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A recent study shows the association between bank vole abundance variations and the incidence of Lyme Borreliosis and Puumala hantavirus infections. The study, carried out by the University of Jyväskylä, the University of Oulu, National Institute for Health and Welfare and and Natural resources institute Finland, showed that the abundance variations of the reservoir hosts of wildlife originated pathogens have importance in estimating the risks these pathogens pose to humans. The results of the work can be taken into account in the risk communication by health authorities. The paper was published in Scientific Reports in 9th August 2021.

Bank vole is the most common rodent species in Northern Europe and the reservoir host for Puumala hantavirus and Borrelia bacteria causing Lyme Borreliosis.

The study quantifies the associations between the time series of bank vole abundance as well as Lyme Borreliosis and Puumala hantavirus infection incidences in Central Finland and Northern Savo regions.

The study shows that the 3-year abundance fluctuations of bank vole are reflected into Puumala hantavirus and Lyme Borreliosis infection incidences. Puumala infection incidence followed the bank vole abundance fluctuations with a couple of months time lag. Hence, the infections are common when bank voles are abundant.

The relationship between bank vole abundance fluctuations and the incidence of Lyme borreliosis was complex. Lyme borreliosis incidence followed the vole abundance variations with longer, approximately one year lag.

One year lag is expected based on the lifecycle of Borrelia -bacteria: A tick larva acquires Borrelia -bactreria while feeding on an infected vole and moults to an infectious nymph and an adult typically one and two years later, respectively.

In Finland, two tick species circulate Borrelia-bacteria. Typically, Ixodes ricinus -tick bites human as a nymph, which explains the one-year lag between bank vole abundance and Lyme borreliosis incidence. Ixodes persulcatus -tick often bites human as an adult. The study identified also a two-year time lag between bank vole abundance and Lyme borreliosis incidence, which may have resulted from bites of adult I. persulcatus ticks.

The study also showed that the strength of the association between Lyme Borreliosis and approximately one year earlier bank vole abundance changed over ther study period. Also the  bank vole population dynamics changed over the study period, which may have affected the infestation of ticks on voles and thus the circulation of Borrelia -bacteria between voles and ticks, consequently affecting the proportion of ticks carrying the bacteria and thus the human infection incidence.

Photo: Ilze Brila, University of Jyväskylä