Johne's disease

JD is caused by the bacteria Mycobacterium avium paratuberculosis.

Several strains of the organism are found in Australia:

• The cattle strain (also called 'C strain' or 'bovine strain') is found mainly in dairy cattle, but also in beef cattle, alpaca, goats, deer and camels, and rarely in sheep.
• The sheep strain (also called 'S strain' or 'ovine strain') is found mainly in sheep and goats, but increasingly in beef cattle that have co-grazed with infected sheep in southern Australia.
• The bison strain (also called 'B strain') has been found in a small number of beef cattle cases in Queensland.

The disease occurs worldwide. However, Australia has relatively little JD compared with most developed agricultural countries. JD is more commonly found in southern states of Australia, and particularly in dairy cattle herds and in sheep flocks in high-rainfall areas.

• Cattle
• Buffalo
• Sheep
• Goats
• Alpaca
• Llama
• Camels
• Deer

The bacteria (Mycobacterium avium paratuberculosis) live and multiply in the small intestine and lymph nodes and are shed in the faeces. They are very slow-growing.

JD is primarily spread when infected animals excrete the bacteria in faeces and susceptible animals ingest the bacteria. Animals can be infected at any age, but juveniles, especially cattle, are more likely than adults to become infected.

Animals often do not show any symptoms of JD for many years. Apparently healthy carrier animals with no signs of disease, as well as clinical cases, can shed the organism.

The bacteria can be transmitted from an infected female animal to its offspring in colostrum or milk. However, the spread is more commonly from faecal contamination of the teats, udder and environment.

Infected bulls, rams and other adult males can also spread infection via their faeces. Ingestion of contaminated pasture, food, milk or water can then cause infection.

Although the bacteria live and multiply in host animals' intestines, they can survive in the outside environment (in water, in faecal matter and on pastures) for several months. In wet conditions, the bacteria can survive in the environment for up to 1 year.

JD has a long incubation period. Clinical disease is not usually seen until at least 2 years after infection (except in deer, which can show disease much earlier). Animals are likely to excrete the bacteria before developing clinical signs. Infection causes a thickening of the intestinal wall, which reduces normal absorption of nutrients from grazing and leads to clinical signs.

Clinical signs in cattle include:

• gradual loss of weight despite a normal, or increased, appetite (wasting)
• chronic diarrhoea (scouring)
• bottle jaw (soft fluid swelling under the lower jaw)
• a drop in milk production (in dairy cattle).

Clinical signs in sheep include:

• gradual loss of weight despite a normal, or increased, appetite (wasting)
• chronic diarrhoea (scouring), but not routinely
• failure of ewes to recover body condition after weaning, followed by wasting
• severe decline in body condition (in wethers).

The progression and severity of disease may be stimulated by stress, especially nutritional stress during lactation for cows and ewes.

Animals showing clinical signs due to JD will inevitably die.

The impacts of JD on productivity vary according to stress and husbandry systems.

• Extensively grazed beef herds typically show lower levels of clinical JD, except during drought or under other stresses.
• Dairy cattle that are under nutritional stress and are potentially exposed to high levels of bacteria have a higher prevalence of infection and higher incidence of clinical JD.
• In sheep, the incidence of clinical JD can be high, especially when cell grazing is used.

The JD risk status of an animal or herd/flock is commonly required for export trade certification. If your market success may be affected by JD, you should protect against entry of infection and manage any current or potential infection to prevent spread of the disease.

Spread of JD between properties is usually due to the movement of animals, either by deliberate movement or by animals straying.

However, spread between adjoining properties without known animal movement may occur due to:

• movements of water (creeks or flooding)
• sheep faecal pellets rolling through fencelines
• undetected straying, particularly of sheep.

The highest risk of spread of JD into and within Queensland is the movement of livestock from:

• high-risk populations interstate, especially dairy cattle and sheep from areas that have a high prevalence of infected flocks
• properties where infection is known or suspected.

There is risk of infection throughout the year.

Legal obligations

Monitor the health of your livestock. If you become aware of or suspect clinical JD in an animal, you must report this as soon as practicable to:

• Biosecurity Queensland on 13 25 23
• or
• the Emergency Animal Disease Hotline on 1800 675 888.

As an animal owner, you have a general biosecurity obligation to take all reasonable and practical measures to prevent or minimise the effects of biosecurity risks such as JD. This means you are legally required to reduce the risk of JD and limit its spread when dealing with possible carriers.

Risk profiles

The current national approach to JD focuses on managing this endemic disease.

Each livestock industry has developed risk-profiling tools for managing JD biosecurity risks. These voluntary tools help to identify herds and flocks that have a low risk of infection They include the:

• Johne's Disease Assurance Score (J-BAS) for beef cattle
• Johne's Disease Dairy Score (JDDS) for dairy cattle
• Australian Johne's Disease Market Assurance Programs (MAPs) for goats and sheep.

However, when you are planning to buy livestock or otherwise introduce livestock to your property, do not just rely on a risk-profiling score. Also ask further questions about the JD history in the herd/flock and property.

Testing

Pre-clinical infection with JD is difficult to detect by testing because the bacteria grow very slowly and there is a slow onset of a specific immune response.

To get the best value from testing for the purpose of monitoring a herd or flock, test a large number of animals. An all-negative result indicates a low risk of infection in the herd or flock and in animals derived from that population.

A positive result to a blood test usually, but not always, indicates infection. Infection can be confirmed only by a positive result from a culture of faeces or tissue, or from microscopic examination of tissues.

You do not need to report a positive result to a blood test during assurance testing unless there is other evidence such as symptoms of clinical disease or historical links to known disease.

Treatment

There is no treatment for JD, and animals showing clinical signs inevitably die. However, there are vaccines that can help protect against the spread and clinical disease of JD in sheep, goats and cattle.

Risk management

The national industry bodies for beef, dairy, sheep, goats and alpaca have tools and frameworks in place to help producers assess and manage JD risks. These are available through Animal Health Australia.

If you have an infected or suspect herd or flock, you must manage JD as part of your general biosecurity obligation. You need to take all reasonable and practical measures to minimise the likelihood and impacts of JD.

Selling or supplying livestock

When selling or supplying JD-susceptible livestock, provide a health statement/declaration (such as a National Cattle Health Declaration) to the buyer or receiver. The statement/declaration must include clear, complete and accurate information about the health of the livestock, particularly if the livestock is infected or is reasonably presumed to be infected with JD.

The buyer or receiver can then assess the likelihood of the livestock being infected with JD and possible impacts of this (such as spreading JD).

Buying or receiving livestock

When buying or receiving JD-susceptible livestock, insist that the seller or supplier provides a health statement/declaration. Obtain and check the statement/declaration before making a purchase decision and before introducing the livestock onto your property.

Managing an infection

If your herd or flock is infected or is likely to be infected with JD, retain the stock and their effluent and excrement within stock-proof boundaries.

Take these steps to minimise the risk of spreading JD and other impacts of the disease:

• Isolate at-risk animals.
• Retain the herd or flock only for growing out and fattening and slaughter.
• Vaccinate at-risk animals.
• Separate calves from adults.
• Plan paddock rotation to minimise the chance of spreading the infection.
• Decontaminate paddocks after exit.
• Monitor livestock health through regular testing.
• Consult your veterinarian to devise a detailed management plan.
• Do not use or consign for human consumption any animal showing clinical signs due to JD. Such an animal has an increased risk of secondary infections that may be harmful to human health, and is also likely to be unfit for transportation.