Feline leukemia (FeLV) is a complex disease that blocks cats' natural ability to fight off germs. It does not yet have a cure. Most cat owners are familiar with this deadly disease, but many are unaware of how serious it is.

Now cat owners have more options because researchers have discovered so much about FeLV. Your cat's life depends on your efforts to fight the disease. The more you know about FeLV, the better your strategy will be.

Feline leukemia most commonly is transmitted through saliva, but it also is transferred through urine and feces. An infected cat can contaminate a healthy one when the cats eat from the same bowl, groom each other, fight, or share the same litter box. Additionally, female cats can pass the virus to their unborn kittens.

Some cats are more likely to become actively infected than others. Because FeLV spreads through intimate contact, cats living in multicat households have the highest risk of contracting the disease. Those who spend part or all of their time outdoors also are at high risk, because the more time a cat is outside, the more chances it has of contacting an infected animal.

Age also affects cats' vulnerability. Kittens are less able to fight FeLV because their immune systems are not fully developed. Cats more than 10 year old also are more susceptible because their immune systems are weaker. Owners of high-risk cats should be careful to protect their pets. Cats not in the high-risk group are still susceptible to FeLV, however, and also need protection.

Feline leukemia goes through several stages after entering a cat's eye and nose membranes, says William D. Hardy Jr., V.M.D., of Cornell University. It first travels to lymph nodes in the throat, where it begins to reproduce, infecting the cat's white blood cells. At this stage, the cat could fight off the disease if its immune system is strong enough; about 40 percent of exposed cats are able to withstand the virus. Otherwise, the infected blood cells transport the virus to the spleen and intestinal tissue, as well as to other lymph nodes.

Eventually the virus enters the bone marrow and contaminates the cat's growing blood cells. Circulating blood then carries the virus to the salivary glands, respiratory cells, and urinary tract. About 56 days after invading its host, the virus becomes present in the cat's saliva and urine, and can be passed to other cats.

After it is exposed to the disease, a cat's system could react in different ways. Veterinarians have no reliable way of predicting reactions, so they usually can't help cats until it's too late. About 40 percent of exposed cats - usually those who are healthy and stress-free - manage to destroy the disease and become immune to it. The other 60 percent divide into carriers and symptomatic cats:

• In carriers, 30 percent of exposed cats, the immune system weakens the virus enough that it doesn't make the cats sick. The virus is present in a carrier cat's body, but the cat does not show FeLV symptoms. Because of the lack of symptoms, an owner of an FeLV carrier probably will not realize that the cat has the virus. Meanwhile, the carrier cat can infect other cats because the virus is contagious. Carrier cats also are more vulnerable to other germs because their immune systems are busy fighting FeLV.

 
• Symptomatic cats, the other 30 percent of exposed cats, receive the full force of the infection and die in three months to three years. These cats are victims of three different conditions:

 
1. First, FeLV causes various cancers by entering and changing body cells. Altered cells grow together to form tumors in different parts of the body. The tumors eventually will interrupt normal body function.

 
2. The second condition occurs when the disease attacks the growing blood cells in the cat's bone marrow. Because the young blood cells are rendered useless, the cat's total number of blood cells decreases, causing anemia and slower clotting. The cat gradually will become less active until it has too little energy to do anything.

 
3. The last and most devastating condition suffered by infected cats is the suppression of their immune systems. FeLV makes it difficult for cats to fight off infections that would normally be easily destroyed. Most infected cats die from secondary infections rather than tumors or anemia. The most common complications are pneumonia, stomach problems, chronic colds, and skin and mouth infections. FeLV also can cause pregnant cats to abort their litters.

Symptomatic cats may not show any physical signs right away and may inadvertently pass the disease to other cats. Until symptoms start to appear, clinical testing is the only way to detect feline leukemia.

Although we do not have ways to stop the disease, we can slow its progress by treating specific symptoms. Veterinarians can control secondary infections with antibiotics, anemia with blood transfusions, and cancers with chemotherapy. Unfortunately, repeated treatment is expensive and does not completely alleviate a cat's suffering. Instead of prolonging the inevitable, many owners of actively infected cats opt for euthanasia.

Carriers, cats that test positive but do not have any symptoms, can now live normal lives. Before scientists could vaccinate against FeLV, most veterinarians recommended euthanasia for FeLV-positive cats whether or not they had symptoms. This policy was meant to control the spread of the disease. Now that we have a vaccine, owners have the option to keep their infected cats if the cats are not suffering. If making such a decision, an owner must isolate the infected cat as much as possible because carriers can still pass the disease to unvaccinated cats.

Unfortunately, isolating a cat without taking any other preventive measures is unreliable because of the possibility of accidental contact between a healthy cat and a diseased one.

Fighting feline leukemia involves three steps:

• Testing
  Six years after FeLV was discovered, researchers developed a blood test to determine the presence of the virus. Veterinarians generally test cats for FeLV before vaccinating them. (Vaccinating an infected cat will not stop the infection and will give the owner a false sense of security, allowing other cats to be exposed.) Testing is especially important if a cat commonly goes outdoors, is frequently sick or lives with more than one cat.

 
Unfortunately, an FeLV test cannot identify the presence of the virus in latent carriers. If a cat in the high-risk category tests negative, it could be a latent carrier.
 
• Vaccinating
  The FeLV vaccine provides cats with 80 to 90 percent protection from contracting the disease. The vaccine must be re-applied yearly in order to stay effective.

 
• Isolating
  Isolating picks up where testing and vaccinating leave off. To cover the vaccine's protection gap, isolating the cat from unknown cats will reduce chances of exposure and help the vaccine work as thoroughly as it can.

By staying current with your cat's yearly vaccines and keeping it away from unfamiliar cats, you can give your feline friend the best chances possible for an FeLV-free life. The expense of yearly shots and the inconvenience of a housebound cat may add up, but when you compare both with the immeasurable cost of a lost companion, you may find they're worth it.

For more information about feline leukemia and cat care in general, visit Web Resources For Cat Lovers.