Lack of preference for infective faeces in Hymenolepis diminuta-infected beetles (Tenebrio molitor)

The beetle-tapeworm life cycle provides a convenient system to study how host behaviour influences the probability of re-infection because initial and secondary infections can be tracked. The beetle, Tenebrio molitor, is infected with the tapeworm Hymenolepis diminuta when it ingests rat faeces containing tapeworm eggs, which upon hatching undergo five morphologically distinct stages while developing inside the beetle. In a series of preference trials, both individual and groups of previously infected beetles were exposed to baits of infective (faeces with eggs) and uninfective faeces. Beetles did not differ in the amount of time spent or in the number of occurrences at each bait type, suggesting that infected beetles show no preference for infective faeces. This may be a host adaptation to avoid further infection, parasite manipulation to avoid competition for host resources, or both. Further, once infected, beetles are no more or no less likely to become re-infected than uninfected beetles. An analysis of the mean and variance of infection suggests that some individuals are highly susceptible to and some are highly resistant to infection, with males being more variable than females. This could explain the higher load of cysticercoids observed in males.     

Sequential and concurrent exposure of flour beetles ( Tribolium confusum ) to tapeworms ( Hymenolepis diminuta ) and pesticide (diatomaceous earth)

The response of Tribolium confusum to sublethal levels of 2 environmental stressors was studied, i.e., parasitic infection represented by the cestode Hymenolepis diminuta , and a physical stressor represented by the natural pesticide diatomaceous earth (DE). These were applied sequentially (DE, then infection) to detect indirect or carryover effects of DE, and concurrently (DE applied immediately after exposure to parasites and DE presence maintained throughout the infection) to detect direct effects of DE. DE alone, but not parasitism alone, produced significant host mortality, and concurrent treatment with DE and parasitism did not increase mortality over DE alone. Parasite abundance was significantly higher following sequential, but not concurrent, DE exposure. Parasite abundance in mated hosts was significantly higher than in virgin hosts. Parasitic infection resulted in significantly fewer eggs retained in the oviduct of beetles, but there was no difference in the number of eggs that accumulated in the culture medium and no difference in the surface-seeking behavior of beetles. Mating status of beetles in all treatments, and DE exposure in concurrent treatments significantly increased their surface-seeking behavior. Concurrent exposure to DE also resulted in a 4- to 6-fold increase in host egg numbers that accumulated in the culture medium. Although DE exposure increased parasite numbers in the beetles, these 2 stressors otherwise appeared to act independently.     

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.     

杂拟谷盗体内 Wolbachia 感染密度的时间和空间分布状况

为搞清杂拟谷盗体内沃尔巴克氏体(Wolbachia)感染密度的时间和空间分布状况,本研究采用实时荧光定量PCR方法测定了杂拟谷盗不同发育阶段、不同日龄、不同性别和不同身体部位的Wolbachia感染密度。结果表明,杂拟谷盗在卵和成虫期Wolbachia感染密度高于其幼虫和蛹期,成虫腹部的Wolbachia感染密度高于其头部和胸部,而成虫不同日龄和雌雄之间的Wolbachia感染密度均没有显著性差异。本研究明确了Wolbachia在杂拟谷盗体内的时空分布和动态变化规律,这对于揭示寄主与共生菌之间的互作关系有着重要意义。     

Effect of age of the intermediate host Tribolium confusum (Coleoptera) on infection by Hymenolepis diminuta (Cestoda)

A cross-sectional study of 27 cohorts of Tribolium confusum aged 2-78 wk was done to examine effects of host age on exposure to eggs of Hymenolepis diminuta under standardized conditions. Pre-exposure, fasting, and postexposure mortality were low, sex ratio was equal, and fecundity of hosts was high during the first 30 wk, followed by increasing mortality and male bias of the sex ratio, and declining fecundity, in older beetles. These changes in the host were not associated with pronounced changes in infection results. Prevalence of infection was higher in females than males, but was unaffected by age in both sexes. Intensity of infection was similar between sexes in beetles up to 30 wk old, and thereafter declined in females, but not in males. Age-related changes in hosts were gradual, but unexpected levels of short-term variation in infection results suggest that some undetermined proximate factors may override general host age effects on the infection process.     

