Climate warming and disease risks for terrestrial and marine biota

Emerging Vibrio species: an unending threat to public health in developing countries

Discharge of inadequately treated sewage effluents into the environment in developing countries has increased over the years, leading to deterioration of water quality of major watersheds in developing nations and consequently an increased incidence of emerging pathogens such as Vibrio species, the prevalence of which has been generally underestimated in developing nations. This review underscores the need for a proactive approach to risk factors for emerging Vibrio infections, so as to establish adequate prevention measures.

Role of microorganisms in the evolution of animals and plants: the hologenome theory of evolution

We present here the hologenome theory of evolution, which considers the holobiont (the animal or plant with all of its associated microorganisms) as a unit of selection in evolution. The hologenome is defined as the sum of the genetic information of the host and its microbiota. The theory is based on four generalizations: (1) All animals and plants establish symbiotic relationships with microorganisms. (2) Symbiotic microorganisms are transmitted between generations. (3) The association between host and symbionts affects the fitness of the holobiont within its environment. (4) Variation in the hologenome can be brought about by changes in either the host or the microbiota genomes; under environmental stress, the symbiotic microbial community can change rapidly. These points taken together suggest that the genetic wealth of diverse microbial symbionts can play an important role both in adaptation and in evolution of higher organisms. During periods of rapid changes in the environment, the diverse microbial symbiont community can aid the holobiont in surviving, multiplying and buying the time necessary for the host genome to evolve. The distinguishing feature of the hologenome theory is that it considers all of the diverse microbiota associated with the animal or the plant as part of the evolving holobiont. Thus, the hologenome theory fits within the framework of the 'superorganism' proposed by Wilson and Sober.

Climate extremes promote fatal co-infections during canine distemper epidemics in African lions

Extreme climatic conditions may alter historic host-pathogen relationships and synchronize the temporal and spatial convergence of multiple infectious agents, triggering epidemics with far greater mortality than those due to single pathogens. Here we present the first data to clearly illustrate how climate extremes can promote a complex interplay between epidemic and endemic pathogens that are normally tolerated in isolation, but with co-infection, result in catastrophic mortality. A 1994 canine distemper virus (CDV) epidemic in Serengeti lions (Panthera leo) coincided with the death of a third of the population, and a second high-mortality CDV epidemic struck the nearby Ngorongoro Crater lion population in 2001. The extent of adult mortalities was unusual for CDV and prompted an investigation into contributing factors. Serological analyses indicated that at least five “silent” CDV epidemics swept through the same two lion populations between 1976 and 2006 without clinical signs or measurable mortality, indicating that CDV was not necessarily fatal. Clinical and pathology findings suggested that hemoparsitism was a major contributing factor during fatal epidemics. Using quantitative real-time PCR, we measured the magnitude of hemoparasite infections in these populations over 22 years and demonstrated significantly higher levels of Babesia during the 1994 and 2001 epidemics. Babesia levels correlated with mortalities and extent of CDV exposure within prides. The common event preceding the two high mortality CDV outbreaks was extreme drought conditions with wide-spread herbivore die-offs, most notably of Cape buffalo (Syncerus caffer). As a consequence of high tick numbers after the resumption of rains and heavy tick infestations of starving buffalo, the lions were infected by unusually high numbers of Babesia, infections that were magnified by the immunosuppressive effects of coincident CDV, leading to unprecedented mortality. Such mass mortality events may become increasingly common if climate extremes disrupt historic stable relationships between co-existing pathogens and their susceptible hosts.

Coral pathogens identified for White Syndrome (WS) epizootics in the Indo-Pacific

BACKGROUND

White Syndrome (WS), a general term for scleractinian coral diseases with acute signs of advancing tissue lesions often resulting in total colony mortality, has been reported from numerous locations throughout the Indo-Pacific, constituting a growing threat to coral reef ecosystems.

METHODOLOGY/PRINCIPAL FINDINGS

Bacterial isolates were obtained from corals displaying disease signs at three ws outbreak sites: Nikko Bay in the Republic of Palau, Nelly Bay in the central Great Barrier Reef (GBR) and Majuro Atoll in the Republic of the Marshall Islands, and used in laboratory-based infection trials to satisfy Henle-Koch's postulates, Evan's rules and Hill's criteria for establishing causality. Infected colonies produced similar signs to those observed in the field following exposure to bacterial concentrations of 1x10(6) cells ml(-1). Phylogenetic 16S rRNA gene analysis demonstrated that all six pathogens identified in this study were members of the gamma-Proteobacteria family Vibrionacae, each with greater than 98% sequence identity with the previously characterized coral bleaching pathogen Vibrio coralliilyticus. Screening for proteolytic activity of more than 150 coral derived bacterial isolates by a biochemical assay and specific primers for a Vibrio family zinc-metalloprotease demonstrated a significant association between the presence of isolates capable of proteolytic activity and observed disease signs.

CONCLUSION/SIGNIFICANCE

This is the first study to provide evidence for the involvement of a unique taxonomic group of bacterial pathogens in the aetiology of Indo-Pacific coral diseases affecting multiple coral species at multiple locations. Results from this study strongly suggest the need for further investigation of bacterial proteolytic enzymes as possible virulence factors involved in Vibrio associated acute coral infections.

An ancient and variable mannose-binding lectin from the coral Acropora millepora binds both pathogens and symbionts

Corals form the framework of the world's coral reefs and are under threat from increases in disease and bleaching (symbiotic dysfunction), yet the mechanisms of pathogen and symbiont recognition remain largely unknown. Here we describe the isolation and characterisation of an ancient mannose-binding lectin in the coral Acropora millepora, which is likely to be involved in both processes. The lectin ('Millectin') was isolated by affinity chromatography and was shown to bind to bacterial pathogens as well as coral symbionts, dinoflagellates of the genus Symbiodinium. cDNA analysis of Millectin indicate extensive sequence variation in the binding region, reflecting its ability to recognise various mannose-like carbohydrate structures on non-self cells, including symbionts and pathogens. This is the first mannose-binding lectin to show extensive sequence variability as observed for pattern recognition proteins in other invertebrate immune systems and, given that invertebrates rely on non-adaptive immunity, is a potential keystone component of coral defence mechanisms.

