Dog ticks prefer humans as hosts when temperatures rise

As temperatures rise as a result of climate change, ticks carrying the deadly bacterial disease Rocky Mountain spotted fever (RMSF) may shift their feeding preferences away from dogs and toward humans, according to new research.

The findings, which were presented today (Nov. 16) at the Annual Meeting of the American Society of Tropical Medicine and Hygiene, are an ominous sign of how climate change (and the expected rise in average temperatures) may increase people's risk of contracting tick-borne diseases.

"Our work indicates that when the weather gets hot, we should be much more vigilant for infections of RMSF in humans," Dr. Laura Backus, a veterinarian and doctoral student, who led the study at the University of California, Davis School of Veterinary Medicine, said in a statement. "We found that when temperatures rose from about 74 to 100 degrees Fahrenheit [23 to 38 degrees Celsius], brown dog ticks that carry the disease were 2.5 times more likely to prefer humans over dogs."

RMSF is a disease caused by the bacteria Rickettsia rickettsii, which is carried by ticks that usually feed on dogs. In Arizona, where several RMSF outbreaks have been reported in recent years, and other Southwestern states, it's transmitted primarily by the brown dog tick (Rhipicephalus sanguineus), according to the Centers for Disease Control and Prevention (CDC). Within the first five days of illness, antibiotics can clear it up, but left untreated, the disease can be fatal. The occurrence of RMSF in the U.S. has risen substantially in recent years from 495 cases in 2000, to a peak of 6,248 cases in 2017, according to CDC data. 

Predicting RMSF outbreaks is difficult, said Backus, but knowing what causes ticks to go after humans could help. "If we can identify the situations, the environmental factors, that lead to humans getting bitten more often, we can then hopefully be able to identify and intervene faster and reduce cases," Backus told Live Science. Earlier work by other researchers had suggested that temperature might be one of those factors, that is, that brown dog ticks may be more aggressive toward humans in hot weather, Backus said.

To test the hypothesis, Backus and colleagues recruited some brave human and canine volunteers for a unique experiment. The researchers set up two large wooden boxes, one containing a dog, the other, a human, connected by a clear plastic tube. They released 20 ticks at a time into the middle of the tube and, for 20 minutes, observed whether the ticks, which choose their hosts by smell, moved toward the dog, toward the human or stayed put. Mesh barriers prevented the ticks from reaching their intended hosts. The researchers performed the experiment at two different temperatures, about 74 F (23.3 C), or room temperature, and about 100 F (37.8 C). They used two types of brown dog ticks: tropical ticks, found in the Southern U.S., and temperate ticks, found throughout the continental U.S. They did 10 trials at each temperature for each type of tick.

Looking at all of the trials combined, the tropical ticks were more likely to move toward dogs at room temperature, with an average of 5.2 of the 20 ticks moving toward the dog and 2.9 ticks crawling toward the human, Backus said. At high temperatures, however, they shifted their preference toward humans: An average of 4.4 ticks moved toward the dog, while 7.5 ticks moved toward the person. At the higher temperature, the ticks were also more likely to make a choice, rather than staying in the middle of the tube.

The results were less clear for the temperate ticks. At higher temperatures, significantly fewer temperate ticks chose dogs. Slightly more of the temperate ticks also chose humans at the higher temperature, but that increase was not statistically meaningful, Backus said. 

The researchers aren't sure what is causing the ticks to shift host preferences. "At hotter temperatures, they might be more eager to find a host, because at higher temperatures, they're more likely to dry out and die faster. And so that might be why they're going toward the host more aggressively and more quickly for survival," Backus said. But this doesn't explain why the ticks would prefer humans more than dogs at higher temperatures, she added.

The results have implications for how climate change could affect the incidence of RMSF. When it's hot out, humans are more likely to be bitten by brown dog ticks, which can carry the disease, and as the climate warms, there are likely to be more hot days, Backus said. "There's certainly a concern based on this that climate change translating to more hot weather events is going to lead to more disease outbreaks," she added.

Originally published on Live Science.