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8 Life-Threatening Infectious Horse Diseases

by ihearthorses

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An infectious disease is one that is spread between members of a species either by direct contact or a secondhand vector such as an insect or wild animal. The 8 horse diseases listed below are potentially deadly, making prevention and early detection essential. Luckily, each has its own set of signs and symptoms as well as measures you can take to prevent infection.

1. West Nile

mosquito drinks blood out of man

Infection with deadly West Nile disease is most common in the late summer or early fall in the northeast and Mid Atlantic regions when mosquitoes are rampant. The insects become vectors for the virus when they take a blood meal from an infected bird. They then transmit the disease when they move on to the next host.

Not all animals exposed to West Nile become ill, but horses seem to be especially susceptible. The virus affects the central nervous system causing encephalitis – inflammation of the brain.

Signs of West Nile:

  • loss of appetite
  • depression
  • fever
  • weakness or paralysis of hind limbs
  • impaired vision
  • head pressing
  • convulsions (seizures)
  • inability to swallow
  • walking in circles
  • hyperexcitability
  • coma


An equine vaccination is available, but is not 100% effective against all strains of the disease. Reducing your horse’s risk of exposure to mosquitos is essential and can be done by reducing their breeding grounds. Mosquitos love stagnant water that collects in gutters, troughs, bird baths, recycling containers, old tires, wading pools, etc. Clean out and empty any water-collecting containers frequently. You may also opt to use an equine-safe insecticide spray.

2. Tetanus

tetanus horse diseases

Horses generally contract tetanus when an untreated wound becomes contaminated with the bacterium Clostridium tetanii which can be found in soil and manure. The bacteria are extremely hardy and can survive for long periods of time without oxygen. Tetanus proves deadly in 50-75% of cases.

Signs of Tetanus:

  • Muscular stiffness and spasms
  • Difficulty moving and eating
  • Tail often held straight out
  • Development of an anxious expression due to facial spasms
  • Sweating
  • In advanced cases the horse will collapse with spasms, convulsions and death from respiratory failure


Vaccination against tetanus and good first aid practices are key to prevention. Prevent wounds by keeping barns and turn-out areas free from potential hazards.

3. Strangles (Streptococcus equi)

woman hand caress pet equine

Strangles is highly infectious from horse to horse and caused by the bacterium Streptococcus equi equi. It causes abscessation of the lymphoid tissue of the upper respiratory tract and is transmitted by direct contact with infected horses, insects that have come in contact with contaminated nasal drainage, human contact from horse to horse and infected equipment or medical supplies.

In a small percentage of cases, ‘bastard’ strangles may occur when abscesses spread to other areas of the body. This is almost always fatal.

Signs of Strangles:

  • Fever (103°–106°F)
  • Nasal discharge
  • Depression
  • Difficulty swallowing
  • Respiratory noise
  • Extended head and neck
  • Swollen lymph nodes


Vaccination and quarantine of affected horses to prevent spreading the disease to other animals.

4. Equine Influenza (Flu)


Equine influenza is a highly contagious virus that is endemic in the US, meaning it circulates continuously in the equine population. Influenza can be spread through direct contact with an infected horse or contamination in the environment. The incubation period of 1-3 days allows the virus to spread rapidly throughout animals in close proximity before symptoms ever appear.

Signs of Equine Influenza

  • Fever
  • A harsh, dry cough of sudden onset that persists for 2-3 weeks or more
  • Clear nasal discharge progressing to thick, green-yellow discharge
  • Lethargy/depression
  • Loss of appetite


While there are vaccinations available, it is difficult to provide full immunity against various strains of virus. New and traveling horses should be quarantined for at least 14 days as a safety precaution.

5. Equine Herpesvirus (EHV)/Rhinopneumonitis

Vet and horse health chech

EHV is another one of the highly contagious horse diseases. It is characterized by respiratory infections, paralysis, abortions, inflammation of the spinal cord, and occasionally death in young horses. It spreads through nasal secretions, contact with infected horses, and contaminated feed and water utensils.

Signs of EHV:

  • Nasal discharge
  •  In coordination
  •  Hind limb weakness
  • Loss of tail tone
  • Lethargy
  • Urine dribbling
  • Head tilt
  • Leaning against a fence or wall to maintain balance
  • Inability to rise


All horses should be vaccinated against Type 1 (EHV1) and Type 4 (EHV4), the two most clinically significant strains. In addition, new animals or those that have recently traveled should be quarantined and shared instruments such as grooming supplies should always be washed between use on each animal.

6. Potomac Horse Fever

Horse drinking water diseases

This disease occurs most often during summer months in the northern U.S. and Canada and is associated with pastures bordering water sources. PHF is transmitted when immature flatworms are ingested through drinking or by infected insects (such as mayflies).

Signs Of PHF: 

  • loss of appetite
  • fever
  • depression
  • decreased intestinal sounds
  • diarrhea
  • mild colic
  • laminitis



7. Equine Infectious Anemia (EIA)

horse flies and deer flies

Equine infectious anemia is a horse disease that is transmitted by blood-sucking insects such as horse flies, deer flies and mosquitoes or by passage across the placenta directly to the foal. It is potentially fatal and there is no cure or effective treatment.

Signs of EIA: 

  • Most infected horses show no symptoms but remain contagious for life
  • Fever
  • Depression
  • Decreased appetite
  • Fatigue
  • Reduced stamina or weakness
  • Rapid weight loss


Since there is no vaccine and no cure for EIA, preventive measures must be taken. Do not reuse instruments or needles on more than one horse without sterilization, test and quarantine new or travelling horses, and test all horses at least annually.

8. Equine Rabies

bat rabies horse diseases

Although relatively rare, horses can get and transmit Rabies. It is spread primarily by the bite of a raccoon, bat, skunk, fox or coyote, and is most common in the northeast U.S. and Texas. Rabies is always fatal in unvaccinated horses.

Signs of Rabies:

  •  Colic
  • Obscure lameness
  • Ataxia
  • Paralysis
  • Incontinence
  • Tremors
  • Fever
  • Depression
  • Aggression
  • Increased sensitivity to touch
  • Convulsions
  • Bruxism (teeth grinding)


Vaccination and additional precautions in endemic areas such as securing trash and palatable horse feeds so as to reduce wildlife entering your property.

Are there any other horse diseases you would like your fellow equestrians to know about? Let us know in the comments below!

H/T to MannaPro.com

8 Life-Threatening Infectious Diseases In Horses-02

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