Feline Leukemia Virus (FeLV): Symptoms, Prognosis and Treatment

Medically reviewed by: Kinga Wojtczak, Doctor of Veterinary Medicine, specialist in canine and feline diseases

Feline leukemia (FeLV) is an infectious and still incurable disease that is widespread among cats. Did you know that, statistically, 25% of cats with clinical signs of feline leukemia (FeLV) will die within one year, and the remaining 75% will fight on for another 2–3 years? What is this insidious, contagious disease? What are the symptoms of feline leukemia, and how can you protect your cat from infection? You will find the answers to these questions in my article. The year is 1964. William Jarrett, a Scottish veterinary pathologist, and his team make a breakthrough discovery. The researchers observe tiny virus particles budding from the membrane surface of neoplastic lymphoblasts isolated from a cat with lymphoma. Experimental injections of the virus cause a malignant neoplastic process to develop in completely healthy cats! It turns out that lymphoma in domestic cats, a disease diagnosed for a long time, is caused by an infectious agent: the feline leukemia virus. This revelation revolutionized scientists' existing approach to epidemiology and set off rapid development of the field in both veterinary and human medicine.

What is feline leukemia (FeLV)?

The feline leukemia virus (FeLV), the retrovirus responsible for this disease, belongs to the oncornaviruses (that is, viruses capable of causing tumors). As the infection progresses, immune system function becomes impaired, and anemia and/or lymphoma develop (you will learn more about lymphoma from the article "Lymphoma in Cats: Symptoms and Prognosis"). FeLV occurs worldwide, and in some countries it is still considered the cause of most fatal diseases in domestic cats. Its prevalence in Europe is low (<1%), but in some areas it may exceed 20%. Today, thanks to widely available diagnostic tests, infection control and routine vaccination, the scale of this dangerous problem has decreased considerably.

A bit of history: where did FeLV come from?

The approximate "birth" of the virus is dated to 10 million years ago (the late Pleistocene). This infectious particle is believed to have evolved from a virus found in an ancestor of the rat, and cats were most probably infected by the alimentary route (through food) or through bite wounds inflicted by rats. Because of the dry desert climate of North Africa, the spread of the virus within the cat population was strongly limited.

How can my cat catch feline leukemia?

Today the situation is somewhat different. Feline leukemia virus infections are reported worldwide, and the disease spreads through the cat population as a result of close contact between infected and susceptible cats. The virus is transmitted through saliva and also through blood. Social behaviors typical of cats, such as mutual grooming, sharing food and water bowls, and fighting, greatly promote the spread of the disease. Iatrogenic transmission of the virus is also possible, e.g., through contaminated needles or during blood transfusions. Fleas may also be potential vectors of the virus, but it is not known whether this route of spread matters in vivo. The virus may be present in all body fluids, secretions and excretions, but it is not transmitted in urine and feces. Because of the way the virus spreads, the disease is more common in outdoor cats, especially intact (unneutered) toms (roaming, fighting). Cats of any age can get sick, but kittens are the most susceptible to infection.

How feline leukemia develops

Factors that favor the development of feline leukemia

Whether or not the clinical form of leukemia develops after contact with the virus depends on several factors:

1. Above all, immune status: cats with weakened immunity, frail cats and cats treated with glucocorticoids are more prone to developing all kinds of infections, including leukemia.
2. The cat's age: young kittens up to 16 weeks of age are the most susceptible. Immunity increases with age.
3. The virulence of the virus.
4. The infectious dose.
5. The intensity of the infection.

After infection, which most often occurs through the mouth or nose, the virus seeks to increase its "population" as quickly as possible. This is called viral replication, and it takes place in the lymphoid tissue of the mouth and throat (oropharynx). The way the virus enters the body, multiplies and spreads is a highly calculated "war strategy," and each stage of the expansion shows a remarkable ability to evade the body's defenses. It is astonishing how this clever retrovirus uses the components and mechanisms of individual tissues and organs for its own purposes.

What does a cat's body do to protect itself from the virus?

