41st Annual Meeting of the Society for Invertebrate Pathology
and 9th International Conference on Bacillus thuringiensis

Parasites can affect the outcome of biological invasions in different ways. Outbreaks of parasites may lead to host population crashes and resultant community change. But parasites do not only act on host population density. We present studies of short-term, behavioural effects of parasites and their effect on invasions. We focus on parasite regulation of crustacean invasions. Using empirical studies and mathematical modelling, we show that two parasites play keystone roles in UK amphipod invasions. Firstly, the microsporidian Pleistophora mulleri may facilitate invasion by two smaller species of amphipod; it has no direct effect on the survival of the native G. d. celticus, but infected animals are less likely to prey on the two smaller invaders. Secondly, the acanthocephalan Echinorynchus truttae may promote coexistence, as infection of the invading species Gammarus pulex reduces its predation on native G. d. celticus. Microsporidia may also drive crayfish invasions. We provide evidence from sequence data that the invading signal crayfish has acquired Thelohania contejeani (porcelain disease) from the native. However, whilst the invader may suffer little from the infection, transmission to the native can cause reduced activity and mortality and so increase the rate of extinction of this species.

To date, only one genera (Nucleospora) within one family (Enterocytozoonidae) of microsporidians contain species that are parasitic within the nuclei of their host cells. All described intranuclear species of the genus Nucleospora exist in fish. We have recently discovered the first intranuclear microsporidian parasite within an invertebrate, the European edible crab (Cancer pagurus). Whilst infected crabs displayed no obvious external symptoms, histologically, infected hepatopancreatic tubules were characterised by the presence of varying numbers of hypertrophic and eosinophilic nuclei within epithelial cells. Parasite stages appeared as eosinophilic granular accumulations causing margination of host chromatin. In advanced cases, degeneration of the tubule epithelia occurred, with parasites and sloughed epithelial cells appearing in tubule lumens. Ultrastructurally, all life stages of the parasite were observed within host nuclei. Uninucleate meronts were not detected though bi-nucleate stages were observed. Multinucleate plasmodia (sporogonal plasmodia) contained up to 22 nuclei in section. Significantly, late stage plasmodia contained multiple copies of apparatus resembling the polar filament and anchoring disk that appeared to associate with individual plasmodial nuclei. As such, aggregation and early assembly of sporoblast components took place within the intact sporogonial plasmodium, a unique feature of the family Enterocytozoonidae. Liberation of sporoblasts from plasmodia or the presence of liberated sporoblasts was not observed in this study. However, maturing and mature spores were observed in direct contact with the host nucleoplasm. In many cases, large numbers of spores were observed within a given section of host nucleus. Spores measured 1.3 ± 0.02 x 0.7 ± 0.01 mm. By considering the shared features of this parasite with microsporidians of the family Enterocytozoonidae and the unique presence of this parasite within the nucleoplasm of hepatopancreatocytes from a decapod crustacean, it was proposed that this parasite is the type species of a new genus of microsporidian (genus Enterospora). Further work is now required to provide molecular taxonomic data for comparison of Enterospora to the two other genera, Enterocytozoon and Nucleospora that reside within the Enterocytozoonidae and with other hepatopancreatic microsporidians of crustacean hosts.

Twelve microsporidian species infecting caddis fly larvae including genera Episeptum, Paraepiseptum (formerly Pyrotheca, Cougourdella), Zelenkaia (provisionally designated genus), Issia and Amblyospora (formerly Thelohania) were characterised. All studied species belonging to genera Episeptum, Paraepiseptum and Zelenkaia are host-species specific parasites infecting fat body and oenocytes of their hosts. Their spores are not infective for the original host, their life cycle involves an intermediate host and (or) transovarial transmission. Data obtained by rDNA sequencing showed that microsporidia from Trichoptera form several separate groups within a large clade uniting microsporidia from crustacea (first of all Copepoda and Cladocera) and insects with aquatic larval stages (Diptera - Culicidae, Simuliidae). It occurs that the presently known microsporidia from Trichoptera have no common and direct ancestor in their evolution. It is hypothetized that microsporidia invaded individual groups independently, but always from ancestors parasitizing crustacea. The presence and mosaic distribution of crustacean microsporidia across all clades of phylogeny trees suggests that the model of their radiation from crustacean hosts was probably repeated in other groups of hosts who acquired microsporidia from crustacea living with them in the same environment.The life cycles of microsporidia switching insects and crustacea as their hosts (e.g. mosquito Amblyospora spp.) are probably relics of the original radiation event. Phylogeny data suggests that microsporidia from Trichoptera could be of similar type.

Parasites have a negative effect on the reproduction and survival of individual Daphnia, which is translated in population level effects. The Daphnia mostly do not evolve as fast as their parasites, however, adaptive genetic changes occur upon infection. In their ‘arms race’ against the fast evolving parasites, there will be selection in the Daphnia against defence mechanisms, other than those that are abundant in the momentary interaction, as Daphnia parasites adapt to specific abundant host genotypes. The antagonistic interactions between the waterflea Daphnia and its parasites are a key structuring force in natural populations, driving their coevolution. Direct empirical demonstration of long-term host-parasite coevolution, in particular Red Queen dynamics, is difficult. Here we capitalize on the fact that dormant stages of both parasites and hosts of our model system, the waterflea Daphnia and its micro-parasites, are conserved in lake sediments and thus provide an archive of past evolutionary dynamics. This allowed us to reconstruct host – parasite coevolution in a natural setting. We document evidence for fast temporal adaptation of the parasite, which supports the idea of ongoing coevolution between the host and the parasite.

