Horse Fever: What Temperature Is Too High and When Should You Call a Vet?

Horse Fever: What Temperature Is Too High and When Should You Call a Vet?

By Dr Duncan Houston

Finding an elevated temperature in your horse can mean very different things.

A horse that has just completed strenuous exercise on a hot afternoon may be overheating. A horse standing quietly in the stable with the same temperature, reduced appetite and nasal discharge may have an infectious disease.

The number matters, but the circumstances, associated signs and temperature trend matter just as much. The first step is determining whether your horse has a true fever or is struggling to lose excess heat.

Quick Answer

Most healthy adult horses at rest have a rectal temperature around 37.2°C to 38.3°C, or 99°F to 101°F. Many practical equine biosecurity protocols use 38.6°C, or 101.5°F, as the upper screening limit.

A resting temperature above 38.6°C should be confirmed and monitored closely. Contact your vet if it persists, rises, or occurs with lethargy, reduced appetite, coughing, nasal discharge, diarrhoea, colic, lameness or neurological signs. A horse with suspected heat stress should be actively cooled immediately, especially if the temperature is above 39.4°C, or 103°F. (MSD Veterinary Manual)

What Is a Normal Temperature for a Horse?

Published reference ranges vary slightly. The MSD Veterinary Manual lists resting temperatures of approximately:

• Mares: 37.3°C to 38.2°C, or 99.1°F to 100.8°F

• Stallions: 37.2°C to 38.1°C, or 99°F to 100.6°F

In everyday equine practice, temperatures up to approximately 38.6°C, or 101.5°F, are often used as the upper end of the resting screening range. (MSD Veterinary Manual)

This does not mean that every temperature below 38.6°C is automatically normal.

A horse whose usual temperature is 37.4°C may have a meaningful change when the reading rises to 38.4°C, particularly if the horse is quiet, off food or showing another sign of illness.

Temperature can also be temporarily influenced by:

• Recent exercise

• Hot or humid weather

• Transport

• Excitement or stress

• Time of day

• Age

• Heavy rugs or poor ventilation

The most useful comparison is therefore your horse’s normal resting baseline, measured under similar conditions.

Is 38.6°C or 101.5°F a Fever in a Horse?

A temperature of 38.6°C is best treated as a decision point, not a diagnosis.

For a bright horse that has just been exercised, travelled or become excited, it may represent a temporary physiological rise. Allow the horse to rest in a shaded, well-ventilated area and recheck.

For a horse that has been resting quietly, particularly one that is dull or not eating normally, the same temperature may be the first sign of illness.

During quarantine, an infectious disease outbreak or known exposure, the Equine Disease Communication Center recommends reporting temperatures above 101.5°F to a veterinarian. Temperatures should also be taken before administering fever-reducing medication. (Equine Disease Communication Center)

How to Take Your Horse’s Temperature Safely

A rectal temperature is the most practical measurement for horse owners.

1. Use a digital rectal thermometer reserved specifically for the horse.

2. Apply a small amount of water-based lubricant or petroleum jelly.

3. Have a competent handler hold the horse.

4. Stand close beside the hindquarter, not directly behind the horse.

5. Lift the tail gently and insert the thermometer into the rectum.

6. Angle the tip slightly against the rectal wall so it does not sit inside a ball of manure.

7. Hold the thermometer securely until it signals that the reading is complete.

8. Remove it carefully, record the result and clean the thermometer thoroughly.

Do not attempt this on a horse that is kicking, frightened or unsafe to handle. Your own safety comes first. A second experienced handler or veterinary assistance may be needed. (Virginia Tech Extension)

If a temperature is unexpectedly high or low, confirm the reading once. Check that the thermometer was inserted correctly and functioning normally, but do not repeatedly delay veterinary contact when the horse is clearly unwell.

Fever Versus Hyperthermia: What Is the Difference?

A fever and hyperthermia both raise body temperature, but the underlying mechanisms are different.

A true fever

A fever occurs when inflammatory signals cause the brain to raise the body’s internal temperature set point.

The horse’s body is deliberately trying to become warmer. This commonly occurs with:

• Viral, bacterial or fungal infection

• Tissue inflammation

• Immune-mediated disease

• Some cancers

• Certain medication reactions

• A transient response after vaccination

A horse developing a fever may initially shiver, seek warmth or appear cold despite having an elevated temperature. External cooling alone does not correct the altered temperature set point. The underlying cause still needs to be identified and treated. (MSD Veterinary Manual)

Hyperthermia

Hyperthermia occurs when the horse produces or absorbs more heat than it can release.

The brain’s temperature set point has not changed. The horse is simply becoming dangerously hot.

