Navigating parasite webs and parasite flow: emerging and re-emerging parasitic zoonoses of wildlife origin

Wildlife are now recognised as an important source of emerging human pathogens, including parasites. This paper discusses the linkages between wildlife, people, zoonotic parasites and the ecosystems in which they co-exist, revisits definitions for 'emerging' and 're-emerging', and lists zoonotic parasites that can be acquired from wildlife including, for some, estimates of the associated global human health burdens. The paper also introduces the concepts of 'parasite webs' and 'parasite flow', provides a context for parasites, relative to other infectious agents, as causes of emerging human disease, and discusses drivers of disease emergence and re-emergence, especially changes in biodiversity and climate. Angiostrongylus cantonensis in the Caribbean and the southern United States, Baylisascaris procyonis in California and Georgia, Plasmodium knowlesi in Sarawak, Malaysia, Human African Trypanosomiasis, Sarcoptes scabiei in carnivores, and Cryptosporidium, Giardia and Toxoplasma in marine ecosystems are presented as examples of wildlife-derived zoonotic parasites of particular recent interest. An ecological approach to disease is promoted, as is a need for an increased profile for this approach in undergraduate and graduate education in the health sciences. Synergy among scientists and disciplines is identified as critical for the study of parasites and parasitic disease in wildlife populations. Recent advances in techniques for the investigation of parasite fauna of wildlife are presented and monitoring and surveillance systems for wildlife disease are discussed. Some of the limitations inherent in predictions for the emergence and re-emergence of infection and disease associated with zoonotic parasites of wildlife are identified. The importance of public awareness and public education in the prevention and control of emerging and re-emerging zoonotic infection and disease are emphasised. Finally, some thoughts for the future are presented.     

Arctic parasitology: why should we care?

The significant impact on human and animal health from parasitic infections in tropical regions is well known, but parasites of medical and veterinary importance are also found in the Arctic. Subsistence hunting and inadequate food inspection can expose people of the Arctic to foodborne parasites. Parasitic infections can influence the health of wildlife populations and thereby food security. The low ecological diversity that characterizes the Arctic imparts vulnerability. In addition, parasitic invasions and altered transmission of endemic parasites are evident and anticipated to continue under current climate changes, manifesting as pathogen range expansion, host switching, and/or disease emergence or reduction. However, Arctic ecosystems can provide useful models for understanding climate-induced shifts in host-parasite ecology in other regions.     

Focus: Zoonotic Disease: An Ecologically Framed Comparison of The Potential for Zoonotic Transmission of Non-Human and Human-Infecting Species of Malaria Parasite

The threats, both real and perceived, surrounding the development of new and emerging infectious diseases of humans are of critical concern to public health and well-being. Among these risks is the potential for zoonotic transmission to humans of species of the malaria parasite, Plasmodium, that have been considered historically to infect exclusively non-human hosts. Recently observed shifts in the mode, transmission, and presentation of malaria among several species studied are evidenced by shared vectors, atypical symptoms, and novel host-seeking behavior. Collectively, these changes indicate the presence of environmental and ecological pressures that are likely to influence the dynamics of these parasite life cycles and physiological make-up. These may be further affected and amplified by such factors as increased urban development and accelerated rate of climate change. In particular, the extended host-seeking behavior of what were once considered non-human malaria species indicates the specialist niche of human malaria parasites is not a limiting factor that drives the success of blood-borne parasites. While zoonotic transmission of non-human malaria parasites is generally considered to not be possible for the vast majority of Plasmodium species, failure to consider the feasibility of its occurrence may lead to the emergence of a potentially life-threatening blood-borne disease of humans. Here, we argue that recent trends in behavior among what were hitherto considered to be non-human malaria parasites to infect humans call for a cross-disciplinary, ecologically-focused approach to understanding the complexities of the vertebrate host/mosquito vector/malaria parasite triangular relationship. This highlights a pressing need to conduct a multi-species investigation for which we recommend the construction of a database to determine ecological differences among all known Plasmodium species, vectors, and hosts. Closing this knowledge gap may help to inform alternative means of malaria prevention and control.     

