Hamster Polyoma Virus: Separating the Facts from the Fiction.

by Liz Johnson

History

In the seventies, unusual outbreaks of infectious lymphomas were first seen in Syrian hamsters. We now understand the cause of this disease, how to recognise it and how to deal with it.

Hamster polyoma virus, abbreviated as HaPV, is a relatively new disease in Syrian hamsters. It probably came from European hamsters some decades ago – a short time in virus terms. HaPV is not common, but infections of colonies of both Syrian and European hamsters have been reported in Europe and the United States. It has not been seen in any other species, hamster or otherwise.

Classification

HaPV belongs to a diverse family of viruses called the Papovaviridae. It was put here because of its structure, not the disease it causes. In fact papovaviruses cause a huge range of diseases quite dissimilar to HaPV. For this reason, it is much better to use the correct name hamster polyoma virus as this immediately tells us that it causes multiple lymphomas. This is especially important when talking to your vet.

How the virus works

The likely source of virus is infected hamster urine and this is why good hygiene and husbandry are crucial. Once a hamster is infected, the virus spreads through many body systems without causing symptoms for 4-30 weeks. Sometimes, but not always, HaPV will go on to cause multiple internal lymphoma tumours, as the name “polyoma” suggests. The hamster may then develop tumours of the hair follicles called “trichepitheliomas”. These are often seen in groups around the eyes, mouth, feet and anus.

Why does the disease sometimes seem different?

When HaPV infects a new group of hamsters, it can be very virulent and infect as much as 80% of individuals. Once the virus is in the group, however, the infection rate falls as young hamsters become resistant. Eventually, only the old or sick may be susceptible. The unfortunate few are more likely to develop the trichepitheliomas, although we are not sure why.

These differences in disease and infection rate have lead to great confusion and suggestion that several viruses are involved. In fact we know definitely it is the same virus, but different hamsters respond differently for a variety of reasons such as age, health and immunity.

It is a feature of viruses that with time they become less virulent. HaPV is no exception and we should be heartened by this. Remember, we have already coped with it for 30 years – at least!

What to look for

Infected hamsters lose weight and condition. Lumps may be felt within the abdomen (lymphomas). These can be in may different organs from the liver to the spleen. Wart-like nodules may be seen on the skin (trichoepitheliomas). In addition, as the hamster is weakened, it may develop Demodectic mange which will appear as hair loss, with scaliness of the skin and itching.

How can we and our vets diagnose HaPV?

This is a fairly simple process, even without tests. Firstly lymphomas are extremely rare in young hamsters and should immediately raise suspicions. Secondly, trichoepitheliomas are ALWAYS caused by HaPV. If the hamster has these, it is a definite diagnosis.

Tests on tissue from lymphomas and trichoepitheliomas may not reveal virus. This is common with HaPV and is a result of the way the disease works and the difficulty of the tests. This is an important point to remember. There is currently NO efficient laboratory test.

Will the vet know about HaPV?

Hamsters are not as numerous as say, cats and dogs. For this reason, not all vets are hamster experts. They should, however, know the significance of a polyoma virus. If you suspect HaPV, tell your vet immediately and, if necessary, it is perfectly acceptable to describe what you know, or even show this article.

Treatment

Unfortunately there is no treatment for individual hamsters yet. Culling the entire group is advised, followed by rigorous disinfection of cages etc, by specifically ANTIVIRAL agent such as VIRKON (widely available from vets and pet shops)

It is also wise to wait some months before restocking. HaPV is quite tough, with some reports of recurrence even after thorough disinfection. It is better to be cautious.

The implications for the future

On a much more encouraging note, it can safely be said that there is no need for hamster owners to panic and the Syrian population will NOT be wiped out! There are such myths floating around, which although well intentioned, are not based on fact, understanding or knowledge.

Viruses become less virulent with time. To an individual owner or breeder, HaPV can cause devastating loss, which should not be underestimated. The hamster population as a whole, however, is quite safe and over the years, HaPV will become less of a problem. This is a fundamental fact of virus evolution.

The key is to spread awareness of the disease, something we can all be involved in. It may be hard, but action must be taken as soon as an infection is supected. Finally, the value of good husbandry and hygiene cannot be overemphasised.

We can do much to control the problem, but it is essential that EVERYBODY takes responsibility.

Liz Newbery Johnson
BSc(Hons) Microbiology, MRes. (Biomedical research) Currently studying at Glasgow Vet School.

Please can I acknowledge my main source text:
Pathology of Laboratory Rodents and Rabbits. Authors: Percy & Barthold ISBN 08138 2551-2
-containing an excellent, comprehensive chapter on hamster husbandry and pathology, which I would recommend to interested parties.

Many thanks also to friends and researchers at Glasgow Vet School, particularly Professor Os Jarrett and Dr Joyce Ferguson

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