Feline Leukemia Virus (FeLV)

Provided by the Cornell Feline Health Center, Cornell University

What is feline leukemia virus?
Feline leukemia virus (FeLV), a retrovirus, so named because of the way it behaves within infected cells. All retroviruses, including feline immunodeficiency virus (FIV) and human immunodeficiency virus (HIV), produce an enzyme, reverse transcriptase, which permits them to insert copies of their own genetic material into that of the cells they have infected. Although related, FeLV and FIV differ in many ways, including their shape: FeLV is more circular while FIV is elongated. The two viruses are also quite different genetically, and their protein constituents are dissimilar in size and composition. Although many of the diseases caused by FeLV and FIV are similar, the specific ways in which they are caused differs.

How common is the infection?
FeLV-infected cats are found worldwide, but the prevalence of infection varies greatly depending on their age, health, environment, and lifestyle. In the United States, approximately 2 to 3% of all cats are infected with FeLV. Rates rise significantly – 13% or more – in cats that are ill, very young, or otherwise at high risk of infection.

How is FeLV spread?
Cats persistently infected with FeLV serve as sources of infection. Virus is shed in very high quantities in saliva and nasal secretions, but also in urine, feces, and milk from infected cats. Cat-to-cat transfer of virus may occur from a bite wound, during mutual grooming, and (though rarely) through the shared use of litter boxes and feeding dishes. Transmission can also take place from an infected mother cat to her kittens, either before they are born or while they are nursing. FeLV doesn’t survive long outside a cat’s body – probably less than a few hours under normal household conditions.

What cats are at greatest risk of infection?
Cats at greatest risk of infection are those that may be exposed to infected cats, either via prolonged close contact or through bite wounds. Such cats include:

  • Cats living with infected cats or with cats of unknown infection status
  • Cats allowed outdoors unsupervised, where they may be bitten by an infected cat
  • Kittens born to infected mothers
  • Kittens are much more susceptible to infection than are adult cats, and therefore are at the greatest risk of infection if exposed. But accompanying their progression to maturity is an increasing resistance to FeLV infection. For example, the degree of virus exposure sufficient to infect 100% of young kittens will infect only 30% or fewer adults. Nonetheless, even healthy adult cats can become infected if sufficiently exposed.

    What does FeLV do to a cat?
    Feline leukemia virus adversely affects the cat’s body in many ways. It’s the most common cause of cancer in cats, it may cause various blood disorders, and it may lead to a state of immune deficiency that hinders the cat’s ability to protect itself against other infections. The same bacteria, viruses, protozoa, and fungi that may be found in the everyday environment – where they usually do not affect healthy animals – can cause severe illness in those with weakened immune systems. These secondary infections are responsible for many of the diseases associated with FeLV.

    What are the signs of disease caused by FeLV?
    During the early stages of infection, it’s common for cats to exhibit no signs of disease at all. However, over time – weeks, months, or even years – the cat’s health may progressively deteriorate or be characterized by recurrent illness interspersed with periods of relative health. Signs can include:

  • Loss of appetite
  • Slow but progressive weight loss, followed by severe wasting late in the disease process
  • Poor coat condition
  • Enlarged lymph nodes
  • Persistent fever
  • Pale gums and other mucus membranes
  • Inflammation of the gums (gingivitis) and mouth (stomatitis)
  • Infections of the skin, urinary bladder, and upper respiratory tract
  • Persistent diarrhea
  • Seizures, behavior changes, and other neurological disorders
  • A variety of eye conditions
  • In unspayed female cats, abortion of kittens or other reproductive failures
  • What are the two stages of FeLV infection?
    FeLV is present in the blood (a condition called viremia) during two different stages of infection:

  • Primary viremia, an early stage of virus infection. During this stage some cats are able to mount an effective immune response, eliminate the virus from the bloodstream, and halt progression to the secondary viremia stage.
  • Secondary viremia, a later stage characterized by persistent infection of the bone marrow and other tissue. If FeLV infection progresses to this stage it has passed a point of no return: the overwhelming majority of cats with secondary viremia will be infected for the remainder of their lives.
  • How is infection diagnosed?
    Two types of FeLV blood tests are in common use. Both detect a protein component of the virus as it circulates in the bloodstream.

