Mycobacterium avium paratuberculosis

Johne's disease (JD) is a serious disease of cattle, sheep, goats, alpaca, llama, camels and deer. It produces chronic diarrhoea or ill thrift, leading to emaciation and eventually death.

There are two known strains of the organism:

• a cattle strain, which infects mainly cattle, alpaca, goats, deer and camels to cause bovine Johne´s disease (BJD)
• a sheep strain, which infects mainly sheep and goats to cause ovine Johne´s disease (OJD).

Johne's disease is notifiable under legislation. If you suspect the disease in animals, you must report it to Biosecurity Queensland.

The disease occurs worldwide. Australia is fortunate in having relatively little JD compared with most developed agricultural countries. In this country, it occurs in particular areas of the southern states, and particularly in dairy cattle herds and sheep flocks in high rainfall areas.

Queensland is regarded as having a very low prevalence of JD. It is extremely important that everyone involved with livestock takes great care to avoid introducing the disease into Queensland.

Young animals are infected by ingestion of M paratuberculosis either when suckling their dam or grazing contaminated pasture. The bacteria invade the mucosa of the small intestine and lymph nodes. After a period of time, usually years, infected animals may develop clinical signs.

Clinical signs of JD infection are a gradual loss of bodyweight despite a normal appetite. During a period of several weeks, diarrhoea develops in cattle, but not routinely in sheep. Animals showing signs of disease will inevitably die. There is no treatment.

The bacteria may be found in the colostrum, milk and faeces of infected animals. Apparently healthy carrier animals with no signs of disease as well as clinical cases can shed the organism.

Bacteria are transmitted from an infected female animal to its off-spring in colostrum or milk or from faecal contamination of the teats and udder. Ingestion of contaminated food or water can also cause infection.

Bacteria survive in faecal material and on pastures where other animals can pick up the infection. In wet conditions, bacteria can survive in the environment for up to a year.

Blood and faecal samples are collected from live animals to test for JD. The blood test is rapid but unreliable as an individual animal test because false positives and negatives can occur. The blood test is suitable only as a herd test. Faecal culture provides a reliable result but the test takes up to three months to complete. Tissue samples of the small intestine can be collected at autopsy for histological examination and culture.

There is no treatment for JD and animals showing clinical signs inevitably die. A vaccine is available to aid protection against ovine Johne's disease (OJD) in sheep and goats, and one is being developed for use in cattle.

The laboratory tests for JD are unreliable or take a long time to complete; therefore, eradication of the disease is not easy. Leaving properties or paddocks empty of stock for a period of time can eliminate the bacteria from the environment. The period of time will depend on environmental conditions.

A National Johne's Disease Control Program (NJDCP) has been in place in Australia since 1996. Specific control and management plans are available for OJD and BJD. The Australian Johne's Disease Market Assurance Program, an audited quality assurance program, incorporates animal health risk assessment, testing and movement controls. It provides a source of low-risk animals for the various industry sectors in the jurisdictions where the disease is known to occur.

In Queensland, the disease is prevented through legislation and measures designed to keep the disease out of the state and detect it promptly should it enter.

It has been speculated that the mycobacterium responsible for JD in animals is linked to Crohn's disease in people. Considerable research has been conducted and an association between the two conditions remains to be proved.