What is the most contagious disease in cats?
Featured in Houston Pet Talk, June 2024 issue
By Dr. Noaker
If you google "what is the most contagious disease in cats," you should get the answer FeLV, which stands for "feline leukemia disease." Most cat owners have heard of FeLV, but what is it?
It is a complicated disease caused by a type of virus called a retrovirus. Here are a few facts:
It is spread through infected saliva (bite wounds, mutual grooming, sharing food bowls), nasal secretions, blood, urine, feces (sharing litter boxes), in utero, and through milk, infecting kittens.
The virus does not survive very long in the environment under normal household conditions.
Kittens are at a greater risk due to their immature immunity.
Types of FeLV Infections
Abortive Infection
Some cats (20-30%) can fight the virus and completely eliminate it before it encodes itself into their genome (DNA). In this case, the test for FeLV will be negative, yet the cat will have antibodies against it and are now considered to be immune to the disease. You and your vet may never know this even happened!
Regressive Infection
About 30-40% of cats will have a partial immune response after exposure. The virus will get encoded into their genome, but their immune system prevents viral replication. There will not be any viral particles in the blood, and the cat cannot infect other cats. However, if the infected cat becomes immunosuppressed from an illness or medications, the virus can reactivate, becoming infective to other cats and putting the cat at risk of showing clinical signs.
Progressive Infection
30-40% of cats exposed can develop progressive disease, and this is the worst prognosis. These cats are at a high risk of suffering a fatal disease. The virus invades the bone marrow, allowing for continual viral replication, and they can infect other cats. Kittens are at a higher risk of developing progressive disease than adult cats because of their immature immune systems.
How Do We Diagnose FeLV?
Enzyme-Linked Immunosorbent Assay (ELISA)
This test detects free viral particles in the blood and can be performed at your vet’s office.
Indirect Immunofluorescent Antibody Assay (IFA)
This test is sent out to a commercial laboratory to confirm a positive ELISA test. The IFA detects the virus in white blood cells. A positive result indicates that there is an active infection with viral replication. This means that it's either an early infection or a progressive infection. Two things can happen:
If the cat goes on to develop a regressive infection, subsequent ELISA and IFA tests will give negative results because the virus is no longer replicating.
With a progressive infection, both tests will remain positive weeks to months later.
Polymerase Chain Reaction (PCR)
This test can detect the virus in the cat’s genome, even if the virus is not replicating. This test will remain positive, even in cats with regressive disease.
Testing Considerations
All of these tests can give a false positive result with a recent infection. If you suspect that a cat has been exposed, the tests should be repeated in 3-6 weeks. Positive tests should be repeated in 6-12 weeks to determine if the FeLV infection is progressive or regressive.
