Management of Feline Retrovirus-Infected Cats

Katrin Hartmann, Munich, Germany

In domestic cats, three retroviruses have been identified: feline immunodeficiency virus (FIV), feline leukemia virus (FeLV), and feline foamy virus (FeFV). All three are global and widespread but differ in their potential to cause disease. FeFV (previously known as feline syncytium-forming virus, FeSFV), a spumavirus, is not associated with disease; thus routine testing is not performed.

FIV, a lentivirus that shares many properties with human immunodeficiency virus (HIV), can cause an acquired immune deficiency syndrome in cats leading to increased risk for opportunistic infections, neurologic diseases, and tumors. In most naturally infected cats, FIV infection does not cause a severe clinical syndrome, and with proper care FIV-infected cats can live many years. Many in fact die at an older age from causes completely unrelated to their FIV infection. In a study of naturally FIV-infected cats, the rate of progression was variable, with death occurring in about 18% of infected cats within the first 2 years of observation (about 5 years after the estimated time of infection). An additional 18% developed clinical signs, but more than 50% remained clinically asymptomatic during the 2 years of observation. FIV infection has little impact on a cat population and does not reduce the number of cats in a household. Thus overall survival time is not necessarily shorter than in uninfected cats, and quality of life is usually fairly high over an extended period of time.

FeLV, an oncornavirus, is the most pathogenic of the three viruses. Historically, it was considered to account for more disease-related deaths and clinical syndromes than any other single infectious agent in cats. More recently, prevalence and consequently the importance of FeLV as a pathogen in cats have been decreasing, mainly because of testing and eradication programs and the use of FeLV vaccines. The death rate of persistently viremic cats in multicat households is approximately 50% in 2 years and 80% in 3 years but is lower in cats kept strictly indoors in single-cat households. Despite the fact that persistent FeLV viremia is associated with a decrease in life expectancy, many owners elect to provide treatment for the clinical syndromes that accompany infection. With proper care, many FeLV-infected cats kept only indoors may live for many years with good quality of life.

Therefore a decision for treatment or for euthanasia of a cat should never be based solely on the presence of a retrovirus infection. FIV- and FeLV-infected cats are subject to the same diseases that befall cats free of those infections, and illness in a given cat may not be related to the retrovirus infection. However, in all cats, healthy or sick, FIV and FeLV status should be known because these infections affect health status and long-term patient management.

Management of Retrovirus-Infected Multicat Households

When a cat in a multicat household is diagnosed with a retrovirus infection, all cats in that household should be tested to determine their infection status. If positive and negative cats are identified within the same household, the owner must be informed of the potential danger to uninfected cats and told that the best method of preventing spread of infection is to isolate the infected individuals and prevent them from interacting with housemates. However, despite this admonition, the overall risk of transmission between these cats is not high for either infection. FIV transmission in households with a stable social cat environment is rare because FIV is transmitted mainly through biting and fighting; if no fights occur, FIV is not likely to be transmitted. Long-term studies of households with FIV-infected cats have shown that a few additional cats may become FIV-positive over time, but in some households no transmission occurred over many years. All cats in these households should be neutered, and new cats should not be introduced into the household because this may lead to fights and to increased risk for transmission, even between cats that have lived together peacefully for a long time. The benefit of FIV vaccination of FIV-negative cats living with a FIV-positive cat is controversial. The vaccine currently available has not been tested under these conditions. Usually the FIV subtype of the infected cat is unknown, and cross-protection by the vaccine against some FIV subtypes (e.g., the frequently found subtype B) is uncertain. Because the vaccine contains whole virus cats respond to vaccination by producing antibodies indistinguishable from those present during natural infection. Therefore vaccinated cats are antibody positive, and assessment of their true infection status can be difficult. Before vaccination is considered, ideally the veterinarian should ensure that the virus in the infected cat can be detected by polymerase chain reaction (PCR) (which is possible in only 50% to 90% of cases). Only if the virus strain of the infected cat is detectable by PCR is it possible to identify later another cat in the household that may become infected despite vaccination.

