Controlling Echinococcus multilocularis-ecological implications of field trials

Ecoepidemic modeling and dynamics of alveolar echinococcosis transmission

Alveolar echinococcosis, transmitted between definitive hosts and intermediate hosts via predation, threatens the health of humans and causes great economic losses in western China. In order to explore the transmission mechanism of this disease, an eco-epidemiological lifecycle model is formulated to illustrate interactions between two hosts. The basic and demographic reproduction numbers are developed to characterize the stability of the disease-free and endemic equilibria as well as bifurcation dynamics. The existence of forward bifurcation and Hopf bifurcation are confirmed and are used to explain the threshold transmission dynamics. Numerical simulations and bifurcation diagrams are also presented to depict rich dynamics of the model. Numerical analysis suggests that improving the control rate of voles will reduce the risk of transmission, while the high predation rate of foxes may also lead to a lower transmission risk, which is different from the predictions of previous studies. The evaluation of three control measures on voles implies that, when the fox's predation rate is low (high), the chemical (integrated) control will be more effective.

A systematic review and meta-analysis on anthelmintic control programs for Echinococcus multilocularis in wild and domestic carnivores

Human alveolar echinococcosis (AE), caused by the tapeworm Echinococcus multilocularis, is one of the most dangerous zoonoses in the Northern hemisphere. In Europe, the parasite's life cycle is sylvatic, involving small rodents as intermediate hosts and red foxes as the major definitive hosts. Given the severity of this disease in humans and the high levels of environmental contamination with E. multilocularis in endemic areas, it seems crucial to implement control measures in order to prevent human AE. This systematic review identifies previous anthelmintic control programs targeting E. multilocularis in wild and domestic carnivores and evaluates the effectiveness of the different strategies implemented. A search through six databases identified 302 scientific papers for the period 1950–2015, of which only 17 were retained according to the inclusion criteria set. These 17 papers focused on control of E. multilocularis by baiting foxes in highly endemic areas of Europe or Japan, with the exception of one study focused on dogs in Alaska. The papers highlighted differences in baiting types, baiting frequency, choice of control areas and length of treatment period. Overall, these studies resulted in a sharp and statistically significant decrease in parasite prevalence, confirmed by the absence of overlap between confidence intervals for the pooled risk differences of control and treated areas. A monthly baiting frequency was proven to be highly effective at decreasing E. multilocularis prevalence in foxes, even in highly endemic areas and in a short period of time. Nevertheless, when foxes were not fully dewormed, the parasite showed a strong capacity to rapidly recover its initial prevalence.

The fox baiting approach appears to be the most useful method for controlling the sylvatic life cycle of E. multilocularis, but it require a cost/benefit analysis before it is likely to be accepted by stakeholders.

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This systematic review evaluated the effectiveness of the control programmes of E. multilocularis.

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Differences in baiting types, baiting frequency, selection of control areas and treatment period were highlighted.

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A monthly baiting was proved to be efficient to decrease E. multilocularis prevalence in foxes.

Dynamic, spatial models of parasite transmission in wildlife: Their structure, applications and remaining challenges

Individual differences in contact rate can arise from host, group and landscape heterogeneity and can result in different patterns of spatial spread for diseases in wildlife populations with concomitant implications for disease control in wildlife of conservation concern, livestock and humans. While dynamic disease models can provide a better understanding of the drivers of spatial spread, the effects of landscape heterogeneity have only been modelled in a few well-studied wildlife systems such as rabies and bovine tuberculosis. Such spatial models tend to be either purely theoretical with intrinsic limiting assumptions or individual-based models that are often highly species- and system-specific, limiting the breadth of their utility. Our goal was to review studies that have utilized dynamic, spatial models to answer questions about pathogen transmission in wildlife and identify key gaps in the literature. We begin by providing an overview of the main types of dynamic, spatial models (e.g., metapopulation, network, lattice, cellular automata, individual-based and continuous-space) and their relation to each other. We investigate different types of ecological questions that these models have been used to explore: pathogen invasion dynamics and range expansion, spatial heterogeneity and pathogen persistence, the implications of management and intervention strategies and the role of evolution in host-pathogen dynamics. We reviewed 168 studies that consider pathogen transmission in free-ranging wildlife and classify them by the model type employed, the focal host-pathogen system, and their overall research themes and motivation. We observed a significant focus on mammalian hosts, a few well-studied or purely theoretical pathogen systems, and a lack of studies occurring at the wildlife-public health or wildlife-livestock interfaces. Finally, we discuss challenges and future directions in the context of unprecedented human-mediated environmental change. Spatial models may provide new insights into understanding, for example, how global warming and habitat disturbance contribute to disease maintenance and emergence. Moving forward, better integration of dynamic, spatial disease models with approaches from movement ecology, landscape genetics/genomics and ecoimmunology may provide new avenues for investigation and aid in the control of zoonotic and emerging infectious diseases.

