FeLV is a gammaretrovirus that infects domestic cats and results in immunosuppression, hematologic abnormalities, and neoplasia, including lymphoma and leukemia. It continues to have clinical significance despite its declining prevalence due to excellent testing and vaccination.

Approximately 30-40% of the exposed cats globally establish persistent (progressive) viremia, and another comparable percentage acquire transient (regressive) infection, and approximately 20-30% of them clear the virus without integration (abortive infection).

FeLV is further classified into several subgroups, such as FeLV-A, FeLV-B, FeLV-C, and FeLV-T, with different clinical manifestations. FeLV-A occurs in all affected cats and causes the primary infection.

FeLV-B predisposes to tumor formation, whereas FeLV-C predisposes to fatal anemia. Subgroup FeLV-T shows tropism to T-cells, leading to immune dysfunction. The subtype that arises in a particular cat is based on mutation and recombination events after infection, further making prognosis and progression of disease complex.

How It Spreads

Major transmission modes of feline Leukemia include friendly close contact, saliva through grooming, shared food/water/litter, and bite wounds. Vertical transmission occurs in utero or via nursing. Transmission via fleas, blood transfusion, or environment is rare and inefficient.

Who Is at Higher Risk

• Young kittens, particularly those more than 16 weeks old, are at the highest risk of FeLV.
• Outdoor, free-roaming, sheltered, or colony cats and multi-cat households have higher transmission.
• Intact males, mixed-breed cats, and cats in endemic regions show an elevated risk of Feline Leukemia Virus.

How Do Cats Get Feline Leukemia?

As mentioned earlier, the most common causes of FeLV in cats are viral exposure through intimate contact, bites, and vertical routes.

Immune response determines outcome:

• Abortive: cat clears virus without proviral integration.
• Regressive: provirus integrates, antigen may disappear.
• Progressive: ineffective immune control, persistent viremia, and high viral shedding risk; leads to marrow suppression, tumors, and immune dysfunction.

Subgroups of FeLV (A, B, C, T) influence disease outcome. FeLV-C is associated with non-regenerative anemia, FeLV-B and others with lymphoma risk.

Symptoms in Cats

In progressive infections, typical clinical signs of feline Leukemia include:

• Anemia, pale mucous membranes
• Lethargy, chronic weight loss
• Poor coat condition
• Persistent fever
• Peripheral lymphadenopathy
• Chronic stomatitis/gingivitis
• Persistent infections, upper respiratory, urinary, skin, GI, ocular, neurologic
• Development of lymphoma or leukemia
• Immunemediated disease, neuropathies
• Reproductive failure

In regressive or abortive infections, cats may remain asymptomatic for long periods unless reactivation occurs under stress or immunosuppression