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Feline Leukemie virus (FeLV)

Feline Leukemie virus (FeLV)

Feline leukemia virus (FeLV)
is a retrovirus that infects cats. As a retrovirus, the genetic information of FeLV is carried by RNA instead of DNA. FeLV is usually transmitted between infected cats when the transfer of saliva or nasal secretions is involved, for example when sharing a feeding dish. If not defeated by the animal’s immune system, the virus can be lethal.

The disease is a virus, not a cancer. The name stems from the fact that the first disease associated with the virus was a form of leukemia. By the time it was discovered that the virus was not the same as leukemia, the misnomer had already found its way into the vocabulary of pet owners.


  1. Transmission
  2. Progression
  3. Prevention
  4. Viral structure
  5. Suggested Therapies and Treatments
  6. Approved Treatment
  7. Comparison with FIV
  8. Comparison with HIV
  9. External links
  10. References

Cats infected with FeLV can serve as sources of infection. Cats pass the virus between themselves through saliva and close contact, by biting another cat, through a litter box or food dish used by an infected cat, and from milk during nursing. Transmission can also take place from an infected mother cat to her kittens, either before they are born or while they are nursing.

The virus is very weak, and can survive only about 2 hours in a dry environment, and about 48 hours in a damp environment (such as a litter box).

FeLV causes immunosuppression in pet cats, and there is also evidence for existence of the virus in larger wild cat populations also (e.g. Lynx, Cheetah, and Lion). Overwhelming epidemiologic evidence suggests FeLV is not transmissible to either humans or dogs. This statement is based on the fact that approximately one pet dog in five lives with a cat, and all pet cats live with humans (some 60 million pet cats in the USA). It is species-specific, and does not infect other animals, such as dogs (in fact, there is apparently no canine version of this disease at all).

Approximately 0.5% of pet cats are persistently infected with FeLV, but many more pet cats (>35%) have specific IgG antibodies which indicate prior exposure and subsequent development of immunity instead of infection. Transmission of FeLV is mainly via saliva and friendly behaviours, such as sharing feeding bowls and mutual grooming (as distinct from fighting and biting).

There is strong evidence kittens under 4 months of age are susceptible to infection, but by eight months are resistant – hence it is a good idea to keep young pet kittens indoors where virus exposure is minimal or non-existent until about 8 months of age.

Kittens can be born with it, having contracted it from their mother while “in utero”. Infection is far higher in city cats, stray or owned, than in rural cats: this is entirely due to the amount of contact the cats have with each other.

The disease’s effect has a wide range: the cat can fight off the infection and becoming totally immune; can become a carrier (like a Typhoid Mary) that never gets sick itself, but infects other cats; to a mid-level case in which the cat has a compromised immune system.

Four subgroups of FeLV exist: A; B; C, and T, but only subgroup A is transmissible between cats. The other subgroups arise de novo and as results of recombination with an endogenous DNA feline sequence. Hence, there is very good evidence this virus is quite ancient, and may well have evolved more than one time over the last 10,000,000 years.

There are many possible outcomes as to how successfully the cat’s immune system will react to the virus. About forty percent of cats extinguish the virus. Sixteen percent fight it off due to minimal exposure to it. The other twenty-four percent resist the virus at phase four, which will be described later. All of this usually occurs between sixteen to eighteen weeks after the FeLV infection begins. About twenty percent are able to put the virus into a latent stage, in which the virus will remain until the cat becomes stressed causing the FeLV to re-emerge. About five to ten percent of cats go through a sequestered stage in which viremia is limited, intermittent, or absent altogether. Approximately thirty percent of cats go through the disease from start to finish, normally resulting in death.

