While most insect encounters result in surface-level bites or stings, some organisms are adapted to sustained life within human tissue layers. True biological invasion by arthropods and parasitic worms is a documented, though rare, medical phenomenon. It is important to distinguish this from the many non-parasitic conditions that can mimic the sensation of crawling.
Actual Organisms That Burrow Beneath the Skin
A small number of organisms have evolved specific mechanisms to penetrate and thrive within the epidermis or dermis. The most common example is the microscopic mite, Sarcoptes scabiei, which causes the highly contagious skin condition known as scabies. These arachnids burrow into the outermost layer of the skin, the stratum corneum, where the female mite excavates a tunnel and deposits eggs. The intense, often nocturnal itching is primarily a hypersensitivity reaction to the mites, their eggs, and their fecal matter. Diagnosis involves identifying the characteristic fine, wavy burrow tracks, typically found between the fingers, on the wrists, or around the waist.
Parasitic worms also cause invasion, particularly the hookworm larvae responsible for cutaneous larva migrans, commonly called “creeping eruption.” These larvae, typically from cat or dog hookworm species (Ancylostoma), penetrate exposed skin when a person contacts contaminated soil or sand. Since the larvae cannot complete their life cycle in a human host, they migrate aimlessly within the upper layers of the epidermis. This migration creates intensely itchy, reddish-brown, snake-like tracks that advance by several millimeters per day.
Another serious invader is the human botfly (Dermatobia hominis), whose larvae cause a condition called cutaneous myiasis. The adult female fly captures a blood-feeding insect, like a mosquito, and glues her eggs onto it in a process called phoresis. When the vector lands on a person, the warmth of the skin causes the eggs to hatch, and the larva quickly burrows into the subcutaneous tissue, where it resides for up to 10 weeks. The presence of the growing larva creates a painful, boil-like nodule, or “warble,” that often features a central opening through which the larva breathes and excretes waste. Patients sometimes report a distinct, shooting pain or even feel the movement of the larva shifting positions beneath the skin.
Common Conditions Mistaken for Subcutaneous Invaders
The sensation of something crawling on or under the skin is known scientifically as formication. This common symptom often occurs without any actual biological infestation. Formication is a form of paresthesia, a tactile hallucination where a person feels a physical sensation without an external cause. It frequently stems from the irritation of peripheral nerves due to various internal medical conditions, such as neuropathies caused by diabetes or shingles. Hormonal fluctuations and substance use and withdrawal are also major factors; the intense formication associated with stimulant abuse is colloquially referred to as “cocaine bugs.”
Other conditions are mistaken for parasitic invasion because of their visible skin manifestations, not just the sensation they cause. Severe allergic reactions, such as urticaria (hives), can cause intensely itchy, raised welts that may be misidentified as bite marks. Common skin irritations like intense dry skin (xerosis) or eczema also lead to constant itching and inflammation. Individuals may scratch and pick until they create lesions easily mistaken for wounds caused by burrowing organisms.
When the Sensation is Psychological
In some cases, the belief in a subcutaneous infestation becomes a fixed, unshakable conviction despite repeated medical evidence to the contrary. This condition is formally recognized as Delusional Parasitosis (DP), or delusional infestation. Classified as a delusional disorder, the physical sensation of formication often serves as a catalyst for this belief. Patients often present with the “matchbox sign,” bringing in samples of skin scrapings, lint, or scabs that they believe are the actual organisms, a phenomenon known as the specimen sign.
Morgellons disease is a specific variant of Delusional Parasitosis where the person believes that fibers or threads are emerging from their skin lesions. A large study by the Centers for Disease Control and Prevention (CDC) found that the fibers collected by patients were primarily textile materials, concluding that the condition is an unexplained dermatopathy most consistent with delusional infestation. Since the belief is a delusion, sufferers typically reject the diagnosis and often refuse psychiatric care, seeking out multiple medical professionals in an attempt to prove the reality of their infestation.