The brown dog tick (Rhipicephalus sanguineus) rarely bites people, far preferring the taste of dog. But global warming could be changing that, exposing people to dangerous diseases as a result.
In the spring of 2007, three men in France became seriously ill after sustaining bites from disease-infected dog ticks. The bites occurred after the hottest April since 1950, said Didier Raoult, a professor at the University of Marseille School of Medicine in France.
The incident reminded Raoult of two other recent cases. A 2004 outbreak of Rocky Mountain spotted fever in Arizona was also associated with dog ticks. And during the exceptionally hot summer of 2003, a man died after 20 brown dog ticks bit him at once.
Raoult suspected that, in each of these cases, hot weather made dog ticks turn on humans. He decided to investigate.
First, he and colleagues visited the site of the most recent outbreak: a house in southern France, where a 50-year-old woman had been living with her tick-infested dog.
The dog died shortly before the outbreak began. The woman said the ticks became particularly aggressive in April 2007.
'Perfect storm scenario'
In less than an hour of searching, the researchers collected 218 brown dog ticks from around the house and garage. Of the 133 ticks they tested, nearly 30 percent were infected with varieties of Rickettsia, a genus of bacteria that causes Rocky Mountain spotted fever, Mediterranean spotted fever, and other life-threatening diseases. In the general dog tick population, fewer than 1 percent are usually infected.
Next, the doctors examined two of the patients — a 25-year-old man and a 30-year-old man. Their symptoms were similar: fevers, headaches, night sweats, rashes, and loss of vision. Both were diagnosed with severe spotted fevers, but the species of pathogens they carried were different: Tests showed that the older man was infected with Rickettsia conorii, the agent of Mediterranean spotted fever, while the younger carried R. massiliae, an emerging pathogen.
"You had a set of conditions at this particular household that created, for lack of a better term, a perfect-storm scenario," said Christopher Paddock, a pathologist with the Centers for Disease Control and Prevention in Atlanta. "There were lots of ticks, lots of infected ticks, warm temperatures, and the dog dying, which is the normal host. All that adds up to ticks feeding on people and transmitting infection."
To see how much the temperature, in particular, mattered, Raoult and two colleagues turned themselves into human guinea pigs. They incubated 500 brown dog ticks at 77 degrees Fahrenheit and 500 at 104 degrees Fahrenheit. Then, they placed the ticks on their own arms.
"They take a very long time to attach," Raoult said, bravely. "It's not like a mosquito. They don't have time to bite you."
After an hour, about half of the ticks incubated at 104 degrees tried to burrow in, Raoult said. None of those incubated at 77 degrees did. The results appeared in November in the journal PLOS Neglected Tropical Diseases. Raoult suspects thirst drives the ticks to seek human blood at higher temperatures.
As global climate warms, dog ticks might be more likely to bite people, and tick-transmitted diseases might become more common, the researchers concluded.
Those conclusions are reasonable, Paddock agreed. But he was wary of inferring too much from this particular study. Temperature is just one factor that affects tick behavior, for one thing. And a sudden surge from 77 degrees to 104 degrees is too extreme to mimic a realistic global warming scenario.
"We're witnessing small incremental changes over time," Paddock said. "We don't know what sort of changes we'll see on a global scale."