Transmission

Viremia is the condition when feline leukemia virus is present in the blood.  There are two stages of viremia during the course of disease, and these are important to know because this is how an owner can get two different, conflicting test results. 

The primary viremia is the stage before the virus infects the bone marrow.  It is at this point that the cat’s immune system is still trying to fight off the infection.  The secondary viremia is when the virus has invaded the bone marrow and established a permanent infection in the cat.

Transmission of FeLV requires intimate moist contact with an infected cat.  The virus is shed in saliva, tears, urine, and feces.  The most common route is contact with infected saliva through grooming, licking, biting, and shared dishes and litter pans.  Kittens can be infected by their mother in utero or during nursing, and FeLV can be obtained through a blood transfusion.

If a cat has been exposed to the virus, there are several possible outcomes, which depend on how well the cat’s immune system responds to the virus challenge.

1.  IMMUNITY   The cat’s immune system mounts a response and eliminates the infection.

During this period, the cat may actually develop a mild form of illness with fever, poor appetite, lethargy, and swollen neck lymph nodes lasting for three to ten days.  This scenario is more likely to occur in an adult cat than a kitten.  Natural immunity is developed that will last for an unknown period of time.

2.  INFECTION  The cat’s immune system is overwhelmed by the virus.

The cat becomes permanently infected with the virus and excretes it in its saliva.  After an initial sickness, the cat usually recovers for a period of weeks to years.  These cats test positive on a screening test for FeLV.  Vaccination of these cats does not cause problems but will not help the cat in any way.

3.  LATENCY     The cat harbors the virus, but it is not easily detected by testing.

The FeLV inserts a copy of its own genetic material into the cat’s cells.  These cats do not shed virus in their saliva or other body secretions, but queens may still pass the virus in utero and through the milk.   These cells may later be transformed into cancer cells or cells which do not function normally.  This genetic change remains undetected for some period of time during which the cat appears normal.

Another 30% of cats do not produce immunity but also do not become persistently infected immediately.  In these cats, the virus hides in the bone marrow for up to 30 months.  Eventually, these cats either overcome the virus or become persistently infected.

4.  CARRIER      The cat becomes an immune carrier.

The virus becomes hidden in the cat’s epithelial cells.  The FeLV is multiplying but is not able to get out of the epithelial cells because the cat is producing antibodies against the virus.  The cat appears normal.  “Carrier" cats that are vaccinated but develop a FeLV-related disease will appear to be a "vaccine failure.”

Finally, some cats can develop a latent or sequestered infection. This probably happens to less than 5-10% of cats. These cats, whose virus is hiding in sites such as the bone marrow, will rarely be contagious and are unlikely to develop illness. They will not test positive on routine testing.

Age is a very important factor in determining what will happen after a cat is exposed to FeLV.  Almost all FeLV-exposed kittens less than 8 weeks of age will have persistent infection and show signs of disease during the acute phase.  As kittens get older, the probability of becoming persistently infected after exposure lessens as they reach adulthood.

There are three disease categories associated with FeLV:

1. Leukemia – white blood cell cancers.

2. Lymphosarcoma – a lymph tissue cancer involving various different organs such as lymph nodes, kidneys, liver, gastrointestinal tract, bone marrow, and blood to name a few.  The location of the tumor will dictate the cat’s clinical signs.

3. Noncancerous disease – a wide variety of unrelated diseases usually related to the immunosuppressive action of this virus.  Because FeLV interferes with the cat’s ability to fight off infectious diseases, the cat may experience chronic infections, chronic gingivitis and tooth problems, and slow wound healing.

Here is a short list of some of the complications of FeLV infection and the reasons they affect FeLV- infected cats.

Anemia:  FeLV often affects cells in the bone marrow and as a result, most FeLV-infected cats have a nonregenerative anemia.

Immune-mediated Diseases:  Large amounts of the FeLV antigens combine with the cat’s own antibodies to form complexes, which can be deposited in the kidneys, blood vessels, or joints.

