Brucellosis in Horses

Key takeaways

Brucellosis is an uncommon bacterial disease that is transmitted to horses from cattle or wild animals through contact with waste products or other bodily substances.

  • It is also transmissible to humans
  • The bacteria cause infections of the bursa at the withers (fistulous withers) or at the poll (poll evil)
  • Trauma to the bursa often precedes infection
  • Symptoms include swelling and bursting of the affected bursa, fever, stiffness, and lethargy
  • Immediate veterinary attention is required
  • This is a reportable disease in many countries, and some countries require euthanasia of infected animals
  • Diagnosis is based on physical examination and bloodwork
  • Treatment options include antibiotics, surgery, draining the bursa, and wound care
  • Prevention requires horses not be stabled with cattle or pastured on ground that has pastured cattle
  • Prognosis with early treatment is good
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A closer look: Brucellosis in Horses

Brucellosis is a serious disease that can harm horses, cause abortion of foals, and be transmitted to humans. Testing and careful management of cattle populations have rendered this disease far less prevalent in recent years and in some countries, it has been eradicated altogether.

Horses suspected of having brucellosis require immediate veterinary attention. In many countries, this is a disease that must be reported to governmental agencies or to appropriate governing bodies.

Horses that are stabled with or close to cattle are most at risk. Horses that pasture in fields where cattle have grazed are also at risk.

Horses that have contact with wild animals or the waste products of wild animals such as elk or bison, or with pigs or the waste products of pigs, are also susceptible.

Risk factors

Brucellosis typically affects either the poll or the withers, and sometimes both. If left untreated, the infection causes the bursa to swell. When the bursa gets too full, it bursts, draining clear, straw-colored liquid.

Swelling typically appears on one side of the body, and sometimes on both sides or along the midline.

In more severe cases, the structures attached to the withers or the poll such as the nuchal ligament (the large ligament used for raising the head) or the dorsal vertebral spines (the bony points along the spine) are affected by the rupture of the bursa. If these structures are damaged and the tissues in them die, the mobility of the horse is significantly impacted.

In some cases, brucellosis infects other joints, such as the hocks or pasterns. In these cases, bone infections or severe arthritis can develop.

Brucellosis can also cause abortions and infertility in affected breeding horses.

Possible causes

Brucellosis is a bacterial infection. The most common bacteria found in affected horses are Brucella abortus and Brucella suis.

These bacteria are transmitted from infected cattle, and to a lesser extent other animals such as bison, elk, and pigs, through urine, feces and other bodily fluids. Typically, the horse is infected by eating infected food or pasture, drinking infected water, or coming in contact with the placenta or other bodily discharge from infected animals.

The bacteria are moderately hardy. They are destroyed by direct sunlight but can survive in manure, urine and water for up to 6 weeks. In colder conditions, the bacteria survive even longer.

Once in the body, the bacteria infect joint bursae, the sac of fluid that provides the cushioned, gliding movement of joints. The specific bursae that are affected are either at the withers (called fistulous withers and affecting the point between the shoulder blades) or the poll (called poll evil and affecting the point behind the ears). In some cases, injury or trauma, typically from ill-fitting tack, precedes the infection.

Main symptoms

Testing and diagnosis

Diagnosis is primarily based on physical examination and bloodwork. X-rays can help identify the extent of the infection, and whether bone infection has occurred. In some cases, a culture of the fluid is examined in the laboratory to determine what strain of bacteria is involved.

In many countries, this is a reportable disease. Cases of brucellosis must be reported to government agencies or other appropriate governing bodies within a few days of an outbreak.

Steps to Recovery

Treatment options depend on the location of the horse. In some areas, treatment of brucellosis is forbidden due to government regulations. Affected horses must be euthanized in these cases.

If treatment is permitted, treatment options include:

  • Antibiotics
  • Draining of the built up fluid from the bursa
  • Cleaning and protecting wounds left from ruptured bursae
  • Surgical removal of the bursa in chronic or severe cases
  • In very severe cases where the connecting structures are affected, surgical removal of all dead tissue

It is unlikely that a horse is the source of transmission to other horses, so there is no need to quarantine horses with brucellosis.

Brucellosis is highly transmissible to humans. Heightened hygiene protocols including hand washing, N95 masks, and glove wearing are required when handling infected horses.

Horses with brucellosis that are treated before the bursa bursts have a good prognosis. Horses have a more guarded prognosis in cases where the bursa ruptures, particularly when the surrounding structures have been affected. Long term administration of antibiotics is often necessary to control the infection. In some cases, horses become permanently infected despite treatment.


Prevention of brucellosis requires that horses avoid contact with the bodily waste of cattle and other animals. Strategies include:

  • Avoiding stabling cattle and horses together
  • Avoiding keeping cattle and horses together in the same pasture
  • Avoiding keeping cattle and horses in adjacent pastures
  • Avoiding keeping horses in pastures that have been used by cattle in the past
  • Applying strict hygiene measures when handling cattle and then horses
  • Ensuring tack fits properly
  • Avoiding interaction between horse and wild animals such as bison and elk

Is Brucellosis in Horses common?

Brucellosis is a rare condition in horses. In many countries, brucellosis has been completely eradicated.

Typical Treatment

  • Antibiotics
  • Draining abscesses
  • Surgery

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