Premium Cattle Health Scheme

This disease is a chronic, progressive, wasting condition that affects ruminants and is caused by the organism Mycobacterium avium subspecies paratuberculosis.

Strains of the organism appear to be specific to the different species of ruminants and if cross infection does occur it does so rarely. The infectious agent is shed in large numbers in faeces; it can be found in colostrum and can pass into the womb to infect the developing calf.

Animals are infected by ingesting the agent and young animals are considered to be the most susceptible to infection. However, clinical signs of diarrhoea and weight loss usually occur some time after 18 months of age.

In heavily infected herds this leads to a high rate of wastage in cattle at two to four years of age. Infection is nearly always introduced to a herd by purchasing infected replacement breeding stock, including bulls.

The absorbed ELISA for the detection of serum antibody (blood or milk test) and the culture of the bacterium from faeces are both valuable tests for the diagnosis of Johne's disease. However, they can only be used to detect infected animals in the later stages of the disease, once clinical disease becomes apparent and for a short period prior to this.

This means a simple test and cull programme is not enough, and it must be supplemented by the removal of offspring of any positive dam from the breeding herd in an effort to exclude animals before they show signs of the disease.

Because of the difficulties with testing and because the infection can survive in the environment for a limited time, control and eradication is more difficult than for the other diseases in the scheme. However, the on-going losses due to the disease and the risk to herds purchasing cattle from infected herds means that an effort should be made to eradicate the disease from infected herds.

Vaccination is useful in heavily infected herds to reduce the number of cases and therefore to reduce the amount of infection in the environment. Vaccination will not remove the infection from the herd, but it is an aid in the control of Johne's disease.

There is likely to be an increasing need for herds that sell breeding stock to be accredited free of Johne's disease.