Spider Bites

Written by Jeffrey Davis on June 14, 2011, 11:23 p.m.

Spider Bites

Those of us who have staffed ERs have heard this chief complaint from our patients many times. The patient commonly presents with a papule on the skin that is tender and red. As common as this complaint is its rarely due to a spider. Of the thousands of spider species that exist around the world only a few spider pose a threat to humans. Those spiders that do pose a risk are widow and false black widow spiders, recluse spiders, Australian funnel web spiders, and Phoneutria spiders. Good news though, the last two don’t exist in the United States.

Bites usually present as a solitary lesion they can be painful or painless, there can be death of the tissue due to envenomation (particularly with recluse spiders) or if the venom enters the bloodstream the effects can be widespread (systemic).

Here’s the rub when it comes to diagnosing spider bites according to the medical literature:
A presumptive diagnosis of a spider bite is most often based on the history and clinical presentation. However, the diagnosis of a spider bite can be considered definitive ONLY if both of the criteria below are fulfilled:
  1. A spider was observed inflicting the bite.
  2. The spider was recovered, collected, and properly identified by an expert entomologist.
The chances of those criteria being met are slim to none.

There’s a common expression, when you hear hoof beats look for horses rather than zebras. So as physicians we look for other reasons.
  1. Infections: MRSA (a common staph infection) very often starts with a single red, swollen, tender lesion. Lyme’s disease (spread by ticks) also have a characteristic target-like lesion. Herpes can also present with a single lesion.
  2. Other bites and stings: Ants, fleas, bedbugs, ticks, mites, mosquitoes, and biting flies can also sting or bite humans.
  3. Other common rashes: Poison oak, ivy and other plants can present with single lesions (although linear spread of lesions is more common.)

So here’s what I recommend. Next time you have a red, tender, swollen lesion that looks like it could be a bite visit with your doctor so he or she can review the symptoms and decide if a visit is needed. Most insect bites will only have a localized reaction that subsides in a few days. Staph infections will typically worsen an may need to be drained and treated with antibiotics. It never hurts to apply a topical antibiotic ointment to one of these lesions early on. Just remember that many people can have an allergic reaction to the neomycin in triple antibiotic so using polysporin is the better choice.