The tick Ixodes scapularis is present in 45.7 per cent of US counties

Study finds ticks are in nearly half of US counties
 
The black-legged tick responsible for spreading Lyme disease is now present in nearly half of the counties in the US, new research has found.

It is now present in 44.7 per cent more counties than it was just under two decades ago, when scientists last mapped the tick's geographical spread.

Lyme disease is the most commonly reported vector-borne disease in the US and cases have risen steadily, both in number and geographical spread, across the east of the country.

Scientists from the Centers for Disease Control and Prevention decided to update the 1998 map of these two ticks, to find out more about the changing disease risks to humans.

Their findings show that the tick Ixodes scapularis is present in 45.7 per cent of US counties, while its close relative I. pacificus is present in 3.6 per cent. Combined, the two ticks were identified in 1,531 counties across 43 states.

In the east of the US, I. scapularis is primarily responsible for spreading the bacteria that causes Lyme disease in humans and I. pacificus, is the primary vector in the far west of the country.

The majority of the change has been seen in the northern central and north eastern states, while distribution in the south has remained quite stable, according to the research, which has been published in the Journal of Medical Entomology.

Researchers say their findings show strong support for systematic sampling to assess tick density in specific areas where ticks can be expected to arrive or increase in number in the near future. They say more work is needed to determine the future expansion of I. scapularis.

The full study has been published in the Journal of Medical Entomology: http://jme.oxfordjournals.org/content/early/2016/01/15/jme.tjv237

Image (c) Scott Bauer, U.S. Department of Agriculture