Discordant FeLV Test Results in Cats With Hematologic Disease

Joyce S. Knoll, VMD, PhD, DACVP (Clinical Pathology), Cummings School of Veterinary Medicine at Tufts University

ArticleLast Updated December 20233 min read
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In the Literature

Kornya M, Bienzle D, Beeler-Marfisi J. Discordant FeLV p27 immunoassay and PCR test results in 21 cats with hematologic disorders. J Feline Med Surg. 2023;25(7):1098612X231183297. doi:10.1177/1098612X231183297

The Research …

Depending on immune response, exposure to FeLV may result in clearance of the infection (ie, abortive infection), persistent but contained infection without clinical signs or persistent viral shedding (ie, regressive infection), or sequestration of the infection in the spleen, lymph node, small intestine, urinary tract, or mammary glands (ie, focal infection; rare).1,2 Progressive infection occurs in cats without an appropriate immune response and results in extensive viral replication and shedding and development of disease.3 Abortive and regressive infections are most common and can be confirmed via p27 antigen assays (eg, point-of-care test kits), PCR for proviral DNA, and measurement of neutralizing antibodies.4,5

This study analyzed signalment, presenting complaint, result of FeLV diagnostic testing, final diagnosis, treatment, and outcome of 21 cats with discordant FeLV tests results (ie, positive FeLV p27 antigen test but negative nonquantitative PCR assay for proviral DNA). Immune-mediated hematologic disease (ie, immune-mediated hemolytic anemia [IMHA], nonregenerative IMHA, immune-mediated bicytopenia, immune-mediated pancytopenia) was diagnosed in 18 cats. Acute lymphocytic leukemia, hemotrophic mycoplasma infection, or pyothorax was diagnosed in the 3 remaining cats. It was unclear why test results were discordant. Of all cats, 18 (including the 2 with infectious disease) were treated and discharged, and 3 were euthanized. Of the 18 cats discharged, 16 were available for follow-up and survived 4 months to 6 years; 4 of the 16 were weaned from medication, and 12 continued to receive immunosuppressive therapy. Following disease recovery, 7 cats were p27 negative on antigen testing.

… The Takeaways

Key pearls to put into practice:

  • Clinical signs and laboratory abnormalities typically affect interpretation of positive FeLV antigen tests. Progressive infection with a guarded prognosis may be assumed in cats with clinical signs suspicious for FeLV; however, this may be an incorrect assumption in some cats with hematologic disease and could adversely affect treatment decisions.

  • Precursor-targeted IMHA (formerly, nonregenerative IMHA) results in selective destruction of erythroid precursors in bone marrow and is characterized by a persistent nonregenerative anemia and left-shifted erythropoiesis. This condition closely resembles nonregenerative anemia and/or bone marrow suppression seen in some cats with progressive FeLV infection. Although immunosuppressive therapy can successfully treat IMHA and precursor-targeted IMHA, medications for retrovirus-infected cats typically have low efficacy and can result in significant adverse effects.

  • Clinically ill FeLV-positive cats should undergo further testing to determine whether illness is due to retroviral infection or a concurrent, unrelated disease process. Progressive infection can be confirmed by a positive FeLV immunofluorescence assay test performed on a fresh (ie, not anticoagulated) blood smear or bone marrow or a second positive FeLV antigen test performed 12 weeks later. Alternatively, an immediate negative PCR test for FeLV proviral DNA can rule out abortive or focal infection. In cats with regressive or progressive infections, proviral DNA can typically be detected by PCR within 2 weeks of exposure (before circulating antigen is detectable [>30 days]), with decreasing levels of proviral DNA in cats with regressive infections. The presence of neutralizing antibodies can rule out progressive infection, but this test is not available in all countries.