Fatal pneumonia epizootic in musk ox (Ovibos moschatus) in a period of extraordinary weather conditions

The musk ox is adapted to extreme cold and regarded as vulnerable to the impacts of climate change. Population decline is proposed to occur due to changes in forage availability, insect harassment, parasite load, and habitat availability, while the possible role of infectious diseases has not been emphasized. The goal of the present article is to describe an outbreak of fatal pasteurellosis that occurred in the introduced musk ox population of Dovrefjell, Norway in 2006, causing the death of a large proportion of the animals. The epizootic coincided with extraordinary warm and humid weather, conditions that often are associated with outbreaks of pasteurellosis. The description is based on long series of data from the surveillance of the musk ox population, weather data from a closely located meteorological station, and pathoanatomical investigation of the diseased animals. It is concluded that the weather conditions likely were the decisive factors for the outbreak. It is suggested that such epizootics may occur increasingly among cold-adapted animals if global warming results in increased occurrence of heat waves and associated extreme weather events, thereby causing population declines and possibly extinctions.

Blood parasites in owls with conservation implications for the Spotted Owl (Strix occidentalis)

The three subspecies of Spotted Owl (Northern, Strix occidentalis caurina; California, S. o. occidentalis; and Mexican, S. o. lucida) are all threatened by habitat loss and range expansion of the Barred Owl (S. varia). An unaddressed threat is whether Barred Owls could be a source of novel strains of disease such as avian malaria (Plasmodium spp.) or other blood parasites potentially harmful for Spotted Owls. Although Barred Owls commonly harbor Plasmodium infections, these parasites have not been documented in the Spotted Owl. We screened 111 Spotted Owls, 44 Barred Owls, and 387 owls of nine other species for haemosporidian parasites (Leucocytozoon, Plasmodium, and Haemoproteus spp.). California Spotted Owls had the greatest number of simultaneous multi-species infections (44%). Additionally, sequencing results revealed that the Northern and California Spotted Owl subspecies together had the highest number of Leucocytozoon parasite lineages (n = 17) and unique lineages (n = 12). This high level of sequence diversity is significant because only one Leucocytozoon species (L. danilewskyi) has been accepted as valid among all owls, suggesting that L. danilewskyi is a cryptic species. Furthermore, a Plasmodium parasite was documented in a Northern Spotted Owl for the first time. West Coast Barred Owls had a lower prevalence of infection (15%) when compared to sympatric Spotted Owls (S. o. caurina 52%, S. o. occidentalis 79%) and Barred Owls from the historic range (61%). Consequently, Barred Owls on the West Coast may have a competitive advantage over the potentially immune compromised Spotted Owls.

Detection and discovery of crustacean parasites in blue crabs (Callinectes sapidus) by using 18S rRNA gene-targeted denaturing high-performance liquid chromatography

Recently, we described a novel denaturing high-performance liquid chromatography (DHPLC) approach useful for initial detection and identification of crustacean parasites. Because this approach utilizes general primers targeted to conserved regions of the 18S rRNA gene, a priori genetic sequence information on eukaryotic parasites is not required. This distinction provides a significant advantage over specifically targeted PCR assays that do not allow for the detection of unknown or unsuspected parasites. However, initial field evaluations of the DHPLC assay suggested that because of PCR-biased amplification of dominant host genes it was not possible to detect relatively rare parasite genes in infected crab tissue. Here, we describe the use of a peptide nucleic acid (PNA) PCR hybridization blocking probe in association with DHPLC (PNA-PCR DHPLC) to overcome inherent PCR bias associated with amplification of rare target genes by use of generic primers. This approach was utilized to detect infection of blue crabs (Callinectes sapidus) by the parasitic dinoflagellate Hematodinium sp. Evaluation of 76 crabs caught in Wassaw Sound, GA, indicated a 97% correspondence between detection of the parasite by use of a specific PCR diagnostic assay and that by use of PNA-PCR DHPLC. During these studies, we discovered one crab with an association with a previously undescribed protist symbiont. Phylogenetic analysis of the amplified symbiont 18S rRNA gene indicated that it is most closely related to the free-living kinetoplastid parasite Procryptobia sorokini. To our knowledge, this is the first report of this parasite group in a decapod crab and of this organism exhibiting a presumably parasitic life history.

Virulence-transmission trade-offs and population divergence in virulence in a naturally occurring butterfly parasite

Why do parasites harm their hosts? Conventional wisdom holds that because parasites depend on their hosts for survival and transmission, they should evolve to become benign, yet many parasites cause harm. Theory predicts that parasites could evolve virulence (i.e., parasite-induced reductions in host fitness) by balancing the transmission benefits of parasite replication with the costs of host death. This idea has led researchers to predict how human interventions-such as vaccines-may alter virulence evolution, yet empirical support is critically lacking. We studied a protozoan parasite of monarch butterflies and found that higher levels of within-host replication resulted in both higher virulence and greater transmission, thus lending support to the idea that selection for parasite transmission can favor parasite genotypes that cause substantial harm. Parasite fitness was maximized at an intermediate level of parasite replication, beyond which the cost of increased host mortality outweighed the benefit of increased transmission. A separate experiment confirmed genetic relationships between parasite replication and virulence, and showed that parasite genotypes from two monarch populations caused different virulence. These results show that selection on parasite transmission can explain why parasites harm their hosts, and suggest that constraints imposed by host ecology can lead to population divergence in parasite virulence.

Diseases leading to accelerated decline of reef corals in the largest South Atlantic reef complex (Abrolhos Bank, eastern Brazil)

Although reef corals worldwide have sustained epizootics in recent years, no coral diseases have been observed in the southwestern Atlantic Ocean until now. Here we present an overview of the main types of diseases and their incidence in the largest and richest coral reefs in the South Atlantic (Abrolhos Bank, eastern Brazil). Qualitative observations since the 1980s and regular monitoring since 2001 indicate that coral diseases intensified only recently (2005-2007). Based on estimates of disease prevalence and progression rate, as well as on the growth rate of a major reef-building coral species (the Brazilian-endemic Mussismilia braziliensis), we predict that eastern Brazilian reefs will suffer a massive coral cover decline in the next 50 years, and that M. braziliensis will be nearly extinct in less than a century if the current rate of mortality due to disease is not reversed.

