Shape variation of cysticercoids of Hymenolepis diminuta (Cyclophyllidea) from fed, partially fed, and fasted Tribolium confusum (Coleoptera)

Quantitative studies of a crowding effect on cysticercoids of Hymenolepis diminuta in the intermediate host are few and limited in scope. In this study, we developed a technique to rapidly collect morphological information on large numbers of parasites, and verified the utility of geometric models for simple and accurate estimation of cysticercoid size for quantitative studies. These models were tested using measurements from 4,899 H. diminuta obtained from 666 Tribolium confusum exposed 1-4 wk previously. Length, width, and depth of the body and cercomer (when present) can be used in conjunction with these models to provide the most accurate estimation of parasite size. However, parasite body length alone can be used, with adjustment for effects of host diet and infection intensity, to predict the remaining measurements in incomplete specimens. Parasites that developed in higher intensity infections, or in hosts with reduced food intake, were narrower and had a proportionately shorter cercomer. Host age, sex, and mating status, and parasite age also had statistically significant, but small-magnitude, effects on parasite shape.     

Do heavy burdens of Schistocephalus solidus in juvenile threespine stickleback result in disaster for the parasite?

The diphyllobothriidean cestode Schistocephalus solidus typically infects threespine sticklebacks that are too small to allow the parasite to reach a mature size. As a result, the parasite must allow further growth of its host to reach the size at which it becomes competent to infect and reproduce in the definitive host. At times, however, intensity of infection can be high, leading to crowding among parasites and to heavy burdens causing mortality among hosts. Our data show that, during a previously observed epizootic, large percentages of plerocercoids (average 75% per host, 82% among all parasites pooled) did not grow to become massive enough in 1-yr-old threespine sticklebacks to be capable of establishment and maturation in the definitive host. Massive deaths of 1-yr-old sticklebacks due to infection during the epizootic resulted in the great misfortune of a disaster for a large number of parasites, resulting in dramatically reduced transmission of S. solidus.     

Proximate factors affecting the larval life history of Acanthocephalus lucii (Acanthocephala)

The growth and eventual size of larval helminths in their intermediate hosts presumably has a variety of fitness consequences. Therefore, elucidating the proximate factors affecting parasite development within intermediate hosts should provide insight into the evolution of parasite life histories. An experimental infection that resulted in heavy intensities of an acanthocephalan (Acanthocephalus lucii) in its isopod intermediate host (Asellus aquaticus) permitted the examination of parasite developmental responses to variable levels of resource availability and intraspecific competition. Isopods were infected by exposure to egg-containing fish feces, and larval infrapopulations were monitored throughout the course of A. lucii development. The relative rate of parasite growth slowed over time, and indications of resource constraints on developing parasites, e.g., crowding effects, were only observed in late infections. Consequently, the factors likely representative of resource availability to larval parasites (host size and molting rate) primarily affected parasite size in late infections. Moreover, at this stage of infection, competitive interactions, gauged by variation in worm size, seemed to be alleviated by greater resources, i.e., larger hosts that molted more frequently. The relatively rapid, unconstrained growth of young parasites may be worse for host viability than the slower, resource-limited growth of larger parasites.     

Parasites and the regional distribution of bumblebee species

Parasites and regional processes may be important to structure local species assemblages In particular, it has been hypothesized that widely distributed and abundant species should harbour more parasite species which could give them a competitive advantage in local species assemblages Empirical evidence bearing on these points are scarce and mainly restricted to vertebrate hosts or plants The aim of this study was to provide data in insect hosts and to test whether the patterns in field populations conform with those correlates expected from the parasite-host distribution hypothesis We investigated species assemblages of bumblebees at 12 different sites in a mesoscale region with their parasites over two consecutive years Parasites included dipteran and hymenopteran parasitoids. nematodes, mites, and protozoa The mean number of parasite species per host species ranged from 1 to 8 To account for sampling effort, all data were corrected for sample size effects The number of parasite species per average host individual (parasite load) ranged from 0 09 to 0 75 In cross-species comparisons, the number of parasite species per host species was positively correlated with regional distribution, i e the number of sites a host species occupied m the region, and with the average local host abundance The same relationships were found for parasite load In addition, parasite load correlated positively with average colony size of the host species, but not with body size of the individuals Bumblebee species were bimodally distributed When separated into widely-distributed and locally-occurring species, common hosts harboured more parasite species than rare ones Moreover, workers of common species individually had higher parasite loads From these results, we conclude that some of the necessary preconditions for parasites being able to affect the distribution and occurrence of their hosts are met in bumblebees The findings support a general pattern that parasite loads correlate positively with local abundance and geographical distribution of their hosts, also on mesoscales usually considered in ecological studies     

