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How Do Ticks Get on You?

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How Do Ticks Get on You?
A blacklegged (or deer) tick in the questing posture, waiting for a host.

A blacklegged (or deer) tick in the questing posture, waiting for a host.

Photo: CDC/James Gathany

Although you may occasionally suffer the misfortune of finding a tick in your hair, that tick didn’t jump on your head. Ticks don’t jump, and they don’t hang around in the trees waiting to drop on you. People often ask me, “If ticks don’t jump, how do ticks get on you?”

Ticks, as you likely know, are blood-feeding parasites. Almost all ticks ambush their hosts, using a behavior called questing. A tick in search of a blood meal will crawl up a plant stem, often on a tall piece of grass, and extend its front legs. This is called the questing posture. The blacklegged tick in the photo above is in the questing posture, waiting for a host.

Why do ticks wait in this position? On its front legs, a tick has special sensory structures, most notably the Haller’s organs, with which it can detect a nearby host. In 1881, a scientist named G. Haller published the first description of these structures, although he misunderstood their purpose. Haller believed these structures were auditory sensors (ears), when in fact they proved to be olfactory sensors (noses). So when a tick sits on a blade of grass with its front legs extended, it is effectively sniffing the air for your scent.

What’s remarkable, however, is just how well the tick can smell you, and sense even your slightest movement. The Haller’s organ can detect the carbon dioxide you exhale with each breath, and the ammonia in your sweat. With legs outstretched, the tiny tick can pick up on all the foul odors people produce, from bad breath to belches, and it can even smell your farts. But even the most well-groomed and properly behaved hiker can’t avoid detection by the Haller’s organ, because it can also sense changes in temperature as you approach.

Once a tick knows you are approaching, it waits to grab hold of your leg as you brush past the vegetation. Most ticks behave passively in this regard, relying on you to come to them. But some, particularly those in the genus Hyalomma, will make a mad dash in your direction as soon as they smell you coming.

Scientists use this behavior to their advantage when sampling an area for ticks. The researcher drags a square of white felt across the ground. Any ticks in its path will sense the movement and grab onto the felt, where they are visible against the white backdrop and can be counted or collected.

Understanding this tick behavior will help you minimize your risk of tick bites. Take care not to walk through areas of thick or high vegetation, and keep your legs covered and treated with an effective tick repellent. Wearing a hat will be almost no help in preventing tick bites, unless you tend to do handstands in the tall grass. When you find a tick on your upper body or in your hair, it is nearly always because the tick managed to crawl there from your leg. Do a thorough, full body tick check immediately upon returning indoors, and you can remove most ticks before they’ve enjoyed a meal of your blood (and possibly infected you with a disease-causing pathogen).

Sources:

  • Tick Biology, Dr. Larisa Vredevoe , UC Davis Department of Entomology and Nematology. Accessed online June 10, 2013.
  • Ticks (Acari: Ixodida), by Lewis B. Coons and Marjorie Rothschild, The University of Memphis. Encyclopedia of Entomology, edited by John L. Capinera.
  • On the structure of “Haller’s organ” in the Ixodiodea, by George Henry and Faulkiner Nuttall, from PARASITOLOGY, Vol. I. No. 3, October, 1908. Accessed via Google Books, June 10, 2013.
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