Real Rider Resource
Potomac horse fever is a serious, non-contagious infectious disease that can cause fever, depression, appetite loss, diarrhea, colic, altered gut sounds, swelling and laminitis. A horse with fever, diarrhea or new hoof pain needs a prompt veterinary call. Do not wait for every sign to appear.
Reviewed August 28, 2026.
What Potomac horse fever is
The American Association of Equine Practitioners describes Potomac horse fever as a non-contagious infectious disease associated with Neorickettsia risticii and Neorickettsia findlayensis. Cases are most often recognized in summer and fall, although weather and regional conditions can shift the season.
The name is misleading if it makes riders think the disease belongs only near the Potomac River. It has been identified in multiple parts of the United States and Canada. Local case history, freshwater environments, aquatic-insect activity and season matter more than the name.
Signs riders may notice
Signs vary from horse to horse. Diarrhea may be absent, mild or severe, and fever can appear before obvious colitis. Watch the whole horse rather than waiting for one textbook pattern.
General and digestive changes
- Fever, depression or unusual quietness
- Reduced appetite or refusal to eat
- Reduced or altered gut sounds
- Mild to moderate colic
- Diarrhea ranging from absent to severe
- Signs of dehydration or rapid decline
Complications and additional signs
- Laminitis, even when diarrhea is not obvious
- Swelling along the limbs or underside of the body
- Toxic-looking or abnormal mucous membranes
- Abortion risk in pregnant mares
- Reluctance to move, turn or bear weight normally
These signs overlap with other infectious and noninfectious problems. A checklist cannot diagnose Potomac horse fever. Veterinary assessment and testing determine the next steps.
The first-hour response
- Stop work and call. Tell the veterinarian the horse's temperature, appetite, manure changes, colic signs, movement, hoof comfort and when the change began.
- Pause movement. Do not haul the horse unless your veterinarian directs it. Use a quiet area with safe footing and keep unnecessary people away.
- Separate equipment. Potomac horse fever itself is not generally contagious, but fever and diarrhea have other possible causes. Use dedicated buckets, tools and handling routes until the diagnosis is clearer.
- Ask about laminitis prevention immediately. Do not wait for obvious hoof pain before discussing the feet with the veterinarian.
- Do not improvise treatment. Avoid forced water, feed, oral products, leftover medication or home remedies. Follow the veterinarian's instructions for that horse.
Why water, lights and aquatic insects matter
The organism cycles through aquatic hosts and insects. Horses are believed to become infected when they accidentally ingest infected aquatic insects, often around freshwater environments and insect hatches. Risk can rise near streams, rivers, ponds, canals and heavily irrigated areas, especially when lights attract insects toward stalls, feed or water.
No single barn step guarantees prevention, but practical exposure checks include:
- Reviewing streams, ponds, canals, drainage areas and irrigation ditches near horse housing.
- Keeping feed and water containers clear of dead insects and organic debris.
- Cleaning waterers consistently and checking them after heavy insect activity.
- Turning off or relocating unnecessary exterior lights near stalls, feed and water when it can be done safely.
- Using screens or physical barriers where practical and maintaining ordinary manure and barn hygiene.
Diagnosis and veterinary care
Your veterinarian may consider the horse's signs, season, location and water or insect exposure, then use laboratory testing such as PCR on blood or fecal samples. Timing affects interpretation, and other causes of fever, colitis and laminitis must remain in the differential diagnosis.
Veterinary treatment may include an appropriate antimicrobial and individualized supportive care for dehydration, inflammation, endotoxemia and laminitis risk. This is not a condition to manage with a retail product or a copied barn protocol. Early veterinary involvement matters, but no article can predict an individual horse's response or outcome.
Where vaccination fits
Potomac horse fever vaccination is risk-based rather than a universal core vaccine. AAEP notes that vaccination may not be fully protective, partly because field strains vary and protection can be incomplete or short-lived.
Build the schedule with the veterinarian who knows the horse and local history. Geography, freshwater exposure, prior cases, travel, age, pregnancy and season can change the recommendation. Do not copy another barn's schedule or assume a vaccinated horse cannot become ill.
What to do when a case is reported nearby
- Confirm the report through EDCC, the state veterinarian or another official source.
- Ask your veterinarian whether the location changes your horse's risk or vaccine plan.
- Review water sources, irrigation, aquatic-insect activity and lights near feed or water.
- Make sure every regular handler knows where the thermometer is and which signs require a call.
- Watch horses closely without assuming a nearby report means your barn was exposed.
Frequently asked questions
Is Potomac horse fever contagious from horse to horse?
Potomac horse fever is generally classified as non-contagious. However, fever and diarrhea can have contagious causes, so separate the sick horse, equipment and traffic until your veterinarian identifies the problem and gives biosecurity instructions.
Can a vaccinated horse still get Potomac horse fever?
Yes. Vaccination may reduce risk but is not fully protective against every field strain. Ask your veterinarian whether local history, water exposure and season justify a risk-based schedule for your horse.
Does Potomac horse fever always cause diarrhea?
No. AAEP guidance describes diarrhea as variable from absent to severe. Fever, depression, appetite loss, altered gut sounds, colic, swelling or laminitis may appear with or without obvious diarrhea.
Which Potomac horse fever signs require an urgent veterinary call?
Call promptly for fever, depression, reduced appetite, diarrhea, colic, reduced gut sounds, limb or belly swelling, a stronger digital pulse, hoof heat, reluctance to move or a rocked-back stance. Do not wait for every sign to appear.
Should I give electrolytes or barn products before the veterinarian arrives?
Do not force feed, water or oral products into a sick horse or improvise a treatment plan. Call your veterinarian, describe the signs and follow the instructions given for that horse. No Draw It Out product treats Potomac horse fever.
Primary references
This article provides general horse-care education and is not a substitute for veterinary diagnosis, treatment or emergency advice. Contact your veterinarian promptly for fever, diarrhea, colic, hoof pain, reluctance to move, rapid decline or any concerning change.
