Whole blood testing finds more low positive cats with feline leukemia

Abstract

Objectives Thousands of cats in the USA are newly diagnosed with feline leukemia virus (FeLV) each year, and known FeLV-infected cats are increasingly adopted from shelters. This study investigated optimal sample types to identify FeLV-infected cats and predictors of long-term survival in a cohort of FeLV-positive cats followed for up to 8 years after diagnosis. Methods Previously, 127 FeLV p27 antigen-positive cats were enrolled in a prospective study. Whole blood, plasma and serum were collected at enrollment and monthly for 6 months. All sample types were tested on SNAP FIV/FeLV Combo Test (SNAP) monthly, and results from microtiter plate ELISA (PetChek) for p27 antigen and a quantitative RT-PCR (qPCR) for proviral DNA were used for confirmation and classification of infection status (high positive, low positive or cryptic/negative). After the 6-month testing phase, cats entered a lifetime survival monitoring phase. Owner-reported status in the current study extended previous survival results by 4 years. Results Testing anticoagulated whole blood on SNAP at enrollment identified five and nine more FeLV-infected low positive cats (n = 29) than plasma or serum, respectively. Although some low positive (n = 11) cats demonstrated variable test results on SNAP with plasma and serum, others (n = 17) were SNAP positive with all three sample types and classified as low positive based on PetChek and qPCR results. After an additional 4 years of monitoring, low positive cats had not reached a median survival, with 19/29 (66%) cats still alive compared with 2/90 (2.2%) high positive cats. Conclusions and relevance Anticoagulated whole blood on SNAP was a sensitive indicator of FeLV infection relative to plasma and serum and therefore should be the preferred diagnostic sample for FeLV antigen testing. Combining the results of whole blood antigen testing, PetChek and qPCR identified cats as high positive, low positive or cryptic/negative, with high positive cats having higher risk for early mortality. Use of these diagnostic tools facilitates the management of FeLV as a chronic condition.