Every year, some 300,000 Americans are diagnosed with Lyme disease.
Thousands also suffer from babesiosis and anaplasmosis, tick-borne ailments that can occur alone or as co-infections with Lyme disease.
But how is Lyme disease transmitted?
In eastern and central North America, blacklegged ticks are the primary vectors for Lyme disease, babesiosis, and anaplasmosis. The pathogens that cause these illnesses are widespread in nature; ticks acquire them when they feed on infected animals.
Now scientists revealed that our risk of getting sick is higher when small, fast-living mammals abound.
Richard S. Ostfeld, a scientist at the Cary Institute of Ecosystem Studies, has found a pattern in his researched on Lyme disease since 1992. “Ticks that fed on certain rodents and shrews were much more likely to pick up multiple pathogens, making the environment riskier for people.”
To investigate why mammals differ in their ‘reservoir competence’ or ability to transmit pathogens to ticks, Ostfeld and his co-authors looked at life history traits for nine mammals known to harbor Lyme disease, babesiosis, and anaplasmosis.
They also looked at the role of mammal population density. As ‘sit and wait’ parasites, ticks are much more likely to encounter animals with dense populations. This, in turn, could help pathogens evolve to exploit specific hosts, resulting in more effective transmission rates.
For Lyme disease and anaplasmosis, fast life history features were a strong predictor of an animal’s ability to transmit infection to ticks. Body size was inversely related to reservoir competence. Raccoon, skunk, opossum, squirrel, and deer infected fewer ticks than their mouse, chipmunk, and shrew counterparts.
“This is consistent with past research on Lyme disease, West Nile virus, and Eastern Equine encephalitis. There is evidence that animals that mature early and have frequent, large litters invest less in some immune defenses, making them better pathogen hosts,” Ostfeld noted.
Population density was the best predictor of species’ abilities to transmit all three pathogen groups, with animals that ticks encountered most frequently being the most effective at transferring infection.
“Fast life history and high population density often go hand-in-hand. In rodents and shrews, pathogen adaptation and poor immune defense may be working together to amplify disease spread,” explained co-author Felicia Keesing of Bard College.
In our struggle to manage the ever-growing list of tick-borne diseases, Ostfeld said, “we need to understand which animals magnify human disease risk. Our results suggest when generalist pathogens emerge, small mammals with large populations and a fast pace of life warrant careful monitoring.”
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“Life History and Demographic Drivers of Reservoir Competence for Three Tick-Borne Zoonotic Pathogens,” Richard S. Ostfeld mail, Taal Levi, Anna E. Jolles, Lynn B. Martin, Parviez R. Hosseini, Felicia Keesing, PLOS ONE September 18, 2014