Mites are very small Arthropods that are just barely large enough to be visible by the naked eye (< 1mm in length.) or microscopic. They are ectoparasites that are mainly a nuisance to humans, parasitize various animals and can act as scavengers. However some are important in the transmission of various diseases as well as acting as allergens.

Mites are simple in their body structure as they generally have an unsegmented body. However they are very small and have unusual shapes that can make them difficult to detect and identify requiring an experienced Microbiologist / Entomologist to make the identification.

Mites belong to the class Arachnida and the subclass Acari. They are closely related to the Ticks. There are many species but we will concentrate on those species that are clinically significant in humans. They are as follows:

  • Sarcoptes scabiei (cause of scabies)
  • Demodex species (implicated in rosacea)
  • House dust mites (act as allergens that can lead to hay fever, asthma, and skin reactions)
  • Chiggers (trombiculid mites – cause a rash called trombiculosis)

Sarcoptes scabiei (itch mite)

Sarcoptes scabiei

Sarcoptes scabei – photo courtesy of CDC

Demodex species

Demodex species are mites that are extremely small so require a microscope to visualize. The species most common to humans is folliculorum. They generally do not cause disease in humans and are considered commensals and not parasites. When they do cause disease the condition is called demodicosis. Elongated with 4 pairs of short legs located in the front, they are adapted to live inside of hair follicles. Nymphs will have only three pairs of legs.

Demodex species – Illustration by W. Vientos

When demodicosis occurs it is usually as a result of the hypersensitivity of the individual to the mites especially when the population of the Demodex is abnormally high on the skin.

House Dust Mites

House Dust Mite – Illustration by W. Vientos

House Dust Mites are microscopic mites that feed on the dead skin cells of humans and thrive in conditions that are warm and humid. These mites are the cause of allergies that can in sensitive individuals be severe enough to induce asthmatic attacks. The inhalation of the feces and mites in dust act as allergens. House Dust Mites are found in bedding, rugs, sofas and other that may act as a food source for the organisms. Frequent cleaning will reduce the load of dust mites and help eliminate allergic reactions in sensitive individuals.

Chiggers (Trombiculid mites)

Trombiculidae are mites commonly referred to as Chiggers and are most easily seen under a microscope because of their small size (400um). They can cause an allergic reaction leading to intense itching. Interestingly the adult mites are not the stage that cause the allergic reaction. It is in fact the larvae of the mite that produce the reaction in humans and animals by embedding their mouthparts into the skin. This is accomplished by the injection of digestive enzymes on to the skin causing the breakdown of skin cells and the formation of a hole in the skin called a stylostome. Parts of the inner skin a then made available for the larvae to feed on. This will lead to intense itching and swelling of the area. Once feeding is completed the larvae will drop off the host to the ground where they become nymphs to later mature into adult mites. While the larvae has six legs … the adult mite will have eight. The nymph and adult stage do not generally cause humans any discomfort and feed on vegetative matter.

Though Chiggers is the more common name used to describe them, they also go by various other names such as harvest mites; spider mites; red-bugs, bush-mites, and berry mites.

They are found in various areas where vegetation is found (woodlands, along lakes and streams, orchards, where there are berry bushes, and even on lawns).

Site reaction to the bite of a Chigger. This type of immunologic reaction will illicit intense itching – photo courtesy of CDC/ M. A. Parsons

Images of Various Mites

Parasitoid mites – these mites were infesting a homeless person whom had taken residence near natural hosts of these mites. The mites had induced a skin reaction that caused intense itching. Video by W. Vientos