Feline leukemia virus (FeLV)

Feline leukemia virus is a common and important cause of illness and death in cats. Cats who are infected with the virus are at high risk to develop diseases as anemia and cancer. Approximately 80-90% of the with FeLV-infected cats dies within four years after the diagnosis was made.

One of the most significant results of the infection is the suppression of the immune system. The virus infects, damages and kills the white blood cells which are responsible for the immune system. Because of this, the cat is very sensitive to other diseases and infections, secondary to the infection with the leukemia virus.

The leukemia virus belongs to the same family as the feline immunodeficiency virus (cat AIDS).

Who is at risk?

The virus does not survive long outside and therefore mainly spreads in cats with prolonged and intensive contact. The risk for contamination with leucosis is bigger in a place where there are many cats are living together in a limited area (shelters etc.) Young cats, especially those younger than six months are particularly prone to get permanently infected with FeLV.

How it spreads?

The primary source of the virus is saliva of an infected cat. The virus is spread when saliva is exchanged between cats, including by sharing food bowls or licking each other. Infection can also occur in fighting and bites or contact with infected urine or feces. The virus can also be transmitted from the mother cat to her kittens during pregnancy or after birth by drinking contaminated milk. This last possible transmission is quite rare because the virus often results in prenatal death when the mother cat is infected, resulting in resorption or abortion of the dead kittens.

Not all cats that are exposed to the virus become persistent leukemia infected. Either they are exposed to too small an amount of the virus or their immune system is successful in eliminating the virus. In the majority of the cats to the virus enters through the mouth or the nose. At the level of the mouth and the nose, the virus multiplies before it spreads through the bloodstream. The spread mainly happens to the bone marrow.

Cats who can eliminate the virus do this during the initial phase of infection (I.e. 4 to 12 weeks after infection). Once there are traces found of the virus in the bone marrow, this reflects a persistent infection and the cat is infected for the rest of her life!

Very rarely the infection is limited to a certain part of the body such as the mammary tissue. In such a situation, we speak of a “localized infection.”


A wide range of chronic and recurrent diseases will develop in cats with the leukemia virus. There is a progressive deterioration in the condition of these animals over time. The clinical symptoms are varied including fever, drowsiness, loss of appetite and weight loss. Also, respiratory problems and skin and digestive disorders are common.

Infected cats may exhibit different symptoms simultaneously.

Anaemia occurs in about 25% of infected cats. The virus could infect the red blood cells in the bone marrow, and is causing a decrease in the number of red blood cells or even the abnormal production of red blood cells which are not functioning normally. In other cases, the virus induces the breakdown of red blood cells by the immune system of the cat! Cats who exhibit anemia have symptoms of weakness and drowsiness.

In about 15% of infected cats, there is cancer developing. The most common form of cancer is lymphoma, a cancer of the lymphocytes (a type of white blood cell) which results in the formation of tumors or leukemia in which tumor cells are present in the bloodstream. The formation of tumors can develop in different locations in the body such as the intestines, the kidneys, the eyes and the nose. However, in leukemia, there is tumor proliferation in various lymph nodes and other places in the body.


Leukemia in cats

Treatment of leukemia

To date, there is not a single therapy which can eliminate the leukemia virus. There can only be an attempt to optimize the quality of life of infected animals and the effects of infection (such as suppression of immunity, anemia and cancer) to the extent of treatment possible.

Thorough and continuous supportive therapy with the treatment of secondary diseases is essential for a leukemia-infected cat. As a result of the suppression of the immune system prolonged courses of antibiotics are required. The response to therapy in these animals is often substandard and occurs much slower.

Potential therapeutic agents are steroids, anabolic steroids and multivitamins, which will stimulate the appetite as well. Antiviral medications such as AZT, which prove to be effective in people with AIDS do not seem to show any beneficial effect in the leukosis infected cat. A recent treatment with recombinant interferon in cats appears to be promising. Cats with a lymphoma may show temporary improvement in the use of chemotherapy. These include chemotherapeutic drugs that must be absorbed through the mouth, but can also be administered by injection.

Infected cats should not be given raw meat as this increases the risk of infection with Toxoplasma gondii. This parasite is a risk for cats with a suppressed immune system where it can lead to uveitis (inflammation of the internal structures of the eye), or nervous symptoms such as seizures and collapsing.

It is advisable anyway to vaccinate the infected cat for cat flu.

Prevention of fleas is appropriate to eliminate the risk for Haemobartonella felis (a blood parasite that causes anemia).

The regular deworming of these animals is a must!


There are various types of vaccines on the market that provide protection against the leukemia virus. The purpose of these vaccines is to protect cats that come into contact with the virus, and thus prevent that the animals get infected. The leukemia vaccination aims to stimulate the immune response of the animal and to prevent the virus to invade the body.

Unfortunately, no vaccine is 100% effective.

Vaccination is highly recommended in animals at high risk to have contact with infected cats, such as outdoor cats and cats in contact with a lot of stray cats.

Sometimes it is advisable to test the cat prior to vaccination, for example when introducing a new cat in the family. An infected cat does not have to look sick but can be infected and be a risk to other cats in the family.

It is also, of course, advisable to keep with leukemia infected cats separate from non-vaccinated cats.

Control of the disease

Since the virus is quite contagious and is transmitted primarily by prolonged contact, cats from a family where there is an infected cat present are at a high risk of infection by washing each other and eating from the same eating bowls. Uninfected cats should be separated from infected cats. Furthermore, it is also advisable to keep infected cats indoors in order to counter the risk of spreading the virus. To keep a cat who is used to go outside, indoors, might not be easy but the disadvantages outweigh the risks!

When breeding cats it is also very important to test cats before breeding to eliminate leukemia infected animals for breeding.

Cats that live solo and just stay indoors have no risk of infection with the virus leukemia. However, it is possible that these cats were infected with the virus through mother cat and that it therefore months to years later start to exhibit symptoms of cat leukemia, although they never came out or came into contact with other cats since they were kittens.

Test for feline leukemia

There are test kits on the market, which detect one of the viral proteins in the blood of FeLV-infected cats. It is often tested simultaneously to the feline AIDS virus. In certain situations, there may emerge false positive and false negative results so in those cases it is recommended to carry out the test a second time for confirmation or to perform another test.

Other tests include:

  • Virus Isolation: Detection of the virus in the blood plasma;
  • Immunofluorescence: detects viral antigens (proteins) in the white blood cells;
  • PCR (polymerase chain reaction) detects the genetic material of the virus, but can only be performed in specialized laboratories.

Prognosis for infected cats

The prognosis for sick infected cats is poorly. The infections associated with the disease are often very serious.

The prognosis for cats that test positive but who are still in good condition at the time of the test is, however, reserved. Most of those infected cats develop over time leukemia related fatal diseases. However, the timing of the appearance of symptoms is also highly variable over time, varying from a few months to years. It is extremely important to isolate these cats in order to avoid the risk of spread of the virus as well as to reduce the risk of secondary infections. Euthanasia is sometimes unavoidable.