Feline Leukemia Virus (FeLV)

 

FeLV is a serious worldwide disease of the feline world that was first diagnosed in the 1960′s. Many cats that get exposed to the virus develop antibodies and are able to fight it off. This is especially true for cats that are free of parasites, are current on their routine vaccines, and are fed a good diet. Cats that have minimal exposure to other cats are at significantly less risk of getting this disease.  Cats in households with several other cats are at greater risk. Humans and dogs do not get this disease, nor do they get FIV.


Cause

It is caused by a retrovirus (FIV is also caused by a retrovirus) that is spread from cat to cat by saliva and respiratory secretions. It is found in the urine, but this is not readily transmitted this way.

Overall infection rates range from 1% – 8% in healthy cats, up to 21% in sick cats. Younger cats are more susceptible to this virus, and resistance develops as your cat ages.

The virus does not live more than a few hours outside a cats body unless it is in a moist environment, like a water bowl. This means that cats that share litter pans and feeding bowls, along with cats that groom each other and fight, are at risk. Kittens born to mothers that have the virus are infected in the womb.


Stages

The whole process from initial infection to the shedding of the virus in body fluids takes from 2-6 weeks.

1. The virus replicates in the lymphatic tissue in the oral cavity.

2. If the immune system is not able to stop the problem at this stage, the virus spreads to white blood cells that circulate in the body.

3. These white blood cells spread the virus to lymph nodes in the rest of the body. Most cats ( 60-80%) make antibodies at this stage to prevent further replication of the virus.

4. If these antibodies are not made, the virus spreads through the circulation to the bone marrow, where it will remain for the rest of the cats life.

5. White blood cells and platelets that are normally made in the bone marrow pick up this virus and bring it back into the circulation.

6. The virus is spread to the salivary glands, the tear glands, and the urinary bladder. It is at this stage that the virus can be shed and infect other cats.

To summarize, several outcomes are possible if your cat is exposed to this virus:

  • It makes antibodies and fights off the virus
  • It becomes a carrier of the virus without showing any symptoms initially. These cats can spread the virus to other cats easily because they show no signs of illness. After a variable period of time these cats will probably develop one of the diseases associated with the virus.
  • The virus weakens the immune system and various problems of a chronic nature (anemia, infections, etc.) develop.
  • It causes a serious cancer of the lymph nodes, called lymphoma or lymphosarcoma. This is why it was called Feline “Leukemia” Virus when first discovered.

It is not understood why some cats can make antibodies to FeLV and never get it, while others succumb to the virus. Many factors are postulated to be involved, including stress, diet, and of course, genetics.


Symptoms

There are no specific set of symptoms that tell us for certainty that a cat has FeLV. Cats that have this virus will commonly have fevers (>103 degrees F), poor appetites, are lethargic, suffer from recurring infections, and will have experienced weight loss. Some even have skin conditions. The ability of this virus to cause immunosupression makes cats more susceptible to Demodex and Scabies.

These symptoms are quite variable though, and are also present with other diseases likehyperthyroidismliver diseasesugar diabeteskidney disease, and feline hyperthyroidism, so a correct diagnosis is important. Cats that are carriers of the disease may not have any symptoms.

An occasional cat with FeLV will have uneven pupils, called Anisocoria

Cats that have FeLV are susceptible to other diseases, notably FIA (Feline Infectious Anemia). FIA is caused by a blood parasite called Hemobartonella. This parasite will either cause anemia by itself, or worsen the anemia caused be FeLV.

Several disease syndromes are associated with this disease, especially since the virus is immunosuppressive:

Anemia can occur because the virus attacks the bone marrow and prevents the production of red blood cells. Anemia is diagnosed by a blood sample that counts the red blood cells. Pets that are anemic tend to be lethargic and have poor appetites. Cats that are anemic due to FeLV need their red blood cells checked every 3 months on a routine basis. Fortunately, this test is inexpensive and can be performed in our hospital in a few minutes. This report from our laboratory is from a cat that is very ill with FeLV. The white blood cells (WBC) and red blood cells (RBC) are very low. The low WBC helps verify that this cat indeed does have a virus that is effecting, and also warns us that this cat is susceptible to secondary bacterial infections.


