Potomac Horse Fever: Symptoms, Vital Sign Changes, and When to Act

Potomac Horse Fever (PHF) can strike suddenly and escalate quickly. Although the disease appears most often in late summer in areas teeming with aquatic insects, every horse owner should know the earliest warning signs. Acting fast can turn a potential emergency into a straightforward veterinary visit instead of a laminitis crisis. See how those early signs unfold in real barns by reading our companion article, Potomac Horse Fever Case Studies: What Real Scenarios Reveal About Early Detection and Recovery.

In this post you will learn what Potomac Horse Fever is, which changes in vital signs signal danger, and precisely when to call your veterinarian so you can move from noticing something off to treatment in progress before complications develop.

What Is Potomac Horse Fever?

Potomac Horse Fever (PHF) isn’t something you hear about every day, but when it strikes, it can turn into an emergency fast. Originally identified near the Potomac River in the 1980s, this disease is now found across many parts of North America, especially during the warmer months.

PHF is caused by a tricky bacterium called Neorickettsia risticii, which lives inside aquatic insects and flukes (tiny parasitic flatworms). Horses get infected when they accidentally ingest these insects or their larvae, often by drinking from contaminated water sources or eating feed exposed to bugs like mayflies, caddisflies, or dragonflies.

What makes PHF difficult is that it doesn’t spread directly from horse to horse. Instead, it hides in the environment particularly in areas near rivers, ponds, and irrigated pastures. That means any horse with outdoor access to water or open feed can be at risk, especially in late spring through early fall.

The biggest concern with Potomac Horse Fever isn’t just the diarrhea or fever, it’s the potential for laminitis (inflammation of the hoof). Laminitis can develop even after other symptoms start improving, which is why this illness requires careful monitoring and fast action.

In short: PHF is a gastrointestinal and systemic infection that can go from mild to life-threatening in less than 48 hours.

Common Symptoms of Potomac Fever in Horses

Potomac Horse Fever can sneak up on even the most observant owner. A horse may look only slightly “off” at breakfast—leaving a bit of grain or standing quietly—and by mid-afternoon can be running a high fever or showing watery diarrhea. Recognising these early hints is critical; prompt treatment keeps PHF from snowballing into dehydration and laminitis.

Below are the signs you’re most likely to see. Use them alongside your daily vital-sign log so you can act while the disease is still in its earliest stage.

Symptom	What to Watch For	Why It Matters	Learn More
Fever	Rectal temperature jumps above 103 °F (39.4 °C), sometimes topping 106 °F	Often the very first clue; nothing else may look wrong yet	The Horse’s Vital Signs
Lethargy & inappetence	Dull attitude, slow to greet feed, leaves hay behind	Combines with fever to shout “infection”	How to Tell if Your Horse Is Sick
Watery diarrhea	Loose manure that can progress to profuse, watery output within 12–24 h of the fever spike	Drives fluid and electrolyte loss that worsens colic risk	—
Colic-like signs	Pawing, looking at the flank, mild rolling	Early gut pain often precedes diarrhea; can mislead diagnosis	Case timeline in Potomac Horse Fever Case Studies
Dehydration	Tacky gums, skin-tent > 2 s, dark urine	Rapid fluid loss can turn fever into a crisis	How to Tell if a Horse Is Dehydrated
Early laminitis	Shifting weight, reluctance to move, increased digital pulses—even after fever improves	The most feared complication; can appear 24–72 h after other signs	Comprehensive Guide to Equine Lameness

Not every horse shows every symptom, and they may arrive out of order. A single high fever paired with subtle appetite change is reason enough to isolate the horse, start temperature checks every two hours, and call your veterinarian. Early intervention keeps survival rates high and hoof complications low, often turning a potential emergency into a manageable seven- to ten-day recovery.

How Potomac Fever Affects Vital Signs

One of the most reliable ways to spot Potomac Horse Fever early is by tracking vital signs. Even before diarrhea shows up, a horse’s temperature and heart rate may already be elevated, and these small changes can tell you a lot about what’s going on internally.

Here’s what you need to know:

Temperature

A sudden spike in temperature is usually the earliest warning that Potomac Horse Fever is brewing. Rectal readings often climb above 103 °F and can push past 106 °F. Because a horse may simply look quiet or “a bit off,” skipping routine checks is an easy way to miss the danger window. Make twice-daily readings part of your barn habit with the step-by-step method in The Horse’s Vital Signs.

If your horse develops an unexplained fever—no recent vaccine, no strenuous workout, no heat stress—put PHF on your radar right away, especially during summer and fall. For real-world timelines showing how fast that temperature can rise, see the Potomac Horse Fever case studies.

Heart Rate

A resting heart rate for an adult horse typically falls between 28 and 44 beats per minute. In Potomac Horse Fever cases, that number can jump into the 60s or higher even while the horse is standing quietly. The elevation reflects the combined stress of fever and systemic inflammation, signalling that the body is working overtime. Brush up on pulse-check technique and normal ranges with Average Heart Rate for a Horse: What’s Normal and How to Check It so you can catch this red flag before other symptoms appear.

Respiratory Rate

Breathing may also become faster or shallower. Normal respiratory rate is 8–16 breaths per minute. A horse with PHF might breathe quicker due to fever, discomfort, or early dehydration. If you’re unsure what normal breathing looks or feels like, this is a great time to revisit the basics. Our guide on checking a horse’s vital signs breaks down what to look for, and how to track these numbers confidently before a health emergency arises.

Gut Sounds

Listen carefully: gut sounds often go quiet with PHF. That’s a major warning sign. A silent belly usually means gut motility has slowed or stopped, a serious issue when paired with diarrhea or colic-like behavior.