Simulating control of a focal wildlife outbreak of Echinococcus multilocularis

The parasitic tapeworm Echinococcus multilocularis is the causative agent of alveolar echinococcosis, a serious zoonotic infection present in Europe that can be fatal. The United Kingdom currently has E. multilocularis free status but the possibility of introduction exists, most likely via an imported or returning dog or other deliberately introduced animal that has not had anthelmintic treatment. We have developed a model to predict the probability of successfully eliminating a focal outbreak of E. multilocularis using a programme of anthelmintic bait distribution. We investigated three different potential control programmes, each with 36 monthly campaigns commencing five, ten or 15 years after disease introduction over an area of 2827km². We assumed equilibrium disease prevalence of 30%, 40% and 55% based on the range of values reported across Europe. However, for all of these scenarios, equilibrium had not been reached at five to 15 years after introduction and simulated local prevalence values were between 0.5% and 28%. We found that it is possible to eliminate the disease with a 38%-86% success rate if control is started five years after introduction, dropping to 0% to 56% if control is delayed until 15 years after introduction, depending upon the prevalence equilibrium. We have also estimated the costs involved in these programmes to be from €7 to €12 million (2013 prices).

Historical Aspects of Echinococcosis

Echinococcosis is a zoonosis whose history dates back to antiquity. This article provides an overview on the general history of echinococcosis, including the elucidation of Echinococcus life cycles and the long controversy on the aetiology of the cystic and alveolar forms of echinococcosis (CE and AE), lasting about 100years since the middle of the 19th century. Furthermore, selected historical aspects of some fields of echinococcosis research are discussed and compared with our current knowledge, such as geographic distribution and epidemiology of CE (Echinococcus granulosus) and AE (Echinococcus multilocularis), clinical aspects and pathology, diagnosis in humans and animals, treatment (with focus on chemotherapy), control and basic research. A short paragraph is devoted to the neotropical forms of echinococcosis, caused by Echinococcus vogeli and Echinococcus oligarthrus. In this context the achievements of some ancestral pioneers of echinococcosis research are particularly highlighted and appreciated. Finally, the role of associations, international organizations (World Health Organization and others) and international working groups in echinococcosis research and control is briefly outlined. The retrospective reveals both the admirable achievements of our ancestors and the scientific progress of more recent times. But, it also shows the gaps in our knowledge, skills and resources that we need to control or even eradicate echinococcosis.

Risk factors for Echinococcus coproantigen positivity in dogs from the Alay valley, Kyrgyzstan

Echinococcosis, caused by the zoonotic cestodes Echinococcus granulosus (sensu lato) and Echinococcus multilocularis, is highly endemic in the Central Asian Republic of Kyrgyzstan, and is being identified increasingly as a public health problem, especially amongst pastoral communities. As domestic dogs are considered to be the main source of human infection, the identification of potential transmission pathways is of relevance when considering implementing an echinococcosis control scheme. The current report describes the results of an analytical study of canine Echinococcus coproantigen enzyme-linked immunosorbent assay (ELISA) prevalence in the Alay valley of southern Kyrgyzstan prior to the commencement of regular praziquantel dosing of dogs. A logistic regression model using a form of Bayes modal estimation was used to identify possible risk factors for coproantigen positivity, and the output was interpreted in a Bayesian context (posterior distributions of the coefficients of interest). The study found that sheepdogs had lower odds of coproantigen positivity, as did dogs in households with donkeys, where owners had knowledge of echinococcosis, and households which engaged in home slaughtering. Surprisingly, there was no evidence of an association between free roaming or previous praziquantel dosing and coproantigen positivity, as has been found in previous studies. Possible reasons for these findings are discussed in the context of the epidemiology of echinococcosis and potential intervention approaches.

