Lyme disease ticks follow rivers in Midwest

ATLANTA – Ticks capable of carrying the bacterium that causes Lyme disease have been slowly migrating along rivers in the Midwest, a trend that may help predict future areas at risk for the disease, say researchers from the University of Illinois.

“The movement of the ticks along riverbeds in Illinois, combined with the existence of animals capable of serving as hosts to both and the bacteria can be seen as a harbinger of the migration of Lyme disease in the Midwest,” said M. Roberto Cortinas, a research associate at the university’s College of Veterinary Medicine.

“Ixodes scapularis,” the tick that carries Lyme disease was first seen in the Midwest in Wisconsin in the early 1970s. By the late 1980s researchers began to observe the tick moving south.

By 1990, when the university began tracking its migration, “I. scapularis” had been found along the northern Mississippi River.

How they do it. The researchers track the movement of the ticks through a number of methods. In the beginning, they focused on deer hunting season, collecting ticks from animals brought in by hunters and recording information on where the hunters found the deer.

More recently they have employed other methods including dragging, in which a piece of cloth is dragged over vegetation to collect ticks, and trapping mice.

Cortinas’ research, which he began in the late 1990s, is founded on the tick risk map developed for Wisconsin and northern Illinois by Dr. Marta Guerra. The research focuses on the migration of the along the Illinois River, which merges with the Mississippi River just north of St. Louis.

Migration path. The ticks appear to be migrating south-southwest, following the river towards the Mississippi. So far they have been collected as far south as Peoria County.

“Unlike the heavily wooded Northeast, the north-central United States is not generally a hospitable environment for ticks,” said Cortinas. “These river corridors are like oases in the desert, where the ticks can survive and move along.”

While the movement of the ticks can predict the eventual arrival of the disease, they are not the only element of the equation, said Cortinas.

Two other elements are necessary. First, small mammals, such as white-footed mice or chipmunks, which serve as a reservoir for the bacteria, must be present for the ticks to become infected.

Deer, the preferred host for adult ticks must be present and are also important in the dispersal of ticks into new areas.

Conclusions. While the researchers expect to see a continual migration of the ticks down the Mississippi, they do not expect to see Lyme disease follow them all the way to the Gulf of Mexico.

As the ticks move south, the small mammals are often replaced by lizards as the blood source for the ticks. Unlike the small mammals, lizards are not effective reservoirs for the bacteria that cause Lyme disease. Without this reservoir the disease cannot exist in the area.

“Is there a southern limit beyond which the disease will not follow? We don’t entirely know where that will take place, but part of the reason for this study is to find out,” said Cortinas.