 Gastrointestinal Disease:  Cancers of the stomach or intestines, as well as FeLV-associated changes in the intestinal wall, can cause anorexia, vomiting, diarrhea, and weight loss.  Parasites and bacteria can multiply more easily and cause diarrhea in FeLV-infected cats.

 Neurologic Disease:  Seizures, blindness, paralysis, and changes in behavior can be seen in FeLV-infected cats.  These symptoms may be caused directly by FeLV or by parasites (Toxoplasma gondii) and fungi (Cryptococcosis), which occur more often in immunosuppressed animals.

 Platelet Disorders:  A decrease in the number of platelets (thrombocytopenia) or a dysfunction of platelets sometimes occurs in cats infected with FeLV.

 Neoplasia:  Usually the cancer is in the form of tumors of white or red blood cells and includes lymphosarcoma, lymphoid leukemia, and myeloid leukemia.

Diagnosis

Testing is the basis for diagnosing and managing FeLV infections.  There are several types of tests used to diagnose the infection.  The enzyme-linked immunosorbent assay (ELISA) test is the most common screening test and can be done in the veterinarian’s office.  This test detects a protein component of the virus as it circulates freely in blood serum.  That is why this test is best done using serum rather than tears, blood, or saliva because the results are more reliable.  The ELISA test detects the primary and secondary viremia stages.  Tests using tears or saliva are recommended only as screening tests for catteries and should not be used to screen individual cases.

Any positive ELISA test results should be confirmed using the immunofluorescent antibody test  (IFA) test, usually done at a commercial laboratory.  The IFA test detects circulating viruses in the secondary viremia stage only.  This test detects a protein component of the virus as it circulates in the bloodstream within infected white blood cells.  It is for this reason that this test uses blood smeared on a slide onto which a fluorescently tagged antibody is reacted.  This special antibody is made in the laboratory and can be seen through a special microscope.

Because both of these tests look for actual viral antigens, vaccinated cats or cats that are immune due to previous infection will not show a positive test.  Also, kittens can be reliably tested because maternal immunity does not affect test accuracy.

The American Association of Feline Practitioners (AAFP) has published recommendations for FeLV testing.  Since it is possible to have results on ELISA and IFA tests that do not agree, the AAFP testing protocol should be followed in order to determine the status of all cats.  The FeLV status of all cats should be known because this is the mainstay of FeLV control and is not replaced by vaccination.  When cats should be tested:

• Cats that are sick, regardless of previous negative results, should be tested.  While many signs such as fever, stomatitis, vomiting, and diarrhea are obvious indicators of illness, other signs are subtle and may include changes in behavior, grooming, and eating habits.  This includes sick cats that have been vaccinated for FeLV.
• All new adoptions, cats and kittens, should be tested whether they will be entering a household with other cats or not.
• All cats in multi-cat households because infected cats can remain asymptomatic for years, during which time they may transmit the virus to uninfected cats.
• Testing should be done when cats have had known or possible exposure to other cats of unknown infection status.  This may include cats that go outdoors unsupervised, or cats that have been in fights and have been bitten.  These cats should be tested no sooner than 60 days post-exposure.
• If the ELISA and IFA test results are not the same, the cat should be tested at 4-8 week intervals for at least 90 days.
• All cats with a negative result should be retested 90 days later to be sure they were not recently infected prior to the first test, when viremia was not yet present.
• If an FeLV infected cat is found in a multi-cat household or cattery, that cat should be removed from the others. All other cats should be tested at 90-day intervals until all cats have had two consecutive negative tests.