Metazoan parasites of whitefish Coregonus clupeaformis (Mitchill) and cisco C. artedii Lesueur from Southern Indian Lake, Manitoba

Metazoan parasites of whitefish Coregonus clupeaformis and cisco C. artedii from Southern Indian Lake, Manitoba were studied to reveal: species composition, differences with host age, sex, and location and season of capture. Whitefish hosted 19 species, 18 of which were also in cisco with generally lower intensity levels. Parasites exhibited definite patterns of abundance with host age and season, the primary causes being dietary and behavioural. No differences in parasite abundance existed between host sexes. Ranking of cisco parasites was significantly different between two sampling sites while whitefish parasites did not differ. Whitefish and cisco from sites 40 miles (64 km) apart had significantly different abundances of Tetracotyle intermedia but not Triaenophorus crassus . An increase in the abundance of copepod-vectored cestodes with a concomitant decrease in abundance of amphipod-vectored parasites is predicted after flooding and diversion.     

Parasitism, immune response and reproductive success in the house martin Delichonurbica

Parasites often exert strong selection pressures on their hosts that have evolved anti-parasite defences to counter the negative effects of parasites. We studied the relationship between intensity of parasitism, one aspect of host immune response, and host reproductive success, using the house martin bug Oeciacushirundinis and its house martin Delichonurbica host as a model system. Experimental manipulation of parasite load of nests during laying of the first clutch altered the intensity of parasitism. Parasites reduced the reproductive success of their hosts measured in terms of body condition and survival of nestlings. Host immune response, measured as the concentration of gammaglobulins and total plasma proteins, was positively associated with parasite reproduction, estimated as the number of juvenile parasites, but was only weakly related to the intensity of adult parasites. The concentration of gammaglobulins was negatively related to nestling body mass, implying a trade-off between immune function and body condition. Parasite reproduction thus exerts a cost on hosts by increasing the immune response. Received: 25 August 1997 / Accepted: 3 November 1997     

The effects of parasite age and intensity on variability in acanthocephalan-induced behavioural manipulation

Numerous parasites with complex life cycles are able to manipulate the behaviour of their intermediate host in a way that increases their trophic transmission to the definitive host. Pomphorhynchus laevis, an acanthocephalan parasite, is known to reverse the phototactic behaviour of its amphipod intermediate host, Gammarus pulex, leading to an increased predation by fish hosts. However, levels of behavioural manipulation exhibited by naturally-infected gammarids are extremely variable, with some individuals being strongly manipulated whilst others are almost not affected by infection. To investigate parasite age and parasite intensity as potential sources of this variation, we carried out controlled experimental infections on gammarids using parasites from two different populations. We first determined that parasite intensity increased with exposure dose, but found no relationship between infection and host mortality. Repeated measures confirmed that the parasite alters host behaviour only when it reaches the cystacanth stage which is infective for the definitive host. They also revealed, we believe for the first time, that the older the cystacanth, the more it manipulates its host. The age of the parasite is therefore a major source of variation in parasite manipulation. The number of parasites within a host was also a source of variation. Manipulation was higher in hosts infected by two parasites than in singly infected ones, but above this intensity, manipulation did not increase. Since the development time of the parasite was also different according to parasite intensity (it was longer in doubly infected hosts than in singly infected ones, but did not increase more in multi-infected hosts), individual parasite fitness could depend on the compromise between development time and manipulation efficiency. Finally, the two parasite populations tested induced slightly different degrees of behavioural manipulation.     

