Top 5 Most Deadly Horse Diseases

Posted by Gabby Gufler, Fri, Nov 20, 2015

Infectious horse diseases are those which horses can get from each other or via a transmitter such as a mosquito or tick, which may transmit the disease from horse to horse. Knowledge is the key to prevention. Here are some of the most common horse diseases:

Potomac Horse Fever

This disease is most often seen in spring, summer, and early fall and is associated with pastures bordering water sources such as creeks or rivers. PHF is an acute enterocolitis syndrome producing mild colic, fever and diarrhea in horses of all ages, as well as abortion in pregnant mares.

The bacterium responsible for the disease, Neorickettsia risticii, has been identified in flatworms that develop in aquatic snails (who knew!). When the water warms up, infected immature flatworms are released from the snail into the aquatic environment. These immature flatworms can be ingested by horses drinking the water, but more commonly they are picked up by aquatic insects. Infected insects (such as mayflies) will hatch en masse and might carry the organism to horses to ingest as they graze (

Signs of PHF:

  • Loss of appetite
  • Fever
  • Depression
  • Decreased intestinal sounds
  • Diarrhea
  • Mild colic

Affected horses might also develop signs of laminitis. PHF can be mild to life-threatening, so if you think your horse is showing signs of PHF you should contact your veterinarian immediately. PHF can be diagnosed by laboratory identification of the organism in a blood or manure sample from the horse by polymerase chain reaction (PCR). If caught early, it can be treated successfully with oxytetracycline.

Prevention: Several vaccines are commercially available. These might not completely prevent illness, but they may reduce its severity if the horse is exposed to the organism. Consult with your veterinarian to decide the best course of action.

Equine Herpesvirus (EHV)/Rhinopneumonitis

With the recent uptick in EHV in 2015, most horse owners are somewhat aware of the dangers of this highly contagious virus. EHV is characterized by respiratory infections, paralysis, abortions, inflammation of the spinal cord and occasionally death in young horses. EHV is extremely contagious, spreading through nasal secretions, contact with infected horses, and contaminated feed and water utensils. Type 1 (EHV1) and Type 4 (EHV4) are the most clinically important.

Signs of EHV:

  • Nasal discharge
  • Loss of 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

If you think your horse may have been exposed to the virus (while traveling or at a show), start isolation procedures immediately to prevent it from spreading through your whole herd. Check the temperatures of all horses on your farm several times a day; if fever is detected, check for EHV1 and consult your equine veterinarian for further guidance.

Prevention: There are two things you can do to help prevent an EHV outbreak on your farm:

  1. Vaccinate: While there are several vaccines available, unfortunately there is no licensed vaccine that has a claim for protection against the neurological strain of the virus (EHM). Consult your veterinarian for further guidance.
  2. Implement Biosecurity Practices on your farm. This includes quarantining any new animals on the farm, or those that have traveled recently, before introducing them to your herd and washing instruments such as grooming supplies between use on each animal.

Equine Influenza (Flu)

Equine influenza is one of the most common infectious diseases of the respiratory tract in horses. This is a highly contagious virus that can be contracted through direct contact with an infected horse or indirectly via a contaminated environment. Infected horses incubate the disease for 1–3 days before displaying symptoms, which is why outbreaks can spread so rapidly. Unfortunately, influenza is endemic in the US, which means it circulates continuously within the equine population.

Signs of Equine Influenza

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

Prevention: Implement good biosecurity practices, including quarantining of newly arrived or traveling horses for at least 14 days. There are also vaccinations available that can be used before exposure. Consult your veterinarian regarding vaccinating your horse.

Streptococcus equi (Strangles)

Strangles is an infectious horse disease characterized by abscessation of the lymphoid tissue in the upper respiratory tract. Streptococcus equi equi is the bacterium which causes the disease, and is transmitted via direct contact with infected horses or subclinical shedders, or indirectly via contact with water troughs, hoses, feed bunks, pastures, stalls, trailers, tack, grooming equipment, nose wipe cloths or sponges, attendants’ hands and clothing, or insects contaminated with nasal discharge or pus draining from the lymph nodes of infected horses.

Signs of Strangles:

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

In some outbreaks and in a small percentage of cases, these abscesses spread to other parts of the body (a condition known as “bastard” strangles, which is nearly always fatal). As with the other diseases above, contact your vet if you think your horse is showing signs of Strangles. Due to being so contagious, affected horses should be separated and cared for by separate caretakers wearing protective clothing.

Prevention: Vaccination is often the best preventive measure for Strangles. Contact your veterinarian for guidance.

Tetanus (Lockjaw)

Tetanus is caused by the bacterium Clostridium tetani, which can be found in soil and manure. This bacterium is found in just about every environment and can survive for long periods of time. Wound contamination is generally what leads to infection; a clean wound is not as likely to result in tetanus.

Tetanus bacteria do not need oxygen and multiply rapidly in the damaged tissues. They produce a toxin (tetanus toxin), and it is this neurotoxin which causes the classical signs of tetanus.

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

Prevention: Tetanus is a preventable disease, and vaccination is the key to prevention. Good first aid practices are also important, including keeping wounds clean and ensuring your turn-out areas are safe, clean, and clear of dangerous items that could cause injury. Consult your veterinarian to ensure you are taking the necessary prevention measures on your farm.


Gabby Gufler

Gabby Gufler graduated from Truman State University in 2013 with a BS in Animal Science & Nutrition and a minor in Equine Science. Gabby currently works on Manna Pro’s marketing team, and enjoys competing regularly with her six horses.