Feline Leukemia

What is feline leukemia virus?

Feline leukemia virus (FeLV), a retrovirus, is the most important infectious disease agent producing fatal illness in domestic cats today.

How is this virus spread from cat to cat?

The feline leukemia virus is excreted in saliva and tears and possibly the urine and feces of infected cats. Prolonged, extensive cat- to-cat contact is required for efficient spread, because the virus is rapidly inactivated by warmth and drying.

For how long can a cat survive with FeLV disease?

A cat with FeLV disease may live for several weeks to several months, depending on how advanced the disease is at the time of diagnosis. However, it is impossible to tell how long any particular cat will survive.

Can a cat become immune to FeLV?

A significant percentage of adult cats that are exposed to the virus develop immunity and do not become persistently viremic (i.e., will not carry the virus indefinitely in the blood and bone marrow). Usually those cats live out a normal life span. However, in some the virus may remain sequestered for a variable period of time somewhere in the body. It is thus conceivable that FeLV might break out and cause disease at a later date, after the cats have been stressed, or perhaps medicated with drugs that suppress the immune system.

Can my children or I become infected with FeLV if our cat is infected? What about our dog?

Although the possibility that FeLV can be transmitted to human beings and cause disease cannot be ruled out completely, there certainly is no evidence to date that transmission does occur, despite decades of extensive research. Also, there is no known association of FeLV with acquired immune deficiency syndrome (AIDS) in human beings. It is true that FeLV can be grown in human cells in culture; the same is true of other infectious disease agents that nevertheless do not produce disease in human beings. Similarly there is no evidence that FeLV is carried by, or causes any illness, in dogs.

CLINICAL SIGNS

What are the clinical signs? Common clinical signs produced by FeLV include anemia, jaundice, depression, weight loss, decreased appetite, diarrhea or constipation, blood in the stool, enlarged lymph nodes, respiratory distress, decreased stamina, excessive drinking and urination, fetal resorption, abortion, infertility, birth of "fading" kittens, and a syndrome resembling panleukopenia ("cat distemper"). FeLV also interferes with the cat's natural ability to ward off infectious disease agents, so that almost any severe, chronic illness may lead your veterinarian to suspect FeLV.

Cancer occurs in some FeLV-infected cats. In those cats the tumor masses may cause such problems as respiratory distress; intestinal inflammation with diarrhea, vomiting or constipation; liver or kidney disease; cloudy eyes; and neurologic abnormalities.

FeLV TESTS

What do the FeLV test results mean?

Two types of FeLV blood tests are in common use: the enzyme-linked immunosorbent assay (ELISA, or kit test, which can be performed in your veterinarian's office) and the immunofluorescence assay (IFA, also called the Hardy test or slide test, which must be sent out to a diagnostic laboratory). Both tests detect a protein component of the virus as it circulates in the bloodstream, either free in the serum (ELISA test) or within infected white blood cells (IFA test). Interpretation of a positive or negative test depends on which of these two tests your veterinarian has selected.

The feline leukemia virus is present in the blood (a condition known as viremia) during two different stages of the infection. The ELISA test can detect the primary viremia - the stage before the bone marrow has become infected, when the cat's immune system still has an opportunity to ward off the virus. The ELISA test can also detect the virus in the secondary viremia stage - when the virus has invaded the bone marrow, thus establishing a firm and lifelong foothold within the cat.

By contrast, the IFA test detects circulating viruses primarily during the second stage. Once the infection progresses to that stage, it has passed a "point of no return." Thus the majority of cats testing positive by the IFA test remain positive for life. Those cats, as well as most of those that are ELISA-positive, are shedding FeLV in the saliva and are infectious for other cats.

A negative FeLV test does not imply immunity to FeLV, nor does it indicate that a cat has never been exposed to the virus (i.e., it is a test for virus, not the antibody to the virus). A negative test can be found in cats that

(1) have never been exposed to FeLV (which is what we all hope for),

(2) are incubating the virus at an earlier stage than either test can detect (such cats might test positive later on),

(3) have overcome a previous infection and thus have not become persistently viremic, or

(4) were infected with the virus previously and have developed disease, but for some reason do not have virus in the bloodstream at the time of testing.

