Potomac Horse Fever: Symptoms, Transmission, and What Actually Saves Horses

By Dr Duncan Houston

Potomac Horse Fever is one of the few equine diseases where a horse can look mildly off in the morning and be seriously ill by the next day.

That is why it catches people out.

It does not always start dramatically. It starts with a fever, a quieter horse, maybe a slight drop in appetite. Then diarrhea develops, dehydration follows, and in some cases laminitis appears quickly after.

The difference between recovery and a complicated case often comes down to how quickly it is recognised and treated.

Quick Answer

Potomac Horse Fever is caused by Neorickettsia risticii and is acquired when horses ingest infected aquatic insects or snails, usually near freshwater environments. It causes fever, depression, diarrhea, and can lead to laminitis. It is not contagious between horses. Early treatment with oxytetracycline is highly effective, but delays increase the risk of complications and poor outcomes.

What Causes Potomac Horse Fever?

Potomac Horse Fever is caused by a bacterium called Neorickettsia risticii.

It is not spread directly between horses.

Instead, the organism lives within aquatic ecosystems and uses intermediate hosts.

Key transmission pathway

• bacteria infect aquatic organisms

• insects or snails carry the organism

• horses ingest infected material while grazing or eating contaminated hay

Important point

Horses do not need to drink from a river to get infected.

Insects can contaminate:

• pasture grass

• hay

• feed areas

Where and When It Occurs

Potomac Horse Fever is most common in:

• areas near rivers, creeks, ponds, or irrigation systems

• regions with standing or slow-moving water

Seasonal pattern

• late spring through early fall

• peak risk in warmer months

Clinical insight

You should think about PHF whenever:

• it is summer

• your horse is near water

• and diarrhea appears suddenly

How It Spreads (and Why That Matters)

PHF is not contagious between horses.

That means:

• isolating horses does not stop spread

• outbreaks often affect individual horses rather than entire barns

What matters instead

Environmental exposure:

• aquatic insects

• contaminated forage

• water proximity

What Are the Early Signs?

Early signs are often subtle.

Early stage

• fever (often first sign)

• lethargy

• reduced appetite

• mild colic signs

Decision checkpoint

A horse with fever in summer should always be taken seriously, even before diarrhea appears.

Progression of Disease

Developing stage

• diarrhea begins

• dehydration develops

• worsening depression

Advanced stage

• severe diarrhea

• endotoxemia

• laminitis risk increases

• weakness or collapse in severe cases

The Laminitis Risk

This is one of the most important complications.

PHF can trigger:

• inflammation

• endotoxemia

• disruption of blood flow to the hoof

Clinical reality

Laminitis is not a rare complication in PHF.
It is one of the main reasons this disease becomes serious.

Severity Framework: How Worried Should You Be?

Mild

• fever

• mild depression

• no diarrhea yet

Action:
Immediate veterinary evaluation recommended

Moderate

• fever

• diarrhea

• reduced appetite

• dehydration starting

Action:
Urgent treatment required

Severe

• profuse diarrhea

• marked depression

• dehydration

• early laminitis signs

Action:
Aggressive treatment and monitoring

Critical

• laminitis

• severe endotoxemia

• recumbency

Action:
Emergency care required

When Is This an Emergency?

Treat as urgent if:

• fever above 102.5°F

• sudden diarrhea

• depression

• signs of colic

• reduced appetite in summer months

Treat as emergency if:

• diarrhea is severe

• horse is weak or dehydrated

• signs of laminitis appear

• condition worsens over hours

How Is PHF Diagnosed?

Diagnosis is based on:

• clinical signs

• time of year

• exposure risk

Confirmatory testing

• PCR testing on blood or feces

Limitations

• blood tests alone can be unreliable

• timing affects results

Clinical insight

Treatment often starts based on suspicion, not waiting for confirmation.

Treatment: What Works Best

1. Oxytetracycline

This is the most important treatment.

• given intravenously

• highly effective when started early

Clinical takeaway

Early treatment dramatically improves outcomes.

2. Supportive Care

Includes:

• IV fluids

• anti-inflammatories

• endotoxin management

• electrolyte support

3. Laminitis Prevention

High-risk cases may require:

• early anti-inflammatory therapy

• careful monitoring of feet

• supportive management

Prognosis

Good prognosis

• early diagnosis

• prompt treatment

• mild to moderate disease

Guarded prognosis

• delayed treatment

• severe diarrhea

• laminitis development

Clinical insight

Most horses can recover if treated early.

Is There a Vaccine?

Yes, but it has limitations.

What it does

• may reduce severity

• may reduce risk

What it does not do

• guarantee protection

Practical takeaway

Vaccination is useful, but it does not replace monitoring.

Prevention: What Actually Helps

Environmental management

• reduce exposure to standing water

• manage pasture conditions

Insect control

• reduce insect attraction

• use fans and airflow

• manage lighting at night

Feed management

• remove uneaten hay

• avoid contamination

Common Mistakes Owners Make

• ignoring early fever

• waiting for diarrhea before acting

• assuming vaccination guarantees protection

• delaying veterinary treatment

• underestimating laminitis risk

The biggest mistake is waiting.

What Should You Do Right Now?

If your horse:

• has a fever in summer

• seems off

• shows early colic signs

Act immediately.

Do not wait for diarrhea.

Frequently Asked Questions

Can horses spread PHF to each other?

No. It is not contagious between horses.

Does every horse near water get PHF?

No, but risk is higher.

Is the vaccine reliable?

It helps, but it is not fully protective.

Can PHF cause laminitis?

Yes, and this is one of the most serious complications.

What matters most?

Early recognition and treatment.

Final Thoughts

Potomac Horse Fever is a disease where timing determines outcome.

The early stage looks mild.
The progression can be rapid.
The complications can be serious.

The key is simple:
recognise it early
treat it quickly
monitor closely

That is what protects the horse.

If you are unsure whether a horse’s fever, diarrhea, or behavior change could be early Potomac Horse Fever, ASK A VET™ can help guide immediate next steps and treatment decisions.