Monday, October 13, 2014

Disease Monograph: Feline Leukemia - Part 2: Diagnosis, Prevention and treatment

Diagnosis of Feline Leukemia

To diagnose this disease, there are two types of blood test. One is called an enzyme-linked immunosorbent assay (ELISA test) and the other is called an immunofluorescent antibody (IFA) test. It is highly recommended to test cats prior to giving the leukemia vaccine to ensure that the cat is not already positive. If your cat is positive for FeLV, the vaccine will not help your cat fight the infection. The ELISA test is a quick and readily-available color change test that detects FeLV virus presence in the blood by testing for a protein that the virus creates when it replicates. This test can detect the virus at any stage of infection and can display positive results within a few days of infection. False positives can occur, due to the nature of this disease, so if your cat has only been tested once and tests positive, it may be a good idea to wait 2-3 months and re-test. Additionally, if you test a cat immediately upon bringing it into the household, you should test again about 60 days after its last possible exposure to the disease (whether it came from a shelter, was outdoors as a stray, or came from a breeder or friend).

In some cases, the tears or saliva of a cat can be tested with the ELISA method, but it turns positive only in late stages of infection and can return a false negative result. It can also return false positive results due to the nature of the testing procedure, so it is not commonly performed.

The IFA test is a test for FeLV proteins (antigens) in blood cells that appear at a later progression of the disease. This test may return a false negative result if the cat is in the early stages of the disease. If the IFA test is positive, it generally means that the cat will be unable to fight off the virus. This test is usually sent to a reference lab to confirm positive test results from the veterinary hospital laboratory.

A third type of blood test exists, but is not commonly used for regular screening. This third test is a PCR test and indicates whether there is any FeLV DNA present in the blood.

According to the American Association of Feline Practitioners (AAFP), the recommendation is that the FeLV status of ALL cats should be known, because Feline Leukemia Virus is responsible for the illness and death of more cats than any other disease condition.  Cats who have had a recent known or possible exposure, or cats that are ill should be tested before entering into a home with other cats.

The Six Stages of FeLV infection

1. The virus enters the cat and infects the white blood cells in the tonsils which then travel to the lymph nodes and copy themselves
2. The virus enters the blood stream and spreads throughout the body
3. They lymphoid system (the root of antibody response) becomes affected and further spreads the virus
4. The virus takes over the immune system and the intestinal tract becomes infected.
5. The bone marrow becomes infected if the cat cannot fight off the infection. At this point, the virus has become a part of the cat for the rest of its life. The virus is now carried in newly-made white blood cells that are produced by the marrow. Red blood cell production may be affected, as well.
6. The infection spreads to the cells that line the body cavities and organs, including salivary glands, stomach, esophagus, intestines, trachea, kidney structures, bladder, pancreas and lungs.

The outlook for cats infected with FeLV is grim. Eighty to ninety percent of cats that test positive do not live more than 4 years after diagnosis (on average, about 2.5 years). The other 17% are carriers that may be outwardly healthy, but can spread the disease to other unvaccinated cats. Cats that test positive and appear healthy should be kept in a low-stress environment to help the body conserve energy to fight the disease, but there are no other definitive preventive measures that will stop the disease progression.


This disease is extremely contagious in kittens. Neonatal kittens are 100% susceptible to contracting the virus after one exposure. By 8 weeks, their risk of infection drops to 85% from one exposure. Only about 30-40% of cats become immune after exposure. This means that 60-70% of unvaccinated cats (6 of 10) will contract the virus.

Several different vaccines exist for feline leukemia virus prevention: inactivated virus, recombinant canarypox vector and genetically engineered subunit vaccines. Once a cat has been vaccinated, it takes about 2-3 weeks before the cat develops immunity, so vaccination for Feline Leukemia is best done well before a cat will be at high risk for exposure. Most Feline Leukemia vaccines only provide immunity for 12 months, and therefore need to be boostered annually. The vaccine used at Exclusively Cats Veterinary Hospital is a vaccine that only needs boostering every 2 years, to help reduce the risk of negative side effects. Some cats vaccinated with feline leukemia virus vaccines have had negative side effects - usually mild, but up to and including vaccine induced sarcomas. Your cat is not at risk from contracting FeLV from the vaccine. Since 1992, none of the FeLV vaccines available are capable of causing active infection. Additionally, the vaccine will not cause your cat to test positive on a Feline Leukemia test. While no vaccine is 100% effective, use of this vaccine in high risk cats is highly protective (80-90%) and strongly recommended.

The virus does not survive long outside of the body, only lasting 2 hours at most, and in reality probably only a few minutes, in a dry environment, keeping the litterbox clean and dry is important if there is a feline leukemia positive cat in a multi-cat environment. The virus is also easily killed with bleach and household detergents.


Many times, a cat diagnosed with FeLV infection is already feeling sick, and in these cases, a pet owner may opt for euthanasia to end the cat's suffering. Since there is no cure for FeLV infection, and since 80% or more of positive cats will die within three years (most of which occur within 6 months) this may be the most humane option in cases where the cat is severely ill.

Propionobacterium acnes - a potential FeLV treatment
If a cat is diagnosed with FeLV infection and is apparently healthy, the best course of action is to limit the cat's exposure to other cats, keep the cat indoors, limit and prevent stress in the cat's life, feed a high quality diet, and keep her (non-FeLV) vaccinations up-to-date (remember that this disease decreases the effectiveness of a cat's immune system, making her more susceptible to other diseases). Quick response to secondary infections or conditions will help keep the infected cat healthy, longer, as well, so a close relationship with a veterinarian is important. For example, if an infected cat's red blood cell count begins to drop, the cat can receive various treatments aimed at stimulating red blood cell production or circulation - B12 or iron injections, erythropoetin treatment, or even a blood transfusion, if needed.

Additionally, there has been some development of experimental treatment protocols combining the

Polyprenyl immunostimulant - a potential FeLV treatment
use of immunostimulants, antivirals such as interferon and drugs that are in development for human AIDS treatment (these are usually tested in cats before moving on to human trials). However, there have not been many clinical use trials for these treatments, so information about their general effectiveness remains vague and anecdotal. Chemotherapy for FeLV associated cancers is available, but FeLV positive cats do not respond to chemotherapy as well as non-FeLV positive cats, and survival time once cancer develops is usually only about 6 months, despite chemotherapeutic treatment.

FeLV versus FIV

Some people confuse Feline Leukemia Virus (FeLV) and Feline Immunodeficiency Virus (FIV) because they are both retroviral diseases that affect the immune system. Feline Leukemia Virus infection, however, is almost two times more common than FIV. There are two very significant differences between the two diseases:

1) FeLV frequently causes severe illness, especially in young cats, while FIV tends to remain latent or non-illness-causing in the body for 8-9 years or so. It may never cause illness in some cats.
2) FeLV is much more easily transmitted. FIV tends to cause disease primarily following a bite wound, while FeLV can be transmitted through casual contact.

References and Further information:

Disease Information Fact Sheet: Feline Leukemia Virus
Journal of Feline Medicine and Surgery (2013) 15, Supplementary File