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.

Prevention:

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.

Treatment

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

http://jfm.sagepub.com/content/suppl/2013/08/14/15.9.785.DC1/5_Fact_sheet_5.pdf

Journal of Feline Medicine and Surgery (2013) 15, Supplementary File

Disease monograph: Feline Leukemia Part 1: What is it?

Feline Leukemia Virus is a disease is transmitted between cats by a virus. The Feline Leukemia virus is a retrovirus from a family of viruses called oncornaviruses - viruses that cause the development of cancers (among other effects). A retrovirus is a virus that interacts with the genetic material of its host in order to reproduce. Feline Leukemia Virus (FeLV) causes immunosuppression, or failure of the body's natural defenses, in many feline species - the domestic cat is most susceptible, but large cat species such as lynxes, Florida panthers, cheetahs and lions have also been reportedly infected. This virus is not known to pass between non-feline species, so humans and dogs appear to be safe.

How is Feline Leukemia transmitted?

Electron micrograph of FeLV - from Wikipedia

The virus is shed in the saliva of an infected cat, and to some degree in respiratory secretions. Most cats receive the virus through the mouth. Generally, it is considered that the virus is transmitted through long term contact with an infected individual - such as sharing litterboxes and food bowls, mutual grooming, or through a bite. Mating behavior may also transmit the virus. Additionally, kittens can contract FeLV from their mother in utero and be positive from birth, or they can contract the virus through their mother's milk. Petting a feline leukemia positive cat and then petting a non-infected cat will not transmit the disease.

The frequency of FeLV positive cats varies greatly between areas. In some areas, the incidence can be as high as 70% in large multi-cat homes where cats have outdoor access. In single-cat, indoor homes, the prevalence is around 3% (3 in 10 cats). FeLV tends to be more common in urban cats (up to 40%) than in rural cats (up to 6%), because cats live in higher population densities and therefore have more contact with each other.

Healthy cats that test positive should not be allowed to roam outside, because they can infect other cats they come across. Additionally, other cats that live in the same household should be vaccinated, and it is recommended that the FeLV positive cat not have contact with the other cats because no vaccine is 100% effective, and the FeLV positive cat may spread other diseases that their weakened immune system allows them to contract.

Signs and symptoms

Not all cats that contract the feline leukemia virus actually develop "leukemia" (cancer of the white blood cells - specifically in the bone marrow), but leukemia is one of the cluster of diseases and symptoms that can develop when a cat is infected with this virus. Common symptoms of FeLV include poor appetite, poor coat and skin condition, fever, lethargy, weight loss, and anemia. Anemia is present in about 25% of all FeLV-related illness. Fifty percent of infected cats that become ill develop immunosuppression, or immune system failure, to some degree.

Various organ diseases can develop, such as liver or intestinal disease. Many cats may experience prolonged healing times after surgery or wounds. Additional signs may include uneven pupil size (anisocoria), swollen lymph nodes (lymphadenopathy), various secondary infections (bladder, skin, respiratory tract - bacterial or viral origins), gingivitis, stomatitis, diarrhea, jaundice, and a type of cancer called lymphosarcoma. Cats that are infected with FeLV are 50 times more likely to develop lymphosarcoma than non-infected cats. Lymphosarcoma (sometimes called lymphoma) is a cancer that consists primarily of a type of white blood cell called "lymphocytes". It can affect almost any organ in the body, but in young cats, it often appears in the chest cavity. In older cats, lymphoma of the intestinal tract is more common. Often in intestinal cases, there is no obvious mass or lump to find, but instead a generalized thickening of the intestine.

• Pale gums (or pale nail beds)
• Jaundice - yellow color to eyes, mouth, ears or other exposed skin and/or extremely dark, staining urine

  

  Jaundice coloring the roof of a cat's mouth

• Enlarged lymph nodes
• Chronic infections of respiratory tract, bladder or skin
• Gingivitis
• Weight loss/poor appetite
• Fever
• Poor coat
• Progressive weakness/sleepiness/lethargy/reclusiveness/hiding
• Diarrhea
• Vomiting
• Dehydration
• Breathing difficulty
• Reproductive problems - sterility, miscarriage

There are four possible paths the disease can take in an infected cat. These potential outcomes depend on the amount of virus the cat is exposed to, the strain of the virus (there are 4 unique strains of FeLV), the cat's initial health level (the effectiveness of his immune system and the presence of any other health complications), and age.

If a cat tests positive for FeLV, one of the following will occur:

1) IMMUNITY - Some cats are able to fight off the virus and become immune. Some of these cats completely eliminate all traces of the virus from their bodies. The cat may have some mild signs of illness - fever, poor appetite, large lymph nodes, lethargy for up to 10 days and then recover. This outcome occurs about 40% of the time, and is more likely in adults than in kittens.

2) PROGRESSIVE INFECTION - The second possibility is that the cat may become infected and develop a compromised immune system. The cat may have a short illness and recover enough to be outwardly healthy for 2-3 years in 50% of the cases. This outcome occurs about 30% of the time, and is more common in kittens than in adult cats. The bone marrow, which produces new blood cells, becomes infected and each new blood cell that is produced will contain more virus, creating a "persistent viremia" or constant presence of virus in the bloodstream. The infection is progressive, gradually taking hold in various body organs. Lymphoma is the final stage of the disease.

3) LATENT INFECTION -  Up to 30% of the time, cats will develop a latent infection. These cats may test positive at first, and then test negative 3-4 weeks later. The virus does not kill the cat's cells once it is inserted into the cellular DNA, but sits in wait. Eventually the body may reject these abnormal cells or the cat will become sick. While healthy, these cats will continue test negative on FeLV tests, but if vaccinated, will not be protected. Pregnant cats with latent infection may test FeLV negative but produce FeLV positive kittens. A latent infection may eventually result in immunity or may develop into active infection at some point in the future. Many cats with lymphoma (a common end-stage of FeLV) do still carry some FeLV DNA and are suspected by some to be latently infected with Feline Leukemia Virus.

4)  CARRIER STATE - Others may experience a "regressive infection" and continue to harbor the virus or particles of virus in a few cells, but the virus cannot replicate. In these cats, there is rarely any sign of illness, and they do not shed the virus. This outcome is extremely rare, occuring only about 1-2% of the time. These cats will test positive on ELISA test but negative on IFA tests. They may remain asymptomatic for a long time, even years. These cats are considered "carriers", and while they may never get sick, they can infect other cats. Eventually, these cats may develop active infection or develop latent infection.