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Feline Leukemia 

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Feline leukemia (FeLV) is a contagious, immunosuppressive gammaretrovirus that is associated with a wide array of disease conditions affecting multiple organ systems and susceptibility to opportunistic infections in cats (Chhetri et al, 2013;Torres et al. 2005). This virus can cause both cytoproliferation and cytosuppressive disease (Torres et al. 2005). The most common mode of infection is through bites, although other less common modes of transmission such as nursing, mutual grooming or sharing dishes can occur (Chhetri et al, 2013). Risk factors for infection include male gender, adulthood, and outdoor access (Hartmann; 2012). Not all cats that have been exposed to FeLV develope infection. It is estimated that ~60% of FeLV-exposed cats devlope regressive infection marked by an effective and durable immune responce that contains and posibly extinguishes viral replication (Torres et al. 2005).  

FeLV infections are chronic in nature and develop through different disease stages (Hartmann, 2012). There are four stages of infection. Abortive infection occurs after exposure when the virus starts to initially replicate in the local lymphoid tissue in the oropharyngeal area (Hartmann, 2012). it is at this stage that cats can develop an effective immune responce against the virus. These cats never become viremic and become part of the estimated 60% of cats that do not become infected adter exposure (Hartmann, 2012; Torres et al, 2005).

The next stage is regressive infection. This stage developes after an effective immune responce when virus replication and viremia are contained prior to shortly after bone marrow infection. this stage is charcterized by the intefration of proviral DNA into the hosts cellular chromosomal DNA (Hartmann, 2012). After initial infection, replicating FeLV spreads systemically through infected mononuclear cells (lymphocytes and monocytes). After about three weeks of viremia, bone marrow cells become infected, and infected hematopoietic precursor cells develop into infected granulocytes and platelets that circulate in the body (Hartmann, 2012). Even if bone marrow cells become infected, a certain percentage of cats are able to clear viremia. However, they cannot completely eliminate the virus from the body, even if they terminate viremia because the information for virus replication (proviral DNA) is present in bone marrow stem cells (Hartmann, 2012). Although proviral DNA remains present within the cellular genome, no virus is actively produced. Therefore, regressively infected cats do not shed FeLV and are not infectious to others (Hartmann, 2012). 

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Progressive infection occurs when the FeLV virus is not contained early in the infection. In this stage extensive virus replication occurs, first in the lymphoid tissues, followed by the bone marrow and mucosal and glandular epithelial tissues (Hartmann, 2012). Progressively infected cats remain persistently viremic and are infectious to other cats for the remainder of their life (Hartmann, 2012). These cats develop FeLV-associated diseases, and most of them will die within a few years. Regressive and progressive infections can be distinguished by repeated testing for viral antigen in peripheral blood; regressively infected cats will turn negative at latest 16 weeks after infection, while progressively infected cats will remain positive (Hartmann, 2012).   

The last stage of infaction is focal infection or atypical infection. This stage is characterized by a persistent atypical local viral replication (e.g., in mammary glands, bladder, eyes). Focal infections have been reported in up to 10% of eperimentally infected cats and is probably rare in nature (Hartmann, 2012).  

The signs and symptoms very depending of the type of infection. The three types of infection are: FeLV-A, FeLV-B, and FeLV-C; and cats can be infected with one, two, or all three types (PetMD). All cats infected with FeLV have FeLV-A; which severely weakens the immune system. FeLV-B is only present in about 50% of FeLV-infected cats, and is primarily associated with tumors and other abnormal tissue growths. The least common type of infection is FeLV-C;which only occurrs in about 1% of FeLV-infected cats. This type of infection is associated with non-regenerative anemia (Hartmann, 2012; PetMD). 

Even though, FeLV is a major oncogene that causes different tumors in cats, the most clinically important consequence of both feline leukemia infection is immunosuppression. Immunosuppression can lead to secondary infectious diseases; which account for most of the clinical signs (Hartmann, 2012).

 

For information on the provention and treatment of feline leukemia click here. 

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References:

  1. Chhetri BK, Berke O, Pearl D, Bienzle D. Comparison of the geographical distribution of feline immunodeficiency virus and feline leukemia virus infections in the United States of America (2000–2011). BMC Veterinary Research. 2013; 92: 1-6. 

  2. Torres AN, Mathiason CK, Hoover EA. Re-examination of feline leukemia virus: host relationships using real-time PCR. Virology. 2005; 332: 272-283. 

  3. Hartmann K. Clinical Aspects of Feline Retroviruses: A Review. Viruses. 2012; 4: 2684-2710. 

  4. PetMD; Feline Leukemia Virus Infection (FeLV) in Cats; c1999-2015. [cited November 16, 2015]. Available from: http://www.petmd.com/cat/conditions/infectious-parasitic/c_ct_feline_leukemia# 

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