Mice to blame for the spread of Lyme dis­ease

Lax groom­ing habits. Pas­sive im­mune sys­tems. End­less off­spring can po­ten­tially carry on dis­ease, sci­en­tists say

The Hamilton Spectator - - HEALTH - LINDSEY BEVER

The crit­ters run through for­est floors through­out the eastern United States, snatch­ing up acorns and other tree seeds, berries and bugs.

White-footed mice — known for their wide eyes and ears, long tails and snow-white bel­lies and the feet from which they get their name — are of­ten over­looked by hu­mans, hid­ing out by the bil­lions in U.S. forests, shrubby thick­ets and even wooded wet­lands.

But there’s one crea­ture that knows them well: the tick.

Sci­en­tists say white-footed mice, which are pri­mary car­ri­ers of the Lyme bac­terium Bor­re­lia burgdor­feri, are a highly pop­u­lar host of black-legged ticks — which con­se­quently makes them a key cul­prit in the spread of Lyme dis­ease.

For Lyme dis­ease trans­mis­sion, “es­sen­tially, the only way peo­ple can get in­fected is through a tick bite,” said Richard Ost­feld, se­nior sci­en­tist at Cary In­sti­tute of Ecosys­tem Stud­ies.

Sci­en­tists say that white-footed mice are pos­ing a par­tic­u­larly high risk to hu­mans this year. A boun­ti­ful acorn har­vest a cou­ple of years ago gave them the sus­te­nance needed to re­pro­duce in greater num­bers and cli­mate change may be push­ing them to ex­pand their range to­ward the north.

“That’s some­thing of a worry be­cause where the mice go, so too go the in­fected ticks,” said Ost­feld, who is co-head­ing the Cary In­sti­tute’s Tick Project, along with his wife, Feli­cia Keesing, a bi­ol­ogy pro­fes­sor at Bard Col­lege in New York.

Ost­feld said there are ar­eas in the United States where Lyme dis­ease is rare and, in those places, few or none of the white-footed mice are in­fected.

But in an en­demic area such as one that ex­tends from Vir­ginia to Maine, at least half and some­times up to 90 per cent of the mice are in­fected with Lyme bac­te­ria.

There are about 30,000 re­ported cases of Lyme dis­ease in the United States each year, ac­cord­ing to the Cen­ters for Dis­ease Con­trol and Pre­ven­tion.

How­ever, be­cause many cases are never re­ported, stud­ies sug­gest that the num­ber may be closer to 300,000, ac­cord­ing to the health pro­tec­tion agency.

The CDC re­ported that in 2015, 95 per cent of Lyme dis­ease cases that were re­ported came from 14 eastern states.

The dis­ease is typ­i­cally spread when im­ma­ture ticks, which are called nymphs, bite mice that are in­fected and then bite hu­mans.

Ticks typ­i­cally at­tach them­selves to hid­den ar­eas on the hu­man body such as the groin, armpits and scalp, and must main­tain the bite for 36 to 48 hours be­fore they can spread the in­fec­tion, ac­cord­ing to the CDC.

What are white-footed mice?

Un­like typ­i­cal grey house mice, white-footed mice, known as Peromyscus leu­co­pus, are tawny brown, aside from the white that cov­ers their bel­lies and feet. They are fast. And they are small, with tails as long as their bod­ies.

Ost­feld said these ro­dents are “gen­er­al­ists” when it comes to what they choose to eat and where they live.

Al­though white-footed mice pre­fer acorns, they will eat other things. Still, it’s the acorns that play a big part in their abil­ity to thrive, sci­en­tists say.

“There’s this very in­ter­est­ing and rather com­pli­cated set of eco­log­i­cal con­nec­tions where the mice and, of course, ul­ti­mately the oak trees, sit at the cen­tre of this im­por­tant risk to hu­man health,” said Clive Jones, emer­i­tus ter­res­trial ecol­o­gist at the Cary In­sti­tute of Ecosys­tem Stud­ies.

Jones, who has stud­ied the con­nec­tion be­tween acorn pro­duc­tion and Lyme dis­ease in oak forests in the eastern United States, said that the more acorns there are in the fall, the greater the risk of Lyme dis­ease to hu­mans two years later. Be­cause 2015 was a good year for acorns, 2017 could be a bad year for hu­mans.

