Spider Bites

Symptoms, Treatment & Identification

Illustration by Joshua Seong. © Verywell, 2017.

Let's get this out of the way: That red bump is probably not a spider bite. Odds are, it's a skin infection. The most common bacterial infections are either staphylococcus aureus or streptococcus. Both of those create red bumps that get hot, swollen, and spread out.

If it is a spider bite and you got it in the United States, it's very unlikely that you will die from it. Of all the spiders in North America, only two are medically significant (capable of actually making you sick): the black widow and the brown recluse.

Plenty of skin rashes and sores get diagnosed in the emergency department as spider bites, but the reality is that most are not. Skin boils from infections are often mistaken for spider bites in the United States, even by doctors.

One case study of an outbreak of bites in military barracks—at least what authorities thought were bites—turned out to be MRSA. All the more reason to be skeptical of a diagnosis of spider bite in the emergency department. If you didn't see the spider bite you, then the chance that it's an actual spider bite is pretty slim. It's almost impossible to identify a spider bite just by how it looks.

Symptoms of Black Widow Spider Bites

In the United States, black widow spiders are considered the most venomous. They have a toxin capable of a systemic reaction that affects muscle and nerve function. Black widow spider bite symptoms can be used to diagnose these bites and include:

  • "Fang" marks (tiny twin holes): telltale signs that are only visible right after the bite happens, before any swelling or redness occurs
  • Sweating
  • Chills
  • Headache
  • Dizziness
  • Muscular cramps (legs, stomach, etc.)
  • Rapid pulse
  • Exhaustion

Other countries have a widow spider called the redback spider. It's a good idea to consider any shiny black spider with a red mark to be in the widow family.

Another type of widow spider, the brown widow, generally has a weaker toxin and can be found worldwide.

Symptoms of Brown Recluse Spider Bites

Despite the terrible wounds often associated with brown recluse, they are much less likely to cause significant injury than black widows. Skin infections may cause boils, which are often misdiagnosed as brown recluse spider bites (take a look at spider bite pictures to help you tell the difference).

It's easier to figure out when a bite is not from a recluse than when it is. One tool to rule out brown recluse bites is to use the mnemonic NOT RECLUSE. This mnemonic helps point out things that are not present in brown recluse bites.

  • Numerous: If there are more lesions than just one or two, they're not from a brown recluse. Brown recluse bites come in ones and sometimes two.
  • Occurrence: The way that a brown recluse bite usually occurs is through disturbing the spider. Most of the time it's hidden away in a closet or an attic, possibly in boxes. Occasionally, the spider can find its way into the bed and bite the patient in his or her sleep. If the occurrence was during gardening, it's probably not a brown recluse bite.
  • Timing: If the bite didn't show up from April to October, it's very unlikely that it's a brown recluse bite.
  • Red center: Brown recluse bites are almost never red and inflamed in the center of the lesion. Usually, they're pale, blue, or purple. They can be red around the periphery, but if it's red in the center, it's probably not a brown recluse.
  • Chronic: If it takes longer than three weeks for a small bite or three months for a bigger bite, it's a 95 percent chance that it's not a brown recluse bite. If your doctor is still telling you that's the problem, it's time for another opinion.
  • Large: The biggest recluse lesions (the tissue that's dying) have been documented to be smaller than 10 centimeters across (four inches). They can be red and swollen well past that area, but the necrosis won't be any bigger.
  • Ulcerates too early: For non-medical folks, this one might be hard to remember. Brown recluse bites take at least a week to ulcerate (break the skin and crust over). If it's crusty before seven days, it's probably not a brown recluse bite.
  • Swollen: Brown recluse bites are flat or sunken in the middle, not swollen. If it's swollen (especially if it's red), it's not a brown recluse. The exceptions to this rule are bites on the face, especially the eyelids. Those swell significantly.
  • Exudes moisture: Brown recluse bites are dry, not moist. If it has pus oozing out of it, it's an infection rather than a spider bite.

Brown recluse spiders are only found in the Southeast United States and are very difficult to identify, even by trained arachnologists. There are several other species of recluse spider that are similar but less studied than the brown recluse. Brown recluse spiders are often described as having a violin-shaped mark on the back of their midsections, but that mark can be inconsistent and very faint.

The Other Spiders

There are more than 37,500 species of spider in the world. Almost all spiders are venomous. That's how they hunt. Most spiders are too small, or their venom too weak, to be dangerous to humans. Some spiders are pretty well-known and seem to get blamed for most of the spider bites out there, even though there's no evidence to support that those lesions are even from spiders.