Zoonoses

The numbers of microbial species that can infect human beings are shown to be 1415, of which 868 species (61%) are zoonotic. Since most of the emerging pathogens (75%) are originated from other animals, public health sectors should be vigilant against the emergence of new zoonotic diseases. Only 33% of zoonoses can spread from human to human after introduction into human population. Various factors such as human demography, ecological change, global transportation and climate change are responsible for the emergence of zoonoses. Even a slight change in the ecological niche where pathogenic organisms thrive would result in the increase of the incidence of the disease.     

Projected climate and land use change alter western blacklegged tick phenology,seasonal host‐seeking suitability and human encounter risk in California

Global environmental change is having profound effects on the ecology of infectious disease systems, which are widely anticipated to become more pronounced under future climate and land use change. Arthropod vectors of disease are particularly sensitive to changes in abiotic conditions such as temperature and moisture availability. Recent research has focused on shifting environmental suitability for, and geographic distribution of, vector species under projected climate change scenarios. However, shifts in seasonal activity patterns, or phenology, may also have dramatic consequences for human exposure risk, local vector abundance and pathogen transmission dynamics. Moreover, changes in land use are likely to alter human–vector contact rates in ways that models of changing climate suitability are unlikely to capture. Here we used climate and land use projections for California coupled with seasonal species distribution models to explore the response of the western blacklegged tick (Ixodes pacificus), the primary Lyme disease vector in western North America, to projected climate and land use change. Specifically, we investigated how environmental suitability for tick host‐seeking changes seasonally, how the magnitude and direction of changing seasonal suitability differs regionally across California, and how land use change shifts human tick‐encounter risk across the state. We found vector responses to changing climate and land use vary regionally within California under different future scenarios. Under a hotter, drier scenario and more extreme land use change, the duration and extent of seasonal host‐seeking activity increases in northern California, but declines in the south. In contrast, under a hotter, wetter scenario seasonal host‐seeking declines in northern California, but increases in the south. Notably, regardless of future scenario, projected increases in developed land adjacent to current human population centers substantially increase potential human–vector encounter risk across the state. These results highlight regional variability and potential nonlinearity in the response of disease vectors to environmental change.     

Tracking host sources of Cryptosporidium spp. in raw water for improved health risk assessment

Recent molecular evidence suggests that different species and/or genotypes of Cryptosporidium display strong host specificity, altering our perceptions regarding the zoonotic potential of this parasite. Molecular forensic profiling of the small-subunit rRNA gene from oocysts enumerated on microscope slides by U.S. Environmental Protection Agency method 1623 was used to identify the range and prevalence of Cryptosporidium species and genotypes in the South Nation watershed in Ontario, Canada. Fourteen sites within the watershed were monitored weekly for 10 weeks to assess the occurrence, molecular composition, and host sources of Cryptosporidium parasites impacting water within the region. Cryptosporidium andersoni, Cryptosporidium muskrat genotype II, Cryptosporidium cervine genotype, C. baileyi, C. parvum, Cryptosporidium muskrat genotype I, the Cryptosporidium fox genotype, genotype W1, and genotype W12 were detected in the watershed. The molecular composition of the Cryptosporidium parasites, supported by general land use analysis, indicated that mature cattle were likely the main source of contamination of the watershed. Deer, muskrats, voles, birds, and other wildlife species, in addition to sewage (human or agricultural) may also potentially impact water quality within the study area. Source water protection studies that use land use analysis with molecular genotyping of Cryptosporidium parasites may provide a more robust source-tracking tool to characterize fecal impacts in a watershed. Moreover, the information is vital for assessing environmental and human health risks posed by water contaminated with zoonotic and/or anthroponotic forms of Cryptosporidium.     

