Climate Change Surprise: Deadly Dog Ticks Moving to Humans

Disease risks are spreading as the climate warms, but this was unexpected: Ticks carrying spotted fever bacteria suddenly prefer you to Fifi when it’s hot

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The brown dog tick experiment at UC Davis School of Veterinary Medicine in California.
The brown dog tick experiment at UC Davis School of Veterinary Medicine in California. Some relief for Fifi, if not for humans.Credit: Don Preisler-UC Davis School of Veterinary Medicine

It’s one thing for mosquitoes to range northward as the hemisphere warms. We could expect that. It’s another thing for the mounting heat to change the target host of parasites. Now we learn that the brown dog tick, carrier of bacteria causing deadly Rocky Mountain spotted fever, suddenly prefers humans to dogs when it gets hotter.

When temperatures rose from about 74 to 100 degrees Fahrenheit (23 to 37.8 degrees Celsius), brown dog ticks carrying spotted fever germs were 2.5 times more likely to prefer humans to dogs, says a study presented at the American Society of Tropical Medicine and Hygiene annual meeting by Dr. Laura Backus of the UC Davis School of Veterinary Medicine, who led the research.

This could help explain the dramatic increase in cases of spotted fever in the United States in recent decades, though other explanations are possible. Indigenous residents of Arizona and northern Mexico have been heavily afflicted in the last 10 years, leading the Centers for Disease Control and Prevention to urge vigilance in Arizona – primarily, watch out for ticks because preventing them from biting in the first place is best; also because early detection and administration of antibiotics (within about a week of infection) is key to surviving spotted fever

Once infection takes hold, the fatality rate for RMSF victims can exceed 20 percent, the study points out. It also bears mention that over 80 percent of people who catch it wind up in the emergency room, the CDC stresses. Complications can include damage to the vascular system, the heart, the lungs, the brain and, ultimately, kidney failure.

Dog ticks can infest households and kennels, and may only be observed when there are so many they’re crawling up the walls. Before feeding they are small, a couple of millimeters; when sated, they can reach a centimeter in size. They’re brick red to brown in color.

Brown dog ticks are found worldwide – this isn’t an American pest. Cousins of our friend the spider, these are highly resilient arachnids (not insects). The kids only have six legs, but by the nymph and adult stage they have the full arachnid complement of eight legs.

As their name implies, dog ticks prefer to parasitize dogs if available. If not, they’ll settle for other animals, including us. The starving will eat anything.

The sudden appetite for human blood by the dog tick when it gets hot is really bad news given the acceleration of global warming and increasing probability of days hotter than 38 degrees Celsius.

How did Backus and colleagues prove the sudden predilection for humans? They set up wooden boxes 3 feet tall by 2 feet wide, which were then connected to each other by a clear plastic tube. They conducted a series of tests that involved putting a human in one box, a dog in the other and ticks in the clear plastic tube between them, and waiting to see which way the ticks went.

Ticks find their prey by smell. At 23.3 degrees Celsius, the brown dog tick (tropical lineage, the researchers specify) primarily chose dogs. At 37.8 degrees Celsius, they chose the human meal.

This type of tick is prevalent in the southern United States – including Florida, Arizona, southern California and Georgia. But as the average temperature warms, the ticks, like mosquitoes before them, are expected to expand their range northward.

A study published last year estimated that 1 billion more people (than now) could become exposed to various mosquito-borne diseases by 2080 under the extreme warming scenario – which is the way the world is headed. Greenhouse gas emissions have continued to climb, coronavirus lockdowns notwithstanding.

The weird news is that temperate-lineage brown dog ticks also carry spotted fever bacteria, and at hotter temperatures seem to suddenly be grossed out by dogs – there was a “pronounced decrease in their preference” for Fifi. However, they didn’t necessarily hare for the humans. They shifted from “pro-dog to neutral.”

The bottom line is that as average temperatures rise, Rocky Mountain spotted fever is likely to become more prevalent in the U.S., including in areas infested by the temperate type of tick. Also note that early symptoms of this fever are sort of like flu and COVID-19 – including fever, headache and muscle pain. In fact, these are also some of the symptoms of many a mosquito-borne disease. Finding an arachnid culprit lurking on your dog, self or curtains can be helpful in expediting diagnostics.

So stay safe, comb your dog thoroughly, a lot, dare to examine very closely what may look like a mole in its fur – and if you realize the “mole” has legs, be careful how you remove it. If you shriek and jerk the arachnid, it will just leave its mouth parts in your dog (or yourself), which will likely get infected. So don’t do that. Grab it using tweezers, as close to its mouth as possible, and gently but steadily pull it off. Or you can burn it with a lit cigarette. It won’t like that and will withdraw its maw from your dermis and drop off.

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