Incidence of hantavirus infections in Belgium

Temporal Dynamics of Two Pathogenic Hantaviruses Among Rodents in Hungary

Hantaviruses are worldwide pathogens, which often cause serious or even fatal diseases in humans. Hosts are predominantly in the form of rodents and soricomorphs; however, bats are also described as an important reservoir. In Hungary, representatives of two human pathogenic species of the genus Orthohantavirus are present: the Dobrava-Belgrade orthohantavirus and Puumala orthohantavirus. In Hungarian forests, the dominant rodent species are Apodemus flavicollis, Apodemus agrarius, Apodemus sylvaticus, and Myodes glareolus, all of which are natural reservoirs comprising different hantaviruses. The aim of the study was to survey the prevalence of hantaviruses among rodent populations and examine the potential relationship regarding population densities, years, sex, and seroprevalence. Rodents were trapped at 13 sampling plots in a forest reserve located in the Mecsek Mountain range, Hungary, from March to October between 2011 and 2014. Rodent serum samples were tested for IgG antibodies against Dobrava-Belgrade virus and Puumala virus by enzyme-linked immunosorbent assay (ELISA) using a recombinant nucleocapsid protein. During the 4-year sampling period, 2491 specimens were tested and 254 (10.2%) proved seropositive for orthohantaviruses. In 2011, the seroprevalence among Apodemus spp. and M. glareolus was 17.2% (114/661) and 3.9% (3/77), respectively, although this rate had reversed itself in 2014. Seropositivity was substantiated in 18.4% (12/65) of Myodes voles, while only 3.6% (13/359) of the tested Apodemus rodents were found to be IgG positive. Seroconversion was observed in 58 cases, while seroreversion was only detected in 3 individual cases. A significant difference among the number of infected males and females was identified in the first 2 years of our study. Winter survival with respect to rodents was not negatively affected due to the hantavirus infection. Hantavirus seroprevalence was not directly influenced by host abundance. Consequently, we assume that high rodent density alone does not lead to an increased risk of hantavirus infection among the rodent host population.

Modelling human Puumala hantavirus infection in relation to bank vole abundance and masting intensity in the Netherlands

This paper deals with modelling the relationship between human Puumala hantavirus (PUUV) infection, the abundance and prevalence of infection of the host (the bank vole), mast, and temperature. These data were used to build and parametrise generalised regression models, and parametrise them using datasets on these factors pertaining to the Netherlands. The performance of the models was assessed by considering their predictive power. Models including mast and monthly temperature performed well, and showed that mast intensity influences vole abundance and hence human exposure for the following year. Thus, the model can aid in forecasting of human illness cases, since (1) mast intensity influences the vole abundance and hence human exposure for the following year and (2) monitoring of mast is much more feasible than determining bank vole abundance.

Identification of factors influencing the Puumala virus seroprevalence within its reservoir in aMontane Forest Environment

Puumala virus (PUUV) is a major cause of mild to moderate haemorrhagic fever with renal syndrome and is transmitted by the bank vole (Myodes glareolus). There has been a high cumulative incidence of recorded human cases in South-eastern Germany since 2004 when the region was first recognized as being endemic for PUUV. As the area is well known for outdoor recreation and the Bavarian Forest National Park (BFNP) is located in the region, the increasing numbers of recorded cases are of concern. To understand the population and environmental effects on the seroprevalence of PUUV in bank voles we trapped small mammals at 23 sites along an elevation gradient from 317 to 1420m above sea level. Generalized linear mixed effects models(GLMEM) were used to explore associations between the seroprevalence of PUUV in bank voles and climate and biotic factors. We found that the seroprevalence of PUUV was low (6%–7%) in 2008 and 2009, and reached 29% in 2010. PUUV seroprevalence was positively associated with the local species diversity and deadwood layer, and negatively associated with mean annual temperature, mean annual solar radiation, and herb layer. Based on these findings, an illustrative risk map for PUUV seroprevalence prediction in bank voles was created for an area of the national park. The map will help when planning infrastructure in the national park (e.g., huts, shelters, and trails).