Species loss on spatial patterns and composition of zoonotic parasites

Species loss can result in the subsequent loss of affiliate species. Though largely ignored to date, these coextinctions can pose threats to human health by altering the composition, quantity and distribution of zoonotic parasites. We simulated host extinctions from more than 1300 host–parasite associations for 29 North American carnivores to investigate changes in parasite composition and species richness. We also explored the geography of zoonotic parasite richness under three carnivore composition scenarios and examined corresponding levels of human exposure. We found that changes in parasite assemblages differed among parasite groups. Because viruses tend to be generalists, the proportion of parasites that are viruses increased as more carnivores went extinct. Coextinction of carnivore parasites is unlikely to be common, given that few specialist parasites exploit hosts of conservation concern. However, local extirpations of widespread carnivore hosts can reduce overall zoonotic richness and shift distributions of parasite-rich areas. How biodiversity influences disease risks remains the subject of debate. Our results make clear that hosts vary in their contribution to human health risks. As a consequence, so too does the loss (or gain) of particular hosts. Anticipating changes in host composition in future environments may help inform parasite conservation and disease mitigation efforts.     

Public Health Risk Assessment Linked to Climatic and Ecological Change

Disturbances of climatic and ecological systems can present risks to human health, which are becoming more evident from health studies linked to climate variability, landuse change and global climate change. Waterborne disease agents, such as Giardia cysts and Cryposporidium oocysts have been positively correlated with rainfall. El Niño-related extreme weather conditions can have a significant impact on vector- and water-borne diseases. The linkages between weather, terrestrial ecology and human health have been discovered for some diseases, such as rodent-borne hantavirus. Marine ecology also plays a role in determining human health risks, such as from cholera, and other enteric pathogens. Deforestation and ensuing changes in landuse, human settlement, commercial development, road construction, and water control systems singly, and in combination have been accompanied by increases in or emergence of diseases like malaria and schistosomiasis in some regions of the world. Long-term climate change may increase the frequency of heat waves and potentially air pollution episodes, increase the number of extreme weather events, cause coastal flooding and salination of fresh water aquifers, and displace coastal settlements. Ultimately, a two-pronged approach (empirical and modeling studies) is required to better understand these linkages between climato-logical and ecological change as determinants of disease.     

The vulnerability of animal and human health to parasites under global change

The term 'global change' is used to encompass all of the significant drivers of environmental change as experienced by hosts, parasites and parasite managers. The term includes changes in climate and climate variability, atmospheric composition, land use and land cover including deforestation and urbanisation, bio-geochemistry, globalisation of trade and transport, the spread of alien species, human health and technology. A subset of land use issues relates to the management of protective technologies in relation to residues in food and the environment and the emergence of resistance. Another is the question of changing biodiversity of both parasites and their associated natural enemies, and the effects on the host--parasite relationship and on parasite management. A framework for studying impacts of global change is proposed and illustrated with field data, and CLIMEX and simulation modelling of the cattle tick Boophilus microplus in Australia. Parasitology suffers from the perception that the key impacts of global change will be driven by changes at lower trophic levels, with parasitic interactions being treated as secondary effects. This is incorrect because the environment mediates host-parasite interactions as much as it affects parasites directly. Parasitologists need to strive for holistic solutions to the management of animal and human health, within a wider context of overall management of those systems, if they are to make a meaningful contribution to global efforts aimed at coping with global change.     

Experimental warming drives a seasonal shift in the timing of host‐parasite dynamics with consequences for disease risk

Multi‐species experiments are critical for identifying the mechanisms through which climate change influences population dynamics and community interactions within ecological systems, including infectious diseases. Using a host–parasite system involving freshwater snails, amphibians and trematode parasites, we conducted a year‐long, outdoor experiment to evaluate how warming affected net parasite production, the timing of infection and the resultant pathology. Warming of 3 °C caused snail intermediate hosts to release parasites 9 months earlier and increased infected snail mortality by fourfold, leading to decreased overlap between amphibians and parasites. As a result, warming halved amphibian infection loads and reduced pathology by 67%, despite comparable total parasite production across temperature treatments. These results demonstrate that climate–disease theory should be expanded to account for predicted changes in host and parasite phenology, which may often be more important than changes in total parasite output for predicting climate‐driven changes in disease risk.     

