Sheep tick (Ixodes ricinus). © INRA, CHAUBET Bernard

Tick-fighting tactics

The Siberian chipmunk: a major Lyme disease reservoir

Researchers at INRA, the French Museum of Natural History (MNHN), and the Pasteur Institute discovered that the Siberian chipmunk, a striped rodent introduced to Europe in the 1960s, amplifies the transmission of Lyme disease by native rodents.

Updated on 10/14/2016
Published on 02/26/2013

Lyme disease, otherwise known as borreliosis, is caused by bacteria in the genus Borrelia. These bacteria are hosted by rodents and transmitted to humans by the bite of the tick Ixodes ricinus.

Lyme disease emerged over 40 years ago and is becoming endemic; around 25,000 new cases occur in France each year. Given its epidemiological importance, researchers at INRA, the French Museum of Natural History (MNHN), and the Pasteur Institute decided to take a look at the role played by the Siberian chipmunk (Tamias sibiricus barberi), which was introduced into Europe’s forests in the 1960s.

Senart Forest study site

The researchers focused on rodent populations found in the Senart Forest, near Paris. Siberian chipmunks were released in the woods by unknown individuals in the 1970s. At present, the forest harbors one of the largest Siberian chipmunk populations in France (10,000 to 15,000 individuals). For the sake of comparison, samples were taken not only from Siberian chipmunks but also from two native rodents: the bank vole (Myodes glareolus) and the wood mouse (Apodemus sylvaticus), which are reservoir hosts for Borrelia bacteria.

Role played by the Siberian chipmunk

The study found that, as compared to native rodents, the Siberian chipmunk significantly enhances the likelihood that Lyme disease will be transmitted to humans, which is estimated using the environmental density of infected tick nymphs. Not only is the prevalence of Borrelia infection higher in chipmunks, but chipmunks also carry a relatively larger number of ticks. Indeed, within the rodent pool, Siberian chipmunks were responsible for 80% of total infected nymphs, while the bank vole and the wood mouse generated just 10% each. Furthermore, the chipmunk was found to be infected with three types of Borrelia (Borrelia burgdorferi ss, B. afzelii, and B. garinii), while the native rodents were infected with just one (B. afzelii).

Unexpected consequences of a species introduction

The results of this study underscore that introducing a new species capable of hosting ticks and Borrelia into a local rodent community can have major consequences for Lyme disease transmission. Given that introduction events are more and more common across the globe, it is crucial to understand how non-native species can serve as hosts and thus contribute to disease emergence.

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Marsot M, Chapuis J-L, Gasqui P, Dozières A, Masséglia S, et al. (2013) Introduced Siberian Chipmunks (Tamias sibiricus barberi ) Contribute More to Lyme Borreliosis Risk than Native Reservoir Rodents. PLoS ONE 8(1): e55377. DOI: 10.1371/journal.pone.0055377

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Lyme disease

Also called Lyme borreliosis, Lyme disease is caused by several species of pathogenic bacteria belonging to the Borrelia burgdorferi species complex. These bacteria are transmitted by tick bites, and their main vector in Europe is the tick species Ixodes ricinus.

Lyme disease is the most common vector-borne disease in humans in the Northern Hemisphere; for example, there are 25,000 new cases in France each year. The natural host reservoirs of Borrelia are rodents and birds.

In humans, Lyme disease causes variable clinical symptoms. In general, a rash forms around the bite mark and spreads outward, forming a characteristic bull’s eye. However, the rash does not always appear, making the Lyme disease diagnosis more difficult.

Treatment can be effective if the disease is identified rapidly and antibiotics are given immediately. However, left untreated, Lyme disease can worsen over the course of several years and cause neurological symptoms (e.g., facial paralysis and meningitis), musculoskeletal problems, and autoimmune disorders, such as inflammatory arthritis.

For further information, see the interview given by Muriel Vayssier-Taussat on Lyme disease to the French journal Pour La Science: “Les outils pour améliorer le diagnostic sont pourtant là [We already have the tools for improving Lyme disease diagnoses].” It appears in issue number 460 (February 2016). Read the full interview (in French) here.