  • ELISA (enzyme-linked immunosorbent assay) and similar tests can be performed in your veterinarian’s office. ELISA-type tests detect both primary and secondary stages of viremia.
  • IFA (indirect immunofluorescent antibody assay) tests must be sent out to a diagnostic laboratory. IFA tests detect secondary viremia only, so the majority of positive-testing cats remain infected for life. Each testing method has strengths and weaknesses. Your veterinarian will likely suggest an ELISA-type test first, but in some cases, both tests must be performed – and perhaps repeated – to clarify a cat’s true infection status.
  • How can I keep my cat from becoming infected?
    The only sure way to protect cats is to prevent their exposure to FeLV-infected cats.

  • Keep cats indoors, away from potentially infected cats that might bite them. If you do allow your cats outdoor access, provide supervision or place them in a secure enclosure to prevent wandering and fighting.
  • Adopt only infection-free cats into households with uninfected cats.
  • House infection-free cats separately from infected cats, and don’t allow infected cats to share food and water bowls or litter boxes with uninfected cats.
  • Consider FeLV vaccination of uninfected cats. (FeLV vaccination of infected cats is not beneficial.) Discuss the advantages and disadvantages of vaccination with your veterinarian. FeLV vaccines are widely available, but since not all vaccinated cats will be protected, preventing exposure remains important even for vaccinated pets. FeLV vaccines will not cause cats to receive false positive results on ELISA, IFA, or any other available FeLV tests.
  • I just discovered that one of my cats has FeLV, yet I have other cats as well. What should I do?
    Unfortunately, many FeLV-infected cats are not diagnosed until after they have lived with other cats. In such cases, all other cats in the household should be tested for FeLV. Ideally, infected and non-infected cats should then be separated to eliminate the potential for FeLV transmission.

    How should FeLV-infected cats be managed?

  • Confine FeLV-infected cats indoors to reduce their exposure to other infectious agents carried by animals, and to prevent the spread of infection to other cats in the neighborhood.
  • Spay or neuter FeLV-infected cats.
  • Feed nutritionally complete and balanced diets.
  • Avoid uncooked food, such as raw meat and eggs, and unpasteurized dairy products because the risk of food-borne bacterial and parasitic infections is much higher in immunosuppressed cats.
  • Schedule wellness visits with your veterinarian at least once every six months. Although a detailed physical examination of all body systems should be performed, your veterinarian should pay special attention to the health of the gums, eyes, skin, and lymph nodes. A complete blood count, serum biochemical analysis, and a urine analysis should be performed at every examination. Additionally, your cat’s weight should be accurately measured and recorded, as weight loss is often the first sign of deterioration.
  • Closely monitor the health and behavior of your FeLV-infected cat. Alert your veterinarian to any changes in your cat’s health immediately.
  • There is no scientific evidence that alternative, immunomodulator, or antiviral medications have any positive benefits on the health or longevity of healthy infected cats.
  • How long can I expect my FeLV-infected cat to live?
    It’s impossible to accurately predict the life expectancy of a cat infected with FeLV. With appropriate care and under ideal conditions, infected cats can remain in apparent good health for many months, although most succumb to a FeLV-related disease within two or three years after becoming infected. If your cat has already experienced one or more severe illnesses as a result of FeLV infection, or if persistent fever, weight loss, or cancer is present, a much shorter survival time can be expected.

    My FeLV-infected cat died recently after a long illness. How should I clean my home before bringing in a new cat?
    Feline leukemia virus will not survive outside the cat for more than a few hours in most environments. However, FeLV-infected cats are frequently infected with other hardier infectious agents, and these may pose some threat to a newcomer. Thoroughly clean and disinfect or replace food and water dishes, bedding, litter pans and toys. A dilute solution of household bleach (4 ounces of bleach in a gallon of water) makes an excellent disinfectant. Vacuum carpets and mop floors. Any new cats or kittens should be properly vaccinated before entering the household.

    Can people become infected with FeLV?
    Epidemiological and laboratory studies have failed to provide evidence that FeLV can be transmitted from infected cats to humans. Regardless, FeLV-infected cats may carry other diseases. At greatest risk of infection are elderly or immunosuppressed people (e.g., those with AIDS, or receiving immunosuppressive medications such as chemotherapy), infants, and unborn children. It’s recommended that pregnant women, people with suppressed immune systems, the very young, and the very old avoid contact with FeLV-infected cats.

     

    Feline Immunodeficiency Virus (FIV)

    (Reprinted with permission from Cornell Feline Health Center)

    New conclusions about FIV-infected and non-infected cats living together:
    Study #1
    Study #2

    What is Feline Immunodeficiency Virus?
    Virologists classify feline immunodeficiency virus (FIV) as a lentivirus (or “slow virus”). FIV is in the same retrovirus family as feline leukemia virus (FeLV), but the viruses differ in many ways including their shape. FIV is elongated, while FeLV is more circular. The two viruses are also quite different genetically, and the proteins that compose them are dissimilar in size and composition. The specific ways in which they cause disease differ, as well.