Synthesising 30 years of mathematical modelling of Echinococcus transmission

BACKGROUND

Echinococcosis is a complex zoonosis that has domestic and sylvatic lifecycles, and a range of different intermediate and definitive host species. The complexities of its transmission and the sparse evidence on the effectiveness of control strategies in diverse settings provide significant challenges for the design of effective public health policy against this disease. Mathematical modelling is a useful tool for simulating control packages under locally specific transmission conditions to inform optimal timing and frequency of phased interventions for cost-effective control of echinococcosis. The aims of this review of 30 years of Echinococcus modelling were to discern the epidemiological mechanisms underpinning models of Echinococcus granulosus and E. multilocularis transmission and to establish the need to include a human transmission component in such models.

METHODOLOGY/PRINCIPAL FINDINGS

A search was conducted of all relevant articles published up until July 2012, identified from the PubMED, Web of Knowledge and Medline databases and review of bibliographies of selected papers. Papers eligible for inclusion were those describing the design of a new model, or modification of an existing mathematical model of E. granulosus or E. multilocularis transmission. A total of 13 eligible papers were identified, five of which described mathematical models of E. granulosus and eight that described E. multilocularis transmission. These models varied primarily on the basis of six key mechanisms that all have the capacity to modulate model dynamics, qualitatively affecting projections. These are: 1) the inclusion of a 'latent' class and/or time delay from host exposure to infectiousness; 2) an age structure for animal hosts; 3) the presence of density-dependent constraints; 4) accounting for seasonality; 5) stochastic parameters; and 6) inclusion of spatial and risk structures.

CONCLUSIONS/SIGNIFICANCE

This review discusses the conditions under which these mechanisms may be important for inclusion in models of Echinococcus transmission and proposes recommendations for the design of dynamic human models of transmission. Accounting for the dynamic behaviour of the Echinococcus parasites in humans will be key to predicting changes in the disease burden over time and to simulate control strategies that optimise public health impact.

Integrating movement ecology with biodiversity research - exploring new avenues to address spatiotemporal biodiversity dynamics

Movement of organisms is one of the key mechanisms shaping biodiversity, e.g. the distribution of genes, individuals and species in space and time. Recent technological and conceptual advances have improved our ability to assess the causes and consequences of individual movement, and led to the emergence of the new field of 'movement ecology'. Here, we outline how movement ecology can contribute to the broad field of biodiversity research, i.e. the study of processes and patterns of life among and across different scales, from genes to ecosystems, and we propose a conceptual framework linking these hitherto largely separated fields of research. Our framework builds on the concept of movement ecology for individuals, and demonstrates its importance for linking individual organismal movement with biodiversity. First, organismal movements can provide 'mobile links' between habitats or ecosystems, thereby connecting resources, genes, and processes among otherwise separate locations. Understanding these mobile links and their impact on biodiversity will be facilitated by movement ecology, because mobile links can be created by different modes of movement (i.e., foraging, dispersal, migration) that relate to different spatiotemporal scales and have differential effects on biodiversity. Second, organismal movements can also mediate coexistence in communities, through 'equalizing' and 'stabilizing' mechanisms. This novel integrated framework provides a conceptual starting point for a better understanding of biodiversity dynamics in light of individual movement and space-use behavior across spatiotemporal scales. By illustrating this framework with examples, we argue that the integration of movement ecology and biodiversity research will also enhance our ability to conserve diversity at the genetic, species, and ecosystem levels.

