Timing of host-vector-pathogen activity and emergence of Lyme disease under climate change

Ecosystems provide important services to the human society, yet, enhancing wildlife can also have its drawbacks for human health and well-being. A major challenge with current climate change is the emergence of tick-borne diseases (TBD), and the World Health Organization identifies Lyme disease as a priority TBD. Climate warming has positive direct effects on tick abundance at northern latitudes, however the importance of indirect climatic effects through host populations is not well known. An important theme in the climate effect literature is the extent to which timing of seasonal activity (phenology) of interacting species responds similarly to warming, making hosts either more or less seasonally available to ticks. The main aim of #TimeLyme is to provide a better understanding of the emergence of TBD at the northern range limits. The #TimeLyme project aim to quantify how incidence and seasonal timing of Lyme disease in humans has developed over years across Norway (1995-2019). We will quantify how timing of migration of red deer affects seasonal niche overlap with ticks and in turn disease incidence. We will also quantify variation in small mammal abundances and timing of activity, tick load and infection prevalence depending on climate. Lastly, we provide a synthesis by quantifying the relative role of direct climate effects on ticks and indirect effects through hosts (small mammals, birds, deer) for Lyme disease incidence. Societal impact is secured by actively involving Norwegian Institute for Public Health and Norwegian Veterinary Institute in the project. The understanding of Lyme borreliosis from the TimeLyme application provide a basis for predicting how climate change will affect incidence. We have written an invited book chapter ("Climate and prediction of tick-borne diseases facing the complexity of the pathogen-tick-host triad at northern latitudes"), which is largely based on elements from the application for a book on "Climate, ticks and disease» edited by Pat Nuttall. This is a collection of expert reviews coming in November 2021. WP1. We have come a long way in analyzing whether there has been a change in seasonal timing of cases of Lyme borreliosis under climate change. We see signs of a clear change, and there are also clear demographic effects on the incidence of Lyme disease. Cases of Lyme disease vary widely between children, adolescents and adults. WP2. In this work package, we focus on the importance of deer, and the features of deer, for the occurrence of ticks and Lyme borreliosis under climate change. It is assumed that the sheep tick (Ixodes ricinus) is a generalist, ie that it feeds on many different species of animals. Nevertheless, we have little quantitative knowledge of whether all host animals are equally suitable for different life stages of ticks. Deer are considered important reproductive hosts for adult ticks, but it has long been discussed whether other medium-sized species of mammals can fulfill the same function. This is relevant if, for example, one decides to regulate the number of deer in order to reduce the number of cases of Lyme disease. A mitigation measure such as reducing the number of deer will have less effect if the adult tick sucks blood from other medium-sized mammals. We have analyzed and published data on the extent to which different medium-sized mammals are suitable host animals for different stages of ticks (Parasites and Vectors 2021). Roe deer was clearly the species with the highest incidence of adult ticks. There were very few ticks on badgers, even though they live in typical tick habitat. We speculate if this is due to their thick fur. Surprisingly, 2/3 of the adult tick on red fox was encapsulated and dead in the subcutaneous tissue. This suggests that the red fox has an immune system that kills the tick. Squirrels had high numbers of tick larvae and nymphs, but only one adult tick was found on 17 squirrels. The work therefore supports the importance of deer as host animals for adult ticks, and that other medium-sized mammals cannot fulfill the same function for ticks. WP3. As planned, we have captured shrews and small rodents in spring and autumn, as well as flagging for ticks. We are now in the process of processing the data, also from previous years. WP4. We have begun to collect relevant empirical information for our theoretical models. We have also had a physical meeting with Jessica Metcalf from the University of Princeton about this work.

Ecosystems provide important services to the human society, yet, enhancing wildlife can also have its drawbacks for human health and well-being. A major challenge with current climate change is the emergence of tick-borne diseases (TBD), and the World Health Organization identifies Lyme disease as a priority TBD. Climate warming has positive direct effects on tick abundance at northern latitudes, however the importance of indirect climatic effects through host populations is not well known. An important theme in the climate effect literature is the extent to which timing of seasonal activity (phenology) of interacting species responds similarly to warming, making hosts either more or less seasonally available to ticks. The main aim of #TimeLyme is to provide a better understanding of the emergence of TBD at the northern range limits as basis for mitigation. #TimeLyme has access to unique databases: The novelty of WP1 lies in quantifying how incidence and seasonal timing of Lyme disease in humans has developed over years across Norway (1995-2019). In WP2, based on extensive field data, we quantify how timing of migration of red deer affects seasonal niche overlap with ticks and in turn disease incidence, and its link to climate. In WP3, we use a combination of field and molecular techniques to quantify variation in small mammal abundances and timing of activity, tick load and infection prevalence depending on climate. Lastly, in WP4, we provide a synthesis by quantifying the relative role of direct climate effects on ticks and indirect effects through hosts (small mammals, birds, deer) for Lyme disease incidence. #TimeLyme is ambitious, yet feasible, designed to assess how climate affects the emergence of tick-borne diseases, hence, linking consequences of climate change to both nature and society. Societal impact is secured by actively involving Norwegian Institute for Public Health and Norwegian Veterinary Institute for co-design and co-production of knowledge.