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Feline Leukemia and FeLV
the Feline Leukemia Virus

What is the Feline Leukemia Virus?

Feline leukemia virus (FeLV), a retrovirus, produces an enzyme that allows for a reversal of genetic transcription, from RNA to DNA rather than the usual DNA to RNA, the newly transcribed viral DNA being incorporated into the host cell's DNA strand for the production of new RNA retroviruses. Human HIV, Feline FIV, and FeLV are all retroviruses.

Although related, FeLV and FIV also differ in their shape: FeLV is circular while FIV is more elongated. The two viruses differ genetically, and their protein consituents are dissimilar. Although many of the diseases caused by FeLV and FIV are similar, there are also many differences. The disease caused by this virus is a kind of Leukemia that affects the lymphocytes, a type of blood cell. Other retroviruses are implicated in various cancers.

How common is the infection?

FeLV-is a world wide condition, rate of infection varies greatly depending on age, health, environment, and living conditions and lifestyle. In the United States, approximately 2 to 3% of all cats have a FeLV infection. Pet cats have a .5% level of infections. A much larger percentage (35% +) have antibodies which suggest that they were exposed and successfully fought off the infection and developed immunity rather than infection. Rates rise significantly to13% or more in cats that are sick, very young, or at high risk.

How is FeLV spread?

Cats infected with FeLV serve as sources of infection. Virus are released in very high numbers in saliva and nasal secretions, but also in feces, urine, and milk from infected mothers. Cat-to-cat transfer of virus may occur from a bite, from licking each other, and rarely by sharing litter boxes and feeding and water dishes. Infection can come from a mother cat to her kittens, either before birth or while nursing.

There is strong evidence that kittens under 4 months are susceptible to infection, by 8 months they are much more capable of fighting it off. FeLV has not been shown to survive long outside a cat's body, probably less than a few hours under normal household conditions.

What increases risk of infection for Cats?

Cats at greatest risk are those exposed to infected cats through prolonged close contact or through bite wounds. These cats include:

Kittens are much more likely to get infected than adult cats, and are at the greatest risk if exposed. As they grow older they become more resistant to FeLV infection. The level of infection of kittens is more than 3 times more likely than a healthy adult. Although they can fight the infection, healthy adult cats can become infected if sufficiently exposed.

What does FeLV do to a cat?

Feline leukemia virus damages a cat's body in many ways. It is the most common cause of cat cancers. it causes some blood disorders, and it seriously undermines the cat's immune reactions and ability to fight off infections.

The same pathogens that live in the cat's everyday environment, but don't affect a healthy cat, can cause severe illness in the animals with compromised immune systems. These are secondary infections and cause many of the the diseases associated with FeLV.

What are the signs of disease caused by FeLV?

During the early stages of infection, no symptoms may be present. It is possible that no symptom ever show if the cat fights off the FeL and develops immunity. It not, over time, sometimes weeks, months, or even years, the cat's health may deteriorate gradually or show periods of being well in between periods of being sick. Some symptoms are:

What happens when a cat is infected?

Primary viremia, is the first stage of virus infection. During this period some cats can overcome the infection, eliminate the virus from the bloodstream, and never progress to the second stage of infection.

Virus enters the cat, often through the pharynx and infects the epithelial cells and specific types of white blood cells. These go to the lymph nodes and start replicating.

The virus then enters the blood stream and is carried to the whole body.

The lympoid system, which is responsible for producing antibodies that attack infected and cancerous cells, become infected and the virus continues to be distributed in the body.

The hemolmphatic system and intestines become infected. At this point the virus has taken over the host's immune system and has caused Viremia

The immune system is now severely compromised and has trouble fighting the virus.

Secondary viremia, a later stage characterized by persistent infection of the bone marrow and other tissue.

If FeLV infection progresses to this stage it has passed a point of no return: the overwhelming majority of cats with secondary viremia will be infected for the remainder of their lives.

The bone marrow becomes infected the virus continues replicating and after a few days infected white blood cells are released.

The body is overwhelmed by the infection and various epithelial cells and mucus membranes become infected. Salivary glands, pharyx, bladder, stomach, esophagus, intestines, trachea, kidney, bladder, pancreas, lung tissue and sebaceous ducts from the muzzle become infected.

