Feline Leukemia Virus: What Vets Should Know

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Feline Leukemia Virus (FeLV) was first identified in 1964 by William Jarrett and colleagues. He observed virus particles budding from the membranes of malignant lymphoblasts obtained from a feline lymphoma patient. FeLV, a gammaretrovirus affecting domestic felines, is a member of the Oncornavirus subfamily of retroviruses. It is characterized by a protein core containing single-stranded RNA, enclosed within a lipid envelope. 

Related: Feline Leukemia Virus and Feline Immunodeficiency Virus 

FeLV-infected cats are globally distributed. However, the prevalence of infection significantly varies based on factors such as age, health status, environment, and lifestyle. In the United States, approximately 2-3% of all cats are infected with FeLV. This percentage rises considerably to 13% or more in cats that are ill, very young, or otherwise susceptible to infection. 

Figure 1. Structural Components of Feline Leukemia Virus 

Source: https://veteriankey.com/feline-leukemia-virus-infection/ 

Routes of transmission of feline leukemia virus 

  • Direct contact: Saliva (most common), nasal secretions, or blood transfusion 
  • Indirect contact: Sharing of food and/or water bowls, litter boxes, toys, or grooming tools. 
  • In utero (transplacental transmission) 
  • Post-natal through the milk of an infected cat 

Courses of feline leukemia virus infection 

Currently, veterinary professionals have identified three courses of infection: progressive, regressive, and abortive. 

Abortive infection 

Abortive infections are characterized by the release of viruses that do not integrate into the host cell genome as proviruses. In these rare instances, the infected animal typically mounts a sufficient immune response. This encompasses both humoral and cell-mediated immunity. This immune response effectively clears the virus, leading to clinical recovery. Abortive infections can only be detected through the presence of antibodies. 

Progressive infection 

There is integration of provirus and persistence of viremia/antigenemia, leading to positive results in both molecular and serological tests. Following FeLV exposure, some cats, particularly young ones, develop progressive FeLV infection, characterized by persistent antigenemia and viremia, resulting in high levels of persistent viral shedding. Cats with progressive FeLV infection pose a significant risk of transmitting the virus to other susceptible cats. 

Regressive infections 

Regressive infections occur when the feline leukemia virus (FeLV) integrates its proviral DNA into the host cell genome. However, active viral replication is suppressed. In these cases, infected cats exhibit positive results in polymerase chain reaction (PCR) tests, indicating the presence of viral DNA. Remaining negative in enzyme-linked immunosorbent assays (ELISA) due to the absence of circulating viral proteins. Despite the lack of active infection, regressive infection is associated with a reduced life expectancy due to the potential development of FeLV-related diseases. 

Important predisposing factors of transmission 

Several factors can influence FeLV transmission. These include: 

  • Viral load of an infected cat 
  • Immune status: cats with a weakened immune system have increased susceptibility to infection.  
  • Environmental factors: crowded living conditions and multiple cat households can facilitate transmission. 
  • Stress: stress can suppress the immune system, making cats more vulnerable to infection. 
  • Viral strain: different strains of FeLV may vary in their ability to infect and cause disease. 

Pathogenesis of feline leukemia virus 

Oronasal inoculation 

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Replication in the oropharyngeal lymphoid tissue 

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The virus is carried by monocytes and lymphocytes to peripheral tissue (including the spleen, lymph nodes, epithelial cells of the intestine and bladder, salivary glands, and bone marrow); the virus also appears in the excretions and secretions of these tissues 

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FeLV-containing leukocytes and platelets can appear in the peripheral blood 

Table 1. FeLV subgroups 

FeLV subgroup Characteristics 
FeLV-A Present in almost all naturally infected cats.  
FeLV-B Increases a cat’s risk for neoplastic diseases 
FeLV-C Strongly associated with the development of erythroid hypoplasia that predisposes severe anemia.  
FeLV-T Predisposes lymphoid depletion and immunodeficiency by infecting and destroying T lymphocytes.  

While all FeLV subgroups can be detected by commonly used diagnostic test kits for FeLV, they cannot be distinguished from each other. It usually takes 30 or more days after infection before the viral antigen can be detected.  