The following scenarios are possible:

Fighting off the virus

In cats with a properly functioning immune system, viral replication is inhibited by cellular mechanisms. Such cats will fight off the virus within a few months, and the immunity gained this way will protect them against another infection for as long as several years. So this is a win: the cat stays healthy.

Spread of the virus through the blood

If for some reason the immune response is too weak to fight off the virus, the virus spreads through the body. The retrovirus brazenly uses blood cells (lymphocytes and monocytes) to spread and travels inside them to various tissues (e.g., the thymus, spleen, lymph nodes and salivary glands). This phase, in which the virus spreads through the body and is present in the blood, is called viremia. In most cats it lasts about 3–6 weeks (at most 16 weeks). What happens to our pet during this time? At first, there may be:

• noticeable weakness,
• fever,
• enlarged lymph nodes.

In addition, the unsuspecting cat sheds the virus into the environment and is itself a source of infection. Unfortunately, that is not the end of the pathogen's rampage through the body. The virus greedily seeks to take over the bone marrow, a kind of "command center." From there it can send infected young blood cells to the front unhindered. But the clever body is not about to give up. A kind of "arms race" begins, during which our cat's immune defenses launch a host of mechanisms aimed at eliminating the enemy. And indeed, most animals clear the viremia shortly after the virus reaches the bone marrow. Then, too, the battle is won! The body has not only cleared the virus from the blood but eliminated it completely. As a reward, very strong immunity against infection develops, which places such cats in the VIP group = a very low risk of disease. And once again the cat ends up healthy.

Bone marrow infection

However, it may also happen that after about 3 weeks of "siege," i.e., viremia, the virus decides to take the fortress at all costs. Its plan is a true feat of strategy: on the offensive, it invades the bone marrow and enters the precursors of the individual blood cells. On the defensive, it integrates its genetic material into the genome of the host cells. The cat's body has 2 options: either destroy its own cells together with their contents, which given the scale of the expansion may mean shooting itself in the foot (there would be huge losses), or leave the cells alone, which is a win-win for the virus: now it can roam freely through the body and shamelessly produce new generations of the pathogen. And so the virus massively infects newly produced granulocytes and platelets. A large-scale, intense viremia begins, accompanied by involvement of the lymphoid tissue (hence another name for the disease, lymphatic leukemia) and the salivary glands. The malicious pathogens ruthlessly plan the future of their "offspring": after all, they must somehow infect new hosts, and saliva is perfect for that. 1 ml of a cat's saliva contains more than a million virus particles! In some lucky cats, even now, after the bone marrow has been invaded, the viremia may be overcome. It has to happen quickly, because the longer the viremia lasts, the less likely this becomes. But it does not come without losses… The body of a cat whose bone marrow is already infected cannot eliminate the virus, because the virus insidiously integrates its genetic material into the bone marrow stem cells. At this point what is called latent infection begins. In such cats no active virus particles are produced; the pathogen is in a sense "dormant," waiting for favorable conditions (such as weakened immunity, stress or pregnancy) under which it will be reactivated and attack again. Clinically, such a cat is healthy: it has no symptoms, and the results of diagnostic tests detecting FeLV are negative. However, it is a kind of "time bomb" that may go off at any moment. It does not always explode, though. Occasionally, though extremely rarely, the disease is overcome for some reason and the virus is eliminated completely. Sometimes the virus also stays dormant for the cat's entire life, leukemia never develops, and other cats are not infected. Such cats are referred to as healthy carriers.

Reactivation of dormant FeLV

What happens if the virus is reactivated after all? Then the aggressive, enraged pathogen reappears in the blood (and this time it stays there for good): this is called persistent viremia. Such animals become a veritable machine for multiplying and spreading the pathogen. As if that were not enough, a whole range of clinical signs appears, and such cats are officially considered to have leukemia. Clinical signs may not appear until several years later, but from the time of diagnosis cats usually survive up to 3 years.