Here are some North American spiders that get a bad rap without much evidence to support their reputations:

  • Hobo Spiders (Tegenaria agrestis): These guys were introduced into the Northwest United States from Europe in the 1980s. Since then, they've been blamed for instances of necrotic arachnidism, medical jargon for tissue death by spider envenomation. A study published in the Journal of Medical Entomology in 2011 found no medical significance from hobo spider envenomation. 
  • Wolf Spiders (Lycosidae): Originally from Australia, wolf spiders are commonly thought to be very dangerous. On the contrary, case presentations of presumed wold spider bites in the United States have reported nothing significant. A study of 45 definite wolf spider bites in Australian showed that they "cause minor effects, no more severe than most other spiders." Wolf spiders didn't cause necrosis like the brown recluse and most of the damage was mechanical, not from venom. 

Other parts of the world have much more dangerous species than the United States. Australia has the red back spider (similar to a black widow), white tail spiders, and deadly funnel-web spiders.

Determining whether a bite is from a spider may be impossible. Studies of brown recluse bites suggest that patients seek treatment more than three days after their bites, making it nearly impossible to identify the culprit. Black widow bites are often identified only by symptoms of its venom, without any visible local bite.

Local reactions to bites from all kinds of toxic bugs look the same: redness, swelling, itching, and pain. Be concerned if a local reaction continues to get worse for more than 24 hours. Look for redness spreading away from the bite, drainage from the bite, increase in pain, numbness/tingling, or a discoloration around the bite that looks like a halo or a bullseye.

You should also call a doctor if you are not up to date on your tetanus vaccination.

How to Treat a Spider Bite

You can treat all bug bites the same unless it's a brown recluse or a black widow bite. Call a doctor or go to the emergency department if symptoms persist more than 24 hours or get worse, or if you think it's a brown recluse or black widow. Some articles on the internet tell you to treat venomous spider bites differently than non-venomous bites, but as you'll see, there's no such thing as a non-venomous spider bite.

If you go to the doctor for a reaction, don't start out by telling him or her it's a spider bite—even if you think that's what it is. Doctors are people, too, and they are susceptible to the power of suggestion just like the rest of us. Instead, tell the doctor, "I have this rash (bump, boil, red spot, black spot, etc.) and I was wondering if you could tell me what it is and what I can do for it."

There are many home remedies for treating insect and spider bites, but do they help? Most of these have not been shown to provide any real benefit. Meat tenderizer (papain) has even been implicated in allergies and asthma reactions to its protein. Suction syringes that are sold to extract toxins do not work and are a complete waste of money.

Allergic Reactions to Spider Bites

Anaphylaxis is always the biggest concern with any type of bug bite. If the victim exhibits any signs of allergic reaction or anaphylaxis shortly after a bug bite, call 911. Other symptoms can include:

Seek medical treatment if symptoms appear in parts of the body away from the bite.

Sources:

Gaver-Wainwright MM, Zack RS, Foradori MJ, Lavine LC. Misdiagnosis of spider bites: bacterial associates, mechanical pathogen transfer, and hemolytic potential of venom from the hobo spider, Tegenaria agrestis (Araneae: Agelenidae). J Med Entomol. 2011 Mar;48(2):382-8.

Isbister GK, Framenau VW. Australian wolf spider bites (Lycosidae): clinical effects and influence of species on bite circumstances. J Toxicol Clin Toxicol. 2004;42(2):153-61.

Livshits, Z., Bernstein, B., Sorkin, L., Smith, S., & Hoffman, R. (2012). Wolf Spider Envenomation. Wilderness & Environmental Medicine23(1), 49-50. doi:10.1016/j.wem.2011.11.010

Pagac BB, Reiland RW, Bolesh DT, Swanson DL. Skin lesions in barracks: consider community-acquired methicillin-resistant Staphylococcus aureus infection instead of spider bites. Mil Med. 2006 Sep;171(9):830-2.

Payne, K., Schilli, K., Meier, K., Rader, R., Dyer, J., & Mold, J. et al. (2014). Extreme Pain From Brown Recluse Spider BitesJAMA Dermatology150(11), 1205. doi:10.1001/jamadermatol.2014.605

Robinson, J., Kennedy, V., Doss, Y., Bastarache, L., Denny, J., & Warner, J. (2017). Defining the complex phenotype of severe systemic loxoscelism using a large electronic health record cohortPLOS ONE12(4), e0174941. doi:10.1371/journal.pone.0174941

Stoecker, W., Vetter, R., & Dyer, J. (2017). NOT RECLUSE—A Mnemonic Device to Avoid False Diagnoses of Brown Recluse Spider BitesJAMA Dermatology. doi:10.1001/jamadermatol.2016.5665

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