Common causes include:

• Strenuous exercise

• Hot and humid weather

• Poor stable or trailer ventilation

• Prolonged transport

• Dehydration

• Obesity or poor fitness

• A thick coat

• Reduced or absent sweating

• Rare disorders such as malignant hyperthermia

Hyperthermia requires immediate active cooling. Delaying because of concerns about using cold water can allow heat-related injury to progress. (University of Minnesota Extension)

How Can You Tell Fever From Overheating?

Temperature alone cannot reliably separate the two.

Hyperthermia is more likely when:

• The horse has just exercised

• The weather is hot or humid

• The horse has been transported

• The stable or trailer is poorly ventilated

• The horse is sweating heavily

• The horse is known to have anhidrosis

• The temperature begins falling promptly with rest and active cooling

Fever is more likely when:

• The horse has been resting

• There is no obvious environmental heat load

• The temperature remains elevated despite rest

• The horse is dull or not eating

• There is coughing, nasal discharge or swollen lymph nodes

• There is diarrhoea, colic or reduced manure production

• There is a painful wound, swollen limb or injection site

• Other horses on the property are also unwell

A horse may also have both fever and heat stress. For example, a febrile horse transported in hot weather may struggle to regulate its temperature. When the distinction is uncertain and the horse is dangerously hot, begin cooling while contacting your vet.

What Causes Fever in Horses?

Fever is a sign, not a diagnosis. The underlying cause can range from a mild self-limiting inflammatory response to a serious contagious or systemic disease.

Respiratory infections

Common possibilities include:

• Equine herpesvirus

• Equine influenza

• Strangles

• Bacterial pneumonia

• Pleuropneumonia

• Other viral respiratory infections

Fever may appear before coughing or nasal discharge becomes obvious. This is one reason routine temperature monitoring is valuable during travel, competition or an outbreak.

Gastrointestinal and systemic infections

Fever may occur with:

• Equine coronavirus

• Salmonellosis

• Clostridial enterocolitis

• Potomac horse fever in endemic regions

• Tick-borne infections

• Bloodstream infections

Some gastrointestinal infections initially produce only fever, reduced appetite and depression. Diarrhoea or colic may not appear until later. (Center for Equine Health)

Localised bacterial infections

A hidden bacterial infection may develop in:

• A puncture wound

• A swollen limb

• An abscess

• A joint or tendon sheath

• The lungs or chest

• The uterus after foaling

• An injection site

• The hoof

The external wound may appear small while the deeper infection is much more significant.

Inflammatory and immune-mediated disease

Less commonly, recurrent or unexplained fever may result from:

• Immune-mediated disease

• Medication reactions

• Internal inflammation

• Neoplasia

• Inflammation affecting an organ or blood vessel

When fever keeps returning or remains unexplained, a systematic veterinary investigation is needed rather than repeated empirical treatment. (MSD Veterinary Manual)

Vaccination reactions

Some horses develop temporary soreness, reduced appetite, lethargy and a low-grade fever after vaccination. These reactions are usually self-limiting, but severe swelling, colic, hives, breathing difficulty, marked depression or a persistent high temperature requires veterinary attention. (AAEP)

How Worried Should You Be?

These levels are practical screening guides rather than absolute diagnostic cut-offs.

Lower concern

The horse is bright, eating and behaving normally, and:

• Temperature is no higher than approximately 38.6°C, or 101.5°F

• There has been recent mild exercise, excitement or transport

• Temperature returns towards baseline with rest

Allow the horse to rest, remove unnecessary rugs and repeat the temperature under calm conditions.

Moderate concern

The horse has a resting temperature of approximately 38.7°C to 39.3°C, or 101.6°F to 102.7°F, or the temperature repeatedly rises above the horse’s normal baseline.

Isolate the horse from others, stop exercise and contact your vet the same day if the reading remains elevated or any other sign develops.

High concern

Contact your vet promptly when:

• Resting temperature is 39.4°C, or 103°F, or higher

• Fever is rising over several readings

• The horse is dull, not eating or not drinking

• There is coughing, nasal discharge, diarrhoea, colic or lameness

• The horse has a painful swollen limb or wound

• More than one horse is febrile

• Fever returns after medication wears off

Critical concern

Treat the situation as an emergency when:

• Temperature reaches 40.6°C, or 105°F, or higher

• An overheated horse is not improving with active cooling

• Heart rate and breathing remain markedly elevated after exercise stops

• The horse becomes weak, uncoordinated or collapses

• There is severe respiratory distress

• The gums are dark red, purple, blue, very pale or dry

• There are seizures or altered awareness

Heatstroke may cause organ damage, shock, gastrointestinal injury, kidney injury and laminitis. Immediate cooling and veterinary treatment are required. (University of Minnesota Extension)

When Is an Elevated Temperature an Emergency?