Here’s how it works: Adult ticks, which mostly feed upon white-tailed deer, drop off and lay their eggs on the for­est floor, Jones said. The eggs hatch out the next year into lar­vae, which at that point are gen­er­ally not in­fected by the bac­te­ria that causes Lyme dis­ease. The lar­vae get in­fected when they feed on an an­i­mal that car­ries the bac­te­ria, which is most of­ten white-footed mice.

Jones said the lar­val ticks then moult into in­fected nymphs that can in­fect hu­mans the fol­low­ing year.

How many acorns there are in the fall de­ter­mines how many mice there are the next year that can in­fect lar­val ticks — acorns are a ma­jor food source for mice, Jones said. He said heavy acorn pro­duc­tion two years ago re­sulted in many mice a year ago that be­came hosts for in­fected lar­val ticks. That re­sulted in many in­fected tick nymphs this year, he said.

Lax groom­ing habits. Pas­sive im­mune sys­tems. End­less off­spring that can po­ten­tially carry on dis­ease, sci­en­tists say.

“For rea­sons we don’t un­der­stand, mice are what we call ex­tremely tol­er­ant of both tick in­fes­ta­tions and Lyme bac­te­ria in their bod­ies,” said Ost­feld.

“The mice just aren’t fas­tid­i­ous. They just don’t re­ally care about be­ing bit­ten by lots of ticks,” he added.

In ad­di­tion, white-footed mice have blood­streams that Ost­feld calls “a breed­ing ground for all kinds of in­fec­tious agents,” in­clud­ing pathogens that cause babesio­sis and anaplas­mo­sis, other tick-borne dis­eases.

How? Be­cause, he said, “mice sim­ply don’t mount a strong im­mune re­sponse once they get ex­posed. So the mice are not pro­tect­ing them­selves phys­i­o­log­i­cally against ei­ther ticks on the out­side of their bod­ies or against these mi­cro­bial pathogens that get in­side their bod­ies and swim around in their blood­streams.”

That said, preda­tors of white­footed mice, such as owls, hawks, bob­cats, foxes and weasels, won’t get Lyme dis­ease by eat­ing an in­fected ro­dent be­cause the bac­te­ria can­not sur­vive di­ges­tive tracts of birds or mam­mals.

It’s not clear why this par­tic­u­lar ro­dent is so tol­er­ant of mi­crobes, but it may be in part to con­serve en­ergy.

“Why spend your pre­cious en­ergy mount­ing an ex­pen­sive im­mune re­sponse to avoid sick­ness if you could spend that same en­ergy crank­ing out ba­bies as fast as you can be­fore you drop dead in the talons of an owl or the mouth of a fox?” Ost­feld said.

“So we think it’s an evolved strat­egy to al­lo­cate en­ergy to­ward faster re­pro­duc­tion.” In­deed, they do. Re­search shows that white­footed mice reach ma­tu­rity at about one month and can then start to re­pro­duce.

Af­ter fer­til­iza­tion and a ges­ta­tion pe­riod of about 22 days, mother mice can have any­where from three to six ba­bies.

Ost­feld said that as soon as a day af­ter birthing a lit­ter, fe­male mice can be ready to re­pro­duce again, mean­ing they may be nurs­ing one lit­ter while car­ry­ing an­other. These mice can do this again and again through­out their lives, which typ­i­cally last about six to 10 months, sci­en­tist say.

“They have a live fast, die young kind of life­style,” Ost­feld said.

What does it all mean for hu­mans?

“We know that this cryp­tic an­i­mal that most peo­ple don’t even know is around plays a crit­i­cal role in our health,” Ost­feld said.

The sci­en­tist said a key to pro­tect­ing hu­man health is to take mea­sures to try to con­trol the white-footed mouse pop­u­la­tion, and one way to do that is by sup­port­ing nat­u­ral preda­tors by keep­ing their habi­tats in­tact.

“It’s a tough nut to crack be­cause these mice are ev­ery­where and are hard to con­trol, hard to man­age,” he said. “But I’m op­ti­mistic that we can re­duce rates of Lyme.”


Richard Ost­feld of the Cary In­sti­tute of Ecosys­tem Stud­ies says black­legged tick pop­u­la­tions can ex­plode based on a boom-and-bust cy­cle of acorns and white-footed mice, which ticks like to feed on.

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