07/15/2024
July 15th is Feline Leukemia Day. Feline leukemia virus (FeLV) is one of the most common and important infectious diseases in cats, affecting between 2-3% of all cats in the United States and Canada. Infection rates are significantly higher (up to 30%) in cats that are ill or otherwise at high risk (see below). Fortunately, the prevalence of FeLV in cats has decreased significantly in the past 25 years since the development of an effective vaccine and accurate testing procedures.
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. Transmission occurs through the saliva, f***s, milk, and urine of an infected cat. The most common transmission route is through saliva. FeLV is transferred through grooming, licking, biting, shared dishes, and shared litter pans. An infected mother can transfer it to her kittens during pregnancy or nursing. Close cat-to-cat contact is required to transmit the disease. FeLV can also be transmitted through a blood transfusion. Young cats, especially those under 4-6 months of age, are the most susceptible to FeLV since their immune systems are not fully mature. FeLV cannot be transmitted to people, dogs, or other animals.
After exposure to FeLV, a cat’s body can react to the virus in a few different ways. 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.
FeLV adversely affects a cat's body in many ways. It is the most common cause of cancer in cats, may cause various blood disorders, and may lead to a state of immune deficiency that hinders a cat's ability to protect itself against other infections. Because of this, common bacteria, viruses, protozoa, and fungi that usually do not affect healthy cats can cause severe illness in FeLV-infected cats. These secondary infections are responsible for many of the diseases associated with FeLV.
During the early stages of infection, it is common for cats to exhibit no signs of disease at all. Over time, however, (weeks, months, or even years) an infected cat's health may progressively deteriorate or he/she may experience repeating cycles of illness and relative health. Signs can include:
• Loss of appetite
• Progressive weight loss
• Poor coat condition
• Enlarged lymph nodes
• Persistent fever
• Pale gums and other mucus membranes
• Inflammation of the gums (gingivitis) and mouth (stomatitis)
• Infections of the skin, urinary bladder, and upper respiratory tract
• Persistent diarrhea
• Seizures, behavior changes, and other neurological disorders
• A variety of eye conditions
• Abortion of kittens or other reproductive failures
A cat with progressive infection is most at risk of developing these clinical signs, but a cat with regressive infection can also develop FeLV associated diseases if the virus reactivates.
FeLV is diagnosed via blood testing that is available at most veterinary practices. The types of blood tests most commonly used for diagnosis:
-ELISA (enzyme-linked immunosorbent assay): This test identifies FeLV proteins in your cat’s blood and can identify cats with early infections. This test can be done in house at most veterinary practices.
-IFA (indirect immunofluorescent antibody assay): This test must be sent to a diagnostic laboratory and can detect when the disease has progressed into the secondary stage. Generally, cats with an IFA-positive test result have a poor long-term prognosis.
-PCR (polymerase chain reaction): This can detect if the virus has been incorporated into the cat’s genome, even if the virus is not currently replicating. This test will remain positive for regressively infected cats.
All new cats or kittens should be tested for FeLV. Any cats that have been exposed to a known FeLV-infected cat, cats with outdoor access, and any cats that are ill, should also be tested. It is important to note that all of these tests can give a false positive result with very recent infection, so if there is concern over recent exposure to FeLV the test should be repeated in 3-6 weeks to ensure accuracy. Most positive tests should have follow up testing performed 6-12 weeks later to determine if the FeLV infection is progressive or regressive.
The only sure way to protect cats from FeLV is to prevent their exposure to FeLV-infected cats. Vaccination for FeLV is available, and although it will not protect 100% of cats vaccinated, it is recommended to reduce the risk of FeLV infection for cats at risk of exposure, such as indoor/outdoor cats. This vaccine is also now considered a core vaccine for kittens, due to their higher risk of developing progressive infection. Owners contemplating FeLV vaccination for their uninfected cats should consider the cats' risk of exposure to FeLV-infected cats and discuss the advantages and disadvantages of vaccination with a veterinarian. Since not all vaccinated cats will be protected by vaccination, preventing exposure remains important even for vaccinated pets.
Unfortunately, there is no cure for FeLV. If your cat tests positive for FeLV and has a progressive infection, he will remain infected for the rest of his life. A FeLV vaccination is not beneficial if your cat is already infected. FeLV infected cats need to remain indoors and should be neutered. Partner with your veterinarian to help your cat feel well for as long as possible and protect them from secondary infection. Together you can help manage your cat’s condition, including the symptoms and any FeLV-associated diseases that may develop.
For more information:
https://www.vet.cornell.edu/departments/cornell-feline-health-center/health-information/feline-health-topics/feline-leukemia-virus
https://catfriendly.com/feline-diseases/felv/
