Black-legged ticks feeding on a five-line lizard
Graham Hickling
By Harini Barath
Lyme disease is one of the most devastating tick-borne infections in the United States, affecting more than 300,000 people each year. It is also one of the most mysterious: the creature that spreads it – the black leg tick – lives across the country. However, the northeastern United States is home to many more cases than anywhere else. Now, researchers have identified an unexpected reason: lizards.
Black leg ticks (Ixodes scapularis), also known as deer ticks, carry corkscrew-shaped bacteria that cause Lyme disease. Ticks catch pathogens – spirochetes that belong to the genus Borrelia– when they suck the blood of animals such as rats, deer and lizards. In the next stage of their life cycle, ticks can attach themselves to an unfortunate human. But each host transmits the microbes differently. Reptiles are worse transmitters than mammals, so ticks that lived on reptiles are less likely to make people sick.
The north-south divide in the cases of Lyme is a very sharp line along the border between Virginia and North Carolina. The researchers raised the hypothesis that the disparity in the cases stems from ticks feeding on different hosts in the two regions.
To test the idea, Jean Tsao, a disease ecologist at Michigan State University, and colleagues conducted an extensive study of eastern ticks – their abundance, behavior and hosts – for more than 2 years at eight field locations across the United States. They found a clear divide in the preferred hosts of ticks and behavior in southern Virginia, matching the pattern of tick infections and Lyme disease.
Tick tracker
The distribution of Lyme disease–causing bacteria in black-footed ticks in the United States. The counties where ticks carry pathogens are orange and light blue, the counties in which ticks carry ticks in dark blue and green, but there is no record of tick infections.
US Centers for Disease Control and Prevention; Journal of Medical Entomology; Entomological Society of America
The big difference seems to be linked to a particular host. In the northeast, black-legged ticks cling to small mammals such as the white-legged mouse, which are famous for transmitting Lyme disease bacteria to insects. But in the south, ticks prefer to feed on lizards, especially lizards. These slender, smooth-scaled reptiles usually live on leaves and branches that have fallen to the ground – the so-called litter – and are particularly weak transmitters of Lyme pathogens. Therefore, fewer southern ticks are infected and fewer people get sick – the team reported last week in PLOS Biology.
The researchers took the right approach to solving the mystery, says Andrea Swei, a disease ecologist at San Francisco State University who was not involved in the study. “They are comparing apples to apples here, and that allows them to say a lot about host association patterns in a large geographic area.”
In a previous study, Tsao and his colleagues observed that ticks in the northeast and southeast also look for hosts differently. In the south, the insects stayed under the debris of the forest to avoid dehydration due to the heat. Northern ticks were more outgoing, climbing leaves and branches, where they were much more likely to encounter and bite humans. This, combined with fewer lizards, makes ticks “a double hit” in the northeast, says Tsao.
“The peculiarities of the tick’s ecology have consequences for human health,” says study co-author Howard Ginsberg, an ecologist at the Patuxent Wildlife Research Center of the US Geological Survey. He hopes the work will inform efforts to track and reduce the spread of Lyme disease.
Climate change can alter these patterns, notes Swei. Observations show that northeastern ticks are already expanding their distribution. At the same time, the researchers speculate that warming the climate may alter the behavior of the tick and the presence of certain hosts, raising the patterns of incidence of Lyme disease. It is important to keep an eye on the regions around the north-south divide, says Swei. “As this zone changes,” she says, “it would really change the risk of disease for people living on that border.”