Evasion of the haemocytic defence reaction of certain insects by larvae of Hymenolepis diminuta (Cestoda)

Insects dispersing taeniid eggs: Who and how?

Taeniosis/cysticercosis and echinococcosis are neglected zoonotic helminth infections with high disease burden caused by tapeworms which circulate between definitive and intermediate host reflecting a predator-prey interaction. Taeniid eggs can remain vital for months, allowing arthropods to mechanically transport them to intermediate hosts. However, the multiple routes that arthropods provide as carriers of taeniid eggs are still often unregarded or not considered. This review focuses on the prevalence and importance of arthropods as carriers and spreaders of taeniid eggs in the epidemiology of taeniosis/cysticercosis and echinococcosis. Current scientific knowledge showed a relevant role of houseflies (Muscidae), blowflies (Calliphoridae), dung beetles (Scarabaeoidea), darkling beetles (Tenebrionidae), ground beetles (Carabidae) and skin beetles (Dermestidae) in the spread of taeniid eggs in the environment, which may favor the infection of new hosts through the direct ingestion of an insect or of contaminated food and water. At last, key research challenges are highlighted, illustrating that further knowledge on the topic is needed to develop and improve guidelines and actions to prevent taeniid infections worldwide.

Comparison of the effects of multiple variables on the levels of infection of the rat tapeworm, Hymenolepis diminuta, in its intermediate host, the confused flour beetle Tribolium confusum

The interaction of the rat tapeworm, Hymenolepis diminuta, with its intermediate beetle host, Tribolium confusum, is a well-studied model system. However, there is so much variation in the methods and designs of the studies that it is difficult to draw comparisons. This study simultaneously compared several aspects of the infection protocol including beetle age, sex, density and mating status; parasite egg condition, infection environment humidity and the times for the three steps of infection: starvation, feeding and post-feeding development. Although statistically limited by low prevalence and intensity levels, we were able to detect the relative effects of the variables. The effects of these variables on prevalence (percent infection) and intensity (mean number of cysticercoids) do not necessarily correlate with each other. Egg condition, reduced starvation times, higher beetle density and longer development times reduced prevalence. However, differences in intensity were only detected with older beetles. When coupled with survivorship data, our study found that our current infection protocol is optimal for infection success. However, the results suggest extending the study to other intermediate hosts and the inclusion of additional variables.

Timing of sub-lethal insecticide exposure determines parasite establishment success in an insect-helminth model

Environmental toxicants are pervasive in nature, but sub-lethal effects on non-target organisms and their parasites are often overlooked. Particularly, studies on terrestrial hosts and their parasites exposed to agricultural toxicants are lacking. Here, we studied the effect of sequence and timing of sub-lethal exposures of the pyrethroid insecticide alpha-cypermethrin on parasite establishment using the tapeworm Hymenolepis diminuta and its intermediate insect host Tenebrio molitor as a model system. We exposed T. molitor to alpha-cypermethrin (LD20) before and after experimental H. diminuta infection and measured the establishment success of larval tapeworms. Also, we conducted in vitro studies quantifying the direct effect of the insecticide on parasite viability. Our results showed that there was no direct lethal effect of alpha-cypermethrin on H. diminuta cysticercoids at relevant concentrations (LD10 to LD90 of the intermediate host). However, we observed a significantly increased establishment of H. diminuta in beetles exposed to alpha-cypermethrin (LD20) after parasite infection. In contrast, parasite establishment was significantly lower in beetles exposed to the insecticide before parasite infection. Thus, our results indicate that environmental toxicants potentially impact host-parasite interactions in terrestrial systems, but that the outcome is context-dependent by enhancing or reducing parasite establishment depending on timing and sequence of exposure.