Phenology of recruitment and infection patterns of Ascocotyle pachycystis, a digenean parasite in the sheepshead minnow, Cyprinodon variegatus

In northwest Florida, Cyprinodon variegatus are parasitized by Ascocotyle pachycystis, a digenean parasite that forms metacercarial cysts in the lumen of the bulbus arteriosus. Field experiments revealed that fish accumulated parasites at an uneven rate within the highly seasonal trematode recruitment period. Older (= larger) fish had higher rates of parasite recruitment and higher parasite prevalences and densities (numbers of metacercariae per individual fish) than did younger (= smaller) fish. Nearly all adults were parasitized (prevalence range 70-100%), and parasite densities ranged from zero to 6800 per fish. Parasite distributions were clumped (= aggregated) in fish of all age classes but were less heterogeneous in early juveniles and adults than they were in late juveniles. Parasites affected the population dynamics of sheepshead minnows by causing reduced winter survivorship, as evidenced by an increase in the average size of fish and a decrease in the average infection intensity over the winter. This revised version was published online in July 2006 with corrections to the Cover Date.     

High parasite infection level in non‐native invasive species: it is just a matter of time

The enemy release hypothesis is often used to explain the success of non‐native species invasions. Growing evidence indicates that parasite or pathogen species richness increases over time in invasive non‐native species; however, this increase should not directly translate into release from enemy pressure as infection intensity of parasites (number of parasites per host) has a more profound impact on host fitness. The changes in intensity of parasitic infections in invasive non‐native species have not yet been thoroughly analysed in newly colonized areas. The goal of this study was to determine whether gastrointestinal parasite (nematode and trematode) infection intensity has increased with time since the populations of American mink Neovison vison were established and how host demographic parameters affect infection intensity. We tested the enemy release hypothesis by substituting space for time, evaluating parasite abundance in American mink at six sites along a chronosequence of mink invasion history. Nematode and trematode abundance increased with time since mink introduction, from a few parasites on average per mink after 16 yr, to 200–250 parasites per mink after 34 yr. The rate of increase in parasite abundance varied among demographic groups of mink (sex and age). Both nematodes and trematodes were more abundant in males than in females, and in subadults than in adults. Higher nematode abundance negatively affected body condition of mink. Our results provide evidence that non‐native species are released from enemy pressure only in the first phase of invasion, and that infection is modulated by host demographics and season. These results contribute to the evaluation of the long‐term patterns of parasite accumulation in invasive non‐native species after their colonization of new territories.     

Vertebrate diets derived from trophically transmitted fish parasites in the Bothnian Bay

Parasites that are transmitted through predator–prey interactions may be used as indicators of trophic relationships between organisms. Yet, they are rarely used as such in the construction of topological (predator–prey) food webs. We constructed food webs of vertebrate trophic interactions using observed diet alone, trophically transmitted parasites alone, and the combination of the two based on data from 31 species of fish from the Bothnian Bay, Finland. The fish food web contained 530 links derived from observed diet, 724 links inferred from parasitism, and 1,058 links calculated from a combination of both stomach contents and parasites. This sub-web constructed from stomach contents had a mean of 17.1 links per fish species, while that using parasites had 23.4 links per fish. Combining the two diet indicators yielded 34.1 links per fish species, illustrating the complementarity of the two methods. Mean number of prey species per fish species was 12.5 using observed diet items, 15.8 using parasites, and 24.5 using both measures together. Mean number of predators per fish species was 7.4 using observed diet, 11.7 using parasites and 15.0 using both. A positive correlation was found between the mean number of parasites and the number of prey taxa in the diet among the fishes. Omnivorous fish had the highest diversity of both parasite species and prey items, while benthophagous fish had among the lowest. Mean total abundance and mean total prevalence of parasites correlated positively with fish size, with piscivores being the largest with the highest abundance and prevalence, while planktivores and benthivores had the lowest. Trophically transmitted parasites may be used to help construct vertebrate sub-webs and derive information about food web processes. Parasites alone provided equivalent if not more information than observed diet. However, resolution is improved by using parasites and observed diet together.     

