1. FeLV-A : This subgroup is the form of FeLV that is involved in horizontal transmission of FeLV between cats. The FeLV-A subgroup is found in all infected cats. The pathogenicity of FeLV-A is low, but it is highly infectious. 2. FeLV-B : This subgroup is less prevalent than FeLV-A. The FeLV-B virus forms within an individual cat, when FeLV-A virus recombines with endogenous FeLV-related sequences. Lymphoid malignances are often associated with the development of FeLV-B. 3. FeLV-C : This subgroup is also less prevalent than FeLV-A. Like FeLV-B, FeLV-C arises in the individual cat. However, its formation is caused by mutations or deletions in the FeLV-A surface glycoprotein gene. The development of aplastic anemia is typically associated with FeLV-C. 4. FeLV-T : Like FeLV-B and FeLV-C, FeLV-T also arises in an individual cat. It is caused by mutations or deletions in FeLV-A surface glycoproteins, like FeLV-C. Immunodeficiency is typically associated with FeLV-T, because it is a T-cell tropic cytopathic virus and causes lymphoid depletion. Feline leukemia is not regarded as a zoonotic disease. Some studies have suggested that human infection with FeLV could be a theoretical possibility; however, there are no reports of transmission to a human and no documented cases of FeLV-associated human leukemia (Lister, 2015). While FeLV does not infect humans, it has been shown to affect wild felids; there are reports of symptomatic disease with impacts on local wildlife populations (Petch et al., 2022). future. These cats were previously described as transient viremic followed by latent infection. 3. Progressive infection : Persistent viremia is used to describe FeLV-infected cats with prolonged viral circulation. These cats will have multiple positive tests, repeatable weeks or months apart. Progressively infected cats shed high numbers of virus; these cats are most capable of infecting other cats. They are at a high risk of death within the next several months to years, although some cats with progressive infection may go on to live long lives. In studies of unvaccinated, cats, approximately 30% to 40% go on to develop progressive infection after FeLV exposure (Lutz, 2012). 4. Focal or atypical infection : Although rare in naturally infected cats, viral replication can be confined to one anatomical region of the body. These cats may test weakly or intermittently ELISA positive for FeLV. This is unlikely to be of clinical significance in real-world scenarios. Most cats with FeLV will go on to develop immunodeficiency. Infected cats often develop anemia and opportunistic infections, which eventually prove fatal (Litster, 2015a). However, FeLV can also lead to neoplastic disease. Malignant neoplasia can also develop in cats with progressive or reactivated FeLV. This most frequently manifests as hematopoietic neoplasia, such as leukemia or lymphoma. Less commonly, myeloproliferative disorders may occur (Litster, 2015a). properties of FeLV. Dysphagia or dyspnea may be observed in cats with mediastinal lymphoma (Fenimore, 2014). Owners may also notice vague signs, such as weight loss, anorexia, or lethargy. If the virus is replicating in the bone marrow, blood count abnormalities—such as anemia, leukopenia, and thrombocytopenia—may be observed on screening laboratory tests.
All immunologically naïve cats have some susceptibility to FeLV. However, the highest susceptibility to FeLV virus is observed in young kittens, especially those up to approximately four months of age. Adult cats typically demonstrate some degree of resistance to infection, making a new FeLV infection less of a risk in this population. It is important to note, however, that biological behavior can differ between FeLV strains, and immune responses can vary significantly among cats. Therefore, it is impossible to precisely predict the likelihood of infection following an infectious dose of FeLV for any individual cat. All cats with possible exposure should be considered at risk of contracting the disease (AAFP, 2013a). Feline leukemia, like other retroviruses, is an enveloped RNA virus. The virus contains an enzyme that reverse- transcribes the viral genome into a DNA form referred to as the FeLV provirus. This DNA provirus is then integrated into the host cat’s genome, allowing it to replicate within the host cells (Hofmann-Lehmann, 2016). Feline leukemia virus can be divided into four major subgroups. The distinctions between subgroups are based on the virus envelope and the properties that this envelope confers (Hofmann-Lehmann, 2016). The four subgroups of FeLV are: Pathophysiology of FeLV In exposed cats, FeLV infection typically begins in the oropharynx. Viral replication initially occurs in the tonsils and the adjacent lymphoid tissues. Next, virus is spread throughout the body in the lymphatic system. At this time, any one of four possible courses of disease may be observed (Hofmann-Lehmann, 2016; Litster, 2015a): 1. Abortive infection : This occurs when a cat is able to mount an effective immune response against FeLV. Abortive infections are often associated with exposure to low doses of FeLV, allowing the cat to more easily combat the infection. Cats with an abortive infection will not test positive for FeLV at any time, because their immune system mounts a response before a significant quantity of circulating FeLV antigen is present. In the past, these cats were referred to as regressor cats. 2. Regressive infection : Most adult cats exposed to FeLV fall into this category. After initial infection with FeLV, the virus reaches the bone marrow in these cats and triggers a transient infection. These cats will temporarily test FeLV-positive, and they may shed infectious FeLV virus during this time. Within three to 16 weeks of infection, however, these cats will mount an immune response that suppresses the virus. At this time, ELISA testing will convert to negative, because there is no significant circulating FeLV antigen present in the bloodstream. These cats are unlikely to show clinical signs of infection, although latent viral replication may be reactivated in the Clinical signs of FeLV The clinical signs of FeLV are variable, depending on which organ system is affected by the disease. Commonly observed signs of FeLV include neoplasia, recurrent infections caused by immunosuppression, or evidence of hepatic, reproductive, or neurologic disease. Many FeLV-infected cats present with signs of leukemia or lymphoma. These neoplasias are caused by the oncogenic
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