Once bitten, animals develop resistance that shrinks tick population

Just in time for tick season, new research is shining a light on how animals develop resistance to tick bites, which points toward the possibility of developing more effective vaccines against the tiny, disease-carrying bloodsuckers.

In a study of “acquired tick resistance” among deer mice, rabbits and cattle, researchers at Washington State University found that once host animals were exposed to ticks, they developed resistance to bites that dramatically shrank the tick population going forward. That’s important because population size is a crucial element of tick ecology: More ticks mean more disease and other tick-associated problems in humans, wildlife and livestock.

“A lot of attention goes into trying to figure out what makes tick populations increase or decrease—what makes them more abundant here and less abundant there?” said Jeb Owen, an associate professor of entomology at WSU and the corresponding author of the new paper published in the journal Ticks and Tick-Borne Diseases. “If we understand those phenomena, we can try to find ways to take advantage of that to limit tick-associated problems.”

Most tick research relies on experiments using lab animals such as house mice or guinea pigs, which are not the targets of ticks in the wild. In order to “bridge the gap between lab models and tick ecology,” the new study used populations of three species that are hosts for the Rocky Mountain wood tick in the wild: deer mice, rabbits and cattle. The animals were housed at the U.S. Department of Agriculture facilities at the University of Idaho, and the research was conducted by a team of scientists from WSU and the USDA.

Researchers fed the ticks on the animals at three life stages—larvae, nymphs and adults—and compared tick feeding, development and reproduction on animals that had been previously infested by ticks against those that had not. In each case, previously infested hosts developed resistance, and the tick population was reduced at all life stages. On average, nearly 23% fewer ticks on tick-exposed hosts reached adulthood and adult females produced 32% fewer larvae. Subsequent simulations indicated that acquired immunity in the hosts could reduce the mean population growth of three-host ticks by 68%.

“Cumulatively, across the life cycle, the impacts are very large,” Owen said. “This gives us new insight into why tick populations go up and down.”

Problems associated with ticks are wide-ranging. Tick-borne diseases in humans include Lyme disease, Rocky Mountain spotted fever and others—some of which can be fatal. Some ticks can cause a “red-meat allergy” that leaves people violently ill after eating meat. The arachnids transmit disease to livestock and wildlife, and can cause direct harm such as anemia and reduced growth. A related consequence among livestock is that when animals are infected, they may simply spend less time eating because they’re dealing with irritation from tick bites.

“If the animal is spending all its time rubbing ad scratching and licking because it’s irritated, there are effects on growth because it’s eating less,” Owen said.

The findings suggest that the natural immune response might be replicated through the development of a vaccine. Such an effort would require more research, and the new study points to several areas where more research is needed, including how quickly animals develop anti-tick resistance and how long it lasts. But the paper’s findings shed new light on what drives tick populations—adding to a body of research that has so far focused more on the effects of weather and other “abiotic” factors in the environment.

“This phenomenon of acquired tick resistance has the potential to play a really profound role in governing tick population dynamics,” Owen said.