Fluctuating asymmetry in an insect host: a big role for big parasites?

Parasites are expected to be associated with host developmental instability because developmentally unstable hosts may be more susceptible to, or more frequently exposed to, parasitic infections compared with developmentally stable ones, or because parasites may directly disrupt host development. In this study, we analysed the relationship between developmental stability in the weta Pleioplectron simplex (Orthoptera) and infections by hairworm and mermithid parasites. These parasites have a long development in their host and the size of adult worms exceeds the length of the host by a considerable amount (15–20 cm). For one character (femora) we found that fluctuating asymmetry was significantly higher in parasitized individuals compared with unparasitized ones, in each of two samples collected in different years. Because no relationship was observed between the level of fluctuating asymmetry and the parasite size or number, however, infection by hairworm and mermithid parasites could be more a consequence of developmental instability than a cause. For other characters (tibia, external and internal spines of femora), the level of fluctuating asymmetry between parasitized and unparasitized individuals was not significantly different. Our results are discussed in relation to ecological constraints met by hairworm and mermithid parasites to complete their life cycle.     

The helminth parasites of the bullhead Cottus gobio (L.) and the stone loach Noemacheilus barbatulus (L.) from the River Avon, Hampshire

Specimens of Cottus gobio and Noemacheilus barbatulus from the River Avon, Hampshire, were examined between July 1970 and July 1971. Both fish were found to act as hosts for Nicolla gallica (adult), Pomphorhynchus laevis (adult) and Triaenophorus nodulosus (plerocercoid). The incidence and intensity of infection were lower in N. barbatulus than in C. gobio , and this was related to differences in host diet. A small number of N. barbatulus were also found to harbour Proteocephalus torulosus . Seasonal cycles of incidence and intensity were evident in N. gallica , with a peak occurring in the winter. P. laevis and T. nodulosus showed no such cycles. Increasing host size was associated with a higher infection of both N. gallica and P. laevis , but not T. nodulosus . The diet of the host was considered to be important in influencing this increase. Differences in host reproductive condition affected the population levels of N. gallica only, where an increased parasite burden was evident in gravid female fish. This was considered to influence the observed increase in incidence and intensity of this parasite during the winter. None of the parasites showed any seasonal maturation cycles. There was no evidence of any interactions between the parasite populations. Host diet and the availability of infective larvae were considered to be the most important features influencing the number of parasites which became established, and the rate at which this occurred. Temperature probably acted indirectly on the establishment of N. gallica , by controlling the availability of the larval stages, and also, by influencing the host reproductive condition, on the length of time for which the parasite was retained by the host.     

Constraints on parasite fecundity and transmission in an insect-STD system

Parasites that are sexually transmitted (causing sexually transmitted diseases, or STDs) can have important effects on host population dynamics, but we know almost nothing for such parasites about constraints on fecundity and transmission. In this study, we have examined the effect of two potentially important constraints in one of the few empirically well-studied animal-STD systems, the ladybird, Adalia bipunctata , and its sexually transmitted mite, Coccipolipus hippodamiae . Using a factorial design, we manipulated: (i) within-host competition, by varying infection intensity; and (ii) host condition, by introducing dietary stress. Infection with C . hippodamiae significantly reduced ladybird survival whether or not diet was restricted, and restricting diet led to reduced survival regardless of infection status. Increased infection intensity and reduced host condition (dietary stress) both independently constrained per capita rates of parasite egg production and the development of infective larvae. Furthermore, when host condition was compromised, significantly fewer larvae were transmitted per adult mite during copulation. The effect of infection intensity on per mite transmission was more complex: there was no significant effect when hosts were fed normally, but when the ladybirds were nutritionally stressed higher infection intensity was associated with a slight increase in the numbers that were transmitted (despite the fact that mite fecundity was reduced under these conditions). These results indicate that host condition and within-host competition may both play an important role in shaping the epidemiology of the A . bipunctata – C . hippodamiae system, by influencing the parasite's basic reproductive ratio (R₀) and the rate of epidemic spread. Our data also extend general insights into STD ecology, by highlighting the importance of constraints on disease dynamics that are likely to be widespread.     