 

Chronic wounds or infections are another common problem associated with FeLV. This occurs when the virus again attacks the bone marrow like in anemia, but this time it effects the white blood cells. White blood cells are needed to fight an infection. Symptoms of this problem could be lethargy, poor appetite, swelling, draining wounds or soreness when petted. This is a chronic non-healing infection on the front leg of a cat, typical of a cat that is immunosuppressed.


 

Malignant cancer of the lymph nodes occurs on occasion. This is a serious complication of the disease and requires specialized medical and surgical care. Pets with this syndrome of the FeLV virus might show signs of weight loss, have poor hair coats and poor appetites.   Our surgeon is holding an enlarged popliteal lymph node just prior to surgical removal for analysis. The popliteal lymph node is located on each of the rear legs opposite the knee.


 

Malignant cancers of internal organs in the abdomen. These are not usually diagnosed until they have grown significantly, verifying the importance of routine exams of FeLV positive cats.

This tumor was in the small intestines. We have a complete case study describing how we diagnose such a tumor and how we treat it.

 

Associated disease like FIA can occur due to immunosupression. This is a parasite of the red blood cells that can worsen the anemia that might already exist. It is treated with the use of special antibiotics, but may be difficult to control due to the effect the FeLV has on the immune system.

Diseases of internal organs like the liverkidneys, brain, lung, GI tract, and eyes are also associated with FeLV. Diseases of these organs are also treated with symptomatic care.


Testing Recommendations

Testing for FeLV is our first line of defense.

Our hospital follows the recommendations of the american association of Feline Practitioners/Academy of Feline Medicine. It has been updated several times, and this web page reflects current information.

Their recommendations are summarized as follows:

  • The FeLV status of all cats should be known
  • Testing and identifying positive cats is the mainstay of managing this disease
  • All new kittens and adult cats should be tested before introduction into any house- Kittens can be tested at any age
  • Vaccination with FeLV vaccine does not interfere with the FeLV test
  • The ELISA (Enzyme linked immunosorbant assay) test is the preferred screening test
  • All positive screening tests should be repeated
  • The IFA (Immunofluorescent antibody) test should be used to confirm a positive ELISA test
  • All cats with negative test results that are exposed to FeLV positive cats should be retested not sooner than 28 days after exposure
  • Testing of outdoor cats or those at risk for exposure to the virus should be performed annually
  • An FeLV test should be performed on every sick cat regardless of vaccine status or results of prior tests
  • Any cat in a multiple-cat household found to be FeLV positive should be isolated from other cats
  • FeLV positive cats can live a high quality life for months to years

The ability to identify FeLV positive cats has been a significant factor in the reduction of FeLV exposure and infection.


Testing Procedure

No test procedure for this disease is foolproof. Some cats can have false negatives on the test and others can have false positives. This is why all diagnostic tests are interpreted in light of other findings. This is illustrated clearly in our page on the diagnostic process.

In general, a negative test means there are no detectable virus particles in the bloodstream at the time of the test. There is no guarantee that your cat will not get this disease at some time in the future, though. A positive test should be verified, especially in cats that have no symptoms of the disease. The FeLV vaccine will not cause a positive test result.

Our in house FeLV test is the ELISa (Enzyme Linked Immunosorbant assay) test. It is the recommended test for this disease, and detects virus particles, called antigens, in the blood. It is very accurate, and checks for evidence of the virus (antigen) in blood, saliva, or tears. Most experimental cats will have a positive test 28 days after exposure, although it can be significantly longer in some cats.

One of our doctors might consider additional testing, using the PCR test,  if this test is negative. This might occur in cats that are less than 12 weeks old, or cats that have been recently exposed to the FeLF virus.

Cats that are positive on this test should be rechecked in 6-8 weeks since some of them can become negative. Or, positive cats can be checked with the IFA or PCR test, and if positive on these tests, FeLV associated disease is most likely.

