Ticks May Fascinate Biologists, but the Blood Suckers Worry Health Officials

National Wildlife, August-Sept, 2000 by Peter Jaret

The six-year-old girl was playing happily on a midsummer day when her fingers began to go numb. Soon she was staggering and falling. By the time her terrified parents rushed her to the Medical College of Georgia in Augusta, she could barely stand or sit up. As the doctors searched for a cause, the little girl's speech became slurred, her breathing labored. Baffled by the spreading paralysis, the medical team prepared to put her on a respirator.

Then a pediatrician, recalling something she had learned in medical school, took a fine-tooth comb and examined the girl's scalp. Hidden in the long blonde hair was what she had suspected: a blood-engorged tick, indicating that the symptoms might be from a rare disease called tick paralysis. The tick, Dermacentor variabilis, releases a toxin that interferes with nerve transmission. The girl's case, notable because tick paralysis is often confused with other disorders, was written up in a recent issue of The New England Journal of Medicine.

Tick paralysis is not the only tick-related ailment that can go undiagnosed. Most of the illnesses that ticks either directly cause in humans or infect them with have been identified only in the past 20 years. As more people choose to live close to wooded areas where ticks flourish, experts say, cases of these diseases are likely to increase. And the incidence of tick-borne infections may be on the rise in areas where oak and other forest trees are growing back, providing habitat for ticks and their hosts.

The most common disease carried by ticks is Lyme disease, first identified in the mid-1970s. In 1998, 16,801 Americans were diagnosed with Lyme disease, the highest number of cases ever reported. Transmitted by ticks infected with the bacterium Borrelia burgdorferi, Lyme disease can be treated with antibiotics if it's caught early. But if it goes undetected, it can lead to joint damage, heart irregularities and even neurological problems.

Rocky Mountain spotted fever, one of the first tick-borne infections to be identified, is the second most common. It strikes about 300 to 400 Americans a year, and as many as 5 percent of cases may prove fatal. Symptoms include high fever, muscle pain, malaise and hemorrhaging into the skin.

Then there's ehrlichiosis, which also causes severe flulike symptoms that can sometimes prove deadly. Two human forms of the disease were identified in the 1980s. Last year researchers added a third when a form of ehrlichiosis known to afflict dogs also showed up in humans. All told, about 600 Americans have been diagnosed with one or another form of ehrlichiosis since the disease was discovered.

Other tick-borne diseases in the United States include tularemia, which causes skin sores; Colorado tick fever, which usually results only in mild illness; and babesiosis, a rare but sometimes fatal disease caused by a protozoan. "Odds are there are other tick-borne illnesses out there that we haven't identified yet," says Lance Durden, an entomologist and cocurator of the U.S. National Tick Collection in Statesboro, Georgia.

What makes ticks the bearers of so much microbial bad news is their thirst for blood. Scientists have identified more than 850 tick species, and though their life cycles vary widely, all survive by drinking the blood of their hosts.

Consider Ixodes scapularis-also known as the black-legged or deer tick- which is the main vector for the Lyme disease bacterium in the United States. It drinks only three meals of blood during its entire life cycle, feeding each time on a different host. In midsummer, larval ticks emerge from eggs deposited the autumn before under leaves on the forest floor. At this stage, the creatures are no bigger than the period at the end of this sentence. Crawling on top of the leaf litter, they grab onto a passing reptile, bird or mammal and, burrowing their mouthparts into the skin, take their first blood meal.

Ticks are known to feed on at least 80 species of mammals or birds. A recent study in Sweden found that ticks can infest migratory birds, which can carry diseases thousands of miles, potentially introducing them to new locales.

One of the favorite dining spots for Ixodes scapularis is the white- footed mouse, a major reservoir for the form of bacteria, called a spirochete, that causes Lyme disease. In some parts of the Northeast, half of the mouse population carries the Lyme spirochete. When a tick dines on an infected mouse, the germ passes into the gut, where it remains for the rest of the tick's life.

After drinking their fill, immature ticks drop back to the forest floor and go through a molting stage, emerging the next spring as nymphs about the size of a pinhead. Now able to climb, they scale blades of grass or small saplings to wait for their next meal. Recent studies have found that a single tick can be infected with up to three different organisms that cause human disease-and it can transmit all three in one bite.

"There's no doubt ticks can be very dangerous," says entomologist Robert Lane of the University of California at Berkeley. "But for those of us who study them, they're also enthralling."