The relationship between Nosema bombi and its bumble bee hosts has long been controversial. While its route of infection within hosts is well-known, transmission routes among hosts have remained unclarified. Furthermore, the impact of the parasite on host fitness (its virulence or pathogenicity) has been reported as high, non-existent or even, counter-intuitively, beneficial. Most of this confusion is due to a lack of controlled experiments. Recent work in a number of groups has considerably advanced our knowledge. In this talk I will review our current understanding and give directions for future work on this important host-parasite relationship

Microsporidia from freshwater Amphipods were only recently the focus of intensive research. Based on a literature review, an overview will be first presented, including data on molecular systematic, transmission (vertical vs. horizontal), fitness effects for the host (feminization…), host ranges and geographical distribution. Parasitism as a component of the success and dynamic of invasions, as well as the potential impact of alien parasites on the local fauna are regarded as important factors. Therefore, this paper will also present ongoing research on a newly characterised microsporidia, Microsporidium sp. D, infecting the invasive amphipod Dikerogammarus villosus. Originating from the Ponto-Caspian area, D. villosus invaded almost all large rivers of Europe in less than 30 years. PCR-RFLP typing showed that M. sp. D followed its host almost all along its invasion route. In an attempt to test if M. sp. D is a way to control the invader or/and a risk for the local fauna, results about transmission (vertical vs. horizontal) and potential impact on host fitness as well as prevalence and pathogenicity to the local fauna will be presented for two invaded areas contrasting by their invasion history and local fauna: Burgundy (France) and Eastern Poland.

Phylogenetically, Microsporidia are now considered highly specialised parasitic fungi. They are all intracellular parasites with a characteristic and unique mode of infection. Microsporidia may infect all life forms and undoubtedly, only a small fraction of the actual number of species have been characterised. In Hymenopteran pollinators, microsporidia infections have been described from four host species only: Nosema apis infecting the European honey bee, Apis mellifera; Nosema ceranae infecting the Asian honey bee, Apis cerana; Nosema bombi, infecting Bombus spp. and Antonospora scoticae infecting Andrena scoticae. N. apis and N. ceranae are cross infective between hosts. However, N. apis does not do well in A. cerana, whereas there is a worldwide process of N. ceranae replacing N. apis in A. mellifera. N. bombi has recently become of particular interest for conservationists, since this parasite may be distributed to areas assumed free from this parasite, thereby presumably endangering endemic bumble bee spp. Furthermore, within-genome rRNA variability in N. bombi suggests that to characterize intraspecific genetic variants in the Microsporidia based on RNA sequences is not straight forward. A. scoticae infects the fat body tissue of A. scotica and may occur with an extreme prevalence in its host.

Lymantria dispar, a well-known defoliator of broadleaved forests, is host for a variety of entomopathogenic microsporidia. In a series of laboratory and semi-field experiments, we studied horizontal transmission of Endoreticulatus schubergi, a midgut parasite, Vairimorpha disparis, a fat body parasite, and Nosema lymantriae a parasite causing systemic infections. Despite the fact, that the three species infect different target tissues, only two main horizontal transmission pathways were identified. Endoreticulatus schubergi is transmitted via spore-laden feces, V. disparis via decomposing cadavers; N. lymantriae uses both transmission pathways. We developed a simulation model that describes stage specific development and mortality of uninfected, latent and infectious hosts and the two main transmission pathways. Results of the laboratory experiments were used to calculate parameters and fit equations for spore release via feces and survival of hosts, which both served as input variables. The number and percentage of infected larvae served as output variable and was compared to the results of the semi-field studies.

The new microsporidium was isolated from the endemic, Taiwanese, atyid shrimp, Caridina formosae (Decapoda, Atyidae) from northern Taiwan. The conspicuous symptom of the infected shrimp is the opaque, white xenomas, found usually around the alimentary canal in the haemocoel, the dorsal part of abdomen underneath the dorsal median carina, and on the gill. A fully developed xenoma consisted of a hard and thick capsule and filled with sporophorous vesicles. The parasites within a sporophorous vesicle were synchronous in development, while among sporophorous vesicle were different in development stages. Fresh spores were pyriform, measuring 6.53 x 4.38 μm. The spore contained a nucleus and isofilar polar-filament with 9-11 coils. The phylogenetic analysis of small subunit rDNA showed that this isolate is closely related to the species of the genus Dictyocoela, a group of microsporidia from crustacean. However, the identities of the SSUrDNA sequences were only around 81%. Therefore, we propose that this isolate is a new species but needs more morphological and molecular evidences to clarify taxonomic position.

Thaumetopoea processionea is an oak pest of high socio-economic impact. Larvae possess urticating setae that cause contact dermatitis and more severe reactions in humans. Infestations in suburban areas and gregarious behavior make inoculative release of microsporidia an interesting option for control of this insect. We studied the occurrence of microsporidia in T. processionea larvae from various locations in Eastern Austria using light microscopy. Microsporidia of the genera Endoreticulatus, Nosema, Cystosporogenes and Vairimorpha were detected in nine out of 18 populations at prevalence between 1.9% and 15.4%. Spores were isolated and stored in liquid nitrogen. All isolates were tested for infectivity to a laboratory host, Lymantria dispar. One Endoreticulatus sp. successfully infected L. dispar larvae and developed infections like in T. processionea. This allowed easy production of inoculum as well as studies with an non-hazardous laboratory host. Endoreticulatus infection was restricted to the gut of the host; development of disease was slow but resulted in significantly elevated mortality. Efficient horizontal transmission was shown in the laboratory. We attempted an inoculative release of Endoreticulatus sp. by spraying laboratory-produced spores in aqueous suspension on small, isolated oak trees infested with T. processionea. Inoculation was successful, however at a low level – maximum prevalence was 9.5%.