Cuban parasitology in review: a revolutionary triumph

Although the population of Hispaniola, Cuba's most similar neighbour in the Caribbean, continues to be threatened by parasitic diseases (including malaria), many tropical parasitic infections in Cuba have been eliminated or controlled. However, some parasitic infections remain important in the Cuban population, and the occurrence of vectors and the high possibility of introduction of parasites mean that Cuban diagnosticians must remain alert. Some key aspects of human parasitology in Cuba are reviewed here, including historical information, comparative data from Hispaniola and Jamaica, and how Cuba strives to maintain and improve its control against parasitic infections. Data from recent key novel parasitology research conducted in Cuba are also described.     

基于土地利用变化的四川省凉山州移民生态效应研究

土地利用变化是自然演变与人类活动共同作用的结果,大量的人口流动一定程度上将影响土地利用方式,进而对区域生态环境质量产生影响作用。因此,研究人口流动对土地利用变化的影响及生态效应,能为合理调控区域人地关系,促进区域生态环境与经济的平衡发展提供理论支持。基于长时序高空间分辨率土地利用数据,以具有典型移民现象的四川省凉山州为研究区,针对移民驱动下的土地利用时空变化特征及其诱发的生态效应进行研究分析。结果表明:(1)2000-2020年间,受人口迁移影响,凉山州土地资源的利用结构和程度都发生了相应改变,以耕地、草地、林地及不透水面的面积变化最为显著。在人口迁入较多的西昌、冕宁和德昌等县市内,供人类生活、生存所需要的耕地和建筑用地的数量显著增加;相反,对于大量人口迁出的喜德、昭觉和美姑等地区,人类活动强度的降低为生态用地的恢复和重建提供了条件,生态用地面积不断上涨。(2)受土地利用类型、地形条件、人口分布等因素的影响,凉山州整体生态环境质量呈西高东低、北高南低的空间分布格局。2000-2020年,整体生态环境质量仅下降了0.36%,总体较为稳定。(3)生态环境质量改善区域集中于东部的越西、美姑及昭觉等县市,而生态环境质量降低的区域主要位于盐源、会里、西昌和德昌等县市。生态环境质量的改变与区域移民活动存在紧密联系,其中由移民迁入引发的耕地和不透水面对生态用地的挤占,对生态环境质量退化贡献比重达87.97%,而随着移民迁出为生态工程的实施提供条件,凉山州内生态用地面积共增加775.84 km²。     

流域生态系统服务簇变化及影响因素——以大清河流域为例

生态系统服务簇是多种生态系统服务的组合,是生态系统多功能性中的主导功能表征。识别生态系统服务簇的空间和功能变化及影响因素,可为基于主导功能实施分区管理策略、统筹土地多功能性,进而为整体提升区域生态系统服务提供参考。本研究以大清河流域为例,选取6种生态系统服务（水资源供给服务、粮食供给服务、水源涵养服务、水质净化服务、土壤保持服务、固碳服务）,在乡镇尺度上探讨了生态系统服务簇的变化特征,研究了影响其变化的主要因素。结果表明：（1）依据主导的生态系统服务类型,大清流域可以分为3个生态系统服务簇,生态调节服务簇（B1）,农产品供给服务簇（B2）和人居环境簇（B3）;（2）2000-2015年,生态调节服务簇的空间格局变幅最大,乡镇变化率为19.6%,而农产品供给服务簇和人居环境簇的空间稳定性较强,变化率小于5.0%;生态系统服务簇类型发生转化的区域主要位于服务簇交界处;（3）尽管服务簇在空间上有变化,但生态调节服务簇、农产品供给服务簇和人居环境簇的生态系统服务呈增长趋势,平均增幅高达8.62%;（4）自然本底条件和生态保护政策是驱动生态调节服务簇变化的主要因素,农产品供给和建设用地面积的变化分别是驱动农产品供给服务簇和人居环境簇时空变化的主要因素。该案例研究结果为管理土地多功能属性、整体提升国土空间功能提供了有效的途径。     

Human impacts decouple a fundamental ecological relationship—The positive association between host diversity and parasite diversity

