Multisystem Diseases: feline leukemia virus infection



Feline leukemia virus (FeLV) causes a complex group of diseases, many of which are fatal. About 50% of cats infected with the leukemia virus develop cancer. The most common type of cancer affects lymphoid tissues (e.g., lymph nodes) causing solid tumors called lymphosarcomas. FeLV can also cause cancer of the blood cells and their precursors in the bone marrow. Veterinarians refer to this group of disorders as myeloproliferative diseases. All myeloproliferative diseases caused by feline leukemia virus are serious. Depending on which blood element is affected, profound anemias, low white blood cell counts (which increase susceptibility to infection), or low platelet levels (which can cause uncontrollable bleeding) may occur. Some myeloproliferative disorders are true leukemias that cause cancerous cells to circulate in the bloodstream.

Cats affected with myeloproliferative disorders often show signs of illness such as progressive weight loss, poor appetite, fever, and pale mucous membranes (caused by anemia). Since such signs often accompany other serious but treatable diseases (e.g., feline infectious anemia) and since laboratory tests such as blood counts and bone marrow examinations are imperative for diagnosis, a veterinarian’s help is necessary if you suspect such a problem in your cat.


Lymphosarcoma (malignant lymphoma) is the most common tumor (abnormal growth of solid tissue) in cats. The solid growths it produces have been found in almost every organ and tissue of cats because the lymphocyte cells from which the cancer cells arise are present in most body tissues. The signs of its presence depend on the site of tumor formation.

The most common site of cancerous growth is the abdomen. Here kidney involvement may produce signs of kidney failure. Persistent diarrhea, constipation, and/or vomiting may occur with intestinal or lymph node involvement. Malignant lymphoma also is frequently found in the chest, resulting in fluid formation, compression of the windpipe, and signs that may be mistaken for other respiratory or heart diseases. These include difficulty breathing and coughing.

Generalized lymph node enlargement occurs in other cases. Liver and spleen lymphosarcoma can occur, and when tumor formation occurs in unusual sites (e.g., brain, spinal cord, skin, eyes) the accompanying signs may be bizarre, ranging from behavior abnormalities and urinary incontinence to paralysis or seizures, skin growths, irregular pupil size, and/or blindness. Blood abnormalities (e.g., leukemia, anemia) may or may not accompany the solid tumor formation of lymphosarcoma. Clearly, a cat with signs that may indicate lymphosarcoma will require a veterinarian’s care. Although a cure is not to be expected, many treatments are available that can comfortably prolong an affected cats’ life.

Feline leukemia virus infection also causes health problems unrelated to cancer. It may suppress an infected cat’s immune system, thereby causing loss of resistance to other infectious diseases (e.g., feline infectious peritonitis, upper respiratory infections, haemobartonellosis, toxoplasmosis). Infected cats also have more frequent and longer lasting bacterial infections. It is not unusual for FeLV-infected cats to have abscesses that fail to heal normally, chronic diarrhea or to have chronic gum inflammation (gingivitis). Feline leukemia may encourage the cat’s immune system to recognize the self as foreign, causing antibody- mediated damage to the cat’s own body tissues.

Cats with this problem may develop kidney inflammation, nerve damage, arthritis, or self- destruction of the red blood cells or platelets. Female cats infected with FeLV have many reproductive system disorders, including infertility, uterine inflammation, loss of kittens due to resorption before birth, spontaneous abortion in later pregnancy, or death of the kittens shortly after their birth.


Feline leukemia virus is classified as a retrovirus. Retroviruses possess the ability to enter a cat’s body cells and use the cat’s own genetic machinery (DNA) to multiply. Feline leukemia virus is a particular kind of retrovirus called oncornavirus (tumor-producing ribonucleic acid virus) thought to have evolved millions of years ago from a leukemia virus of mice. Infected cats excrete (shed) newly formed virus mainly in their saliva, although it is also found in the blood, tears, respiratory secretions, urine, and stool.

The virus is passed from cat to cat by contact with infectious materials. Female cats can transmit the virus to their kittens through colostrum, milk, and across the placenta. Sharing a litter pan with an infected cat, receiving infected blood by transfusion, or, in theory, even getting a bite from a flea from an infected cat could result in leukemia virus infection. Most often, though, transmission occurs by contact with saliva during mutual grooming, biting, or sharing of food and/or water bowls. One drop of ingested saliva can contain up to 400,000 infective viral particles!

Despite this fact, prolonged (days to weeks) close contact between cats (especially adults) is usually required for transmission of FeLV and transmission via bite wounds is thought to be uncommon.


Not all cats exposed to the leukemia virus become infected, and of those who become infected most do not become permanently sick.

