By: Dr. Laura Noaker, VERGI 24/7 Animal Emergency and Critical Care Hospital

If you Google “what is the most contagious disease in cats”, you should get the answer of 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

• 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 

• There are three types of infections

1. 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!

2. Regressive infection – the best way to explain this is 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 is at risk of showing clinical signs. 

3. 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 system

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 vets 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 cats 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. 

• A third test, called a polymerase chain reaction (PCR), can detect the virus in the cats genome, even if the virus is not replicating. This test will remain positive, even in cats with regressive disease.

All of these tests can give a false positive result with even 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. 

What are the clinical signs?

FeLV is the most common cause of cancer in cats and can cause various blood disorders and can suppress the immune system. Many infectious diseases that do not normally affect a healthy cat can cause severe illness in cats infected with FeLV. 

Common clinical signs:

• Persistent fever

• Loss of appetite

• Progressive weight loss

• Enlarged lymph nodes

• Poor coat condition

• Pale gums 

• Gingivitis (inflammation of the gums)

• Skin, urinary and upper respiratory infections

• Persistent diarrhea

• Eye conditions

• Neurological disorders

• Abortion of kittens/reproduction failure 

Treatment and prevention:

There is no cure for FeLV. Veterinarians can only treat the signs and symptoms. The best way to treat FeLV is to prevent your cat from getting exposed. Keeping your cats indoors and isolated from potentially infected cats is recommended. Outdoor cats are usually infected from fighting and receiving bites from infected cats. 

Prior to introducing a new cat into your home, take the cat to your vet for testing. A positive test result on a new cat means that the cats should be housed separately until the cat can be retested. Food and water bowls and litter boxes should not be shared between FeLV-infected cats and non-infected cats. It is important to remember that cats with regressive disease can re-activate the infection at a later date. This is why any sick cat, regardless of previous negative testing, should be taken to your vet and repeat the FeLV test. 

Is vaccination helpful?

It is available but will not protect 100% of cats but it is recommended for cats at risk for exposure. It is also considered as a core vaccine for kittens. Since it doesn’t protect 100% of cats, it is still important to reduce exposure to new cats and have new cats tested. It is important to note that vaccines will not cause false positive FeLV results on ELISA, IFA, or any other available FeLV tests.

What to do if your cat tests positive:

A positive test can be devastating news to receive but remember, a number of these cats may develop an abortive or a regressive form of the disease. Do not be pressurred into euthanizing your cat unless it is clinically very ill and suffering! Many cats can live for years. Your job will be to monitor your cat for weight loss, reduced or absent appetite, activity level, elimination habits, eye or mouth issues and overall behavior. Any deviations from normalcy should warrant a visit to your family vet.

FeLV is a complicated disease but in many cases, a cat showing no clinical signs can go on to lead a healthy life for many years.