High seroprevalence of Francisella tularensis with low titers in Oklahoma equids from 2021-2023 suggests widespread exposure in the state's equine population.

Abstract

Objective: To determine the seroprevalence of Francisella tularensis in horses in Oklahoma and to describe factors associated with seropositivity.

Methods: In this observational study, 238 frozen equine serum samples were tested for F tularensis by microagglutination assay (MAT). Samples were collected from 36 of 77 Oklahoma counties from 2021 through 2023. Exclusion criteria included insufficient sample quantity, cross-reactivity to Brucella abortus, and county outside of Oklahoma. The Fisher exact test was used to compare the proportion of positive tests by year, river basin, and topographic region. Additionally, a field investigation was conducted on a farm with a recent case of F tularensis in a foal. Ten equids were evaluated by physical examination and tested for F tularensis by MAT. Forty ticks were collected from 8 of 10 animals and were tested for F tularensis by real-time PCR.

Results: Of the 238 serum samples in the observational study, 212 met the inclusion criteria, of which 51 of 212 (24.1%) tested positive for antibodies against F tularensis. Eight of 10 animals from the field investigation tested positive, with no clinical abnormalities. There was no association between seropositivity and any tested factor. All ticks tested negative for F tularensis by PCR.

Conclusions: A high proportion (24.1%) of Oklahoma equines carry antibodies against F tularensis. None of the investigated factors were associated with seropositivity.

Clinical relevance: Low-level positive MAT results are common in horses in Oklahoma. Additional work is needed to determine disease incidence in Oklahoma horses, the optimal MAT diagnostic threshold for horses, and implications for human health.