Once the virus has entered the cat, there are six phases to a FeLV infection:

  • Phase one is when the virus enters the cat, usually through the pharynx where it infects the epithelial cells and infects the tonsillar B-lymphocytes and macrophages. These white blood cells then filter down to the lymph nodes and begin to replicate.
  • In phase two, the virus enters the blood stream and begins to distribute throughout the body.
  • Phase three starts when the lymphoid system (produces antibodies to attack infected and cancerous cells) becomes infected with further distribution throughout the body.
  • Phase four is the main point in the infection, where the virus can take over the body’s immune system cause viremia. During this phase the hemolymphatic system and intestines become infected.
  • If the cat’s immune system does not fight off the virus, then it goes onto phase five where the bone marrow becomes infected. At this point, the virus will stay with the cat for the rest of its life. In this phase, the virus replicates and is released four to seven days later in infected neutrophils (white blood cells), and sometimes lymphocytes, monocytes (white blood cell formed in the bone marrow), and eosinophils (another white blood cell).
  • At phase six the cat’s body is overwhelmed by infection and mucosal and glandular epithelial cells (tissue that forms a thin protective layer on exposed bodily surfaces and forms the lining of internal cavities, ducts, and organs) become infected. The virus replicates in epithelial tissues including salivary glands, oropharynx, stomach, esophagus, intestines, trachea, nasopharynx, renal tubules, bladder, pancreas, alveolar ducts, and sebaceous ducts from the muzzle.

Cats diagnosed as persistently infected by ELISA testing may die within a few months or may remain asymptomatic for up to 4 years. The fatal diseases are leukemias, lymphomas, and non-regenerative Anemias. Although there is no known cure for the virus infection, in 2006 the United States Department of Agriculture approved Lymphocyte T-Cell Immunomodulator as a treatment aid for FeLV and/or FIV infections.

Vaccines for FeLV are available, though no currently available vaccine offers 100% protection from the virus. [1] Serious side effects have also been reported as a result of FeLV vaccination; in particular, a small percentage of cats who received FeLV vaccines subsequently developed vaccine-associated sarcomas, an aggressive tumour, at the injection site. [2] The development of sarcomas with the use of the old FeLV and other vaccines may be due to the inflammation caused by aluminium adjuvants in the vaccines.[3]

Merial produces a recombinant vaccine consisting of canarypox virus carrying FeLV gag and env genes (sold as PUREVAX FeLV in the USA and Eurifel FeLV in Europe). This is thought to be safer than the old vaccine as it does not require an adjuvant to be effective. Although this is a live virus, it originates from a bird host and so does not replicate in mammals.[4]

Viral structure
Feline Leukemia Virus (FeLV) is an RNA retrovirus or oncornavirus first described by W. Jarrett (et al, Nature 202:566) at University of Glasgow, School Veterinary Medicine, in 1964. The virus comprises 5′ and 3′ LTR’s and three genes: Gag (structural), Pol (enzymes) and Env (envelope and transmembrane); the total genome is about 9,600 base-pairs.

See the entry on retroviruses for more details on the life cycle of FeLV.

Suggested Therapies and Treatments
A FeLV infected cat may not actually display any sign of sickness. The virus does not actually kill the cat – it compromises the immune system. This makes it very easy for the cat to contract other infections that run out of control: a simple nail injury can become a full-blown, out-of-control infection. It is these infections and the likelihood of cancer that are the main treatment problem. A FeLV-positive cat can die at six months, two years or 10 years after the initial infection with the virus.

Because cats normally may not appear to be sick until they are in an advanced state of illness, it is vital that cat owners pay close attention to their pets’ day-to-day condition. Cats should be examined regularly for painful or swollen areas. Any cat showing these symptoms should be examined by a veterinarian. Treatment may consist of a course of antibiotics for bacterial infections, sometimes for a few weeks due to the difficulty of eliminating the infection.

It may be advisable to take away any scratching pads and posts: when cats sharpen their claws, they strain the base of their claws, causing the skin to open up a bit. This is enough to let in the bacteria that will infect the cat.

Approved Treatment
In 2006, the United States Department of Agriculture issued a conditional license for a new treatment aid termed Lymphocyte T-Cell Immune Modulator[5]. Lymphocyte T-Cell Immune Modulator is manufactured by T-Cyte Therapeutics, Inc. and exclusively sold by IMULAN BioTherapeutics, LLC.

Lymphocyte T-Cell Immune Modulator is intended as an aid in the treatment of cats infected with feline leukemia virus (FeLV) and/or feline immunodeficiency virus (FIV), and the associated symptoms of lymphocytopenia, opportunistic infection, anemia, granulocytopenia, or thrombocytopenia. The absence of any observed adverse events in several animal species, suggests that the product has a very low toxicity profile.