If an FeLV-infected cat has lived for some time in an otherwise FeLV-negative household, the other cats that have been in contact with the FeLV-shedding cat already will have been infected. Thus these cats most likely are immune to new infection, and an owner may elect to keep all of the cats together. However, studies in cluster households have shown that eventually other cats in these households may become FeLV viremic. This is most likely the result of reactivation of a long-persisting latent infection, but the risk of new infections cannot be excluded totally as virus-neutralizing antibodies may not persist for life. The risk of becoming viremic in adult FeLV antigen-negative cats is approximately 10% to 15% if they live with an FeLV-shedding cat for several months. If owners refuse to separate housemates, the uninfected cats should be administered FeLV vaccination to enhance their natural level of immunity. However, owners should be informed that vaccination may not provide sufficient levels of protection in these environments of high viral exposure.

Management of Individual Retrovirus-Infected Cats

The most important life-prolonging advice the veterinarian can give to owners of retrovirus-infected cats is to keep the cats strictly indoors. This avoids spread to other cats in the neighborhood and prevents exposure of the immunosuppressed, retrovirus-infected cat to infectious agents carried by other animals. In FIV-infected cats, secondary infections cause clinical signs and may lead to progression of the FIV infection. This is probably not the case in FeLV infection, in which the retroviral infection is more pathogenic and progresses relatively independently of cofactors.

“Routine vaccination” of retrovirus-infected cats is subject to much discussion. Although there is no scientific proof that retrovirus-infected cats are at increased risk from modified-life virus (MLV) vaccines, inactivated vaccines are recommended out of concern that MLV vaccines given to immunosuppressed animals may regain pathogenicity. FIV-infected cats are susceptible to secondary infection and while in an early stage of infection are able to mount appropriate levels of protective antibodies after vaccination; thus regular vaccination would seem indicated. However, it is the author’s opinion that vaccines should not be given to FIV-infected cats if strictly kept indoors because immunosuppression and immune stimulation can lead to progression of FIV infection by altering the balance between the immune system and the virus. Stimulation of FIV-infected lymphocytes is known to promote virus production in vitro. In vivo vaccination of chronically FIV-infected cats with a synthetic peptide was associated with a decrease in the CD4/CD8 ratio. Thus the potential tradeoff to protection from infection with vaccination is progression of FIV infection secondary to increased virus production. If FIV-infected cats are kept strictly indoors, the risk of secondary infections may be lower than the possible adverse effects of vaccination. In some countries or states legal requirements for rabies vaccination may supersede these issues.

In contrast, FeLV-infected cats should be administered routine vaccinations. Studies investigating the immune response to rabies vaccination demonstrated that FeLV-infected cats may not be able to mount adequate immune responses. Therefore protection in an FeLV-infected cat after vaccination is not comparable with that in a healthy cat, and more frequent vaccinations than usually recommended must be considered (e.g., every 6 months), especially in cats allowed to go outside.

Retrovirus-infected cats should have routine health care visits at least semiannually to promptly detect changes in health status. A complete blood count (CBC), biochemistry profile, and urinalysis should be performed at least annually (CBC every 6 months in FeLV-infected cats to detect anemia or other cytopenias associated with FeLV infection). Intact male and female retrovirus-infected cats should be neutered to reduce stress associated with estrus and mating behavior and the desire to roam outside the house and interact aggressively. Surgery generally is well tolerated by asymptomatic retrovirus-infected cats, but perioperative antibiotic administration should be used for all surgeries and dental procedures. Because both viruses live for only minutes outside of the host and are susceptible to all disinfectants (including common soap) simple precautions and routine cleaning procedures prevent transmission in the hospital. Retrovirus-infected cats can be housed in the same ward as other hospitalized patients but in individual cages. They may be immunosuppressed and should be kept away from cats with other infectious diseases and therefore not be placed in a “contagious disease ward” with cats suffering from infections such as respiratory viruses.

If retrovirus-infected cats are sick, prompt and accurate identification of the secondary illness is essential. Treatment recommendations for specific situations are given in Boxes 272-1 and 272-2. Often the clinical signs of retrovirus-infected cats are not caused by the retrovirus infection and so intensive diagnostic testing for secondary diseases should proceed early in the course of illness to allow for appropriate therapeutic intervention. Many cats with a retrovirus infection respond as well as uninfected cats to appropriate medications, although a longer or more aggressive course of therapy (e.g., antibiotics) may be needed. Glucocorticoids or other immunosuppressive as well as bone marrow–suppressive drugs should be avoided. Griseofulvin has been shown to cause bone marrow suppression in FIV-infected cats and should not be used.

Box 272-1 Suggested Treatment Recommendations for FIV-Infected Cats

Treatment with or without Clinical Signs

If no clinical signs are present: