Abstract
Zoonotic diseases, caused by pathogens transmitted between other vertebrate animals and humans, pose a major risk to human health. Rodents are important reservoir hosts for many zoonotic pathogens, and rodent population dynamics affect the infection dynamics of rodent-borne diseases, such as diseases caused by hantaviruses. However, the role of rodent population dynamics in determining the infection dynamics of rodent-associated tick-borne diseases, such as Lyme borreliosis (LB), caused by Borrelia burgdorferi sensu lato bacteria, have gained limited attention in Northern Europe, despite the multiannual abundance fluctuations, the so-called vole cycles, that characterise rodent population dynamics in the region. Here, we quantify the associations between rodent abundance and LB human cases and Puumala Orthohantavirus (PUUV) infections by using two time series (25-year and 9-year) in Finland. Both bank vole (Myodes glareolus) abundance as well as LB and PUUV infection incidence in humans showed approximately 3-year cycles. Without vector transmitted PUUV infections followed the bank vole host abundance fluctuations with two-month time lag, whereas tick-transmitted LB was associated with bank vole abundance ca. 12 and 24 months earlier. However, the strength of association between LB incidence and bank vole abundance ca. 12 months before varied over the study years. This study highlights that the human risk to acquire rodent-borne pathogens, as well as rodent-associated tick-borne pathogens is associated with the vole cycles in Northern Fennoscandia, yet with complex time lags.
Highlights
Zoonotic diseases—diseases caused by pathogens transmitted between non-human vertebrate animals and humans—pose a substantial health threat to humans[1,2,3]
Lyme Borreliosis (LB) persists in these regions, with approximately 6000–7000 human infections annually, mainly caused by B. afzelii that is transmitted by generalist ticks I. ricinus and I. persulcatus[42,43] in Finland[44]
LBlab was non-linearly associated with bank vole abundance 12 months earlier, and this association changed over the years as there was an interaction between bank vole abundance with 12-month lag and each of year and year[2] (Table 1; Fig. 2A,B)
Summary
Zoonotic diseases—diseases caused by pathogens transmitted between non-human vertebrate animals and humans—pose a substantial health threat to humans[1,2,3]. L.) bacteria, including rodent-associated Borrelia afzelii, which cause tick-borne Lyme Borreliosis (LB) disease in humans, are transmitted by Ixodes spp. ticks from wildlife reservoir hosts to h umans[16]. The findings are not unequivocal with some studies showing positive but others showing no association between rodent abundance and following year human disease[38,39,40] It remains enigmatic how the circulation of rodent-associated tick-borne pathogens is maintained in over very low reservoir host densities that may last over extended time periods, which characterise the multiannually fluctuating vole populations in Northern Europe[41]. One of the most common rodent species in Northern Europe is the bank vole (Myodes glareolus)[45], which is considered as a hyperreservoir (species that carry out two or more zoonotic pathogens) of zoonotic p athogens[7], including PUUV46 and tick-borne Borrelia afzelii, one of the agents causing LB47,48. The tick life-cycle necessarily introduces time lags in pathogen transmission; ticks typically acquire the pathogen in the larval stage when feeding on a reservoir host and can transmit the pathogen to humans in subsequent life stages, either as a nymph (typically in the following year) or as an adult (typically two years later)[52,53]
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