Discovered in the 1960’s, the feline leukemia virus (FeLV) is a retrovirus belonging to the subfamily orthoretrovirinae. It is in the same family as the feline immunodeficiency virus (FIV). More deaths occur from the feline leukemia virus than any other pathogenic disease in cats. It is said that approximately 2%-3% of cats in the United States are infected with FeLV.
Four subgroups of FeLV exist A, B, C, and T, but only subgroup A is transmissible between cats. The other subgroups arise de novo or as results of recombination with an endogenous feline DNA sequence. Hence, there is very good evidence this virus is quite ancient and may well have evolved more than one time over the last 10,000,000 years.
FeLV is an oncovirus, a name that refers to a virus which causes cancer, but FeLV also causes immunosuppression, allowing other diseases to take hold more easily.
How is FeLV transmitted?
Large quantities of the virus are shed via saliva and respiratory secretions. Smaller amounts of virus are shed in the urine, feces, and milk. The virus is most frequently transmitted during mutual grooming, sneezing, nose-to-nose contact, sharing food bowls, mating, in-utero, via the milk, bites, and sharing litter trays.
Kittens under twelve months of age are most susceptible to FeLV infection due to their immature immune systems.
The virus is extremely fragile and is quickly destroyed in the environment (such as food bowls, cages, litter trays etc). It is highly susceptible to heat and disinfection.
What does FeLV do?
Once the virus enters the body, it replicates in the lymphoid tissue surrounding the site of initial penetration. Having been infected, there are three possible outcomes, each with equal (33%) likelihoods of occurrence:
The cat develops a transient viremia. The virus is present in the blood and saliva for less than twelve weeks. The cat develops neutralising antibodies, which destroy the disease. The cat is no longer infected and cannot transmit the disease to other cats. It does not become sick and has a normal life expectancy.
The cat develops a persistent viremia. The virus is present in the blood and saliva for over twelve weeks. The cat doesn’t mount an effective immune response and is susceptible to many potentially fatal diseases. Mortality ranges from six months to three years.
The cat develops a latent infection. It produces neutralising antibodies to destroy the virus, but the virus isn’t extinguished completely and persists in the bone marrow and T-cell lymphocytes. The cat typically doesn’t develop diseases or cancers related to FeLV. The virus can be reactivated during times of stress or concurrent illness.
What are the symptoms of feline leukemia?
Clinical signs vary widely, depending on the disease type and on which organs are involved. Some symptoms caused (directly or indirectly) by FeLV include:
Around 80% of cats die within a year of infection, and almost all will die within two to three years.
How is feline leukemia diagnosed?
There are two types of tests available:
ELISA (enzyme-linked immunosorbent assay): This test can be performed in your veterinarian’s office. It can detect an antigen known as p27 in whole blood (blood which hasn’t been separated into components), blood serum (the straw coloured liquid portion of blood), blood plasma, saliva, and tears of the cat. It is possible to get a false positive or a false negative. In the case of a positive from whole blood, a second test performed on blood plasma or blood serum is recommended.
IFA (immunofluorescence assay): This test needs to be sent out to a laboratory. Testing detects an antigen in leucocytes (white blood cells) or platelets.
How is feline leukemia treated?
There is no cure for FeLV, and treatment is usually targeted at managing FeLV-related diseases and keeping the cat as healthy as possible. Your veterinarian will tailor treatment and care of the FeLV-positive cat according to its individual circumstances. Keeping your cat in a stress-free environment, feeding a nutritious diet, and avoiding exposure to disease are all important keys to helping a cat with FeLV. Treatments/management may include:
Regular check-ups with your veterinarian to carefully monitor your cat’s health.
Different vaccination schedule for the FeLV-positive cat such as only using a killed vaccine instead of a modified live vaccine.
Interferon may be prescribed by your veterinarian. Interferon is a natural protein released by cells which have been invaded by viruses and assist the immune response by inhibiting viral replication.
AZT (azidothymidine) is an antiviral drug used in humans with HIV. It can produce quite severe side effects in cats. Your cat will need to be closely monitored by his veterinarian.
Administration of antibiotics where necessary for secondary bacterial infections.
Fluid therapy to treat dehydration.
Administration of vitamins and minerals.
Chemotherapy to manage lymphoma.
Keeping your cat indoors.
Keeping your cat as stress-free as possible.
Prevention of feline leukemia:
To prevent exposure in the first place, it is advised you keep your cat either indoors or contained within a cat enclosure.
There is a vaccine for FeLV. This is recommended for cats who are at high risk of exposure to the virus. No vaccine is 100% effective, so it should not be assumed that a cat is completely safe from infection once it is vaccinated
Avoid having FeLV-positive and -negative cats together.
Always test new cats for FeLV before introducing them to resident cats.