To scratch an itch

Though itching is a universal human affliction, science still cannot fully explain the exact mechanism by which we perceive an itch—or why scratching provides such fast relief. What we do know is that several chemicals produced by the body can cause itching, the most notorious being histamine. We also know that, though itching and pain sensations are carried by different nerve fibers, there must be interconnections. Very hot or cold applications, for example, can make an itch stop. And oral painkillers can often help relieve it. A study that scanned brain activity in people with deliberately induced itching found that when they were allowed to scratch, areas of their brains associated with unpleasant sensations, such as itching, became less active, while areas associated with pain became more so.

Itching is a symptom that can signal a near-limitless variety of problems, from trivial to life-threatening. If you have a persistent itch, here's how to proceed.

Itching can be generalized or limited to one or more specific locations. If the itch is localized and accompanied by a rash, here are the main culprits to investigate, in approximate order of likelihood:

• Common insect bites. Bites from mosquitoes or no-see-ums usually require only a cold compress or steroid cream to ease discomfort. Spider bites can cause blistering and ulceration in addition to itching and might require topical antibiotics. If you're stung by a bee, carefully remove the stinger. Consider seeking medical attention when inflammation and swelling extend well beyond the bite site, since you might require oral antihistamines or corticosteroids.
• Less common insect-related itching. Scabies (which is diagnosed with a skin biopsy), head and pubic lice (crabs), bedbug bites (mostly on the torso), and flea bites (usually below the knees of dog owners) all cause severe itching. Each requires a specific eradication technique. Over-the-counter permethrin shampoos generally eliminate head and pubic lice and scabies. Call a professional exterminator to get rid of bedbugs. As for fleas, the remedy is to rid your pet of the critters. (Oddly, one of the most consequential insect bites, that of the deer tick that transmits Lyme disease, rarely causes itching.)
• Fungal infections. If your itchy rash looks flat and brownish-red and is in a dark, warm, moist spot like your groin, armpits, or feet, you probably have a fungal infection. Over-the-counter fungicidal creams, ointments, and powders are the initial treatment of choice, though stubborn infections might require oral prescription medications.
• Skin allergy. The red, blistery, sometimes weepy and intensely itchy condition of contact dermatitis is, as its name indicates, caused by direct contact with an allergen. Common culprits are shampoos, heavy metals (especially nickel) in jewelry, elastic in underwear, liquid detergents, poison ivy, and poison oak. But the cause can be difficult to pin down, since it might be as obscure as a single ingredient in a hair spray. Steroid creams can provide some relief until you find and banish the culprit.

With or without a rash, itches that have spread over your whole body sometimes indicate more serious conditions. Common causes include:

• Dry skin. This is the most common cause of total body itching and is especially likely in the dehydrated air of heated winter interiors. The main danger of such itching is scratching, which can lead to skin infections caused by resistant bacteria. Faithful use of emollient creams can help, especially after bathing or showering.
• Hives. Also known as urticaria, this itchy, generalized rash of red skin bumps or wheals is usually caused by a reaction to medications or foods. Penicillin is a common offender among drugs, and shellfish among foods. If you can identify the cause, stay away from it for good, because it might cause an even more severe reaction the next time. But sometimes the source is never found, in which case you have idiopathic urticaria. Antihistamines and corticosteroids are often necessary.
• Systemic illnesses. More rarely, generalized itching, usually without a rash, can signal conditions that include anemia or thyroid, liver, or kidney disease. And at times it can be a tip-off to lymphoma or an underlying internal cancer. If you experience prolonged itching over your whole body that you cannot attribute to an allergic reaction or dry skin, a visit to your doctor is in order.