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You are here: Agriculture » Pests, Diseases and Weeds » Animal Diseases » Sheep » Why Control OJD disease

Why Control Ovine Johne's Disease?

Note Number: AG1127
Published: June 2008
Updated: January 2011

 

Since Ovine Johne’s Disease (OJD) first appeared in Australia in 1980, the number of infected sheep flocks has slowly increased. It is no longer considered possible to eradicate OJD from Australia, and models predict that OJD will spread extensively if left uncontrolled.

Controlling OJD on individual farms and nationally is beneficial to Australia’s sheep industry because:

• OJD can cause significant economic loss on farms where clinical disease occurs
• Producers in areas with little or no known OJD want to limit the entry of sheep from higher risk areas
• controlling OJD reduces the likelihood of a negative impact on our export markets.

Why manage OJD on farm?

There are two main ways OJD can affect the profitability of a sheep farm:

1. Through decreased production and production losses
2. Through reduced marketing opportunities of sheep

Controlling OJD will limit these impacts on your sheep flock.

Loss of production due to OJD

A major economic effect of OJD is the increase in mortality within a flock. Owners of flocks that are infected with OJD and undertake no appropriate management measures may, over time, experience heavy losses as the level of infection within the flock increases. Annual death rates in Australian sheep flocks of up to 17% have been seen.

OJD is often not diagnosed in a flock until a significant proportion of the flock is already infected.  The disease has a long incubation period (the time period from initial infection of the sheep until it shows signs of disease) and infected sheep can be shedding the bacteria in their manures for a considerable period (even years) prior to showing signs of disease. Therefore, a large number of sheep in a flock can be infected before disease is detected. As sheep are more likely to show signs of clinical disease if they are stressed, death rates in a flock may increase significantly during times of stress (eg. drought) and OJD is often diagnosed in flocks during or after such periods of stress. 

In these heavily infected flocks, sheep may start to waste and die from 14-15 months of age or earlier.

Australian research has also shown that OJD infected sheep may start to suffer from weight loss nearly 12 months before they die and that sheep in early stages of the disease produce less wool than their flock mates.
Fertility rates can fall, while susceptibility to other diseases such as internal parasites can increase.  Cull rates may need to increase to remove high-risk mobs.

However it’s not all bad news. Vaccination is proving to be extremely useful in assisting farmers to control the spread of OJD within a flock by reducing, over time, the level of infection on the farm and, therefore, the economic loss due to OJD.

Other management strategies, such as culling of high risk sheep and grazing management, can be used in conjunction with vaccination to minimize the lost production due to OJD.

Financial assistance measures are available to assist with managing OJD. Financial subsidies are available to obtain professional advice for the development of a Property Disease Management Program (PDMP), for vaccine and to undertake flock testing. (Refer to Information Note: AG1042 Financial assistance measures for OJD in Victoria)

In addition many of the control strategies used for OJD, such as worm control, maintaining boundary fences, improved grazing and culling management, can assist in maintaining a healthy flock. 

Controlling OJD may improve your flock’s marketability

With the changes to the Victorian OJD Prevalence Areas coming into affect in January 2011, there will be no restriction on the trade of sheep from flocks affected by OJD within Victoria.  However, access to interstate markets and other markets (such as the live export trade) may be restricted. These impacts are particularly important for sheep studs and sellers of store sheep.

Under the national Assurance Based Credits (ABC) scheme, developed by the sheep industry, sheep flocks have several options to enhance their marketability through vaccination and testing. The program is based on a points scheme that indicates the risk of OJD infection. (Refer to Agriculture Note: AG1156 Assurance Based Credits (ABC) Scheme and the Sheep Health Statement).

By controlling OJD and developing a Property Disease Management Program (PDMP), owners of infected flocks can reduce the impact of OJD in their flock, minimise the risk of spreading disease to neighbouring flocks, and enhance marketability of low risk sheep (this will depend on your current markets).

So whilst trading will not be restricted it is still important to monitor your flock for OJD and implement control strategies.

Other impacts of OJD

Disease spread - flocks owned by neighbours and trading partners can be at risk of infection if OJD is left uncontrolled.

Public health - research to date has failed to substantiate a link between Johne’s disease in animals and Crohn’s disease in humans. This potential link is often given as a reason to control OJD, as a precaution if future research shows that there is a relationship between Crohn’s and Johne’s disease.

The impacts of OJD on your sheep flock can be avoided or reduced:

• by preventing OJD from entering your sheep flock.
• by controlling OJD and implementing a Property Disease Management Program (PDMP) with strategies such as vaccination.
• by finding out whether vaccinating your flock against OJD, testing your flock or implementing other control strategies would protect your enterprise from the effects of OJD.

Further Information

Further information about OJD can be obtained from animal health staff at your nearest DPI office or on the DPI web site: http://www.dpi.vic.gov.au/farming/OJD

Acknowledgements

The previous version of this note was revised by Leanne Dries (June 2008) and Chris Hourigan and was published in February 2005.

Reviewed by:

Rachael Holmes, Biosecurity Victoria. January 2011.