Scratching an Itch: Insights into Inflammation and Bacterial Defense

Scratching an insect bite or rash might seem tempting, but it only provides brief relief and may worsen the itch over time, according to recent research. The act of scratching is a natural reaction to skin irritants, which can increase skin inflammation and worsen conditions like eczema.

Researchers from the University of Pittsburgh highlight that why scratching causes increased swelling and whether this evolutionary response has benefits are not well understood. To explore the connection between itching, scratching, and inflammation, the researchers led by Professor Daniel H. Kaplan applied an allergen to mice ears, inducing allergic-contact dermatitis, a common skin reaction.

The experiment involved two groups of mice. One group was ‘normal,’ while another had a genetic modification to lack an itch-sensing neuron. Some mice were allowed to scratch freely, and others wore an Elizabethan-style collar, often called the ‘cone of shame’ by pet owners, preventing them from scratching.

The findings revealed that scratching activated mast cells, described by the Cleveland Clinic as part of the body’s ‘alarm system,’ which releases histamine and other chemicals causing itchiness and inflammation. Mice allowed to scratch freely released more of the signaling molecule substance P, leading to more mast cells activation, increased inflammation, and a prolonged itch-scratch cycle. Mice that didn’t scratch or lacked the itch-sensing neuron had less inflammation.

Despite potential benefits, such as defending against skin bacteria like Staphylococcus aureus, chronic scratching can damage skin more than help, according to Kaplan. While scratching might reduce some bacterial levels, the prolonged inflammation might not be worth it.

The researchers also explored whether scratching impacts the skin microbiome. It was observed that scratching decreased levels of Staphylococcus aureus, a bacterium commonly linked to skin infections. These findings pose a paradox, as scratching an itch is often satisfying, yet it seems to have evolved due to an underlying advantage in bacterial defense.

For those battling persistent skin itches, the American Academy of Dermatology advises using cold compresses and 1% hydrocortisone cream for relief. The study’s detailed findings were published in the journal Science.