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Climate change could turn some ticks into pacifiers for humans instead of canines.
At temperatures around 38 ° C (100 ° Fahrenheit), some brown dog ticks attracted more people than dogs, the experiments show. Ticks can carry the pathogen that causes the deadly Rocky Mountain spotted fever. The finding suggests that warmer weather could lead to a further spread of the disease from ticks to humans, the researchers reported Nov. 16 at the annual meeting of the American Society of Tropical Medicine and Hygiene.
“We can expect outbreaks of more frequent and major disease from Rocky Mountain spotted fever when heat is produced and when we get hot more often,” says Laura Backus, a researcher at the University of California, Davis School of Veterinary Medicine. Patients with Rocky Mountain spotted fever may die if they do not receive antibiotic treatment within five days. Between 5 and 10 percent of infected people succumb to the disease.
Previous research conducted in Europe has suggested that ticks are more aggressive to people who get hot. To find out whether brown reed host preference depends on temperature, Backus and colleagues captured infants and adults from two genetically distinct groups, or lineages, of the species Rhipicephalus sanguineus. One lineage came from a hot region of Arizona and was considered a tropical tick. The other lineage, from Oklahoma, tolerates colder weather and is considered temperate.
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The scientists compared the behavior of ticks at an ambient temperature of 23 ° C (74 ° F) and an overwhelming 38 ° C. In 10 separate trials at each temperature, the scientists placed around 20 ticks from both lineages and groups of age. center of a plastic tube and gave them the option to move towards a human or a dog.
Usually, the tick prefers dogs to humans. But at higher temperatures, adult tropical ticks showed a preference for the human. More than twice as many ticks moved toward humans at the warmest temperature compared to room temperature. The change in temperature had no significant effect on temperate ticks or nymphs of any of the lineages.
To test whether temperature affects whether brown dog ticks are more attracted to humans than dogs, the researchers set up an experiment in which they placed a dog in a large wooden box (shown) and one person in another. They then connected the boxes with a clear plastic tube and observed if the ticks placed in the tube crawled towards the person or the dog.Don Preisler / UC Davis Faculty of Veterinary Medicine
It is unclear why adult tropical ticks have changed their preference to high temperature. It may have something to do with how they feel about their hosts. “It’s possible that your chemical receptors in your body are activated differently at high temperatures, but it’s still unclear,” Backus adds.
In the last two decades, the United States has seen a steady increase in tick-borne diseases (SN: 11/15/18). In 2000, there were 495 cases of rickettsiosis from spotted fever, a category that includes Rocky Mountain spotted fever, reports from the U.S. Centers for Disease Control and Prevention. Now, 4,000 to 6,000 tick-stained fevers count each year in the United States.
Different types of ticks can carry the bacteria that causes the Rocky Mountain spotted fever. But the brown dog tick is responsible for spreading in the southwestern United States.
“The finding is very exciting, as it demonstrates the difference in behavior between the two lineages,” says Kathleen Walker, an entomologist at the University of Arizona in Tucson who was not associated with the study. It adds more tropical ticks to the north as temperatures rise due to global warming.
Backus says his team would like to study brown dog mites from more regions in the future, a step toward having a larger data set to look for patterns. “And if humans prefer high temperatures, as this study finds, that helps us understand why we’re seeing more outbreaks of spotted fever in the Rocky Mountains,” he says.
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