How Environment Affects Bacteria in Ticks

Using computer modeling, researchers have investigated how environmental factors affect the occurrence of human-pathogenic bacteria found in the European sheep tick. The research was published today in PeerJ.

The researchers collected sheep ticks in the Swiss Alps, and they identified all possible pathogenic bacteria living inside them. A number of Borrelia species, which cause Lyme borreliosis, and Rickettsia species, which cause spotted fever, were found in the ticks.

According to the researchers, the modeling did not show any single environmental factor to clearly promote or impair the pathogens. Individual pathogens borne by ticks had different reactions to their environmental conditions, which makes it difficult to assess the impact of climate change.

“For example, temperature has a negligible effect on the kinds of pathogenic bacteria ticks have,” explains Tuomas Aivelo of the University of Helsinki. “More significant are factors related to the humidity of the surroundings, such as the aspect and slope of the sites. Borrelia afzelii is more common in slopes facing north, which are more humid than the hotter southern ones, while the Rickettsia species were more prevalent in steeper slopes that are also drier due to stronger water run-off.”

Collecting samples from different elevational gradients is a practical way of studying the effects of environmental conditions.

“Despite the short distances, ticks from varying habitats are easily comparable. A change of a single kilometer in the vertical is the equivalent of a roughly 1,000-kilometer transition between south and north,” Aivelo says. “In the lowest reaches of the study area, the average temperature corresponds with a hotter-than-average Finnish summer day, whereas the summer temperatures in the upper reaches of the area over a kilometer higher resemble an average summer in central Finland.”

The researchers found that the ticks were likely to be infected with many pathogens simultaneously. In other words, they may promote co-infection.

“On the other hand, ticks infected with Spiroplasma bacteria had much fewer human pathogens, such as Borrelia bacteria,” Aivelo says. “A Spiroplasma infection is known to protect mosquitos against many parasites and pathogens, making a similar phenomenon possible also in ticks.”