Establishment Success of the Beetle Tapeworm Hymenolepis diminuta Depends on Dose and Host Body Condition

Parasite effects on host fitness and immunology are often intensity-dependent. Unfortunately, only few experimental studies on insect-parasite interactions attempt to control the level of infection, which may contribute substantial variation to the fitness or immunological parameters of interest. The tapeworm Hymenolepis diminuta—flour beetle Tenebrio molitor model—has been used extensively for ecological and evolutionary host–parasite studies. Successful establishment of H. diminuta cysticercoids in T. molitor relies on ingestion of viable eggs and penetration of the gut wall by the onchosphere. Like in other insect models, there is a lack of standardization of the infection load of cysticercoids in beetles. The aims of this study were to: (1) quantify the relationship between exposure dose and establishment success across several H. diminuta egg concentrations; and (2) test parasite establishment in beetles while experimentally manipulating host body condition and potential immune response to infection. Different egg concentrations of H. diminuta isolated from infected rat feces were fed to individual beetles 7–10 days after eclosion and beetles were exposed to starvation, wounding, or insertion of a nylon filament one hour prior to infection. We found that the establishment of cysticercoids in relation to exposure dose could be accurately predicted using a power function where establishment success was low at three lowest doses and higher at the two highest doses tested. Long-term starvation had a negative effect on cysticercoid establishment success, while insertion of a nylon filament and wounding the beetles did not have any effect compared to control treatment. Thus, our results show that parasite load may be predicted from the exposure dose within the observed range, and that the relationship between dose and parasite establishment success is able to withstand some changes in host body condition.

Longitudinal study of parasite-induced mortality of a long-lived host: the importance of exposure to non-parasitic stressors

Hosts face mortality from parasitic and environmental stressors, but interactions of parasitism with other stressors are not well understood, particularly for long-lived hosts. We monitored survival of flour beetles (Tribolium confusum) in a longitudinal design incorporating cestode (Hymenolepis diminuta) infection, starvation and exposure to the pesticide diatomaceous earth (DE). We found that cestode cysticercoids exhibit increasing morphological damage and decreasing ability to excyst over time, but were never eliminated from the host. In the presence of even mild environmental stressors, host lifespan was reduced sufficiently that extensive degradation of cysticercoids was never realized. Median host lifespan was 200 days in the absence of stressors, and 3-197 days with parasitism, starvation and/or DE. Early survival of parasitized hosts was higher relative to controls in the presence of intermediate concentrations of DE, but reduced under all other conditions tested. Parasitism increased host mortality in the presence of other stressors at times when parasitism alone did not cause mortality, consistent with an interpretation of synergy. Environmental stressors modified the parasite numbers needed to reveal intensity-dependent host mortality, but only rarely masked intensity dependence. The longitudinal approach produced observations that would have been overlooked or misinterpreted if survival had only been monitored at a single time point.

Hymenolepis diminuta infections in tenebrionid beetles as a model system for ecological interactions between helminth parasites and terrestrial intermediate hosts: a review and meta-analysis

The cestode Hymenolepis diminuta (Cyclophyllidea) uses a variety of insects as its intermediate host, where ingestion of eggs results in development in the hemocoel of a cysticercoid that is infective to a rat definitive host. Species in 2 genera, Tenebrio and Tribolium (Coleoptera: Tenebrionidae) have been used extensively as laboratory intermediate hosts. This review examines experimental studies on ecological aspects of the relationship between H. diminuta and tenebrionid beetles, including the acquisition and establishment of the parasite, host effects on the parasite, and parasite effects on the host. A meta-analysis of infection results from the literature revealed strong relationships across host species and strains between (1) prevalence and intensity of infection, (2) efficiency of cysticercoid production and exposure conditions, and (3) variance in abundance or intensity of infection relative to their respective means. The underlying mechanisms producing these patterns remain elusive. Comparative studies are infrequent, and the use of divergent methodologies hampers comparisons among studies. In spite of these problems, there is much to recommend this as a terrestrial host-parasite model system. It represents those relationships in which mostly minor, but occasionally major, responses to parasitic infection occur, and in which host genetics and environmental conditions can serve as modifying factors. Moreover, this is a tractable experimental system, and is backed by an extensive literature on host biology.

