Feline leukemia virus (FeLV)

Feline leukemia virus (FeLV) is one of the most important infectious diseases of cats worldwide.

• Affected cats can develop anemia (a low red blood cell level), cancers, and/or suppression of the immune system.
• The disease worsens over time and is usually fatal. The virus can infect domestic and wild cats (such as lions).
• Widespread testing and vaccination efforts have helped to reduce the prevalence of the disease over the past 30 years, and in 2010 approximately 3% of the cats in the United States were infected.
• There is increased risk for the disease among outdoor cats, unneutered males, and cats with other diseases (especially respiratory disease, mouth diseases, and abscesses).

Transmission

FeLV is a type of virus called a retrovirus, meaning it can be incorporated into a cat’s genome and may not be cleared over time.

• Persistently infected cats shed infectious viral particles in saliva, as well as in feces, urine and milk. Any close contact among cats can spread FeLV, including bite wounds, mutual grooming, or sharing feeding dishes and litter boxes.
• Transmission can also take place from an infected mother cat to her kittens, either before they are born or while they are nursing.
• FeLV does not survive long outside a cat’s body – probably less than a few hours under normal household conditions – so it is unlikely for a cat to be infected from the environment without prolonged close contact with an infected cat.
• Cats at greatest risk of FeLV infection are those that may be exposed to infected cats, either via prolonged close contact or through bite wounds.
  • Such cats include cats living with infected cats or with cats of unknown infection status, cats allowed outdoors unsupervised where they may be bitten by an infected cat, and kittens born to infected mothers.
• Though any cat exposed to the virus can develop an FeLV infection, kittens are at a greater risk than adult cats due to their immature immune system.

Outcomes of Exposure

After exposure to FeLV, a cat’s body can react to the virus in a few different ways, leading to abortive, regressive, or progressive infections.

• In some instances, a cat can mount an effective immune response against the virus and completely eliminate it before the virus becomes incorporated into the cat’s genome. This is considered an abortive infection, and all direct testing for the virus will be negative.
  • These cats will have antibodies against FeLV and are considered immune to the disease.
• Abortive infections were once considered quite rare but studies using newer testing methods show that at least 20-30% of cats exposed to FeLV develop an abortive infection.
• These cats will never test positive for FeLV using any routine tests or show clinical signs of the disease, so owners and veterinarians may never be aware that they were ever infected by the virus.

About 30-40% of cats have a partially effective immune response following exposure to the virus and develop a regressive infection.

• In these infections, the virus is incorporated into the cat’s genome, but the immune system prevents prolonged viral replication, so there are no viral particles present in the cat’s blood after the initial infection.
• While a cat has a regressive infection, it cannot actively infect other cats with the disease, and it is very unlikely to experience clinical signs from FeLV. However, it is possible for the virus to reactivate and start replicating again, especially if the cat becomes immunosuppressed through illness or medications.
• When this happens, the cat is again infectious to other cats and at risk of developing clinical illness.

Progressive FeLV infection carries the worst prognosis, and cats with progressive FeLV are at high risk of developing potentially fatal associated diseases.

• With a progressive infection, a cat’s bone marrow becomes infected with the virus and allows for continual viral replication.
• Cats with progressive FeLV infection shed viral particles and can infect other cats.
• Studies suggest that 30-40% of cats exposed to FeLV develop a progressive infection, but kittens are at much higher risk of developing progressive disease than cats exposed to the virus as an adult.

Cats infected by feline leukemia virus can develop a number of different disorders, including:

• Anemia
• Cancer (especially lymphoma and leukemia)
• Suppression of the immune system (increasing the risk of other infections)
• Immune-mediated disease (in which the cat’s immune system causes damage to its own cells)
• Reproductive problems (loss of pregnancy and “fading kitten” syndrome)
• Intestinal inflammation, neurologic disorders (including nerve dysfunction and blindness)
• Stomatitis (severe inflammation in the mouth)

These disorders can be worsened by the presence of other infectious diseases, such as feline panleukopenia or calicivirus.