The pathogenesis of nephropathia epidemica: new knowledge and unanswered questions

Puumala virus (PUUV) causes an acute hemorrhagic fever with renal syndrome (HFRS), a zoonosis also called nephropathia epidemica (NE). The reservoir host of PUUV is the bank vole (Myodes glareolus). Herein we review the main clinical manifestations of NE, acute kidney injury, increased vascular permeability, coagulation abnormalities as well as pulmonary, cardiac, central nervous system and ocular manifestations of the disease. Several biomarkers of disease severity have recently been discovered: interleukin-6, pentraxin-3, C-reactive protein, indoleamine 2,3-dioxygenase, cell-free DNA, soluble urokinase-type plasminogen activator, GATA-3 and Mac-2 binding protein. The role of cytokines, vascular endothelial growth hormone, complement, bradykinin, cellular immune response and other mechanisms in the pathogenesis of NE as well as host genetic factors will be discussed. Finally therapeutic aspects and directions for further research will be handled.

Modelling zoonotic diseases in humans: comparison of methods for hantavirus in Sweden

Because their distribution usually depends on the presence of more than one species, modelling zoonotic diseases in humans differs from modelling individual species distribution even though the data are similar in nature. Three approaches can be used to model spatial distributions recorded by points: based on presence/absence, presence/available or presence data. Here, we compared one or two of several existing methods for each of these approaches. Human cases of hantavirus infection reported by place of infection between 1991 and 1998 in Sweden were used as a case study. Puumala virus (PUUV), the most common hantavirus in Europe, circulates among bank voles (Myodes glareolus). In northern Sweden, it causes nephropathia epidemica (NE) in humans, a mild form of hemorrhagic fever with renal syndrome.Logistic binomial regression and boosted regression trees were used to model presence and absence data. Presence and available sites (where the disease may occur) were modelled using cross-validated logistic regression. Finally, the ecological niche model MaxEnt, based on presence-only data, was used.In our study, logistic regression had the best predictive power, followed by boosted regression trees, MaxEnt and cross-validated logistic regression. It is also the most statistically reliable but requires absence data. The cross-validated method partly avoids the issue of absence data but requires fastidious calculations. MaxEnt accounts for non-linear responses but the estimators can be complex. The advantages and disadvantages of each method are reviewed.

In Search for Factors that Drive Hantavirus Epidemics

In Europe, hantaviruses (Bunyaviridae) are small mammal-associated zoonotic and emerging pathogens that can cause hemorrhagic fever with renal syndrome (HFRS). Puumala virus, the main etiological agent carried by the bank vole Myodes glareolus is responsible for a mild form of HFRS while Dobrava virus induces less frequent but more severe cases of HFRS. Since 2000 in Europe, more than 3000 cases of HFRS have been recorded, in average, each year, which is nearly double compared to the previous decade. In addition to this upside long-term trend, significant oscillations occur. Epidemic years appear, usually every 2-4 years, with an increased incidence, generally in localized hot spots. Moreover, the virus has been identified in new areas in the recent years. A great number of surveys have been carried out in order to assess the prevalence of the infection in the reservoir host and to identify links with different biotic and abiotic factors. The factors that drive the infections are related to the density and diversity of bank vole populations, prevalence of infection in the reservoir host, viral excretion in the environment, survival of the virus outside its host, and human behavior, which affect the main transmission virus route through inhalation of infected rodent excreta. At the scale of a rodent population, the prevalence of the infection increases with the age of the individuals but also other parameters, such as sex and genetic variability, interfere. The contamination of the environment may be correlated to the number of newly infected rodents, which heavily excrete the virus. The interactions between these different parameters add to the complexity of the situation and explain the absence of reliable tools to predict epidemics. In this review, the factors that drive the epidemics of hantaviruses in Middle Europe are discussed through a panorama of the epidemiological situation in Belgium, France, and Germany.

