by John Chitty, BvetMed CertZooMed Cbiol MlBiol MRCVS
I read with interest Dr Deeney's article on Aleutian Disease in the last edition. This has been a very emotive subject within the ferret world and, as with most articles on this subject, Dr Deeney's piece highlights how little we really do know about this disease in the ferret.
One of the major problems we have is in identifying how important Aleutian Disease (AD) is as a disease of ferrets. A lot of this relates to difficulties in diagnosing the disease. Firstly, there are no "classic" signs of AD; those typically listed include hindquarter paresis/ paralysis, melaena, lethargy, and liver/ spleen enlargement. In the ferret these are seen in many disease syndromes, including lymphoma which is becoming very common now. A few years ago I carried out post-mortem examination on seven ferrets with hindquarter paralysis; six had spinal abscesses and one had a spinal haemorrhage. None had histopathological evidence of AD. So we should be wary of diagnosing AD on clinical appearance.
Examination of serum proteins by electrophoresis may be of great help. Typically, gamma globulin level of >20% is characteristic of AD-infected ferrets (Besch-Williford (1987)). However, this test is not routine in many laboratories and may be expensive to perform. These levels may also be raised by any other stimulus of antibody production (eg recent vaccination, other infectious agents, and tumours of antibody-producing white blood cells). There are also AD positive ferrets that do not have raised gamma globulins (Besch-Williford (1987)). This has also been seen in ferrets in our practice.
Blood testing (CEP), similarly, is little help in these situations. As Dr Deeney reported, the finding of antibody is synonoymous with having disease. It merely indicates exposure to the virus. This form of testing is also rarely performed on the sick ferret as it is expensive to test the single animal (as opposed to group testing). Ideally all dead ferrets should have a post-mortem examination with histopathological examination of tissues. This way typical AD lesions may be found and correlated with the preceding clinical signs and testing results. However, typical lesions may be seen without clinical signs (Porter et al (1982)) so the finding of mild lesions may not guarantee the diagnosis! Similarly Lloyd (1999) reports that there may be no histopathological changes.
So, ideally to diagnose AD with certainty, we need several if not all of the following:-
Typical clinical signs
Raised gamma globulin levels
Positive antibody test
Does this matter? It certainly does. A lot of AD diagnosis has, in the past, been based on partial evidence and may not have been accurate. Without an accurate knowledge of the prevalence of AD we are unable to say how important it really is.
This brings us to the routine testing of ferrets for AD antibodies.
This is something I have carried out in many ferrets over several years, and have become ever more confused by the results we have been finding. As Dr Deeney reported many ferrets do fluctuate between positive and negative, and it is difficult to decide what these results mean.
The main problem appears to be that ferrets simply do not produce the same level of antibody in response to the virus as mink. Porter et al (1982) report rises in gamma globulin in experimentally infected ferrets of between 0.5-1g/dl, whereas in mink it is much higher (Porter et al (1980)). Therefore, tests designed for detecting these high levels of antibody in mink may not be sensitive enough to detect the much smaller amounts in ferrets. This may explain why some ferrets' results do indeed fluctuate; some days they produce enough antibody to be detected, some days they don't. Like Dr Deeney we believe in the test's specificity, ie a positive test IS a positive test, however, we have seen too many ferrets show inconsistent results to be confident about the test's sensitivity so we now interpret the test as positive and "not positive".
What does this mean regarding screening?
The ideal screening test should be very sensitive. You need to be confident that those animals that are tested negative really are. From the results gained in our tests, I am not sure we can say this about AD.
What about the saliva test? Recently we tested nine ferrets using both saliva and blood tests. Six tested negative on both; one was positive on both while two were positive on blood but not on saliva.
This is a very small sample size but it may suggest that the saliva test is less sensitive than the blood test. This may be a feature of the test or it may be due to the impracticalities of the test procedure. It was very difficult collecting saliva from conscious ferrets, and there was a genuine risk of them swallowing the swabs. For this reason alone I find it hard to justify using this test again.
So what should we do?
We need more information about AD. A lot of our information comes from the mink, yet this may not be accurate. Mink produce more antibody than ferrets, which may be why the tests are so variable in ferrets. It is also the reason for the disease; mink over-produce antibody and the resulting clumps of antibody and virus cause problems in the small blood vessels of the kidneys, heart, lungs, etc. The virus doesn't cause the disease; it is the response of the mink's immune system causing disease. But, if ferrets don't produce that much antibody normally, how will they produce the same disease? We know that ferrets can get clinical AD. But how frequently? How severely? Should we worry more about other diseases such as lymphoma or distemper?
Are there differences relating to different strains of AD? There should be. Porter et al (1982) describe differences between ferrets inoculated with ferret strains and mink strains of AD (and neither produced lesions as severe as those seen in mink). The current tests do not distinguish between strains of virus and it may be that different strains are more significant than others.
Should we cull positive ferrets? If it is proven that positive ferrets WILL go on to have disease and infect other ferrets then we should cull. However, we don't know this. I have been working with a small colony of ferrets which have been maintained in isolation since testing positive. All but one are still very healthy FIVE YEARS on. The sick ferret is doing well on medication for its cardiac disease, and has normal blood proteins. None of them have clinical AD and it is hard to see why they should have been culled.
Can they infect others? Porter et al (1982) describe the finding of low levels of virus in the spleen 180 days post-infection. However, longer term studies have not been done. Nor have studies on the excretion of virus been performed. Similarly we need to assess the effects of stress and disease on their response to AD. We also need to consider whether en-masse testing is appropriate; if ferrets do excrete AD and they do excrete it like mink then bringing together many ferrets for testing sessions may be an ideal opportunity for disease spread; it should be noted that AD can be spread by aerosol in mink! (de Geus et al (1996)).
It seems very simple to recommend culling positive ferrets; "we don't know so lets' play safe". However, this solution is not always palatable (consider the debate surrounding Foot and Mouth culling) and we need to research AD more thoroughly before we can be sure this is the correct response. This starts with ferret vets so we can find exactly how important and prevalent this virus really is.
We then need a more sensitive test. Using immunofluorescence, Porter et al (1982) found 42% of 214 healthy ferrets to have low levels of antibody. We do not know if this is the situation in the UK, as we do not have this test available to us.
In summary, I find it hard to recommend control measure for something about which so little is known.
I also find it hard to recommend culling positive ferrets when I am unsure if they will develop disease, or if they are a risk to other animals.
I am equally unsure that many of the ferrets I have tested and passed as negative really are free of AD. Just as I can re-test a positive ferret and find it negative a couple of weeks later, perhaps I should re-test the negatives?
John Chitty, BvetMed CertZooMed Cbiol MIBiol MRCVS
Besch-Williford, CL (1987) "Biology and Medicine of the Ferret" Veterinary Clinics of North America: Small Animal Practice 17 (5): 1155-1185
de Geus, B, van Eck, J, van de Louw, A, et al (1996) "Transmission of Aleutian Disease Virus by Air" Scientifur 20 (4); 350-354
Lloyd, M (1999) "Ferrets; Health, Husbandry and Diseases" Blackwell
Porter, DD, Larsen, AE, Porter, HG (1980) "Aleutian Disease of Mink" Advances in Immunology 29; 261-286
Porter, HG, Porter, DD, Larsen, AE (1982) "Aleutian Disease in Ferrets" Infection and Immunity 36 (1); 379-386