# Global distribution of Neorickettsia risticii, the causative agent of potomac horse fever: a systematic review

## TL;DR

This paper reviews the global distribution of Neorickettsia risticii, the bacterium causing Potomac Horse Fever, and highlights gaps in its surveillance and understanding.

## Contribution

The first systematic review consolidating global evidence on the distribution and host range of Neorickettsia risticii.

## Key findings

- N. risticii prevalence in horses varies widely, with serological surveys showing 5.45%–26.6% and PCR positivity 0%–14.38%.
- The bacterium has been detected in invertebrates and wildlife such as bats, snails, and aquatic insects.
- Research is heavily concentrated in North America, with significant gaps in global surveillance.

## Abstract

Potomac Horse Fever (PHF), also known as equine neorickettsiosis (EN) or equine monocytic ehrlichiosis is an acute, potentially fatal infectious disease in horses caused by the monocytotropic rickettsia bacterium Neorickettsia risticii. This obligate intracellular bacterium is maintained throughout the life cycle of digenetic trematodes which utilize multiple intermediate and definitive hosts. Research on N. risticii is scattered across multiple continents, with most studies originating from North and South America. Therefore, a systematic review is needed to consolidate global evidence, clarify the geographic distribution and host range of this pathogen, and identify knowledge gaps to guide future surveillance, diagnosis, and control strategies. Therefore, we conducted the first systematic review of the distribution of Neorickettsia risticii in horses and invertebrates. A comprehensive search of five electronic databases was performed to retrieve studies that reported the occurrence of N. risticii. Retrieved records were screened and assessed following the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) 2020 guidelines. Twenty-seven eligible studies published between 1990 and 2024 were included in the qualitative synthesis of this review. The reported prevalence in horses varied widely by diagnostic method: serological surveys detected N. risticii antibodies in 5.45%–26.6% of sampled populations, whereas molecular assays reported PCR positivity ranging from 0%–14.38%. Neorickettsia risticii DNA has also been detected in a variety of invertebrate and wildlife hosts, including bats, trematodes, snails, aquatic insects, and soft ticks. These findings highlight the complex ecology of N. risticii and its presence across diverse host groups, although its prevalence remains generally low. The distribution of studies was geographically skewed towards North America, revealing significant gaps in surveillance in other regions.