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.

Without a host to harbor it, the FeLV virus is weak. Household cleaners kill the virus almost immediately. Environments that are not precisely the right temperature and humidity will kill the virus, too. Even in an ideal environment - one that is cool and damp - FeLV rarely lasts more than three days. Because the virus is so weak, the chances of owners transferring the disease from cat to cat on their hands of clothing is small. Although owners should take care to wash their hands after petting unfamiliar cats, they need not panic if an oversight occurs.

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.

One reason scientists have not yet found a cure for feline leukemia is because the virus acts differently from most other viruses. When a normal virus attacks a cell, it enters the cell's nucleus, the "command center," and takes control. Effectively, the virus hijacks the cell, forcing it to obey the virus's commands. Once it has assumed control, the virus begins to multiply, commanding the captured cell to assist in producing thousands of duplicate copies of the virus. When the new viruses are complete, they burst out of the cell, destroying it.

Feline leukemia hijacks cells the same way, but unlike normal viruses, FeLV may command its victim to do more than one task. The captured cell will become either a virus producer or a cancer producer. When an FeLV-captured cell becomes a virus producer, it is not destroyed when it completes a set of new viruses. Instead, the cell lives on to produce more batches of viruses. Because the producers are not destroyed, FeLV progresses much faster than many other diseases.

When captured cells become cancer-producers, a transformation uncommonly caused by most viruses, the cells mutate and cluster together, causing tumors. Scientists still do not know why FeLV creates cancerous cells.

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.

 
In some carriers, the virus may retreat temporarily, hiding in the cat's bone marrow. In this stage, the virus is dormant; the cat is not contagious and does not have to constantly fight the virus. These "latent" carrier cats live normal lives until the virus re-emerges. Stress often causes the virus to come out of the bone marrow and renew its attack. If a cat doesn't eat a proper diet, if it becomes ill, or if something makes it constantly nervous (like the presence of a rival cat), the cat is under stress. Because the immune system does not function as well when stressed, the virus may choose that time to pop out and start fighting again. In both carriers and latent carriers, stress may weaken the cats' systems enough to allow FeLV to completely affect the cats' bodies.
 
• 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.

Attempts to destroy the FeLV virus within cats have been unsuccessful. A drug called AZT can prevent the disease from taking hold, but it must be given between 14 and 21 days after the cat was exposed to the virus. Because it is difficult to pinpoint the exact time a cat was exposed, and because AZT often produces severe side effects, the drug is not very useful.

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. The chance of contraction increases with the number of cats.

 
• 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.

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. Because a veterinarian cannot identify a latent carrier until the virus re-emerges, the best way to handle a suspected latent carrier is to:

• Vaccinate the cat, in case it is not a latent carrier.

 
• Keep the cat as healthy and stress-free as possible.

 
• Keep the cat away from other cats as much as possible.

 
• Watch for symptoms, such as sluggishness, poor appetite or recurring colds or infections.

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.