Equine neorickettsiosis: A global perspective of the natural habitat of the bacteria and clinical disease.

Abstract: Equine neorickettsiosis (EN) is an infectious, non-contagious systemic disease of horses caused by the closely related obligatory intracellular bacterial species Neorickettsia risticii and N. findlayensis. Clinical cases are considered endemic in multiple regions across the United States and Canada, as well as in parts of South America, including Uruguay and Brazil. Neorickettsia spp. are obligate endosymbionts of digenean trematodes, which have complex life cycles involving a molluscan first intermediate host, a wide range of invertebrate or vertebrate second intermediate hosts, and a vertebrate definitive host in which sexual reproduction occurs. Horses serve as aberrant hosts following ingestion of aquatic insects parasitized with Neorickettsia-infected trematodes. Horses develop nonspecific clinical signs such as fever, lethargy, and anorexia. Gastrointestinal manifestations including altered intestinal motility, colic, and watery diarrhea are common, and some horses experience acute laminitis and abortions in pregnant mares. Molecular detection of nucleic acids is currently the preferred diagnostic method due to its rapid turnaround time and high analytical sensitivity. Antibiotic treatment is recommended in suspected cases, particularly in endemic areas, even before confirmatory test results is obtained. Early therapy is associated with a favorable prognosis for EN-associated colitis. However, effective long-term disease prevention strategies remain limited, and the development of broadly protective vaccines capable of addressing strain diversity and providing durable immunity represents an important ongoing research priority.