Update on Feline Leukemia Virus

Update on Feline Leukemia Virus
by Arnold Plotnick MS, DVM, ACVIM, ABVP
Published in the Catnip Newsletter 12/2010

Despite advances in diagnosis and vaccination, this virus remains one of the most lethal, contagious infections affecting domestic cats.
            Theresa Bachu hovered nervously in the exam room.  As an avid cat rescuer in New York City, she’s seen many of the cats and kittens she’s rescued test positive for feline leukemia virus.  This particular scraggly bunch – a mother and her four kittens just taken off the street –  all had upper respiratory infections, and all were at increased risk for having the virus.  My staff and I drew blood from all five felines, and after ten tense minutes, I had the test results Theresa wanted to hear: all five cats tested negative, and nowcould be placed for adoption. 

            Feline leukemia virus (FeLV) was first recognized in 1964. It quickly became known as one of the most deadly infections to affect domestic felines.  The virus causes profound suppression of the cat’s immune system.  In addition, the virus can transform healthy lymphoid cells (cells involved in the immune response) into malignant cancerous cells. Although widespread testing and vaccination has markedly decreased the rate of infection over the last 20 years, the ability of infected cats to readily shed and transmit FeLV through saliva ensures that this virus will continue to plague cat owners, breeders, and veterinarians for many years to come.

            A variety of factors increase the risk of infection (sidebar 1). Male cats are at increased risk, as are mixed breed cats, and cats that go outdoors.  Young cats and those that reside in multicat households are also at increased risk.  Repeated exposure to a cat that is known to be infected with FeLV greatly increases the risk of contracting the virus.

            FeLV is transmitted in two general ways: between unrelated adult cats 

(“horizontal” transmission), and from a mother cat to her kittens (“vertical” transmission). “The virus is present in high amounts in the saliva and milk of infected queens, and transmission from an infected queen to her kittens is believed to be the most significant source of infection”, says Dr. Michael Stone, board certified internist at the Cumming School for Veterinary Medicine at Tufts University.  Horizontal transmission occurs most commonly via the oral/nasal route, by the sharing of food and water dishes, through mutual grooming, and through sharing a communal litterbox.  “FeLV can also be transmitted through bite wounds, explaining the increased prevalence of FeLV in unneutered males that roam outdoors and engage in territorial battles”, adds Dr. Stone.

            Once the virus enters the body, it replicates in lymph tissue near the site of entry. Typically, this occurs in the oral, nasal, and pharyngeal (throat) tissues.  During this stage of viral replication, cats may show signs of illness – fever, lethargy, diarrhea, and lymph node enlargement.  From here, one of four general scenarios can occur:

  • Progressive infection – the immune system fails to contain the virus. Viral replication occurs in the oral/nasal tissues, and infected cells transport the virus to other tissues, such as the spleen, thymus, and other lymph nodes. The bone marrow becomes infected, as do the salivary glands, and many virus particles are shed in the saliva. These cats frequently succumb to a FeLV-associated illness in a few months or years. 
  • Regressive infection – the cat mounts an effective immune response.  Viral replication occurs near the site of entry of the virus, but the infection is contained and the virus does not invade the bone marrow.  Viral particles or viral proteins cannot be detected using the common screening tests. However, viral DNA can be detected via PCR testing (described below). These cats are at little risk of developing FeLV-associated diseases. 
  • Abortive exposure – after exposure, the cat mounts an effective immune response, and the virus is presumably eliminated from the body. Virus particles or proteins cannot be detected by any test currently employed.
  • Focal infection – a rare scenario, in which the FeLV infection is restricted to certain tissues, such as the spleen, a lymph node, or a mammary gland. 

            There is some controversy regarding the four outcomes of FeLV infection described above.  In the past, many cats were believed to be able to completely clear the virus from their body – a scenario that fits most closely with “abortive exposure”.  Newer research, however, suggests that most cats actually remain infected for life following exposure to the virus. These cats test negative on the common screening tests, and virus particles cannot be cultured from the tissues.  However, a specialized DNA test – known as PCR testing – confirms the presence of viral DNA integrated into the cat’s genome.  These cats are unlikely to ever clear the virus, since the viral DNA has now integrated itself into the cat’s own DNA.  These cats are unlikely to shed virus particles in their saliva, and are unlikely to develop FeLV-associated diseases. The clinical significance of these cats that test negative on the screening tests, but test positive on PCR tests, is not yet clear.  

            Diagnosis of FeLV is based on detection of a protein present on the virus particle.  This protein, called p27, is an antigen – a substance that stimulates the immune system to produce antibodies.  Most cats that are infected with FeLV have abundant p27 antigen in their body fluids.  The most common in-clinic tests detect p27 antigen in the bloodstream.  Most cats, if exposed to FeLV, will test positive within 30 days using these antigen tests.  

