Mites and chiggers and infectious disease

Definition

Mites are small-to-microscopic arachnids. They have a body without a constriction between the cephalothorax and abdomen, mandibles adapted for piercing, and, usually, four pairs of short legs in the adult and three in the juvenile form (larvae). The scabies mite, Sarcoptes scabiei, is a member of a family of parasitic mites that cause scabies in humans. Chiggers are the larvae of a certain type of mite of the family Trombiculidae. These very small, reddish mites feed on humans and other animals only when they are in the larval stage.

Mites

S. scabiei var. hominis, the human itch-mite that causes scabies, is in the arthropod class Arachnida, subclass Acari, family Sarcoptidae. Mites are among the most diverse and successful of all the invertebrate groups. An estimated 48,200 species have been identified and described. They have exploited an incredible array of habitats, and because of their small size (most are microscopic), they go largely unnoticed. Many live freely in soil or water, but many species live as parasites on plants and animals.

Disease signs and symptoms. The scabies mite, S. scabiei, causes scabies in humans. Persons with scabies usually have no symptoms during the first two to six weeks of infestation; however, infected persons can spread scabies during this time. Severe itching, especially at night, and a pimple-like, itchy rash are the earliest and most common symptoms of scabies. These symptoms are caused by sensitization to the proteins and feces of the parasite. The itching and rash may affect much of the body or be limited to common sites such as the wrist, elbow, armpit, webbing between the fingers, nipples, penis, waist, belt line, and buttocks. The rash also can include tiny blisters and scales. Scratching the rash can cause skin sores, which can become infected with bacteria.

Transmission route. The microscopic scabies mite burrows into the upper layer of the skin, where it lives and lays its eggs. The burrows appear as tiny raised serpentine lines that are grayish or skin-colored and are a centimeter or more in length. Scabies is usually spread by direct, prolonged, skin-to-skin contact with a person who already has scabies. It is spread easily to sexual partners and to persons in the same household. Scabies is sometimes spread indirectly by sharing the clothing, towels, or bedding used by an infested person. Animals do not spread human scabies.

Diagnosis and treatment. Diagnosis of a scabies infestation is usually made based on the customary appearance and distribution of the rash and the presence of burrows. Whenever possible, the diagnosis should be confirmed by identifying the mite, mite eggs, or mite fecal matter. This can be done by carefully removing a mite from the end of its burrow using the tip of a needle or by obtaining a skin scraping for microscopic examination.

Products used to treat scabies are called scabicides because they kill scabies mites; some also kill eggs. Scabicide creams or lotions are prescription medications that should be applied to all areas of the body. Following treatment, the infected person should wear clean clothes. Treatment also is recommended for household members and sexual contacts.

Chiggers

Chiggers are the juvenile form of a certain type of mite of the family Trombiculidae. These very small mites feed on humans and other animals only when they are in the larval stage. Chiggers are classified in the phylum that includes the arachnids and are in the order of Acari and the suborder Prostigmata. The scientific name for the chigger is Eutrombicula alfreddugesi.

Chiggers are invertebrates with four pairs of legs. They are less than 1/150th of an inch long and are reddish-brown. The juvenile forms have six legs, although harmless adult mites have eight legs. Chiggers reproduce by laying eggs. They are found from Central Mexico to Canada and are commonly encountered in woodlands, along the periphery of swamps, in shrub thickets, and in grass that has not been mowed.

Transmission route. Chiggers are parasites that rely on blood from a host for food, and humans are especially vulnerable. Unlike scabies mites, chiggers do not burrow into the skin. They feed at the base of a hair follicle or in a skin pore. They are well known for the rash they cause in humans upon attachment.

A common myth about chiggers is that they burrow into and remain inside the skin. Chiggers insert their feeding structures into the skin and inject enzymes that cause the destruction of host tissue. Hardening of the surrounding skin results in the formation of a feeding tube called a stylostome, which works like a straw for the feeding chigger and also irritates the surrounding skin, producing intense itching because of the body’s reaction to the stylostome and the chigger’s saliva. A species of chigger in the Pacific Islands and in East Asia is a vector for Japanese river fever.

Disease signs and symptoms. A chigger bite is not noticeable, but the bite becomes annoying and itchy after a few hours because of the injection of digestive enzymes into the skin. Intense itching within one or two days of the bite is the most common symptom. The bite area may become red and raised, resembling a blister. The itching persists for several days, and complete resolution of the skin lesions can take up to two weeks. Chigger bites do not produce any long-term complications.

Diagnosis and treatment. A chigger bite is diagnosed from symptoms. The chigger’s feeding tube that creates many of the symptoms cannot be removed from the person’s skin. However, the person’s body will eventually break down the skin tissue that forms the tube; healing then begins in a process that can take from ten days to three weeks. Relief from the itching may be found with topical hydrocortisone cream, Benadryl ointment, or calamine lotion. Prolonged scratching may lead to skin wounds that may become infected with bacteria.

Impact

Globally, mites are important nuisance pests. The biting and bloodsucking behavior of the scabies mite, for example, causes considerable discomfort, and a few species also cause serious allergic reactions, such as asthma. Scabies occurs worldwide and affects people of all races and social classes. It can spread rapidly under crowded conditions where close body contact is frequent. Institutions such as nursing homes, extended-care facilities, and prisons are often sites of scabies outbreaks.

According to the World Health Organization (WHO) in 2023, 400 million cases of scabies occurred worldwide every year. The organization also stated that an estimated 5 to 50 percent of children in resource-poor areas are affected by scabies, and scabies infections are most common in hot, tropical countries. With better detection methods and treatments, however, having a scabies infection can amount to nothing more than temporary distress.

Chigger bites do not produce any long-term complications. However, because of the intense itching it causes, prolonged scratching of the itches may lead to skin wounds that may become infected by bacteria.

Bibliography

Atkinson, P. W., ed. Vector Biology, Ecology, and Control. New York: Springer Science, 2010.

Chosidow, O. “Clinical Practices: Scabies.” New England Journal of Medicine 354 (2006): 1718-1727.

Maguire, J. H., R. J. Pollack, and A. Spielman. “Ectoparasite Infestations and Arthropod Bites and Stings.” In Harrison’s Principles of Internal Medicine, edited by Joan Butterton. 17th ed. New York: McGraw-Hill, 2008.

"Scabies." World Health Organization, 31 May 2023, www.who.int/news-room/fact-sheets/detail/scabies. Accessed 4 Feb. 2025.

Wilson, B. B., and M. E. Mathieu. “Mites (Including Chiggers).” In Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, edited by Gerald L. Mandell, John F. Bennett, and Raphael Dolin. 7th ed. New York: Churchill Livingstone/Elsevier, 2010.