Symptoms of feline leukemia

What does the aftermath of the battle look like in our purring friend? Unfortunately, it is scorched earth, with the disease taking many forms and causing a wide variety of symptoms. This is due to the nature of the virus (oncogenicity), the way it acts (its effect on the immune system) and where it lives (the bone marrow and its cells). So we may be dealing with different clinical signs and accompanying diseases.

Risk of tumor development

The feline leukemia virus has an oncogenic effect. FeLV infection is most often accompanied by the development of lymphomas (you will find more about lymphoma in the article "Lymphoma in Cats: Symptoms and Prognosis"). But that is not the whole repertoire of our infamous protagonist. It contributes to the development of leukemia and other tumors of the hematopoietic system. Sick cats also develop osteochondromas, olfactory neuroblastomas or keratinizing skin tumors. Tumors of the hematopoietic system (myeloproliferative diseases)

• Leukemias. The clinical signs result from a deficiency of normal blood cells. That is why acute myeloid leukemias are most often accompanied by anemia, sepsis, an enlarged liver with jaundice, and an enlarged spleen. The diagnosis should be based on a complete blood count and a bone marrow examination. Unfortunately, the prognosis for a cat with myeloid leukemia is poor.
• Myelofibrosis: constant stimulation of the bone marrow by the virus leads to fibrosis of the marrow and increased production of neoplastic cells. For the diagnosis, examination of material obtained by core biopsy is of the greatest value.
• Fibrosarcomas. They are characterized by rapid growth. Disseminated nodules appear in the skin and subcutaneous tissue and metastasize to the lungs and other organs. But beware! These tumors should not be confused with feline post-vaccination fibrosarcomas (injection-site sarcomas; FeLV has no influence on their development).

Other tumors

• Fibrochondromas: cartilaginous outgrowths on the surface of bones. Usually benign, but because of their location (they sometimes develop near the spine) they can cause severe clinical signs
• Spontaneous olfactory neuroblastomas: malignant, aggressive tumors of the nasal and pharyngeal epithelium with a tendency to metastasize
• Cutaneous horn: a benign proliferation of keratinocytes. The role of the virus in its development is still unclear
• Iris melanoma: the role of FeLV in its pathogenesis is still controversial

Non-neoplastic hematologic disorders

Because of the virus's suppressive effect on the bone marrow, hematologic disorders are common. Anemia and the signs that come with it:

• drowsiness,
• loss of appetite,
• listlessness,
• pale mucous membranes,
• jaundice,
• dehydration,
• an enlarged spleen.

The disease is often complicated by a secondary infection with Mycoplasma haemofelis, which further worsens the signs of anemia. Platelet abnormalities, which may contribute to excessive bleeding in cats. Neutropenia that responds to treatment with glucocorticoids. It usually occurs in cycles and means that there are too few neutrophils (neutrophilic granulocytes) in the blood. Because the "first line of defense" is weakened, cats with neutropenia show:

• recurrent fever,
• chronic bacterial infections,
• chronic gingivitis with hyperemia (redness) and a purulent (pus-filled) exudate.

Panleukopenia-like syndrome. It resembles feline panleukopenia, but in cats with FeLV it is accompanied by anemia. The clinical signs are typical of enteritis.

Weakened immune system

The retrovirus that causes feline leukemia considerably weakens cats' immune systems. As in people infected with HIV, secondary bacterial, viral or fungal infections are very common. Such a cat becomes defenseless even against common pathogens that a healthy cat's body would easily cope with.

Skin diseases

These are often linked to immunosuppression. Traumatic wounds, an ordinary allergy or even external parasites strongly predispose the cat to secondary infections, abscesses, inflammation of the external ear canal (otitis externa) or miliary dermatitis.

Immune-mediated diseases

1. Immune-mediated hemolytic anemia.
2. Glomerulonephritis.
3. Uveitis with deposition of immune complexes in the iris and ciliary body.
4. Polyarthritis.

