ETIOLOGY AND EPIDEMIOLOGY

Brucellosis in horses is caused by infection with species of the genus Brucella, especially B. abortus and B. suis. There are no reports of successful natural or experimental infection of horses with Brucella melitensis or Brucella canis.¹⁵ Brucella spp., are nonmotile, aerobic, intracellular gram-negative cocci or short rods that are usually arranged singly or less frequently in pairs, short chains, or small groups.³ They require complex media for growth in culture, and many strains require supplementary carbon dioxide (CO₂) for optimal growth on primary isolation. On clear media, colonies are transparent, raised, and convex with a smooth, shiny surface. They appear as a pale honey color by transmitted light.³ The optimum temperature for growth is 37° C (98.6° F), but growth occurs between 20° and 40° C (68°-104° F).³

Brucella abortus infections have been reported worldwide but have been effectively eradicated from several European countries, Japan, and Israel. Cattle are the most common natural hosts for B. abortus, but the organism has also been isolated from horses, American bison, buffalo, and yaks.³ Most horses with brucellosis have a history of contact with cattle.^7,8,12 In 1937, Duff¹⁶ examined 85 horses with fistulous withers; B. abortus was isolated from 80% of these horses. Of the horses with brucellosis, 92% had reported contact with cattle, and 56 horses were from farms where cattle had brucellosis.¹⁶ Brucella suis most often infects swine but may also infect horses and cattle.

Brucella spp., may be transmitted by ingestion, inhalation, or direct contact through skin abrasions or mucous membranes. It is presumed that most horses acquire infection from cattle. B. abortus has been isolated from equine feces and urine and from aborted equine fetuses, suggesting that horses are a potential source of infection to cattle.^12,17,18 However, experimental infections indicate that horses do not excrete the organism in sufficient numbers to infect susceptible cattle in close contact.^18,19

There is no apparent age, gender, or breed susceptibility to the organism in horses, although most cases have been reported in horses older than 3 years.⁶ Brucellosis is considered a zoonotic disease (see Public Health Considerations).

PATHOGENESIS AND CLINICAL FINDINGS

Many horses that are seropositive for B. abortus, which suggests exposure or infection with the organism, exhibit no recognizable clinical signs. In horses that develop clinical disease, B. abortus appears to have a predilection for infection of the tendons, muscles, bones, and joints of horses. It is often associated with septic bursitis of the supraspinous bursa over the second and third dorsal vertebral spinous processes (fistulous withers) or supraatlantal bursa over the first and second cervical vertebra (poll evil). However, B. abortus is not the only potential etiologic agent in horses with fistulous withers. In geographic areas with a low prevalence of infection in cattle, B. abortus is rarely isolated from horses with fistulous withers.^6,20

Fistulous withers secondary to B. abortus infection is characterized by a profuse serofibrinous exudate with varying degrees of necrosis and formation of distended fistulous tracts of varying size (Figs. 40-1 and 40-2). The onset of clinical signs may be abrupt or insidious. Early clinical signs may include localized pain, heat, and swelling of the bursa without obvious external fistulation or exudate. Lethargy and general stiffness may be present.⁵ The bursa ruptures, and purulent exudate drains from the fistula. There may be apparent healing, fibrosis, and refistulation. The exudate often contains multiple bacteria, including Streptococcus spp., Staphylococcus aureus, and Actinomyces bovis. Horses with fistulous withers that are seropositive to B. abortus are significantly more likely than seronegative horses with fistulous withers to have radiographic evidence of osteomyelitis of underlying dorsal spinous processes.⁶

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