Ectoparasites

Parasites are organisms that feed, grow, or find shelter in or on a host organism without contributing to the survival of the host. Ectoparasites are parasites, usually insects, that reside on the exterior of the host organism, as opposed to endoparasites, which live inside organs, tissues, or cells. Human ectoparasites live on or in the skin and usually feed on blood or keratin from their human host. Most ectoparasites go through at least part of their life cycle on their host organism and are members of the insecta class of in-vertebrates—lice, fleas, and flies—or the arachnida class—ticks, scabies, and others.

Ectoparasites are very common and were responsible for transmitting the infectious agents that caused some of the worst plagues in human history. Today, they continue to cause significant morbidity and even mortality in many parts of the world. The human diseases caused by ectoparasites are due either to the infestation itself, or a virus, bacteria, or parasite carried by the ectoparasite vector.

Many ectoparasites are simply a nuisance, such as lice of the head, body, or pubic area. Lice cause itching, irritation, and occasionally secondary bacterial infections, but rarely any more serious disease. Other ectoparasites such as scabies and the tungiasis (tunga penetrans) flea cause disease when the female lays her eggs in the skin of human hosts. This causes intense itching and discomfort and can also lead to secondary infections. Similarly, certain tropical species of flies can intentionally or accidentally deposit their eggs under human skin. These eggs develop into larva which cause pain and irritation and eventually exit the skin, much to the surprise of the unsuspecting host. Scabies and lice are the most common ectoparasites encountered in North America and are a common cause for presentation to primary care providers.

Many other types of chiggers, mites, and blister beetles inhabit environments from mattresses to jungles, and have effects from allergic irritation to blistering and infection. Certain types of ticks can cause paralysis in children by injecting a potent neurotoxin into the blood as it feeds. Over several days, the effects of tick paralysis may present as a feeling of unsteadiness and progress to weakness in the limbs and possible respiratory failure. This type of tick paralysis is rare, but residents and visitors to tick-infested areas should be aware of the risks, and inspect their bodies for ticks after outdoor activities.

The most important route by which ectoparasites cause disease in humans is by acting as vectors for dangerous bacteria. For example, Lyme disease is caused by a bacteria transmitted in North America by the Ixodes ticks found primarily in New England states. Lyme disease presents initially as a bulls-eye rash, and if not treated, it may lead to nervous disorders, heart problems, or joint swelling and pain. Over 15,000 cases of Lyme disease are reported in the United States each year; while treatment is available and effective, many cases go unnoticed.

Another very important family of diseases transmitted by ticks are rickettsia (also known as Ehrlichia). [Page 584]These are very small bacteria that are responsible for a variety of diseases including the typhus group (murine typhus, epidemic typhus, and scrub typhus), the spotted fever group (Rocky Mountain spotted fever, rickettsial pox, and others), as well as Q fever and trench fever. Each of these diseases is carried by a different type of ectoparasite tick, louse, or mite, which is, in turn, normally carried by a mammal such as a mouse, dog, squirrel, or human. The diseases caused by rickettsia range from mild to life threatening, but all are treatable with antibiotics if properly diagnosed.

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