Hydration Indicators

Watch the gums, skin, and overall demeanor. Sticky gums, dry mouth, or skin tenting longer than 2 seconds all suggest dehydration. Horses with Potomac Fever lose fluids rapidly, especially once diarrhea starts.

When to Call the Vet

With Potomac Horse Fever, waiting a day or two to see if your horse “snaps out of it” is a dangerous game. The disease can go from subtle to severe overnight, and early intervention saves lives.

Here’s when you should pick up the phone:

Fever Over 102°F

A persistent fever, especially when combined with a change in behavior or gut function, is enough reason to call. Don’t wait for diarrhea to appear — by then, the infection is already advanced.

Heart Rate Over 60 bpm at Rest

This indicates serious systemic stress. If your horse hasn’t been working and their heart rate is that high, something’s not right internally.

Diarrhea or Reduced Gut Sounds

Once diarrhea starts, things can spiral fast. Horses become dehydrated, electrolytes drop, and the risk of laminitis increases. If gut sounds are absent in any quadrant or manure is suddenly loose, act quickly.

Signs of Laminitis

Even after the fever breaks, keep checking hooves. If your horse shifts weight, walks stiffly, or seems sore on the front feet, it could be early laminitis. Call your vet immediately if you see these signs.

What Your Vet Might Do

Once on-site, your vet may:

• Take a blood sample for PCR testing

• Start IV fluids to prevent dehydration

• Administer oxytetracycline (an antibiotic shown to be effective against PHF)

• Recommend anti-inflammatory meds to protect against laminitis

Recovery often depends on how soon treatment starts which is why recognizing the early changes in vitals is so crucial.

Prevention Tips

While Potomac Horse Fever can’t be completely eliminated, you can take meaningful steps to reduce the risk, especially during peak season (late spring through early fall). Prevention isn’t just about vaccines; it’s about daily habits and environmental awareness.

1. Vaccination

There is a vaccine for PHF, and while it isn’t 100% effective, it can lessen the severity of symptoms if your horse does get infected. It’s often given in spring before mayflies emerge and sometimes boosted in late summer depending on your region. Review how it fits into your broader schedule with our guide to the 5-Way Equine Vaccine, then ask your vet about timing for your region.

2. Manage Exposure to Insects

Since horses get PHF by ingesting aquatic insects, your prevention strategy should include minimizing contact with those bugs.

• Keep feed and water covered, especially at night when insects are most active.

• Don’t feed horses near ponds, creeks, or irrigation ditches.

• Turn off barn lights at night to avoid attracting flying insects like mayflies and caddisflies.

Daily observation matters too; our checklist How to Tell if Your Horse Is Sick can help you catch subtle changes before they snowball.

3. Monitor Water Sources

Provide fresh, bucket-or automatic-waterer supply and discourage drinking from natural standing water where insects breed. Clean troughs often to prevent bug buildup, and back this up with simple hydration checks from How to Tell if a Horse Is Dehydrated so you know your prevention plan is working.

Keep Learning: Related Guides

• A Treatise on the Horse and His Diseases: An in-depth manual covering a wide range of equine health issues, written for horse owners and caretakers.

• Practice of Equine Medicine: A detailed look at diagnosing and treating common equine illnesses, useful for both students and experienced handlers.

• Browse All Books: Explore our full library of equine textbooks, historical guides, and veterinary references to support your learning.

  
  

Final Thoughts

Potomac Horse Fever is one of those conditions that doesn’t give you much time to hesitate. What starts as a quiet fever can quickly escalate into dehydration, diarrhea, and even laminitis. But with the right knowledge — and a habit of checking vital signs — you’re in a much better position to catch it early and act fast.

If there’s one takeaway, it’s this: trust the small signs. A slightly elevated heart rate, a dip in appetite, or subtle behavior changes might be all the warning you get. That’s why understanding your horse’s body — not just on the surface, but inside and out — is such a critical part of modern horse care.

Want to go beyond the basics? Our Equine Anatomy Certification helps horse owners, students, and equine professionals build real confidence in reading the body systems behind these signs,and responding with clarity, not guesswork.

Stay alert. Stay informed. And keep learning. Your horse deserves it.

Potomac Horse Fever F.A.Q:

What time of year is Potomac Horse Fever most common?

Potomac Horse Fever tends to spike in late spring through early fall, especially in July and August when aquatic insects like mayflies and caddisflies are most active. If your horse has pasture access near water, stay extra vigilant during these months.

Learn more about tracking vital signs seasonally.

Can Potomac Horse Fever cause long-term damage?

Yes. Even after surviving the initial infection, some horses experience lasting issues, particularly from laminitis triggered during or after PHF. That’s why early detection and supportive care are so important — to prevent complications that could impact long-term soundness.

To understand how PHF affects internal systems, explore our Equine Anatomy Certification.

How long is the recovery time after treatment?

Most horses recover from PHF within 7 to 14 days with prompt veterinary care. However, recovery from secondary issues like dehydration, laminitis, or gut imbalances may take longer. Daily monitoring of temperature, hydration, and mobility is key during this period.

Can PHF occur even in horses that are vaccinated?

Yes. The PHF vaccine helps reduce severity, but it doesn't always prevent infection entirely. Vaccinated horses may still develop Potomac Fever, though symptoms are usually less intense and resolve faster with treatment.

Is there a test to confirm Potomac Horse Fever?

Yes, your veterinarian can perform a PCR test using a blood or fecal sample to confirm PHF. However, because early treatment is critical, many vets begin antibiotics based on clinical signs before test results return.