The shortage of cases of Lyme disease in the southern United States may be due in part to what black-legged ticks bite in the southern regions.
Although black-footed ticks (Ixodes scapularis) claim much of the eastern half of the country as their home, the Lyme disease they spread is largely concentrated in the northeast and increasingly in the midwest.
It is well known that ticks in the Northeast tend to cling to white-footed mice. This relationship turns out to be a blessing for Lyme disease. When infected with the bacterium Borrelia burgdorferi, which causes Lyme disease, these mice spread it very efficiently to ticks, which can then transmit it to humans.
But ticks residing in the south are different. They are more likely to bite lizards called skinks, which are poor transmitters of bacteria, the researchers reported on Jan. 28 in PLOS Biology.
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This study “shows that there is this really interesting change” from north to south in the predominant tick host, says Shannon LaDeau, a disease ecologist at the Cary Institute of Ecosystem Studies in Millbrook, New York, who was not part of the research team. “It looks like this is reducing transmission” in the south of the bacteria that cause Lyme disease.
An estimated 476,000 people are diagnosed with Lyme disease each year in the United States, according to insurance data from 2010 to 2018. In about 70 to 80 percent of cases, a rash in the tick bite area is an early sign of the disease. ; other symptoms include fever, fatigue, and pain. Most people recover with early treatment with antibiotics. If the diagnosis is lost, the infection can spread in the body and cause arthritis and nerve pain (SN: 22/06/19).
Finding and removing ticks after a hike is a part of controlling Lyme disease. Understanding the behavior of ticks and their relationship to the environment can inform other methods of prevention.
Black-legged ticks need blood meals to progress during various stages of development. The larvae that hatch from the eggs are the first to look for a host for the blood; this is when they can first become infected with the Lyme bacterium. The next blood meal is in the nymph stage. Infected nymphs such as larvae can spread the bacterium to other hosts, including people.
There has been a long debate about the difference between cases of Lyme disease between the north and the south, says research ecologist Howard Ginsberg at the Patuxent Coastal Field Station at the University of Rhode Island in Kingston. The ticks are in the south, so "why isn't there a lot of Lyme disease?"
One possible reason is that tick nymphs in the north look for hosts above or above layers of leaves, which puts them in the way of passing hikers. But nymph ticks in the south are more likely to be left in the trash, reducing the chance of such encounters, researchers reported in tick-borne diseases in 2019. Ticks may remain below the leaves in the warmer south until they avoid drying out. .
Ginsberg says this host-seeking behavior and the results of the new study help explain the north-south difference. In 2011 and 2012, he and his colleagues captured host animals in live traps and collected and tested ticks at eight sites in the eastern half of the United States. “We tried to catch everything that was crawling on the ground to which the tick could add,” he says.
In the north, the most common guests were mice, while in the south, ticks selectively attached to skins, Ginsberg says. At the Massachusetts site, for example, 75 percent of the larvae and 93 percent of the nymphs were removed from the mice, accounting for 79 percent of the captured host animals. The team did not catch strokes.
But at the Florida site, although about 40 percent of the animals captured were mice, they only had 3 percent of the larvae and less than 1 percent of the nymphs. Meanwhile, skinks, which made up 28 percent of the captured host animals, had 92 percent of the larvae and 98 percent of the nymphs. The team also found that ticks from northern sites were much more likely to become infected with Lyme bacteria than ticks from southern sites.
Understanding the ecological context of Lyme disease can help identify targets to try to reduce human risk, LaDeau says. For example, the possibility of vaccinating mice against the Lyme bacterium (SN: 8/9/17) may be more useful in the north.
Differences observed from north to south also influence predictions of how climate change could affect Lyme disease. Black-footed ticks moved further north, bringing Lyme disease to Canada, in part due to warming. Perhaps the biting behaviors and patterns in the south will eventually expand to Maryland, Delaware and Virginia, reducing cases of Lyme disease there, Ginsberg says. He will need more research to find out how climate change will affect skink populations and how warming can change tick behavior, he says.