    How common is the infection?
    FIV-infected cats are found worldwide, but the prevalence of infection varies greatly. In the United States, approximately 1.5 to 3 percent of healthy cats are infected with FIV. Rates rise significantly -15 percent or more – in cats that are sick or at high risk of infection. Because biting is the most efficient means of viral transmission, free-roaming, aggressive male cats are the most frequently infected, while cats housed exclusively indoors are much less likely to be infected.

    How is FIV spread?
    The primary mode of transmission is through bite wounds. Casual, non-aggressive contact does not appear to be an efficient route of spreading FIV; as a result, cats in households with stable social structures where housemates do not fight are at little risk for acquiring FIV infections. On rare occasions infection is transmitted from an infected mother cat to her kittens, usually during passage through the birth canal or when the newborn kittens ingest infected milk. Sexual contact is not a major means of spreading FIV.

    What does FIV do to a cat?
    Infected cats may appear normal for years. However, infection eventually leads to a state of immune deficiency that hinders the cat’s ability to protect itself against other infections. The same bacteria, viruses, protozoa, and fungi that may be found in the everyday environment–where they usually do not affect healthy animals–can cause severe illness in those with weakened immune systems. These secondary infections are responsible for many of the diseases associated with FIV.

    What are the signs of disease caused by FIV?
    Early in the course of infection, the virus is carried to nearby lymph nodes, where it reproduces in white blood cells known as T-lymphocytes. The virus then spreads to other lymph nodes throughout the body, resulting in a generalized but usually temporary enlargement of the lymph nodes, often accompanied by fever. This stage of infection may pass unnoticed unless the lymph nodes are greatly enlarged.

    An infected cat’s health may deteriorate progressively or be characterized by recurrent illness interspersed with periods of relative health. Sometimes not appearing for years after infection, signs of immunodeficiency can appear anywhere throughout the body.

  • Poor coat condition and persistent fever with a loss of appetite are commonly seen.
  • Inflammation of the gums (gingivitis) and mouth (stomatitis) and chronic or recurrent infections of the skin, urinary bladder, and upper respiratory tract are often present.
  • Persistent diarrhea can also be a problem, as can a variety of eye conditions.
  • Slow but progressive weight loss is common, followed by severe wasting late in the disease process.
  • Various kinds of cancer and blood diseases are much more common in cats infected with FIV, too.
  • In unspayed female cats, abortion of kittens or other reproductive failures have been noted.
  • Some infected cats experience seizures, behavior changes, and other neurological disorders.
  • How is infection diagnosed?
    Antibody tests detect the presence of antibody in the blood of infected cats.

    Positive Results

  • Because few, if any, cats ever eliminate infection, the presence of antibody indicates that a cat is infected with FIV. This test can be performed by most veterinary diagnostic laboratories and also is available in kit form for use in veterinary clinics. Since false-positive results may occur, veterinarians recommend that positive results be confirmed using a test with a different format.
  • Infected mother cats transfer FIV antibodies to nursing kittens, so kittens born to infected mothers may receive positive test results for several months after birth. However, few of these kittens actually are or will become infected. To clarify their infection status, kittens younger than six months of age receiving positive results should be retested at 60-day intervals until they are at least six months old.
  • Negative Results

  • A negative test result indicates that antibodies directed against FIV have not been detected, and, in most cases, this implies that the cat is not infected. Nevertheless, it takes eight to 12 weeks after infection (and sometimes even longer) before detectable levels of antibody appear, so if the test is performed during this interval, inaccurate results might be obtained. Therefore, antibody-negative cats with either an unknown or a known exposure to FIV-infected cats-such as through the bite of an unknown cat-should be retested a minimum of 60 days after their most recent exposure in order to allow adequate time for development of antibodies.
  • On very rare occasions, cats in the later stages of FIV infection may test negative because their immune systems are so compromised that they no longer produce detectable levels of antibody. Polymerase chain reaction (PCR) tests are designed to detect short segments of a virus’s genetic material. While antibody-based tests are ideal screening tests for infection, in certain situations (such as confirming infection in antibody-positive kittens or determining infection of cats vaccinated with antibody-producing FIV vaccines), PCR-based tests, in theory, would be superior. Although PCR testing methods offer promise and are being actively explored, at this time unacceptable numbers of false-positive and false-negative results prevent them from routinely being recommended.
  • How can I keep my cat from becoming infected?
    The only sure way to protect cats is to prevent their exposure to the virus. Cat bites are the major way infection is transmitted, so keeping cats indoors – and away from potentially infected cats that might bite them – markedly reduces their likelihood of contracting FIV infection. For the safety of the resident cats, only infection-free cats should be adopted into a household with uninfected cats.