ELISA tests are also available to detect virus in secretions: saliva and tears. There is some degree of variability in the tests, and some positive animals may be missed. At this time saliva and tear tests are probably best reserved for screening purposes (e.g., to test a number of cats, to determine whether FeLV is present in a household or cattery) and for testing cats that are difficult to bleed.

I received two different test results for my cat. What does this mean?

This is an all-too-common problem and is called "discordant test results". As explained above, both the IFA and ELISA tests determine the presence of FeLV in the blood of cats, but in different "compartments" of the blood (serum vs. white blood cells).

If the cat is positive by ELISA and negative by IFA at the same time, it may merely be that the virus is at the primary viremia stage, which ELISA can detect and IFA usually cannot. A cat might also test differently at two different times with the same test; a reversal of a positive test to negative eight weeks later is especially common for the ELISA test. That indicates that the cat has successfully contained the spread of the virus, so that it never reaches the secondary stage of viremia.

Some healthy cats may remain ELISA-positive, IFA-negative for a prolonged period of time. Those cats are still carrying FeLV as a sequestered infection but are apparently not shedding it in saliva, and thus will not transmit the virus to their feline companions.

It is also possible for a cat in the early stages of the infection to test IFA-negative and become positive later on. Once a cat has become IFA-positive, it is usually positive for life (persistently viremic). Whenever there is a discrepancy between tests, one must repeat the tests to be sure that consistent results are obtained.

Finally, when two tests fail to agree, there is a slim possibility that one of them is incorrect. This is not something to base all your hopes on, but it is true that the tests are not 100 percent accurate, and once in a great while you might receive an incorrect result.

My cat had a positive FeLV test, but she seems completely healthy. Will she become sick and die?

Even if two or more successive tests reveal your cat to be truly positive, it will not necessarily die. An FeLV-positive healthy cat may live for months or years; the life expectancy is impossible to predict. Your cat is probably shedding virus that could infect other cats, however, and you should take precautions to reduce the chance of spreading the disease (see "Prevention"). In addition, the body's reaction to the virus may protect it from the primary FeLV disease problems but not from the immune-system suppression that the virus also can cause. Your cat thus may be much more susceptible to other infectious diseases and will require careful monitoring and immediate treatment should illness become apparent.

TREATMENT

Is there any cure?

To date there is no cure for FeLV infection or disease. A variety of chemotherapeutic regimens have been developed, and in certain cases those regimens can produce a temporary remission, depending on the physical condition of the cat and the type of disease that is present. Those drug therapies may allow the cat to continue in a reasonably healthy state for a period of several weeks to several months. However, it must be understood that those are only remissions and not permanent cures. Chemotherapeutic drugs are very potent, and their effects must be monitored carefully, to avoid overdosing the patient.

Various antiviral compounds including interferon may also be used to treat cats with FeLV infection. Those compounds, while still experimental, are generally safer to use than chemotherapeutic agents, and may reduce the amount of virus present in the blood of the cat, and may extend the period of remission of clinical disease. As yet, antiviral compounds do not produce permanent cures for FeLV infection or disease. Hopefully, additional research will produce effective antiviral therapies that will cure FeLV disease.

Are massive doses of vitamin C effective against FeLV?

There is no scientific documentation that vitamin C cures cats of leukemia. Controlled studies of feline viral rhinotracheitis, canine distemper, and human respiratory infections have failed to show effectiveness of high doses of vitamin C. Of course, a multivitamin and mineral supplement may be helpful to any sick animal that is not eating properly; however, there is little evidence to support claims that such a supplement can cure any of those conditions. Other than providing general support to the animal's health, vitamin and mineral supplements, in our estimation, are not effective in preventing the spread of FeLV within a cattery and certainly will not cure an individual cat of its infection.