Latitudinal variation of immune defense and sickness behavior in the white-crowned sparrow (Zonotrichia leucophrys)

There is a general trend that parasitism risk declines as latitude increases. Host populations breeding at high latitudes should therefore invest less in costly immune defenses than populations breeding in temperate or tropical zones, although it is unknown if such an effect is mediated by environmental (photoperiodic) or genetic factors or both. Acquired immune function (humoral, cell-mediated) and behavioral sickness responses to lipopolysaccharide (LPS; mimics bacterial infection) were assessed in two subspecies of white-crowned sparrow (Zonotrichia leucophrys) that breed at different latitudes in western North America. Zonotrichia l. gambelii (GWCS) is a high-latitude breeder (47-68 degrees N) while Z. l. pugetensis (PWCS) breeds at temperate latitudes (40-49 degrees N). Captive males of each subspecies were acclimated to (1) a short day (non-breeding) photoperiod (8L:16D), (2) the breeding photoperiod of PWCS (16L:8D), or (3) the breeding photoperiod of GWCS (20L:4D). Photoperiod was manipulated because shorter day lengths may enhance immune function. In support of a genetic effect, humoral responses to diphtheria-tetanus vaccination were significantly higher in PWCS compared to GWCS, regardless of photoperiod. There were no differences in cell-mediated responses to phytohemagglutinin (PHA) between subspecies or among photoperiods. For sickness responses to LPS, a significant interaction between photoperiod and subspecies was found, with long day GWCS producing stronger sickness responses (losing more weight, eating less) than short day GWCS and PWCS on all day lengths. However, these effects were influenced by photoperiodic changes in body condition. In conclusion, we find evidence for genetic control of immune responses across latitude, but no support for environmental (photoperiodic) regulation.

Riding the wave: reconciling the roles of disease and climate change in amphibian declines

We review the evidence for the role of climate change in triggering disease outbreaks of chytridiomycosis, an emerging infectious disease of amphibians. Both climatic anomalies and disease-related extirpations are recent phenomena, and effects of both are especially noticeable at high elevations in tropical areas, making it difficult to determine whether they are operating separately or synergistically. We compiled reports of amphibian declines from Lower Central America and Andean South America to create maps and statistical models to test our hypothesis of spatiotemporal spread of the pathogen Batrachochytrium dendrobatidis (Bd), and to update the elevational patterns of decline in frogs belonging to the genus Atelopus. We evaluated claims of climate change influencing the spread of Bd by including error into estimates of the relationship between air temperature and last year observed. Available data support the hypothesis of multiple introductions of this invasive pathogen into South America and subsequent spread along the primary Andean cordilleras. Additional analyses found no evidence to support the hypothesis that climate change has been driving outbreaks of amphibian chytridiomycosis, as has been posited in the climate-linked epidemic hypothesis. Future studies should increase retrospective surveys of museum specimens from throughout the Andes and should study the landscape genetics of Bd to map fine-scale patterns of geographic spread to identify transmission routes and processes.

Once introduced, diseases may spread quickly through new areas, infecting naive host populations, such as has been documented in Ebola virus in African primates or rabies in North American mammals. What drives the spread of the pathogenic fungus Batrachochytrium dendrobatidis (Bd), which causes chytridiomycosis, is of particular concern because it has contributed to the global decline of amphibians. We modeled the spatiotemporal pattern of the loss of upland amphibian populations in Central and South America as a proxy for the arrival of Bd and found that amphibian declines in Central and South America are best explained by Bd spreading through upland populations; we identified four separate introductions of Bd into South America. Climate change seriously threatens biodiversity and influences endemic host–pathogen systems, but we found no evidence that climate change has been driving outbreaks of chytridiomycosis, as has been posited in the climate-linked epidemic hypothesis. Our findings further strengthen the spreading-pathogen hypothesis proposed for Central America, and identify new evidence for similar patterns of decline in South American amphibians. Our results will inform management and research efforts related to Bd and other invasive species, as effective conservation actions depend on correctly identifying essential threats to biodiversity, and possible synergistic interactions.

The spread of chytrid fungus, rather than climate change, best explains amphibian declines in Central and South America, based on an analysis of observed epidemics with predictable spatiotemporal patterns in four of five mountain ranges.

Cellular responses in sea fan corals: granular amoebocytes react to pathogen and climate stressors

Background

Climate warming is causing environmental change making both marine and terrestrial organisms, and even humans, more susceptible to emerging diseases. Coral reefs are among the most impacted ecosystems by climate stress, and immunity of corals, the most ancient of metazoans, is poorly known. Although coral mortality due to infectious diseases and temperature-related stress is on the rise, the immune effector mechanisms that contribute to the resistance of corals to such events remain elusive. In the Caribbean sea fan corals (Anthozoa, Alcyonacea: Gorgoniidae), the cell-based immune defenses are granular acidophilic amoebocytes, which are known to be involved in wound repair and histocompatibility.

Methodology/Principal Findings

We demonstrate for the first time in corals that these cells are involved in the organismal response to pathogenic and temperature stress. In sea fans with both naturally occurring infections and experimental inoculations with the fungal pathogen Aspergillus sydowii, an inflammatory response, characterized by a massive increase of amoebocytes, was evident near infections. Melanosomes were detected in amoebocytes adjacent to protective melanin bands in infected sea fans; neither was present in uninfected fans. In naturally infected sea fans a concurrent increase in prophenoloxidase activity was detected in infected tissues with dense amoebocytes. Sea fans sampled in the field during the 2005 Caribbean Bleaching Event (a once-in-hundred-year climate event) responded to heat stress with a systemic increase in amoebocytes and amoebocyte densities were also increased by elevated temperature stress in lab experiments.

Conclusions/Significance

The observed amoebocyte responses indicate that sea fan corals use cellular defenses to combat fungal infection and temperature stress. The ability to mount an inflammatory response may be a contributing factor that allowed the survival of even infected sea fan corals during a stressful climate event.

Effects of food abundance, age, and flea infestation on the body condition and immunological variables of a rodent host, and their consequences for flea survival

Temporal variation in body condition and immunological variables of animals that harbor parasites may explain patterns of variation in infestation, as well as parasite impact on the host. We emulated such variability in Sundevall's jirds by manipulating food availability and flea infestation in juveniles and adults and examining how these changes affect survival of fleas on their hosts. Body condition of food-restricted jirds deteriorated, but there was no change in their immunological variables. Adult jirds were in better body condition and had higher immunocompetence than juveniles, however there were no significant effects of flea infestation on any of the variables examined. The main effects of flea infestation were a decrease in the response to phytohaemagglutinin injection, and an increase in the negative effects of food restriction on body mass. Flea survival was higher on juveniles, but fleas did not respond to temporal variability in body condition and immunocompetence of the jirds. We concluded that changes in body condition and immune responses due to growth or variability in food abundance are more important than changes caused by the fleas themselves. Flea infestation is more detrimental to jirds when they are not able to compensate for mass loss through increased food consumption.