From individual heterogeneity to population-level overdispersion: quantifying the relative roles of host exposure and parasite establishment in driving aggregated helminth distributions

In most host-parasite systems, variation in parasite burden among hosts drives transmission dynamics. Heavily infected individuals introduce disproportionate numbers of infective stages into host populations or surrounding environments, causing sharp increases in frequency of infection. Parasite aggregation within host populations may result from variation among hosts in exposure to infective propagules and probability of subsequent establishment of parasites in the host. This is because individual host heterogeneities contribute to a pattern of parasite overdispersion that emerges at the population level. We quantified relative roles of host exposure and parasite establishment in producing variation in parasite burdens, to predict which hosts are more likely to bear heavy burdens, using big brown bats (Eptesicus fuscus) and their helminths as a model system. We captured bats from seven colonies in Michigan and Indiana, USA, assessed their helminth burdens, and collected data on intrinsic and extrinsic variables related to exposure, establishment, or both. Digenetic trematodes had the highest prevalence and mean abundance while cestodes and nematodes had much lower prevalence and mean abundance. Structural equation modeling revealed that best-fitting models to explain variations in parasite burden included genetic heterozygosity and immunocompetence as well as distance to the nearest water source and the year of host capture. Thus, both differential host exposure and differential parasite establishment significantly influence heterogeneous helminth burdens, thus driving population-level patterns of parasite aggregation.     

A model of encounters between host and parasite populations

A simulation model of the encounter between host and parasite populations is described. The model is two-dimensional in that it represents hosts and parasites as sums of random numbers. It allows for the manipulation of host and parasite numbers, areas of interaction, congruity of geographic ranges, parasite infectivity, and reproduction, or non-reproduction, of the parasite. The model generates parasite distributions (number of hosts vs. parasite/host classes) and their parameters (prevalence, mean number of parasites/host, variance/mean ratio as a measure of aggregation), and thus reveals the manner in which these parameters vary under different encounter conditions, i.e. their "behavior". Simulation results indicated that the behavior of parasite population mean, prevalence, and degree of aggregation was primarily a function of the rate at which infective stages were supplied to the system. In cases in which infective stages were continuously available, prevalence rose rapidly to nearly 100%, with increasing infectivity and parasite numbers, and the populations were not particularly aggregated. When infective stages were introduced in single large waves, both mean and prevalence remained low and the parasite populations were highly aggregated. Model results were compared with published data sets. The latter were also seen to fall into the two general categories of parameter behavior.     

Observations on the routes of infection of Oswaldocruzia filiformis (Nematoda: Trichostrongylidae) in amphibia

The oral, percutaneous and subcutaneous routes of infection of Oswaldocruzia filiformis were investigated in amphibia. Tadpoles of Bufo bufo and Rana temporaria can be infected with O. filiformis when kept temporarily in a suspension of infective larvae in water. Larval stages and subadults were found in tadpoles. All stages of the parasite, including egg-producing females, were found after metamorphosis of the host. However, under natural circumstances infection of tadpoles seems unlikely. Oral infections in metamorphosed hosts of both species were successful in 97.5% of the host animals used. The first eggs appeared 29 days after infection in the faeces. The oral route seems to be normal for O. filiformis in amphibia. Experiments on percutaneous infections did not reveal actual penetration of larvae in or through the skin nor a subsequent migration through host tissues. Sometimes a few larvae were found in the stomach and intestine, but in these particular cases the experimental conditions did not totally exclude the possibility of oral infections. Consequently, the percutaneous route of infection is not plausible for O. filiformis. Subcutaneous inoculation of infective larvae seems to be a possible way of establishing experimental infections. Erratic localisation of the parasite in the enlarged gall bladder of the host was observed.     

Manipulation of host food availability and use of multiple exposures to assess the crowding effect on Hymenolepis diminuta in Tribolium confusum

We studied the 'crowding effect' in Tribolium confusum infected with Hymenolepis diminuta. Manipulations included age and number of parasites, and diet, sex, age and number of exposures of hosts. Volume per parasite was unaffected until an intensity of at least 5-10 parasites per host, then declined approximately inversely as intensities increased. Parasite size was affected by host sex but not age or reproductive status. Host diet affected parasite size and the impact of crowding. Daily gain in parasite volume peaked partway through the developmental period and preceded the first evidence of a crowding effect. Parasites that established during a second exposure had a transient developmental delay but eventually grew as large or larger than parasites from a single exposure with the same total intensity. Parasites responded to crowding by differential allocation of resources. Cercomer volume decreased even with slight crowding, the capsule surrounding the scolex was not reduced until crowding became more severe, and scolex width was reduced only in the most extreme conditions. The data support the hypothesis that the crowding effect in this system is driven primarily by nutrient, rather than space limitations.     