Immunosuppression by larvae ofMoniliformis moniliformis(Acanthocephala) in their cockroach host (Periplaneta Americana)

The results of 4 different assays for haemocytic immune responsiveness in the cockroach,Periplaneta americana, have shown that the presence of larvae ofMoniliformis moniliformiswithin the haemocoele depresses the insect's immune reactivity. Thus, the proportion of haemocytes that phagocytose fluorescent latex beadsin vivo, the proportion of haemocytes that can be stimulatedin vitroby theβ1,3-glucan, laminarin, to produce phenoloxidase, and the number of haemocytic aggregates producedin vivoin response to zymosan stimulation, are significantly depressed compared with control, unparasitized cockroaches. Also, when cockroaches are injected intra-haemocoelically with hatched oncospheres of the tapeworm,Hymenolepis diminuta, a higher prevalence and intensity of tapeworm larvae are found in insects already parasitized byM. moniliformis. All of these assays show that depression rather than total suppression of the immune response occurs; in the wild, this may be sufficient to ensure that the cockroach does not succumb to unrelated infections during the long developmental period of the parasite.

Phagocytosis of microvilli of the metacestode ofHymenolepis diminutabyTenebrio molitorhaemocytes

A limited number of metacestodes ofHymenolepis diminutafromTenebrio molitorhad host plasmatocytes and cell debris of coagulocyte origin associated with the microvillar surface. There was evidence of phagocytosis of microvilli by plasmatocytes, involving clathrin-like endocytotic vesicles. In contrast to these otherwise ‘normal’ cysticercoids, occasional batches were ‘sticky’ and, on flushing into saline, rapidly developed large bladders possibly associated with osmotic stress. In addition, numerous small surface blebs were present. These ‘surface-stressed’ cysticercoids also had host plasmatocytes and cell debris associated with the bladder bases and the blebs, and phagocytosis of microvilli was observed. In no case had precipitation of haemolymph, melanization or encapsulation (features typical of the insect immune response) occurred. The possibility that the limited host response indicates a spatially restricted impairment of the surface compatibility of the parasite, and thus insufficient to elicit the full immune response, is discussed.

Induction of cysteine proteinase in the encapsulation of Hymenolepis diminuta eggs in the American cockroach, Periplaneta americana

Haemocytes play an essential role in defending invertebrates against pathogens and parasites that enter their haemocoel. In the present study, the cockroach, Periplaneta americana was able to encapsulate Hymenolepis diminuta eggs within 24 h after injection. Proteolytic activity of egg capsules was determined by gelatin zymography. A gelatinase-type proteolytic enzyme with molecular weight about 65 kDa was present at the time of capsule formation. Enzyme activity was obviously inhibited by leupeptin but not by ethylenediamine tetraacetic acid (EDTA) or 1,10-phenanthroline or phenylmethanesulfonyl fluoride (PMSF). Hence, we tentatively characterized this enzyme as a cysteine proteinase. The specificity of the cellular immune response in vivo and the increased cysteine proteinase activity coincided with the capsule size and encapsulation process. The possible function of this cysteine proteinase activity during encapsulation of the H. diminuta eggs by P. americana is discussed.

Effects on the development of Dipylidium caninum and on the host reaction to this parasite in the adult flea (Ctenocephalides felis felis)

Temperature was found to be a major factor affecting the development of Dipylidium caninum and the presence of a host reaction of adult Ctenocephalides felis felis to D. caninum. Adult fleas reared at 30-32 degrees C contained fully developed metacestodes when they emerged from their cocoons. However at lower temperatures, D. caninum could not complete development until the flea hosts had spent some time on their mammalian hosts. It was the surface temperature of the mammals (31-36 degrees C) and not the fleas' blood meals which resulted in the metacestodes completing their development. This development of D. caninum was therefore independent of the flea development. At 20 degrees C, a larger and more prolonged host reaction was mounted than at higher temperatures. The larval flea diet had a small effect on the subsequent cestode development and the adult fleas' reaction to it.