If there is no cure for FeLV, why did my veterinarian prescribe steroid treatment?

Therapy with a steroid (such as prednisolone) acts to decrease the numbers of some circulating white blood cells (lymphocytes). A cat with leukemia may have an increased number of abnormal (cancerous) lymphocytes circulating in its bloodstream; therefore steroid treatment may help to destroy them. Prednisolone may also act directly against the cells of some solid tumors (such as lymphosarcoma) that are caused by FeLV. Steroids also inhibit the cells that are normally responsible for destroying senescent red blood cells; that effect may help to combat the anemia and excessive red blood cell destruction that often accompany FeLV.

It is important to remember that because steroids and FeLV both suppress the immune system, an FeLV-positive cat undergoing steroid therapy is especially vulnerable to other infections.

PREVENTION

Is there a vaccine to protect my cat against FeLV infection?

Several vaccines are now available to aid in the protection of your cat against FeLV infection. The vaccines are produced by various methods, and either contain the inactivated ("killed") whole virus, or a subunit protein of the virus. The principle of protection is the same for each of these vaccines.

Are the FeLV vaccines safe?

The FeLV vaccines are as safe as other commonly used feline vaccines. As with any vaccine in animals or humans, some reaction to the vaccine may occur in a relatively small number of vaccinations. The vast majority of cats vaccinated with FeLV vaccines will experience no reaction at all. Occasionally, your cat will experience some malaise for a few hours or for a day or two after vaccination. On rare occasions, an allergic reaction to one of the components of the vaccine may occur which will result in fever, diarrhea, and malaise. This allergic reaction can be treated by your veterinarian.

How effective are the FeLV vaccines?

The FeLV vaccines are reasonably effective in preventing persistent FeLV infection should your vaccinated cat be exposed to the virus. No vaccine is 100 percent effective, and this is true for the FeLV vaccines. The immune response produced by these vaccines will protect most exposed cats from becoming infected with the virus. Occasionally after exposure to the feline leukemia virus, a vaccinated cat will develop a transient viremia (temporarily become FeLV positive for up to 12 weeks), but the immune response produced by the vaccine will control the virus such that these cats will not develop clinical disease. Unfortunately, a small percentage of FeLV-vaccinated cats will not be protected against exposure to FeLV.

What age should my cat be vaccinated for FeLV?

Kittens should be vaccinated twice starting at nine to ten weeks of age, with the second dose of the vaccine given three to four weeks later. Your cat should receive annual revaccinations ("booster" vaccinations) against FeLV.

If my cat has been vaccinated, is it safe to have this cat live with a FeLV-positive cat?

The FeLV vaccines are not 100 percent effective, and thus a degree of risk occurs when a vaccinated cat is housed with a persistently-infected cat (FeLV-positive cat). It is recommended that FeLV-positive cats not be housed with FeLV-negative cats, even those that have been vaccinated. Certainly, a cat vaccinated against FeLV will have a far greater chance of successfully withstanding an exposure to FeLV than an unvaccinated cat.

Will vaccination interfere with the FeLV diagnostic tests?

No, vaccination will not interfere with either the ELISA or IFA diagnostic tests. The vaccines do not contain living virus, and the diagnostic tests detect a specific protein within the virus. Antibodies against FeLV, produced as a result of vaccination, are not detected by the diagnostic tests.

One of my cats was just diagnosed FeLV-positive. How can I protect my other cats from getting the disease?

In either a cattery or a multicat household, the most effective procedure is to test by IFA and remove all FeLV-positive cats. The remaining FeLV-negative cats should then be vaccinated and retested every three to six months for the next year, and any that become positive during that time should be removed. The household cannot be considered "free" of FeLV until all remaining cats have tested negative in two sequential tests taken at least three months apart. No new cats should be brought into the household until all the cats already there test negative repeatedly. All new cats should test negative initially, be quarantined for at least two months, and retest negative before being allowed to mingle with other resident cats.

