9 Things You Should Know About Feline Leukemia

9 Things You Should Know About Feline Leukemia


So, you adopted a pair of adorable kittens from a kind neighbor. When you took them to the vet for their first checkup, they did great in the physical exam, but their blood samples showed something odd: The cats were positive for feline leukemia (FeLV). What does that mean? And what does it say about these sweet kitties’ future? Here are some fast facts about this disease, which occurs in about two to three percent of cats in the United States.


9 Things You Should Know About Feline Leukemia

1. Feline leukemia is not cancer

In humans, leukemia is a cancer of the blood and bone marrow, but feline leukemia is a virus. The feline leukemia virus can cause cancers, however.

2. There are three types of feline leukemia infections

FeLV-A occurs in all cats infected with the virus. It causes the suppression of the immune system that makes the disease so dangerous. FeLV-B occurs in about half of infected cats, and it causes tumors and other abnormal tissue growth. FeLV-C is by far the rarest, found in only about one percent of infected cats, and causes severe anemia. FeLV-positive cats can have one, two or all three types of infections.

3. The virus is short-lived but highly contagious

The most common ways cats get infected with FeLV are through mutual grooming and bite wounds. Saliva and nasal secretions have a high concentration of the virus. Mother cats can transmit leukemia to their kittens through their milk, but some kittens seem to fight off the infection. It’s rare, but occasionally the virus is transmitted through shared litter boxes and food dishes. The good news is that the virus doesn’t survive long outside the body — just a few minutes or so under normal conditions. (Hoarding environments and other overcrowded and unsanitary conditions can greatly increase the risk of infection.)

4. Repeated or continuous exposure is necessary for infection

According to the Cat Owner’s Home Veterinary Handbook, healthy adult cats can fight off the virus for a long time, even with long-term exposure. Kittens and young cats have less resistance, though. The virus won’t even appear in a blood test until the cat has been exposed for at least four weeks.

5. A cat exposed to FeLV doesn’t necessarily get infected

About 30 percent of exposed cats don’t get infected at all. Another 30 percent develop a transient infection — the virus is present in their blood and saliva for less than 12 weeks, after which they are cured and aren’t contagious to other cats. Between five and 10 percent of exposed cats develop a latent infection: The virus is no longer in their blood and saliva, but it still lives in their bone marrow. These cats can start shedding the virus again under stressful situations or transmit the virus to their kittens in utero or through breast milk. But 30 percent of exposed cats do develop a persistent infection; they remain contagious and have a much shorter life expectancy.

6. Infected mother cats have trouble with pregnancy

A mother cat infected with FeLV has a higher chance of spontaneous abortion of kittens, resorption of the fetuses and stillbirth. Feline leukemia is also a possible cause of some cases of “fading kitten syndrome.”

7. About a third of persistently infected cats develop cancer

Of the 30 percent of exposed cats who develop a persistent FeLV infection, about a third develop a virus-related cancer. The most common type is lymphosarcoma, also known as lymphoma, masses that can be found in the lymph nodes in the groin, chest, armpits and neck. Lymphosarcoma sometimes spreads to other organs such as the eyes, brain, kidneys and other organs. The virus can also cause the blood cancer known as leukemia, but that’s much less common.

8. FeLV-positive cats can have good lives

Leukemia-positive cats must be kept indoors and fed the best possible diet. Regular vet checkups are crucial for maintaining health. They may not live as long as other cats, but they can enjoy a good quality of life as long as they receive excellent care. Because FeLV-positive cats’ immune systems are weak, they need aggressive treatment of any infections.

9. Immune-boosting therapies may help

Some vets recommend using immune system-boosting medications like interferon and Lymphocyte T-Cell Immunomodulator, but the jury is still out on how effective these treatments are.

Do you have an FeLV-positive cat? What have you done to keep him or her as healthy as possible? Have you used any of the newer immune-strengthening therapies, and if so, have they had any effect? Please share your stories in the comments!

What is Feline Leukemia Virus (FeLV)?

“FeLV does not pose a risk to other species of animals or people.”

Feline leukemia virus (FeLV) is one of the most important infectious viruses of cats. It was first discovered in cats with a form of leukemia, hence its name. FeLV is the cause of a variety of diseases, not just leukemia. Like all viruses, FeLV is a tiny microorganism consisting of nucleic acid and a few proteins and glycoproteins in a simple structure. Viruses can only replicate themselves inside living cells. FeLV is specific to members of the cat family and does not pose a risk to other species of animals or people.


How common is FeLV?

FeLV infection is found worldwide. In general, around 1-2% of the cat population is persistently infected with this virus, and many more are exposed. The proportion of cats infected differs according to the geographical location, environment and the life-style of the cat. Infection is more common in colonies of cats where there is close contact between individuals.

What disease does the virus cause?

feline_leukemia_virus_disease_complex-1FeLV invades and replicates in various cells, including cells in the cat’s immune system and blood-forming tissues. To replicate, the nucleic acid (genetic code) of FeLV inserts itself into the nucleic acid of the cells it has invaded. The result can be death of the cell or a mutation or change in its genetic code. Such a change can make the cell potentially cancerous; the cancerous change may not occur for months or years after infection.