Treatment

Cats that test positive for FeLV may live for months to years depending on the stage of the viremia and illness status of the cat.  Owners should never assume that euthanasia is the only option for a cat with FeLV.  That should be addressed on an individual basis in consultation with the veterinarian.  In many cases, it is possible and feasible to keep a FeLV-positive cat and ensure good quality of life through the combined efforts of the owner and the veterinarian.  Because positive cats are capable of transmitting their infection to other cats, they should not live with other cats nor should they be allowed to roam outside.  This not only protects other cats from FeLV, but also protects the positive cat against the many diseases and illnesses they may contract due to their increased susceptibility.

The mainstays of treating FeLV positive cats are protecting them from exposure to other diseases, ensuring good nutrition, reducing stress, controlling parasites, and early and aggressive treatment of any symptoms that appear.  There is no specific treatment for FeLV and no known cure.  A large number of therapies have been investigated for FeLV positive cats, but most have not shown encouraging results.  A drug that stimulates the immune system, known as interferon, can be given orally to cats without side effects and may be helpful in many cases.

Specific cancers associated with FeLV have their own chemotherapy treatment protocols.  Some forms of leukemia (blood cancer) are responsive to some chemotherapy and some are not.  Because the virus is not affected by treatment, the cat will always remain infected with FeLV and relapse is possible and should be expected.

Prevention

None of the available vaccines provide 100% protection.  Almost all of them are derived from an inactivated (killed) virus or portions of the virus.  Most of these vaccines have one or two additives called adjuvants that enhance the antigenicity (ability of the virus to cause an immune response) of the killed virus.  In general, it is recommended that cats who go outdoors, to shows, are boarded, or otherwise have contact with cats other than those in their household should be vaccinated against FeLV.  Remember, all cats should be FeLV tested before they are vaccinated.  Cats that are entirely indoors may not need to be vaccinated against FeLV.  Vaccination is something you should discuss with your veterinarian.

Cats already infected with the feline leukemia virus will not be helped by the vaccine, but it does not hurt them either.  Vaccination does not affect feline leukemia test results, so a newly adopted cat with unknown vaccination status can be tested with confidence in the results.

It is essential that the risk of exposure of a FeLV-negative cat to a potentially FeLV-positive cat be minimized.  In single cat households, this is best accomplished by keeping the cat indoors.  When a new cat is being introduced to a house after the loss of a feline leukemia positive cat, extensive environmental disinfection is not necessary.  The feline leukemia virus only lives at most a few hours outside the cat.  Normal cleaning of dishes, litter boxes, and bedding should be sufficient.

In multi-cat households, all FeLV-positive cats should be removed or totally separated from the other cats.  Movement of cats in and out of the household or cattery should be minimized.  All cats should be retested at 90-day intervals until all cats have had two consecutive negative tests.  In cattery situations, only FeLV-negative cats should be used for breeding animals.

We hope this article helps you better understand how feline leukemia virus infection affects cats.  The information provided here should not substitute for a visit to your veterinarian.

Addie DD, et al. “Long-term impact on a closed household of pet cats of natural infection with feline coronavirus, feline leukaemia virus and feline immunodeficiency virus.” Vet Rec 2000 Apr 8;146(15):419-24.>

 Greene CE and Calpin JP. “Feline Leukemia Virus Infection.” In Morgan RV, (ed), Handbook of Small Animal Practice, 2^nd ed. 1992 Churchill Livingstone Inc. New York, NY:1197-98.>

 Hartmann K, et al. “Treatment of feline leukemia virus (FeLV) infection.” Vet Microbiol 1999 Sep 1;69(1-2):111-3.>

 Hofmann-Lehmann R, et al. “Feline leukaemia provirus load during the course of experimental infection and in naturally infected cats.” J Gen Virol 2001 Jul;82(Pt7):1589-96.>

 McCaw DL, et al. “Immunomodulation therapy for feline leukemia virus infection.” J Am Anim Hosp Assoc 2001 Jul-Aug;37(4):356-63.>

 Pedersen NC. “Immunogenicity and efficacy of a commercial feline leukemia virus vaccine.” J Vet Intern Med 1993 Jan-Feb;7(1):34-9.>