University of Pittsburgh research explains why scratching worsens itches

University of Pittsburgh research explains why scratching worsens itches

The childhood warning to avoid scratching bug bites and rashes is backed by cellular science, according to research from the University of Pittsburgh. While scratching provides immediate relief, it can trigger a self-sustaining cycle of inflammation and itching that prolongs the discomfort.

Dr. Daniel Kaplan, a dermatologist whose lab focuses on immune reactions in the skin, investigated this phenomenon by studying allergic contact dermatitis. This common type of itch is typically triggered by irritants such as nickel in jewelry or poison ivy.

The "Cone of Shame" Experiments

To isolate the effects of scratching from the itch itself, Kaplan’s team used mice. The researchers applied a rash-inducing irritant to the ears of the animals. In normal mice, the act of scratching led to an influx of inflammatory immune cells at the site, which increased swelling.

The team compared these results to mice bred with defective itch-sensing nerve cells, which experienced a much milder rash. To confirm that the scratching was the primary cause of the increased inflammation, researchers placed normal mice in veterinary collars, often called cones of shame, to prevent them from scratching. These mice showed significantly fewer inflammatory cells and less swelling than those that were able to scratch.

Kaplan noted that this evidence mirrors human experience. He stated that a mosquito bite is typically gone in five or 10 minutes for most people if ignored. However, if a person begins scratching, the bite can become your friend for a week as it becomes more inflamed and itchier.

The Cellular "Double Whammy"

The biological driver of this cycle involves mast cells, which are first responders for the immune system. These cells release compounds to fight toxins or germs, but they also release histamine, which triggers allergic reactions. While allergens are known to activate mast cells, pain does as well.

Kaplan observed that people tend to scratch until it starts to hurt. When this happens, pain-sensing nerve cells release a chemical messenger known as substance P. In findings published last year, Kaplan’s team reported that substance P activates mast cells through a molecular pathway different from the one used by allergens. This dual activation creates a double whammy that intensifies the inflammation of the bite or rash.

Evolutionary Trade-offs

The tendency to scratch is widespread across species, occurring in humans, other mammals, and even fish. One theory suggests scratching helps creatures remove parasites like mites or fleas. Kaplan also looked into whether mast cells help defend against Staphylococcus aureus, a common skin bacterium.

In experiments where mice were infected with the bacteria, those that scratched had lower levels of the germ on their ears. This suggests that the resulting inflammation or other mast cell compounds may provide some germ protection. Despite this, Kaplan stressed that scratching is deleterious and advised that people should avoid scratching, even if it is easier said than done.

Breaking the Cycle

Beyond the physical damage of skin wounds, scarring, and infections, excessive scratching can lead to anxiety and stress. Some individuals experience a phenomenon where scratching in one area causes them to itch elsewhere on the body. Furthermore, the release of serotonin—a pain-fighting chemical—can sometimes make an itch feel more intense.

To break the itch-scratch cycle, dermatologists recommend several strategies:

• Topical Treatments: Hydrocortisone cream, calamine lotion, and oatmeal baths are recommended for contact dermatitis and bug bites.
• The "Cheat Code": Kaplan suggests menthol-containing creams, which can fool the skin into sensing cold rather than itch.
• Physical Alternatives: If the urge to scratch is irresistible, rubbing, patting, tapping, gently pinching the skin, or holding the area tightly is preferred over using fingernails.
• Skin Maintenance: Using scent-free hydrating cleansers and applying moisturizing cream daily helps prevent dry skin, which can exacerbate itching.

Medical treatments vary by cause. Antihistamines can manage some mast-cell triggered itches, while corticosteroids are used for psoriasis and eczema. For itches caused by nerve pain, such as multiple sclerosis, doctors may prescribe anti-seizure medications or antidepressants.

Looking forward, drug companies are developing MRGPRX2 blockers. These target the specific molecular pathway linked to scratching identified by Kaplan’s team. He hopes this understanding will eventually lead to improved treatments for chronic skin diseases like eczema.