Up to 70% of stock hit by Johne's

Article By: Emma Tankersley Date: 9/07/2002 09:07:10 Source: Rural News

JOHNE'S disease in New Zealand may affect 60%-70% of sheep flocks and dairy herds, according to Dave West from Massey University’s Institute of Veterinary, Animal and Biomedical Sciences.

West chaired a Meat New Zealand funded session on Johne’s disease at last month’s New Zealand Society of Animal Production conference.

Johne’s is a chronic intestinal disease caused by infection with the bacteria Mycobacterium avium subspecies paratuberculosis.

West says Johne’s is difficult to control because it has a long incubation period, can be easily spread by apparently healthy but infected animals, and the organism can survive for a long time outside a host.

Diagnostic tests to detect clinical and subclinical - when the infected animal doesn’t show symptoms - forms of the disease can be unreliable.

It is believed animals are infected by ingesting the bacterium early in life, but the clinical disease does not usually occur until after year two in cattle and sheep.

Offspring may also be infected while in the uterus if the mother is heavily infected.

Only a small proportion, usually less than 1% of animals in a flock or herd develop clinical signs. Over a period of several weeks, affected animals lose weight, develop chronic diarrhoea and die. There is no known cure.

The cost of Johne’s disease in the cattle, sheep and deer industries is about $29.2 million - relatively small when compared to the value of the industries. However, West says there is a social stigma associated with presence of the disease on farms. For this reason, New Zealand opted to treat the disease as a manageable condition and give farmers a range of control options.

This contrasts with Australia, which identifies flocks as infected or low risk, and uses zoning to help control the spread of the disease. The disease is eradicated from sheep farms by destocking for up to 18 months. West says this level of control has a high financial cost and increases the stress and negative stigma associated with the disease.