Clinical symptoms of feline leukemia virus 

FeLV-infected cats may manifest symptoms of illness that may be a result of the viral infection, secondary to FeLV-related immunosuppression, or due to an unrelated illness.  

Immunosuppression associated with FeLV can increase a cat’s risk of bacterial, fungal, viral, and protozoal infections. While FeLV-infected cats may be at increased risk of infectious disease, it should not be assumed that all infectious diseases occur because of the presence of FeLV. 

Immunosuppression may also increase an FeLV-infected cat’s risk of anemia-causing infectious agents. Any abnormalities observed in CBC results should be subjected to a comprehensive investigation, irrespective of FeLV status, to identify and treat any underlying treatable conditions. 

FeLV-infected cats are also at risk of developing immune-mediated diseases (glomerulonephritis, polyarthritis, etc.), particularly when antibody-antigen complexes are formed because of abundant FeLV antigens and low anti-FeLV IgG antibodies.  

FeLV-associated oncogenicity can develop when there is low blood concentrations of complement system proteins in FeLV-positive cats. The complement system is an important factor in some forms of antibody-mediated tumor cell lysis.    

Diagnostic tests for FeLV 

Three types of FeLV tests are commonly used. These include the following: 

Enzyme-Linked Immunosorbent Assay (ELISA) 

ELISA tests targeting the FeLV p27 protein are readily available for both in-clinic and reference laboratory use. In-clinic tests, utilizing whole blood, serum, or plasma samples, are commonly employed for FeLV screening in veterinary clinics and animal shelters. To ensure diagnostic accuracy, confirmation of positive in-clinic screening results is recommended. 

Antigenemia (presence of p27 capsid antigen) is a marker of infection and, in most, but not all cats, a parameter for viremia (presence of replication-competent virus). 

Indirect Immunofluorescent Assay (IFA) 

IFA tests for the presence of structural core antigens, including FeLV p27, in the cytoplasm of cells. An IFA test necessitates the submission of a sample to a diagnostic laboratory and cannot detect FeLV infection until bone marrow involvement occurs. 

False-negative test results of IFA may occur due to technical error, leukopenia, or limited bone marrow involvement. Conversely, false-positive results are primarily attributable to technical error. Similar to ELISA, IFA may not detect regressive infections, which occur when the immune system successfully suppresses viral replication. This limitation arises from the fact that insufficient viral antigen is produced during regressive infection to be detected by these tests. 

Polymerase Chain Reaction (PCR) 

A PCR test can detect the presence of FeLV genetic material integrated into the cat’s genome. This test can identify infections even when the virus is not actively replicating. This makes it useful for detecting regressive infections where viral levels may be low or undetectable by other tests. 

Figure 2. Feline Leukemia Virus Testing Flowchart 

Prevention and control 

FeLV vaccines are highly effective in preventing infection in susceptible cats. However, vaccination does not guarantee complete protection, especially in cats already infected or exposed to high viral loads. Therefore, minimizing exposure is important, even for vaccinated pets. 

Veterinarians consider the FeLV vaccine to be a core vaccine for kittens because of their high risk of developing a progressive infection. The Feline Veterinary Medical Association  (formerly American Association of Feline Practitioners, AAFP) 2020 Feline Retrovirus Management Guidelines recommend that all kittens receive a two-dose FeLV vaccination series as part of their initial vaccinations. Administer a booster vaccination one year later. 

Treatment for feline leukemia virus 

Currently, there is no cure for FeLV infection. Vaccines do not cure FeLV infection. The treatment of infected cats aims to manage symptoms and secondary infections. These may include antibiotics for bacterial infections, blood transfusions for severe anemia, antivirals, steroids, chemotherapy, and nutritional support.  

References 

  • Feline Leukemia Virus Brochure. (n.d.). Cornell University College of Veterinary Medicine. https://bit.ly/3DiY95H 
  • Fusco, G., Marati, L., Pugliese, A., Levante, M., Ferrara, G., De Carlo, E., Amoroso, M. G., & Montagnaro, S. (2023). Prevalence of feline leukemia virus and feline immunodeficiency virus in cats from southern Italy: A 10-year cross-sectional study. Frontiers in Veterinary Science, 10, 1260081. https://doi.org/10.3389/fvets.2023.1260081 

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Editor’s note: this post was originally published on December 20, 2024.

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