Potomac horse fever

Infection of Neorickettsia risticii (N. risticii) -Caitlin Logan 

Reservoir 

• aquatic insects such as mayflies, caddisflies, damselflies, dragonflies 
• freshwater or operculate snails

 Portal of entry:

• consuming infected aquatic insects
• exposure to freshwater or operculate snails 

Portal of exit

• incubation period 1-3weeks
• horses have not been documented to remain infected long-term
•  no zoonotic transmissions, non-contagious through natural horse to horse contact
• Mortality rate 5-30%

 Susceptible hosts:

• First Intermediate host – snails
• Second intermediate host- aquatic insects
• Definitive hosts- bats, birds
• Accidental Hosts- equines

Mode of transmission:

Consuming infected aquatic insects 

• Equine eating foliage near water and accidentally eating an infected mayfly on the leaf.
• Equine eating hay near water trough and accidentally consuming dead affected caddisfly.

Whole body blood transfusions from infected donor

• Equine being treated for anemia, accidentally given donor blood infected with N. risticii.

Transplacental

• Pregnant equine infected with N. risticii aborts fetus. Necropsy of fetus reveals aborted fetus lesions with hepatic discoloration and increased content within the small and large intestines.

Exposure to freshwater or operculate snails

•  Equine eating foliage near freshwater accidentally consumes infected snail.

How we can control the transmission of Potomac Horse Fever? 

Potomac Horse Fever (PHF) does not affect other animals or humans. PHF has a vaccine available, but the vaccine does not fully protective against the disease, however it has proven highly effecting at reducing the severity of the clinical signs and recommended in areas with increased risk for PHF. Horses with diarrhea should be quarantined until contagious diseases are ruled out. Minimize insect ingestion in stabled horses by turning barn lights off at night, which attracts insects.

Resources: 

• “Potomac Horse Fever (PHF).” Cornell University College of Veterinary Medicine, www.vet.cornell.edu/animal-health-diagnostic-center/about/news/potomac-horse-fever-phf. Accessed 30 Sept. 2024.

• “Potomac Horse Fever - Potomac Horse Fever.” Merck Veterinary Manual, www.merckvetmanual.com/digestive-system/infectious-diarrheal-diseases-in-horses/potomac-horse-fever#Diagnosis_v3263548. Accessed 30 Sept. 2024.

• “Potomac Horse Fever.” AAEP, 26 Sept. 2024, aaep.org/resource/potomac-horse-fever/#:~:text=Potomac%20Horse%20Fever%20(PHF)%20is,breed%20or%20sex%20of%20horse. Accessed 30 Sept. 2024.

• “Potomac Horse Fever.” Potomac Horse Fever Fact Sheet , equinediseasecc.org/Content/ContentDocs/Potomac%20Horse%20Fever%20Factsheet_FINAL_2021.pdf. Accessed 30 Sept. 2024.

• Shaw, Sarah D, and Henry Stämpfli. “Diagnosis and Treatment of Undifferentiated and Infectious Acute Diarrhea in the Adult Horse.” The Veterinary Clinics of North America. Equine Practice, U.S. National Library of Medicine, Apr. 2018, www.ncbi.nlm.nih.gov/pmc/articles/PMC7134835/. Accessed 30 Sept. 2024.