Diagnostic tests for Ovine Johnes Disease
Note Number: AG0966
Published: October 2003
Updated: Novevember 2009
Understanding the strengths and weaknesses of the diagnostic tests for ovine Johnes disease (OJD) is important.
The two main tests used to detect ovine Johnes disease (OJD) in live sheep are: (1) faecal culture that detects the OJD bacteria in animal manure, and (2) a blood test that detects antibodies to OJD.
Faecal culture to grow the OJD bacteria
Infected sheep can shed the OJD bacteria in their manure. Faecal culture involves collecting manure and attempting to grow the bacteria in a laboratory. If OJD bacteria grow, the disease must be present in the flock. However, faecal culture will still not detect every infected sheep. Faecal culture takes at least 12 weeks to complete because the OJD bacteria grow very slowly.
Pooled faecal culture (PFC)
This involves the collection of single faecal pellet samples from up to 50 sheep and the pooling of these samples for culture in the laboratory.
In most circumstances, faecal culture tests are preferred to blood tests as it is possible to detect infection at an early stage of the disease. An advantage of PFC is that it enables flock testing for less than half the cost of blood testing. Testing seven pools of 50 sheep (350 sheep per flock) by PFC is considered equivalent to blood testing 875 sheep.
Serial faecal culture (SFC)
This involves the collection of three faecal samples from an individual sheep at 10 to 14 day intervals. SFC may be used to test valuable individual animals where slaughter is undesirable. This technique goes some way to overcoming the problem of infected sheep not shedding OJD bacteria all the time.
Polymerase chain reaction
Polymerase chain reaction (PCR) tests work by detecting sequences of DNA that are specific to a type of bacteria. It is currently used to: (1) confirm the bacterium that has grown in culture is the Johnes disease-causing bacteria, and (2) determine whether the bacteria is the strain that affects cattle or sheep.
It is hoped that in the near future PCR can be used directly on manure rather than a culture of manure. This would decrease the time taken to obtain results significantly.
Blood tests for OJD
Blood testing is not used routinely in Victoria as faecal culture is the preferred test for OJD. The most commonly used blood test for OJD in sheep and goats is the agar gel immunodiffusion (AGID) test. This test is most reliable in the later stages of infection, when the antibody response occurs.
If antibodies are detected, the animal is highly likely to be infected. Only two in every 1000 non-infected sheep will falsely test positive. However, the AGID test only detects about 30% of infected sheep. This makes the test useful for determining if a flock is infected, provided large numbers of animals are tested, but less reliable as an individual animal test.
Concentrating testing efforts on sheep that are wasting improves the chance of detecting disease, because these animals are more likely to have been infected for long periods of time, and therefore to have developed an antibody response. In this instance, the AGID test can detect at least 60% of infected sheep.
Confirming infection
A flock is not classified as infected solely by a sheep returning a positive blood test; a necropsy must be conducted to determine if the sheep is truly infected.
Sheep with advanced OJD show characteristic lesions on post-mortem examination. These include thickening of the gut wall, prominent lymphatics and swollen lymph nodes. Infection is confirmed by a veterinary pathologist, who must identify both the characteristic lesions and the presence of the OJD bacteria by microscopic examination. Culture of gut tissues may be necessary to confirm OJD if the microscopic examination is inconclusive.
Abattoir Inspection
Abattoir inspection is performed when sheep are slaughtered in a meatworks. Accredited inspectors become highly skilled at detecting the signs of Johnes disease. The advantage of abattoir inspection is that hundreds of sheep can be examined in an hour and the costs of inspection are relatively low.
Further References
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
This note was developed by Susan Bibby, October 2003.
It was reviewed by:
Tracy Bradley August 2004.
David Champness, Animal Health Field Services. November 2009.
ISSN 1329-8062
Published and Authorised by:
Department of Primary Industries
1 Spring Street
Melbourne, Victoria
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