"I Think I Got Bitten By a Spider"

The release of "Spiderman, the Movie" seems to be an opportune time to discuss the true nature of spider bites. In the movie a man is bitten by a radioactive spider and subsequently evolves spider-like abilities. This is obviously pure fantasy but, many people suffer from arachnophobia, a morbid fear of spiders, and erroneously feel the need to get expert medical opinion for every suspected bite. 

"I'm afraid it was poisonous" is a common statement accompanying a visit for a suspected bite. That statement is both right and wrong. All except 2 species of spiders are venomous. However, only a small percentage of spiders have the anatomic capability, that is, adequate fang structure, to penetrate human skin. Of the one's that do, only a few have adequate venom to cause a noteworthy reaction. 

Typical spider venom is a potion of digestive enzymes. Spiders don't chew their food, they drink it by predigesting it with their venom. It is this digestive action that causes the skin reaction in humans. The vast majority of reactions are a simple, transient skin irritation and that do not require any medical attention. Occasionally more severe reactions occur that include skin breakdown, blistering, and scab formation. Even under these circumstances, simple home remedies are adequate, like cleansing and applying an ice pack and an off-the-shelf antibiotic ointment. Professional medical intervention is appropriate when there is a sign of secondary infection of the wound as signaled by findings such as an expanding red border, pus drainage, or red streaks extending away from the wound. 

The above is what to expect from the average bite of commonly encountered spiders in the United States. However, there are 2 notorious spiders which account for the greatest proportion of serious reactions. These are the Brown Recluse (Loxosceles reclusa) and the Black Widow (Latrodectus mactans).

The Brown Recluse Spider

The Brown Recluse, also called the Fiddle-back Spider due to its distinctive black violen shaped marking on its back, is not aggressive by nature and only bites when it is disturbed or in self defense. Half of the time, the bite is not even felt by bitten person and they don't even recall coming in contact with a spider. People contact the spider by intruding into its domain which is, as expected by its name, reclusive. They are encountered in sheds, attics, crawl spaces, woodpiles, piles of rags, etc. 

After a genuine envenomation from a Brown Recluse, pain develops in six to eight hours and the bite site begins to look bruised. Over the next few days, the skin breaks down and ulcerates beneath a blood blister. This process can continue over a period of weeks and the resulting wound can take many months to finally heal to a scar. Rarely, surgical treatment such as skin grafting is required for wounds that stubbornly refuse to heal. Small wounds such as those under a dime size, are treated as any other spider bite. 

Cortisone type steroids were once used to treat the advancing edge of an ulcer but this practice has never shown any objective benefit and so is not employed any longer. Another drug, dapsone, has been used experimentally for severe bites. Unfortunately, this drug has its own side effects which may cause more problems than it solves by virtue of its adverse effect on blood cells. So patients are carefully chosen and must undergo monitoring of blood counts while on this agent.

On extremely rare occasions, the bite of a brown recluse may deposit venom directly into a small blood vessel. This can cause a total body reaction identical to that of a rattlesnake bite. This "one in a million" event produces fever, vomiting, blood cell rupturing, kidney failure, and potentially progress to coma and even death.

The Black Widow Spider

The Black Widow, so called due its instinctive devouring of the male of her species after mating, inhabits similar habitat as the Brown Recluse. It has a very distinctive, round, shiny, black body with the characteristic red hourglass marking on its underside.

The venom of the Black Widow is a peculiar exception to rule in that it is not composed of digestive enzymes, but is a very potent nerve agent. Like other spiders, its bite may not be perceived by a human victim, but the toxic nature of its venom will make itself known within the hour. Typically muscle spasms begin around the site of the bite which then progresses to the muscles of the entire body, particularly those that make up the abdominal wall. The picture painted by a victim of a black widow bite is so distinctive that most Emergency Physicians, who are usually the first to see it, can make the diagnosis without benefit of any history of a bite at all. More severe envenomations can produce dangerously high blood pressure, fever, and vomiting.

Treatment is done in the hospital setting as potent muscle relaxants and pain medication are usually required to control symptoms. In addition there is also an antivenin for use when necessary. The antivenin is derived from horses so it is not administered to every bite victim due to the potential for immune reactions that can be harmful to the recipient. It is generally used on individuals who are unstable or resistant to conventional treatment.

In conclusion, most spider bites are relatively simple events requiring only home remedies. The dreadful fear of dire consequences are generally unfounded. That is not to say that deaths do not occur, they do.  Across the country there are four or five deaths annually from venomous spiders. That should hopefully convey some measure of reassurance as this means one is 40 to 50 times more likely to succumb to a strike from a random bolt of lightning!

– Michael J. Varney, M.D.