How to Tell an Insect Bite From a MRSA Infection

It's easy to confuse their early symptoms

It is not unusual for people to say they have a spider bite even when they didn't actually see a spider or other insect.

It's a logical thought, especially if the rash covers a small area, the person has no history of a contact allergy, and there are pimple-like bumps instead of the rash you might expect from eczema, allergy, or an infection.

The one clue that a wound is not a spider bite is the presence of a pustule. A pustule is a small pimple or boil filled with pus. A spider bite may fill with fluid, but it's not usually pus. 

While pus may suggest acne, it could be a more serious condition called community-acquired methicillin-resistant Staphylococcus aureus (MRSA).

This article explains some key differences between MRSA and spider bites. It also describes what different types of insect bites can look and feel like.

What bit me?

Verywell / Jessica Olah

Understanding MRSA

MRSA is a type of bacteria that causes significant and sometimes life-threatening infections of the skin and other organs. It used to occur only in hospitals, where bacteria can become resistant to antibiotics.

In recent years, however, MRSA can be found in community settings such as schools, gyms, 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, harming your lungs, urinary tract, and other 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. Most are silent carriers who don't develop any symptoms.

Bite Symptoms

Spider bite symptoms can vary from person to person as well as by the species. Typically, spider bites result in a fluid-filled blister that can burst and form an open ulcer.

It is possible to have a few separate bites. In that case, you might see red or purplish discoloration of the surrounding skin. While pus doesn't usually develop in the blister, it can when the blister is open and ulcerated.

Other types of insect bites have the following features:

  • Mosquito bites leave a red, puffy bump.
  • Flea bites can look like several red bumps surrounded by a red "halo." They are itchy and occur mostly around the feet and ankles.
  • Chigger bites appear as welts, blisters, or hives. They can be found 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 to check for Lyme disease. Tick bites can sometimes cause a painful, burning sensation but are most often not felt or 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 tell the difference 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 merge 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 MRSA with a simple culture of pus or tissue.

Treatment usually involves oral antibiotics such as trimethoprim-sulfamethoxazole and doxycycline. Occasionally, an abscess may be need to be drained.

It's vital to get proper medical care quickly, because a delay could result in potentially life-threatening complications or even death.

Summary

It can be hard to tell the difference between a spider bite and MRSA. The most important clue may be the kind of blister that forms. MRSA blisters contain pus and spider bites contain fluid. Eventually, MRSA boils often expand and spread.

MRSA is an antibiotic-resistant bacteria that can be found in lots of community settings. If it's not treated, it can enter the bloodstream and cause life-threatening infections.

If you're not sure whether you have been bitten by a bug or you have a MRSA infection, the best thing to do is visit your healthcare provider for tests.

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?

  2. Centers for Disease Control and Prevention. MRSA and the workplace.

  3. Nemours. KidsHealth. First aid: Spider bites.

  4. Nemours. KidsHealth. What are bedbugs?

  5. Merck Manual Consumer Version. Staphylococcus aureus Infections.

Additional Reading