The premises should be routinely scrubbed with detergent or disinfectant and wiped down with a solution containing four ounces of household bleach per gallon of water (bleach is an excellent disinfectant for viruses and other infectious disease agents). All food and water bowls, bedding material, and litter pans should be thoroughly cleaned and disinfected. Better yet, they should be replaced.

My one and only cat just died of FeLV. Is it safe to bring a new cat into my home?

Feline leukemia virus is relatively unstable and will not survive outside an infected cat for an appreciable length of time. The Cornell Feline Health Center recommends a waiting period of at least thirty days after removal of an FeLV-positive cat before a new cat is acquired. Other precautions that should be taken are identical to those described above to protect healthy cats. Thoroughly disinfect or replace the food dishes, litter pans, and bedding that were used by the infected cat. Floors that are covered with tile or other hard surfaces should be cleaned and then disinfected with dilute bleach solution (4 oz. household bleach to 1 gal. water). Thorough vacuuming of rugs, plus the thirty-day quarantine, should be sufficient to eliminate the virus from carpeting in the household.

Should FeLV-positive cats be destroyed?

Removal of persistently FeLV-positive (positive on the IFA test) cats from a household is the only proven effective method for FeLV control. The question naturally arises: what is to be done with such cats after their removal? In the past, some have recommended euthanasia (because there is no reliable means of eliminating the virus from the cat's body, the cat itself must be destroyed, to destroy the virus). Euthanasia has also been put forth as the only effective means for preventing further spread of FeLV within the cat population at large. The question of euthanizing a positive cat is one that must be addressed in each individual case, in consultation with the attending veterinarian. Vaccination against FeLV does not completely replace testing and removal as the method of choice for controlling FeLV.

If you own only one cat and it is FeLV-positive, euthanasia is not necessary from the standpoint of controlling virus transmission, so long as you keep your cat indoors and away from all other cats. You must remember, however, that in time the cat may develop an FeLV-related illness and become so uncomfortable that euthanasia becomes the only humane course of action.

If you have only a few cats and are reluctant to have a positive one destroyed, particularly if it is clinically healthy, a strict intrahousehold quarantine program may permit you to protect your other cats from infection. The FeLV-positive cat must be prevented from having any contact with the negative cats, perhaps by housing it in a separate room within the house. Separate feeding utensils and litter pans should be provided, and hands should be thoroughly washed and clothing (including shoes) after handling and caring for the positive cat. The positive cat should never be allowed outdoors, where it might come into contact with FeLV-negative cats and transmit the virus.

A favorite breeding queen (now pregnant) has tested positive for FeLV. Must she be destroyed? If so, can I wait until after her kittens are born?

Feline leukemia virus is transmitted from carrier queens to their kittens either in utero or after birth. A very high percentage of kittens born to infected queens will succumb to FeLV infection or FeLV-related disease. In our estimation it is absolutely essential that you establish a test-and-removal program, so that all persistently infected animals are removed from the cattery. Continuing to breed FeLV-positive queens merely expands the problem and in essence signs the death warrant of kittens born to those queens.

What is the Cornell Feline Health Center doing to combat FeLV?

Our research on FeLV has involved basic studies of the virus itself; attempts to develop more effective vaccines; and evaluations of the effectiveness of current FeLV vaccines, therapies, and diagnostic tests. Our efforts are directed toward eliminating forever the threat of this devastating viral infection.

©1997 by Cornell School of Veterinary Medicine. All rights reserved.

For more information on Feline Leukemia, check the following sites:

Feline Leukemia - From South Paws Animal Hospital
Chloe's Page - Includes several resources and additional links
Feline Leukemia - From the Bengal Cat Society
Feline Leukemia - From The Animal Clinic
Living With A FeLV+ or FIV+ Cat - An EXCELLANT site!
Fighting FeLV - Another EXCELLANT site by Beth Zimmerman
Newer Methods For Treating a FeLV+ Cat