Academic literature on the topic 'Host-parasite relationships ; Insects – Parasites'

It is probably a truism to say that what seems to be a single organism is, in fact, an assemblage of organisms – for there can hardly be an individual living outside a specialized laboratory that does not have commensals or parasites living within. The insects, the most diverse and numerous group of organisms on earth are no exception, and with micro- and macro-parasites from a wide range of taxa generate a remarkable range of interspecific associations. In some cases the insect is the sole host, in others it is an intermediate host or vector. It is the latter relationship which attracts much attention when insects and arachnids transmit infectious agents to humans, their animals or crops. Knowledge of the parasites of insects provides us with an opportunity to develop novel control methods for pests. Despite the diverse and widespread nature of insect infections, their impact on human well-being and the opportunity they give us to understand the complexity of the natural world, the subject remains a surprisingly neglected field. In this volume we endeavour to draw the veil from the 'black-box' approach to the insect stages of parasite life cycles to reveal some of the complexities of these relationships and how they are currently being analysed.

The catalogue includes more than 700 parasites of domestic and wild animals recorded at the National Veterinary Laboratory, Papua New Guinea, since data began to be gathered at the end of World War 2. It incorporates some information already published and data on parasites, particularly of indigenous fauna, not recorded previously in the country. Wildlife host species include wild pig, deer, bats, murine rodents, marsupials, monotremes, birds, reptiles, amphibians, fishes and invertebrates. The range of parasites in domestic and many wild animals shows great affinity with that found in Australia. Some notable exceptions amongst domestic animal parasites are the endoparasites Trichinella papuae, Capillaria papuensis and Mammomonogamus laryngeus and the economically significant ectoparasites Chrysomya bezziana, Tropilaelaps mercedesae and Varroa jacobsoni that are not recorded in Australia. Unusual host-parasite associations include the larvae of the insects Chrysomya spp. and Lucilia sp., parasites of warm-blooded animals, infesting, respectively, cold-blooded crocodiles and cane toads, and the mammalian mite, Sarcoptes scabiei, on an avian host, cassowaries. No host switching of helminths was seen between domestic and wild animals, or between populations of deer, wild pigs and wallabies when grazing together. The economic importance of certain parasites for domestic animals, the potential threats from introduced or newly-discovered parasites, and the relationship between some parasites and their wildlife hosts, are discussed. Information is presented in two tables: a parasite–host list that includes the location of a parasite in or on a host as well as a list of references of relevance to the country, and a host–parasite list that contains the distribution of the parasites according to province or locality.

Abstract Temporary but substantial reductions in voluntary food intake routinely accompany parasite infection in hosts ranging from insects to humans. This “parasite-mediated anorexia” drives dynamic nutrient-dependent feedbacks within and among hosts, which should alter the fitness of both hosts and parasites. Yet, few studies have examined the evolutionary and epidemiological consequences of this ubiquitous but overlooked component of infection. Moreover, numerous biomedical, veterinary, and farming practices (e.g., rapid biomass production via high-calorie or high-fat diets, low-level antibiotics to promote growth, nutritional supplementation, nonsteroidal anti-inflammatory drugs like Ibuprofen) directly or indirectly alter the magnitude of host anorexia—while also controlling host diet and therefore the nutrients available to hosts and parasites. Here, we show that anorexia can enhance or diminish disease severity, depending on whether the current dietary context provides nutrients that bolster or inhibit immune function. Feedbacks driven by nutrition-mediated competition between host immune function and parasite production can create a unimodal relationship between anorexia and parasite fitness. Subsequently, depending on the host’s diet, medical or husbandry practices that suppress anorexia could backfire, and inadvertently select for more virulent parasites and larger epidemics. These findings carry implications for the development of integrated treatment programs that consider links between host feeding behavior, nutrition, and disease severity.