Collaborative control initiatives targeting zoonotic agents of alveolar echinococcosis in the northern hemisphere

Alveolar echinococcosis is one of the most important lethal zoonotic helminth infections in the northern hemisphere. Currently, the threat to public health is increasing, as evidenced by the rising prevalence rate of alveolar echinococcosis, as well as the invasion of urban areas by infected wild foxes. This threat is further increased due to the involvement of pet dogs, and probably cats, as emerging sources of infection. These increased threats to public health also have associated economic risks; therefore, there is a need for effective and sustainable methods of control. In this paper, initiatives to control alveolar echinococcosis by targeting its definitive hosts through anthelmintic baiting campaigns initiated by local residents who used local resources for bait production, distribution and collection of fecal samples for diagnosis are described. Further, when such distribution programs are coupled with the use of GIS-based maps, the optimum distribution of bait was obtained. These programs have also included the use of intravital diagnostic analyses of infection rates, which have been overseen by the Forum on Environment and Animals (FEA), and also allowed a nationwide monitoring of echinococcosis in difinitive hosts. In addition, a government initiative requiring mandatory reporting of echinococcosis in dogs to health authorities was recently initiated in Japan. Overall, the results of this study have shown that use of collaborative control initiatives targeting zoonotic agents of alveolar echinococcosis can be an effective method for reducing the threat of lethal echinococcosis in the northern hemisphere.

Ecological and biological factors involved in the transmission of Echinococcus multilocularis in the French Ardennes

In order to identify the respective importance of the ecological and biological factors involved in the transmission of Echinococcus multilocularis, we estimated grassland vole intermediate host (Microtus sp. and Arvicola terrestris) population densities, in relation to the diet of the definitive host (red fox, Vulpes vulpes) and with the prevalence of E. multilocularis in the fox population. The study was conducted in the Ardennes, north-eastern France, which is an area with a high incidence of alveolar echinococcosis. Surface index methods showed that Microtus was the most abundant intermediate host in the area. Furthermore, Microtus was present in one-third of the 144 faeces and 98 stomach content samples examined and represented more than two-thirds of the rodent occurrences. Red fox predation on Microtus was significantly correlated with Microtus relative abundance. In contrast, the relative abundance of A. terrestris was very low. This species, as well as Clethrionomys glareolus and Apodemus sp., was little consumed. E. multilocularis prevalence in foxes was determined from carcasses and reached 53% (95% confidence interval 45–61%). Intensity of infection varied from 2 to 73,380 worms per fox, with 72% of the sampled worm burden harboured by 8% of the sampled foxes. The selected explanatory variables (sex, year, age class, health and nutritional condition, and season) failed to predict prevalence rate and worm burden. The high prevalence rate in foxes indicates the possibility of intense E. multilocularis transmission, apart from periods, or in landscapes, favourable to large population outbreaks of grassland rodents.

A stochastic model of Echinococcus multilocularis transmission in Hokkaido, Japan, focusing on the infection process

Echinococcus multilocularis causes human alveolar echinococcus. In Japan, high prevalence of E. multilocularis among the fox population has been reported throughout Hokkaido. Accordingly, control measures, such as fox hunting and the distribution of bait containing Praziquantel, have been conducted. This study developed a transmission model for individuals in the fox population and included a stochastic infection process to assess the prevalence of E. multilocularis. To make our model realistic, we used the worm burden for each individual in the fox population. We assumed that the worm burden depends on the number of protoscoleces in a predated vole and the number of infection experiences. We carried out stochastic simulations with 1,000 trials for the situations of Koshimizu and Sapporo, Hokkaido, Japan. The distribution of the worm burden among foxes obtained using the model agreed with dissection data. The simulation indicates that a careful choice of season is necessary for an effective distribution of Praziquantel-containing bait. A stochastic model for E. multilocularis, which can assess the range of the prevalence in the fox population, would be helpful in analyzing their complex life-cycle and also in designing control strategies.

Research on targeting sources of alveolar echinococcosis in Japan

Echinococcus multilocularis is a fatal zoonotic parasite in the Northern Hemisphere. Recently, it has become endemic in many countries in Asia, especially in the northern island of Hokkaido in Japan. The increasing threat of public health due to alveolar echinococcosis has compelled researches for sensitive diagnosis and effective control. This paper reviews on the epidemiology, diagnosis and control of echinococcosis specifically in Japan. International collaborative responses by researchers and government initiatives such as mandatory reporting system for veterinarians who diagnose echinococcosis in dogs are presented. Successful control measures in Japan using anthelmintic fortified baits for foxes are described. Assessment of prevalence rates during control campaigns is analyzed favoring the use of intravital diagnosis rather than the traditional necropsy method from hunting or trapping activities of wild foxes. The novel concept of "endogenous development" by local resident volunteers towards sustainable control of echinococcosis is stressed.

