Ask a Veterinarian: Feline Leukemia Virus (FeLV) - The Trupanion Blog
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Ask a Veterinarian: Feline Leukemia Virus (FeLV)

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What is Feline Leukemia Virus?

Feline leukemia virus (normally abbreviated “FeLV”) is a contagious organism that is one of the most common infections of cats around the world. It is part of a family of viruses called “retroviruses” that are able to invade a cat’s body, becoming integrated into the cat’s own DNA. Because of this ability to invade, FeLV is termed an “oncogene.” Oncogenes are genes in the body that can undergo a transformation into cancer cells.

Although it is one of the most common infections in cats, it only affects approximately 2% of all cats in the U.S. and this percentage continues to drop because of the effectiveness of preventative vaccination protocols.

Unlike other viruses that can make us temporarily sick–think of the “flu,” FeLV can cause a number of different problems when our cat becomes infected.

How can my cat catch this virus?

There are two main ways a cat can catch FeLV. A pregnant infected mother cat can infect her kittens before they are born (transplacental) or after birth via nursing (virus is shed into the milk). The second way a cat catches FeLV is through very close contact with an infected cat. The virus is shed in blood (exchanged when cats fight), urine, feces, and nasal secretions. Young cats under 2 years old are far more susceptible to catching FeLV. And don’t think because your cat is an indoor cat that they are safe from the virus. Basically young, indoor/outdoor or outdoor cats that might run into a feral or FeLV carrying cat are most likely to catch the virus.

Mature cats and especially cats that have been given the preventative FeLV vaccine are often completely immune to the virus.

What kind of symptoms can FeLV cause?

When a young cat first catches FeLV they might become very sick with a fever, have a poor appetite, lethargy, weight loss, enlarged lymph nodes, and dehydration. In other situations, the initial illness after exposure to FeLV is not that dramatic. Instead what happens is the virus lays latent and later, differing illnesses develop. Common secondary infections because of a suppressed immune system include dental infections, tongue/mouth infections, eye troubles, upper respiratory tract infections, and abscesses. Symptoms associated with these infections include fever, drooling, loss of appetite, weight loss, unkempt appearance, ocular discharge, nasal discharge, sneezing, and general malaise.

Secondary cancers that develop because of latent FeLV include leukemias (white blood cell cancer), lymphomas and fibrosarcomas. All of these cancers are treatable but unfortunately prognosis is most often worse when you have FeLV + cancer. Symptoms vary widely but in general include lethargy and often loss of appetite. Lymphoma of the intestinal system is common and symptoms most often include vomiting, diarrhea, loss of appetite and weight loss.

Prevention of FeLV – Vaccination

After discussing the specific pros and cons of vaccination to prevent FeLV in your cat, your veterinarian will be able to give you great specific advice. In almost all cases and especially in young cats, FeLV vaccination is recommended to provide protection. Indoor-only cats are much less likely to run into FeLV but they can still slip outdoors so often vaccination does make the most sense.

Treatment and Prognosis of FeLV

Although many newer treatments exist for our FeLV + cats, nearly 80% of infected cats will die within three years of being diagnosed. This statistic is startling and sad, but with aggressive treatments and advanced care from veterinary internal medicine and oncology specialists when your FeLV + cat becomes ill, there is definitely more hope than there once was. Newer chemotherapy treatments and treatments with drugs like azidothymidine (AZT), feline interferon, staphylococcal protein A (SPA) are used to combat the cancers and infections FeLV + cats are more susceptible to.

One of the main keys to extending the lifespan of your FeLV cat is to get veterinary care at the first sign of any illness. This can be tough with cats because they love to hide their troubles. Watch for subtle signs like sleeping in an odd location, not grooming as much as normal—the cat will suddenly have a “rougher” hair coat or more dander than normal, change in behavior patterns, and decreased interest in food or treats.


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