Getting to the Bottom of a Bite

Toxic reactions to bites are uncommon in horses, but when they do occur the consequences can be serious.
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Getting to the Bottom of a Bite
Brown recluse bite sites can develop into open wounds with tissue loss and secondary infection. | Photo: Courtesy Julie Rys
Toxic reactions to bites are uncommon in horses, but when they do occur the consequences can be serious. Let’s jump right in and meet Venus, a 12-year-old Thoroughbred mare. Put on your veterinary detective hat, and see what type of bite you think is responsible for her clinical signs.

Venus’ owner found her one morning standing head-down, breathing heavily, unwilling to move, and with a severely swollen hind leg. She had seemed normal the previous evening. The veterinarian’s initial physical exam revealed the mare had an elevated heart rate (80 beats per minute, normally 40), respiratory rate (24 breaths per minute, normally 12), and temperature (104.5°F, normally 99-101°F). Venus was exquisitely painful; her breathing was shallow and panting, and when forced to move she hopped on her left hind leg. The right hind was three times its normal size from the pastern to the stifle. The swelling was warm and very painful to the touch. After careful examination of the leg, the veterinarian discovered a tiny red spot on the lateral aspect of her metatarsus (outside of the cannon bone). The skin immediately surrounding this spot was weeping a clear yellow fluid.

Initial rule outs when faced with a limb with acute swelling and severe lameness should be abscess, infection/inflammation, and fracture. The most common cause, by far, is a simple subsolar abscess. Your veterinarian should perform a hoof tester evaluation and a careful hoof exam. On Venus, these were normal. Next to eliminate is a long bone fracture, which may be associated with severe swelling of the surrounding muscle masses. However, the presence of a high fever is not consistent with fracture, and Venus did begin to bear weight on the affected limb. That leaves us with the oozing red spot, high fever, severe pain, and sudden swelling and lameness. These are consistent with infection/inflammation or cellulitis of unknown origin.

Cellulitis describes a condition where something—trauma, bacterium, toxin—sets off a local cellular reaction that causes the body to release fluid, white blood cells, and a variety of chemicals. These chemicals enter the circulation and cause systemic signs of illness. At this point we have not determined the definitive cause of Venus’ clinical signs, but we have reasoned that her cellulitis is associated with inflammation and, potentially, infection

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Written by:

Chrysann Collatos, VMD, PhD, Dipl. ACVIM-LA owns High Desert Veterinary Service, in Reno, Nevada.

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