Felled loblolly and slash pine trees were infested with Ips calligraphus (Germar) (80.2% of Ips population), I. avulsus (Eichhoff) (11.2%), and I. grandicollis (Eichhoff) (8.2%). A complex of beneficials associated with the Ips spp. broods consisted of 27 known or suspected insect predator species and 10 species of parasites. Predators comprised 98.8% and parasites 1.2% of the total beneficial insects collected. The predators Lonchaea sp. (Diptera: Lonchaeidae), Aulordum spp. (Coleoptera: Colydiidae), staphylinids and histerids (Coleoptera), and Scoloposcelis mississippensis (Drake and Harris) (Hemiptera: Anthocoridae) comprised 44.7, 6.8, 6.0, and 4.3%, respectively, of the total beneficial insect complex. The most abundant parasite was Roptrocerus eccoptogastri Ratzeburg, which accounted for 37.3% of all parasites but only 0.5% of the total beneficial insect complex. Abundance of both the beneficial insect complex and their Ips hosts was highest in trees felled in May and lowest in trees felled during August, suggesting a possible densitydependent relationship between the beneficials and the Ips spp. populations. Plegaderus sp. was the only species to show consistent preference for host tree, being more abundant in slash pine than on loblolly.

Insect societies play a crucial role in the functioning of most ecosystems and have fascinated both scientists and the lay public for centuries. Despite the long history of study, we are still far from understanding how insect societies have evolved and how social cohesion in their colonies is maintained. Here we suggest inquiline social parasites of insect societies as an under-exploited experimental tool for understanding sociality. We draw on examples from obligate inquiline (permanent) social parasites in wasps, ants and bees to illustrate how these parasites may allow us to better understand societies and learn more about the evolution and functioning of insect societies. We highlight three main features of these social parasite–host systems—namely, close phylogenetic relationships, strong selective pressures arising from coevolution and multiple independent origins—that make inquiline social parasites particularly suited for this aim; we propose a conceptual comparative framework that considers trait losses, gains and modifications in social parasite–host systems. We give examples of how this framework can reveal the more elusive secrets of sociality by focusing on two cornerstones of sociality: communication and reproductive division of labour. Together with social parasites in other taxonomic groups, such as cuckoos in birds, social parasitism has a great potential to reveal the mechanisms and evolution of complex social groups. This article is part of the theme issue ‘The coevolutionary biology of brood parasitism: from mechanism to pattern’.

AbstractThe quantification of host–parasite associations from field data is a fundamental step towards understanding host–parasite and host-parasite–pathogen dynamics. For parasites that are not rigid host specialists, exemplified in this paper by ticks, the interpretation of host–parasite association data is difficult. Interpretations of tick collection records have largely assumed that off-host collection records offer a valid basis from which to make claims about the host specificity or generality of tick species. A simple simulation analysis of rudimentary tick–host interactions in a hypothetical 50 × 50-cell habitat demonstrates that perceptions of tick–host relationships can be strongly biased by spatial patterns. Regardless of their true level of host specificity or generality, it seems that: (i) more abundant ticks will be perceived as generalists, while rarer species will be considered specialists; and (ii) tick species that have patchy, strongly aggregated distributions will be more likely to be perceived as host specialists than species that have more dispersed or uniform distributions. Since all available evidence suggests that abundances and spatial patterns vary between tick species, there is no way of assessing the true validity of claims about host specificity without first undertaking detailed research on the relative abundances and spatial and temporal patterns of both tick and host distributions.

AbstractUnicellular flagellates of the family Trypanosomatidae are obligatory parasites of invertebrates, vertebrates and plants. Dixenous species are aetiological agents of a number of diseases in humans, domestic animals and plants. Their monoxenous relatives are restricted to insects. Because of the high biological diversity, adaptability to dramatically different environmental conditions, and omnipresence, these protists have major impact on all biotic communities that still needs to be fully elucidated. In addition, as these organisms represent a highly divergent evolutionary lineage, they are strikingly different from the common ‘model system’ eukaryotes, such as some mammals, plants or fungi. A number of excellent reviews, published over the past decade, were dedicated to specialized topics from the areas of trypanosomatid molecular and cell biology, biochemistry, host–parasite relationships or other aspects of these fascinating organisms. However, there is a need for a more comprehensive review that summarizing recent advances in the studies of trypanosomatids in the last 30 years, a task, which we tried to accomplish with the current paper.

