Shoe boils, also known as olecranon bursitis, capped elbows or elbow sores, are a common condition seen in horses. They occur when the horse repeatedly puts pressure on its elbow joint, often when lying down for extended periods on shallow bedding or with long-heeled horseshoes.

This prolonged pressure causes inflammation and swelling, leading to the formation of a fluid-filled sac beneath the skin, typically located on or near the point of the elbow. [1][2]

While most cases of shoe boil develop gradually and are relatively painless, sudden injury to the elbow or infection can cause acute shoe boil, which may result in pain, heat in the area, and even lameness.

Many cases resolve on their own once the underlying cause is identified and addressed. Recurrence of shoe boils is common, so the condition often requires ongoing management. [3]

Working with a veterinarian to develop a treatment plan can help prevent recurrence and infection in affected horses. [3]

Bursitis in Horses

Bursae are fluid-filled sacs found between tissues like bones, tendons, and muscles. They serve to reduce friction and cushion these structures during movement, providing protection and support for the horse’s body. [4]

Bursitis refers to inflammation of the bursa. The most common locations for bursitis in horses are in the shoulder, elbow, and hock. [1] Bursitis and injury to a bursa may result in pain, limited motion, and decreased functional mobility.

Two types of bursitis can occur in horses: [1][5]

  • Congenital bursitis: Inflammation of a normal bursa that horses are born with
  • Acquired bursitis: Formation and inflammation of a new bursa beneath the skin, where one didn’t exist before

The olecranon bursa, where shoe boils form, is located over the prominent olecranon process of the ulna in the elbow joint in horses. This area is particularly susceptible to injury and inflammation in horses. [4]

What are Shoe Boils in Horses?

Shoe boils in horses are characterized by swelling on the elbow joint, which may be painful or sensitive to the touch. In some cases, the skin overlying the swelling may become ulcerated or infected, leading to further complications.

Shoe boils are described as either acute or chronic:

  • Acute Shoe Boil: An acute shoe boil refers to the initial stage when a visible swelling is first noticed on the horse’s elbow. The affected area may be hot and painful to the touch, paired with mild lameness. If managed appropriately, the swelling may resolve without recurrence. [1][3]
  • Chronic Shoe Boil: If left unmanaged or if the underlying cause is not resolved, recurrent inflammation can lead to chronic shoe boils. Chronic cases are characterized by persistent or recurring swelling. Eventually fluid in the elbow joint builds up excessively and tissues lining the joint capsule thicken as fibrous tissue accumulates. [6]

The term “shoe boil” originated from the idea that these swellings were caused by pressure from the horse’s hoof when lying down on hard surfaces, such as stall floors or uneven ground. However, they can occur even in horses that are not shod.

Septic Bursitis

Septic bursitis is a more severe form of shoe boil involving a bacterial infection. Infection of a bursa can happen either through the bloodstream (hematogenous) or by bacteria directly entering the bursa, such as during joint injections or spread from a contaminated skin wound.

Symptoms of septic bursitis may include: [1][7]

  • Painful, inflamed skin
  • Swelling that may rupture
  • Heat at the point of the elbow
  • Overgrown tissue the site of injury