Hildegunn Viljugrein

Our understanding of how migratory wildlife populations affect incidence of infectious diseases spilling over into humans is limited. Ticks are expanding their distribution towards northern latitudes, causing emergence of tick‐borne diseases. Deer serve as reproduction hosts for adult ticks, supporting the tick populations. However, in northern areas, deer populations are partially migratory, and migrants occupy high elevation summer ranges that are unsuitable for ticks. Migration can thus theoretically lead to escape from exposure to ticks and to deer feeding fewer ticks, which lower disease hazard to humans. Combining data on Ixodes ricinus tick abundance with GPS‐tracking data of red deer ( Cervus elaphus ), we quantified the distributional overlap of ticks and red deer along elevational gradients in Norway. Furthermore, we correlated both deer density and the proportion of migratory deer with the incidence of Lyme disease in humans. We found that migratory deer summer ranges had colder climate and overlapped less with tick distribution than those of resident deer. Deer density consistently increased Lyme disease incidence in humans. However, we found only weak evidence that deer migration negatively affected Lyme disease incidence. Our study provides a rare quantification of how host availability, in terms of both host density and migratory movement, affects risk of a zoonotic disease.

Competing objectives are common in the management of wildlife diseases. However, the extent to which disease mitigation conflicts with other goals has rarely been quantified. Chronic wasting disease (CWD) is a lethal cervid disease widely distributed in North America and recently detected in Europe. After the detection of CWD among reindeer (Rangifer tarandus) in Norway in 2016, intensified and male‐biased harvesting was used to manage CWD, but this may have adverse side effects. Based on extensive surveillance data (2001‒2022) gathered by the Norwegian Institute for Nature Research (NINA) and local management in addition to harvest statistics, we quantified how CWD management in Hardangervidda, Norway, changed harvest rates and affected population size, operational sex ratio, and age structure, and, in turn, whether recruitment, sex ratio of calves, and calving dates were affected. We used a formal reindeer quality standard to define adverse management. The estimated harvest rate of adult males reached >40% annually; however, the population size was only moderately reduced because the harvest of adult females was similar or decreased. The sex ratio changed from approximately 1:2.2 to 1:6.5 (males ≥3 years: females ≥1 year), which brought the population from good to poor quality according to the reindeer quality standard. The proportion of prime‐aged (≥5 years) males among adult males (≥2 years) in the harvest decreased from >60% to approximately 30%. Demographic changes led to limited side effects in the short‐term, with a delay of 4‒5 days in calving dates and a slightly more female‐biased sex ratio among the harvested calves but without a marked change in early recruitment. Wildlife populations are increasingly threatened by severe disease outbreaks that may require intensive management. Our study highlights the importance of extensive surveillance to document the potential adverse effects on population abundance, demography, and demographic side effects. The fairly limited short‐term adverse impacts can be traded against a higher likelihood of future sustainability without severe diseases; however, this may become an issue of concern if the disease becomes endemic. Balancing these trade‐offs would benefit from good communication between veterinary sciences and wildlife ecology experts.

Lyme disease is an emerging infectious disease and the most common vector‐borne zoonosis in the northern hemisphere. The pathogen that causes Lyme disease in Europe is vectored by the generalist tick Ixodes ricinus, and the emergence of Lyme disease is partly linked to how climate warming affects tick distribution and abundance. However, we lack long‐term data on tick infestations and infection prevalence in the main hosts involved in the transmission cycle. Here, we quantified the temporal trends (2014–2022) of I. ricinus infestations and the prevalence of Borrelia burgdorferi sensu lato in small mammalian hosts and linked annual variation to host abundance and climate in Norway. We found that tick infestations for both larvae (21% per year [95% CI 18–25]) and nymphs (18% [11–26]), and infection prevalence (14% [8–20]) increased over the period and were negatively associated with rodent abundance. Additionally, warmer years were associated with increased larval tick infestations on hosts. The combination of a temporal increase in both larval tick infestation and infection prevalence in hosts likely results in increased production of infected nymphs. Thus, we provide one mechanistic step toward understanding the Lyme disease emergence at northern latitudes of Europe.