Lymphocyte T-Cell Immune Modulator is a potent regulator of CD-4 lymphocyte production and function.[6] It has been shown to increase lymphocyte numbers and Interleukin 2 production in animals.[7]

Lymphocyte T-Cell Immune Modulator is a single chain polypeptide. It is a strongly cationic glycoprotein, and is purified with cation exchange resin. Purification of protein from bovine-derived stromal cell supernatants produces a substantially homogeneous factor, free of extraneous materials. The bovine protein is homologous with other mammalian species and is a homogeneous 50 kDa glycoprotein with an isoelectric point of 6.5. The protein is prepared in a lyophilized 1 microgram dose. Reconstitution in sterile diluent produces a solution for subcutaneous injection.[8]

The cat should receive as much immune-system support as possible. Nutritional supplements that may support the cat’s immune system include: colostrom (a milk product), Chinese mushroom extracts, plant-based extracts (purple coneflower, for example), and cat vitamins.[citation needed]

A FeLV-positive cat may also have persistent gingivitis and other dental problems (persistent gingivitis is a prime symptom of FeLV and FIV).

The use of interferon is recommended for use against FeLV. Interferon alpha is available in the United States. Interferon-ω (omega) is sold in Europe at least under the name Virbagen Omega and manufactured by Virbac. When used in treatment of cats infected with FeLV in non-terminal clinical stages (over the age of 9 weeks) there have been substantial improvement in mortality rates; in non-anaemic cats, mortality rate of 50% was reduced by approximately 20% following treatment.[citation needed]

The drug is quite expensive dependent on body-weight, but will be covered by most pet insurers.

The use of inteferons are not recommended for cats currently using steroids for symptomatic treatment. A 2 week withdrawal period is recommended before starting treatment, and steroids should ideally not be used at any point after the treatment, as they will effectively reverse the efficacy of the drug (inteferon-ω simulates natural leukocyte response, and steroids will greatly hamper this).

For American pet owners, one way to deal with leukemia or anemia associated with FeLV is blood transfusions. It is also recommended that the cat undergo a battery of tests such as a bone marrow aspiration, abdominal ultrasound, chest x-ray, or thoractic endoscopy. With the results from these, the disease’s effects may be seen on the viscera and any cancerous tumours arising from the disease may be taken care of. Following transfusion, it is important to keep the cat on antibiotics and steroids.

Comparison with FIV
FeLV and Feline immunodeficiency virus are in the same family, and are sometimes mistaken for one another. However, the viruses differ in many ways. Their shapes are quite different: FeLV is more circular while FIV is elongated. The two viruses are also quite different genetically, and their protein coats differ in size and composition. Although many of the diseases caused by FeLV and FIV are similar, the specific ways in which they are caused also differs.

Comparison with HIV
Dr. Don Francis at the Center for Disease Control in Atlanta theorized that AIDS was caused by a sexually transmitted virus on the model of FeLV. As director of CDC’s AIDS Laboratory Activities, he worked closely with the Institut Pasteur to eventually demonstrate that HIV was the cause of AIDS.



Known as FIV or cat AIDS, FIV is an infectious disease caused by a  retrovirus belonging to the lentivirus family. It is in the same family as the FeLV virus, and is similar to the HIV virus in humans. FIV infects both domesticated cats, lions, tigers, pumas & cheetahs.

FIV attacks the cells of the immune system, leading to FAIDS (feline acquired immune deficiency syndrome). This compromises the cat’s ability to fight off infections.

It was first discovered in 1986 in a colony of cats in California, and is found worldwide.

What is a virus?

A virus is a ultramicroscopic infectious agent which consists of either DNA or RNA wrapped in a protein coat. It is only able to replicate inside living cells. The phrase “a piece of bad news wrapped in protein” is a very well known saying regarding viruses.

Viruses are not living organisms, they don’t respirate, process nutrients or generate waste products.

What does FIV do & what are the symptoms?

FIV attacks the cat’s immune system which makes it vulnerable to secondary bacterial, viral, fungal & protazoal infections.

Stage 1: Once inside the body, FIV is carried to the regional lymph nodes where it replicates in the white blood cells known as T lymphocytes (CD4+ lymphocyte). It then spreads to other lymph nodes throughout the body. At this time there may be an acute illness which is characterised by fever, leukopenia, anaemia, malaise & swollen lymph nodes, lasting a few weeks. During this initial stage it may go unnoticed that the cat is unwell.

