The Facts of Feline Leukemia

Feline leukemia is a cancerous disease caused by feline leukemia virus (FeLV). FeLV causes diseases other than leukemia including immunodeficiency and additional cancers. Cats may not start to show signs of disease for months or years after being infected with FeLV. Infection with FeLV is a major cause of illness and death in domestic cats.

What are the characteristics of feline leukemia virus?

FeLV is a type of virus called a retrovirus. Other notable feline diseases in the same family are feline immunodeficiency virus (FIV) and human immunodeficiency virus (HIV the virus that causes AIDS).

Retroviruses are species-specific. This means a feline retrovirus will only infect cats; a human retrovirus will only infect humans. Retroviruses are made up of RNA. In the host the RNA is transcribed into DNA and incorporated into the DNA of the host’s cells.

Retroviruses are fragile being easily inactivated by ultraviolet light, heat, detergents, and drying. Retroviruses are widespread in nature. As a matter of fact they have been around for so many millions of years parts of a feline retrovirus are actually incorporated into every cat’s DNA. This is called “endogenous” FeLV DNA. This is passed from generation to generation.

There are three subgroups of FeLV and each tends to cause a different type of disease:

  1. FeLV-A is found in all naturally infected cats and is easily transmitted. For this reason this is the FeLV we use for making vaccines against FeLV.
  2. FeLV-B is found in about of naturally infected cats. It is formed when FeLV-A combines with the endogenous FeLV DNA.
  3. FeLV-C is rare. FeLV-C results from mutations of FeLV-A.

How common is FeLV infection?

It is estimated that 2-3% of healthy cats are infected with FeLV. Approximately 20-30% of the healthy cats living in infected multi-cat households and catteries are infected.

How is the FeLV transmitted?

Large amounts of FeLV are excreted in the saliva. Therefore the most common mode of transmission is through nose-to-nose contact, mutual grooming, and shared food and water bowls. Bites are a very efficient way to transmit FeLV.

FeLV can also be found in lesser amounts in tears, urine, and feces. Thus litter boxes could be a source of infection in multi-cat households or catteries. FeLV can also be transmitted across the placenta (in utero) and through the milk.

It takes large amounts of virus to infect an adult cat so usually prolonged contact is necessary for transmission.

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What happens to a cat after being exposed to FeLV?

If the cat becomes infected from the exposure 2-4 weeks later in the acute stage of infection large numbers of the virus can be found in the bloodstream. Cats in the acute phase usually do not show signs of disease. If they do show signs of disease, they are usually mild fever, slight lethargy, and swollen lymph nodes. When an adult cat is exposed to FeLV four things can happen:

  • 30% of adult cats will not be infected due to inadequate exposure.
  • 30-35% of adult cats have a transient infection; over the course of 6 months or so the cats will eventually eliminate the virus from its body.
  • 5-10% of adult cats will develop latent infections; these cats will not be able to eliminate 100% of the virus but will be able to hold it in check. This is called a latent infection. These cats usually show no signs of infection and usually do not shed virus in their saliva or other body secretions. Queens however may still pass the virus in utero or through the milk.
  • 30% of adult cats will become persistently infected; these cats will not develop an adequate immune response and will remain permanently infected with FeLV. These are the cats that will become ill and die of FeLV-related diseases usually within 2-3 years of infection. These cats will shed large amounts of virus in their saliva.

Age is a very important factor in determining what will happen after a cat is exposed to FeLV. Almost all FeLV-exposed kittens less than 8 weeks of age will have persistent viremia and show signs of disease during the acute phase. As kittens get older there is the probability of becoming persistently infected after exposure lessens until it reaches approximately 30% in adulthood.

The prevalence of FeLV infection is highest in cats between 1 and 6 years of age with a mean age of 3 years. Male cats are twice as likely to be infected. This may be due to the frequency in which intact males roam and fight.

What diseases are caused by FeLV?

FeLV can cause:

  • Immunodeficiency
  • Anemia
  • Immune-mediated diseases
  • Reproductive problems
  • Gastrointestinal disease
  • Neurologic disease
  • Platelet disorders
  • Lymphadenopathy (enlarged lymph nodes)
  • Neoplasia

What are the clinical signs of disease?

The clinical signs of disease are going to be variable because so many body systems can be affected. Loss of appetite, fever, weight loss, and weakness are the first clinical signs of disease commonly seen in infected cats.

How is FeLV infection diagnosed?

Blood tests are commonly used to test both asymptomatic and symptomatic cats for FeLV antigen. The ELISA test can be performed in a veterinarian’s office. The ELISA test can be performed on blood serum saliva or tears. Using serum will reduce the possibility of obtaining a false positive test (a positive test in an uninfected animal).

Can FeLV be prevented?

Because of the advances in medical science and veterinary care there are several vaccinations available today that protect your cat against FeLV. A few of these vaccines include Fel-O-Guard Plus and Fel-O-Vax. Remember an ounce of prevention is worth a pound of cure. Guard your cat against the danger of feline leukemia.