Integrated approaches and empirical models for investigation of parasitic diseases in northern wildlife

A decade of research has yielded a multidisciplinary approach for detection, prediction, and potential mitigation measures.

The North is a frontier for exploration of emerging infectious diseases and the large-scale drivers influencing distribution, host associations, and evolution of pathogens among persons, domestic animals, and wildlife. Leading into the International Polar Year 2007–2008, we outline approaches, protocols, and empirical models derived from a decade of integrated research on northern host–parasite systems. Investigations of emerging infectious diseases associated with parasites in northern wildlife involved a network of multidisciplinary collaborators and incorporated geographic surveys, archival collections, historical foundations for diversity, and laboratory and field studies exploring the interface for hosts, parasites, and the environment. In this system, emergence of parasitic disease was linked to geographic expansion, host switching, resurgence due to climate change, and newly recognized parasite species. Such integrative approaches serve as cornerstones for detection, prediction, and potential mitigation of emerging infectious diseases in wildlife and persons in the North and elsewhere under a changing global climate.

Effects of climate change on Arctic marine mammal health

The lack of integrated long-term data on health, diseases, and toxicant effects in Arctic marine mammals severely limits our ability to predict the effects of climate change on marine mammal health. The overall health of an individual animal is the result of complex interactions among immune status, body condition, pathogens and their pathogenicity, toxicant exposure, and the various environmental conditions that interact with these factors. Climate change could affect these interactions in several ways. There may be direct effects of loss of the sea ice habitat, elevations of water and air temperature, and increased occurrence of severe weather. Some of the indirect effects of climate change on animal health will likely include alterations in pathogen transmission due to a variety of factors, effects on body condition due to shifts in the prey base/food web, changes in toxicant exposures, and factors associated with increased human habitation in the Arctic (e.g., chemical and pathogen pollution in the runoff due to human and domestic-animal wastes and chemicals and increased ship traffic with the attendant increased risks of ship strike, oil spills, ballast pollution, and possibly acoustic injury). The extent to which climate change will impact marine mammal health will also vary among species, with some species more sensitive to these factors than others. Baseline data on marine mammal health parameters along with matched data on the population and climate change trends are needed to document these changes.

Determination of the effects of temperature on viability, metabolic activity and proliferation of two Perkinsus species, and its significance to understanding seasonal cycles of perkinsosis

The range of water temperatures in which Perkinsus species can survive and proliferate remains ill-defined, particularly at lower temperatures. The in vitro viability, metabolic activity, and proliferation of 3 isolates each of P. marinus and P. olseni trophozoites at 28°C, and at 15 and 4°C, after transfer from 28°C, were compared. Both species showed declines in metabolic activity and proliferation from 28°C to 15°C. At 4°C, both species had viability after 30 days incubation time (P. marinus 49%, P. olseni 58%), but limited metabolic activity and no proliferation. Perkinsus marinus viability was further compared when transferred directly from 28°C, 18°C and progressively from 18°C (0·5°C/day) to 2, 4 and 6°C and maintained for up to 4 months. Viability was highest under progressive transfer (77% and 54% after 30 and 60 days exposure to test temperatures). The decrease in P. marinus viability at the lower temperatures in vitro only partially explains decreasing parasite infection intensities in eastern oysters in the colder months of the year. Moreover, the significant decrease in parasite infection intensities in late winter and early spring, as temperatures increase, is likely due to an active process of elimination by oyster host defences.

Within-host disease ecology in the sea fan Gorgonia ventalina: modeling the spatial immunodynamics of a coral-pathogen interaction

We develop a spatially explicit model for the within-host interactions between a fungal pathogen and the immune response by its coral host. The model is parameterized for the recent epizootic of Aspergillus sydowii in the sea fan Gorgonia ventalina, but its structure is adaptable to many other diseases attacking corals worldwide, fungal infections in other invertebrates and plants, and opportunistic fungal infections in vertebrates. Model processes include pathogen growth and spread through consumption of host tissue, chemotactic attraction of undifferentiated host amoebocytes to infections, and amoebocyte differentiation into various cell types that attack the pathogen. Sensitivity analysis shows that the spread rate of a single localized infection is determined primarily by the pathogen's potential rate of host tissue consumption and by the host's ability to replenish the pool of undifferentiated amoebocytes and sustain a long-term response. The spatial localization of immune responses creates potentially strong indirect interactions between distant lesions, allowing new infections to grow rapidly while host resources are concentrated at older, larger infections. These findings provide possible mechanistic explanations for effects of environmental stressors (e.g., ocean warming, nutrient enrichment) on aspergillosis prevalence and severity and for the observed high spatial and between-host variability in disease impacts.

The role of biotic factors in the transmission of free-living endohelminth stages

The transmission success of free-living larval stages of endohelminths is generally modulated by a variety of abiotic and biotic environmental factors. Whereas the role of abiotic factors (including anthropogenic pollutants) has been in focus in numerous studies and summarized in reviews, the role of biotic factors has received much less attention. Here, we review the existing body of literature from the fields of parasitology and ecology and recognize 6 different types of biotic factors with the potential to alter larval transmission processes. We found that experimental studies generally indicate strong effects of biotic factors, and the latter emerge as potentially important, underestimated determinants in the transmission ecology of free-living endohelminth stages. This implies that biodiversity, in general, should have significant effects on parasite transmission and population dynamics. These effects are likely to interact with natural abiotic factors and anthropogenic pollutants. Investigating the interplay of abiotic and biotic factors will not only be crucial for a thorough understanding of parasite transmission processes, but will also be a prerequisite to anticipate the effects of climate and other global changes on helminth parasites and their host communities.

Evolution of pathogens in a man-made world

Human activities have resulted in substantial, large-scale environmental modifications, especially in the past century. Ecologists and evolutionary biologists are increasingly coming to realize that parasites and pathogens, like free-living organisms, evolve as the consequence of these anthropogenic changes. Although this area now commands the attention of a variety of researchers, a broad predictive framework is lacking, mainly because the links between human activities, the environment and parasite evolution are complex. From empirical and theoretical examples chosen in the literature, we give an overview of the ways in which humans can directly or indirectly influence the evolution of different traits in parasites (e.g. specificity, virulence, polymorphism). We discuss the role of direct and indirect factors as diverse as habitat fragmentation, pollution, biodiversity loss, climate change, introduction of species, use of vaccines and antibiotics, ageing of the population, etc. We also present challenging questions for further research. Understanding the links between anthropogenic changes and parasite evolution needs to become a cornerstone of public health planning, economic development and conservation biology.