Factors affecting the development of Dipylidium caninum in Ctenocephalides felis felis (Bouché, 1835)

Ctenocephalides felis felis larvae were infected with Dipylidium caninum at a range of temperatures from 20 degrees - 35 degrees C at 3 mm Hg saturation deficit (SD) and 30 degrees C at 8 mm Hg SD. Hosts were subsequently dissected at 6, 9 and 12 days after infection. Four replicate experiments were performed and results of development, and host reactions analysed by the Genstat computer programme. These were found to depend on the temperature and saturation deficit of the environment. Unlike previous findings, parasite development and host reaction were found to be independent of host development. Host reaction was more marked and prolonged at 20 degrees - 25 degrees C than at higher temperatures. No perceptible growth of the parasite occurred at 20 degrees C. The development patterns of growth at the higher temperatures were similar but shifted in time so that faster growth occurred at higher temperatures. Rate of growth was fastest at 35 degrees C, despite the fact that this temperature was unfavourable to the hosts, all of which died at the time of pupation.

Humoral mechanisms in the immune response of insects to larvae of Hymenolepis diminuta (Cestoda)

Previous work has shown that larvae of Hymenolepis diminuta are not encapsulated by the haemocytes of the locust, Schistocerca gregaria, but are encapsulated by haemocytes of the cockroach, Periplaneta americana. The specificity of the cellular immune response in vivo, and of the agglutinating activity of the sera against vertebrate erythrocytes in vitro, is known to differ between the two species of insect. Accordingly, the sera of each of these species was tested for its agglutinating activity in vitro against a pure suspension of a known number of freshly-hatched oncospheres of H. diminuta; it was found that serum of P. americana strongly agglutinated oncospheres while the serum of S. gregaria had no effect. The agglutinating activity of cockroach serum was only partially removed by preadsorption with either human, rat or rabbit erythrocytes, all of which are strongly agglutinated by the serum. Although the function in vivo of this agglutinating activity directed against oncospheres is not known, it is suggested that this observation--that both the haemocytes and serum of one insect species respond to the parasite whereas the serum and haemocytes of another species apparently do not--may be of some relevance to the immune recognition mechanisms of insects.

The influence of Hymenolepis diminuta on the survival and fecundity of the intermediate host, Tribolium confusum

An experimental study of the effects of parasitism by H. diminuta on the intermediate host, Tribolium confusum, is described. No density-dependent constraints on parasite establishment within individual hosts are evident, although a reduction in cysticercoid size at high parasite burdens is demonstrated. The relationship between parasite burden, host mortality and host fecundity is investigated. Host mortality is linearly related to parasite burden, whereas the relationship between parasite burden and host fecundity is non-linear. There is no difference in viability between eggs from infected and uninfected females. The generative causes of these effects are not investigated experimentally, although it is postulated that survival is related to the degree of damage to the midgut wall caused by parasite penetration, and fecundity to the biomass of parasites harboured by the host. The significance of these effects is discussed in relation to the overall dynamics of the host-parasite association.

Evasion of the insect immune response by Moniliformis dubius (Acanthocephala): further observations on the origin of the envelope

The envelope around larvae of Moniliformis dubius appears to protect the parasite against immune recognition and encapsulation by the insect host's haemocytes. The origin of this envelope has been the subject of controversy although most evidence suggests it is parasite-derived. If host-derived, the envelope would be expected to share surface properties with host tissue. Thus, experiments were undertaken, transplanting parasites and host tissue to other insects and using haemocytic encapsulation as an assay for immune recognition, in order to compare the response to host tissue and to the parasite's envelope. Parasites without their envelopes, and pieces of tissue (ventral nerve cord) from the experimental host (the locust Schistocerca gregaria) were recognized as foreign and encapsulated in the cockroach, Periplaneta americana. The majority of parasites with their envelopes were unencapsulated or only partially encapsulated on transfer to their normal host, P. americana, indicating that the envelope does not have surface similarity to locust tissue. Cockroach-derived parasites with or without envelopes were not encapsulated in S. gregaria, suggesting that the larva itself can evade or inhibit the locust's recognition mechanism. However, since larvae which develop in S. gregaria are enclosed in an envelope, the formation of the envelope would seem to be an inherent feature of the parasite's development.