Harvesting and culling are methods used to monitor and manage wildlife diseases. An important consequence of these practices is a change in the genetic dynamics of affected populations that may threaten their long‐term viability. The effective population size (N_e) is a fundamental parameter for describing such changes as it determines the amount of genetic drift in a population. Here, we estimate N_e of a harvested wild reindeer population in Norway. Then we use simulations to investigate the genetic consequences of management efforts for handling a recent spread of chronic wasting disease, including increased adult male harvest and population decimation. The N_e/N ratio in this population was found to be 0.124 at the end of the study period, compared to 0.239 in the preceding 14 years period. The difference was caused by increased harvest rates with a high proportion of adult males (older than 2.5 years) being shot (15.2% in 2005–2018 and 44.8% in 2021). Increased harvest rates decreased N_e in the simulations, but less sex biased harvest strategies had a lower negative impact. For harvest strategies that yield stable population dynamics, shifting the harvest from calves to adult males and females increased N_e. Population decimation always resulted in decreased genetic variation in the population, with higher loss of heterozygosity and rare alleles with more severe decimation or longer periods of low population size. A very high proportion of males in the harvest had the most severe consequences for the loss of genetic variation. This study clearly shows how the effects of harvest strategies and changes in population size interact to determine the genetic drift of a managed population. The long‐term genetic viability of wildlife populations subject to a disease will also depend on population impacts of the disease and how these interact with management actions.

Heterogeneous aggregation of parasites between individual hosts is common and regarded as an important factor in understanding transmission dynamics of vector-borne diseases. Lyme disease is vectored by generalist tick species, yet we have a limited understanding of how individual heterogeneities within small mammal host populations affect the aggregation of ticks and likelihood of infection. Male hosts often have higher parasite and infection levels than females, but whether this is linked to sexual body size dimorphism remains uncertain. Here, we analysed how host species, sex, and body mass influenced Ixodes ricinus tick infestations and the infection prevalence of Borrelia burgdorferi sensu lato (s.l.) in three species of small mammals involved in the enzootic transmission cycle of Lyme disease in Norway from 2018 to 2022. Larval and nymphal ticks were found on 98% and 34% of all individual hosts, respectively. In bank voles and wood mice, both larval and nymphal tick infestation and infection probability increased with body mass, and it increased more with mass for males than for females. Tick infestation in the common shrew increased with body mass and was higher in males, while pathogen infection was higher in females. Sex-biases in infestation did not correspond with level of sexual body mass dimorphism across species. This study contributes to our understanding of how individual heterogeneity among small mammalian hosts influences I. ricinus tick aggregation and prevalence of B. burgdorferi s.l. at northern latitudes.

Lyme borreliosis, the most common vector‐borne disease in Europe and North America, is attracting growing concern due to its expanding geographic range. The growth in incidence and geographic spread is largely attributed to climate and land‐use changes that support the tick vector and thereby increase disease risk. Despite a wide range of symptoms displayed by Lyme borreliosis patients, the demographic patterns in clinical manifestations and seasonal case timing have not been thoroughly investigated and may result from differences in exposure, immunity and pathogenesis. We analysed 25 years of surveillance data from Norway, supplemented by population demography data, using a Bayesian modelling framework. The analyses aimed to detect differences in case seasonality and clinical manifestations of Lyme borreliosis across age and sex differentiated patient groups. The results showed a bimodal pattern of incidence over age, where children (0–9 years) had the highest incidence, young adults (20–29 years) had low incidence and older adults had a second incidence peak in the ages 70–79 years. Youth (0–19 years) presented with a higher proportion of neuroborreliosis cases and a lower proportion of arthritic manifestations compared to adults (20+ years). Adult males had a higher overall incidence than adult females and a higher proportion of arthritis cases. The seasonal timing of Lyme borreliosis consistently occurred around 4.4 weeks earlier in youth compared to adults, regardless of clinical manifestation. All demographic groups exhibited a shift towards an earlier seasonal timing over the 25‐year study period, which appeared unrelated to changes in population demographics. However, the disproportionate incidence of Lyme borreliosis in seniors requires increased public awareness and knowledge about this high‐risk group as the population continues to age concurrently with disease emergence. Our findings highlight the importance of considering patient demographics when analysing the emergence and seasonal patterns of vector‐borne diseases using long‐term surveillance data.