Environmental change and disease dynamics: effects of intensive forest management on Puumala hantavirus infection in boreal bank vole populations

Intensive management of Fennoscandian forests has led to a mosaic of woodlands in different stages of maturity. The main rodent host of the zoonotic Puumala hantavirus (PUUV) is the bank vole (Myodes glareolus), a species that can be found in all woodlands and especially mature forests. We investigated the influence of forest age structure on PUUV infection dynamics in bank voles. Over four years, we trapped small mammals twice a year in a forest network of different succession stages in Northern Finland. Our study sites represented four forest age classes from young (4 to 30 years) to mature (over 100 years) forests. We show that PUUV-infected bank voles occurred commonly in all forest age classes, but peaked in mature forests. The probability of an individual bank vole to be PUUV infected was positively related to concurrent host population density. However, when population density was controlled for, a relatively higher infection rate was observed in voles trapped in younger forests. Furthermore, we found evidence of a "dilution effect" in that the infection probability was negatively associated with the simultaneous density of other small mammals during the breeding season. Our results suggest that younger forests created by intensive management can reduce hantaviral load in the environment, but PUUV is common in woodlands of all ages. As such, the Fennoscandian forest landscape represents a significant reservoir and source of hantaviral infection in humans.

Hantaan virus surveillance in small mammals at firing points 10 and 60, Yeoncheon, Gyeonggi Province, Republic of Korea

We used epidemiological data and indirect fluorescent antibody tests to determine the Hantaan virus (HTNV) antibody-positive (Ab+) prevalence in small mammals captured at firing point 10 (FP-10) and firing point 60 (FP-60), Gyeonggi Province, near the demilitarized zone, Republic of Korea (ROK), from 2001 to 2005. We used these data, combined with the partial M segment amplified from HTNV recovered from lung tissues of Apodemus agrarius, to clarify the genetic diversity and phylogenetic relationships among HTNV strains in the ROK. Of the eight species of rodents and one insectivore species captured, A. agrarius accounted for 93.4% and 88.5% at FP-10 and FP-60, respectively. Only two species of rodents, A. agrarius and Micromys minutus, were HTNV Ab+. The overall HTNV Ab+ prevalence for A. agrarius captured at FP-10 and FP-60 was 23.3% (121/520) and 14.5% (94/647), respectively. The hantaviral reverse transcription-polymerase chain reaction-positive rate of Ab+ A. agrarius was 74.2% (167/215), and the phylogenetic trees, based on the 269-nucleotide G2-encoding M segment, demonstrated that HTNV strains from FP-10 and FP-60 were distantly segregated from HTNV of other geographic regions in Korea and China. These data are useful in the development of risk reduction strategies for the prevention of hantavirus infections among military personnel, especially during training or the event of hostilities, and civilian populations.

Hantaviruses and their hosts in Europe: reservoirs here and there, but not everywhere?

Five hantaviruses are known to circulate among rodents in Europe, and at least two among insectivores. Four (Dobrava, Saaremaa, Seoul, and Puumala [PUUV] viruses) are clearly associated with hemorrhagic fever with renal syndrome (HFRS). PUUV, the most common etiological agent of HFRS in Europe, is carried by the bank vole (Myodes glareolus), one of the most widespread and abundant mammal species in Europe. This host-virus system is among hantaviruses also the most studied one in Europe. However, HFRS incidence varies throughout the continent. The spatial as well as temporal variation in the occurrence of HFRS is linked to geographic differences in the population dynamics of the reservoir rodents in different biomes of Europe. While rodent abundance may follow mast seeding events in many parts of temperate Europe, in northern (N) Europe multiannual cycles in population density exist as the result of the interaction between rodent populations and specialist predator populations in a delayed density-dependent manner. The spatial distribution of hantaviruses further depends on parameters such as forest patch size and connectivity of the most suitable rodent habitats, and the conditions for the survival of the virus outside the host, as well as historical distribution patterns (phylogeographies) of hosts and viruses. In multiannually fluctuating populations of rodents, with population increases of great amplitude, one should expect a simultaneous build-up of recently hantavirus-infected (shedding) rodents. The increasing number of infectious, virus-shedding rodents leads to a rapid transmission of hantavirus across the rodent population, and to humans. Our review discusses these aspects for PUUV, the only European hantavirus for which there is a reasonable, yet still far from complete, ecological continental-wide understanding. We discuss how this information could translate to other European hantavirus-host systems, and where the most important questions lie for further research.