Climate change and infectious diseases

The worldwide upturn in the occurrence of both new (emerging) and reemerging or spreading infectious diseases highlights the importance of underlying environmental and social conditions as determinants of the generation, spread, and impact of infectious diseases in human populations. Human ecology is undergoing rapid transition. This encompasses urbanization, rising consumerism, changes in working conditions, population aging, marked increases in mobility, changes in culture and behavior, evolving health-care technologies, and other factors. Global climate change is becoming a further, and major, large-scale influence on the pattern of infectious disease transmission. It is likely to become increasingly important over at least the next halfcentury, as the massive, highinertial, and somewhat unpredictable process of climate change continues. The many ways in which climate change does and will influence infectious diseases are subject to a plethora of modifying influences by other factors and processes: constitutional characteristics of hosts, vectors and pathogens; the prevailing ambient conditions; and coexistent changes in other social, economic, behavioral, and environmental factors. This global anthropogenic process, climate change, along with other unprecedented global environmental changes, is beginning to destabilize and weaken the planet's life-support systems. Infectious diseases, unlike other diseases, depend on the biology and behavior—each often climate-sensitive—of two or more parties. Hence, these diseases will be particularly susceptible to changes as the world's climate and its climate-sensitive geochemical and ecological systems undergo change over the coming decades.

Thermal tolerance in a south-east African population of the tsetse fly Glossina pallidipes (Diptera, Glossinidae): implications for forecasting climate change impacts

For tsetse (Glossina spp.), the vectors of human and animal trypanosomiases, the physiological mechanisms linking variation in population dynamics with changing weather conditions have not been well established. Here, we investigate high- and low-temperature tolerance in terms of activity limits and survival in a natural population of adult Glossina pallidipes from eastern Zambia. Due to increased interest in chilling flies for handling and aerial dispersal in sterile insect technique control and eradication programmes, we also provide further detailed investigation of low-temperature responses. In wild-caught G. pallidipes, the probability of survival for 50% of the population at low-temperatures was at 3.7, 8.9 and 9.6 degrees C (95% CIs: +/-1.5 degrees C) for 1, 2 and 3 h treatments, respectively. At high temperatures, it was estimated that treatments at 37.9, 36.2 and 35.6 degrees C (95% CIs: +/-0.5 degrees C) would yield 50% population survival for 1, 2 and 3 h, respectively. Significant effects of time and temperature were detected at both temperature extremes (GLZ, p<0.05 in all cases) although a time-temperature interaction was only detected at high temperatures (p<0.0001). We synthesized data from four other Kenyan populations and found that upper critical thermal limits showed little variation among populations and laboratory treatments (range: 43.9-45.0 degrees C; 0.25 degrees C/min heating rate), although reduction to more ecologically relevant heating rates (0.06 degrees C/min) reduce these values significantly from approximately 44.4 to 40.6 degrees C, thereby providing a causal explanation for why tsetse distribution may be high-temperature limited. By contrast, low-temperature limits showed substantial variation among populations and acclimation treatments (range: 4.5-13.8 degrees C; 0.25 degrees C/min), indicating high levels of inter-population variability. Ecologically relevant cooling rates (0.06 degrees C/min) suggest tsetses are likely to experience chill coma temperatures under natural conditions (approximately 20-21 degrees C). The results from acute hardening experiments in the Zambian population demonstrate limited ability to improve low-temperature tolerance over short (hourly) timescales after non-lethal pre-treatments. In flies which survived chilling, recovery times were non-linear with plateaus between 2-6 and 8-12 degrees C. Survival times ranged between 4 and 36 h and did not vary between flies which had undergone chill coma by comparison with flies which had not, even after factoring body condition into the analyses (p>0.5 in all cases). However, flies with low chill coma values had the highest body water and fat content, indicating that when energy reserves are depleted, low-temperature tolerance may be compromised. Overall, these results suggest that physiological mechanisms may provide insight into tsetse population dynamics, hence distribution and abundance, and support a general prediction for reduced geographic distribution under future climate warming scenarios.

Coral reefs under rapid climate change and ocean acidification

Atmospheric carbon dioxide concentration is expected to exceed 500 parts per million and global temperatures to rise by at least 2 degrees C by 2050 to 2100, values that significantly exceed those of at least the past 420,000 years during which most extant marine organisms evolved. Under conditions expected in the 21st century, global warming and ocean acidification will compromise carbonate accretion, with corals becoming increasingly rare on reef systems. The result will be less diverse reef communities and carbonate reef structures that fail to be maintained. Climate change also exacerbates local stresses from declining water quality and overexploitation of key species, driving reefs increasingly toward the tipping point for functional collapse. This review presents future scenarios for coral reefs that predict increasingly serious consequences for reef-associated fisheries, tourism, coastal protection, and people. As the International Year of the Reef 2008 begins, scaled-up management intervention and decisive action on global emissions are required if the loss of coral-dominated ecosystems is to be avoided.

Seasonality of infectious diseases

Seasonality, a periodic surge in disease incidence corresponding to seasons or other calendar periods, characterizes many infectious diseases of public health importance. The recognition of seasonal patterns in infectious disease occurrence dates back at least as far as the Hippocratic era, but mechanisms underlying seasonality of person-to-person transmitted diseases are not well understood. Improved understanding will enhance understanding of host-pathogen interactions and will improve the accuracy of public health surveillance and forecasting systems. Insight into seasonal disease patterns may be gained through the use of autocorrelation methods or construction of periodograms, while seasonal oscillation of infectious diseases can be easily simulated using simple transmission models. Models demonstrate that small seasonal changes in host or pathogen factors may be sufficient to create large seasonal surges in disease incidence, which may be important particularly in the context of global climate change. Seasonality represents a rich area for future research.

Identification of a ciliate (Oligohymenophorea: Scuticociliatia) associated with brown band disease on corals of the Great Barrier Reef

A ciliate associated with the coral disease brown band (BrB) was identified as a new species belonging to the class Oligohymenophorea, subclass Scuticociliatia. The ciliates were characterized by the presence of large numbers of intracellular dinoflagellates and displayed an elongated, tube-shaped body structure. They had uniform ciliature, except for three distinct cilia in the caudal region, and were typically 200 to 400 microm in length and 20 to 50 microm in width.

