Some infectious ticks prefer humans to dogs when the heat is on

It sounds like good news for dogs but bad news for humans.

A new study by researchers at the University of California, Davis (UC Davis), School of Veterinary Medicine found that a variety of tick that carries the bacteria that causes Rocky Mountain spotted fever (RMSF)—a disease that can be fatal to both dogs and humans—are more than twice as likely to shift their feeding preference from dogs to humans when temperatures rise.

For the study, the researchers constructed two large wooden boxes three feet tall by two feet wide, then connected them with a clear plastic tube.

In one box, a human, in the other, a dog, and brown dog ticks (Rhipicephalus sanguineus sensu lato) in the clear plastic tube between them. The researchers then observed whether the ticks (which choose hosts to feed on based on smell) preferred dogs or humans. They conducted their observations at 20-minute intervals—first at temperatures of around 74°F, then at 100°F.

They found that when temperatures rose from about 74 to 100 degrees, the brown dog ticks that carry the disease were 2.5 times more likely to choose the human to feed on.

Cases of RMSF and related diseases have risen dramatically over the past two decades, including a rash of deadly RMSF outbreaks among indigenous communities in Arizona and northern Mexico, where the majority of cases are spread by the brown dog tick.

And with research showing that climate change will lead to more 100°F days across the US in the coming years, the number of cases is only likely to grow.

NEWStat contacted Laura Backus, MPH, DVM, who led the study, to find out more.

NEWStat: Why are brown dog ticks more aggressive toward humans than dogs in warm weather?

Laura Backus: We don’t really know why higher temperatures affect which host the brown dog tick wants. Previous research shows that hot temperatures make brown dog ticks more aggressive to hosts in general—moving faster toward them and attaching more quickly—but it’s not clear why their preferences would change.

NEWStat: You draw a distinction between brown dog ticks of temperate lineage and brown dog ticks of tropical lineage. How do these differences play into your findings?

LB: Both lineages carry RMSF and typically feed on dogs. [And both] are found in areas with high densities of dogs. In general, the tropical lineage is located closer to the equator, in areas where the average temperature is greater than 68°F, while the temperate lineage is located farther from the equator. There are areas where they overlap in the US, however, including in California and Arizona. Our results showed that the tropical lineage was more affected by high temperatures than the temperate lineage: the temperate lineage did shift its preference, with fewer ticks going toward the dogs and more toward humans at the high temperature, but it wasn’t as significant. This means the effect of hot weather on Rocky Mountain spotted fever and other diseases that the brown dog tick carries might be different depending on geographic location and the local tick population.

NEWStat: On the surface, this appears to be good news for dogs. Have you seen any decrease in cases of RMSF in dogs in regions where brown dog ticks are more common?

LB: Unfortunately, it probably doesn’t mean that ticks are going to start rejecting dogs altogether. Particularly with the tropical lineage—the number choosing the dog only decreased a small amount, while the number choosing the human increased markedly, so the risk to dogs wasn’t really lower, but the risk to humans was much higher. Because dogs are likely to have higher exposure to the ticks than people, we expect that what it really means is that the ticks will be more likely to feed on humans than they were before, but the risk to dogs isn’t likely to go down.

NEWStat: What are the implications of your findings for veterinarians and pet owners?

NEWStat: With increasing hot weather events, tick control for dogs becomes an even more important part of reducing disease risk for both dogs and their humans. Even if ticks are more willing to feed on humans when it’s hot, ticks feeding on dogs are what keep the brown dog tick population going, so you can interrupt that cycle and reduce risk with tick prevention. Spay/neuter programs, especially in areas where there are large populations of free-roaming dogs or high densities of dogs, are also critical. It’s also important to recognize that the brown dog tick is able to carry and spread RMSF, as we didn’t realize it was a vector for the disease in the US until the last 20 years or so.

In 2019, the National Institutes of Health released a strategic plan to further research into ticks and pathogens they carry in response to the rise in tick-borne diseases in the US. Additional studies are sure to yield more information on the effects on both humans and animals.

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