Call your vet immediately when an elevated temperature occurs with any of the following:

• Laboured or noisy breathing

• Rapid deterioration

• Weakness, stumbling or incoordination

• Reduced tail tone or urine dribbling

• Seizures, circling or unusual behaviour

• Collapse or inability to stand

• Severe colic

• Profuse or watery diarrhoea

• Signs of laminitis

• A heart rate that remains above approximately 60 beats per minute at rest

• A respiratory rate that remains very high after exercise stops

• A foal that is weak, not nursing or lying down excessively

• Multiple affected horses

A neonatal foal with sepsis may develop either fever or an abnormally low temperature. Do not rely on the absence of fever to rule out serious illness in a young foal. (MSD Veterinary Manual)

Important Warning for Horses in Hendra Virus Risk Areas

In Australia, particularly in Queensland and northern New South Wales, fever accompanied by rapid deterioration, breathing difficulty, nasal discharge or neurological signs must be treated with additional caution.

Avoid close contact with nasal secretions, saliva, blood and other bodily fluids. Keep people, pets and other horses away, and call your veterinarian before attempting further examination or treatment. Where possible, move the healthy horses away rather than moving the sick horse through the property. (NSW Health)

What Should You Do Right Now?

1. Stop exercise

Do not continue riding, lunging or travelling a horse with an unexplained elevated temperature.

Move the horse to a safe, shaded and well-ventilated location.

2. Record the circumstances

Write down:

• The temperature

• The time

• Whether the horse recently exercised

• Weather conditions

• Recent travel

• Vaccination history

• Recent medication

• Appetite and water intake

• Manure and urine production

• Any coughing, discharge, swelling, wounds, lameness or neurological signs

This information can significantly improve your vet’s initial assessment.

3. Check the other vital signs

When safe, record:

• Heart rate

• Respiratory rate

• Gum colour

• Capillary refill time

• Gut sounds

• Sweating pattern

• Hydration

• Behaviour and appetite

A moderately elevated temperature becomes much more concerning when combined with a high heart rate, abnormal breathing, altered gums or depression.

4. Decide whether heat is involved

Recent exercise, transport, high humidity, direct sunlight, poor ventilation or abnormal sweating all make hyperthermia more likely.

When overheating is suspected, start cooling immediately.

5. Cool an overheated horse correctly

Use continuous cool or cold water over as much of the body as possible. Concentrate on the neck, shoulders, back, chest, abdomen and major muscle groups.

Also:

• Move the horse into shade

• Use fans where safely available

• Offer cool, clean water

• Remove rugs, sheets and unnecessary equipment

• Continue monitoring temperature and behaviour

Cold water is safe for an overheated horse. Whole-body continuous hosing is effective, and routinely scraping every application of water off is not necessary. (Center for Equine Health)

Recheck the temperature approximately every 5 to 10 minutes during active cooling. Continue until the temperature is approaching 38.5°C, or 101.3°F, and the horse’s breathing, heart rate and behaviour are clearly recovering.

Monitor for rebound heating because heat stored in large muscles may continue moving into the circulation after the first temperature reduction. (MSD Veterinary Manual)

6. Isolate a horse with suspected fever

When infection is possible:

• Separate the horse from the group

• Stop all horse movement on and off the property until veterinary advice is obtained

• Use separate buckets, tack, thermometers and grooming tools

• Avoid nose-to-nose contact

• Handle the sick horse after the healthy horses

• Wash hands and change contaminated clothing or footwear

• Record the temperatures of exposed horses

During an outbreak or known exposure, twice-daily temperature checks approximately 12 hours apart can help detect additional cases early. (Equine Disease Communication Center)

7. Contact your vet

Tell your vet:

• The highest temperature recorded

• Whether it was measured at rest

• Whether the temperature is rising or falling

• How the horse responded to cooling

• The horse’s other vital signs

• Any recent travel, competition or new arrivals

• Whether other horses are affected

• Any medication already administered

Should You Give Phenylbutazone or Flunixin?

Do not automatically give leftover phenylbutazone, flunixin or another anti-inflammatory medication before speaking with your vet.

These medications can reduce fever and make the horse appear temporarily better without addressing the underlying cause. They can also alter the temperature pattern your vet needs to interpret.

When your veterinarian has already provided a treatment plan, take and record the temperature before each approved dose. The EDCC specifically recommends measuring temperature before fever-reducing medications during infectious disease monitoring. (Equine Disease Communication Center)

Do not give human fever medication to a horse.

How Will a Vet Find the Cause?

The investigation depends on the temperature pattern and the horse’s other signs.

Your vet may assess:

• Recent travel and exposure to other horses

• Vaccination and medication history

• Respiratory tract

• Lymph nodes

• Chest and abdominal sounds

• Limbs, feet, joints and wounds

• Injection sites

• Neurological function

• Reproductive tract in mares

• Other horses on the property

Initial testing may include blood and urine analysis. Depending on the findings, further investigation may involve nasal swabs, PCR testing, faecal testing, ultrasound, radiographs, fluid sampling, cultures or other infectious disease tests.

