BY ALEX ASHWOOD

BRAHMAN NEWS SEPTEMBER 2008 Issue #160

Johne’s disease (JD) is caused by a bacterium (mycobacterium avium paratuberculosis) which slowly but progressively infects domestic and non domestic ruminants. JD is a serious disease to the grazing industries since it can reduce profits, increase social costs and introduce negative animal welfare issues.

Stock exposed to the bacterium contract JD in their early stages of growth (up to 12 months) but remain asymptomatic usually for several years before showing weight loss and diarrhoea. In some cases the onset of clinical symptoms can be a considerable period after exposure to the disease.

Placing the stock at risk and not managing the spread of the disease can cause serious economic and social costs to producers, particularly those enterprises highly dependent on stud stock sales. Commercial properties with infected herds can consign stock to the meat works, however, clinically affected stock command low prices due to their poor condition and presentation. With a better understanding of the nature of the disease and appropriate preventative management, producers can reduce the threat of contracting JD and minimise the potential economic losses associated with the disease.

OCCURRENCE OF JD

The disease mainly occurs in moist environments particularly where there are high concentrations of stock (stocking densities/hectare). Producers purchasing from locations with a known incidence of JD need to ensure they are obtaining stock from non infected herds.

Movements and transfers of stock from infected herds can be a serious threat to the herd health of individual properties. For instance, the use of dairy or beef stock with an unknown JD status in ET programs can pose a serious threat to the herd as well as the subsequent expensive progeny.

DESCRIPTION OF JD

JD is a sinister disease because it develops slowly and can be misdiagnosed due to the relatively late development of clinical signs. JD is characterised as an inflammatory bowel disease which causes low production since stock are not absorbing the nutrients provided by pastures and supplements. As the disease progresses, the appetite of the stock declines and the stock show signs of severe weight loss and chronic scouring.

The clinical signs of JD are diarrhoea appetite loss and a severe reduction of body condition.

The onset of clinical symptoms are quite variable and subject to a variety of factors including behavioural and environmental stress. The average age of stock showing clinical signs is 4-5 years but can vary between 2-8 years. The shedding of bacteria again is variable and ranges between 6-18 months prior to clinical signs of JD.

TRANSFER OF JD

The major route of infection is from the intake of infected manure. Once the bacterium is in the environment the potential for infection is greatly increased. This is because with clinical cases the rate of shedding of bacterium is high and the increased levels of manure grossly enhances the chances of transmission.

Stock most susceptible to infection are calves less than 6 months of age. Where the bacterium survives well (moist environments) there is a greater risk of calves becoming infected from manure on teats, stock camp sites, and pastures.

A lesser route for transmission is the transfer of bacterium to the foetus via the placenta from infected cows. A further important avenue for infection is from milk and colostrum. Obviously milk from infected cows should not be used to rear calves. Herd practices such as feeding stored colostrum and multiple suckling can increase the potential spread of the disease in infected herds.

IDENTIFICATION OF JD

JD is difficult to diagnose early in infection where there are no symptoms. The bacterium hides in the lymph nodes in the bowel in a dormant state. The body’s immune system only produces JD antibodies when the bacterium starts to multiply and moves to the cells lining the gut wall.

Blood tests (eg ELISA) detect antibodies to the bacterium and are only useful to identify infected animals when antibodies are raised. They are most reliable when animals are showing symptoms of JD. The other main test used is detection of the bacterium in the faeces by culture of the dung. This test also is most reliable late in infection when large numbers of the organism are present (shedding) in the dung.

A small number of non infected animals can be positive to the antibody test (false positives) but this does not occur with cultures so manure samples are used to verify positive blood test results. The confidence in the herd status is increased by testing at least 50 animals in the herd. The more time the herd is tested negative the greater the likelihood that the herd is free from infection and emphasis should be on keeping infection out by controlling straying and checking the herd status of any introductions. In Australia, zoning is used to indicate areas where more BJD has been detected. Protected and Free zones have little or no known BJD while residual and Control zones have higher known numbers of infected herds. 

Another tool to indicate risk of infection is individual herd status which reflects whether the herd has been tested and how often. The Cattle Market Assurance program (Cattle MAP) is a voluntary BJD accreditation program for individual herds wishing to promote their BJD herd status and tested herds are classified as MN1, MN2 or MN3. Check Test and “Beef Only” are other BJD herd statuses with less rigorous eligibility criteria that are also used to promote herd status or facilitate stock movement to other areas and states.

PREVENTATIVE MANAGEMENT

There is no treatment for JD and prevention is the only means of control. Key considerations in preventative management include:-

• Sourcing stock from properties with a clean JD herd status. The likelihood of infection is proportional to the number of stock sourced from non tested herds in locations where there is a JD problem.
• The identification and removal of infected stock.
• Implementation of control programs that are practical, economic and achievable.
• Since the main spread of JD is through manure, most successful control programs revolve around manure management and the reduced exposure of young stock to infection.
• Elimination of herd and feed practices that lead to infection is as important as the removal of infected stock.
• Re-stocking of infected pastures and yards by young stock after 12 months of spelling free from infected stock.
• Continuous testing of infected herds to monitor the progress of control measures.

COSTS OF JD

Due to the slow development of the disease, the economic costs of disease are frequently hidden. Nevertheless, obvious losses include low meat prices, the purchase of replacement stock, lost stud stock sales and the costs of preventative management.

Less obvious costs include infertility, low production, reduced feed conversion and increased susceptibility to other diseases.

An unmeasurable expense is the social cost (anxiousness and concern) as a result of dealing with the disease.

A further issue that could impact on meat sales are the public’s perceptions and expectations that wholesome (clean-green) meat products are produced from healthy, well managed stock.

BOTTOM LINE

JD is an untreatable sinister disease which has been detected in cool temperate and moist subtropical environments particularly where there has been a history of dairy farming.

It is probable that with ongoing surveillance and increased national movements that the incidence of JD may be found in other environments and locations.

It is mindful to remember that once the bacterium is in a suitable environment it has the potential to slowly spread throughout the entire herd causing serious economic and social costs.

As with most diseases, prevention is the best control and the most effective means to eliminate the costs of this sinister disease.

QUEENSLAND SITUATION

The QDPI&F have indicated that Queensland is a Protected Zone for Bovine Johnes Disease (BJD) (JD News 2007) as there are no known BJD infected herds.

A survey of dairy herds in 1997 showed that 2.3% of the cows sampled gave a positive reaction, however none of the cows produced a positive dung culture after further faecal testing.

At present a further dairy cow survey is underway and if no infected cows are detected it will imply the BJD is not present in the Queensland dairy industry. The survey involves approximately 20% of the States dairy herds. To date 5115 cows from 143 herds have been blood tested and only 0.39% have shown a positive blood reaction.

It is known that blood tests give a low percentage of false positives, subsequently faecal samples have also been collected for culture analysis.

BJD is a notifiable disease and both the dairy and beef industries are keen that BJD is kept out of Queensland. It is subsequently important that producers source stock from herds that have a free status.