Ruminating on complexity: macroparasites of wildlife and livestock

Recent advances in ecology have improved our understanding of the role of parasites in the dynamics of wildlife populations. However, conditions that prevail in many wildlife systems, such as host movement, contact with livestock, and heterogeneity in the environment of the parasite outside of the host, have largely been ignored in existing models of macroparasite transmission. We need to refine these models if we are to stand a chance of developing effective parasite control strategies. New quantitative approaches enable us to address key complexities and make better use of scarce data, and these should enhance our efforts to understand and control emerging problems of interspecific parasite transmission.

Canid immunity to Echinococcus spp.: impact on transmission

The canid intestinal immune system recognizes the cestode parasite Echinococcus following infection, but it is still unclear the extent to which protection against reinfection occurs. An increasing number of studies have shown that there are convex age-related abundance or prevalence rates in naturally infected populations of dogs with E. granulosus and foxes infected with E. multilocularis. Data from dogs naturally infected with E. granulosus give a better fit to a model that incorporates herd immunity compared to competing models of age-related changes in infection pressure. A theoretical framework suggests that such herd immunity will act as a stabilizing force for Echinococcus populations. Hence a decrease in infection pressure to dogs or foxes will not result in a corresponding decrease in infection pressure to intermediate hosts or to humans. In addition the age structure of the canid population could affect transmission.

Implications of increased susceptibility to predation for managing the sylvatic cycle of Echinococcus multilocularis

The ability to increase the chances that infectious prey are taken by predators is an observed feature of many parasites that rely on one or more predator-prey relationships to complete their life-cycle. In the sylvatic life-cycle of Echinococcus multilocularis - the causative agent of human alveolar echinococcosis-- foxes are the final host, with voles acting as intermediate hosts. Here we review the evidence that E. multilocularis causes increased susceptibility to predation and present a general mathematical model for the sylvatic life-cycle. The ability to increase susceptibility to predation in the intermediate host reduces the sensitivity of the parasite population to adverse conditions. For example, there is no critical density of foxes below which the parasite is expected to die out, even if the effect of the parasite on infected prey is very small. We suggest that increased susceptibility to predation is a plausible explanation for the observed resilience of E. multilocularis during and following field trials of praziquantel baiting.

Mathematical modeling of Echinococcus multilocularis transmission

A mathematical model for the transmission cycle of Echinococcus multilocularis would be useful for estimating its prevalence, and the model simulation can be instrumental in designing various control strategies. This review focuses on the epidemiological factors in the E. multilocularis transmission cycle and the recent advances of mathematical models for E. multilocularis transmission.

Transmission dynamics of Echinococcus multilocularis; its reproduction number, persistence in an area of low rodent prevalence, and effectiveness of control

On the basis of high prevalences of Echinococcus multilocularis in the growing fox populations in Central Europe, its total biomass may have increased significantly in the past 20 years. E. multilocularis is now also found in areas outside the known endemic area in Central Europe. Therefore, E. multilocularis, the causative agent of a serious parasitic zoonosis, might be of major concern for public health and a challenge to control. Some experimental field trials to control E. multilocularis using an anti-worm drug reduced parasite burden in a contaminated region during the control campaign, but failed to eradicate the parasite completely. It was our aim to develop a mathematical model describing the biomass of egg, larval, and adult worm stages of the E. multilocularis life-cycle, and simulate a hypothetical control campaign. Additionally, we derived the reproduction number of this parasite and explored conditions for the persistence of the parasite's life-cycle. Our model shows that while control campaigns rapidly reduce the worm burden in the definitive host, and consequently eggs in the environment, the pool of larvae in the intermediate host remains large. The parasite's life-cycle persists in a region where prevalence in the intermediate host is low (∼1%). Therefore, we conclude that the parasite is likely to re-emerge if control is discontinued on the basis of reduced worm population. Continued treatment of the definitive host is required to eradicate the larval stage of the parasite from the intermediate host population.

Emergency vaccination of rabies under limited resources -- combating or containing?