AbstractMermithid nematodes (Nematoda: Mermithida: Mermithidae) parasitize a wide range of both terrestrial and aquatic invertebrate hosts, yet are recorded in bumble bees (Insecta: Hymenoptera: Apidae: Bombus) only six times historically. Little is known about the specific identity of these parasites. In a single-season nationwide survey of internal parasites of 3646 bumble bees, we encountered six additional instances of mermithid parasitism in four bumble bee species and genetically characterized them using two regions of 18S to identify the specific host–parasite relationships. Three samples from the northeastern USA are morphologically and genetically identified as Mermis nigrescens, whereas three specimens collected from a single agricultural locality in the southeast USA fell into a clade with currently undescribed species. Nucleotide sequences of the V2–V6 region of 18S from the southeastern specimens were 2.6–3.0% divergent from one another, and 2.2–4.0% dissimilar to the nearest matches to available data. The dearth of available data prohibits positive identification of this parasite and its affinity for specific bumble bee hosts. By doubling the records of mermithid parasitism of bumble bee hosts and providing genetic data, this work will inform future investigations of this rare phenomenon.

AbstractThe genus Picobia Haller (Acari: Syringophilidae) is revised. The species of this genus are permanent parasites living in quills of bird body feathers. In total 19 species are recognized. Four new species are described from passerine birds (Passeriformes): Picobia locustella sp. n. from Locustella naevia (Sylviidae), P. biarmicus sp. n. from Panurus biarmicus (Panuridae), P. sturni sp. n. from Sturnus vulgaris (Sturnidae) and P. cissa sp. n. from Cissa chinensis (Corvidae). Two species are redescribed: P. dryobatis (Fritsch) and P. zumpti (Lawrence). A neotype for P. dryobatis is designated. A new diagnosis for the genus Picobia and a key to females are provided. Parsimony analysis of the genus is conducted. Host-parasite relationships are briefly discussed.

Background: Wolbachia is the most common endosymbiotic bacteria in insectborne parasites and it is the most common reproductive parasite in the world. Wolbachia has been found worldwide in numerous arthropod and parasite species, including insects, terrestrial isopods, spiders, mites and filarial nematodes. There is a complicated relationship between Wolbachia and its hosts and in some cases, they create a mutual relationship instead of a parasitic relationship. Some species are not able to reproduce in the absence of infection with Wolbachia. Thus, the use of existing strains of Wolbachia bacteria offers a potential strategy for the control of the population of mosquitoes and other pests and diseases. Methods: We searched ten databases and reviewed published papers regarding the role of Wolbachia as a promising drug target and emerging biological control agents of parasitic diseases between 1996 and 2017 (22 years) were considered eligible. Also, in the current study several patents (WO008652), (US7723062), and (US 0345249 A1) were reviewed. Results: Endosymbiotic Wolbachia bacteria, which are inherited from mothers, is transmitted to mosquitoes and interferes with pathogen transmission. They can change the reproduction of their host. Wolbachia is transmitted through the cytoplasm of eggs and have evolved different mechanisms for manipulating the reproduction of its hosts, including the induction of reproductive incompatibility, parthenogenesis, and feminization. The extensive effects of Wolbachia on reproduction and host fitness have made Wolbachia the issue of growing attention as a potential biocontrol agent. Conclusion: Wolbachia has opened a new window to design a costly, potent and ecofriendly drug target for effective treatment and elimination of vector-borne parasitic diseases.

The ability to mount an efficient immune response should be an important life-history trait as parasitism can impact upon an individual's fecundity and survival prospects, and hence its fitness. However, immune function is likely to be costly as resources must be divided between many important traits. Whilst many studies have examined host resistance to particular parasite types, fewer have considered general immune responses. Studies that have considered general immune responses tend to do so in vertebrate models. However, the complexity of the vertebrate immune system makes the examination of evolutionary aspects of immune function difficult. Using larvae of the genus Spodoptera (Lepidoptera: Noctuidae) as a model system, this study examines' genetic and phenotypic aspects of innate immunity. The aims were to assess the levels of additive genetic variation maintained in immune traits, to consider possible costs that could maintain this variation, and to assess the role of phenotypic plasticity in ameliorating those costs. A key finding of this study was that high levels of additive genetic variation were maintained in all of the measured Immune traits. Analysis of the genetic correlations between traits revealed potential trade-offs within the immune system and between immune components and body condition. In addition, it was shown that larvae living at high densities invest more in immune function than those living in solitary conditions, suggesting that larvae can minimise the costs of immune function by employing them only when the risk of pathogenesis is high.

