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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

Ticks are tiny, biting arachnids that feed on the blood of warm-blooded animals, including humans. They burrow painlessly into the skin with their feeding parts, bite, draw blood and eventually drop off when they become engorged with blood. Only the feeding parts are inserted into the skin. The body, which is dark in color and ranges from the size of a poppy seed to a pencil eraser, remains visible on the skin surface or scalp. Ticks swell and turn bluish-gray when filled with blood. Most tick bites in the United States involve hard ticks (Ixodidae), which have been increasing in number since the middle 1900s.

Secretions from the tick's feeding parts can cause skin reactions, such as raised areas, lumps and growths called granulomas. Fever and paralysis also may develop after tick bites, although paralysis is rare. In addition, ticks can be infected with bacteria, viruses or protozoa. These organisms can be transmitted from the tick to the host (the animal or person) as the tick feeds, causing disease.

Tick-borne diseases include:

  • Lyme disease

  • Babesiosis

  • Human granulocytic ehrlichiosis

  • Tularemia

  • Rocky Mountain spotted fever

  • Colorado tick fever

  • Human monocytic ehrlichiosis

  • Relapsing fever

Ticks live in tall grass and in wooded areas, particularly cool, moist, mature woods with thick undergrowth. They also can be found at the edges of woods near lawns or fields, but rarely in lawns, which are too dry and hot. Ticks wait in the underbrush for an animal or human to brush by, and then grasp the fur or skin and crawl up the leg. They don't fly, jump or drop from trees. They wander the body for 30 minutes to an hour before inserting their feeding parts into the skin.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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