Feline Leukemia Virus


Unlike humans,” leukemia” in cats is caused by a virus.  This virus, called the Feline Leukemia Virus (or FeLV) is part of a group of viruses called retroviruses, which trick the body into producing more of the virus every time it makes new cells.  The disease is not actually leukemia as we think of it in humans, but looks similar in the effects on the body. FeLV targets the cat’s immune system, suppressing it and taking away the cat’s ability to defend against other diseases.  This means that the cat is then open to infection from many different sources, and even bacteria and fungi that would cause no harm to an uninfected cat can cause serious illness. 

What are the symptoms?

Symptoms of FeLV may not show up right away, since the disease has to go to work on the immune system first.  As the disease progresses, the overall health of the cat will worsen, and they may have bouts of serious illness regularly.  Some symptoms include:

  • Loss of appetite
  • Continuing weight loss
  • Inflammation of the gums and mouth
  • Persistent fever
  • Upper respiratory infections
  • Diarrhea
  • Poor coat condition
  • Eye infections
  • Seizures, behavioral changes, and neurological disorders


It is important to understand that there is no treatment or cure for FeLV.  An infected cat may be provided some relief from the symptoms through veterinary care, but ultimately, the virus will be fatal.  The disease progresses quickly and it is unusual for a cat to survive more than a few years with the disease.


How common is it?

FeLV is present in cat populations across the globe, but in the US it is estimated that only 2-3% of all cats are infected with FeLV.


Which cats are at risk?

Any cat that is exposed to the virus may contract FeLV, but animals at higher risk are ones whose immune system is already compromised:  the very young, the very old, and the already sick. 


How is it spread?

FeLV is a contagious virus, spread from cat to cat by saliva and mucus from the nose.  It can also be spread by urine and feces, and nursing kittens can be exposed through infected milk.  It is most commonly spread by contact with another cat’s saliva (grooming each other, sometimes by fighting, or sharing food and water bowls) or is passed from a nursing mom to her kittens.  The virus can survive several hours outside the body. 

Some cats that come into contact with the virus can mount a successful immune response and get rid of the virus before it causes damage.  These cats are almost always adults, as their immune system is much more developed and more able to fight off infection.  However, if the virus infects the bone marrow and other tissues, it enters the secondary stage of infection.  At this point, the cat will not be able to get rid of the virus, and there is no cure.


How can I protect my cat?

The best protection for your cat is to limit its interaction with potentially infected animals.  This means either keeping your cat indoors with other cats that are FeLV negative, or supervising its outdoor time to ensure no contact with strange cats.  If possible, provide your cat with the FeLV vaccine, which your veterinarian can give after ensuring the cat does not have the virus.  This vaccine provides a measure of protection for outdoor cats.  Ideally, the cats your cat comes into contact with on a regular basis should all be tested for FeLV/FIV and vaccinated if possible.

For more information on FeLV and other retroviruses, please visit Cornell University’s website.

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