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

The study assessed the prevalence of Francisella tularensis, a bacterium causing a disease called tularemia, in horses of Oklahoma by testing serum samples. It found widespread exposure in the equine population, although no particular factors influencing seropositivity were detected.

Study Design and Methodology

• The research is an observational study involving the testing of 238 frozen equine serum samples for Francisella tularensis using a microagglutination assay (MAT). This assay is used to measure antibodies in the blood that could indicate the presence of the bacterium.
• Samples were collected across 36 counties in Oklahoma from 2021 to 2023.
• Exclusions were made for samples with insufficient quantity, those showing cross-reactivity to Brucella abortus (a bacterium causing a different disease), and those sourced from outside Oklahoma.
• The Fisher exact test, a statistical tool used for small sample sizes, was used to compare positive test proportions by various factors such as year of sampling, river basin, and topographic region.
• A complementary field investigation was conducted on a farm where a foal had recently tested positive for F tularensis. Ten equids (a family that includes horses and related species) were physically examined and tested for F tularensis. Ticks, possible disease vectors, were collected from these animals and also tested for the bacterium.

Key Findings

• Out of the 212 eligible serum samples, 51 (24.1%) tested positive for antibodies against F tularensis, indicating that a significant proportion of Oklahoma equines have been exposed to the bacterium.
• In addition, 8 out of 10 animals from the field investigation tested positive. However, these animals did not display any clinical abnormalities insinuating the disease may not always result in illness in equids.
• No associations were found between seropositivity and any of the tested factors (year, river basin, and topographic region).
• Interestingly, all ticks tested were negative for the bacterium, implying that they may not play a significant role in its transmission among equids in Oklahoma.
• Many horses in Oklahoma yielded low-level positive results from the MAT, which although indicates exposure to F tularensis, does not confirm active disease.

Conclusions and Further Research

• From the sample studied, 24.1% of Oklahoma equines were found to carry antibodies against F tularensis, suggesting extensive exposure to the pathogen in the state’s equine population.
• No associations between tested variables and the presence of antibodies were discovered, suggesting the need for further investigation to identify the risk factors and transmission dynamics.
• The study also highlights the need to establish the ideal diagnostic threshold for horses and to explore the epidemiological implications for human health as tularemia is a zoonotic disease, i.e., transmissible from animals to humans.