Stage 2: This is the asymptomatic phase which can last for many years. During this stage cat appears healthy & is able to lead a normal life.

Stage 3: As we’ve already discussed, FIV destroys the T lymphocytes, these cells are required for the proper functioning of the immune system.  Eventually when enough T lymphocytes have been destroyed, the immune system loses it’s ability to fight off opportunistic infections & signs of immunodeficiency develop.

Cats show a range of symptoms in this stage, these symptoms may vary from cat to cat. Some of which may include:

  • Weight loss
  • Poor coat condition
  • Anaemia
  • Gastroenteritis
  • Gingivitis & stomatitis
  • Diarrhoea
  • Chronic or recurrent infections of the skin, eyes, urinary tract, respiratory tract etc.
  • Cancer

How is FIV transmitted?

The virus is present in large quantities in the cat’s saliva, and the most common mode of transmission is via bite wounds. Free roaming, entire male cats are at greater risk as they are more likely to become involved in territorial fighting.

Occasionally FIV is passed onto kittens who’s mother is FIV positive. This may happen either in utero or via infected milk.

Cats don’t become infected via mutual grooming, nor will the act of mating, although the biting that goes along with mating may pass on the virus.

How is FIV diagnosed?

FIV is diagnosed based on history, clinical signs & a blood test known as ELISA (enzyme-linked immunosorbent assay), which detects antibodies to FIV. It is possible to get false positive or false negatives from these results for the following reasons:

False positive:

If a cat has received the FIV vaccine it will have a positive test result.

Kittens born to FIV infected mothers may have received antibodies from their mothers milk. This doesn’t mean that the kitten has FIV, just that it’s received antibodies to FIV. Kittens who test positive should be re-tested at a later date.

False negative:

It usually takes several weeks for antibodies to FIV to appear in the blood, if the cat is tested prior to this it will show a negative result.

If the cat is in the later stages of infection it may not be producing antibodies.

A FIV PCR (polymerase chain reaction) test is available in some commercial laboratories. PCR detects the presence of the FIV virus in the blood.

Western blot test or IFA (Immuno-Fluorescent Antibody Test): If a cat has tested positive to FIV it is sometimes recommended to follow up with either a Western Blot Test or an IFA test.

If your cat has tested positive to FIV but you are not sure if it has had the vaccine, or want to be sure it does/doesn’t have the virus then you may be able to request a PCR (polymerase chain reaction) test, which will be able to detect the presence of FIV DNA in the blood.

How is FIV treated?

There is no cure for FIV, once a cat has it, it’s for life. The goal is to provide supportive care to the infected cat. This may include;

  • Regular veterinary check ups.
  • Maintaining proper parasite control.
  • Ensuring that prompt veterinary attention is sought at the first sign of illness.
  • Feed a high quality diet.
  • Limiting their exposure to disease by keeping them indoors & away from neighbourhood or stray cats.
  • The use of anti bacterial & anti fungal drugs
  • Maintain a proper vaccination regime to protect your cat from other infectious diseases.
  • Blood transfusions may be necessary in stage 3.
  • High calorie supplements may be necessary in stage 3.
  • There are other drugs available such as interferon & AZT which have been used in some cats, but it is best to speak to your veterinarian about these.

Should I have my FIV positive cat euthanised?

No, this isn’t necessary. FIV positive cats can live for many years, especially if they are given prompt medical attention when necessary & supportive care.

Prevention of FIV:

Desexing of all pets, not allowing them to free roam & testing all cats used for breeding.

There is now an FIV vaccine which is available, it isn’t 100% effective. Cats given the vaccine will test positive to FIV which may result in confusion, especially in the case of a cat being picked up by a shelter or council. For further information on the vaccine, read here…

Which cats are most at risk of FIV?

Un-neutered, free roaming males are at the greatest risk of FIV. Any cat allowed to free roam is at risk of catching FIV.

Can FIV positive & FIV negative cats live in the same household?

The general opinion is yes, this is okay as long as there isn’t any fighting between the cats. As FIV positive cats are more susceptible to opportunistic infections it is important to ensure the health & vaccination status of ALL cats in the household.

Others suggest either keeping FIV positive cats isolated from FIV negative cats or rehoming the FIV positive cat in a single cat household. Your veterinarian is the best person to speak to in this regard.