Ecology of hantavirus in a changing world

Hantavirus is a genus of virus represented by 45 different species and is hosted by small mammals, predominantly rats and mice. Roughly, half of all hantaviruses cause diseases in humans that vary in morbidity from mild to severe. The natural and anthropogenic changes occurring in the environment appear to be impacting the ecology of hantaviruses and their natural hosts as well as the incidence of hantaviral diseases in humans. Although such studies are limited at this time, there is evidence that natural climate cycles such as El Niño as well as anthropogenic climate change enhance hantavirus prevalence when host population dynamics are driven by food availability. Climate appears to have less of an effect on hantavirus when host populations are controlled by predators. Human alteration to the landscape also appears to enhance hantavirus prevalence when the disturbance regime enriches the environment for the host, for example, agriculture. More long-term studies on multiple species of hantavirus are needed to accurately predict the outcome of changing environmental conditions on prevalence in hosts as well as disease incidence in humans.

Hantavirus outbreak in Western Europe: reservoir host infection dynamics related to human disease patterns

Within Europe, Puumala virus (PUUV) is the causal agent of nephropathia epidemica (NE) in humans, a zoonotic disease with increasing significance in recent years. In a region of Belgium with a historically high incidence of NE, bank voles (the PUUV reservoir hosts), were monitored for PUUV IgG antibody prevalence in nine study sites before, during, and after the highest NE outbreak recorded in Belgium in 2005. We found that the highest numbers of PUUV IgG-positive voles coincided with the peak of NE cases at the regional level, indicating that a PUUV epizootic in bank voles directly led to the NE outbreak in humans. On a local scale, PUUV infection in voles was patchy and not correlated to NE incidence before the epizootic. However, during the epizootic period PUUV infection spread in the vole populations and was significantly correlated to local NE incidence. Initially, local bank-vole numbers were positively associated with local PUUV infection risk in voles, but this was no longer the case after the homogeneous spreading of PUUV during the PUUV outbreak.

Association between habitat and prevalence of hantavirus infections in bank voles (Myodes glareolus) and wood mice (Apodemus sylvaticus)

In order to determine the habitat preferred by Myodes (before Clethrionomys) glareolus and the corresponding Puumala hantavirus seroprevalence in those habitats, we captured rodents simultaneously in three significantly different habitats. We compared trapping success and presence of virus per habitat during an ongoing epidemic in order to test the hypothesis of a density-dependent seroprevalence. Our study showed that bank vole population density, as well as Puumala virus seroprevalence, were habitat dependent. Apodemus sylvaticus was found more vulnerable for deteriorating habitat conditions than M. glareolus and could play a role as vehicle for Puumala virus and as mediator for inter- and conspecific virus transmission.

Population, environmental, and community effects on local bank vole (Myodes glareolus) Puumala virus infection in an area with low human incidence

In this study, the distribution of Puumala hantavirus (PUUV) infection in local bank vole Myodes glareolus populations in an area with low human PUUV infection (nephropathia epidemica [NE]) incidence in northern Belgium was monitored for 2 consecutive years. Bank voles were trapped in preferred habitat and tested for anti-PUUV IgG. Infection data were related to individual bank vole features, population demography, and environmental variables. Rare occurrence of PUUV infection was found and PUUV prevalence was low compared with data from the high NE incidence area in southern Belgium. Small-scale climatic differences seemed to play a role in PUUV occurrence, vegetation index and deciduous forest patch size both influenced PUUV prevalence and number of infected voles in a positive way. The data suggested a density threshold in vole populations below which PUUV infection does not occur. This threshold may vary between years, but the abundance of bank voles does not seem to affect the degree of PUUV seroprevalence further. We found indications for a dilution effect on PUUV prevalence, dependent on the relative proportion of nonhost wood mice Apodemus sylvaticus in a study site. In conclusion, we regard the combination of a dilution effect, a possible threshold density that depends on local conditions, and a higher fragmentation of suitable bank vole habitat in our study area as plausible explanations for the sparse occurrence of PUUV infection and low prevalence detected. Thus, beside human activity patterns, local environmental conditions and rodent community structure are also likely to play a role in determining PUUV infection risk for humans.