Background

Rabies is the most important viral zoonosis from a global perspective. Worldwide efforts to combat the disease by oral vaccination of reservoirs have managed to eradicate wildlife rabies in large areas of central Europe and North-America. Thus, repeated vaccination has been discontinued recently on a geographical scale. However, as rabies has not yet been eradicated globally, a serious risk of re-introduction remains. What is the best spatial design for an emergency vaccination program – particularly if resources are limited? Either, we treat a circular area around the detected case and run the risk of infected hosts leaving the limited control area, because a sufficient immunisation level has not yet been built up. Or, initially concentrate the SAME resources in order to establish a protective ring which is more distant from the infected local area, and which then holds out against the challenge of the approaching epidemic.

Methods

We developed a simulation model to contrast the two strategies for emergency vaccination. The spatial-explicit model is based on fox group home-ranges, which facilitates the simulation of rabies spread to larger areas relevant to management. We used individual-based fox groups to follow up the effects of vaccination in a detailed manner. Thus, regionally – bait distribution orientates itself to standard schemes of oral immunisation programs and locally – baits are assigned to individual foxes.

Results

Surprisingly, putting the controlled area ring-like around the outbreak does not outperform the circular area of the same size centred on the outbreak. Only during the very first baitings, does the ring area result in fewer breakouts. But then as rabies is eliminated within the circle area, the respective ring area fails, due to the non-controlled inner part.

We attempt to take advantage of the initially fewer breakouts beyond the ring when applying a mixed strategy. Therefore, after a certain number of baitings, the area under control was increased for both strategies towards the same larger circular area. The circle-circle strategy still outperforms the ring-circle strategy and analysis of the spatial-temporal disease spread reveals why: improving control efficacy by means of a mixed strategy is impossible in the field, due to the build-up time of population immunity.

Conclusion

For practical emergency management of a new outbreak of rabies, the ring-like application of oral vaccination is not a favourable strategy at all. Even if initial resources are substantially low and there is a serious risk of rabies cases outside the limited control area, our results suggest circular application instead of ring vaccination.

Modeling the transmission of Echinococcus granulosus and Echinococcus multilocularis in dogs for a high endemic region of the Tibetan plateau

Echinococcus granulosus and Echinococcus multilocularis abundance and prevalence data, for domestic dogs of Shiqu County, Sichuan Province, People's Republic of China, were fitted to mathematical models to evaluate transmission parameters. Abundance models, assuming the presence and absence of immunity, were fit for both E. granulosus and E. multilocularis using Bayesian priors, maximum likelihood, and Monte Carlo sampling techniques. When the models were compared, using the likelihood ratio test for nested models, the model assuming the presence of immunity was the best fit for E. granulosus infection, with a purgation based prevalence of 8% (true prevalence interval of 8-19% based on the sensitivity of purgation) and a mean abundance of 80 parasites per dog, with an average infection pressure of 560 parasites per year. In contrast, the model assuming the absence of immunity was the best fit for E. multilocularis infection, with a purgation based prevalence of 12% (true prevalence interval of 13-33% based on the sensitivity of purgation) and a mean abundance of 131 parasites per dog, with an average infection pressure of 334 or 533 parasites per year assuming a 5 or 3 month parasite life expectancy, respectively. The prevalence data for both parasites was then fit to a set of differential equations modeling the transition between infection states in order to determine number of infectious insults per year. Infection pressure was 0.21, with a 95% credibility interval of 0.12 to 0.41, infections per year for E. granulosus and 0.52, with a 95% credibility interval of 0.29-0.77, infections per year for E. multilocularis assuming a 5 month parasite lifespan or 0.85, with a 95% credibility interval of 0.47-1.25 infections per year, assuming a 3 month E. multilocularis lifespan in dogs.