How is infection diagnosed?

Two types of FeLV blood tests are in common use. Both detect a protein of the virus in the bloodstream.

ELISA (Enzyme Linked Immunosorbent Assay) can be performed in a veterinarian's office. ELISA tests detect both primary and secondary stages of viremia.

IFA (Indirect Immunofluorescent Antibody Assay) tests are sent out to a diagnostic laboratory. IFA detect secondary viremia only.

The ELISA test is usually done first. Both test might be done more than once to detect the cat's infection stage and status.

Prevention of FeLV?

Vaccination against FeLV

Since vaccines are not 100% effective, a rate of 75%-85% effectiveness has been noted. This is a significant improvement over no vaccination. The only sure way to protect cats is to prevent their exposure to FeLV-infected cats.

Consider FeLV vaccination of uninfected cats. (FeLV vaccination of infected cats does no good.) FeLV vaccines are widely available, but not all vaccinated cats will be protected, preventing exposure is still important for vaccinated pets.

FeLV vaccines do not cause cats to false positive results on ELISA, IFA.

Serious side effects have also been reported as a result of FeLV vaccination; in particular, a small percentage of cats who received FeLV vaccines later developed vaccine-associated sarcomas, an aggressive tumour, at the injection site.

Keep Healthy Cat Away from Infected Cats

Litterbox Maintenance

Since the virus dies quickly, within 2 hours, in a dry environment, chances of transmission will drop considerably if the litter is kept dry. One way is to clean damp litter after each use!!!

Another way is to use a specialized litter box designed to drain off urine for diabetic testing of sugar. This type of litter box has been shown to reduce or even prevent FeLV infection caused by exposure to the virus in damp litter.

What if my cat is infected?

Many FeLV-infected cats are diagnosed with FeLV while living with other cats. In such cases, all other cats in the household should be tested for FeLV. Ideally, infected and non-infected cats should be kept apart to eliminate the chance for FeLV transmission.

How should FeLV-infected cats be nursed?

Keep FeLV-infected cats indoors to reduce exposure to other infectious agents. This also prevent the spread of infection to other cats in the area.

Neuter FeLV-infected cats.

Pay particular attention to the diet. It should be high quality and nutritionally complete. Uncooked diets and foods should be avoided. A FeLV cat is particularly susceptible to infections and cannot fight off bacteria, fungus and parasites present in raw food as well as a healthy cat.

Regular visits to the vet will help diagnose and treat opportunistic side infections.


No treatment has been shown completely effective, 2 alternatives are known at this point./p>

In the US, Lymphocyte T-Cell Immune Modulator was licensed in 2006 for FeLV and FIV in cats. It is a regulator of CD-4 Lumphocity production and function. It increases lymphocyte numbers and interleukin production in animals.

In Europe Interferon-omega is available in Europe under the name Virbagen Omega. It is used for non-terminal stages and has reduced mortality rate in non anemic cates by 20%.

What is the Life Expectancy of a FeLV-infected cat?

It is difficult to predict the life expectancy of a cat infected with FeLV. With care and under ideal conditions, cats can remain in apparent good health for months. Most die due to a FeLV-related disease within two or three years after infection. If the cat has experienced one or more severe illnesses related to an FeLV infection, or if persistent fever, weight loss, or cancer is present, a much shorter life span is probable.

My FeLV-infected cat died.
What can I do before bringing in a new cat?

Feline leukemia virus are fragile and will not survive outside a cat for long. However, FeLV-infected cats often have more persistant diseases that can survive longer. Clean and disinfect food and water dishes, bedding, litter boxes and toys. Bleach solutions (4 ounces of bleach in a gallon of water) work very well. Clean floors and carpets and any bedding that the cat has contacted. Any new cats should be vaccinated before entering the household.

Can humans become infected with FeLV?

No evidence that FeLV can be transmitted from infected cats to humans has ever been found. Regardless, FeLV-infected cats may carry other diseases. Pregnant women, people with suppressed immune systems including AIDs , very young and very old people should avoid contact with FeLV-infected cats.

emails: Christine

This article is provided for information only. It is not to be used instead of consulting a VET. If your kitty is sick get some help.