How to Tell a Spider Bite From a MRSA Infection

And when it could be another bite entirely

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If you find a little blister, you may assume that you've gotten a spider bite. And, in many cases, you will be right, particularly if the blister is filled with clear fluid. But, if the fluid looks cloudy or pus-like, you may have a potentially serious infection known as methicillin-resistant Staphylococcus aureus (MRSA).

Spider bites can cause one or several rashes or lesions with fluid-filled blisters called bullae. However, other insect bites can too. And although MRSA looks very similar, it typically causes multiple rashes with small, pus-filled bumps called pustules.

This article explains the key differences between MRSA and spider bites, as well as other common bug bites. It also describes treatments and complications of MRSA and when it is time to see a healthcare provider.

What bit me?

Verywell / Jessica Olah

Signs and Symptoms

Spider bites and MRSA are sometimes particularly difficult to tell apart, especially in the early stages when symptoms are just starting to emerge.

This is why it is important to monitor any suspected spider bite for changes in its appearance or numbers. It may not end up being a spider bite at all. And if it's MRSA, fast treatment is needed.

Number of Bumps

While spider bites tend to be solitary, it is possible to get a few separate bites at once.

MRSA may start with one or several bumps, but the rash almost invariably grows in size and amount, causing a cluster of pustules that can merge into a larger pus-filled mass.

Appearance of Blisters

Spider bite symptoms can vary from person to person as well as by species. Typically, spider bites result in fluid-filled blisters (bullae) that can burst and form open ulcers.

If you get several in the same spot, you may experience a purplish discoloration of the skin with swelling, pain, warmth, and hardening.

In contrast, MRSA causes pustules. These bumps form when the immune system sends white blood cells to fight the infection.

This causes inflammation (swelling) at the site of infection and death to nearby tissues. Pus, the collection of the dead bacteria, white blood cells, and tissues, then fills the bumps.

Rash Type Skin Appearance  Progression
MRSA A bump or infected area of skin that is red, swollen, painful, and warm, typically with pus-filled bumps Can progress rapidly and spread over hours or days, causing larger pockets of pus
Spider A red, inflamed bump on the skin that can be itchy or painful and have a fluid-filled blister Does not spread but can form small ulcers when the blister pops

While spider bites are commonly recognized by the telltale blister, other bites can do the same if you are sensitive to them. MRSA can mimic them as well.

Rash Type Description  Features
Mosquito A puffy, reddish, itchy bump or wealth that often has a well-defined, irregular border Does not spread but can darken and harden
Chigger Itchy red bumps that look like pimples, blisters, or small hives, usually found around the waist, ankles, or in warm skin folds Can get bigger and itchier over several days but rarely causes pus unless there is a secondary infection
Tick A red, warm bump or welt, often with visible puncture marks, that is usually not painful or itchy Does not spread but may expand to the size of a dime
Bed bugs Often cause a line or cluster of red, intensely itchy spots on the back of the neck, shoulders, arms, and legs Can spread if the infestation is not treated
Lice Small reddish, itchy bumps, sometimes with crusted blood, that are most often found on the back of the head or ears Can spread if the infestation is not treated

Importance of Identification

Both MRSA and spider bites can lead to serious complications. With spider bites, complications are related to how venomous the particular spider involved is and/or how vulnerable or sensitive you are to the venom. With MRSA, the concern is related to the dissemination (spread) of the infection internally.

These complications, some of which are life-threatening, further underscore the importance of telling the two conditions apart and getting treated quickly, if needed.

Complications of Spider Bites

In North America, four of the most venomous spiders are the black widow, brown recluse, hobo spider, and armed spider. The venoms of these four spiders are neurotoxic, meaning that they attack the nervous system.

Most cases cause headaches, lethargy, irritability, muscle pain, tremors, and impaired coordination.

But, in severe cases, potentially life-threatening complications can rapidly develop, including:

Complications of MRSA

In most cases, MRSA causes a mild skin infection. If left untreated, however, MRSA can cause complications affecting multiple organ systems.

This occurs when the bacteria disseminate beyond the skin through blood and lymph vessels, causing a systemic (whole-body) infection. Some of the possibly life-threatening complications include:

MRSA is easily spread by skin-to-skin contact or contact with a contaminated object or surface. Being aware of and treating an infection as early as possible is important not just for your health, but that of those around you.

When to Call a Healthcare Provider

Knowing the difference between a spider bite and MRSA can help you respond appropriately, particularly in emergency situations. As a rule of thumb, if you're not sure what your blister or rash is but are worried, have it checked out.

Call 911 or seek emergency medical treatment if you experience any of the following after getting or finding what you think might be a bite of any kind:

  • Severe pain, swelling, or redness at the bite site
  • Lightheadedness, dizziness, or fainting
  • Severe muscle or abdominal cramps
  • Nausea or vomiting
  • Difficulty breathing
  • Trouble swallowing
  • High fever
  • Disorientation
  • Loss of coordination
  • Rapid, slowed, or irregular heartbeats
  • Extreme muscle stiffness or spasticity
  • Seizures

Treatment

Spider bites often require no treatment unless you have a severe reaction. The same cannot be said for MRSA because it can be so easily spread.

If you have a small skin eruption caused by MRSA, your healthcare provider may make an incision to drain it.

Until the wound is fully healed, you need to keep it covered and take standard precautions to avoid infecting others. Wash your hands frequently and avoid the shared use of clothing, towels, and personal care items.

Not all MRSA infections need to be treated with antibiotics. And by definition, MRSA is resistant to some of these drugs. Still, there are several that can be used alone or in combination to treat the infection.

Antibiotics with demonstrated activity against MRSA include:

The drugs may be given in orally in pill form (to be taken by mouth). Severe cases may need to be treated intravenously (by delivering the drugs into a vein).

If placed on the appropriate antibiotics, the symptoms of MRSA will usually resolve within seven to 14 days.

If you are prescribed oral antibiotics, you need to take the drugs as prescribed and to completion even if you're feeling better. If you don't, the infection may return. Worse yet, a more resistant strain may emerge and be harder to treat.

Signs MRSA Treatment Is Not Working

With that said, certain MRSA strains are harder to treat and may not respond as quickly or as well. In such cases, you would need to see a healthcare provider if:

  • The infection does not improve after three or four days of antibiotic therapy.
  • The rash begins to spread or worsen.
  • You develop a fever, or your fever get worse.
  • The symptoms return soon after antibiotic therapy is completed.

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.

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

  2. Rahmani F, Kohjasteh SMB, Bakhtavar HE, Rahmani F, Nia KS, Faridaalaee G. Poisonous spiders: bites, symptoms, and treatment; an educational review. Emerg (Tehran). 2014 Spring;2(2):54–8.

  3. Del Brutto OH. Neurological effects of venomous bites and stings: snakes, spiders, and scorpions. Handb Clin Neurol. 2013;114:349-68. doi:10.1016/B978-0-444-53490-3.00028-5

  4. Centers for Disease Control and Prevention. What is MRSA?

Additional Reading

By Daniel More, MD
Daniel More, MD, is a board-certified allergist and clinical immunologist. He is an assistant clinical professor at the University of California, San Francisco School of Medicine and currently practices at Central Coast Allergy and Asthma in Salinas, California.