Cause of Pus Blisters: Bug Bites or MRSA?

Bug bites can cause pus blisters, but so can a potentially serious infection known as methicillin-resistant Staphylococcus aureus (MRSA). While bites and infections can be confused, there are ways to tell them apart.

Spider bites and other insect bites can cause one or several rashes or lesions, with fluid-filled blisters called bullae. 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 insect and spider bites. It also describes treatments and complications of MRSA and when it is time to see a healthcare provider.

Signs and Symptoms of Bug Bites vs. MRSA

Spider bites, insect stings, 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 insect 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.

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.

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. Other issues can arise with different insects.

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

There are five types of venomous spiders that cause the most risk for human, including the black widow, brown recluse, and hobo spider. The spider venoms 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. They include:

• Tachycardia (abnormally rapid heartbeats)
• Bradycardia (abnormally slowed heartbeats)
• Acute renal failure (sudden kidney failure)
• Rhabdomyolysis (the potentially deadly breakdown of muscles)
  
• Stroke (typically caused by a severe spike in blood pressure and a spontaneous brain bleed)

Anaphylaxis (a potentially life-threatening, whole-body allergy) can occur with spider bites.

Complications of Other Bug Bites

Beyond spiders, other insect bites and stings can lead to complications beyond discomfort. The life-threatening anaphylaxis that can occur with spiders also can happen with bee stings and black ants. Other conditions can include:

• Impetigo, which can follow an insect bite or scabies parasite infection
• Cellulitis, a skin infection that can follow bites from fleas, bedbugs, mosquitoes, and other insects
• Lymphangitis, typically caused by bacteria that's spread by tick bites, cat scratches, and more

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:

• Pneumonia (an infection of the lung)
• Endocarditis (an infection of the heart)
• Meningitis (an infection of the lining of the brain and spinal cord)
• Osteomyelitis (a bone infection)
• Septic arthritis (joint pain inflammation caused by a bacterial infection)
• Sepsis (the body's extreme overreaction to an infection)

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 skin color changes (redness, purple) 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, loss of consciousness, and even death can occur without immediate treatment of certain insect bites, including the black widow spider.

Treatment

Insect bites from spiders, mosquitoes, fleas, and more 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:

• Bactrim (trimethoprim-sulfamethoxazole)
• Vancocin (vancomycin)
• Cubicin (daptomycin)
• Zyvox (linezolid)
• Targocid (teicoplanin)

The drugs may be given 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.

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 or lesion does not improve after three or four days.
• The rash begins to spread or worsen.
• You develop a fever, or your fever gets worse.

Symptoms that don't respond to antibiotics, or that return soon after antibiotic therapy is completed, need to be reported to your provider.

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, visit your healthcare provider for tests.