Arachnophobia!: The Truth Behind Spider Bites

by Marissa Camilon, MD 2011 |

They are the stuff of legends, nightmares, horror movies, and, for some people, deeply intense fears. Without a doubt, spiders are freaky little buggers.  But is this fear justified?  Their impact has reached into the medical field: one Australian medical organization notes that 13% of emergency calls are regarding spider bites. We know that we, as a society, freak out (and apparently call our Australian doctors) when we are bitten, but is a spider bite really cause for concern?  While these miniscule monsters strike fear in many, the question remains, “Are they really the medical malady that we think they are?”  

Most people can tell you about the more dangerous of these “eight-legged monsters” which, in the United States, includes the black widow and brown recluse. While their reputations may precede them, we investigated whether they are really a threat to us humans.  In actuality, they rarely are. While spider bites can cause local tissue damage, their effects are usually just that: local. Serious complications can occur but they do so in very rare cases, proving that a spider’s venom may not be powerful enough (or voluminous enough) to spread throughout your body (even if it was going to make you superhuman). For the most part, bites are usually benign to humans, mainly because spiders are too small and don’t have long enough fangs or strong enough muscles to cause serious damage. Spider venom, as malicious as it sounds, is often too small in volume or does not have any pathologic affect on humans (apparently we are not their first food choice, evolutionarily speaking).  

In the medical world, it is actually very difficult to diagnose a spider bite. The vague red, raised bump on your skin is not unique to a specific insect and can actually be caused by a large list of things other than bug bites. To know for sure that this bump is due to a spider bite, a few things are required. First, you essentially need to see it biting you.  It’s the only way to know, without a doubt, that a spider bit you.  Next, whatever symptoms that you are suffering from need to begin around the time that a bite is seen. Lastly, it is extremely helpful for the spider to be caught (or at least clearly identified) so that any specific treatment for those dangerous bites can be appropriately given. Smashed spiders are just as helpful if it is placed in alcohol right after being killed. Unless the bite is seen and the spider is identified, you will need to have very specific symptoms to know that your itchy red bump is truly a spider bite.

Black Widows

Reports of bites from these notorious creatures have been circulating for more than 2000 years. The Lactrodectus mactans, or the Black Widow, is easily identified by the glaring bright red hourglass on the under-side of its body. These brightly colored females are known for envenomating humans. While the trademark red hourglass is infamous in America, there are more than 30 different known species in countries like Australia where the red marking on their bodies can differ and identifying the culprit may be more complex. The venom that gives these spiders their bad reputation contains lactrotoxin, a toxin that affects its target’s nerves and is thus dubbed “neurotoxic.”  When exposed to lactrotoxin, your nerves uncontrollably release substances called neurotransmitters into the synaptic cleft (the gap between two nerves) and are then trapped there (in a normal nerve, these neurotransmitters are quickly reabsorbed). The increased release of neurotransmitters can have a number of effects on yourr bodydepedning on which nerves are affected.  The most common symptoms include muscle spasms and abdominal pain within one hour after getting bitten. In less common cases, people can develop a fever, feel agitated, and experience tingling in their arms or legs. Bites usually occur when we encounter these little creatures hiding in dark places, like a shoe or an outhouse.  

In rare cases, people may experience heart problems or more systemic symptoms. For these people, the antivenom is usually indicated. Why don’t we give everyone this awesome-sounding antivenom?  We reserve antivenom for the most severe cases because the antivenom itself can cause a number of problems. In 75% of people, the antivenom will cause a notable allergic reaction. To find out if you’re part of this allergic group, bitten individuals are often given a “preliminary dose” to test whether they react or not. If you react, we treat the reaction and don’t give you any more antivenom.  Most widow bites, however, are easily managed symptomatically with ice, loose wrapping, and temporary immobilization of the bitten area.

Brown Recluses

While these predators may have reputations as notorious as the Black Widows, the Brown Recluse is not nearly as recognizable. For those of you brave enough to look closely, you will see that this small spider, usually 1.5 cm long, has what is described as a violin or fiddle design on its back and six eyes instead of eight (we don’t blame you for not wanting to get that close!). These spiders are mostly found in the south central USA, though they can travel around the country via luggage and clothing. The Brown Recluse’s venom notably differs from Black widow’s.  This spider’s venom contains sphingomyelinase which causes destruction of local tissue and red blood cells (thus earning the title, “hemolytic”).  

Most bites, which occur after the spider is disturbed in a pile of clothing or linen, occur without the victim noticing but become painful within 2 to 8 hours of the bite. The skin lesion left behind after the bite is the telltale sign of a Brown Recluse. Because of the local cellular damage caused by the venom, the skin shows a small red lesion that is surrounded by a light red ring. The central area can then progress to a bluish color with some skin depression. In severe cases, giant blisters, or bulla, can form in the bite area within 2-3 days then develop into an deep ulcer 3-5 days later. To make matters worse, the damaged tissue, if not cared for and cleaned properly, can become secondarily infected. These cases can take up to 2 weeks to recover fully. These more severe reactions are more likely to occur in overweight individuals and are quite rare (one study showing that only 3 out of 111 bites were severe enough to require a skin graft). Similar to Black Widow bites, uncomplicated bites only require symptomatic treatment, including ice, compression, and regular wound care. For more serious cases, heavy hydration is given to prevent kidney problems and excision of severly infected skin may be performed six to eight weeks after being bitten (one more excellent reason to keep your bites clean!).

The Take Home Message

In some rare cases, a spider bite may be as bad as the horror stories you’ve heard.  As a general rule, however, you don’t have much to worry about from these little buggers. Like most things in nature, remember that the spiders fear you infinitely more than you fear them (in their multiple pairs of eyes, we are the attacking giants). If you are the unlucky recipient of a presumed spider bite, rest assured that the small, itchy red bump is not cause for concern and can be simply treated at home with ice and regular cleaning (saving you the trouble of waiting in the doctors office, standing in a long line in an emergency room, or making a phone call to an Australian spider bite hotline). Luckily (if we can say that), bad spider bites aren’t sublte.  If you develop any of the more severe reactions we discussed (which will be very apparent), pursue one of those three options for further evaluation.  While legends and movies continue to demonize arachnoids as “Eight Legged Monsters,” you can sleep a little bit more soundly at night knowing that we are the real giants in the battle between humans and spiders.

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  1. Braitberg, G et al. “Spider bites: Assessment and management.” Australian Family Physician 2009. 30 (11): 862- 867,
  2. Saucier J. “Arachnid envenomation” Emergency Medicine Clinics of North America 2004. 22: 405-22.
  3. Vetter RS et al. “Medical Aspects of Spider Bites.” Annual Reivew of Entomology 2008. 53: 409-29.


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