Although newborn kittens usually cannot resist a terminal infection, approximately two thirds of healthy cats older than three months of age suppress or throw off the infection. The remaining one third remain infected and actively shed the virus for the rest of their lives. (In the USA only about 2% of randomly tested cats on average are infected.) These cats pose a danger to other cats even if they never show any signs of illness caused by the virus. These cats are called FeLV carriers, and they can be detected by blood tests performed by your veterinarian. Tests performed on saliva or tears are also available, but they miss a significant number of infected cats and may give a false positive result so they are best used only as screening tests. Carrier cats should never be allowed to contact uninfected cats. This means they should be kept indoors, isolated from uninfected cats, and provided with their own litter pans and food and water bowls.

Some veterinarians advocate euthanasia of all infected cats.

Although most cats who become chronically FeLV infected will have trouble with illnesses and die within three years, many live nearly normal lifespans with few apparent illnesses. Some remain visibly healthy. Owners who are able to isolate their infected cats do not need to prematurely euthanize an apparently healthy but FeLV-infected cat. Neither should owners be overly concerned about any danger to themselves, since FeLV virus has never been shown to infect human beings naturally.


The best way to prevent leukemia virus infection in your cat is to avoid virus exposure. The safest cats are those who are kept indoors away from other potentially infected cats. Avoid introducing any new cat to your household until he or she had been blood tested negative for FeLV at least twice over a three- to five-month period. This is most desirable since recently exposed cats could conceivably test negative at first, only to turn positive later. The best strategy is to test the cat before bringing him or her home, then confine him or her to separate quarters until after the repeat testing is completed.

Do not allow your cat to share food or water bowls with other untested cats. It is particularly dangerous to allow your cat to share food and/or water bowls or an elimination area with other untested neighborhood cats.

Providing an outdoor water source only attracts cats who may be FeLV (or other disease) carriers; and although FeLV survives for only three or four hours when it dries at room temperature, it may remain infectious for several days when suspended in water at room temperature.

Neighborhood cats will also compete for territory with your own and the resulting bite wounds may spread FeLV.

Be sure any boarding kennel or grooming parlor you patronize is cautious about infectious diseases. Your veterinarian may request feline leukemia virus testing of your cat before dental procedures in order to provide extra protection against dispersal of contagious saliva in the veterinary hospital.


Vaccines against FeLV have been available since 1985. Some were found to be ineffective and/or unsafe and were removed from the market; new ones have replaced the old. Safe feline leukemia virus vaccines pose no danger to a healthy cat; however, they are unnecessary for cats who will never be exposed to FeLV (cats living indoors alone or with other test- negative cats) and for cats already infected with FeLV.

Also since most adult cats exposed to FeLV—unlike other diseases prevented by vaccination including panleukopenia, rabies, and respiratory infection—will not become sick, the risk of infection will need to be weighed against the cost of vaccination and the potential protective effect of the vaccine. This warrants a thorough discussion with your veterinarian before accepting routine feline leukemia virus vaccination of your cat. Determination of your cat’s infection status prior to vaccination will require blood tests.


Should you have the misfortune to have a cat die of FeLV, wait at least two weeks before introducing a new uninfected cat to a previously contaminated environment. During the waiting period be sure to clean the premises thoroughly with detergent and commercial disinfectant (all common ones including alcohol will kill FeLV). Wash all food and water bowls and litter pans, then disinfect them by soaking them in 0.16% hypochlorite solution (household bleach diluted 4 ounces/gallon water, 32ml/l).

To eliminate any concerns about cleanup, you may want to discard these items. If you have any other cats who have lived with the infected one, follow the procedures for control of feline leukemia virus in catteries.


  1. Blood test all cats for feline leukemia virus by immunofluorescent antibody (IFA) test. This blood test is most effective for detecting cats who will excrete the virus persistently.
  2. Remove all FeLV-infected cats. (This is usually done by euthanasia. If infected cats are not to be removed, no new uninfected cats should be introduced and infected cats should not be allowed to reproduce).
  3. Isolate all cats found negative for FeLV to cleaned and disinfected quarters and provide them with cleaned and disinfected food and water bowls, litter pans, and new toys.
  4. Retest all quarantined cats for FeLV three months after the initial test to detect any cats who may have been incubating the disease.
  5. Repeat the removal of positive (infected) cats and continue the isolation of negative cats.
  6. Continue the retesting, removal of positive cats, and isolation of negative cats until all remaining cats have blood tested negative at least twice at three-month intervals.
  7. Isolate all new cats. Introduce them to other cats in the household only after two negative FeLV tests performed three months apart.