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Spider Bite

Your horse has a pea-sized lump that's very sore. If you suspect spider bite, here's what to do

© Whitney Cranshaw, Colorado State University, Bugwood.org

What You See
It started out 3 weeks ago as a pea-sized lump in the center of a 3-inch-high soft swelling on the side of your horse's chest. You assumed it was a fly bite or bee sting. Then the hair covering the lump fell out, leaving a 50-cent-piece-sized bald spot. The spot was pink, raised about a quarter-inch, and very sore. (Your horse stepped away and/or pinned his ears when you touched it.) Then he started rubbing it furiously on any available surface.

In the ensuing days, the lump gradually grew to its current state-a hard, fist-sized knot. It's apparently still very itchy, because your horse rubs it incessantly, despite your efforts to keep it clean and covered with a variety of antibiotic, anti-itch, and fly-repellent creams and ointments. At times it's sealed closed. But he keeps rubbing it open, allowing a thick, greenish-yellow liquid to spill out.

What Should You Do?
1. Call your veterinarian today.

Why: Although this isn't an emergency requiring urgent care, your horse needs prompt veterinary attention. The history of this particular lesion and your horse's obsession with scratching it-despite your good care-suggest that it's the result of a toxic spider bite, such as from a brown recluse or black widow. Spider bites are a common occurrence in autumn. That's when many creatures move indoors for the winter, often resulting in horses and arachnids becoming unwitting roommates.

When tissues come into contact with spider venom, they're severely damaged and quickly die, resulting in an abscess. In this case, the abscess is a capsule of inflamed tissue clenched around a liquefying hotbed of dead, dying, and infected muscle, connective tissue, and nerve endings. The pain and itch can be intense, as evidenced by your horse's behavior.

If diagnosed earlier-and the type of spider confirmed-then quick and aggressive veterinary treatment, possibly including antivenin, can limit the tissue damage by blocking the destructive action of the venom. At the current stage, however, the venom has done its damage and is long gone. Your only option to resolve the problem with the least possible scarring will be surgical removal of all affected tissue.

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2. Keep your horse from rubbing the area.

Why: Additional trauma from scratching and rubbing can enlarge the abscess by forcing its contents into adjacent tissues. Your horse's efforts to scratch the itch may also bruise and/or abrade the affected area.

How: Keep your horse in crossties under close supervision, occupy him with hand walking, or confine him in a small, tree-free paddock with electric fencing. Or, consider hauling him to your vet's hospital facility, if available, so your horse can be put in a padded recovery stall and/or sedated to keep him from rubbing until his appointment.

3. Withhold food.

Why: Your vet might opt to perform surgery under a short-acting general anesthetic, and there's less chance of complications if your horse has an empty stomach.

How: Pick up all hay and feed and/or bring your horse in from pasture. If he'll be in a stall and is prone to eating his bedding, strip it out. Make sure he has free access to fresh water at all times, so he keeps himself well hydrated.

4. Declare war on spiders.

Why: To lessen the likelihood of a repeat occurrence.

How: Clean out all undisturbed corners of the barn, loafing shed, and any other sheltered areas where your horse spends time, to find and eliminate spider nests. Then keep these areas clear to discourage spiders from setting up housekeeping. Woodpiles are favorite habitats for spiders, so if there are any in or near your horse's digs, remove them. (Wear heavy gloves, long sleeves, and long pants-and watch for angry arachnids.) Also, check hay flakes by giving them a good shake before letting him dig in.

Prognosis
Good. Thanks to a rich blood supply and the good drainage effects of gravity, this particular location should heal normally once all affected tissue has been removed. Your vet may decide to leave the wound open, rather than stitch it closed, so it can drain optimally. Despite how ugly this looks at first, the crater usually fills with healthy, pink tissue within a couple weeks, and within another week or 2, the skin grows in from the edges, leaving little or no scar.

Karen Hayes is an Idaho-based equine practitioner.

This article first appeared in the November, 2000 issue of Horse & Rider magazine.

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