Other syndromes

1. FeLV-associated enteritis: the signs most often include bloody diarrhea, vomiting, oral ulcers, gingivitis, loss of appetite and weight loss. Sometimes nonspecific signs appear, such as rhinitis, dyspnea (difficulty breathing) and lethargy.
2. FeLV-associated feline acquired immunodeficiency syndrome: this may involve diarrhea that is difficult to treat (resulting from inflammation of the small intestine and colon), as well as nonspecific opportunistic infections.
3. Reproductive disorders: in pregnant queens, fetal resorption, abortion and embryonic death are possible. Late in pregnancy, abortions with expulsion of normally developed fetuses are fairly common. They are not uncommonly accompanied by inflammation of the uterus (metritis) caused by bacterial infections
4. Fading kitten syndrome. Some kittens are resistant to infection, but most develop viremia followed by early death, most often in the first 2 weeks of life. This is accompanied by loss of appetite, dehydration, hypothermia and thymic atrophy.
5. Neurological disorders: dilated and unequal pupils (anisocoria), blindness, Horner's syndrome, vocalization, hyperesthesia, paresis and urinary incontinence are just some of the signs that may appear as a result of the virus's action.
6. Liver diseases: jaundice, inflammation of the liver (hepatitis) or liver degeneration.

Diagnosing feline leukemia

The most effective way to fight leukemia is early screening. It not only aims to detect cats infected with the virus but also makes it possible to stop the virus from spreading through the cat population. The main diagnostic methods used in clinical practice are:

• direct immunofluorescence test (FA),
• ELISA test,
• immunochromatographic techniques (ICGA).

The main sample for testing is blood. Viral antigens can also be detected in tears and saliva, but there is a greater risk of false-negative results.

When is the best time to test a cat's blood for feline leukemia?

1. At least once in a lifetime in all cats, regardless of where the kitten comes from, what its history is and whether it has been vaccinated against leukemia. I know from experience that many cats are vaccinated against this disease but have never had a feline leukemia test. Unfortunately, this disease can occur regardless of previous tests and vaccinations.
2. If you notice any symptoms in your cat that suggest the disease, and also when abnormalities associated with FeLV infection have been found (hematologic disorders, tumors, immunosuppression, frequent and recurrent infections).
3. If you already have a cat at home and want to adopt another. Before bringing a new cat into your home (regardless of its age), you should always have it tested for FeLV first. Remember: leukemia is an infectious disease!
4. After contact with other cats, especially if they were not vaccinated: this test is best done about 28 days after possible exposure to the pathogen, or done twice.

My cat is FeLV positive. What next?

A positive feline leukemia test result is not necessarily a death sentence. Confirmatory tests detect only infection with the virus (that is, its presence in the blood), not the disease! If there are no clinical signs of leukemia, cats with FeLV can live for many years. Even if the infection is accompanied by some symptoms, they may be caused by other factors. You should also remember that no test is 100% reliable. That is why the results should be interpreted in light of the animal's current health and possible exposure to the pathogen. Sometimes it is also a good idea to repeat the test. To know how to act when FeLV is detected in a cat, let's consider a few hypothetical situations.

1. The test was done on one of several cats living in the home. We already know how the leukemia virus spreads in a cat population, and we also know that some cats may acquire immunity after a single contact with it. What can we do to protect the rest of our feline companions?

• First and foremost, test all the cats for FeLV. The best way to prevent the virus from spreading is complete isolation of infected cats, with no possibility of contact with healthy ones. And even the acquired immunity of the others does not last forever. The ability to neutralize the virus does not persist for life: over time, viremia may also develop in cats considered immune to infection. Remember that such a cat should not, under any circumstances, have contact with other cats, even those that are not from our own cattery.
• Healthy cats should be vaccinated against leukemia. But here, too, you should remember that in this situation vaccination does not guarantee full protection against infection.
• After several months or years, when the infected cats are gone from the cattery through natural attrition, the remaining cats are considered immune to infection.