Providence virus (PrV) is a member of the Tetraviridae, a family of small, positive sense, single-stranded RNA viruses, which characteristically infect the midgut tissue of heliothine larvae. PrV is the only known tetravirus that replicates in cultured insect cells. The virus comprises a monopartite genome resembling members of the genus Betatetravirus with the capsid precursor protein undergoing autoproteolytic cleavage at its C-terminus consistent with other tetravirus capsid precursor proteins. Analysis of viral cDNA predicted the presence of three potential overlapping gene products (from 5` to 3`): (1) p130, a protein of unrecognized nucleotide or amino acid homology with a 2A-like processing site at its N-terminus; (2) p104, the replicase ORF, which was found to be phylogenetically related to tombus-and umbraviruses replicases. The presence of a read-through stop signal in the p104 ORF was proposed to produce and amino terminal product with a predicted MW of 40 kDa (p40) and (3) the capsid protein precursor (81 kDa) which has two 2A-like processing sites at its N-terminus. Metabolic radiolabelling of viral translation products in persistently infected MG8 cells and in vitro translation of the individual ORFs were performed in order to analyse the expression of PrV gene products. p130 was translated with no evidence of 2A-like processing. Two products of 40 kDa and 104 kDa were translated from the p104 ORF, indicating that the read-through stop signal was likely to be functional. Finally, the capsid protein precursor ORF produced a major translation product of 68 kDa corresponding to the capsid protein precursor as well a peptide of 15 kDa that was attributed to the activity of the second 2A-like site at the N-terminus of the p81 ORF. The subcellular distribution of viral RNA (vRNA) and p40 in MG8 cells was investigated using immunofluorescence and biochemical fractionation. The results showed that p40/p104 and vRNA accumulated in polarized, punctate structures in some but not all MG8 cells and in some cases, co-localization was observed. This thesis concludes that PrV is a novel tetravirus with significant similarities plant carmolike viruses that should be re-classified at the family level.

The success of an invasive alien species is often attributed to the ecological advantage gained from natural enemy release. Numerous factors have been suggested as contributing to the success of Harmonia axyridis as an invasive alien species, including enemy release. This thesis studied the interactions of several parasites with H. axyridis, investigating parasite transmission, growth and virulence as well as host immune responses, thereby shedding light on the potential role of enemy release in the invasion biology of this ladybird. Benefits gained by invasive alien species from enemy release diminish if parasites of native species shift hosts to exploit the novel invader. The fungal ectoparasite Hesperomyces virescens began infecting H. axyridis shortly after it invaded the UK, probably as a result of a host shift from Adalia bipunctata. This study found a rapid increase in H. virescens prevalence over three years in London H. axyridis populations. Laboratory study showed H. virescens transmission and growth to be more efficient on A. bipunctata than the novel host. In addition, reciprocal interspecific transfers of H. virescens strains isolated from A. bipunctata and H. axyridis revealed that the infection characteristics of the fungi from these two hosts differed, suggesting strains may have diverged after the initial shift from A. bipunctata to better exploit the host from which they were derived. Laboulbenialian fungi were previously thought to have negligible impacts on host fitness. A detailed examination of H. virescens infecting H. axyridis found distinct virulence, with infections resulting in a 50% reduction in host lifespan. In addition, chronic H. virescens infection in males caused acceleration in the age-associated decline in body condition while for females, infection triggered fecundity senescence and a faster age-related decline in fertility. While their role in accelerating ageing is debated, the results presented here provide evidence that infectious diseases can drive the ageing process in this insect species. In nature, multiple parasites affecting a single host are common. The effect of co-infection on the virulence caused by two fungal infections was characterised using H. axyridis and A. bipunctata hosts. The ability of two ladybird species to defend against an acute fungal parasite, while infected with the relatively avirulent H. virescens was found to be sex-specific. While for females, the presence of co-infection did not alter the virulence seen in singly infected females, a higher mortality rate existed for co-infected males compared with those infected singly. Previously, H. virescens has been considered to be avirulent, however, this study provides evidence that this chronic fungal parasite may be important when considering the mortality associated with co-infections in the field. The invasive success of H. axyridis has, in part, been attributed to a more vigorous immune ability compared with other competitor species. Previously, field studies have shown that the prevalence of the parasitoid wasp Dinocampus coccinellae in H. axyridis is considerably lower than in the UK primary host of this wasp, Coccinella septempunctata. The extent to which the prevalence asymmetry in the field is driven by differences in host encapsulation response was tested by first comparing the encapsulation ability of C. septempunctata and H. axyridis directed against an artificial implant. Following this, the encapsulation response of D. coccinellae parasitized individuals was assessed and compared between the two host species. While encapsulation ability did not differ between the host species, and D. coccinellae did not affect the immune response of H. axyridis, wasp parasitism did alter the encapsulation ability of C. septempunctata, although it was inconsistent across sexes and populations. Overall, this thesis furthers our understanding of the fungal parasite H. virescens and its association with the notorious invader H. axyridis. The research presented here also demonstrates the use of H. axyridis as a model system in areas other than invasion ecology and furthermore, contributes to understanding the role of infectious disease in the rate of ageing. Finally, sex-specific effects were found across the chapters of this thesis, demonstrating the use of H. axyridis in the study of sex-specific effects of infections.