Thermal stress and coral cover as drivers of coral disease outbreaks

Very little is known about how environmental changes such as increasing temperature affect disease dynamics in the ocean, especially at large spatial scales. We asked whether the frequency of warm temperature anomalies is positively related to the frequency of coral disease across 1,500 km of Australia's Great Barrier Reef. We used a new high-resolution satellite dataset of ocean temperature and 6 y of coral disease and coral cover data from annual surveys of 48 reefs to answer this question. We found a highly significant relationship between the frequencies of warm temperature anomalies and of white syndrome, an emergent disease, or potentially, a group of diseases, of Pacific reef-building corals. The effect of temperature was highly dependent on coral cover because white syndrome outbreaks followed warm years, but only on high (>50%) cover reefs, suggesting an important role of host density as a threshold for outbreaks. Our results indicate that the frequency of temperature anomalies, which is predicted to increase in most tropical oceans, can increase the susceptibility of corals to disease, leading to outbreaks where corals are abundant.

Coral reefs have been decimated over the last several decades. The global decline of reef-building corals is of particular concern. Infectious diseases are thought to be key to this mass coral mortality, and many reef ecologists suspect that anomalously high ocean temperatures contribute to the increased incidence and severity of disease outbreaks. This hypothesis is supported by local observations—for example, that some coral diseases become more prevalent in the summertime—but it has never been tested at large spatial scales or over relatively long periods. We tested the temperature–disease hypothesis by combining 6 years of survey data from reefs across 1,500 kilometers of Australia's Great Barrier Reef with a new ocean temperature database derived from satellite measurements. Our results indicate that major outbreaks of the coral disease white syndrome only occurred on reefs with high coral cover after especially warm years. The disease was usually absent on cooler, low-cover reefs. Our results suggest that climate change could be increasing the severity of disease in the ocean, leading to a decline in the health of marine ecosystems and the loss of the resources and services humans derive from them.

High-resolution satellite data of ocean temperature across the Great Barrier Reef reveal that warm temperature anomalies can drive outbreaks of coral disease under conditions of high coral cover.

Disease effects on reproduction can cause population cycles in seasonal environments

Recent studies of rodent populations have demonstrated that certain parasites can cause juveniles to delay maturation until the next reproductive season. Furthermore, a variety of parasites may share the same host, and evidence is beginning to accumulate showing nonindependent effects of different infections.

We investigated the consequences for host population dynamics of a disease-induced period of no reproduction, and a chronic reduction in fecundity following recovery from infection (such as may be induced by secondary infections) using a modified SIR (susceptible, infected, recovered) model. We also included a seasonally varying birth rate as recent studies have demonstrated that seasonally varying parameters can have important effects on long-term host–parasite dynamics. We investigated the model predictions using parameters derived from five different cyclic rodent populations.

Delayed and reduced fecundity following recovery from infection have no effect on the ability of the disease to regulate the host population in the model as they have no effect on the basic reproductive rate. However, these factors can influence the long-term dynamics including whether or not they exhibit multiyear cycles.

The model predicts disease-induced multiyear cycles for a wide range of realistic parameter values. Host populations that recover relatively slowly following a disease-induced population crash are more likely to show multiyear cycles. Diseases for which the period of infection is brief, but full recovery of reproductive function is relatively slow, could generate large amplitude multiyear cycles of several years in length. Chronically reduced fecundity following recovery can also induce multiyear cycles, in support of previous theoretical studies.

When parameterized for cowpox virus in the cyclic field vole populations (Microtus agrestis) of Kielder Forest (northern England), the model predicts that the disease must chronically reduce host fecundity by more than 70%, following recovery from infection, for it to induce multiyear cycles. When the model predicts quasi-periodic multiyear cycles it also predicts that seroprevalence and the effective date of onset of the reproductive season are delayed density-dependent, two phenomena that have been recorded in the field.

Population structures of two genotypes of Vibrio vulnificus in oysters (Crassostrea virginica) and seawater

Vibrio vulnificus biotype 1 strains can be classified into two genotypes based on the PCR analysis of variations in the virulence-correlated gene (vcg). Genotype has been correlated with human infection for 90% of isolates from human cases having the vcgC sequence type and 87% of environmental strains having the vcgE variant. In this study we examined the dynamics of V. vulnificus populations and the distribution of the two genotypes recovered from oysters and surrounding estuarine wasters. Analysis of 880 isolates recovered from oysters showed a disparity in the ratio of the two genotypes, with those of the vcgE (E) genotype accounting for 84.4% of the population. In contrast, 292 isolates recovered from the waters surrounding the oyster sites revealed an almost equal distribution of the two genotypes. The levels of vcgC (C genotype) strains from both sources increased as a percentage of the population as water temperatures increased, while no culturable V. vulnificus cells were recovered from December through February. Our results suggest that there is a selective advantage for strains of the E genotype within oysters while survival of the C genotype strains may be favored by increased water column temperatures. These data suggest that the low incidence of infections may be due to the comparatively rare consumption of an oyster that contains a greater number of V. vulnificus vcgC genotype strains than of vcgE genotype strains. Levels of the two genotypes as well as seasonal dynamics within both oyster tissue and the surrounding waters may aid in identifying risk factors associated with human infection.

Mixed infections, cryptic diversity, and vector-borne pathogens: evidence from Polygenis fleas and Bartonella species

Coinfections within hosts present opportunities for horizontal gene transfer between strains and competitive interactions between genotypes and thus can be a critical element of the lifestyles of pathogens. Bartonella spp. are Alphaproteobacteria that parasitize mammalian erythrocytes and endothelial cells. Their vectors are thought to be various biting arthropods, such as fleas, ticks, mites, and lice, and they are commonly cited as agents of various emerging diseases. Coinfections by different Bartonella strains and species can be common in mammals, but little is known about specificity and coinfections in arthropod vectors. We surveyed the rate of mixed infections of Bartonella in flea vectors (Polygenis gwyni) parasitizing cotton rats (Sigmodon hispidus) in which previous surveys indicated high rates of infection. We found that nearly all fleas (20 of 21) harbored one or more strains of Bartonella, with rates of coinfection approaching 90%. A strain previously identified as common in cotton rats was also common in their fleas. However, another common strain in cotton rats was absent from P. gwyni, while a rare cotton rat strain was quite common in P. gwyni. Surprisingly, some samples were also coinfected with a strain phylogenetically related to Bartonella clarridgeiae, which is typically associated with felids and ruminants. Finally, a locus (pap31) that is characteristically borne on phage in Bartonella was successfully sequenced from most samples. However, sequence diversity in pap31 was novel in the P. gwyni samples, relative to other Bartonella previously typed with pap31, emphasizing the likelihood of large reservoirs of cryptic diversity in natural populations of the pathogen.