Can I catch FIV from my cat?

No it is not possible to catch FIV from your cat, nor is it possible for your cat to catch HIV from a human. While both viruses come from the same family, they are species specific.

Please note:

The medical articles on this site have not been written by a veterinarian & should not be considered a replacement for a veterinarian visit. The articles are provided for informative purposes only.

Always seek immediate veterinary advice for any problems (health or behavioral) in your pets.

While great care has been made in the creation of these articles, we cannot guarantee the accuracy or omissions on these pages. If in any doubt whatsoever, seek professional medical advice.



What is hypertrophic cardiomyopathy?

Hypertrophic cardiomyopathy (HCM) is the most common heart disease of cats,
whether they are random bred or pedigreed. It is a heart muscle disease in which the papillary muscles (the muscles in the left ventricle that anchor the mitral valve) and the walls of the left ventricle become abnormally thickened.
HCM is often a progressive disease, and a proportion of affected cats develop
heart failure if the muscle hypertrophy and subsequent scarring of the heart muscle significantly affects heart function.
For more information on HCM, see:
What causes HCM in cats?

This is currently unknown in most cats although familial (hereditary) HCM has been
observed in several breeds, such as the Maine Coon and American Shorthair.
Anecdotal information suggests there is familial HCM in many other breeds.
Heart muscle hypertrophy in cats can be caused by other diseases,
such as systemic hypertension (high blood pressure) and hyperthyroidism.
HCM is a primary disease of the heart muscle. Hypertension and hyperthyroidism cause secondary thickening of the left ventricle and so are not causes of HCM (although it is possible that they may exacerbate the disease if they become present in a cat with mild to moderate HCM).
HCM is diagnosed when these other causes are ruled out.
Is HCM genetic?

In Maine Coons and American Shorthairs,
HCM has been confirmed as an autosomal dominant inherited trait,
as it is in humans where over 130 gene mutations in 10 genes
have been found to cause the disease. The disease has variable expression;
meaning some cats are severely affected,
others are only mildly to moderately affected, and some cats may not have
evidence of the disease yet produce affected offspring.
While a specific feline gene mutation has not yet been identified,
research is underway in the Maine Coon cat.
However, since few veterinary cardiologists and geneticists have the expertise to study genes, it is unlikely that the responsible gene or genes for each affected
breed will be found at any time in the near future. If a gene is identified as a cause of HCM in Maine Coon cats, it may not be the same gene responsible for HCM in other breeds. HCM will require investigation in each breed individually.
Can HCM have a nutritional cause?
There is no evidence in cats, humans or other species of animals that
HCM can have a nutritional cause.
How is HCM diagnosed?

HCM is diagnosed using ultrasound of the heart – an echocardiogram.
Echocardiography is a good way to detect moderate to severely affected cats.
However, it may not always detect the mildly affected cats where
changes in the heart can be minimal.
In addition to an echocardiogram, other tests may also be useful in assessing cats with HCM. For example, a chest x-ray is necessary to detect heart failure in cats with severe HCM. An electrocardiogram is useful in cats that have an abnormal heart rhythm. Blood pressure measurement and blood testing for hyperthyroidism is indicated to rule out other diseases that mimic HCM, especially mild to moderate HCM. Ideally, an echocardiogram to test cats for HCM should be performed by a
board-certified radiologist or cardiologist.
Should my cats be tested for HCM and how often should they be tested?

In clinical practice, the most common patients tested for HCM with echocardiography are cats with suggestive clinical signs of heart disease,
such as a heart murmur.
Testing cats used in a pedigreed breeding program is a more difficult endeavor.
Echocardiography is not a perfect tool for diagnosis of HCM – some affected individuals will escape detection and access to good quality ultrasound services may be difficult and expensive for some breeders. At the very least, breeding cats should be ausculted (examined by a vet with a stethoscope)
for heart murmurs or arrhythmias once yearly.
Any cat with an abnormality should have an echocardiogram.
A significant percentage of cats with HCM will not have a heart murmur, however.
Since HCM can occur at any age, a single normal echocardiogram does not guarantee a cat is free of disease. Breeding cats should probably have an echocardiogram yearly during their breeding years.
Examining retired cats periodically is also advantageous as this may allow the identification of affected cats that have offspring in a breeding program.
At what age should a cat be tested for HCM?

