Feline leukemia viral infection is a significant disease among domestic cats. A number of routine and confirmatory tests can be used in combination to confirm a diagnosis of FeLV infection.
How FeLV Infection is Diagnosed
It typically takes at least four weeks of fairly constant exposure to the feline leukemia virus for it to appear in blood samples of affected animals. After 20 weeks of prolonged exposure, roughly 80% of exposed cats probably will become infected.
Most veterinarians presented with a cat showing non-specific signs of systemic illness will conduct an initial database of blood work (complete blood count and serum chemistry panel), a urinalysis and thoracic radiographs (chest X-rays). The results of these tests may be unremarkable even in infected cats. But, they may identify anemia (low circulating red blood cell levels), secondary urinary tract infections, chest masses and/or abnormal fluid accumulation in the thoracic cavity. More advanced confirmatory tests include an enzyme-linked immunosorbent assay (ELISA) test of blood, saliva or tears (blood is the preferred sample), and an immunofluorescent antibody (IFA) test of blood and/or bone marrow. These tests can identify the actual viral antigen in the sample.
Most veterinary practitioners recommend using an in-clinic ELISA test for initial screening, as it is more likely to detect early and transient FeLV infections. If the ELISA is positive, the IFA test is recommended, because it is more likely to identify persistent viremia (presence of the virus in circulation) and viral shedding, with bone marrow involvement. If the results of both tests are positive, the cat is considered to be viremic and will remain infected with FeLV for life. Tests on saliva or tears are not considered to be reliably diagnostic. A polymerase chain reaction (PCR) test of blood or fixed tumor tissues is also available at some referral laboratories.
It is suggested that both the ELISA and IFA tests be repeated in 2 to 3 months unless both tests are positive, to determine whether the virus has been eliminated by the cat’s immune response or whether it has progressed from transient to persistent, permanent infection.
Cats with latent FeLV infection will typically test negative on both the ELISA and IFA tests, because they do not have viral particles in circulating blood. Cats previously vaccinated against FeLV do not test positive on these blood tests as a result of the vaccination.