Aquatic eutrophication promotes pathogenic infection in amphibians

The widespread emergence of human and wildlife diseases has challenged ecologists to understand how large-scale agents of environmental change affect host-pathogen interactions. Accelerated eutrophication of aquatic ecosystems owing to nitrogen and phosphorus enrichment is a pervasive form of environmental change that has been implicated in the emergence of diseases through direct and indirect pathways. We provide experimental evidence linking eutrophication and disease in a multihost parasite system. The trematode parasite Ribeiroia ondatrae sequentially infects birds, snails, and amphibian larvae, frequently causing severe limb deformities and mortality. Eutrophication has been implicated in the emergence of this parasite, but definitive evidence, as well as a mechanistic understanding, have been lacking until now. We show that the effects of eutrophication cascade through the parasite life cycle to promote algal production, the density of snail hosts, and, ultimately, the intensity of infection in amphibians. Infection also negatively affected the survival of developing amphibians. Mechanistically, eutrophication promoted amphibian disease through two distinctive pathways: by increasing the density of infected snail hosts and by enhancing per-snail production of infectious parasites. Given forecasted increases in global eutrophication, amphibian extinctions, and similarities between Ribeiroia and important human and wildlife pathogens, our results have broad epidemiological and ecological significance.

A novel bacterial disease of the European shore crab, Carcinus maenas – molecular pathology and epidemiology

Several rickettsia-like diseases have been reported in arthropods (insects and crustaceans), some of which result in significant losses of economically important species such as shrimp and crabs. This study reports on the molecular pathology of a recently emerged disease of the European shore crab, Carcinus maenas, termed milky disease - named as a result of the unusual milky appearance of the haemolymph (blood). This disease was more prevalent (>26 %) during summer months when the water temperature in a pilot crab farm was approximately 19 degrees C. The putative causative agent of the disease was a Gram-negative bacterium that could not be cultured on a range of agar-based growth media. Diseased crabs showed significant reductions in free blood cell numbers and total serum protein. Such animals also displayed raised levels of glucose and ammonium in blood. Ultrastructural and in situ hybridization studies revealed that the causative agent associated with milky disease multiplied in the fixed phagocytes of the hepatopancreas (digestive gland), ultimately to be released into the haemolymph, where the circulating blood cells showed little response to the presence of these agents. Attempts to induce the infection by short-term temperature stress failed, as did transmission experiments where healthy crabs were fed infected tissues from milky disease affected individuals. Sequence analysis of the 16S rRNA gene from the milky disease bacteria indicated that they are a previously undescribed species of alpha-proteobacteria with little phylogenetic similarity to members of the order Rickettsiales.

Wild deer as a source of infection for livestock and humans in the UK

Wild deer can feature in the epidemiology of a wide range of livestock and human diseases in the United Kingdom by representing a source of disease via various transmission routes. This review highlights current and possible future infections of deer in the UK which may have an impact on livestock and/or human health. Increases in deer abundance as well as range expansion are likely to exacerbate the potential for disease persistence due to the formation of multi-species deer assemblages, which may act as disease reservoirs. Climatic changes are likely to have a direct impact on the presence and abundance of various pathogens and their vectors, so that with a warming climate exotic diseases may play a role in future UK livestock and wildlife disease management. This paper highlights the need for a monitoring strategy for wildlife diseases, in particular infections in wild deer, in the UK.

Sponge-associated microorganisms: evolution, ecology, and biotechnological potential

Marine sponges often contain diverse and abundant microbial communities, including bacteria, archaea, microalgae, and fungi. In some cases, these microbial associates comprise as much as 40% of the sponge volume and can contribute significantly to host metabolism (e.g., via photosynthesis or nitrogen fixation). We review in detail the diversity of microbes associated with sponges, including extensive 16S rRNA-based phylogenetic analyses which support the previously suggested existence of a sponge-specific microbiota. These analyses provide a suitable vantage point from which to consider the potential evolutionary and ecological ramifications of these widespread, sponge-specific microorganisms. Subsequently, we examine the ecology of sponge-microbe associations, including the establishment and maintenance of these sometimes intimate partnerships, the varied nature of the interactions (ranging from mutualism to host-pathogen relationships), and the broad-scale patterns of symbiont distribution. The ecological and evolutionary importance of sponge-microbe associations is mirrored by their enormous biotechnological potential: marine sponges are among the animal kingdom's most prolific producers of bioactive metabolites, and in at least some cases, the compounds are of microbial rather than sponge origin. We review the status of this important field, outlining the various approaches (e.g., cultivation, cell separation, and metagenomics) which have been employed to access the chemical wealth of sponge-microbe associations.

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.

Emerging infectious diseases of plants: pathogen pollution, climate change and agrotechnology drivers

Emerging infectious diseases (EIDs) pose threats to conservation and public health. Here, we apply the definition of EIDs used in the medical and veterinary fields to botany and highlight a series of emerging plant diseases. We include EIDs of cultivated and wild plants, some of which are of significant conservation concern. The underlying cause of most plant EIDs is the anthropogenic introduction of parasites, although severe weather events are also important drivers of disease emergence. Much is known about crop plant EIDs, but there is little information about wild-plant EIDs, suggesting that their impact on conservation is underestimated. We conclude with recommendations for improving strategies for the surveillance and control of plant EIDs.

