great lakes dairy sheep symposium - the Department of Animal ...

• In my personal experience, I have never diagnosed a Suffolk or Hampshire sheep with
OPP though I have had testimony from producers that have purchased positively infected
black-faced rams.
• Certain breeds seem to have a higher rate of infection. Note the breed alliance list at the
end of this paper.
Diagnosis of OPP
Diagnosis of this disease is best provided through finding antibodies in the blood (serology).
Because the virus is slow growing and hides in the cells, the sheep will be slow to make
antibodies to it. The tests available are not perfect but, if used properly, will help in eradication.
Diagnosis with a positive serological test in a clinically affected sheep is only a start. Testing to
determine who may be infected and spreading the virus is the tricky part.
There are two commonly used serological tests available.
• AGID (agar gel immunodiffusion)
- Very specific and rare to get false positives.
- Only about 75% sensitive. The sheep has been infected a while before seroconverting.
- May give false negatives early in infection – or suspects.
- Difficult test to read so the lab technician reading it is very important.
- Use a lab that runs them regularly and has a record of accuracy.
• cELISA (competitive enzyme-linked immunoabsorbent assay)
- More sensitive test therefore can detect antibodies earlier in infection (2 weeks?)
- Less specific due to sensitivity. Other factors can trigger false positives.
- Laboratory differences confuse results.
• Other tests available
- PCR (polymerase chain reaction) reads DNA
- Postmortem ID of virus
Control of OPP
If you decide to embark on an eradication program for your flock, testing every sheep over 6
months of age will tell you what your incidence is. With this information, you can make
decisions whether to cull and retest over a series of 6 month intervals or keep separate flocks and
orphan raise your replacements. Don’t forget the rams.
There are many factors that need to be considered in these decisions – do you have genetics
that you want to save, do you have time to orphan lambs, facilities to keep positive and negative
flocks separate?
• > 50% positive, probably orphan the females, maybe some ram lambs for breeding. Need
to catch the wet lambs prior to nursing, preferably before they hit the ground. Leave the
wether lambs and feeder ewe lambs on the positive dams. The lambs need to be raised in
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