The U.S. Centers for Disease Control estimates that each year there are about 300,000 cases of Lyme Disease, which is vectored by blacklegged ticks. These ticks are also commonly known as deer ticks, which may cause people to believe that ticks are infected with Lyme disease after biting deer. However, while it’s true that deer and other mammals can spread tick populations, they do not carry the disease. Instead, ticks mainly pick up Lyme pathogens from white-footed mice.

While you might suppose that the ticks would harm the mice, new research in the journal Ecology shows that the mice appear to be indifferent to larval tick infestations. Even more surprising, male mice with large tick loads were more likely to survive during a given season, and white-footed mice with hundreds of larval ticks survived just as long as those with only a few ticks.

“Our findings were counterintuitive,” said Richard Ostfeld, one of the study’s authors. “By definition, ticks are parasites. But tick burdens were not correlated with reductions in white-footed mouse survival or overwintering success, and they didn’t slow population growth. It looks like ticks are getting a free lunch.”

Conclusions were based on an analysis of 5,587 ‘capture histories’ recorded between 1995 and 2011. Every 3-4 weeks, from the peak of larval tick activity in midsummer until the end of the mouse breeding season in the fall, mice were trapped. On their first capture, animals were outfitted with ear tags. Each time a mouse was trapped, researchers recorded the number of ticks on the animal, as well as other variables like its tag ID and sex.

“White-footed mice are reservoirs for the agents that cause Lyme disease, anaplasmosis, and babesiosis,” said lead author Michelle Hersh. “Larval blacklegged ticks can become infected when they feed on mice. These ticks can then transmit illness to people during their next blood meal.”

Prior research revealed a positive relationship between white-footed mouse numbers and the abundance of blacklegged ticks infected with the bacterium that causes Lyme disease, but little was known about the impact that feeding larval ticks had on white-footed mouse survival.

To answer this question, Hersh and colleagues developed a model using the long-term ‘capture history’ data. If blacklegged ticks had a negative effect on mice, it was expected that heavy tick burdens would lead to a decrease in mouse survival and fewer feeding opportunities for future ticks. Conversely, if mice were tolerant of ticks, heavily parasitized animals would survive as long as their parasite-free counterparts, and tick feeding opportunities would abound.

“Ticks feed on blood. It seemed obvious that they should have a negative impact on mice, Hersh said. “But our analysis found that larval ticks had few measurable effects on white-footed mouse survival, and none that were negative. This held true even in stressful years, when mouse numbers were high or acorn resources were scarce. There was also an unexpected positive association between tick burdens and within-season survival in male mice.”

The authors provide several scenarios that could explain why infested male mice were likely to survive. Among them: habitat most conducive to mouse survival (i.e. dense vegetation) may also favor ticks, dominant mice with large home ranges may encounter more ticks, and mice with heavy tick burdens may exhibit more risk-averse behavior.

“Our findings underscore the importance of mice as reservoirs for tick-borne pathogens,” said co-author Shannon LaDeau. “From a human health perspective, the indifference that white-footed mice have to blacklegged ticks is bad news. It signals a positive feedback loop that favors the proliferation of parasites.”

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– Mice give ticks a free lunch