Coral microbial communities, zooxanthellae and mucus along gradients of seawater depth and coastal pollution

The high incidence of coral disease in shallow coastal marine environments suggests seawater depth and coastal pollution have an impact on the microbial communities inhabiting healthy coral tissues. A study was undertaken to determine how bacterial communities inhabiting tissues of the coral Montastraea annularis change at 5 m, 10 m and 20 m water depth in varying proximity to the urban centre and seaport of Willemstad, Curaçao, Netherlands Antilles. Analyses of terminal restriction fragment length polymorphisms (TRFLP) of 16S rRNA gene sequences show significant differences in bacterial communities of polluted and control localities only at the shallowest seawater depth. Furthermore, distinct differences in bacterial communities were found with increasing water depth. Comparisons of TRFLP peaks with sequenced clone libraries indicate the black band disease cyanobacterium clone CD1C11 is common and most abundant on healthy corals in less than 10 m water depth. Similarly, sequences belonging to a previously unrecognized group of likely phototrophic bacteria, herein referred to as CAB-I, were also more common in shallow water. To assess the influence of environmental and physiologic factors on bacterial community structure, canonical correspondence analysis was performed using explanatory variables associated with: (i) light availability; (ii) seawater pollution; (iii) coral mucus composition; (iv) the community structure of symbiotic algae; and (v) the photosynthetic activity of symbiotic algae. Eleven per cent of the variation in bacterial communities was accounted for by covariation with these variables; the most important being photosynthetically active radiation (sunlight) and the coral uptake of sewage-derived compounds as recorded by the delta(15)N of coral tissue.

Climate change and developing-country cities: implications for environmental health and equity

Climate change is an emerging threat to global public health. It is also highly inequitable, as the greatest risks are to the poorest populations, who have contributed least to greenhouse gas (GHG) emissions. The rapid economic development and the concurrent urbanization of poorer countries mean that developing-country cities will be both vulnerable to health hazards from climate change and, simultaneously, an increasing contributor to the problem. We review the specific health vulnerabilities of urban populations in developing countries and highlight the range of large direct health effects of energy policies that are concentrated in urban areas. Common vulnerability factors include coastal location, exposure to the urban heat-island effect, high levels of outdoor and indoor air pollution, high population density, and poor sanitation. There are clear opportunities for simultaneously improving health and cutting GHG emissions most obviously through policies related to transport systems, urban planning, building regulations and household energy supply. These influence some of the largest current global health burdens, including approximately 800,000 annual deaths from ambient urban air pollution, 1.2 million from road-traffic accidents, 1.9 million from physical inactivity, and 1.5 million per year from indoor air pollution. GHG emissions and health protection in developing-country cities are likely to become increasingly prominent in policy development. There is a need for a more active input from the health sector to ensure that development and health policies contribute to a preventive approach to local and global environmental sustainability, urban population health, and health equity.

Climate warming and disease risks in temperate regions--Argulus coregoni and Diplostomum spathaceum as case studies

The link between climate changes and disease risks from various pathogens has been increasingly recognized. The effect of climatic factors on host-parasite population dynamics is particularly evident in northern latitudes where the occurrence and transmission of parasites are strongly regulated by seasonality-driven changes in environmental temperatures. Shortened winter periods would increase growth potential of many parasite populations. The ways in which climate warming could affect life history dynamics of the directly transmitted crustacean ectoparasite Argulus coregoni and complex life cycle trematode Diplostomum spathaceum, which frequently cause problems in northern fish farming, are discussed. Increased problems for fish farming are predicted in terms of increased infection pressure from these parasites in future. This would increase problems associated with infections and increase the use of expensive management protocols with high environmental impact.

Climate change and outbreaks of amphibian chytridiomycosis in a montane area of Central Spain; is there a link?

Amphibian species are declining at an alarming rate on a global scale in large part owing to an infectious disease caused by the chytridiomycete fungus, Batrachochytrium dendrobatidis. This disease of amphibians has recently emerged within Europe, but knowledge of its effects on amphibian assemblages remains poor. Importantly, little is known about the environmental envelope that is associated with chytridiomycosis in Europe and the potential for climate change to drive future disease dynamics. Here, we use long-term observations on amphibian population dynamics in the Peñalara Natural Park, Spain, to investigate the link between climate change and chytridiomycosis. Our analysis shows a significant association between change in local climatic variables and the occurrence of chytridiomycosis within this region. Specifically, we show that rising temperature is linked to the occurrence of chytrid-related disease, consistent with the chytrid-thermal-optimum hypothesis. We show that these local variables are driven by general circulation patterns, principally the North Atlantic Oscillation. Given that B. dendrobatidis is known to be broadly distributed across Europe, there is now an urgent need to assess the generality of our finding and determine whether climate-driven epidemics may be expected to impact on amphibian species across the wider region.

The role of microorganisms in coral health, disease and evolution

Coral microbiology is an emerging field, driven largely by a desire to understand, and ultimately prevent, the worldwide destruction of coral reefs. The mucus layer, skeleton and tissues of healthy corals all contain large populations of eukaryotic algae, bacteria and archaea. These microorganisms confer benefits to their host by various mechanisms, including photosynthesis, nitrogen fixation, the provision of nutrients and infection prevention. Conversely, in conditions of environmental stress, certain microorganisms cause coral bleaching and other diseases. Recent research indicates that corals can develop resistance to specific pathogens and adapt to higher environmental temperatures. To explain these findings the coral probiotic hypothesis proposes the occurrence of a dynamic relationship between symbiotic microorganisms and corals that selects for the coral holobiont that is best suited for the prevailing environmental conditions. Generalization of the coral probiotic hypothesis has led us to propose the hologenome theory of evolution.

Climate change, parasitism and the structure of intertidal ecosystems

Evidence is accumulating rapidly showing that temperature and other climatic variables are driving many ecological processes. At the same time, recent research has highlighted the role of parasitism in the dynamics of animal populations and the structure of animal communities. Here, the likely interactions between climate change and parasitism are discussed in the context of intertidal ecosystems. Firstly, using the soft-sediment intertidal communities of Otago Harbour, New Zealand, as a case study, parasites are shown to be ubiquitous components of intertidal communities, found in practically all major animal species in the system. With the help of specific examples from Otago Harbour, it is demonstrated that parasites can regulate host population density, influence the diversity of the entire benthic community, and affect the structure of the intertidal food web. Secondly, we document the extreme sensitivity of cercarial production in parasitic trematodes to increases in temperature, and discuss how global warming could lead to enhanced trematode infections. Thirdly, the results of a simulation model are used to argue that parasite-mediated local extinctions of intertidal animals are a likely outcome of global warming. Specifically, the model predicts that following a temperature increase of less than 4°C, populations of the amphipodCorophium volutator, a hugely abundant tube-building amphipod on the mudflats of the Danish Wadden Sea, are likely to crash repeatedly due to mortality induced by microphallid trematodes. The available evidence indicates that climate-mediated changes in local parasite abundance will have significant repercussions for intertidal ecosystems. On the bright side, the marked effects of even slight increases in temperature on cercarial production in trematodes could form the basis for monitoring programmes, with these sensitive parasites providing early warning signals of the environmental impacts of global warming.