Spider Bite or Skin Infection?

MRSA Skin Infection

Learn the difference between a spider bite and skin infection. © US Dept of Agriculture

Many of my patients complain of "spider bites" as a reason for skin sores that appear red, painful, swollen and itchy. Most of these people never saw the spider that supposedly caused the "bites", but assume nevertheless that a spider bite was the cause of the sores. Physicians also commonly misdiagnose skin lesions as spider bites, even though spider bites only rarely occur. The more likely cause of these sore, especially when there are multiple, is a skin infection caused by community acquired methicillin-resistant Staphylococcus aureus (MRSA).

What Is MRSA?

MRSA is a bacteria that has caused significant, sometimes life-threatening infections of the skin. It used to be a problem only in hospital settings, where the use of antibiotics led to the development of antibiotic-resistant bacteria. However, in recent years MRSA can also be found in community settings, such as schools, gyms, nail salons and health spas. The treatment of MRSA includes antibiotics such as trimethoprim-sulfamethoxazole and doxycycline. The misdiagnosis of MRSA as a spider bite could lead to improper treatment (no antibiotics) and potential life-threatening complications.

Which Spider Bites Can Be Confused with MRSA?

The brown recluse spider (Loxosceles species) is commonly found in the Southwestern and Southern United States. It is characterized by the presence of a dark brown pattern on the middle of their backs that resembles a violin that is bordered by three pairs of eyes.

They are often found in bedding, linens and piles of clothing and often bite people when they are trapped between fabric and the victim’s skin.

Brown recluse spider bites are initially painless. Within a few days, they result in a necrotic, inflammatory skin lesion. Some people experience systemic symptoms, such as body aches, fever, chills, nausea and vomiting.

Less common complications include seizures and kidney failure. Treatment usually involves local wound care, tetanus prophylaxis, antibiotic treatment for secondary infections and surgical debridement of necrotic tissues.

How to Tell the Difference Between Spider Bites and MRSA?

So, how do you know the difference when the appearance is very similar? First of all, if a person doesn't live in an area of the United States where brown recluse spiders are found, it is less likely that a skin sore was caused by a spider bite. If there are multiple or recurrent skin sores, spider bites are also less likely to be the cause. The diagnosis of a MRSA skin infection can only be made by a physician, usually with the use of a skin culture. Treatment usually involves the use of oral antibiotics and occasionally surgical drainage of any skin abscesses.

Learn more about the diagnosis and treatment of spider bites.


Kemerer JJ, Reitz M. Diagnosis of Brown Recluse Spider Bites is Overused. Am Fam Physician. 2007;76:943-7.

Diaz JH, Leblanc KE. Common Spider Bites. Am Fam Physician 2007;75:869-73.

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