Exposure in Horses to Human Tick-Borne Relapsing Fever Agent Borrelia persica, Israel, 2025.

Exposure of horses in Israel to the bacterium Borrelia persica, which causes tick-borne relapsing fever in humans, was investigated, revealing that nearly 10% of horses have been exposed to the pathogen.

Background and Significance

• Tick-borne relapsing fever (TBRF) is an infectious disease primarily affecting humans, caused by Borrelia species such as Borrelia persica.
• B. persica is prevalent in western Asia, including Israel, where it is transmitted by soft ticks of the genus Ornithodoros.
• While humans are the primary affected hosts, there is limited information on the exposure and potential role of other animals, including horses, in the pathogen’s ecology.

Study Purpose

• To assess the degree to which horses in Israel are exposed to B. persica, indicating possible involvement in the transmission cycle.
• To use molecular tools to detect the pathogen’s DNA directly in horses, confirming active or recent infection.
• To perform phylogenetic analysis to understand the relationship between isolates from horses, ticks, and humans.

Methodology

• A total of 301 horses from various regions in Israel were sampled for blood.
• Serological testing was done to detect antibodies against B. persica antigens, which indicates prior or ongoing exposure.
• Polymerase chain reaction (PCR) testing was conducted on blood samples to detect B. persica DNA, signaling current infection.
• Phylogenetic analysis was carried out on positive PCR samples to compare bacterial genetic sequences from horses, humans, and ticks.

Key Findings

• Approximately 9.96% of the horses tested were seropositive for B. persica antigens, indicating significant exposure to the pathogen.
• Only 1 horse (0.33%) was PCR positive, confirming the presence of B. persica DNA and thus an active or recent infection.
• The phylogenetic analysis clustered the B. persica sequences from horses closely with those from humans and ticks, suggesting a shared transmission cycle.

Implications and Conclusions

• Horses in Israel are exposed to the relapsing fever pathogen Borrelia persica, implicating them as possible reservoirs or incidental hosts.
• The low PCR positivity relative to seropositivity suggests that active infections in horses are less common or transient.
• Phylogenetic evidence supports the hypothesis of a transmission cycle involving ticks, humans, and horses, highlighting the need for a One Health approach to control the disease.
• Understanding this tri-host relationship is important for public health and animal health strategies aimed at preventing and controlling tick-borne relapsing fever in endemic regions.