Feline leukemia virus

Feline leukemia virus
Electron micrograph of Feline leukemia virus
Virus classification
(unranked):	Virus
Realm:	Riboviria
Kingdom:	Pararnavirae
Phylum:	Artverviricota
Class:	Revtraviricetes
Order:	Ortervirales
Family:	Retroviridae
Genus:	Gammaretrovirus
Species:	Feline leukemia virus
Synonyms[1]
Feline sarcoma and leukemia virus Feline sarcoma virus

Feline leukemia virus (FeLV) is a retrovirus that infects cats. FeLV can be transmitted from infected cats when the transfer of saliva or nasal secretions is involved. If not defeated by the animal's immune system, the virus weakens the cat's immune system, which can lead to diseases which can be lethal. Because FeLV is cat-to-cat contagious, FeLV+ cats should only live with other FeLV+ cats.

FeLV is categorized into four subgroups, A, B, C and T. An infected cat has a combination of FeLV-A and one or more of the other subgroups.^[2][3] Symptoms, prognosis and treatment are all affected by subgroup.^[2]

FeLV+ cats often have a shorter lifespan, but can still live "normal", healthy lives.^[4]

Signs and symptoms

The signs and symptoms of infection with feline leukemia virus are quite varied and include

Asymptomatic carriers will show no signs of disease, often for many years.^[citation needed]

Progression

The disease has a wide range of effects. The cat can fight off the infection and become totally immune, can become a healthy carrier that never gets sick itself but can infect other cats, or a mid-level case in which the cat has a compromised immune system.^{[citation needed]} Nevertheless, the development of lymphomas is considered the final stage of the disease. Although it is thought that virus protein has to be present to induce lymphomas in cats, newer evidence shows that a high percentage of FeLV-Antigen negative lymphomas contain FeLV-DNA, indicating a "hit-and-run" mechanism of virus-induced tumor development.^[5]

Once the virus has entered the cat, there are six stages to a FeLV infection^{[citation needed]}:

• Stage One: The virus enters the cat, usually through the pharynx where it infects the epithelial cells and infects the tonsilar B-lymphocytes and macrophages. These white blood cells then filter down to the lymph nodes and begin to replicate.
• Stage Two: The virus enters the blood stream and begins to distribute throughout the body.
• Stage Three: The lymphoid system (which produces antibodies to attack infected and cancerous cells) becomes infected, with further distribution throughout the body.
• Stage Four: The main point in the infection, where the virus can take over the body's immune system and cause viremia. During this stage the hemolymphatic system and intestines become infected.

If the cat's immune system does not fight off the virus, then it progresses to:^{[citation needed]}

• Stage Five: The
  eosinophils
  (all white blood cells formed in the bone marrow).
• Stage 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.

Transmission

Cats infected with FeLV can serve as sources of infection of FeLV-A.^[3] Cats can pass the virus between themselves through saliva and close contact, by biting another cat, and (rarely) through a litter box or food dish used by an infected cat.^[6]

Once a cat has been infected with FeLV-A, additional mutated forms of the original FeLV-A virus may arise, as may FeLV subgroups B, C and T.

In addition to domestic cats, some other members of Felidae are now threatened by FeLV (e.g. lynx and Florida panther).^[3] Overwhelming epidemiologic evidence suggests FeLV is not transmissible to either humans^[2] or dogs.^[7]

Approximately 0.5% of pet cats are persistently infected with FeLV, but many more pet cats (>35%) have specific

Kittens can be born with it, having contracted it from their mother while in utero.[6]

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.^[10]

Diagnosis and prognosis

Testing for FeLV is possible with

Cats diagnosed as persistently infected by ELISA testing may die within a few months or may remain asymptomatic for longer; median survival time after diagnosis is 2.5 years.^[6]

FeLV is categorized into four subgroups.^[2][12]

• FeLV-A is responsible for the immunosuppression characteristic of the disease. The vast majority of cats with FeLV has FeLV-A.^[13] An exception was reported in 2013.^[14]
• FeLV-B causes an additional increase in the incidence of tumors and other abnormal tissue growths. About half of FeLV infected cats have FeLV-B.
  endogenous FeLV (enFeLV).^[12]
• FeLV-C causes severe anemia. Approximately 1% of FeLV infected cats have FeLV-C.[13]^[15] It forms by mutation of FeLV-A.^[12]
• FeLV-T leads to lymphoid depletion and immunodeficiency.^[2] It forms by mutation of FeLV-A.^[12]

The fatal diseases are

Serious side effects have also been reported as a result of FeLV vaccination; in particular, a small percentage of cats who received the adjuvanted killed virus vaccine developed vaccine-associated sarcomas, an aggressive tumour, at the injection site.^[17] 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.^[18]

See the entry on retroviruses for more details on the life cycle of FeLV.^{[citation needed]}

Treatment

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Approved US treatment

In 2006, the

Lymphocyte T-Cell Immunomodulator is a potent regulator of CD-4 lymphocyte production and function.^[22] It has been shown to increase lymphocyte numbers and Interleukin 2 production in animals.^[23]

Lymphocyte T-Cell Immunomodulator is a single chain

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 improvements in mortality rates; in non-anemic cats, mortality rate of 50% was reduced by approximately 20% following treatment.[24]

History

FeLV was first described in cats in 1964.^[25] The disease was originally associated with leukemia; however, it was later realized that the initial signs are generally anemia and immunosuppression.^[25] The first diagnostic test became available in 1973, which led to a "test and elimination" regime, dramatically reducing the number of infected cats in the general population.^[26] The first vaccine became available in 1986.^[26]

Comparison with feline immunodeficiency virus

FeLV and

• Feline foamy virus (FFV)
• Feline immunodeficiency virus (FIV)
• Feline vaccination

References

• B. Zharkoy, A. Zolototrubov, D. Fedosov. "Intensity of free radical processes in limphocytes at BLV and FeLV". Journal of Animal Science. XLII,5/2005, p. 272-276.