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

Potomac Horse Fever (PHF) can come on fast and get serious even faster. While it’s more common in certain regions and seasons, every horse owner should know the signs, especially because early action can mean the difference between recovery and complications.

In this post, we’ll walk through what Potomac Fever is, how to spot it, what changes in your horse’s vital signs to look for, and when it’s time to call the vet.

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

PHF can sneak up on you. A horse might look a little off in the morning — maybe not finishing breakfast or seeming quiet, and by the afternoon, they’re running a fever or showing signs of diarrhea. Recognizing the early signs is critical to stopping the illness before it progresses.

Here are the most common symptoms to watch for:

• Fever: One of the first signs. A horse with PHF often spikes a fever over 103°F. You might not see anything else wrong yet which is why daily temperature checks in the summer are a smart habit.

• Lethargy and inappetence: Your horse may seem dull, less interested in hay, or slow to respond. This is often dismissed at first, but combined with fever, it’s a red flag.

• Watery diarrhea: This symptom may arrive within 12–24 hours of the fever and can range from loose manure to severe, watery diarrhea that dehydrates the horse quickly.

• Signs of colic: Mild gut pain, pawing, looking at the flank. These may appear early, especially if gut motility is affected.

• Dehydration: Diarrhea leads to fluid loss, and if not corrected, this can become dangerous. Check gum moisture and perform a skin tent test if you suspect dehydration.

• Early laminitis: Sometimes the fever and diarrhea improve, but the horse starts shifting weight, acting sore on their feet, or lying down more often. That’s laminitis, and it can be devastating if not caught early.

Not every horse will show every symptom, and some may appear out of order. But a combination of high fever, behavior changes, and gut upset should always prompt a closer look.

Remember, the sooner you detect it, the better the outcome. PHF has a good survival rate with early treatment but it becomes riskier the longer you wait.

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

The most common first sign of PHF is a sudden spike in temperature, often climbing above 103°F and sometimes reaching 106°F. Horses may appear quiet or just “off,” so if you're not in the habit of checking temperature regularly, you might miss it.

If you notice a fever with no obvious cause — no recent vaccines, no hard workout, no heat stress, PHF should be on your radar, especially in summer and fall.

Heart Rate

A horse’s normal resting heart rate is around 28–44 bpm. In cases of PHF, this can jump significantly, often into the 60s or higher, even when the horse is standing still.

This increase usually reflects both the fever and internal inflammation. It’s a strong signal that the body is working overtime.

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.

Talk to your vet about whether vaccination makes sense for your area and your horse’s risk level.

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.

3. Monitor Water Sources

Avoid letting horses drink from natural standing water. Provide clean, regularly refreshed water from a controlled source. If you use automatic waterers, clean them often to prevent insect buildup.

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.