How to Tell an Insect Bite From a MRSA Infection

Misdiagnosis Is Common During Early Appearance of Rash

It is not unusual to hear people complain of spider bites even though they may never have actually seen a spider or any other type of insect.

It is not an unfair assumption given that the rash may not be particularly widespread, the person may have no history of a contact allergy, and the outbreak itself may involve pimple-like pustules instead of the types of rash associated with eczema, allergy, or an infection.

The one clue that a "spider bite" is anything but is the presence of a pustule, a small pimple or boil filled with pus. While a spider bite may fill with fluid, pus is not generally involved. 

While pus may suggest a breakout of acne, it is possible that it is a more serious condition called community-acquired methicillin-resistant Staphylococcus aureus, (MRSA).

What bit me?
Illustration by Jessica Olah, Verywell

Understanding MRSA

MRSA is a bacteria that causes significant and sometimes life-threatening infections of the skin and other organs. It used to be a problem that occurred only in hospitals, where the widespread use of antibiotics led to the development of antibiotic-resistant bacteria.

In recent years, however, MRSA can be commonly found in community settings such as schools, gyms, health spas, and even nail salons.

The symptoms of MRSA can vary depending on where you're infected. In most cases, it causes a mild infection of the skin, such as a sore or boil. At other times, it can cause a more serious infection that spreads into the bloodstream, causing injury to the lungs, urinary tract, and other major organ systems.

Because it is so hard to treat, MRSA is sometimes referred to as a "superbug." All told, around 1% of the population has MRSA although most are silent carriers.

Symptoms of Spider and Insect Bites

Spider bite symptoms can vary from person to person as well as by the species of insect. Typically speaking, spider bites will result in the formation of a fluid-filled blister that can burst and lead to the development of an open ulcer.

It is possible to have a few separate bites, generally localized with the red or purplish discoloration of surrounding skin. While pus doesn't usually develop in the blister, it can when the blister is open and ulcerated.

Other types of insect bite are characterized by the following features:

  • Mosquito bites will result in a red, puffy bump of skin.
  • Flea bites can appear with multiple red bumps surrounded by a red "halo." They are itchy and occur mostly around the feet and lower extremities.
  • Chigger bites appear as welts, blisters, or hives, occurring mostly in folds of skin or where clothing is tight against the skin.
  • Tick bites are most often singular. If you develop a spreading red rash around the site of the tick bite, you should see a healthcare provider for evaluation of possible Lyme disease. They can present with a painful, burning sensation but are most often not felt or even noticed.
  • Bed bug bites often appear in a line or group with a dark red center.
  • Lice bites happen mainly on the scalp.

MRSA vs. Spider Bite

There are two main symptoms that should help you differentiate between a spider bite and MRSA:

  • Fluid-filled blister (spider) versus pus-filled blister (MRSA)
  • Single or few lesions (spider) versus multiple lesions (MRSA)

If it is MRSA, a small cluster of pustules will often consolidate into a larger and expanding mass of pus, something that insect bites rarely do. 

When to Call Your Healthcare Provider

If in doubt, call your healthcare provider, who can diagnose ​an MRSA with a simple culture of pus or tissue. Treatment usually involves the use of oral antibiotics such as trimethoprim-sulfamethoxazole and doxycycline. Occasionally, surgical drainage of an abscess may be needed.

Improper or delayed treatment could result in potentially life-threatening complications or even death.

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5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. What is MRSA? Reviewed June 26, 2019.

  2. Centers for Disease Control and Prevention. MRSA and the workplace. Updated August 17, 2015.

  3. Nemours. KidsHealth. First aid: Spider bites. Reviewed July 2018.

  4. Nemours. KidsHealth. What Are Bedbugs? Reviewed February 2019.

  5. Merck Manual Consumer Version. Staphylococcus aureus Infections. Revised June 2019.

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