Human impacts on ecosystems can decouple the fundamental ecological relationships that create patterns of diversity in free‐living species. Despite the abundance, ubiquity, and ecological importance of parasites, it is unknown whether the same decoupling effects occur for parasitic species. We investigated the influence of fishing on the relationship between host diversity and parasite diversity for parasites of coral reef fishes on three fished and three unfished islands in the central equatorial Pacific. Fishing was associated with a shallowing of the positive host‐diversity–parasite‐diversity relationship. This occurred primarily through negative impacts of fishing on the presence of complex life‐cycle parasites, which created a biologically impoverished parasite fauna of directly transmitted parasites resilient to changes in host biodiversity. Parasite diversity appears to be decoupled from host diversity by fishing impacts in this coral reef ecosystem, which suggests that such decoupling might also occur for parasites in other ecosystems affected by environmental change.     

Review of Climate,Landscape, and Viral Genetics as Drivers of the Japanese Encephalitis Virus Ecology

The Japanese encephalitis virus (JEV), an arthropod-born Flavivirus, is the major cause of viral encephalitis, responsible for 10,000–15,000 deaths each year, yet is a neglected tropical disease. Since the JEV distribution area has been large and continuously extending toward new Asian and Australasian regions, it is considered an emerging and reemerging pathogen. Despite large effective immunization campaigns, Japanese encephalitis remains a disease of global health concern. JEV zoonotic transmission cycles may be either wild or domestic: the first involves wading birds as wild amplifying hosts; the second involves pigs as the main domestic amplifying hosts. Culex mosquito species, especially Cx. tritaeniorhynchus, are the main competent vectors. Although five JEV genotypes circulate, neither clear-cut genotype-phenotype relationship nor clear variations in genotype fitness to hosts or vectors have been identified. Instead, the molecular epidemiology appears highly dependent on vectors, hosts'' biology, and on a set of environmental factors. At global scale, climate, land cover, and land use, otherwise strongly dependent on human activities, affect the abundance of JEV vectors, and of wild and domestic hosts. Chiefly, the increase of rice-cultivated surface, intensively used by wading birds, and of pig production in Asia has provided a high availability of resources to mosquito vectors, enhancing the JEV maintenance, amplification, and transmission. At fine scale, the characteristics (density, size, spatial arrangement) of three landscape elements (paddy fields, pig farms, human habitations) facilitate or impede movement of vectors, then determine how the JEV interacts with hosts and vectors and ultimately the infection risk to humans. If the JEV is introduced in a favorable landscape, either by live infected animals or by vectors, then the virus can emerge and become a major threat for human health. Multidisciplinary research is essential to shed light on the biological mechanisms involved in the emergence, spread, reemergence, and genotypic changes of JEV.     

Chagas disease in Mexico: an analysis of geographical distribution during the past 76 years--a review

Literature from 1928 through 2004 was compiled from different document sources published in Mexico or elsewhere. From these 907 publications, we found 19 different topics of Chagas disease study in Mexico. The publications were arranged by decade and also by state. This information was used to construct maps describing the distribution of Chagas disease according to different criteria: the disease, vectors, reservoirs, and strains. One of the major problems confronting study of this zoonotic disease is the great biodiversity of the vector species; there are 30 different species, with at least 10 playing a major role in human infection. The high variability of climates and biogeographic regions further complicate study and understanding of the dynamics of this disease in each region of the country. We used a desktop Genetic Algorithm for Rule-Set Prediction procedure to provide ecological models of organism niches, offering improved flexibility for choosing predictive environmental and ecological data. This approach may help to identify regions at risk of disease, plan vector-control programs, and explore parasitic reservoir association. With this collected information, we have constructed a data base: CHAGMEX, available online in html format.     