Modelling the spatial distribution of Echinococcus multilocularis infection in foxes

Alveolar echinococcosis is a rare but fatal disease in humans and is caused by the fox tapeworm Echinococcus multilocularis. The densities of fox and grassland rodent populations and the interactions between them influence E. multilocularis transmission rates in Europe. Successful rabies control has caused fox populations and E. multilocularis prevalence rates to increase in many European countries. The potential increase of the infection pressure on the human population motivates the monitoring of the infection status of foxes over space and time. Detection of E. multilocularis antigen levels in fox faecal samples collected in the field might provide a pragmatic methodology for epidemiological surveillance of the infection status in wildlife hosts across large areas, as well as providing an indication of the spatial distribution of infected faeces contaminating the environment. In this paper, a spatial analysis of antigen levels detected in faeces collected in the Franche-Comté region of eastern France is presented. In Franche-Comté, rodent outbreaks have been observed to originate in areas rich in grassland. Spatial trends in fox infection levels were modelled here as a function of the composition ratio of grassland in the landscape derived from the CORINE land-cover map. Kriging models incorporating the grassland trend term were compared to a variety of models in which five alternative trend expressions were used: the alternative trend expressions included linear and quadratic polynomials on the x and y coordinates with and without a grassland term, and a constant mean model. Leave-one-out cross-validation indicated that the estimation errors of kriging with a trend models were significantly lower when the trend expression contained the grassland index term only. The relationship between observed and predicted antigen levels was strongest when the estimated range of autocorrelation was within the home range size of a single fox. The over-dispersion of E. multilocularis in foxes may therefore account for the majority of spatial autocorrelation locally, while regional trends can be successfully modelled as a function of habitat availability for intermediate hosts.

Fenced pasture: a possible risk factor for human alveolar echinococcosis in Tibetan pastoralist communities of Sichuan, China

Alveolar echinococcosis, infection caused by the parasitic helminth Echinococcus multilocularis, is a zoonosis strongly linked to climatic and ecological factors. Cross-sectional survey data were used to test a hypothesis that partial fencing of pastures could promote alveolar echinococcosis transmission in semi-nomadic pastoral communities of the Tibetan plateau, PR China. Using multiple stepwise logistic regression with consideration of factors of age and gender, it was shown that partial fencing around the settlements in winter pasture was significantly and independently associated with the risk of human alveolar echinococcosis in the surveyed villages (P = 0.021). The underlying reason may lie in overgrazing, an assumed cause of population outbreaks of small mammal intermediate hosts of the parasite on the Tibetan plateau. Overgrazing may have been exacerbated by the reduction of communal pastures nearby the settlements due to introduction of partial fencing around group tenure pastures acquired by Tibetan pastoralist families.

Assessing anti-rabies baiting--what happens on the ground?

BACKGROUND

Rabies is one of the most hazardous zoonoses in the world. Oral mass vaccination has developed into the most effective management method to control fox rabies. The future need to control the disease in large countries (i.e. Eastern Europe and the Americas) forces cost-benefit discussions. The 'Increase bait density' option refers to the usual management assumption that more baits per km2 could compensate for high fox abundance and override the imperfect supply of bait pieces to the individual fox.

METHODS

We use a spatial simulation, which combines explicitly fox space use (tessellation polygons) and aeroplane flight lines (straight lines). The number of baits actually falling into each polygon is measured. The manager's strategic options are converted into changes of the resulting bait distribution on the ground. The comparison enables the rating of the options with respect to the management aim (i.e. accessibility of baits).

RESULTS

Above 5% (approx. 10%) of all fox groups without any bait (at most 5 baits) relate to the baiting strategy applied in the field (1 km spaced parallel flight lines, 20 baits per km2 distributed) under habitat conditions comparable to middle and western Europe (fox group home-range 1 km2, 2.5 adults; reference strategy). Increasing the bait density on the same flight-line pattern neither reduces the number of under-baited fox group home-ranges, nor improves the management outcome and hence wastes resources. However, reducing the flight line distance provides a more even bait distribution and thus compensates for missed fox groups or extra high fox density.The reference strategy's bait density can be reduced when accounting for the missed fox groups. The management result with the proper strategy is likely the same but with reduced costs.

CONCLUSION

There is no overall optimal strategy for the bait distribution in large areas. For major parts of the landscape, the reference strategy will be more competitive. In situations where set backs are attributed to non-homogeneous bait accessibility the distribution scheme has to be refined zone-based (i.e. increase of the flight line length per unit area). However, increase in bait density above the reference strategy appears inappropriate at least for non-urban abundance conditions of the red fox.