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Feline Leukemia Virus (FeLV): signs, tests, and treatment

Writer: Joel
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Updated on: April 20, 2021
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FeLV (Feline Leukemia Virus) is a retrovirus that usually carries a grave prognosis for cats. It is one of the most common causes of death for the species, with about 85% of all infected felines dying within three years of diagnosis.

While a small number of infected cats may survive the illness, they must often live with it for the rest of their lives.

Feline Leukemia Virus (FeLV): signs, tests, and treatment 2

Clinical signs of Feline Leukemia virus

FeLV has several adverse effects on a cat's body. It is a leading cause of cancer and may often result in various blood disorders.

It also commonly leads to a state of immune deficiency in which the cat loses its ability to fight off infections. As a result, common viruses, bacteria, fungi, and protozoa that usually do not infect cats can cause significant illness. These secondary infections are the primary cause of the many diseases attributed to FeLV.

During the early stages of infection, cats may not exhibit any signs of disease. However, over time, a cat's (a few weeks, months, or even years)  health will steadily decline, or he/she will start to experience repeat cycles of illness. Cats with FeLV may exhibit the following symptoms.

  • Progressive weight loss
  • Loss of appetite
  • Enlarged lymph nodes
  • Poor coat condition
  • persistent fever
  • chronic diarrhea
  • pale coloration of the gums and other mucous membranes
  • abortion of kittens and other reproductive complications
  • behavior changes, seizures, and other neurological conditions

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How FeLV spreads

FeLV spreads through mutual grooming, bite wounds, and blood transfusions. It may also spread when infected mothers nurse their kittens. Adult cats with healthy immune systems have a greater chance of surviving the infection than kittens (who have underdeveloped immune systems) or immune-compromised adult cats.

Since it is so infectious, it is not unusual for cats found positive to be euthanized by shelters or rejected by transfer programs due to poor prognosis and significant risk of transmission.

FeLV tests

FeLV testing can be challenging due to the various stages and infection forms this disease can exhibit.

Most studies site a regressive and progressive disease form. The progressive form, also described as the persistent form, is when the virus cells are proactively replicated by the body and released. Animals with this variation of the disease will likely develop leukemia virus symptoms and die from associated complications.

The recessive/latent infection is when the immune system actively fights off the infection and halts the disease's progression. These animals are less likely to die but will still spread the disease. It is commonly believed that the two forms of the disease may be fluid. This complicates the diagnosis and management process.

ELISA (enzyme-linked immunosorbent assay) tests are a great way to detect FELV since they are not affected by animal vaccination status. However, testing is complicated by the possibility that the animals could fight the infection or develop a latent infection. This means that test would work only at a specific point in time.

A negative test does not satisfyingly rule out the possibility of infection, depending on where the cat may have been exposed. Similarly, a positive test does not mean the cat will be persistently infected and die from the disease. Antibodies may still circulate in the cat's blood long after the cat has fought the infection.

For these reasons, it's advised to carry out an immunofluorescent assay (IFA) laboratory test to confirm the findings of an ELISA test. Several studies have shown that about 33% of all positive in-house ELISA tests provided false results. IFA tests are a reliable indicator of viral replication or actual infection. ELISA tests, on the other hand, can only show exposure and disease development potential.

Feline leukemia virus treatment

Unfortunately, 85% of all cats with a persistent FELV infection die with three years of diagnosis. However, frequent trips to the vet, along with effective management, can keep these cats feeling well for a considerable period and help protect them from secondary infections.  If your cat has been diagnosed with FELV, you should take it to the vet twice a year for laboratory testing, physical examination, and parasite control. This will help your vet identify problems quickly and prevent complications. All FeLV-infected cats should be neutered and kept indoors.

Is there a vaccine for FELV?

There are two types of vaccines available: an adjuvanted killed virus and a nonadjuvanted recombinant infectious vaccine. However, none of these offer complete protection.

Note: FeLV vaccines are generally avoided because of their potential to cause cancerous tumors (sarcomas).

Sarcomas are aggressive, cancerous growths that require invasive procedures to remove. However, the tumors usually can’t be removed and prove fatal. For this reason, your veterinarian may advise you not to give your cat the vaccine.

The vaccines are recommended for cats at high risk of infection, such as

  • Those with outdoors access
  • feral cats
  • cats that aren’t infected but live with an infected cat
  • cats in multi-cat households
  • cats whose status is not known (living in animal shelters or simillar facilities)

How do I protect my cat from FELV

The only guaranteed way to protect your cat is to limit their exposure to FeLV-positive cats. Keeping your cat indoors, for example, will prevent contact with potential carriers. If you wish to take your cat outside, provide adequate supervision or place him/her in a secure closed environment to avoid fighting and wandering.

All cats should be tested for FELV before you bring them into your home, and healthy cats should be separated from those who've been infected, regardless of whether they show symptoms. They also shouldn’t share litter boxes and food and water bowls.

Unfortunately, most owners will not realize their cats have the condition until after they have lived with other cats. In such situations, all cats in the household should be tested for FELV.


While a FELV diagnosis can be disheartening, it is essential to remember that cats with this condition can live happy, relatively normal lives for significant periods. If your cat has been diagnosed with the illness, careful monitoring of appetite, weight, activity level, and behavior will help you give your cat appropriate care and manage the disease.

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