Most, if not all, animals will at some stage in their lives encounter parasites. Some of the most widespread and abundant parasites belong to the Class Trematoda. Trematodes often have a substantial negative impact on individual intermediate host ecology. In this thesis, I investigate ecological and evolutionary consequences of the interaction between the microphallid trematode, Maritrema novaezealandensis, and its intermediate snail and crustacean hosts. Parasites often show a heterogeneous spatial distribution pattern in natural animal host populations. In this study, factors determining the spatial distribution of larval trematodes in Zeacumantus subcarinatus were investigated at two spatial scales (within and among bays). The distribution of shorebird definitive hosts explained a significant amount of the variation in the distribution of trematodes among bays. However, within a bay, other factors override the effect of bird distribution. The influence of larval trematodes on reproduction, survival and population density of Z. subcarinatus was investigated using laboratory and field studies. In the laboratory, it was found that larval trematodes induce castration and mortality of Z. subcarinatus. The field study revealed that the local prevalence of trematodes had a significant negative effect on population density of Z. subcarinatus. Through castration, trematodes act as strong selective agents on snail host life history. The effect of trematodes on life history characteristics (reproductive effort, juvenile growth, size at maturity and susceptibility to trematode infections) were investigated among natural populations of Z subcarinatus. Reproductive effort was not higher for uninfected females from populations where the risk of becoming infected was high. However, offspring from those populations were significantly larger, and laboratory-reared juveniles grew significantly faster than conspecifics from other populations. In addition, size at maturity was negatively correlated with trematode prevalence across snail populations. Z. subcarinatus thus adapts to a high local risk of trematode infection by reaching maturity early, thereby increasing the chance of reproducing. The influence of M. novaezealandensis on the survial of the amphipod host, Paracalliope novizealandiae was examined using experimental infections and field observations. The experimental infections demonstrated that parasite-induced mortality was intensity-dependent. The number of M. novaezealandensis per amphipod was too low to significantly induce host mortality in the field. However, the transmission strategy of this parasite allows it to affect host populations during weather conditions ideal for trematode transmission. Trematode strategies in the second intermediate host are important to the understanding of host-parasite co-evolution and the evolution of parasite life cycles. In this study, potential density-dependent effects at the metacercarial stage on size and fecundity of in vitro adult M. novaezealandensis was examined in both experimentally infected P. novizealandiae and naturally infected Macrophthalmus hirtipes. For this purpose, a method to excyst and cultivate M. novaezealandensis metacercariae to an egg producing stage, was developed. Naturally infected M. hirtipes also harboured larval stages of three other helminths. Crowding effects in the two crustacean hosts were expressed as a decreased volume and smaller egg production of in vitro adult M. novaezealandensis. In addition, interspecific interactions among parasite species were observed in crab hosts. The work in this thesis provides evidence that M. novaezealandensis significantly and negatively affect intermediate host ecology. The heterogeneous distribution of trematodes causes differential effects among host populations with subsequent effects on the life history of snail hosts. In addition, this study demonstrates that parasites interact within their second intermediate host with possible implications for the way parasites exploit their hosts.