Random parasite encounters coupled with condition-linked immunity of hosts generate parasite aggregation

Parasite aggregation is viewed as a natural law in parasite-host ecology but is a paradox insofar as parasites should follow the Poisson distribution if hosts are encountered randomly. Much research has focused on whether parasite aggregation in or on hosts is explained by aggregation of infective parasite stages in the environment, or by heterogeneity within host samples in terms of host responses to infection (e.g., through representation of different age classes of hosts). In this paper, we argue that the typically aggregated distributions of parasites may be explained simply. We propose that aggregated distributions can be derived from parasites encountering hosts randomly, but subsequently by parasites being 'lost' from hosts based on condition-linked escape or immunity of hosts. Host condition should be a normally distributed trait even among otherwise homogeneous sets of hosts. Our model shows that mean host condition and variation in host condition have different effects on the different metrics of parasite aggregation. Our model further predicts that as host condition increases, parasites become more aggregated but numbers of attending parasites are reduced overall and this is important for parasite population dynamics. The effects of deviation from random encounter are discussed with respect to the relationship between host condition and final parasite numbers.     

Local diversity reduces infection risk across multiple freshwater host‐parasite associations

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The host specificity of Gyrodactylus salaris Malmberg (Platyhelminthes, Monogenea): susceptibility of Oncovhynchus mykiss (Walbaum) under experimental conditions

An experimental epidemiological approach was chosen to study the survival and infection dynamics of Gyrodactylus salaris on juvenile rainbow trout, Oncorhynchus mykiss , in the laboratory. A marked heterogeneity in the host stock was apparent. The rainbow trout could be divided into three groups on the basis of parasite survival and infection pattern on individually isolated fish: (1) hosts receptive to initial parasite attachment, but unreceptive to parasite establishment and reproduction; (2) hosts moderately susceptible to parasite establishment and reproduction, but which, after a period of restricted parasite population growth, responded, recovered and eliminated the parasites; and (3) hosts very susceptible to parasite infection and reproduction, but which, after a period of significant parasite population growth, responded, recovered and eliminated the parasites. These different patterns are considered to reflect genetic differences between host individuals. Parasite aggregation was also shown to be an important factor in the outcome of the host-parasite association. The parasites were finally eliminated on the individually isolated hosts, but not on hosts maintained in batches and so host population size and immigration of fresh. previously unexposed, hosts appeared to be important for growth and maintenance of the parasite population. The parasite was not found to cause host mortality. Rainbow trout was a suitable host for G. salaris , capable of transmitting the parasite to new localities as a consequence of stocking programmes or migratory behaviour.     

Heligmosomoides polygyrus (Nematoda): the dynamics of primary and repeated infection in outbred mice

The population dynamics of Heligmosomoides polygyrus were studied in outbred male MF1 mice subject either to primary or repeated experimental infection. Little variability in susceptibility was observed between mice, but heterogeneity increased with both duration and intensity of primary infection; this result indicates that there are differences in parasite survival between hosts. The rate of parasite-induced host mortality was 4 X 10(-4) per parasite per host per parasite lifespan. The mortality rates of male and female larvae during their development in the intestinal wall were estimated as 0.033 and 0.021 per parasite per day respectively, and estimates of the expected lifespans of the adult male and female parasites in primary infection of 11.22 and 9.92 weeks were obtained. Approximately 40% of female worms were observed in copula at any one time, although this proportion was significantly depressed in hosts harbouring fewer than 50 parasites and during the first four weeks of infection. Parasite fecundity was markedly age-dependent; each female worm produced approximately 31,000 eggs during its lifespan. No density dependence in either worm survival or fecundity in primary infection was apparent. The only detectable effect of worm density was in association with spatial distribution in the intestine; high levels of infection were associated with a posterior shift in the location of a proportion of the parasite population. Characterization of the dynamics of primary infection allowed predictions to be made about the expected dynamics of repeated infection. The comparison of predicted results and observed data revealed unequivocal epidemiological evidence for the density-dependent regulation of parasite population growth during repeated infection, affecting both parasite survival and parasite fecundity. The results also demonstrated the existence of two types of host individual in which the dynamics of repeated infection were markedly different. It is concluded that immunological differences between mice (possibly under genetic control) may be responsible for the observed effects; approximately 25% of MF1 mice seem unable to generate any protective immunity against H. polygyrus, whereas 75% become almost completely refractory to reinfection. This experimental system could be used for quantitative investigation of the impact of acquired immunity and genetic heterogeneity on helminth population dynamics. Both are of obvious relevance with respect to the control of infections of medical and veterinary significance.     