Climate changes may lead to tipping points where the epidemiological characteristics of infectious wildlife diseases suddenly change, leading to outbreaks. However, empirically documented cases of the extent to which emerging infectious diseases (EIDs) affect populations of large vertebrates are few, partly because of the limited surveillance of remote wildlife populations. EIDs may potentially lead to higher mortality than mean mortality levels. Here, we quantified the likely impact of digital necrobacillosis on the survival and population dynamics of wild alpine reindeer (Rangifer tarandus) in Hardangervidda National Park (NP), Norway. The outbreak of digital necrobacillosis in 2019 contributed to a decrease in the summer survival rate of juveniles to 51% compared to an average of 88% in 2005–2018 and 77% in 2020. However, since the outbreak mainly reduced juvenile survival, the impact on short‐term population dynamics was moderate, lowering the population growth rate (λ) before harvest from an average value of 1.18 (2005–2018) to 1.08 (2019) before harvest. In 2019, anomalous climate conditions with a high daily mean temperature combined with the highest recorded daily precipitation in late summer were observed. Further studies are required to document the pathological development of digital necrobacillosis across demographic groups and proximate and ultimate causes of mortality.

Climate change has had a major impact on seasonal weather patterns, resulting in marked phenological changes in a wide range of taxa. However, empirical studies of how changes in seasonality impact the emergence and seasonal dynamics of vector-borne diseases have been limited. Lyme borreliosis, a bacterial infection spread by hard-bodied ticks, is the most common vector-borne disease in the northern hemisphere and has been rapidly increasing in both incidence and geographical distribution in many regions of Europe and North America. By analysis of long-term surveillance data (1995–2019) from across Norway (latitude 57°58′–71°08′ N), we demonstrate a marked change in the within-year timing of Lyme borreliosis cases accompanying an increase in the annual number of cases. The seasonal peak in cases is now six weeks earlier than 25 years ago, exceeding seasonal shifts in plant phenology and previous model predictions. The seasonal shift occurred predominantly in the first 10 years of the study period. The concurrent upsurgence in case number and shift in case timing indicate a major change in the Lyme borreliosis disease system over recent decades. This study highlights the potential for climate change to shape the seasonal dynamics of vector-borne disease systems.

Surveillance of wildlife diseases poses considerable logistical challenges compared to that of humans or livestock. Citizen science can enable broader coverage, but building an efficient disease monitoring system that relies on hunters is challenging. Chronic wasting disease (CWD) is a lethal and infectious prion disease of cervids. Improving surveillance is important with the detection of CWD in wild reindeer (Rangifer tarandus) in Norway. This study describes the components of an efficient CWD monitoring system utilizing recreational hunters. We report the success of data capture after 6 years of surveillance. We provide an overview of CWD occurrence among the 24 wild reindeer areas and quantify the likelihood of disease absence in areas without detection. Surveillance aimed to test hunted reindeer aged ≥1 year. With higher quotas and extended hunting seasons, proactive surveillance was implemented in at‐risk areas. There were several challenges of population demarcation and the lack of surveys required for risk‐based sampling. Several specific tools for hunters have been developed, including digital apps for rapid reporting and feedback. Laboratory capacity was expanded, and novel statistical tools were developed for the specifics of the sampled tissues. The surveillance (2016–2021) achieved a sample return rate of 61.5% from a maximum of 22,123 harvested reindeer aged ≥1 year. Among these, 64.1% included both relevant tissues (retropharyngeal lymph nodes and brain), yielding 9412 (42.5%) complete samples of harvested reindeer. Samples originating from harvest constituted ~84% of total wild reindeer samples. CWD was detected in 2 of the 24 wild reindeer management areas. The remaining populations had a probability of CWD‐freedom from 60% to 99% (mean = 77%) at a design prevalence of 0.5%. Utilizing hunters to monitor wildlife disease appears to be the most realistic option for cervid species. However, the logistical and economic constraints are substantial and pose long‐term challenges. Considerable uncertainty about disease occurrence remains even after massive surveillance, and whether management should take preventive actions remains a challenge.