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.Parasites make thecarrion until eventual ingestion by a scavenger.For longest contribution to reproduction when in thematuring parasites in the intestine, the inside of the most compatible combinations of host and parasitehost is exceedingly hostile: survival of adult worms genotypes.This, too, was the explanation proposedis limited to only a few weeks or months.In by Mas-Coma et al.(2001) for hyperendemic fascio-immunocompromised hosts, this brief lifespan is liasis in the Bolivian Altiplano: reproduction by 112 PARASI TI SM AND ECOSYSTEMSselfing from the snail(s) originally introduced has parasite feeding rate and host metabolic loss aregenerated a homogeneous intermediate host reduced; the activity of the host immune responsepopulation highly susceptible to infection.is, to a greater or lesser extent, suppressed.The out-Within host and parasite populations, variations in come, demonstrated by the detailed quantitativeparasite infectivity and host susceptibility may deter- studies on monogeneans such as Gyrodactylus,mine that, at a micro-level , some host environments Pseudodiplorchis, and Protopolystoma species (above),are more hostile than others.This has been docu- shows that during periods of reduced parasite-mented in many systems illustrating local adapta- induced pathology, host-mediated attack on para-tion.Amongst the case studies emphasized in this site populations is also reduced.At higherreview, quantitative evidence is provided by data temperatures, when there are rate increases infrom experimental infections of P.xenopodis in X.lae- parasite activity (including feeding, reproductionvis.Using full-sibling groups of lab-raised hosts (with and transmission), the host immune response is alsoreduced genetic variation), survival to patency for correspondingly upregulated.In endotherms, bydifferent isolates of parasites (from both local and dis- contrast, both pathogenic effects and immune attacktant localities) showed major variation (Jackson and are continuous at constant temperature both areTinsley 2003b).Development within the host at relentless in provoking and generating hostile con-higher or lower temperatures (within the natural sea- ditions within the host micro-environment.It mightsonal range experienced by host and parasite) may be speculated that improved immune defenceresult in significant differences in survival (Jackson against infection was a major selective advantage inand Tinsley 2002).Studies of lifetime reproductive the evolution of endothermy, developing in justperformance showed marked variation between two classes of one animal phylum (and perhapsinfections in survivorship, egg output, etc.(Jackson also in the now-extinct advanced reptiles that wereand Tinsley 2001).All this combines to illustrate, for ancestral to the mammals and birds).However,this natural host parasite system, variations in the while the increased activity of immune defencesdegree of hostility that may exist in nature.will have been a major benefit of endothermy, theIn considering the hostility created by the concomitant change in the thermal environment ofimmune response, there are fundamental differ- parasites will have escalated the evolutionary armsences between endothermic and ectothermic verte- race, so that parasite pathology and reproductivebrate hosts.The components of immune attack are output, in an endotherm, is also relentless.The evo-broadly equivalent (for instance, Xenopus has most lution of endothermy may therefore have been aof the immune attributes better-documented in critical factor in increasing the virulence of parasitemammals), but the intensity of interactions is infections, so that the capacity for creation of highlymodulated in ectotherms by natural temperature hostile environmental conditions has also selected forvariation.In ectotherms, host parasite interactions improved adaptation to survive in and circumventexperience periods of respite at low temperatures: these hostile conditions. CHAPTER 7Parasitism and environmentaldisturbancesKevin D.Lafferty1,2 and Armand M.Kuris2,3Several new diseases have gained celebrity status in recent years, fostering aparadigm that links environmental stress to increased emergence of disease.Habitat alteration, biodiversity loss, pollution, climate change, andintroduced species are increasing threats to the environment that arepostulated to lead to emerging diseases.However, theoretical predictions andempirical evidence indicate environmental disturbances may increase someinfectious diseases but will reduce others.may lack sufficient energy to mount an effective7.1 Introductiondefence (Rigby and Moret 2000), making them moreTo build a predictive framework for how environ-susceptible to opportunistic infections (Scott 1988;mental disturbances can affect parasitic diseases,Holmes 1996).But stress is not just fretting aboutwe limit our scope to those environmental dis-how to make an unreasonable deadline or frustra-turbances that result from human activities.tion over being late for an appointment while sittingAnthropogenic change that may affect parasitein stalled traffic.Toxic chemicals (Khan 1990), mal-communities can be divided into five broad types:nutrition (Beck and Levander 2000), and thermalhabitat alteration, biodiversity loss, pollution,stress (Harvell et al.1999) are all examples of stres-climate change, and introduced species.We do notsors hypothesized to increase individual suscepti-limit ourselves to the facile prediction that environ-bility to infectious diseases.This line of thoughtmental change will lead to increases in parasitism.suggests that environmental stress should aggrav-As we will make clear, there are substantial theoret-ate infectious disease.An opposing predictionical and empirical reasons to expect the oppositeemerges if one considers population dynamics.will also often result from such changes.Abundant species have more parasites (ArnebergWith the possible exception of invasive species,et al.1998a).The likelihood and impact of anenvironmental disturbances can collectively beepidemic increases with host density becauseconsidered as stressors (Lafferty and Kuris 1999)
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