            Because a positive test has potentially dire consequences, any positive test should be confirmed, in order to rule out the possibility of a false-positive result.  A second blood test can be performed, ideally using a test from a different manufacturer. Alternatively, a test can be performed that looks for the p27 antigen within infected blood cells, rather than circulating within the bloodstream.  This test, performed on blood smears or on a bone marrow sample, is known as an IFA (immunofluorescence) test. A positive IFA test not only confirms that the cat is infected with FeLV, but that the cat’s bone marrow has been affected – a progressive infection. Sadly, as we noted above, these cats are likely to succumb to a FeLV-associated illness sometime in the future.

            A relatively new type of test, called a PCR test, is perhaps the most sensitive test for the diagnosis of FeLV. This test can actually detect the presence of viral DNA that has been incorporated into the cat’s own genome.  A recent study using PCR showed that 5 - 10% of cats who tested negative for FeLV (i.e., they had no circulating p27 antigen in their bloodstream) had, in fact, a regressive infection – they had FeLV DNA in their genome.

            The FeLV status of all cats should be known because of the serious health consequences of infection.  “Sick cats should be tested, even if they have tested negative in the past”, says Dr. Tina Waltke, a feline practitioner in New York City.  Cats and kittens should be tested when they are first acquired.  Cats with a known exposure to a FeLV-infected cat, or to a cat with unknown FeLV status, should be tested, as should cats living in households with other cats infected with FeLV. Cats with high-risk lifestyles (i.e. those that go outdoors or those that like to fight) should be tested on a regular basis.  If considering vaccinating a cat against FeLV, the cat should be tested first.  “Vaccinating a cat that is already positive for FeLV, while not harmful, offers no benefit to the cat”, notes Dr. Waltke.  

            The clinical signs of FeLV are varied and non-specific.  Many cats can be infected, yet show no clinical signs.  “In cats that are symptomatic, the most common sign I see is weight loss”, says Dr. Waltke.  The next most common signs are fever, dehydration, upper respiratory signs, diarrhea, conjunctivitis, oral infections, swollen lymph nodes, and abscesses (Sidebar 2).  The virus is immunosuppressive, causing chronic inflammatory conditions and increasing the susceptibility of acquiring secondary and opportunistic infections.

            FeLV can invade the bone marrow, resulting in hematologic abnormalities. Despite the name feline leukemia virus, actual leukemia (cancer of the blood) is just one of many blood cell abnormalities  that can be caused by the virus.  Severe anemia is often seen in cats with FeLV infection.  Leukopenia (a decreased white blood cell count) and thrombocytopenia (decreased platelet numbers) is also commonly reported.  The feline leukemia virus is notorious for causing lymphoma – cancer of the lymph nodes and lymphoid tissues. “Cats that test positive for FeLV are sixty times more likely to develop lymphoma compared to cats that test negative for the virus”, warns Dr. Stone.

            Identification and segregation of infected cats is undoubtedly the most effective means of preventing new infections with FeLV.  This, coupled with vaccination, is clearly responsible for the steady decline in the prevalence of infection seen in the past 20 years. Because close, intimate contact – mutual grooming, sharing food bowls, water bowls, litter boxes – is optimal for transmission of the virus, any cat that tests positive for FeLV should be isolated in a separate room to prevent the infected cat from interacting with its housemates.  If isolation is impossible, uninfected cats should be vaccinated against FeLV, bearing in mind that no vaccine is 100% effective and that vaccination is not as effective as isolation.  

            FeLV-infected cats can live for many months with proper care. A study of over 800 cats with FeLV infection revealed the median survival to be 2.4 years.  Many cats, however, may live for several years with good quality life. No cat should be euthanized based solely on a diagnosis of FeLV infection. Cats infected with FeLV should be kept strictly indoors to prevent spreading the virus to other cats, as well as to reduce their exposure to infectious organisms carried by animals they may encounter outdoors. Good nutrition, proper litter box hygiene, and a stimulating, loving environment are important in keeping infected cats healthy.  FeLV-infected cats should have more frequent veterinary check-ups compared to uninfected cats – at least semi-annually. Body weight should be accurately measured and recorded at each visit, as weight loss is often the first sign of deterioration in a cat’s condition. Many treatment studies have been conducted in cats that have been infected, either naturally or experimentally, with FeLV.  Unfortunately, many of the results are difficult to interpret, or they are not well-controlled studies in which a drug is compared against a standard treatment or placebo. At present, there is no treatment that has proven to be effective in clearing a FeLV infection.  
Sidebar 1: Risk factors for FeLV infection
Male sex
Mixed breed
Not neutered
Young age (less than 7)
Outdoor access
Tendency to fight with other cats
Living in a multi-cat household
Living with cats known to be infected with FeLV
Sidebar 2: The most common clinical signs seen in cats with FeLV
Weight loss (64%)
Fever (42%)
Dehydration (25%)
Rhinitis (18%)
Diarrhea (17%)
Conjunctivitis (17%)
Oral problems (15%)
Enlarged lymph nodes (13%)
Abscesses (12%)