Effect of mebendazole on the larval development of three Hymenolepidid cestodes.

Mebendazole or Telmin (which contains 16.7% mebendazole) markedly retarded the development of Hymenolepis diminuta and H. nana when their intermediate hosts Tribolium confusum were fed on flour mixed with it from day 1 to day 10 p.i. Though retardation also occurred with H. microstoma, the effect of the drug on this parasite was noticeably less pronounced than with the other 2 species of this genus.     

Costly resistance to parasitism: evidence from simultaneous quantitative trait loci mapping for resistance and fitness in Tribolium castaneum

Information on the molecular basis of resistance and the evolution of resistance is crucial to an understanding of the appearance, spread, and distribution of resistance genes and of the mechanisms of host adaptation in natural populations. One potential important genetic constraint for the evolution of resistance is fitness cost associated with resistance. To determine whether host resistance to parasite infection is associated with fitness costs, we conducted simultaneous quantitative trait loci (QTL) mapping of resistance to parasite infection and fitness traits using the red flour beetle (Tribolium castaneum) and the tapeworm parasite (Hymenolepis diminuta) system in two independent segregating populations. A genome-wide QTL scan using amplified fragment length polymorphism (AFLP) markers revealed three QTL for beetle resistance to tapeworm infection. These three QTL account for 44-58% variance in beetle infection intensity. We identified five QTL for fecundity and five QTL for egg-to-adult viability, which accounted for 36-57% and 36-49%, respectively, of the phenotypic variance in fecundity and egg-to-adult viability. The three QTL conferring resistance were colocalized with the QTL affecting beetle fitness. The genome regions that contain the QTL for parasite resistance explained the majority of the variance in fecundity and egg-to-adult viability in the mapping populations. Colocalization of QTL conferring resistance to parasite infection and beetle fitness may result from the pleiotropic effects of the resistance genes on host fitness or from tight linkages between resistance genes and adverse deleterious mutations. Therefore, our results provide evidence that the genome regions conferring resistance to tapeworm infection are partially responsible for fitness costs in the resistant beetle populations.     

Population and infection dynamics of Daubaylia potomaca (Nematoda: Rhabditida) in Helisoma anceps

Daubaylia potomaca is an unusual parasite for several reasons. Specifically, it has a direct life cycle in which it uses a planorbid snail, Helisoma anceps , as the definitive host. In addition, adult females have been shown to be both the infective stage and the only stage documented to be shed from a live, infected host. Finally, adults, juveniles, and eggs have been observed in all tissues and blood spaces of the host, suggesting the parasite consumes and actively migrates through host tissue. The present study examined the population and infection dynamics of D. potomaca in Mallard Lake, a 4.9-ha public access pond in the Piedmont region of North Carolina. In particular, the study examined the role of seasonality on the prevalence and mean intensity of infection of D. potomaca in the snail host. Data collected from August 2008 to October 2009 suggest that prevalence and mean intensity were inversely related in the spring and fall. Prevalence in fall 2008 was 10.3% but increased to 47.3% in spring 2009. Conversely, intensity was high in fall 2008 at 52.4 ± 8.9 worms/infected host but dropped to 3.1 ± 0.3 worms/infected host in spring 2009. During the same time, the parasites within the snails went from highly aggregated populations in the fall to a less aggregated distribution in the spring. It is hypothesized that D. potomaca induces mortality of the snail hosts during the winter, followed by a rapid recruitment event of the nematodes by the snail population after torpor.     

Seasonal population dynamics of the nematode Cystidicoloides tenuissima (Zeder) from the River Swincombe, England

The seasonal population dynamics of adult and larval Cystidicoloides tenuissima were studied in its definitive hosts brown trout, Salmo trutta and juvenile Atlantic salmon, S. salar , and mayfly intermediate host, Leptophlebia marginata , from the River Swincombe, Dartmoor National Park, Devon, U.K. Infective larvae were present in each mayfly generation for almost its entire duration in the steam benthos. The infection parameters (prevalence and mean intensity) and maturation in the fish indicated C. tenuissima was an annual parasite exhibiting a seasonal periodicity and also systematic variation with the host age. Maturation was correlated to river water temperature. Infection parameters increased from September to May, then declined in June and July and remained relatively constant for the rest of the summer. Variation in the fish infection parameters over time, site, and host species appeared to be controlled by transmission related events; the availability of infective larvae, host feeding behaviour and water temperature. The availability of infective larvae and host diet controlled the rate at which parasites were added to the parasite population, but the pattern of gains and losses was determined by a temperature dependent rejection response.     

