Tick host choice explains mystery of US Lyme disease infection rates

Posted: 12th February 2021

A team of US scientists has unlocked one of the mysteries of Lyme disease infection: why is Lyme disease common in the northeastern United States but rare in the southeast? The answer is differences in the hosts that ticks choose and their behaviour, the US-based team have established.

Lyme disease develops in a person after being bitten by a tick that carries the Borrelia bacteria. The ticks acquire the bacteria responsible for Lyme disease from a vertebrate host. One of Lyme disease’s tell-tale symptoms is a bright red circular rash (often called a ‘bulls-eye’ rash) that appears around an insect bite. It’s a serious condition, affecting up to 300,000 people a year in the USA, and many thousands more across the world.

The tick responsible for transmitting the bacteria to humans can be found across the eastern United States, but scientists have long struggled to explain why infection is more prevalent in the north and rare in the south.

To understand the reasons why scientists conducted a detailed and extensive analysis of ticks at eight field sites across the eastern US. They discovered that ticks in the south tended to select lizards as their hosts rather than small mammals.

They established that lizards infected with the Borrelia bacteria are less likely to pass it on to other ticks than small mammals. The fewer ticks that are infected can explain the reduction in human transmission. “North-south differences in tick host-seeking behavior can affect the number of tick bites, potentially resulting in fewer humans being bitten in the south,” the authors conclude.

“Northern blacklegged ticks attach mostly to mice and other mammals, while southern ticks love lizards, especially skinks, “said researcher Howard Ginsberg of the US Geological Survey’s Patuxent Wildlife Research Center. “It’s interesting that this quirk in tick ecology can have such an important effect on human health.”

Researchers also discovered that ticks in northeastern regions have to climb above leaf cover and twigs to find a host, where they can come into contact with humans. Ticks in the south, targeting lizards, can stay below leaf litter where they’re less likely to brush against a human, and transmit the disease.

Researchers believe that the findings could put to bed one of the longest-standing debates on the differing north-south rates of Lyme Disease infection.

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You can read the paper, Why Lyme disease is common in the northern US, but rare in the south: The roles of host choice, host-seeking behavior, and tick density, here.