Potomac Horse Fever: Signs, Creek-Side Risk and When to Call the Vet

What is Potomac horse fever?

Potomac horse fever (PHF) is a serious intestinal illness caused by the bacterium Neorickettsia risticii, which horses pick up by accidentally swallowing infected aquatic insects in water, feed or pasture. It typically causes fever, going off feed, dullness and diarrhea, and it can trigger laminitis. It strikes mostly in summer and early fall, especially on farms near creeks and rivers.

The name comes from the Potomac River region of Maryland and Virginia, where the disease was first recognized in 1979. Today it has been reported in many parts of the US and Canada.

The news: a new case in Tennessee

On Sept. 23, 2026, the Equine Disease Communication Center (EDCC) posted an alert that the Tennessee Department of Agriculture had confirmed PHF in a horse in Washington County, in the state's northeast. The Horse and Horse&Rider reported it on Sept. 24.

The state report says no additional details are available. That means nothing is public about the horse's age, use or outcome, so be wary of anyone claiming otherwise.

It isn't Tennessee's first PHF report this season:

• Late August 2026: The Horse reported that two horses in Blount County tested positive and a third was suspected; all three were euthanized.
• Sept. 2026: the Washington County case above.

Sporadic cases like these are typical. PHF isn't an outbreak that spreads barn to barn; each case reflects local insect exposure.

How horses catch it: snails, flukes and mayflies

The life cycle sounds complicated, but the practical takeaway is simple: the risk comes from water.

1. Freshwater snails carry flukes (tiny flatworms) that harbor N. risticii.
2. The flukes leave the snails and infect aquatic insect larvae in creeks, rivers and ponds.
3. The larvae hatch into adult insects, including mayflies and caddisflies, as well as damselflies, dragonflies and stoneflies.
4. A horse swallows those insects, alive or dead, in its water, hay, grain or grass.

Mayflies and caddisflies are the big concern because they often hatch in huge swarms. They are drawn to lights, so dead insects can pile up in stalls, feed tubs and water buckets near a lit barn.

Why July to September?

Most cases are reported July through September, when aquatic insects hatch in large numbers. Weather can stretch the season from late spring into fall, so a warm October is not a free pass.

Is it contagious?

PHF is not considered contagious from horse to horse. A sick horse does not infect its pasture mates. If several horses on one farm get sick, it usually means they shared the same insect exposure.

Potomac horse fever symptoms to watch for

Signs can be mild or severe, and they often come on fast. The reported incubation period is one to three weeks after exposure.

• Fever, which can be high
• Going off feed (leaving hay or grain)
• Depression or dullness: standing quietly, head low, not interested
• Diarrhea, from cow-pie soft to watery, often a day or two after the fever starts
• Mild colic: pawing, looking at the belly, lying down more
• Dehydration
• Swelling of the lower legs
• Abortion in pregnant mares

Laminitis: the complication to fear

The Merck Veterinary Manual reports that laminitis develops in roughly 20% to 30% of affected horses, and it can be hard to manage. Warning signs include a rocked-back stance, reluctance to walk, heat in the hooves and a strong digital pulse. See our guide to fall laminitis for what founder looks like.

How serious is it?

Reported mortality reaches up to about 30%; the AAEP puts the range at 5% to 30%. Early veterinary care matters. Your vet can confirm PHF with lab testing, such as PCR on blood or manure.

When to call the vet

A feverish horse with diarrhea needs a vet call right away, day or night. Don't wait to see if it passes, and don't try to treat it yourself. PHF shares signs with other serious causes of diarrhea, such as salmonellosis, and only your vet can sort them out.

Before you call, gather:

• Rectal temperature, plus heart and respiratory rates if you know how (see our first aid kit and vital signs checklist)
• When signs started and what the manure looks like
• Whether the horse is eating and drinking
• Any hoof heat or reluctance to move
• Whether the horse lives near a creek, river or pond, and whether any horses have been vaccinated for PHF

If your horse also shows belly pain, our colic signs guide covers what to do until the vet arrives.

Prevention checklist for creek-side barns

No single step prevents PHF, but together these cut down how many insects your horse swallows.

Lights and insects

• Turn off barn and outdoor lights at night during hatch season, or dim them. Lights draw mayflies and caddisflies into stalls and feed rooms.
• If you need lighting, place it away from stalls, feed and water.
• Pay extra attention to stalls near open doors and at aisle ends.

Water and feed

• Skim and scrub water troughs and buckets daily in peak season; dump any with dead insects.
• Cover or empty feed tubs between meals.
• Store hay and grain where swarms can't settle on it.

Pasture and streams

• Limit access to creeks, rivers and ponds from July to September; fence off stream banks if you can.
• Offer clean trough water so horses aren't drinking from streams.

The vaccine

• Ask your vet about the Potomac horse fever vaccine, especially if you live near water or in an area with past cases.

Be realistic about it. The AAEP says vaccination may not be fully protective, and field protection is limited partly because the organism varies. Still, the EDCC and The Horse note that vaccinated horses that do get sick tend to have fewer and milder signs. Your vet can advise whether it makes sense and how to time it before your local peak season.

What to watch next

• Late-season cases: Tennessee's cases came in late August and late September. Watch the EDCC alerts page for your state until the first hard frosts.
• Warm autumns: a long, warm fall can extend insect hatches, so keep the light and water habits going a little longer.
• Spring planning: vaccine timing is decided before the season, so put a PHF question on your spring vet visit list.
• Daily checks: a quick temperature and manure check beats catching PHF late. Our health basics and care guide pages cover the routine.

For more seasonal threats, read our West Nile virus update and browse all trending horse topics.