Field Notes: Trouble With Ticks
Although two diseases spread by deer ticks struck fewer Minnesotans last year than in 2002, state health officials continue to warn people to guard against tick bites this summer.
According to the Minnesota Department of Health, doctors reported 473 cases of Lyme disease to the MDH in 2003, down from 867 cases reported in 2002, the all-time high for the state. Similarly, doctors reported 78 cases of ehrlichiosis, a potentially fatal but less well-known tick-borne illness, compared with 149 cases reported in 2002.
Deer ticks transmit both Lyme disease and ehrlichiosis-now known as human anaplasmosis, or HA. The sesame-seed-sized arthropod thrives in wet, humid conditions. Last summer's dry weather probably kept deer ticks deep in the leaf litter and thus resulted in fewer cases of Lyme disease and HA, according to MDH epidemiologist Dave Neitzel.
"One of the deer tick's greatest challenges in life besides finding hosts to feed on is to maintain its water balance," Neitzel said. "Without enough moisture they dry out and die."
Early symptoms of Lyme disease typically include a characteristic "bull's-eye" rash that appears as a red ring and fever, fatigue, and headache. Left untreated, Lyme disease can cause chronic arthritis, heart problems, and neurological damage.
Early symptoms of HA include fever, chills, and nausea. As many as half of all victims require hospitalization, according to the Centers for Disease Control and Prevention in Atlanta. Two to 3 percent of individuals diagnosed with HA die of the disease.
Last year more than 23,000 people in the United States were reportedly diagnosed with Lyme disease, compared with about 17,000 people in 2002. More than 1,200 cases of HA have been reported in the United States since the disease first was described in 1994.
Deer ticks are thought to pick up Lyme and HA bacteria when they feed on mice and other small mammals. Adult ticks attach themselves to a white-tailed deer or another host, where they mate. When a deer tick bites a person, it is able to transmit the Lyme disease bacteria after being attached 24 to 48 hours, and HA after about one day of feeding.
In Minnesota, the highest risk of Lyme disease is in the east-central part of the state between the Twin Cities and Duluth. The risk of Lyme disease is also high in western and central Wisconsin. The highest risk for Lyme disease and HA occurs from mid-May through mid-July.
The key to preventing tick-borne illnesses is to avoid tick bites, Neitzel said. He recommends avoiding brushy wooded places from mid-May through mid-July and using a tick repellent that contains permethrin on clothing. Standard insect repellants that contain DEET are also an option. Also, check often for ticks and remove them promptly by pulling them slowly but firmly. Monitor the area surrounding bites for one month for early symptoms of Lyme disease.
Two or three cases of babesiosis, a third deer-tick-borne illness are usually reported each year in Minnesota. Most people infected with this parasitic disease have no symptoms or mild illness, but severe infections have been known to occur. Early symptoms include high fever, chills, headache, and fatigue. Last year's decline in reported Lyme disease isn't necessarily an indication that the danger is waning, Neitzel said. "Deer ticks have the ability to come back fairly quickly. Last year's good news doesn't mean that the disease won't come back too."
For more information and risk maps, visit Minnesota Department of Health Diseases and Conditions Web page.