Dog Ticks Prefer Humans As Hosts When It's Hot, Raising Tick-Borne Disease Risks

Can rising temperatures really affect the spread of tick-borne diseases? Researchers think so. In fact, they found that disease-carrying ticks tend to prefer humans as hosts when the temperatures go higher.

A study presented Monday at the Annual Meeting of the American Society of Tropical Medicine and Hygiene (ASTMH) revealed how climate change may increase humans' risks for tick-borne diseases because of the warmer temperatures.

As the news release from ASTMH explained, cases of Rocky Mountain Spotted Fever (RMSF) and other related tick-borne diseases have been increasing in the past 20 years. There has also been evidence that brown dog ticks, which carry these diseases, tend to become more aggressive toward humans in warmer weather.

Tick Experiment

To find out how hot weather may increase RMSF risks among humans, the researchers conducted an experiment wherein they placed humans and dogs in two separate wooden boxes that were connected by a clear plastic tube in between. They placed ticks inside these tubes and observed, under different temperatures, whether the ticks would go to the dog or the human first.

The experiment was done at two different temperatures: 74 degrees Fahrenheit (room temperature) and 100 degrees Fahrenheit. As for brown dog ticks, two types were used in the study: tropical brown dog ticks and temperate brown dog ticks. The researchers conducted 10 trials for each temperature and type of tick.

The researchers found that at the higher temperature, a type of brown dog tick in the tropical lineage clearly shifted its preference from dog to human, the ASTMH news release reported. Meanwhile, the temperate type of brown dog ticks only showed a slight increase in the preference for humans but also displayed a decrease in their preference for dogs.

"Our work indicates that when the weather gets hot, we should be much more vigilant for infections of RMSF in humans," study lead Laura Backus, MPH, DVM said. "We found that when temperatures rose from about 74 to 100 degrees Fahrenheit, brown dog ticks that carry the disease were 2.5 times more likely to prefer humans over dogs."

Neither the dog nor the human volunteers were bitten in the process because mesh barriers were installed to keep ticks from reaching their intended hosts, Live Science reported.

More Human Cases

Although it is still unclear why the ticks shifted their preferences to humans when it became hotter, the findings suggest that the rising temperatures due to climate change may lead to an increase in human RMSF infections.

In the case of the tropical lineage brown dog tick, it can be found mostly in the southern parts of the U.S. But with the rising temperatures, it's possible that their range would increase and move northward, thereby also increasing the chances for human RMSF cases.

As for the temperate lineage brown dog ticks, they are more widespread and can be found in the lower 48 states, the ASTMH news release said. This means that the slight increase in their preference for humans could also lead to increases in the number of RMSF cases in the places where they can be found.

High Alert

As such, the researchers note the importance of having healthcare providers also consider RMSF in diagnosing patients, especially since its symptoms are similar to that of other illnesses. The problem is that while RMSF can be treated in the earlier stages or during the first week, the fatality rate increases to 20% once the infection "takes hold" and complications begin to arise.

In fact, RMSF can be fatal if not treated within the first five days of symptoms, the Centers for Disease Control and Prevention said, so it is of great importance to identify the illness during the early stages.

In 2019, for instance, a 2-year-old boy fell into a coma for a week after being bitten by a tick. The boy eventually recovered but the illness got worse before doctors diagnosed and treated him for RMSF.

"Climate change is moving so quickly that it is critical to keep pace with the many ways it may alter and intensify the risk of a wide range of infectious diseases so we are better prepared to diagnose, treat and prevent them," ASTMH President Joel Breman, MD, DTPH, FASTM said.