Potomac Horse Fever by a Vet – 2025 Update on Symptoms, Transmission & Treatment 🌊🐴
In this article
Potomac Horse Fever by a Vet – 2025 Update on Symptoms, Transmission & Treatment 🌊🐴
By Dr Duncan Houston, BVSc
Introduction
Potomac Horse Fever (PHF) is a potentially fatal illness in horses, first recognized in the 1980s along the Potomac River in Maryland. Since then, the disease has spread across at least 14 U.S. states—including Texas—and is now considered a seasonal, water-borne threat. In 2025, horse owners near creeks, rivers, or irrigated pastures should remain alert during summer and fall when risk is highest. Early diagnosis and veterinary treatment are essential for survival.
What Causes Potomac Horse Fever? 🧬
- Caused by the bacterium Neorickettsia risticii
- Originally misclassified as a tick-borne Ehrlichia—now known to be spread via aquatic insects and snails
- These organisms serve as intermediate hosts for the bacteria, which infect horses when ingested
Where & When Is It Found? 📍
- More common in areas near fresh water: rivers, creeks, ponds, and irrigated fields
- Transmission peaks in late spring through fall—often June through September
- States reporting cases: California, Texas, Kentucky, Maryland, and others
How It Spreads 🔄
- Horses are not contagious to each other
- Infection occurs through accidental ingestion of infected aquatic insects or snails—often found on hay or pasture grass
Clinical Signs to Watch For ⚠️
- Fever (often high, >102.5°F)
- Lethargy, depression
- Decreased appetite
- Colic-like symptoms
- Watery diarrhea (may progress rapidly)
- Dehydration
- Laminitis (founder) may develop in severe cases
- In pregnant mares, abortion has been reported
Diagnosis 🩺
- Serology is unreliable—false positives and negatives are common
- Confirm with:
- PCR test on feces or blood
- Culture of organism from blood (rare)
- Rule out other causes of diarrhea, including salmonella, coronavirus, or colitis
Treatment Options 💊
- IV oxytetracycline is the treatment of choice—must be administered under veterinary supervision
- Supportive care:
- IV fluids for dehydration
- NSAIDs (e.g. flunixin) to control inflammation
- Anti-endotoxemic therapy if laminitis risk is high
- Prognosis is good with early intervention—but deteriorates rapidly if diagnosis is delayed
Is There a Vaccine? 💉
- Yes—but vaccine efficacy is inconsistent
- May help reduce severity of clinical signs
- Often given in spring with EWT boosters in endemic areas
- Still must monitor vaccinated horses—vaccination is not a guarantee
Prevention in 2025 🛡️
- Stable horses away from rivers or ponds during outbreak season
- Turn off barn lights at night to reduce insect attraction
- Clean up uneaten hay that may attract insects
- Use fly sheets, repellents, and fans to deter insect contact
- Test any horse with sudden diarrhea or depression
Case Example: Summer Outbreak
- Two geldings on a riverside pasture in central Texas developed fever and diarrhea within 24 hours of each other
- One horse responded quickly to IV tetracycline and recovered
- The second developed laminitis and required long-term therapeutic shoeing
- Neither horse had been vaccinated for PHF that season
FAQs About Potomac Horse Fever
Q: Can horses spread PHF to each other?
A: No—horses must ingest infected aquatic insects. It is not directly contagious between horses.
Q: Should I vaccinate my horse?
A: Yes, in endemic areas or if you live near water—but monitor your horse regardless, as vaccines are not always protective.
Q: Can PHF cause founder?
A: Yes—laminitis is a serious complication in moderate to severe cases and requires intensive care.
Conclusion
Potomac Horse Fever is a serious and sometimes fatal disease that requires early detection and veterinary intervention. Horses near freshwater sources or in endemic areas are at higher risk—especially during summer and fall. Although vaccines are available, they aren’t foolproof, so biosecurity and rapid treatment remain your best defense. In 2025, horse owners must remain vigilant and proactive during peak PHF season.
Need help creating a PHF prevention or response plan? Contact AskAVet.com or download the app 📱 to speak directly with Dr Duncan Houston and our infectious disease team. 🩺🐴
