Spring is the time of year when many rural properties and paddocks in Tasmania are filled with the sight of newborn and frolicking lambs bleating for their mothers among expanding flocks, but in the aftermath of two wet winters Bruce Jackson, senior vet at the Department of Primary Industries, Parks, Water and Environment, recently issued a warning to the state’s farmers and vets confirming the incidence of Ovine Johnes Disease is on the rise.
From Tasmania’s first diagnosed case in 2001, OJD has continued to spread across the state, but reports have suggested there has been a significantly greater prevalence of the disease during the past five years. It is estimated approximately seven per cent of flocks are now affected, although the actual figure could be as high as 35 per cent according to Jackson, who said an important factor in disease control is open and honest communication between neighbouring producers, as well as a proactive vaccination program.
“The rate of detection of previously unknown infected flocks has increased over the last 18 months, although the flock prevalence is higher in some other parts of Australia where the disease has been established for a longer period. I would encourage vaccinating lambs even if signs of the disease are not yet present in the area because it will only be a matter of time,” he said.
OJD is a wasting disease caused by the sheep strain of the bacterium mycobacterium paratuberculosis which grows mainly in the small intestine. Infection causes the intestine wall to slowly thicken, preventing adequate nutrition being absorbed by the animal. Transmission and infection occurs predominantly when sheep graze on pastures soiled by animal faeces. Death usually occurs within three to six months.
“Due to the long incubation period of the disease in individual sheep, slow onset of cases within flocks and the drought, some producers haven’t realised disease was present until abattoir surveillance detected a case,” said Jackson.
While still to be verified scientifically, fine-wool merinos appear to be more susceptible to OJD than either British breeds or crossbreeds, and anecdotal observations suggest most sheep are infected as lambs, despite adult sheep being equally susceptible. Once clear signs of OJD are evident euthanasia is the best option.
Research into better disease detection is being conducted, and a Polymerase Chain Reaction has been developed for use directly on faecal samples, although the test has yet to receive approval under the Australian and New Zealand Standard Diagnostic Procedures.
Although vaccination in Tasmania is purely voluntary Jackson said the practice had increased dramatically during the past year despite a recognition both by industry and the manufacturer that vaccination will not fully protect against the disease. The manufacturer claims a 90 per cent reduction in mortality, and a 90 per cent reduction in excretion.
“We encourage prevention through good biosecurity practices, use of the Sheep Health Statement and vaccination when the risk of local spread becomes significant. Managers of flocks where OJD is already present should prepare low contamination paddocks for lambing ewes and weaners, as well as vaccinating lambs – preferably at marking – and minimising stress on adult sheep to reduce deaths from the disease,” he said.
Jackson encourages vets in regional and rural areas to consider providing testing services and advice to producers about all aspects of disease prevention, management of diseased flocks, market access issues, and vaccination programs.
“There are several specific roles within the national management program for SheepMAP vets in running the Market Assurance Program, or MAP, mainly for ram sellers, and in testing and certifying low risk groups of sheep on other properties. We would like to see more vets attain SheepMAP accreditation through Animal Health Australia so that producers can have easier access to these services.”
The national OJD Management Plan is currently under review and a public discussion paper that includes four options for disease management beyond 30th June 2012 is available.