Parvoviruses in the Rat

© Ann Storey MSc FIBMS


Introduction

(There has been some outbreaks of litter loss among breeders over the last year or so and a theory that this could be due to parvovirus infection. Because of this I have written the following article.)

Parvoviruses are small round viruses stable to heat, acid, many solvents and disinfectants. They can survive out of the body at room temperature for many days, freezing and temperatures up to 80C. They have been isolated from a wide range of animals including rodents, birds, cats, dogs, pigs and primates including man where it causes a common childhood infection called 'slapped cheek syndrome'. They are species specific which means that under natural conditions rat parvoviruses only attack rats. Parvoviruses mostly infect only tissues containing rapidly dividing cells such as the liver, bone marrow, developing cerebellum, kidneys and lungs . They can cause latent or subclinical infection as well as obvious illness.

Rat Parvoviruses

There are three groups of parvoviruses in rats, each containing a number of strains of virus. However the only strain that seems to produce disease in the rat is the first one described, Rat Parvovirus (RV) also known as Kilham rat virus. The incidence of RV infection in commercial breeding colonies has been shown to be high. It used to be high in laboratory stocks too but as these have become progressively more disease free, infection in these colonies is now very rare.

Parvovirus infection in adult rats is usually asymptomatic, that is the rats appear fit and healthy. However on one reported occasion two groups of non immune rats were mixed with a colony known to be carrying RV. Two weeks later most of the newly introduced rats developed a staring coat, dehydration and cyanotic scrotums (they looked blue!). Out of 150 rats 32 died. RV was isolated from affected rats. Other reported symptoms in adult rats include cyanosis of the extremities, encephalomyelopathy and paralysis.

For the point of view of the breeder the most serious problem is the infection of pregnant does. Unlike a lot of viruses this one crosses the placental barrier effectively, causing foetal death, reduced litter size, jaundice, deformities and hydrocephalus. Usually rats affected early in pregnancy (before the 12th day) are more likely to suffer foetal death and reabsorbtion meaning you might not even know that they have been pregnant, while rats affected after this may be born alive but with problems. Where these late infected kittens die, the doe may go overdue or have other problems with going into labour. The reason for this is that one of the important triggers for labour is believed to be the increased activity of the foetal adrenal glands, but if they are dead then is trigger is removed. While the doe does eventually go into labour, it tends to be late and prolonged, meaning that many owners resort to caesareans.

In colonies where the infection is endemic, most rats develop antibodies and become immune by the time they are 7 months old. Studies have shown that RV is readily transmitted among rats held in confined quarters and that infections are perpetuated by the introduction of non-immune animals.

Transmission of this virus is mostly via respiratory secretions and, due to the viruses' hardiness, off of contaminated surfaces, hands, water bottles etc. The virus can also be transmitted in their milk and faeces although these routes are not so important. The disease can be transmitted by apparently healthy rats. After infection, baby rats may secrete the virus for ten weeks and at least 7 weeks after they have developed antibodies. Newly infected adult rats (which usually do not show symptoms) secrete the virus for a much shorter period of time. In addition some rats become carriers and while they do not appear to excrete the virus during this period, the disease may reactivate if the rat becomes immunosuppressed due to stress, other disease, radiation, drugs etc.

The Fancy's problem

The recent syndrome that has affected several breeders is not new and outbreaks have been around since the late '70s at least. The usual scenario is that a short time after attending a show, a breeder or breeders will suddenly have a run of does losing their litters. Either they reabsorb or they die or they give birth to dead or dying litters. Occasionally a few kittens may survive. This process will go on for two or three months and then cease. Does which have previously lost litters will go on to breed successfully next time (if they haven't had a hysterectomy that is). Now we have no hard evidence that this is RV. There are two other diseases which can cause litter loss like this, they are Sendai virus and metritis. Sendai virus is an acute respiratory virus which can cause pneumonia in rats. Severely infected pregnant does will lose their litters not because the virus infects the kittens, but because the doe is unable to take in enough oxygen to support the pregnancy. However the doe has respiratory symptoms during this time. The second cause, metritis, is due to infection of the uterus with a respiratory pathogen such as Mycoplasma or Pasteurella. This only normally happens with second or subsequent litters. This is because the infection is probably sexually transmitted and the doe was probably infected during her first pregnancy. Does may either become completely infertile or give birth to dead or dying litters. However, unlike RV, about two to three weeks after they have lost the litter they will develop a discharge, they will not get pregnant anymore and without a hysterectomy they will get progressively more sick.

It would appear from this that the outbreaks of litter loss have more in common with RV than the other two possibilities. Of course, it could be another infection, in which case we will only find out if investigations are performed.

Testing

For Parvovirus infection, the most reliable test is indirect fluorescent antibody assay (IFA). This will detect all strains of rat parvovirus and works by exposing infected cells to your rat's serum. To do the test the vet will have to take a blood sample from your rat. They would normally take this from the tail vien. They should not take more than 1ml per 100gm of rat.

Prevention of infection

It has to be said that if you go to shows, or take in other rats then you will be exposing your rats to infection. There is not much you can do to protect your rats from catching it. Quarantining your show stock is a good idea but to work this must be done rigorously. However, if you suspect that your rats are infected then do not take any rats to shows or to another fancier's house etc. Also, because you might carry the virus on you, either don't go or go in fresh clothes, shoes etc and shower. Do not handle any rats while you are there. In the meantime, if you start to lose litters then it is advisable to stop breeding or taking in new rats for a period of two to three months.

Some people have recommended disinfection. The trouble is that most disinfectants that will work against this virus are things such as 10% hypochorite (bleach) which are unsuitable for use on judging table etc. They also do not prevent airborne transmission or transmission via hands, clothes etc. As I have said, whatever the infection is it has been around a long time and as fanciers we have to accept a degree of risk when we exhibit our rats.

Further Reading

Pathology of Laboratory Rodents and Rabbits 2001 2nd Ed. D H Percy and S W Barthold, Iowa State University Press.

The Laboratory Rat, Volume 1 Biology and Diseases, 1979 Eds H J Baker, J R Lindsey, S H Weisbroth, Academic Press.