Back to Viruses
The Parvoviridae are single-stranded DNA (ssDNA) viruses. Having no cell structure of its own it infects the body’s cells to replicate. Parvoviruses preferentially replicate when cells are in a mitotic stage of cell division such as is seen during fetal development.
During the active stage of the infection clinical signs may include:
The modes of transmission are horizontal through exposure to infected urine, feces, nasal secretions, milk, fomites (inanimate objects or materials which can convey disease-producing agents), and through the placental barrier. Although transmission is not typically by aerosol, the viruses it can become airborne on particles of dust during the disturbance of the rats’ bedding or litter.
Parvoviruses target replicating tissue and can cause cell and tissue destruction. This explains why RV, while in its active stage, will interfere with the formation of offspring while they are developing in utero or perinatally. Such interference may result in smaller litters, stillborns, or the resorption of litters.
Antibodies will form within 7-10 days after exposure. It has been concluded through research that the anti-viral antibodies do not clear infection (Ball-Goodrich et al., 2002).
If an exposed female has already built up antibodies they will be passed on to her offspring in colostrum during suckling. After the transference of the antibodies these young rats will test positive, but will not be infected. These babies may then become infected 2-7 months after the effects of maternal antibodies have worn off. At this point they would have less evidence of clinical disease.
If the mother has been infected and has not had time to build up antibodies, then the developing babies can be affected in utero. Infection early in the pregnancy may lead to partial or total resorption of the fetuses. If a dam is infected in the last trimester (with no antibodies present) she may then transfer the virus either in utero or through milk, after they are born, to the surviving offspring.
Infection occurring from the third trimester in utero through the first week of life can result in rats with a more persistent infection. Early infection such as this can take up to six months to clear.
The persistence of the virus depends largely on the age and/or immunocompetence of the rat when it is exposed. The younger the rat is at initial infection the longer the disease will persist. Young rats are immunologically naïve and therefore stay infectious much longer than adults.
Once the rats get past the active infection, typically 60-90 days, the virus does remain in certain tissues including the spleen, lymph nodes, and walls of blood vessels. These areas of infection are not affiliated with the transmission routes (such as the urinary tract, mammary glands, and digestive system) and therefore the virus will no longer be transmitted via urine, milk, or feces. Note that once a rat is exposed it will continue to test positive for RV on serology tests even after the active infection as cleared.
Research has shown that if a normally post-infectious rat is stressed either physically or psychologically there is a possibility that the RV can be reactivated and transmitted to other rats.
Other factors that contribute to infection and transmission issues are the rat’s natural resistance to disease, whether the rat is immunocompromised (e.g. athymic), and the particular strain of RV you are dealing with. In a rat with immune system disorders it would be possible for RV to persistently infect. Particularly virulent strains may persist past the normal predicted times.
RV antibodies can show up on a standard serum test as soon as seven days post-exposure but it is recommended to wait 14-21 days after possible exposure for more accurate testing. The rat being tested should be at least two months of age. Remember that young adult antibody-positive rat may have maternal antibody and not antibody developed as a result of active Infection.
If your vet does not have the equipment to spin the blood then the lab can explain alternate methods. Usually .5mL of whole blood is required to make between .1 - .25mL of serum (minimum for one test is .1 mL). Pooling blood will decrease the accuracy of your results. Do not add anticoagulants. If the sample is diluted with saline please note the ratio (usually 4 parts saline - 1 part whole blood). Samples can be iced and shipped overnight.
The ELISA is used for initial screening with RV as part of the basic series. If the RV test is positive, other more specific tests discriminating among H-1 virus, RV and RPV infections can be run. You can specifically request RV (KRV) testing.
ELISA (enzyme-linked immunoabsorbent assay) - primary screen
IFA (indirect fluorescent antibody assay) - confirmatory test
In the breeding environment, it may affect the outcomes of planned breeding. It is not typically a health issue and can be active in a colony with little or no effect.
In a colony, the RV infection can be kept active by the addition of naïve rats, either in the form of new litters or by bringing in rats from outside sources.
Once rats have shed the virus they are not typically contagious to others, but they will continue to test positive for RV and there is no conclusive proof that it can no longer be transmitted. It is possible for the RV Infection to become reactivated in a rat, thus infecting new additions. It is also possible for new rats to become infected via fomites.
A positive ELISA does not necessarily mean the rat is infectious to others. A positive result can simply mean that the rat has been infected at some point in the past and is no longer shedding the virus. DNA testing performed on excreta can indicate whether or not virus is still being shed. More Invasive (terminal) procedures are required to show whether the rat Is Infected at sites not expected to be associated with transmission. DNA testing may not be a practical option for the pet owner or hobby breeder because it is very expensive and may require euthanasia of the rat.
Parvoviruses are extremely stable in the environment and can remain viable for weeks or months. It is this characteristic that facilitated the worldwide spread of canine parvovirus in the early 1980s. Removing the virus from an environment, especially in a non-lab setting, would be a monumental. The first step would be to thoroughly scrub to remove all debris on cages, accessories, walls, and floors with soapy water then sterilize with bleach solution. No surface, crack, corner, or wire could be ignored if the clearance is to be as complete as possible. Carpet, furniture, and drapes may present a problem during an attempt to disinfect. Dishes, bowls, and water bottles would also need this treatment. Porous items such as wicker nests and wood accessories could potentially harbor the virus and elimination may need to be considered. Food and litter, even though unused may also have been contaminated during normal handling and may also need to be eliminated. This would have to be completed after the quarantine and well before any rats were brought in.
Attempting to clear a colony of KRV would have to start by following a quarantine of 60-90 days for juvenile and adult rats and up to six months for rats suspected of in utero infection or within one week of birth.
Moving the rats exposed as infants (or in utero) to another location would decrease quarantine time. There would have to be a complete moratorium on breeding and acquisition of new rats.
After a full quarantine and a thorough cleaning, the success of the campaign can be tested by bringing several new rats into the colony, distributing them throughout the sub groups and then ELISA testing them two - three weeks [prefer 4 weeks If low-level contamination Is still present] later. Negative testing will not be conclusive proof that the colony is 100% free of active RV but may serve as an indicator.
Keep in mind that it is also possible for the previously infected rats, through reactivation, to transfer the disease to the new rats at any given time causing active infection. Periodic testing of rats that came in after the full quarantine will aid in determining if this has happened.
Posted on December 12, 2003, 16:20,
Last updated on February 23, 2012, 16:57