Johne's disease

Johne's disease (JD) is caused by infection with Mycobacterium avium subsp. paratuberculosis (Mptb), a species of bacteria that grows slowly in the gut wall of infected animals. 

There are two strains of Mptb of significance to Australian sheep and cattle producers. 

1. The cattle strain of Mptb called Bovine Johne’s Disease (BJD) mainly affects cattle, goats, deer and camelids (i.e. alpacas and llamas)
2. The sheep strain called Ovine Johne’s disease (OJD) mainly affects sheep and goats. 

Cross-species transmission (sheep strain infection in cattle and vice versa) does happen, but only very occasionally. Sheep and cattle strains are not considered as separate infectious agents and if cattle is infected with either strain, it is classified as BJD.

Not all animals exposed to Mptb will become infected and not all infected animals with go on to shed Mptb. 

The consequence of exposure in an uninfected animal depends largely on four factors:

1. the JD strain the animal is exposed to
2. the dose, or the number of bacteria the animal is exposed to
3. the age of the exposed animal
4. individual animal factors (genetics and immunity).

Symptoms

Bacterial infection causes the walls of the gut to become thickened, which interferes with the absorption of food. This leads to loss of condition even when the sheep is on good feed. If on poor feed or stressed the disease may be noticed sooner.

An animal becomes infected when it eats or drinks something which has been contaminated by faeces infected by the bacteria. Animals are more likely to be susceptible to infection when young, especially up to 12 months of age and in addition the fetuses of clinically affected dams can become infected.

Infected animals:

• will progressively lose weight despite drenching, antibiotics and a good appetite
• may also have diarrhea and bottle jaw
• may show scouring although it is not a common symptom
• will waste away until they die or are euthanised. 

As the disease has a long incubation period and the thickening of the gut develops slowly over time, an infected animal is more likely not to show clinical signs until they are over 2 years old.

Often skinny sheep and cattle are first noticed when the flock or herd is brought in for shearing, lambing or calving.

Prevention of Bovine Johnes disease

By far the greatest risk to uninfected beef herds is infected dairy cattle shedding bacteria, or land contaminated by dairy cattle shedding bacteria. 

Dairy cattle

1. To eliminate the risk entirely, do not buy dairy cattle.
2. If you are going to purchase dairy calves:
   • do not mix them with calves that are planned to be retained in the herd
   • move them off property by 18 months of age
   • do not run calves that are planned to be retained on land grazed by cattle from the dairy industry for at least twelve months after the dairy cattle have moved off that land.
3. If you are in the business of buying adult dairy cattle do not mix them with cattle under a year of age that are planned to be retained or intended to be sold as breeders.

Agistment

Many producers agist their cattle in coastal areas when feed on the tablelands is in short supply. 

1. If agisting, it is important to ask about the grazing history of the property.
2. Avoid grazing cattle you plan to retain that are less than a year of age on agisted property that has been grazed by adult dairy cattle, particularly in the previous twelve months.

Cross-species transmission with sheep

Sheep flocks infected with sheep strain Mptb present far less of a risk for JD transmission to beef herds than dairy cattle. 

To reduce the risk of beef cattle becoming infected with sheep strain Mptb on mixed enterprises:

• ensure sheep are vaccinated with Gudair®, including any bought in sheep, to reduce the dose of Mptb to which both young sheep and young cattle are exposed. It’s important to know that vaccination will not eradicate the risk of shedding Mptb
• do not co-graze sheep and cattle (i.e. do not graze cattle and sheep in the same paddock at the same time)
• avoid grazing paddocks previously grazed by older sheep with young replacement cattle for the first three months after the older sheep have been moved off the paddock
• if you find a ewe with signs consistent with OJD, cull that animal promptly and mark her lamb to ensure it is not retained as a replacement animal.

According to the current BJD policy, clinical JD in either sheep or cattle can impact the Johne’s Beef Assurance (J-BAS) score of cattle from the same property.

Prevention of Ovine Johnes disease

OJD has a long incubation period and there is no treatment or cure.

Vaccination

On infected properties, a Gudair® vaccination program dramatically reduces the burden of disease and mortalities. 

Current research has shown that on many infected properties, low levels of infection will persist even with a long history of Gudair® vaccination and will increase once vaccination ceases.

Until further information about eradication is available, ongoing vaccination is strongly encouraged. 

If any new sheep are acquired make sure, they have been vaccinated with Gudair®, either as lambs if they are coming from high risk or infected properties, or when they are brought onto the property.

There is no benefit from Gudair® vaccination of lambs you will sell to slaughter. 

Culling

The burden of disease can be reduced by culling any sheep suspected of being infected sooner rather than later and by identifying and culling lambs from any ewes suspected of being infected.

Grazing management

Reduce infection rates in young stock by ensuring weaned lambs don’t graze paddocks for at least three months after they have been grazed by older classes of sheep. 

Consider fencing off low areas or creeks where water may be contaminated.

Buying in sheep

Always ask for a Sheep Health Declaration when buying in sheep, which has a large section describing the OJD risk of those sheep.  

If your property has no history of infection, only buy in sheep with a high level of assurance of having not come off of a property infected with OJD. To do this you can buy in sheep from an area previously classified as low prevalence and/or buying sheep from flocks in SheepMAP.

Neighbouring flocks

Consider the risk that neighbours with sheep present to your property, in particular the potential for movement of water between properties, movement of sheep faeces (e.g. the potential for a summer storm to wash sheep faeces from a neighbour’s sheep camp on the high side of the boundary fence), and straying stock.