Cancers can occur in a variety of tissues, organs and body sites, depending on the type and location of cells that have been infected with FeLV. Such cancers can involve any type of the circulating white blood cells (leukemia) or other cells of the blood-forming tissues. The most common tumor associated with FeLV is that of lymphoid cells known as lymphoma or lymphosarcoma. These tumors may occur at single or multiple sites in the body.

Although the development of cancer is one outcome of FeLV infection, other diseases more commonly develop. In many cats, FeLV infection results in a moderate to severe suppression of the immune system. This means that the infected cat is less able to defend itself against a wide range of infections that would not normally cause a problem in healthy cats. Affected cats may develop various clinical signs, and there is a progressive deterioration in their health over time.

Another common occurrence in FeLV-infected cats is the development of a profound and life-threatening anemia. Other problems, including abortion, severe enteritis (intestinal inflammation), neurological (nervous) disease, and ocular (eye) disease are commonly associated with FeLV infection.

FeLV is usually fatal. Studies have shown that 80-90% of FeLV-infected cats will die within three to four years of initial diagnosis.

How is FeLV transmitted?


“Direct contact between cats is the most frequent method of FeLV infection.”

Direct contact between cats is the most frequent method of FeLV infection. The virus is fragile and cannot survive longer than a few hours outside of the cat. A cat with FeLV sheds a large quantity of the virus in its saliva as well as in other bodily fluids such as nasal secretions, urine and feces. However, FeLV is not a highly contagious virus, and transmission generally requires a prolonged period of close contact between infected and susceptible cats. Close contact activities include mating, mutual grooming, and sharing of litter trays and food bowls. Cat bites by an infected cat can readily transmit infection.

Another potential source of infection occurs when a pregnant cat infected with FeLV gives birth. In this situation, the kittens may be born with FeLV virus or, more likely, are infected when their mother grooms them. Fortunately, most queens infected with FeLV are infertile or there is pre-natal death of the kittens with abortion or resorption of the fetuses.

What happens when a cat is exposed to FeLV?

Not all cats exposed to FeLV will develop persistent infections. Many of the cats that are infected are able to mount an immune response to the virus. This immunity is successful in eliminating the virus in approximately 30% of the adult cats exposed to FeLV. However, there will have been a period of some weeks before they cleared the infection, during which the virus was replicating in those cats. During the period when FeLV was replicating inside those cats’ cells, there may have been cell changes that could lead to disease later in life.

Approximately 70% of cats are unable to mount an effective immune response and eliminate the virus. Following infection, these cats become persistently and permanently infected with the virus and are at the highest risk of developing FeLV-related disease. It is these permanently infected cats that are primarily responsible for the transmission of FeLV to other cats. It can be many months or even years between the initial virus infection and the onset of related clinical disease problems. During this time, virus particles may be continuously shed in the cat’s saliva.

How can FeLV infection be diagnosed?

Diagnosis of FeLV infection is relatively simple. A rapid blood test can be performed which is able to detect viral proteins of FeLV in the blood of an infected cat. This test is very accurate and reliable, although false results may rarely occur. Some cats with only a transient FeLV infection (those cats that are able to mount an effective immune response) will be positive on the blood test. A second test performed eight to twelve weeks after the first test may be required to confirm persistent infection. In some situations, it may be necessary to confirm infection through further blood testing at a specialized laboratory where more sophisticated tests are available.

“Diagnosis of FeLV infection is relatively simple.”

Diagnosis of disease due to FeLV is more difficult because of the variety of signs and symptoms. It is common to have a complicated situation in which other diseases or conditions co-exist with, and are promoted by, the FeLV infection.

Is there any treatment for FeLV infection or disease?

“There is no treatment to eliminate the virus from the body.”

There is currently no specific treatment for FeLV-infected cats. There is no treatment to eliminate the virus from the body. Most FeLV-infected cats will eventually die or be euthanized because of diseases related to their infection. However, many cats showing FeLV-related disease will improve with symptomatic treatment, at least for a time. For example, if FeLV is causing immunosuppression and the patient develops secondary infections, the secondary infections may be treatable, leading to clinical improvement.

How can infection be prevented?

Vaccines are available to protect cats against FeLV infection. Their use is highly recommended for any cat that goes outside at any time and therefore could have contact with FeLV-infected cats. As with other vaccines, an initial course of two injections is required, and regular boosters are necessary to maintain immunity. Your veterinarian will discuss the most appropriate vaccination options for your cat with you. All cats should be tested for FeLV prior to vaccination.

Although vaccination is very helpful in preventing infection with FeLV and thus controlling FeLV-related disease, no vaccine is 100% protective. Do not allow your cat to roam. Where possible do not allow your cat, particularly if a kitten, to come into close contact with known FeLV-infected cats or cats without a known history of proper vaccinations.