    Vaccines to help protect against FIV infection are now available. However, not all vaccinated cats will be protected by the vaccine, so preventing exposure will remain important, even for vaccinated pets. In addition, vaccination may have an impact on future FIV test results. It is important that you discuss the advantages and disadvantages of vaccination with your veterinarian to help you decide whether FIV vaccines should be administered to your cat.

    I just discovered that one of my cats has FIV, yet I have other cats as well. What do I do now?
    Unfortunately, many FIV-infected cats are not diagnosed until after they have lived for years with other cats. In such cases, all the other cats in the household should be tested, as well. Ideally, all infected cats should be separated from the non-infected ones to eliminate the potential for FIV transmission. If this is not possible – and if fighting or rough play is not taking place – the risk to the non-infected cats appears to be very low.

    How should FIV-infected cats be managed?

  • FIV-infected cats should be confined indoors to prevent spread of FIV infection to other cats in the neighborhood and to reduce their exposure to infectious agents carried by other animals.
  • FIV-infected cats should be spayed or neutered.
  • They should be fed nutritionally complete and balanced diets.
  • Uncooked food, such as raw meat and eggs, and unpasteurized dairy products should not be fed to FIV-infected cats because the risk of food-borne bacterial and parasitic infections is much higher in immunosuppressed cats.
  • Wellness visits for FIV-infected cats should be scheduled with your veterinarian at least every six months. Although a detailed physical examination of all body systems will be performed, your veterinarian will pay special attention to the health of the gums, eyes, skin, and lymph nodes. Your cat’s weight will be measured accurately and recorded, because weight loss is often the first sign of deterioration. A complete blood count, serum biochemical analysis, and a urine analysis should be performed annually.
  • Vigilance and close monitoring of the health and behavior of FIV-infected cats is even more important than it is for uninfected cats. Alert your veterinarian to any changes in your cat’s health as soon as possible.
  • There is no evidence from controlled scientific studies to show that immunomodulator, alternative, or antiviral medications have any positive benefits on the health or longevity of healthy FIV-infected cats. However, some antiviral therapies have been shown to benefit some FIV-infected cats with seizures or stomatitis.
  • How long can I expect my FIV-infected cat to live?
    It is impossible to accurately predict the life expectancy of a cat infected with FIV. With appropriate care and under ideal conditions, infected cats can live a normal life span. If your cat has already had one or more severe illnesses as a result of FIV infection, or if persistent fever and weight loss are present, a much shorter survival time can be expected.

    My FIV-infected cat died recently after a long illness. How should I clean my home before bringing in a new cat?
    Feline immunodeficiency virus will not survive outside the cat for more than a few hours in most environments. However, FIV-infected cats are frequently infected with other infectious agents that may pose some threat to a newcomer. Thoroughly clean and disinfect or replace food and water dishes, bedding, litter pans, and toys. A dilute solution of household bleach (four ounces of bleach in 1 gallon of water) makes an excellent disinfectant. Vacuum carpets and mop floors with an appropriate cleanser. Any new cats or kittens should be properly vaccinated against other infectious agents before entering the household.

    Can I become infected with FIV?
    Although FIV is a lentivirus similar to HIV (the human immunodeficiency virus) and causes a disease in cats similar to AIDS (acquired immune deficiency syndrome) in humans, it is a highly species-specific virus that infects only felines. A number of studies have failed to show any evidence that FIV can infect or cause disease in people.

    Why should I have my cat tested?
    Early detection will help you maintain the health of your own cat and also allow you to prevent spreading infection to other cats.

    Under what circumstances should FIV testing be performed?

  • If your cat has never been tested.
  • If your cat is sick, even if it tested free of infection in the past but subsequent exposure can’t be ruled out.
  • When cats are newly adopted, whether or not they will be entering a household with other cats.
  • If your cat has recently been exposed to an infected cat.
  • If your cat is exposed to cats that may be infected (for example, if your cat goes outdoors unsupervised or lives with other cats that might be infected). Your veterinarian may suggest testing periodically (yearly) as long as your cat is exposed to potentially infected cats.
  • If you’re considering vaccinating with an FIV vaccine.
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