Cestode transmission patterns

The paradox of high prevalence but low probability of having an egg develop to an adult has been resolved by the evolution of 3 major and basic strategies involving transmission: evolution of life cycles interpolated into host biology; presentation of infective stages that increase probability of contact between host and parasite; and increase in reproductive potential. The rarity of direct cycles confirms that cycles in themselves, with at least 2 hosts, are a key element of cestode success because they provide a vehicle for dispersal and transmission of infective stages. Transmission is primarily by passive stages that become incorporated through intermediate hosts or accidentally in the food chain. High host specificity results from efficient transmission pathways but may represent a fragile system for the evolution of the species. Probability of transmission is increased through diversity of intermediate hosts, making eggs more susceptible to ingestion and by behavioral manipulation of hosts by parasite stages. Spatial and temporal aspects of transmission may be increased through paratenesis. Asexual proliferation of immature stages is uncommon and is favored where there is selective predation; such proliferation may be part of a transmission strategy of colonial cestodes that require high infrapopulations in order to survive. Hyperapolysis may be part of a transmission strategy used by the Tetraphyllidea, Trypanorhyncha, and Lecanicephalidea to increase proglottid production. The dynamics of transmission for cestodes of humans and domestic animals require a different perspective than those of wild hosts. All strategies are reviewed within the framework of certain cestode morphological and ecological constraints. A total of 11 figures and 48 references complements the text.     

Experimental heteroxenous cycle of Lagochilascaris minor Leiper, 1909 (Nematoda: Ascarididae) in white mice and in cats.

Reports of natural infections of sylvatic carnivores by adult worms of species similar to Lagochilascaris minor in the Neotropical region led to attempts to establish experimental cycles in laboratory mice and in cats. Also, larval development was seen in the skeletal muscle of an agouti (Dasyprocta leporina) infected per os with incubated eggs of the parasite obtained from a human case. In cats, adult worms develop and fertile eggs are expelled in the feces; in mice, larval stages of the parasite develop, and are encapsulate in the skeletal muscle, and in the adipose and subcutaneous connective tissue. From our observations, we conclude that the larva infective for the mouse is the early 3rd stage, while for the final host the infective form is the later 3rd stage. A single moult was seen in the mouse, giving rise to a small population of 4th stage larvae, long after the initial infection.     

Effect of a nuclear polyhedrosis virus on the relationship between Trichoplusia ni (Lepidoptera: Noctuidae) and the parasite, Hyposoter exiguae (hymenoptera: Ichneumonidae)

The effect of a nuclear polyhedrosis virus on the relationship between Trichoplusia ni and the parasite, Hyposoter exiguae, was investigated to determine if the virus could invade and multiply in the tissues of the parasites, if parasites which emerged from virus-infected T. ni larvae had normal emergence, fecundity, and longevity, and if the parasite could serve as a vector for the virus. Light microscopy revealed particles which appeared to be polyhedra within the lumen of the midgut of parasite larvae from virus-infected hosts. Transmission electron microscopy confirmed the presence of polyhedra and free virions within the midgut of the larvae. Polyhedra or free virions were never found within any parasite tissues. Parasite larvae within hosts exposed to virus before parasitization perished when their hosts died of virus infection. Parasite larvae in hosts exposed to virus after parasitization completed their development before their hosts died of virus infection. The proportion of parasites which survived increased as the time between host parasitization and host virus exposure increased. Parasite larvae which developed in hosts exposed to the virus soon after parasitization spent significantly less time in their hosts than did parasites which developed in noninfected hosts. There was no significant difference in time spent in the pupal stage, percent adult emergence, adult longevity with and without food and water, and fecundity of parasites which developed in virus-infected hosts and those which developed in noninfected hosts. Female parasites laid as many eggs in virus-infected hosts as they did in noninfected hosts. Sixty percent of the female parasites which oviposited in virus-infected hosts vectored infective doses of virus to an average of 6% of the healthy hosts subsequently exposed to them. None of the healthy host larvae exposed to male parasites which had been exposed to virus-infected host larvae became infected with the virus. Forty percent of the female parasites which developed in virus-infected hosts transmitted infective doses of the virus to an average of 65% of the healthy host larvae exposed to them. Ninety percent of the male parasites which developed in virus-infected hosts transferred infective doses of the virus to an average of 21% of the healthy host larvae exposed to them.