Persistent or recurrent fever sometimes requires repeated examinations because the source may not be apparent during the first visit. (MSD Veterinary Manual)

Common Mistakes Horse Owners Make

Feeling the ears or muzzle instead of using a thermometer

Warm ears, a dry muzzle or hot skin cannot confirm fever. Use a rectal thermometer.

Calling every post-exercise temperature a fever

Exercise can temporarily raise temperature. The key is whether the horse cools normally and whether the temperature continues falling during recovery.

Assuming the horse is safe because it is still eating

Some horses continue eating during the early stages of significant illness. Appetite is useful, but it is only one part of the assessment.

Giving medication before recording the temperature

This can conceal the trend and complicate infectious disease monitoring.

Failing to isolate early

Waiting for coughing or nasal discharge can allow contagious disease to spread. Fever may be the first detectable sign.

Cooling only the legs or small areas

An overheated horse benefits from continuous cooling over a large body surface, combined with airflow and shade.

Withholding water from a hot horse

A hot horse should be offered cool, clean water. When electrolytes are added to water, plain water should also remain available because some horses refuse flavoured water. (University of Minnesota Extension)

How to Prevent Fever and Heat-Related Emergencies

Not every illness can be prevented, but early detection and sensible management reduce the risk.

Know your horse’s baseline

Take the temperature on several healthy, quiet days. Record it at similar times and under similar conditions.

Keep a thermometer available

Store a labelled digital thermometer and lubricant in both the stable first aid kit and the horse trailer.

Monitor after travel and competition

Check horses returning from shows, clinics, sales, breeding facilities and hospitals. During a known exposure or outbreak, measure temperatures twice daily as directed by your veterinarian.

Use good biosecurity

Avoid sharing:

• Water buckets

• Feed tubs

• Halters

• Bits

• Grooming equipment

• Thermometers

Quarantine new or returning horses where practical.

Reduce heat stress

During hot or humid weather:

• Exercise during the cooler parts of the day

• Reduce intensity and duration

• Provide shade and airflow

• Ensure unrestricted access to water

• Offer salt appropriately

• Watch for reduced sweating

• Clip excessively long coats when appropriate

• Improve trailer and stable ventilation

• Allow time for acclimatisation after moving to a hotter climate

Older, overweight, unfit horses, young foals and horses with poor sweating ability require closer monitoring. (University of Minnesota Extension)

Keep vaccinations appropriate for the horse’s risk

Work with your veterinarian to build a vaccination program based on location, travel, competition, age and exposure. Vaccination does not replace hygiene, isolation and biosecurity. (AAEP)

Frequently Asked Questions

Is 38.6°C or 101.5°F too high for a horse?

It is at the upper end of commonly used resting limits. Recheck it under calm resting conditions. Contact your vet if it persists, rises or occurs with abnormal behaviour or other clinical signs.

Is 38.9°C or 102°F dangerous?

A temperature of 102°F is usually abnormal in a resting adult horse. It is not automatically life-threatening, but it warrants isolation, close monitoring and veterinary advice, particularly when the horse is dull or not eating.

How long should a horse remain hot after exercise?

The temperature, heart rate and breathing should begin falling during rest and active cooling. Failure to improve within approximately 20 minutes, or any weakness, distress or incoordination, requires urgent veterinary attention. (University of Minnesota Extension)

Should I hose a horse that has a fever?

Hosing is essential when hyperthermia or heat stress is suspected. It does not treat the altered temperature set point of a true fever, although shade, ventilation and removal of heavy rugs remain sensible. When the distinction is uncertain and the horse is dangerously hot, begin cooling and call your vet.

How often should I take the temperature?

For a healthy horse, take occasional baseline measurements so you know what is normal. During infectious disease exposure, quarantine or an outbreak, twice-daily monitoring approximately 12 hours apart is commonly recommended. Follow your veterinarian’s instructions for an actively sick horse. (Equine Disease Communication Center)

Final Thoughts

An elevated temperature is a warning sign, not a complete diagnosis.

What matters most is whether the horse was resting or exercising, whether the temperature is rising or falling, and whether there are changes in appetite, behaviour, breathing, movement or gastrointestinal function.

Confirm the reading safely. Start immediate cooling when heat stress is possible. Isolate a resting horse with unexplained fever. Call early when the temperature is high, persistent or accompanied by other signs.

A thermometer is one of the cheapest pieces of equipment in a stable, but used properly, it can provide the first warning of a serious illness or developing outbreak.

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For tailored support interpreting your horse’s temperature readings and associated signs, ASK A VET™ can help you organise the information while you contact your regular veterinarian.