How Do Ticks Get on You?

Adult male Wood Tick, Dermacentor variabilis, (aka Dog Tick, American Dog Tick, Hard Tick). Northern Ontario, Canada.
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Although you may occasionally suffer the misfortune of finding a tick on your leg, you can rest assured that tick didn’t jump on you. Ticks don’t jump. But that knowledge leaves the question open: How do they get onto humans? 

Ticks Ambush Their Hosts

Ticks, as you likely know, are blood-feeding parasites. Almost all ticks use a behavior called questing to ambush their hosts. When in search of a blood meal, a tick will crawl up a plant stem or tall piece of grass and simply extend its front legs. This is called the questing posture. The black-legged tick in the photo is in the questing posture, waiting for a host.

Haller's Organs and a Tick's Keen Sense of Smell

Why do ticks wait in this position? A tick has special sensory structures on its front legs, called the Haller’s organs, with which it can detect an approaching 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. Using its Haller’s organs, a tick can detect the carbon dioxide you exhale with each breath and the ammonia in your sweat. Even the most well-groomed and properly behaved hiker can’t avoid detection by the Haller’s organs, because they can also sense changes in temperature as you approach.

How Ticks Actually Get on You (Without Jumping)

When a tick knows you are nearby, 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. A 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.

Avoiding Ticks

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. 

What to Do About a Tick Bite

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). Be careful when removing them if they've bitten you. Use a tweezer and pull on the arachnid without crushing it. Put it in a container, if possible, and freeze it. Then wash your hands and the site. If you get a rash, fever, infection, or flu-like symptoms, see your doctor, and bring the tick with you if possible. If you believe you may have Lyme disease, be insistent on being tested for it, as it and its complications are best treated in the early stages.

Sources

  • Vredevoe, Larisa. "Tick Biology." UC Davis Department of Entomology and Nematology.
  • Coons, Lewis B., and Marjorie Rothschild. "Ticks (Acari: Ixodida)." In Encyclopedia of Entomology, edited by John L. Capinera. The University of Memphis.
  • Henry, George, and Faulkiner Nuttall. "On the Structure of 'Haller’s Organ' in the Ixodiodea.Parasitology, Vol. I. No. 3, (October 1908). Google books.