The "crowding effect" in the cestode Schistocephalus solidus: density-dependent effects on plerocercoid size and infectivity

The occurrence of the crowding effect was demonstrated in plerocercoids of the cestode Schistocephalus solidus infecting threespine stickleback Gasterosteus aculeatus from Walby and Scout lakes, Alaska. Contrary to an earlier report, relatively large numbers of parasites (>3-4 plerocercoids) were observed to grow large enough in an intermediate host fish to become competent to infect and to mature in the definitive host under any of 3 assumed threshold values and 1 scenario of graded sizes for parasite competency. In Walby Lake, intensity and host body mass were significant predictors of mean plerocercoid mass per host, whereas intensity, host body mass, and combined parasite index were significant predictors in Scout Lake. Slopes of equations expressing the relationship between mean parasite mass and intensity for both lakes were less than 1, implying that processes other than or in combination with simple resource limitation might be producing the observed crowding effect. The causal mechanism for the crowding effect could include exploitative competition, interference competition, and host immune response. There were significant differences in infection between the two lakes, including different distributions of parasite intensities among hosts and different expressions of the crowding effect; however, an explanation of the differences awaits further investigation.     

Histopathological and ultrastructural studies of the tapeworm Monobothrium wageneri (Caryophyllidea) in the intestinal tract of tench Tinca tinca

Monobothrium wageneri is a monozoic caryophyllidean tapeworm of tench Tinca tinca. The pathological changes caused by this parasite within the intestinal tract of wild tench are described for the first time. Parasites were found attached to the anterior third of the intestine in tight clusters comprising up to 109 tapeworms. Infection was associated with the formation of raised inflammatory swellings surrounding the parasites. This host response, combined with the deep penetration of the scolex into the gut wall, formed a very firm seat of parasite attachment. Histopathological changes were characterised by a pronounced fibrogranulomatous lesion that extended through all layers of the intestine. This was accompanied by haemorrhage, oedema, necrosis and degeneration of the muscularis. A marked eosinophilic interface layer between the scolex of the tapeworm and gut wall indicated intimate host-parasite contact. Ultrastructural examinations revealed coniform spinitriches covering the neck and lateral sides of the scolex and capilliform filitriches present on the apical end of the scolex. Numerous glandular cytons (tegumental glands) were recorded throughout the scolex tegument. Large numbers of secretory granules discharged from the glands through a network of processes onto the scolex surface were consistent with distancing the cellular responses of the host. Observations of severe inflammatory lesions, partial intestinal occlusion and the potential for intestinal perforation represent important pathological changes that are consistent with loss of normal gut function. The lesions associated with the attachment of M. wageneri are more severe than those recorded for any other tapeworm of British freshwater fish.     

Competitive Growth Strategies in Intermediate Hosts: Experimental Tests of a Parasite life-History Model Using the Cestode, Schistocephalus solidus