Lernaeocera branchialis (Linnaeus, 1767) is a parasitic copepod possessing a complex dual-host lifecycle. The “definitive” gadoid hosts, including Gadus morhua (Atlantic cod), Melanogrammus aeglefinus (haddock) and Merlangius merlangus (whiting), are infected by the fertilised female, which penetrates the host’s ventral aorta or bulbus arteriosus whilst undertaking extensive metamorphosis and a haematophagous lifestyle. The pathogenic effects of this activity upon the host have been well documented and mortality may occur, especially when multiple parasites are present. These negative impacts on cod, particularly juveniles, by L. branchialis have the potential to adversely affect cod aquaculture in the future, and already vulnerable wild cod stocks. This PhD project therefore, investigated the immune response of wild haddock and cultured-cod post-infection by L. branchialis, and the possible mechanisms by which the parasite modulates/evades the host’s immune response. The systemic immune response of both wild haddock and cultured-cod post-infection by L. branchialis depended on the maturation stage of the parasite, and in the former host species, upon the infection intensity. Wild haddock harbouring fully metamorphosed females showed an increase in circulating thrombocytes and a decrease in serum protein levels however; if multiple mature L. branchialis were present the haddock possessed reduced circulating monocytes, and increased circulating thrombocytes and serum anti-trypsin activity. Infection by L. branchialis was also associated with a suppressive effect on haddock serum spontaneous haemolytic activity. These responses were thought to be due to the host trying to counteract the increased damage caused by the massive increase in size and the feeding of the mature parasite, which is more pronounced when multiple parasites are present, resulting in the increase in some parameters and the ‘consumption’ of others. However, the effect of parasite-derived secretions and other pathogens due to observations on wild fish could not be discounted. The laboratory-infection of cultured-cod from two different sources was also performed in order to study the immune response over time. The two groups of cod showed differences in their immune response to L. branchialis. The first group showed suppressed respiratory burst activity of phagocytes, as the parasite reached the early penella sub-stage, whilst no suppression in phagocyte respiratory burst activity was found in the second group. The parasite was found to migrate along the afferent branchial artery of the cod where a thrombus formed and was present throughout its migration into the ventral aorta. At 14 d post-infection, leukocytes expressing Interleukin 8 mRNA were observed within the free-flowing blood at the periphery of the organising thrombus within the lumen of the ventral aorta. This was speculated to aid the recruitment and activation of leukocytes to the site, and the maturation and neovascularisation of granulation tissue. The infection of the second group subsided with the death of the parasite, and none of the parasites metamorphosed past the early penella sub-stage. The live parasites infecting the first group of cod did not possess IgM or complement component C3 binding on their cuticle, however, both IgM and C3 binding occurred on the dead parasites in the second infection trial. This may highlight the importance of these opsonins and the cytotoxic effect of phagocytes in the elimination of L. branchialis by some cod. However, the first infection was terminated as the parasite reached the early penella sub-stage due to a loss of stock cod prior to the study, so the long-term success of the infection can not be concluded. Therefore, the immune response to infection needs to be determined over the entire metamorphosis of L. branchialis to determine whether the infection was successful or not, and preferably in populations with varying susceptibility to L. branchialis. This will not be possible without further studies into the resistance of different stocks of cultured-cod. Many arthropod parasites, such as ticks and salmon lice, have been previously documented to produce pharmacologically active secretions, aiding host invasion and parasite feeding, preventing the host immune response from working effectively against the parasite, all aimed at improving survival of the parasite. Therefore, the effects of the secretory/excretory products (SEPs) produced during the initial infective stage and by the mature, fully metamorphosed female on the immune response of cultured-cod in vitro, and the location of exocrine glands associated with the oral region of the parasite were investigated. The SEPs from the infective stage of the parasite were found not to affect the intracellular hydrogen peroxide (H2O2) production of phagocytes. The practical difficulties in collecting large quantities of the SEPs from the infective stage meant that their effects could not be tested on the other host immune parameters studied. The SEPs from fully metamorphosed female L. branchialis, however, had a number of suppressive effects on the host immune response in vitro including: 1) suppression of the intracellular production of cytotoxic H2O2 during the respiratory burst of phagocytic leukocytes post-PMA stimulation, 2) suppression of the production of macrophage activating factor by leukocytes with a priming effect on naïve phagocyte function, and 3) suppression of the chemo-attraction ‘power’ of zymosan activated cod serum, i.e. anaphylatoxin activity, on head kidney-derived leukocytes. These effects were dose-dependent, and highlight the capacity of L. branchialis to suppress its host’s innate immune response at the local feeding area. Further work is required to establish the mechanisms by which the parasite-derived SEPs suppress these host immune parameters, and to identify which molecules produced by the parasite are responsible. The correlation between these in vitro results, and systemic immune parameters measured from laboratory-infected Atlantic cod and wild infected haddock are discussed. Host immuno-modulation by other arthropod parasites is mediated by pharmacologically active secretions produced by exocrine glands. Therefore, the exocrine glands of the infective and fully metamorphosed female L. branchialis were also investigated in order to identify those that might be responsible for the secretion of host-modifying products. Adult female exocrine glands were mapped using diaminobenzidine (DAB), most commonly known to stain peroxidases and catalases. These compounds are known to be involved in the neutralisation of harmful free radicals which are released during the respiratory burst and tissue damage. Such products may therefore be important protective secretory components at the site of feeding / infection. Exocrine glands were located in the infective stage associated with the oral region, one pair termed the anterior gland complex (AGC), and the other pair extending either side of the oral cone termed the circum-oral glands (CG). These were further investigated using light microscopy and transmission electron microcopy. The AGC and CGs possessed multi-component secretions and they possessed secretory vesicles, abundant and highly active rough endoplasmic reticulum and Golgi apparatus suggesting that protein is an important component of the secretory products. These glands were also observed in the fully metamorphosed females where they had increased in size within the cephalothorax post-metamorphosis. It is hoped that the identification of these glandular structures, which are thought to secrete within the local vicinity of the oral cone, will aid future studies regarding the identification and secretion kinetics of parasite-derived molecules during the infection and feeding process.

