Detection and phylogenetic relationship of Neorickettsia risticii in horses from Southern Rio Grande do Sul, Brazil.

Abstract

BACKGROUND: Neorickettsia risticii, an obligate intracellular bacterium, is the causative agent of Potomac Horse Fever (PHF) - a systemic, acute, and potentially fatal diarrheal disease. This pathogen is an endosymbiont of digenetic trematodes, with freshwater snails playing a role in its biological cycle. The disease exhibits endemic and seasonal patterns, primarily affecting horses in wetlands during warmer months, and has been documented in the United States, Canada, Europe, and South America. Previous studies confirmed the pathogen's presence in herds from southern Brazil via serology and PCR; however, its genetic sequencing and phylogenetic classification remained unresolved. OBJECTIVE: This study aimed to perform molecular diagnosis, genetic sequencing, and phylogenetic analysis of N. risticii in naturally infected symptomatic horses, as well as to screen clinically healthy animals and Heleobia spp. snails from endemic farms. METHODS: The agent's DNA was investigated in blood and snail samples using nested PCR. RESULTS: Symptomatic animals yielded a 529-bp product (consistent with N. risticii), whereas clinically healthy horses and snails showed no amplification. We report the first genetic sequences of N. risticii from southern Brazil, along with a phylogenetic study of the pathogen. CONCLUSION: Sequence comparison and phylogenetic analysis revealed high similarity among the obtained samples and other sequences from South America. Our findings confirm the agent's circulation in the region and, by enriching the genetic database, encourage further studies to elucidate the etiological agent's epidemiology.