Feline Leukemia is a retrovirus. As a member of the retrovirus family, the feline leukemia virus’s genetic material is transmitted as RNA. Once the virus infects the cell, DNA copies of the virus are transcribed and these copies are inserted randomly into the host’s genetic material. Once the DNA is integrated within the genome, any cell division that occurs will cause both of the new cells to contain the virus.

Feline leukemia is found worldwide. The prevalence of Feline Leukemia in the general pet population in North America, regardless of whether a cat is feral or owned, is thought to be 5%. The incidence of Feline Leukemia in an area is directly proportional to the density of the cat population in that area.

Feline Leukemia Virus is transmitted by prolonged social contact with an infected cat. Although the virus is believed to be shed in the saliva, nasal secretions, and urine, it is extremely sensitive to environmental conditions, causing transmission to be through direct contact only. Direct contact would include mutual grooming and sharing food and water bowls or litter areas. Bite wounds from infected cats will transmit the virus with the wandering and fighting behavior of Tom cats putting them at increased risk of infection. The virus may also be spread from a blood transfusion or vertically from mother to kitten. Vertical transmission is believed to occur in utero or through the ingestion of milk.

Clinical signs of disease are usually a result of anemia, suppression of the immune system, or neoplasia (cancer). Bone marrow suppression is the most common clinical syndrome resulting in a severe nonregenerative anemia (no additional red blood cells are being produced). Feline Leukemia is also the most common cause of low platelet levels and a lack or decrease in white blood cell numbers. Feline Leukemia infected cats have a sixty-fold increase risk of developing lymphoma as compared with feline leukemia negative cats. The most commonly seen lymphoma is that of the digestive tract lining seen clinically as enteritis. Lymphoma may also occur in multiple areas:mediastinum, spinal, kidney and eye forms have also been reported. Due to immunosuppression, secondary infections are more common including bacterial, viral (especially of respiratory tract), protozoal and fungal infections. Prompt attention and aggressive treatment should be conducted when secondary infections are incurred in order to achieve a positive outcome. Unexplained weight loss and long lasting infections should be suspect for the presence of Feline Leukemia. Fetal resorptions, abortions and neonatal deaths (Fading Kitten Syndrome) are common with feline leukemia infection in a Queen. Up to 20% of kittens that survive vertical transmission will become persistently infected and become carrier adults.

Most veterinary clinics conduct routine testing for feline leukemia status and results are usually obtained within minutes. Testing is recommended before proceeding with any vaccination program or introduction of a new cat or kitten into a multiple cat household.

Cats testing positive should be retested at three to six month intervals to detect possible recovery from the virus.

If your cat tests positive for feline leukemia, there are three possible outcomes to the infection:

Approximately 40% of cats testing positive develop an immune response and become resistant to future bouts of viral activity.
Another 30% become latent carriers of the disease, becoming neither fully recovered and yet are not seriously affected. These cats are able of transmitting the virus to other cats as well as vertically to their offspring.
The remaining 30% of cats testing positive for feline leukemia become persistently infected and eventually succumb to the virus. Of these cats, about 8. 3% die within three years of acquiring the virus. Death may be due directly to feline leukemia virus itself or an associated infection.

Several vaccines are presently available for the prevention of Feline Leukemia, including vaccines manufactured from Fort Dodge Animal Health, Pfizer, Merial, etc. An initial series of two vaccines is recommended starting as early as eight weeks of age depending on the vaccine. The second vaccine should be administered three to four weeks after the initial vaccination. Revaccination should be conducted yearly for those cats at risk of exposure. There is no value in vaccinating an individual confirmed to be infected.

According to the Feline Advisory Panel, Leukemia virus vaccines should be considered as a noncore vaccine and should be recommended only for cats at risk of exposure. Cats at risk would include those that are allowed outdoors, those cats living with a Feline Leukemia infected cat, or a cat residing with other cats in which the feline leukemia status is unknown. Vaccination is not generally necessary in shelter or boarding situations unless multiple cats are housed together. When cats are housed individually, there is little risk of exposure.

The American Association of Feline Practitioners recommends that cats infected with Feline leukemia be confined indoors to prevent the spread of the virus to other neighborhood cats. Infected cats should be neutered to prevent sexual aggression and frustration. Surgery is well tolerated in cats showing no clinical signs of disease. Antibiotics should be given to cats undergoing dental or surgical procedures to prevent secondary infections. These antibiotics support health if pet becomes immunosuppressed following the stress of surgery.

Treatment is limited. Feline lymphoma may be treated with chemotherapy and prednisone. AZT is the most widely used antiviral agent for feline and human retroviral infections. AZT prevents the conversion of viral RNA into DNA. Recombinant human interferon has been used with limited success. Interferon typically had an efficacy of 50 days duration, until host antibody production inhibits activity. Secondary infections should be treated aggressively and supportive care may be necessary. Blood transfusions rarely provide any sustained anemia relief since the anemia is nonregenerative and red blood cells have a limited life span.

To date, no studies have indicated the possibility of transmission to humans.


References:

Antech Diagnostics News March 2007.

The 2006 American Association of Feline Practitioners Feline Vaccine Advisory Panel Report. Journal of the American Veterinary Medical Association, Vol 229, No. 9, November 1, 2006.

Kahn, Cynthia Editor. The Merck Veterinary Manual. 9th Edition. 2005. pp 631-635.

Levy, Julie and Cynda Crawford. Veterinary Internal Medicine. Saunders 2005. pp653-659.