1. Our only cat has tested positive for FeLV.

• The infected cat should be kept strictly indoors. Cats diagnosed with FeLV should not be allowed outdoors, and not only because they pose a threat to other cats. Remember that FeLV causes immunosuppression, which puts our cat at a very high risk of developing various infections and diseases.
• Feeding: it is safest to feed a specially balanced, highly nutritious diet. Here you can confidently rely on commercial pet food manufacturers. Raw meat, eggs and unpasteurized milk should be avoided (risk of transmitting bacteria or parasites with the food).
• Check-ups at the veterinary clinic: at least once every six months, sometimes more often.
• Prevention of internal and external parasites and heartworm, as well as preventive vaccinations against infectious diseases.
• Spaying or neutering to avoid the stress associated with heat.

Treating feline leukemia

Feline leukemia: antiviral treatment

Scientists have been working for years to develop an effective treatment for leukemia in cats. Unfortunately, there is still no therapy that would eliminate the virus from the body. Antiviral drugs have of course been introduced into treatment, but for various reasons they have not fulfilled their role. An effective antiviral drug is one that inhibits viral replication and stimulates the immune system to fight the pathogen. In addition, it should be as nontoxic as possible, relatively inexpensive and available in oral form (it has to be given for the rest of the patient's life). Unfortunately, no such drug has been developed. Some hopes were pinned on zidovudine (AZT), which inhibits viral replication fairly effectively, but because of its side effects (severe bone marrow suppression) its use is limited to cats with concurrent acute gingivitis or neurological disorders, and the patient's condition should then be closely monitored, including complete blood counts. Other drugs, such as didanosine, zalcitabine, ribavirin, foscarnet or suramin, either show relatively high toxicity or cause severe bone marrow suppression, which is why clinicians very rarely use them. Although these drugs are used in human medicine (e.g., to treat HIV infection in humans), their effectiveness against FeLV in naturally infected cats has not been proven.

Feline leukemia: immunomodulatory treatment

Immunomodulatory treatment of leukemia consists of giving interferon, which has immunomodulatory and antiviral effects. Several types of interferon are available, the most common being human interferon α and recombinant feline interferon ω (the product Virbagen). Cats treated with interferon have longer survival and less severe clinical signs.

Treating the diseases that accompany FeLV infection

Treatment of leukemia consists mainly of fighting the secondary infections that accompany it. Treatment in infected cats does not differ from therapy in uninfected cats, although in the former it usually takes longer. Although about 3 years usually pass from the diagnosis of FeLV to the cat's death, in reality, if the appropriate recommendations are followed, infected cats can live longer and die in old age of causes completely unrelated to FeLV.

Preventing feline leukemia infection

The first and most basic step in preventing the spread of the feline leukemia virus is monitoring, especially in catteries and large groups of cats. Regular testing to identify infected cats and separate them is a more effective way of controlling the disease than vaccination alone.

Feline leukemia vaccination

Kittens should be vaccinated against leukemia twice, 3 weeks apart, starting at 8–9 weeks of age. Subsequent booster doses are given one year after the second vaccination and then every year. This approach is recommended for outdoor cats that may encounter the virus in their environment. Vaccination against leukemia is inextricably linked with the risk of dangerous, potentially fatal post-vaccination sarcomas. These dangerous tumors occur so often that a special injection protocol has been developed. It is recommended to give the injection in the left hind limb. Why? If a tumor develops, the leg can be amputated as a last resort, saving the animal's life. The decision to vaccinate should be made carefully, taking into account the cat's actual risk of exposure to infection.

Is there cause for concern?

Feline leukemia is a highly infectious disease that takes its toll on our purring companions. The fight against the virus is often doomed from the start, and the course of the disease can be exhausting not only for the patient but also for the owners. That is why we should not forget about it during our cat's routine "check-ups" at the clinic. Let's ask the veterinarian about a test and talk about the risks and the options for preventing FeLV leukemia. Let's do everything we can so that our furry friend never becomes a battleground for the feline leukemia virus. Do you have questions about viral leukemia in cats? Or perhaps your cat is in a risk group and you want to learn more about prevention? Leave a comment below the article now, and I will reply as soon as I can.

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