Influence Of Climate Warming On The Increase In Tick-borne Diseases

This work, carried out by the Unité de recherche sur les maladies infectieuses et tropicales émergentes (CNRS/ IRD/ Université de la Méditerranée) is headed by Didier Raoult.

Dog ticks are mainly prevalent during the spring.  They do not usually bite humans.  The few cases of such bites have been observed during the summer when the ticks were exposed to very high temperatures.  These parasites are vectors of bacteria (rickettsioses) which are the agents for serious infectious diseases in humans such as Rickettsia conorii, or Mediterranean spotted fever.

The team led by Raoult in the Unité de recherche sur les maladies infectieuses et tropicales émergentes, has recently demonstrated the role of climate warming in the increase in tick-borne diseases.  Their study showed that rickettsioses were more common and more severe during the very hot summers of 2003 and 2005, and that a minor epidemic developed in the spring of 2007, which was the warmest in 50 years.

The researchers thus developed an experimental model.  One group of ticks was incubated for 24 hours at 40°C and another group at 25°C.  Both groups were then put in contact with humans.  The results were unquestionable: the affinity of dog ticks for humans was much greater after the parasites had been  kept at a temperature of 40°. Thus the affinity of this group of ticks for humans was disturbed by the rise in temperature. Under the effect of warmth, it was as if the ticks had gone mad and started to bite humans.

These findings may explain the seasonal incidence of bites in high summer, and the increasing number of cases that occur during excessively hot periods.  Episodes of global warming may therefore be associated with epidemics of diseases vectored by ticks whose behavior has been affected by the ambient temperature.