Progressive and regressive infection with feline leukemia virus (FeLV) in cats in southern Brazil: Prevalence, risk factors associated, clinical and hematologic alterations

Enzyme-linked immunosorbent assay (ELISA) for viral antigen is commonly used for the diagnosis of progressive feline leukemia virus (FeLV) infection but is not able to determine the true prevalence of infection when used as the sole test. Additional testing to detect proviral DNA will identify regressive (antigen negative) FeLV infections as well as progressive infections. Therefore, this study aimed to determine the prevalence of progressive and regressive FeLV infection, outcome-associated factors, and hematologic changes. A cross-sectional study was performed on 384 cats selected from routine hospital care. Blood samples were subjected to complete blood count, ELISA for FeLV antigen and FIV antibody, and nested PCR amplifying the U3- LTR region and gag gene, which are conserved in most exogenous FeLV. The prevalence of FeLV infection was 45.6% (CI_95% 40.6-50.6%). The prevalence of progressive infection (FeLV⁺P) was 34.4% (CI_95% 29.6-39.1%), that of regressive infection (FeLV+R) was 10.4% (CI_95% 7.4-13.4%), for discordant but positive results 0.8% (CI_95% 0.75-0.84%), for FeLV⁺P coinfected with FIV 2.6% (CI_95% 1.2-4.0%), and FeLV⁺R coinfected with FIV 1.5% (CI_95% 0.3-2.7%). Male cats were three times more likely to be in the FeLV⁺P group. Cats coinfected with FIV were 4.8 times more likely to belong to the FeLV⁺R group. In the FeLV⁺P group, the main clinical changes were lymphoma (38.5%), anemia (24.4%), leukemia (17.9%), concomitant infections (15.4%), and feline chronic gingivostomatitis - FCGS (3.8%). In the FeLV⁺R group, the main clinical signs were anemia (45.4%), leukemia (18.2%), concomitant infections (18.2%), lymphoma (9.1%), and FCGS (9.1%). Cats in the FeLV⁺P and FeLV⁺R groups showed mainly thrombocytopenia (56.6% and 38.2%), non-regenerative anemia (32.8% and 23.5%), and lymphopenia (33.6% and 20.6%). Hemoglobin concentration, packed cell volume (PCV), platelet count, lymphocytes, and eosinophils in the FeLV⁺P and FeLV⁺R groups had lower medians than the control group (FeLV/FIV-uninfected, healthy). Erythrocyte and eosinophil counts were statistically different among the three groups, with the medians of the FeLV⁺P and FeLV⁺R groups being lower than those of the control group. In addition, the median PCV and band neutrophil counts were higher in FeLV⁺P than in FeLV⁺R. Our results show a high prevalence of FeLV, different factors associated with the course of infection, and more frequent and severe hematologic changes in progressive infections compared with regressive infections.