Climate change may see rise in ticks

Richard Collins An outbreak of what seemed to be rheumatoid arthritis occurred in Old Lyme, Connecticut, in 1975. Dr Allen Steere of Yale Medical School identified it as a previously unknown ailment which he called “lyme arthritis”. About 65,000 Europeans fall victim to this disease each year, with fifty to a hundred cases occurring in Ireland.

An outbreak of what seemed to be rheumatoid arthritis occurred in Old Lyme, Connecticut, in 1975. Dr Allen Steere of Yale Medical School identified it as a previously unknown ailment which he called “lyme arthritis”. About 65,000 Europeans fall victim to this disease each year, with fifty to a hundred cases occurring in Ireland.

The bacterium responsible for lyme borreliosis was isolated in 1981; it is spread by ticks. These flightless creatures are not insects, but relatives of spiders scorpions and mites. They belong to an ancient lineage; a tick specimen, attached to a dinosaur feather in 99-million-year-old amber, was described in Nature Communications last year. There are some 900 tick species worldwide.

Living in the undergrowth, ticks will climb up stalks or blades of grass. “Everything comes to him who waits” is their maxim; these robust creatures can go for up to year without food. Sensitive to carbon dioxide in expelled breath, they quickly detect the presence of a potential host in their vicinity. Waiting, with two front legs extended like grappling hooks, a tick will latch onto fur or clothing. Once on board the host, it seeks out a patch of bare skin, burrows into the flesh with its powerful jaws, and begins sucking the victim’s blood. Remaining in place for hours or days, its body swells balloon-like with ingested blood. Anchored into the wound by its jaws and sticky saliva, a tick is difficult to dislodge.

Hunger, they say, is good sauce. This is true for ticks as well as people, according to scientists at the University of Cincinnati. They deprived some of the little vampires of food for 36 weeks in the laboratory and observed their behaviour. “Starved ticks are more likely to look for a host,” concluded lead researcher Andrew Rosendale.

The hungrier they become, the more determined they are to seek you out and the longer they will gorge themselves on your blood. The more prolonged your encounter with a tick, the more likely you are to contract Lyme.

Hungry animals are forced to take risks. A rat, for example, will seldom venture out in daylight unless it has to; facing starvation, it is “caught between the devil and the deep blue sea”. Ticks, it seems, are no exception to the risk-taking recklessness rule. But how did the Cincinnati research team discover this?

Dog ticks, bred in the laboratory, were placed in experimental glass cylinders. Carbon dioxide in the air acts as a tick dinner-bell. So, by breathing into the containers, the researchers could simulate the approach of potential hosts and observe the ticks’ responses. Those which had been starved for long periods proved much more active than recently-fed ones. When the food stimulus was withdrawn, all the ticks lapsed into a dormant energy-saving state. After three months of starvation, however, their behaviour began to change.

Metabolism doubled, the production of saliva increased and the tick’s immune systems became activated; blood-feeding animals must able to resist the many unwanted bacteria they encounter.

The more they were starved, the more they were priming themselves for the next blood-meal,” says Rosendale.

Tick research is important. Warm conditions suit these parasites. Climate change, therefore, may lead to expansion in their numbers here and elsewhere, with implications for humans.

- Andrew Rosendale et al. Progressive behavioural, physiological, and transcriptomic shifts over the course of prolonged starvation in ticks. Molecular Ecology. 2018.