Prevalence and genetic characterization of feline leukemia virus in portuguese stray cats

With its large stray population, Portugal had the highest prevalence of the Feline Leukemia Virus (FeLV) infection in Europe. Progressive infection is still high, especially in southern Europe. In addition to the viral load, the subgroup is a determining factor in the development of the various syndromes associated with this disease, which can be fatal, justifying the morbidity and mortality of this retrovirus. The aim of the study was to determine the prevalence of FeLV, the viral load and to identify the subgroup of circulating virus in stray cats in the Lisbon metropolitan area. This prospective study was conducted at the Veterinary Teaching Hospital of the FMV-ULusófona, from November 2022 to January 2023. Blood samples from 129 stray cats from the Lisbon metropolitan area were included. All samples were tested for p27 antigen detection and proviral DNA by qPCR. For the detection of FeLV subgroups (A, B and C) nested PCR was performed.

The results showed that at the time of enrollment, 8.5% (n = 11/129) of the cats tested positive for FeLV using p27 antigen and qPCR for proviral DNA. Considering the proviral DNA load, 90.9% (n = 10/11) of the FeLV infections were classified as "likely progressive infection" (> 4.0 × 10⁵ copies/mL). In the remaining animal, the FeLV infection was classified as "likely regressive infection" (≤ 4.0 × 10⁵ copies/mL). In this study, the most common FeLV subgroup was a combination of AB (54,5%, n = 6/11), followed by FeLV-A (45,5%, n = 5/11). FeLV-C subgroup was not detected.

To the best of our knowledge, this is the first study to research FeLV subgroups in Portugal. The prevalence of FeLV in this group was consistent with our expectations. However, the molecular characterization differed significantly from that observed in other countries. These results suggest that stray cats may not be the main epidemiological concern.