Phagocytosis of microvilli of the metacestode of Hymenolepis diminuta by Tenebrio molitor haemocytes

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.

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.

Parasite manipulation of insect reproduction: who benefits?

Host fertility is often curtailed as a result of parasitic infection. The hypothesis that this may confer an adaptive advantage upon the symbionts if nutrients are directed from reproduction and made available for host/parasite maintenance is explored. The suggestion is made that an understanding of the mechanisms underlying the pathophysiology of fecundity reduction may shed light upon the evolutionary implications of this strategy for both parasite and host. To illustrate this the down-regulation of egg production is explored with reference to a particular model system, the association between metacestodes of the rat tapeworm, Hymenolepis diminuta and the mealworm beetle, Tenebrio molitor. Several aspects of host reproductive behavior and physiology are affected by infection in this association, including vitellogenesis. Metacestodes directly inhibit the fat body synthesis of vitellogenin in a stage-specific, density-dependent manner. This inhibition is likely to be orchestrated by a modulator molecule, produced by the parasite. In the ovarian follicles, juvenile hormone III binding to a specific follicular membrane-binding protein is inhibited in infected beetles, resulting in the down-regulation of a cascade of events which enables vitellogenin to pass into the developing oocyte. Data to support the proposed existence of a parasite-induced antigonadotrophin, of host origin, are discussed. Evidence that similar mechanisms operate in Plasmodium-infected anopheline mosquitoes and Onchocerca-infected blackflies is presented in support of the possibility that a parasite-induced reduction in host reproductive fitness is an adaptive strategy and an assessment of who is manipulating whom is made.

Ontogenetic changes in helminth membrane function

During their life-cycle many parasites experience a wide range of environments including free living and those provided by a variety of intermediate and final hosts. The nutritional requirements of parasites are met by physiological processes adapted to exploit the physicochemical characteristics provided by different hosts. In helminth parasites these adaptations are frequently expressed on the tegumentary surface. As an example of adaptations within the Trematoda, the control of monosaccharide transport in Proterometra sp. is described. Environmental sodium, although not directly involved in the uptake process, nevertheless regulates the expression of transport capabilities. In the Cestoda, the uptake of monosaccharides and amino acids is described for Hymenolepis diminuta. The metacestode of this tapeworm inhabits the blood system of an arthropod, and the adult the gut of a mammal. There are quantitative and qualitative differences in the amino acids and monosaccharides in these two environments and these are reflected in the transport mechanisms exhibited by the two forms of the life-cycle. In Echinococcus granulosus the transfer of amino acids, sugars and macromolecules across the membranes of hydatid cysts and protoscoleces is described. The major difference between these two stages in the life-cycle relates to the ability of hydatid cysts to absorb macromolecules, whereas protoscoleces are impermeable to these compounds. The potential for future work is emphasized.