An experimental study of laboratory populations of the stored-products moth, Cadra cautella (Lepidoptera: Phycitidae) and its larval parasitoid, Venturia canescens (Hymenoptera: Ichneumonidae) identified and quantified density- and age-dependent demographic characteristics of the host-parasitoid system. Host imago longevity and fecundity depended on larval weight at pupation. Observed effects of C. cautella larval competition for food on larval mortality, stage duration, and weight at pupation were successfully captured in a mathematical model. Host larval age significantly influenced inter-stage cannibalism and susceptibility to mortality resulting from parasitoid oviposition wounds. Both larval parasitoid developmental rates and adult parasitoid attack rates depended on host larval age. Long-term population experiments of host and host-parasitoid populations revealed that host populations fluctuated with a period slightly in excess of host generation time and that parasitoid populations were in synchrony with host populations.

This chapter highlights the use of consensus techniques and other approaches for understanding historical biogeographic relationships and patterns of associations between parasites and their hosts. It deals primarily with analyses that are often placed under the heading of cospeciation or codivergence, situations in which hosts and their parasites appear to have intimate, long-standing historical connections and in which speciation in the host may result in speciation in the parasite. This type of association appears to obtain for many internal parasites and for certain external parasites, such as lice. In addition, or on the other hand, there is a whole class of host associations, such as those between herbivorous insects and their food plants, in which the relationship of the parasite and the host generally does not show such long-term fidelity but involves many apparent host shifts. These latter situations are often referred to under the more liberal heading of coevolution.