Fistulous Withers

Sarah M. Reuss

■ Definition Fistulous withers is a chronic inflammatory condition of the supraspinous bursa and associated tissues in horses, mules, and donkeys. The supraspinous bursa is located dorsal to the dorsal spinous processes of the second through fifth thoracic vertebrae.

■ Clinical Signs Affected horses may show nonspecific signs including lethargy, fever, and generalized muscle stiffness. These may progress to more specific signs of pain, heat, and swelling of the withers and cervical area. Over days to weeks, these swellings mature and rupture at one or more locations, releasing serous or mucopurulent drainage. The classically described discharge is yellowish-brown with white, yellow, or red flakes (“rice bodies”).² Fistulous tracts may extend to the dorsal spinous processed of the vertebrae, as well as to the scapula, ribs, and occasionally the thoracic cavity. Lesions and drainage may wax and wane.

■ Etiology and Pathophysiology There are two forms of fistulous withers: idiopathic (or typical) and traumatic (or atypical). The more common idiopathic form results from the spread of inflammation and infection of the supraspinous bursa to surrounding structures, including the nuchal liga­ment and/or dorsal spinous processes. Organisms reported to be the primary cause of fistulous withers include Brucella abortus, Actinomyces bovis, and Onchocerca cervicalis. B. abortus has a predilection for synovial structures and is often associated with septic supraspinous bursitis in addition to septic atlantal bursitis (“poll evil”). In 1937, B. abortus was isolated from 80% of fistulous withers cases; 92% of those horses had contact with cattle, and 65% of affected horses were from farms with Brucella-positive cattle.² Currently there is some geographic variability.

The traumatic form of fistulous withers is preceded by an open wound or blunt trauma to the withers and then secondary infection. Infection may result from external contamination or from hematogenous spread to devitalized tissue. Secondary contamination of open wounds with Streptococcus equi subsp. Zooepidemicusⁱ or environmental microorganisms is reported.

■ Diagnosis A careful history must be obtained, including duration and severity of clinical signs and response to previous treatment. Physical examination with careful palpation over the withers may reveal pain, swelling, or draining tracts. CBC may show leukocytosis due to neutrophilia as well as hyper­fibrinogenemia indicating chronic inflammation.

If the swelling is encapsulated and not draining, needle aspiration should be performed to obtain a sample for culture and sensitivity.

Serologic testing for antibodies against B. abortus should be performed in all animals with fistulous withers due to the public health implications. The plate agglutination test is more sensitive and specific than the card test. Brucella titers are considered to be negative when they are less than 1 : 50. Ideally, paired titers should be performed, with any rise in titers performed 2 weeks apart being indicative of current infection.⁸ Chronic infections may not show an increase, however, so a single high titer with compatible clinical signs should be considered diagnostic. Animals should be isolated while awaiting test results, and public health officials should be contacted if Brucella is suspected.

Radiographs should be obtained to determine the extent of the lesions, the presence of osteomyelitis of the dorsal spinous processes, and/or the presence of a foreign body. There may also be swelling or granular opacity of the soft tissue in the withers region.³ Horses with positive B. abortus titers are more likely than seronegative horses to have radiographic evidence of osteomyelitis of the dorsal spinous processes.⁴ A positive contrast fistulogram may help determine the extent of any draining tracts. Ultrasonography may also help delineate draining tracts or foreign bodies.

■ Treatment Medical treatment may be attempted but is often unsuccessful alone. Antimicrobial selection should be guided by culture and sensitivity results. In the absence of a culture, systemic broad-spectrum antimicrobials may be used. This should be combined with lavage of the draining tracts.

Brucella vaccination with the S19 strain has been reported in Brucella-confirmed cases as an effective extralabel treatment in horses. The vaccination regimens described vary from single-dose subcutaneous injection to a series of three subcutane­ous injections 10 days apart. Success rates of 70% have been reported in horses treated with S19 vaccine.² However, sub­cutaneous injection reportedly causes severe local and systemic reaction, including fever, depression, and local abscessation.⁸ Intravenous administration of S19 vaccine has also been reported, but it resulted in death in 3 of 4 horses.⁴ At this time, there are no approved vaccinations for use in horses, and the RB51 vaccine has not been used in equids. Also, the apparent decline in frequency of Brucella in fistulous withers makes vaccination of questionable value.

Medical management alone is rarely effective and usually needs to be combined with aggressive surgical debridement. Reportedly 80% of traumatic fistulous withers treated conser­vatively will have recurrent lesions.⁹ Standing drainage and debridement has been performed, but general anesthesia and aggressive debridement is the preferred treatment.³ Ostectomy and curettage of the dorsal spinous processes may be necessary. Primary closure is difficult to achieve but will provide a more cosmetic result and quicker healing time. Wounds left open to heal by second intention may take months to heal.³ Applica­tion of maggots (freshly emerged, germ-free larvae) in combina­tion with light surgical debridement has been reported in five cases. Four of those resolved successfully, with the remaining case being lost to tetanus before complete healing was achieved.¹⁰

■ Prevention and Control B. abortus infection, if present, carries great public health implications. Horse owners and veterinary staff must be notified of the possible exposure and take appropriate precautions. Brucellosis in humans may cause acute fever with pneumonia, spondylitis, orchitis, or pyelone­phritis. Chronic brucellosis causes a transient fever in humans.¹

Parasite control is an important aspect of prevention of fistulous withers. Horses should also be kept separate from Brucella-positive cattle. Proper saddle and harness fit also decreases the incidence of traumatic fistulous withers.