the risk of infection — before as well as after birth — all progeny of infected cows are highly likely to be infected.
Milk from infected cows is also considered a risk. Artificial insemination and embryo transfer are considered very low-risk sources of contamination.
The most common form of spread from farm to farm is via an infected animal (one generally showing no clinical signs). Environmental spread has to be considered but is relatively uncommon.