Mites are minute arthropods, typically measuring less than a millimeter in length, belonging to the class Arachnida alongside spiders and ticks. These tiny organisms have a simple, unsegmented body plan and possess eight legs in their adult stage. While tens of thousands of species exist in virtually every environment, only a few are parasitic and interact directly with human skin.
Clarifying the “Internal Mite” Misconception
The idea of mites infesting internal human organs is largely a misunderstanding of how these parasites interact with the body. Mites associated with humans are ectoparasites, meaning they live on the surface or in the outermost layers of the skin. Their habitation is confined to structures like hair follicles, sebaceous glands, or the epidermis. They do not survive in the bloodstream, muscle tissue, or major organs.
The confusion arises because some mites, such as the scabies mite, burrow into the skin. This burrowing is strictly limited to the stratum corneum, the dead, outermost layer of the epidermis. This superficial location allows the mite to feed and lay eggs without entering the living, systemic tissues of the host. Extremely rare cases of human acariasis (pulmonary or intestinal) have been reported where environmental mites are found in the lungs or digestive tract. These situations typically involve accidental inhalation or ingestion of storage mites and are considered non-specific invasions, not true systemic infestations.
Principal Species of Human-Associated Mites
The three most common types of mites that interact with humans each occupy a distinct location on the skin surface.
Scabies Mites
The scabies mite, Sarcoptes scabiei var. hominis, is responsible for the highly contagious skin condition known as scabies. The female mite creates characteristic serpentine tunnels within the stratum corneum, where she deposits eggs and fecal matter. This activity triggers a delayed hypersensitivity reaction in the host, leading to intense itching and rash.
Follicle Mites
Demodex folliculorum and Demodex brevis are the most frequent permanent ectoparasites of humans, usually found on the face. D. folliculorum resides primarily in the hair follicles, while the smaller D. brevis penetrates deeper into the sebaceous glands and ducts. These mites are often harmless and considered normal skin fauna. However, their proliferation can lead to demodicosis, which is linked to conditions like rosacea and blepharitis.
Harvest Mites (Chiggers)
Harvest mites, or chiggers (family Trombiculidae), are only parasitic during their six-legged larval stage. These larvae do not burrow or permanently attach to the host. Instead, they attach to the skin for a few days to feed on liquefied skin cells. The chigger injects a digestive enzyme to break down skin tissue, which it then consumes, causing intense irritation and the formation of a red welt.
Mite Life Cycles and Transmission Routes
The life cycle of mites generally includes four active stages: egg, larva (six-legged), nymph, and adult (eight-legged). The total duration of the life cycle varies significantly between species, ranging from a few weeks to several months depending on environmental conditions.
The scabies mite completes its entire life cycle on the human host over 14 to 31 days. The female lays eggs in her skin burrow, which hatch into larvae that migrate to the surface to mature into nymphs and adults. Transmission of Sarcoptes scabiei occurs primarily through direct, prolonged skin-to-skin contact.
Chiggers have a life cycle that is mostly environmental, with only the larval stage being parasitic. Adult chiggers overwinter in the soil and lay eggs that hatch into parasitic larvae. These larvae attach to a passing host from vegetation, feed for up to four days, and then drop off to the ground. They develop into non-parasitic nymphs and adults that feed on small arthropods or plant material.
Demodex mites complete their life cycle entirely within the hair follicles and sebaceous glands. Mating occurs at the follicle opening, and the eggs are laid inside the pilosebaceous unit.
Methods for Identification and Diagnosis
Diagnosing a mite infestation begins with a clinical evaluation of the patient’s symptoms and the distribution of skin lesions. Scabies is often suspected based on severe, nocturnal itching and the presence of characteristic linear burrows, especially in areas like the finger webs or wrist creases. A definitive diagnosis requires the microscopic identification of the mite, its eggs, or fecal pellets.
The standard diagnostic procedure is a skin scraping. Mineral oil is applied to a suspected lesion or burrow, and the area is superficially scraped with a scalpel. The collected material is then examined under a low-power microscope. Dermatoscopy, which uses a magnifying lens with a light source, can also be employed to visualize the mites or their tracks directly on the skin, often showing the “delta sign” of a scabies mite.
For Demodex mites, diagnosis typically involves epilating a few eyelashes or performing a skin surface biopsy. The biopsy uses cyanoacrylate glue to collect material from the skin surface. This sample is then examined under a microscope to determine the density of mites, as a high number per square centimeter is necessary to confirm a diagnosis of demodicosis.