Hantavirus disease (nephropathia epidemica) in Belgium: effects of tree seed production and climate

Recently, human cases of nephropathia epidemica (NE) due to Puumala virus infection in Europe have increased. Following the hypothesis that high reservoir host abundance induces higher transmission rates to humans, explanations for this altered epidemiology must be sought in factors that cause bank vole (Myodes glareolus) abundance peaks. In Western Europe, these abundance peaks are often related to high tree seed production, which is supposedly triggered by specific weather conditions. We evaluated the relationship between tree seed production, climate and NE incidence in Belgium and show that NE epidemics are indeed preceded by abundant tree seed production. Moreover, a direct link between climate and NE incidence is found. High summer and autumn temperatures, 2 years and 1 year respectively before NE occurrence, relate to high NE incidence. This enables early forecasting of NE outbreaks. Since future climate change scenarios predict higher temperatures in Europe, we should regard Puumala virus as an increasing health threat.

Risk factors for hantavirus infection in Germany, 2005

In 2005, a marked increase in hantavirus infections was observed in Germany. Large cities and areas where hantaviruses were not known to be endemic were affected. A case-control study identified the following independent risk factors for infection: occupational exposure for construction workers, living <100 meter from forested areas, and exposure to mice.

Determinants of the geographic distribution of Puumala virus and Lyme borreliosis infections in Belgium

BACKGROUND

Vector-borne and zoonotic diseases generally display clear spatial patterns due to different space-dependent factors. Land cover and land use influence disease transmission by controlling both the spatial distribution of vectors or hosts, and the probability of contact with susceptible human populations. The objective of this study was to combine environmental and socio-economic factors to explain the spatial distribution of two emerging human diseases in Belgium, Puumala virus (PUUV) and Lyme borreliosis. Municipalities were taken as units of analysis.

RESULTS

Negative binomial regressions including a correction for spatial endogeneity show that the spatial distribution of PUUV and Lyme borreliosis infections are associated with a combination of factors linked to the vector and host populations, to human behaviours, and to landscape attributes. Both diseases are associated with the presence of forests, which are the preferred habitat for vector or host populations. The PUUV infection risk is higher in remote forest areas, where the level of urbanisation is low, and among low-income populations. The Lyme borreliosis transmission risk is higher in mixed landscapes with forests and spatially dispersed houses, mostly in wealthy peri-urban areas. The spatial dependence resulting from a combination of endogenous and exogenous processes could be accounted for in the model on PUUV but not for Lyme borreliosis.

CONCLUSION

A large part of the spatial variation in disease risk can be explained by environmental and socio-economic factors. The two diseases not only are most prevalent in different regions but also affect different groups of people. Combining these two criteria may increase the efficiency of information campaigns through appropriate targeting.

Fluctuating rodent populations and risk to humans from rodent-borne zoonoses

The fluctuations in abundance of a wildlife reservoir are an attractive explanation for temporal variation in primary human cases of a zoonosis. This is because high abundance may lead to more contact between humans and animals, but also to outbreaks of disease within the reservoir population. We propose a mathematical framework that sets out the consequences of correlation between reservoir abundance and reservoir prevalence for how numbers of human cases are related to reservoir abundance. The fluctuations of rodent populations are well studied and often dramatic. A review of field studies of rodent reservoirs for plague, hantaviruses, and other zoonoses shows that, at a seasonal time scale, a positive correlation between host abundance and host prevalence is rarely observed. More commonly, there is an inverse relationship or negative correlation such that a seasonal increase in rodent abundance is not accompanied by a corresponding increase in the abundance of infectious animals. Seasonal changes in rodent abundance are hence unlikely to fully explain seasonal variation in primary human cases. The few longer field studies (>5 years) show a positive but delayed relationship between reservoir abundance and reservoir prevalence.

