Prevalence of feline leukemia virus infection and associated diseases in a Portuguese domestic cat population: A 4.5-year cross-sectional study

In a 2019 Pan-European Study, Portugal exhibited the highest prevalence of Feline Leukemia Virus (FeLV) infection (8.8%). Following the coronavirus disease 2019 (COVID-19) pandemic, it is crucial to evaluate how the prevalence of FeLV has evolved. FeLV infection is associated with the highest morbidity rates, primarily due to the increased incidence of diseases that compromise the health of cat populations, which varies according to the lifestyle and background of the cats studied. This study aimed (1) to estimate the prevalence and temporal trends of FeLV and FIV infections among cats presented to a university veterinary hospital in the Lisbon metropolitan area, and (2) to evaluate the clinical associations between FeLV infection, health status, and FeLV-related conditions in cats. Conducted over 4.5 years, from January 2019 to July 2023, this cross-sectional study took place at a teaching hospital and involved 1,124 cats that were tested serologically and/or by qPCR and RT-qPCR for FeLV. Information was gathered on the intrinsic and extrinsic characteristics of the cats, their health status, and any related diseases. The overall prevalence of FeLV was found to be 11.3% (95% CI: 9.5%-13.3%), with 1.8% (95% CI: 1.1%-2.7%) of cats co-infected with FIV, and it peaked in 2020 at 14.1% (95% CI: 7.5%-23.4%), with 2.4% (95% CI: 0.03%-8.2%) co-infected with FIV. Over the 4.5-year period, an increasing number of cats were tested, and more quantification of proviral and viral loads was performed. This indicated a more progressive course in 47.0% (31/66), of sick FeLV-infected cats, who exhibited a higher incidence of FeLV-related diseases. Although there was no significant difference in the average age between positive and negative cats, FeLV-positive cats demonstrated a higher rate of sickness (74.8%, n = 95). To the best of the authors' knowledge, this study represents the largest cross-sectional investigation of FeLV infection prevalence and its health implications conducted in Portugal. Overall, the available data suggest a possible increase in FeLV prevalence in Portugal, concurrent with a declining vaccination rate from 14.2% to 5.0%. The results also highlight notable differences in clinical status between progressive and regressive disease courses, reinforcing the necessity of staging the course of infection at diagnosis to ensure an informed medical approach and realistic prognosis. Efforts should focus on improving vaccination and screening activities, promoting neutering of indoor and outdoor cats, and isolating infected cats.