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Feline leukemia virus is part of a family of viruses called Retroviridae. It consists of a protein core and a single-stranded ribonucleic acid surrounded by a lipoprotein envelope. The basic structure of FeLV is shown below: it consists of an outer envelope and an inner capsid.

There are four different subtypes of the gamma retrovirus FeLV:FeLV-A,FeLV-B, FeLV-C,and FeLV-T.Variation of the major envelope protein, gp70, is associated with the 3 virus sub-groups A, B and C. All cats infected with FeLV-B, FeLV-C, and FeLV-T are co-infected with FeLV-A, with FeLV-A being the only type that is transmitted between animals. The other subtypes influence the clinical expression of disease - FeLV-B may accelerate damage to nerve tissue and lymphoma (a cancer of the lymphatic cells in the immune system such as B-lymphocytes, which are important for antibody production, and T-lymphocytes, important for destroying virus-infected cells); FeLV-C is associated with non-regenerative anemia (a form of anemia where cats cannot replace lost red blood cells with new ones); and FeLV-T is associated with immunodeficiency in cats. The envelope protein, pl5e, which is anchored in the plasma membrane of infected cells, appears to be at least partially responsible for the viral-induced immunosuppression. Antibody to gp70, which is expressed on the outer surface of the virus as well as on the surface of infected cells,is important in natural resistance.

Anderson et al., 2000; Anderson et al., 2001; Essex, 1980; Hardy, 1980; Keel et al., 2008; Ogilvie et al., 1988; Sykes, 2010