This ELISA test also checks for antibodies to FIV (Feline Immunodeficiency Virus) at the same time. This added convenience minimizes cost and requires less blood than if two separate tests were run. In addition, it is highly accurate, and can be performed within 15-20 minutes.

It is a sensitive test, and can occasionally give false positive reports. Conversely, cats with a bad infection (called acute infection) can be negative on this test, when in reality there are positive for FeLV.

Specialized equipment and training is needed to accurately run the ELISA test. The bottle with the dark blue top is the reagent used to start the chemical reaction needed to read the test. A few drops of blood are all that is needed. The test kit the blood will be placed into is called a Snap test, because the right hand side of it is snapped down to complete the test.


After being placed in the reagent solution, the blood is transferred to the diagnostic test kit well. The blood immediately starts flowing towards the white circle in the center of the test kit. It takes 30-60 seconds to reach the white circle

When the blood flow reaches the center circle the kit is activated by pushing down on the elevated area on the right side of the test kit. After a few seconds the blood starts flowing back to the left.

The blood eventually flows all the way back to its starting point. After 10 minutes a blue dot appears, signifying that this cat is negative for both FeLV and FIV.

The three different type of positive results that are possible:

FeLV Positive FeLV and FIV Positive FIV Positive

There is an additional test for the FeLV called the IFA (Immunofluorescent Antibody) test that checks for evidence of the virus (antigen again) in white blood cells and platelets. Cats become positive on this test only after the virus has moved to the bone marrow, which takes at least 3 weeks after the virus is in the bloodstream. A positive test here means the bone marrow is infected, so FelV associated diseases which eventually appear.

This test is used to confirm a positive ELISA test, and signifies a persistent infection. It is not used as an initial screening test like the ELISA test because it can miss the initial stage of virus infection in the blood stream. This test needs to be sent to our outside laboratory, and is used only when one of our doctors feels it is necessary.

PCR ( Polymerase Chain Reaction) is another test, sent to an outside lab, that helps us with this diagnosis. It is most useful in patients that have progressive infection (they are not mounting an adequate immune response) because it detects the presence of DNA even if there is no antigen present.

To review this important aspect of controlling this diseases, current AAFP guidelines for testing are as follows:

All sick cats should be tested for FeLV, no matter what its past history, test results, or vaccine status.

All cats should be tested prior to adoption, especially if there are other cats in the household and they are FeLV negative.

All cats in a household should be tested when a new cat that is FeLV negative enters the household.

Any cat that has recent exposure to an FeLV positive cat, and all outdoor cats, should be tested.

Any cat that has an unknown FeLV status, and any cat that will be a blood donor should be tested.

Treatment

Over the year many different “concoctions” have been tried to treat FeLV (along with FIV and FIP), some with great claims of success. With the advent of the Internet these miracle cures spread rapidly, and since they are in print, somehow have great credibility. With new antiviral medications as a potential to treat viruses in the near future one day these claims may ring true. Until then, it is best to follow the treatment plan that your veterinarian recommends.

Also, any treatment has the potential to cause other problems. For example, in humans, it is very common to take megadoses of Vitamin C (ascorbic acid). Claims abound about how it cures or prevents the common cold (which is caused by a virus). Unlike people, cats can manufacture their own ascorbic acid, so it is not mandatory in the diet like it is in people. To further complicate the issue, something as seemingly harmless as large doses of this water soluble vitamin are one of the potential causes of bladder stones in animals.

There is no current medication that will kill this virus. Treatment is aimed at keeping the immune system as strong as possible and utilizing medication as needed. In reality, we are treating the symptoms of the secondary diseases that occur because of the immunosupression caused by the virus.

As in other viruses, the symptoms associated with FeLV can wax and wane, so it is usually advantageous to treat for several days to help a cat get over what is hopefully a temporary episode. Our nursing staff excel at treating cats with the significant diseases associated with FeLV, and these cats can be treated much better if they are hospitalized.

Some of the more common treatments include:

Antibiotics

Are commonly used to help these immunosuppressed cats fight off bacterial infections. The blood sample above showed a cat with a very low white blood cell count due to the virus. This cat is more susceptible to infections because of this.

