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Veterinary research communications2026; 50(3); 205; doi: 10.1007/s11259-026-11146-y

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

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.
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Publication Date: 2026-03-18 PubMed ID: 41848949PubMed Central: PMC12999721DOI: 10.1007/s11259-026-11146-yGoogle Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This research systematically reviews the global distribution and host range of Neorickettsia risticii, the bacterium causing Potomac Horse Fever, synthesizing data from multiple studies to clarify its prevalence and ecological complexity across various species and regions.

Background

  • Potomac Horse Fever (PHF): An acute infectious disease in horses caused by Neorickettsia risticii, also called equine neorickettsiosis or equine monocytic ehrlichiosis.
  • Pathogen Characteristics: Neorickettsia risticii is a monocytotropic rickettsia, meaning it infects monocytes, and exists as an obligate intracellular bacterium, meaning it lives inside host cells.
  • Life Cycle: This bacterium is maintained throughout the life cycle of digenetic trematodes (parasitic flatworms), which involve multiple intermediate and definitive hosts.
  • Scientific Gap: Research on N. risticii is dispersed, predominantly from North and South America, with little consolidated information on its global distribution.

Objectives

  • Conduct a systematic review to consolidate global evidence regarding the distribution of Neorickettsia risticii.
  • Determine its geographic spread and host range, particularly in horses and relevant invertebrates.
  • Identify gaps in knowledge to guide improvements in disease surveillance, diagnosis, and control.

Methodology

  • Performed a comprehensive search of five electronic databases to collect studies reporting the occurrence of N. risticii.
  • Applied PRISMA 2020 guidelines to screen, select, and assess retrieved studies rigorously to ensure quality and consistency.
  • Included 27 eligible studies published from 1990 to 2024 for qualitative synthesis.

Key Findings – Prevalence in Horses

  • Prevalence varied significantly depending on diagnostic technique:
    • Serological surveys: Detected antibodies against N. risticii in 5.45% to 26.6% of horses tested, indicating exposure to the bacterium.
    • Molecular assays (PCR): Detected presence of N. risticii DNA in 0% to 14.38% of tested populations, indicating current or recent infection.
  • The wide range in detection rates may be due to differences in testing methods, timing of sample collection, geographic locations, or horse populations.

Key Findings – Detection in Other Hosts

  • N. risticii DNA was also found in a variety of invertebrates and wildlife species:
    • Bats
    • Trematodes (the parasitic flatworms involved in lifecycle)
    • Snails
    • Aquatic insects
    • Soft ticks
  • This diversity highlights the complex ecology and transmission dynamics of the bacterium.
  • Despite its broad host range, the overall prevalence in these species was generally low.

Geographical Distribution and Surveillance Gaps

  • The majority of studies originated from North America, skewing the understanding of global distribution.
  • Noticeable gaps exist in research and surveillance in other continents, such as Asia, Africa, and Europe.
  • This uneven distribution of data limits comprehensive understanding of the pathogen’s true global impact and potential risk areas.

Implications and Future Directions

  • Enhanced global surveillance is needed to fill geographic gaps and better track N. risticii distribution beyond Americas.
  • Improved diagnostic standardization across regions and host species could lead to more consistent prevalence estimates.
  • Understanding transmission ecology among various hosts, including wildlife and invertebrates, can inform targeted control and prevention strategies.
  • The systematic consolidation of data supports veterinary health professionals and researchers in prioritizing resource allocation and research efforts to mitigate PHF impacts worldwide.

Cite This Article

APA
Onyiche TE, Peng TL. (2026). Global distribution of Neorickettsia risticii, the causative agent of potomac horse fever: a systematic review. Vet Res Commun, 50(3), 205. https://doi.org/10.1007/s11259-026-11146-y

Publication

Veterinary research communications
ISSN: 1573-7446
NlmUniqueID: 8100520
Country: Switzerland
Language: English
Volume: 50
Issue: 3
PII: 205

Researcher Affiliations

Onyiche, ThankGod Emmanuel
  • Departments of Veterinary Parasitology and Entomology, University of Maiduguri, P.M.B. 1069, Maiduguri, Nigeria. Thankgod.Onyiche@nwu.ac.za.
  • Unit for Environmental Sciences and Management, North-West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom, 2520, South Africa. Thankgod.Onyiche@nwu.ac.za.
Peng, Tan Li
  • Faculty of Veterinary Medicine, Universiti Malaysia Kelantan, Kota Bharu, Pengkalan Chape, 16100, Malaysia. li.peng@umk.edu.my.

Conflict of Interest Statement

Declarations. Ethical approval: Approval was not required. Competing interests: The authors declare no competing interests.

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