Fistulous withers is a skin condition characterized by holes and tracts in the withers. The term is also used to describe any severe withers skin condition. However, “true” fistulous withers isn’t as common as it was years ago.
The classic cause is infection with the organism Brucella abortus, normally found in cattle. The organism thrives in the bursal tissues in the horse’s withers, causing inflammation and swelling that eventually can also involve the nearby bone of the vertebral column. The organism enters the horse’s body either through the mouth, nose or eyes, or through broken skin.
Ironically, one of the most effective methods of preventing the disease in cattle, the RB-51 vaccine, puts horses at risk. Although less dangerous than the natural disease, RB-51 organisms can cause illness in animals and people working with the vaccine. Other causes include:
Strep and Staph bacteria can gain access as a result of irritation/rubs/sores on the withers and/or skin softening and weakening from prolonged contact with sweat and dirt in saddle pads and from incorrectly fitted saddles that rub.
Another cause is the Onchocerca, a tiny worm that has a special affinity for the midline of the belly (“summer dermatitis”) and the thick ligament on the top of the neck. The horse usually picks up the infection from biting flies or midges.
The adult warble fly, which resembles a large bumblebee and lays its eggs on the long hairs of the lower leg/fetlock, can be a problem. Its larvae are particularly common in the shoulder/withers area. They may even migrate to the brain, causing fatal hemorrhage.
Less common causes include skin infections with Habronema or Drachia worms. Normally stomach parasites, these worms can cause lesions and infection. These lumps may open and drain.
You should pay close attention to the health of the withers skin, which is easily bruised. Tissues that are weakened and irritated may be more prone to infection. Monthly use of ivermectin will eliminate the parasites that cause worm-related episodes and prevent reinfection.
However, you’ll need your vet for bacterial infection. Cultures should be obtained prior to starting antibiotics to determine the exact organism involved. X-rays are advisable to see if the infection has extended to bone.
Some vets prefer surgery to remove obviously infected areas, followed by antibiotics. Others like antibiotics alone. However, if the infection involves bone, surgery is a must.
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