If Hemobartonella (FIA- Feline Infectious Anemia) is present in the bloodstream, a special antibiotic will be used to help suppress it.

Immune Stimulators

Medication can be given that can help boost the immune system. Their effects are variable and usually are worth trying in some cases. Immunoregulin and interferon are the common medications here.

Immunoregulin is an intravenous medication made up of a killed bacteria that helps stimulate the immune system.

Cortisone

The most common form of cortisone used in cats with cancer from FeLV is prednisone. It can help reduce the size of a solid tumor (lymphosarcoma) and decrease the number of circulating cancerous cells in a cat with leukemia. Unfortunately, the mechanism that allows prednisone to cause these cancers to temporarily improved also suppresses the immune system to the point that the cat is now much more vulnerable to secondary bacterial infections.

Vitamins

These cats do not eat well and can become anemic, so supplementing with B-complex vitamins might be of some benefit. A high quality cat food should also be fed. assist feeding should be utilized in cats that are not eating anything at all.

Fluids

Cat with fevers and those not eating well will routinely become dehydrated. Giving fluids will substantially help these cats fight off some of the FeLV associated diseases. They will also help counteract the fever that commonly accompanies this virus and also when secondary bacterial infections have set in.

Red Blood Cell Stimulators

A hormone called erythropoetin can be supplemented to help minimize anemia. Unfortunately, the body might eventually makes antibodies to this oral medication and the anemia returns, sometime in a more severe form. Use of this medication requires adhering to specific protocols and close monitoring.

Blood Transfusions

These can be extremely beneficial in anemic cats that are not producing adequate RBC’s due to the virus. It must be fully understood though that this is only a temporary measure.

Appetite Stimulants

Some medications, especially a drug called Periactin (cyproheptadine) can increase the appetite in some cats.

Anabolic Steroids

These medications might help counteract anemia, increase appetite, and promote on overall feeling of well being.

TLC

A warm peaceful environment with plenty of attention are extremely beneficial.

Cats that are positive for the FeLV and are normal acting present a dilemma. These cats have a chance of dying from this virus in several years, and are the potential source of infection for other cats, yet they are perfectly normal otherwise. If the positive cat lives alone, and will not go outside, then it is reasonable to take the watch and wait approach. Much of the decision on these cats depends on the individual circumstances of your household, especially how many other cats you have. FeLV positive cats should never be allowed to roam outside. These cats should not be bred since there is a great likelihood of passing the virus to the kittens.


FeLV Vaccine

The vaccine for FeLV is effective, and all cats that go outside or are at risk should be vaccinated for this virus after testing negative on a blood sample. It should not be administered to FeLV infected cats. The vaccine will not affect the carrier state or the capacity to infect other cats. It will not reverse the deleterious effects of a cat that already has one of the diseases associated with FeLV, and it will not cause an FeLV negative cat to appear positive on testing.

Initially it is given to kittens after 9 weeks of age, and reboostered 2-4 weeks later. If the second vaccine in the 2 vaccine series is given greater than 4 weeks after the first, an additional vaccine needs to be administered 2-3 weeks later. After the initial series, yearly boosters are given. If your cat goes outside frequently, we recommend yearly FeLV testing along with vaccination.

No vaccine can be guaranteed to be 100% effective, so we recommend separating any FeLV positive cat from FeLV negatives cats, even if the negative cats are vaccinated.


Prevention

Cats that are FeLV positive should not be allowed to breed, roam or contact other cats. Keep food bowls and utensils of positive cats away from all other cats, and clean them with bleach. Replace all bedding, food bowls, litter pans etc. when bringing in a new cat to a household that has had an FeLV positive cat.

If one or more cats in a multi cat household is positive they should be removed. The remaining negative cats should be checked every 3-6 months, and if positive, be removed from the house. Do not bring a new cat (make sure it is negative of course) into a household that has a history of cats positive unless the remaining cats are negative on 2 successive tests. Wait at least one month before introducing this new cat into the household.