HCM can affect cats at any age. It has been seen in kittens only a few months of age and in cats over the age of 10. In Maine Coons, most affected male cats have evidence of disease by 2 years of age, and most affected females have evidence of disease by 3 years of age although instances have been documented where the disease has not shown up until 7 years of age.
Ragdolls with severe disease seem to develop it earlier in life,
often at under 1 year of age.
Guidelines for other breeds have not yet been developed.
It is therefore hard to recommend a specific age to start testing.
It may make sense to test most breeding cats for the first time by the age of 2 years.
What do I do if my cat is diagnosed with HCM?
The cat should be removed from the breeding program and all offspring should be
watched closely for the development of HCM. Statistically, 50% of the cat’s offspring would be expected to have the gene that causes HCM if one parent was a heterozygote.
However, the most prudent approach may be not to use any of the
offspring in a breeding program.
The parents of an affected cat should also be examined with ultrasound,
as one of them likely carries the gene mutation for HCM.
In some cases, identification of the affected parent may be difficult,
especially if the disease is mild. In these cases, the most prudent
approach may be to remove both parents from the breeding program.
It is possible for a cat to develop a spontaneous mutation that causes
HCM during embryonic development but this is an unlikely cause in a breed
known to have the problem.

All breeders that are using cats related to an affected cat should be notified that a cat has been diagnosed with HCM. Similarly, pet owners should be notified that a relative has been diagnosed with the disease. Echocardiographic examination of cats related to the affected cat should be performed.
Will we ever eliminate HCM from my breed?
The tools we currently have to diagnose HCM
(i.e., echocardiography and necropsy)
are not perfect and will not allow us to totally eliminate this disease.
However, echocardiographic screening will be able to reduce the incidence of
HCM within a breed if enough breeders are involved.
Breeders should use all the information they can gather about HCM in family lines,
including pedigree analysis based on accurate identification of affected cats.
Any cat that dies suddenly or dies from HCM should have a necropsy
(i.e., post mortem examination).
Most cats with HCM will have a heart that weighs more than 20 grams and
most cats with severe HCM will have a heart that weighs more than 30 grams.
Myocardial fiber disarray, the hallmark microscopic heart muscle abnormality seen in humans with familial HCM is seen in all Maine Coon cats with HCM. Unfortunately, most veterinary pathologists are not trained to recognize this lesion.
In the long term, we will need a genetic test for HCM in each breed. A genetic test would allow us to identify affected cats before they were bred and do so accurately. Since the disease is inherited as an autosomal dominant trait, once a mutation is identified, if all breeders cooperated by testing their breeding cats for the mutation the disease could be eliminated from the breed within several generations. However, the money and resources necessary to identify the gene or genes and to develop a genetic test for each breed are scarce in veterinary medicine. Breeders and cat fanciers can help by supporting research
through organizations such as the Ricky Fund established
by the Winn Feline Foundation.
Can two normal parents produce a kitten with HCM?
Since HCM is known to be an autosomal dominant trait in the breeds where the
inheritance is known, each affected cat must have one affected parent.
However, there are possible situations in which an affected cat may come from
two apparently normal parents.
The first possibility is that one of the parents has been misdiagnosed.
This can happen due to inexperience of the ultrasonographer or poor quality equipment.
It can also happen if a cat’s status is decided on the basis of only one or two ultrasounds early in life. Since HCM can develop at any age, a cat that is normal on ultrasound one year could still have HCM and show signs later in life.
Since the trait has variable expression, not every affected cat will have echocardiographic evidence of HCM. It is therefore possible for a cat to test negative for HCM on ultrasound, and yet still carry the defective gene and pass it to offspring. Finally, it is possible for spontaneous mutations to occur in cats from normal parents. These cats may then pass on their mutation to offspring.
We do not know how often spontaneous mutations causing HCM occur in cats. Statistically, spontaneous mutations are more likely to occur in random bred cats than in pedigreed cats.
What does “HCM free cattery” mean?
There is no universally agreed upon definition of an HCM free cattery. The terminology is currently unclear, as different breeders mean
different things when they use this term.
Ideally, each breed should develop a specific definition and guidelines for use of this designation for catteries.