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.