Analysis of landscape spatial pattern changes in urban fringe area: a case study of Hunhe Niaodao Area in Shenyang City

The urban fringe is areas where land use changes and develops rapidly, analyzing land use changes and driving forces in urban fringe area is of great significance for landscape pattern and ecological environment construction. This study takes the Hunhe Bird Island area of Shenyang City as the research object, uses Landsat remote sensing images from 2005 to 2019 as the basic data, and uses the land transfer matrix and landscape index method to analyze the overall landscape environment and six land types. Using government policies and statistical data to analyze the driving forces of landscape changes. The research results show that the landscape matrix had changed, the overall landscape structure had changed drastically, the ecological environment was unstable, and human interference factors had increased, and the fragmentation phenomenon had become more and more serious. Each land type has different environmental changes. Except for the relatively stable water environment, other land types have different degrees of fragmentation. The reason is that the driving force of landscape pattern changes mainly comes from the influence of human factors, which is closely related to population, economic development and government policies. Therefore, future development and protection strategies are proposed for each land type in the urban fringe area. For the overall development of urban fringe area, it is advocated to make full use of land resources, strengthen the correct judgment and forecast of urban population and economy, formulate the correct urban boundary strategy for sustainable development, avoid the ecological environment problems faced by excessive land development, and realize the “Rational expansion” of cities.

     

Emerging helminth zoonoses

As our ability to recognise and diagnose human disease caused by helminth parasites has improved, so our understanding of the epidemiology and clinical manifestations of these diseases has improved. Humans can develop patent infection with a wide range of helminth parasites, whose natural host is another vertebrate. Rather than focusing on a comprehensive review of zoonotic helminth infections, this review describes in detail examples of zoonotic helminth infections that have newly appeared in human populations, or have existed but are increasing in incidence or geographic range. Examples include intestinal capillariasis, anisakidosis, eosinophilic enteritis, oesophagostomiasis and gnathostomiasis. Potential reasons for the emergence of these infections, including changes in social, dietary or cultural mores, environmental changes, and the improved recognition of heretofore neglected infections often coupled with an improved ability to diagnose infection are discussed.     

Tracking Host Sources of Cryptosporidium spp. in Raw Water for Improved Health Risk Assessment

Recent molecular evidence suggests that different species and/or genotypes of Cryptosporidium display strong host specificity, altering our perceptions regarding the zoonotic potential of this parasite. Molecular forensic profiling of the small-subunit rRNA gene from oocysts enumerated on microscope slides by U.S. Environmental Protection Agency method 1623 was used to identify the range and prevalence of Cryptosporidium species and genotypes in the South Nation watershed in Ontario, Canada. Fourteen sites within the watershed were monitored weekly for 10 weeks to assess the occurrence, molecular composition, and host sources of Cryptosporidium parasites impacting water within the region. Cryptosporidium andersoni, Cryptosporidium muskrat genotype II, Cryptosporidium cervine genotype, C. baileyi, C. parvum, Cryptosporidium muskrat genotype I, the Cryptosporidium fox genotype, genotype W1, and genotype W12 were detected in the watershed. The molecular composition of the Cryptosporidium parasites, supported by general land use analysis, indicated that mature cattle were likely the main source of contamination of the watershed. Deer, muskrats, voles, birds, and other wildlife species, in addition to sewage (human or agricultural) may also potentially impact water quality within the study area. Source water protection studies that use land use analysis with molecular genotyping of Cryptosporidium parasites may provide a more robust source-tracking tool to characterize fecal impacts in a watershed. Moreover, the information is vital for assessing environmental and human health risks posed by water contaminated with zoonotic and/or anthroponotic forms of Cryptosporidium.     

旋毛虫感染与肠黏膜免疫应答作用的研究进展

旋毛虫病是一种常见的人兽共患寄生虫病,也是一种重要的食源性寄生虫病,严重危害着人类的健康。肠黏膜是肠道寄生虫（包括旋毛虫等）进入宿主的重要门户,即机体非特异性抗感染的第一道防线,也是宿主抵御肠道寄生虫入侵的重要固有屏障,后者发挥着固有性免疫和适应性免疫功能的作用。宿主的肠黏膜免疫应答反应决定旋毛虫与宿主相互作用和适应关系。本研究就目前国内外学者研究旋毛虫感染与宿主免疫的现状,分别从肠道黏膜组织学结构、免疫细胞、细胞因子和小肠上皮细胞4个方面,综述一下肠黏膜对旋毛虫感染的免疫应答作用,目的在于揭示宿主肠黏膜对旋毛虫感染的免疫应答机制。