In parasites with a complex life cycle, the fitness of an individual depends on its probability of reaching the final host and on its fecundity. Because larval growth in intermediate hosts may affect both transmission and adult size, selection should optimize growth patterns that are conditional on the presence and number of conspecific competitors. A recent model predicts that the total parasite volume per host should increase with intensity if larvae are able to vary growth depending on the number of conspecifics in the host (Life History Strategy hypothesis, i.e. LHS). Further, we would here expect growth rates to increase with intensity. By contrast, under the simplest alternative hypothesis of Resource Constraints (i.e. RC), the total parasite volume should remain constant. We experimentally infected copepods Macrocyclops albidus with the cestode Schistocephalus solidus to achieve 1, 2 or 3 parasites per host taking care that hosts had similar quality status at each infection level, and compared larval growth trajectories at the three intensity levels. The asymptotic total parasite volume was larger in double and triple infections than in single infections. Furthermore, the asymptotic total parasite volume was significantly larger in triple than in double infections but only in larger copepods that were less constrained by a host-size ceiling effect. These results, together with the fact that growth rates increased with intensity, support the LHS hypothesis: procercoids of a tapeworm may “count” their conspecific competitors in their first intermediate host to harvest its resources strategically until the next step in their complex life cycle. Co-ordinating editor: A. Biere     

Temperature and development in host-parasite relationships

Summary The effects of temperature on growth and development of the cabbage butterfly, Pieris rapae, and three wasp parasites: Apanteles rubecula, Apanteles glomeratus and Pteromalus puparum in Vancouver, Canada, and Canberra, Australia, are examined. We compare the estimates of temperature threshold for development and the number of degree-days above this threshold required to complete development for the immature stages of all species in both localities. Developmental patterns of both the host and its parasites differ between localities. Within the range of temperatures likely to be experienced during the host's breeding season, Australian parasites have longer generation times than their host at low temperatures and shorter generation times at high temperatures. Canadian parasites have shorter generation times, relative to the host, at all temperatures. This may be necessitated by the shorter breeding season available to the Canadian parasites.Besides temperature, parasite development is affected by host size and, in the gregarious species, parasite density. Host larval development is retarded by both Apanteles.All parasites are smaller at higher temperatures and males are smaller than females, but size is also affected by host size and parasite density.Although parasite size, and consequently fecundity, varies greatly, parasites experiencing similar temperatures will have closely similar developmental periods. The ecological significance of these developmental responses is discussed.     

A meta‐analysis of ant social parasitism: host characteristics of different parasitism types and a test of Emery’s rule

Abstract 1. In ant social parasitism, the process by which parasite–host systems evolved and the types of invasion mechanisms parasites use are being debated. Emery’s rule, for example, states that social parasites are the closest relatives to their hosts. The present study uses previously published data to test whether Emery’s rule applies equally to all parasitism types (i.e. xenobiosis, temporary, dulosis, and inquilinism). In addition, this study also investigates other links between parasite–host relatedness and host biology, which has implications for understanding the invasion mechanisms used by certain parasites. 2. We find that xenobiotic parasites typically use distantly‐related host species that are of at least medium colony size. Temporary parasites often have multiple host species that are very closely related to the parasite and hosts with medium‐size colonies. Dulotic parasites frequently have multiple host species that are slightly less related and of any size. Lastly, inquiline parasites tend to have a single, very closely related, host species with medium‐size colonies. 3. Parasites tend to be more closely related to host species if they have a single host species or when the host has a large colony size. In contrast, parasites with multiple host species or hosts of small colony size tend to be less related to their hosts. 4. This study is the first to examine trends in ant social parasitism across all known parasite species. Our meta‐analysis shows that Emery’s rule applies to inquilinism and temporary parasitism, but not to dulosis and xenobiosis. Our results also suggest that both parasitism type and parasite–host relatedness predict the number of hosts and host colony size. It may be that a chemical mimicry mechanism allows invasion of large host colonies, but requires close relatedness of parasite and host, and concentration on a single host species.     

Sex-specific response of a mosquito to parasites and crowding

Host-parasite interactions are significantly influenced by the sex of the host and the environment in which the host is found. Sex-specific responses to parasite infection, however, may change according to the host environment. I examine the combined effect of parasite infection and crowding on males and females of the mosquito Aedes albopictus. At a high larval density, infected males experienced a greater relative reduction in body size than did infected females, whereas the pattern was reversed at low density. This experiment demonstrates the importance of the environment on sex-specific responses to parasites and contributes to a growing body of work examining sources of variation in host-parasite interactions.