Predicting the emergence of human hantavirus disease using a combination of viral dynamics and rodent demographic patterns

The paper proposes a model explaining the spatial variation in incidence of nephropathia epidemica in Europe. We take into account the rodent dynamic features and the replicative dynamics of the virus in animals, high in the acute phase of newly infected animals and low in the subsequent chronic phase. The model revealed that only vole populations with multi-annual fluctuations allow for simultaneously high numbers of infected rodents and high proportions of those rodents in the acute excretion phase during the culminating phase of population build-up. This leads to a brief peak in exceptionally high concentrations of virus in the environment, and thereby, to human exposure. Such a mechanism suggests that a slight ecological disturbance in animal-parasite systems could result in the emergence of human diseases. Thus, the potential risk for public health due to several zoonotic diseases may be greater than previously believed, based solely on the distribution of human cases.

Hantavirus infections in Europe

Hantaviruses are enveloped RNA viruses each carried by a specific rodent species. Three hantaviruses, Puumala, Dobrava, and Saaremaa viruses, are known to cause haemorrhagic fever with renal syndrome. In Europe. Puumala causes a generally mild disease, nephropathia epidemica, which presents most commonly with fever, headache, gastrointestinal symptoms, impaired renal function, and blurred vision, whereas Dobrava infections often also have haemorrhagic complications. There are few available data about the clinical picture of confirmed Saaremaa infections, but epidemiological evidence suggests that it is less pathogenic than Dobrava, and that Saaremaa infections are more similar to nephropathia epidemica caused by Puumala. Along with its rodent host, the bank vole (Clethrionomys glareolus), Puumala is reported throughout most of Europe (excluding the Mediterranean region), whereas Dobrava, carried by the yellow-necked mouse (Apodemus flavicollis), and Saaremaa, carried by the striped field mouse (Apodemus agrarius), are reported mainly in eastern and central Europe. The diagnosis of acute hantavirus infection is based on the detection of virus-specific IgM. Whereas Puumala is distinct, Dobrava and Saaremaa are genetically and antigenically very closely related and were previously thought to be variants of the same virus. Typing of a specific hantavirus infection requires neutralisation antibody assays or reverse transcriptase PCR and sequencing.

Distribution of hantavirus foci in Belgium

During 1999 and 2000, we performed rodent captures on 15 sites all over Belgium to evaluate the presence of hantaviruses in local rodent populations. Viral antibody and RNA detection was performed by ELISA/focus reduction neutralisation test and RT-PCR, respectively. We found hantavirus-positive rodents on 13 out of 15 trapping sites and 3 rodent species were found positive for hantavirus infection. Apart from Puumala virus that was carried by Clethrionomys glareolus, 2 additional rodent species, Microtus arvalis and Apodemus sylvaticus, were found antibody- and/or RNA-positive.

Behavioral, physiologic, and habitat influences on the dynamics of Puumala virus infection in bank voles (Clethrionomys glareolus)

Populations of bank voles (Clethrionomys glareolus) were monitored during a 4-year study in southern Belgium to assess the influence of agonistic behavior, reproductive status, mobility, and distribution of the rodents on the dynamics of Puumala virus (abbreviation: PUUV; genus: Hantavirus) infection. Concordance was high between data from serologic testing and results of viral RNA detection. Wounds resulting from biting or scratching were observed mainly in adult rodents. Hantavirus infection in adults was associated with wounds in the fall, i.e., at the end of the breeding season, but not in spring. In addition, sexually active animals were significantly more often wounded and positive for infection. Hantavirus infection was associated with higher mobility in juvenile and subadult males. Seroconversions observed 6 months apart also occurred more frequently in animals that had moved longer distances from their original capture point. During nonepidemic years, the distribution of infection was patchy, and positive foci were mainly located in dense ground vegetation.