Johne’s Disease (JD) is an infection of the small intestine caused by the bacterium Mycobacterium avium var. paratuberculosis (Mptb).
Herbivores such as cattle, sheep, goats, alpaca and deer can contract JD. ovine (sheep), bovine (cattle) and bison strains of the bacterium have been described in Australia, but these strains are not host-specific. It is no longer considered valid to refer to Ovine JD (OJD) and Bovine JD (BJD) as different diseases.
The disease can possibly also infect humans, but it is not known if this causes illness. The bacterium Mptb has been isolated from human patients with Crohn’s Disease, but it has also been isolated from healthy people, and it has been impossible to demonstrate the presence of it in other Crohn’s patients.
Due to it being a gut infection, Mptb bacteria are excreted in faeces and contaminate the environment.
The higher the stocking density, the greater the levels of contamination and potential infection. The bacteria can survive in the environment for many months – more than a year, if conditions are favourable. Destocking of the premises is therefore not a practical step in managing the disease.
Mptb is a slow-growing bacterium and the onset of disease after infection is also slow. It can take as long as two years from first infection for sheep to show signs of illness – mostly wasting and diarrhoea. Less information is available for goats, but it is likely that this also applies. In the case of cattle, the interval can be as long as five years.
This slowness gives rise to much controversy about the economic impact of JD. In enterprises with a high turnover of stock, it is possible that JD will never become clinically evident, which has an adverse impact on diagnosis. Although it is considered the ‘gold standard’ to grow and demonstrate the bacterium in cultures from samples, it takes about 13 weeks before the bacterium can be identified in the laboratory.
Other diagnostic techniques include molecular methods, e.g. demonstrating the Mptb DNA with a Polymerase Chain Reaction (PCR) test. Although PCR is extremely sensitive, false negative results are possible. Infected animals differ in the rate at which they excrete Mptb bacteria and this rate may also vary during the course of the disease. It is therefore possible that a faecal sample taken for DNA extraction for the PCR may not contain any Mptb bacteria.
There are also various serological methods (e.g. ELISA) which demonstrate an immune response, but are less reliable for drawing a firm conclusion about an animal’s JD status.
There is one commercial vaccine each in Australia for JD in cattle, sheep and goats. No vaccine can be 100% effective, as it relies on the immune competence of the vaccinated animal. However, both Silirum and Gudair vaccines have demonstrated ability to reduce the number of cattle and sheep showing signs of JD, to slow down the appearance of signs of illness in infected herds and flocks, and to reduce the number of bacteria excreted by sick animals.
Producers who have not seen signs of JD in their livestock during the past three to five years and have not introduced new stock onto their premises may assume that their stock are free from the disease. Good animal husbandry should maintain that status, through hygiene and biosecurity.
Inadvertent introduction of the infection in new livestock can be prevented either by maintaining a closed herd or flock, or only allowing animals of a known or certified low JD risk onto the premises.
Producers of different types of livestock have managed JD differently in recent years. There are sheep, goat and alpaca market assurance programs (MAPs) to assist with this process. CattleMAP has recently been replaced by the Johne’s Beef Assurance Score (J-BAS) and the Dairy Score.
J-BAS and Dairy Score are risk profiling tools where the scoring system allows for the assessment of the risk of a herd for JD, based on a biosecurity plan for the property. Although J-BAS is a voluntary tool in most states, it is part of the WA and NT entry requirements, so is necessary for entry into those markets. Under J-BAS, each producer is responsible for their own risk management.
From 1 October 2017, LPA accredited producers will be required to meet new biosecurity and animal welfare requirements, and complete a three-yearly assessment in order to continue to be LPA accredited. This means producers will need to confirm they have a Farm Biosecurity Plan in place and implement best-practice biosecurity practices on-farm.
It is important to note that an enterprise biosecurity plan is intended to manage the risk not only of JD, but of any condition with a potential adverse impact on animal health. The plan should provide contingencies for the introduction of weed seeds, drench resistant worms, lice, etc in order to protect profitability and market access.