Potomac Horse Fever 2025: Vet‑Approved Guide to Diagnosis, Treatment & Prevention

By Dr Duncan Houston BVSc 🩺

Potomac Horse Fever (PHF) is an acute, seasonal disease caused by the bacterium Neorickettsia risticii. Transmission occurs when horses accidentally ingest aquatic insects carrying infected trematodes, typically in spring through autumn. PHF leads to fever, colitis, diarrhea, and laminitis, and can be fatal in up to 30% of severe cases, especially when treatment is delayed :contentReference[oaicite:3]{index=3}.

1. Cause & Transmission 🌿

• PHF is caused by N. risticii, an intracellular Gram-negative bacterium, endemic in North America and parts of Canada and Europe :contentReference[oaicite:4]{index=4}.
• Its lifecycle involves aquatic snails, flukes, and insects like mayflies and caddisflies. Infection usually occurs when horses ingest adult infected insects or contaminated water/feed :contentReference[oaicite:5]{index=5}.
• It's seasonal (late spring to early autumn), and often linked to proximity (<5 miles) to water bodies :contentReference[oaicite:6]{index=6}.
• PHF is not directly contagious between horses, though multiple horses can be infected from shared exposure :contentReference[oaicite:7]{index=7}.
• Rare transmission can occur via blood transfusion or from mare to fetus, possibly causing abortion :contentReference[oaicite:8]{index=8}.

2. Clinical Signs & Risk Factors

Look for these indicators:

• 🏵️ Fever (103–107°F), depression, anorexia, mild colic, and decreased gut sounds :contentReference[oaicite:9]{index=9}.
• Profuse watery diarrhea within 24–48 hours :contentReference[oaicite:10]{index=10}.
• Edema (limbs, ventral), electrolyte imbalances, and dehydration :contentReference[oaicite:11]{index=11}.
• High risk of acute laminitis—20–40% of cases—due to endotoxemia :contentReference[oaicite:12]{index=12}.
• Pregnant mares may abort in late gestation :contentReference[oaicite:13]{index=13}.
• Mortality rates range from 5–30%, influenced by laminitis and treatment timing :contentReference[oaicite:14]{index=14}.

3. Diagnosis & Testing 🧪

• Diagnosis is based on seasonality, clinical signs, and testing :contentReference[oaicite:15]{index=15}.
• PCR on blood and feces is the most reliable early test for bacterial DNA :contentReference[oaicite:16]{index=16}.
• Serology (IFA) is unreliable due to false positives or vaccine interference :contentReference[oaicite:17]{index=17}.
• CBC/chemistry may reveal elevated PCV, electrolyte loss, inflammatory markers :contentReference[oaicite:18]{index=18}.
• Rule out salmonella, clostridia, cyathostomes, and other causes of colitis :contentReference[oaicite:19]{index=19}.

4. Treatment Protocol 💊

• First-line therapy is IV oxytetracycline (6.6 mg/kg q12h for 5–7 days). Rapid response often seen within 12–24h :contentReference[oaicite:20]{index=20}.
• Alternative antibiotics (oral doxycycline or other tetracyclines) can be used if IV not available :contentReference[oaicite:21]{index=21}.
• Supportive care is essential: IV fluids, NSAIDs (flunixin), electrolyte replenishment :contentReference[oaicite:22]{index=22}.
• Laminits prevention: immediate cryotherapy (ice boots) and laminitis management :contentReference[oaicite:23]{index=23}.
• Endotoxin binding (e.g., polymyxin B, Biosponge) and plasma transfusion may be needed in critically ill horses :contentReference[oaicite:24]{index=24}.

5. Prognosis & Recovery

• With prompt treatment, recovery is typical within 3 days; full resolution in 1–2 weeks :contentReference[oaicite:25]{index=25}.
• Laminitis associated with PHF tends to be severe and may be refractory :contentReference[oaicite:26]{index=26}.
• Relapse can occur, requiring a second antibiotic course :contentReference[oaicite:27]{index=27}.
• Fatality is most often due to untreated endotoxemia or laminitis—from 5–30% mortality :contentReference[oaicite:28]{index=28}.

6. Prevention & Vaccination 🛡️

• Inactivated vaccines are available; efficacy is variable due to strain diversity, offering 50–78% protection during controlled trials, but lower in the field :contentReference[oaicite:29]{index=29}.
• Vaccination schedule: 2-dose series in spring, booster every 3–4 months in endemic areas; separate from rabies for better response :contentReference[oaicite:30]{index=30}.
• Prevent insect exposure: turn off barn lights, avoid turnout near water, cover feed/water, use screens and repellents :contentReference[oaicite:31]{index=31}.
• Ensure paddocks are managed to minimize insect habitats and aquatic insect proliferation :contentReference[oaicite:32]{index=32}.

7. Farm & Barn Strategies

• Limit access to riparian areas; rotate pastures away from water bodies :contentReference[oaicite:33]{index=33}.
• Install barn curtains or screens to prevent insect entry :contentReference[oaicite:34]{index=34}.
• Cover feed buckets and keep water sources clean :contentReference[oaicite:35]{index=35}.
• Use misters during emergence of aquatic insects :contentReference[oaicite:36]{index=36}.

8. Monitoring & Biosecurity

While PHF is not contagious from horse to horse, quarantine isolated horses with diarrhea until other causes are ruled out :contentReference[oaicite:37]{index=37}. Monitor herd temperatures daily during seasons of risk.

9. Special Cases: Pregnant Mares & Foals

• Infection during gestation often leads to late-term abortion and fetal lesions :contentReference[oaicite:38]{index=38}.
• Mares in endemic areas should be vaccinated and closely monitored during summer/fall season.

10. Summary Table

11. Ask A Vet Support 📲

Ask A Vet provides:

• 📞 Telehealth consultations for rapid triage and treatment planning
• 📋 Lab test coordination for PCR and monitoring
• 🌱 Vaccination scheduling and risk assessment
• 🧠 Insect control and pasture management guidance
• 🚺 Reproductive advice for pregnant mares in endemic regions

Final Thoughts 🐴

Potomac Horse Fever remains a seasonal threat but is manageable with swift recognition, appropriate antibiotic therapy, and proactive preventative measures. In 2025, integrating veterinary insight, targeted vaccination, and environmental controls ensures your horse stays healthy through summer and fall. Act fast—treatment within 24 hours is critical to prevent life-threatening complications.

Concerned about PHF risk or want a herd-preparedness plan? Download the Ask A Vet app or visit AskAVet.com to connect with expert veterinary support tailored to your region and farm. Stay proactive, stay safe. 🩺🐎

Disclaimer: This article is educational and not a substitute for individualized veterinary care. Always consult your veterinarian for specific advice.