Life cycle and cytochemical study of the endogenous developmental stages of Eimeria akeriana (coccidiida, Eimeriidae) from the Asia Minor gerbil

Three generations of schisonts in the life cycle of Eimeria akeriana, the intestinal parasite of Meriones blackleri, were determined. Gametogony begins in 94 hours, the first oocysts discharge in 5.5--6 days and lasts 14.5 to 15 days after the oocyst administration. A cytochemical study of the distribution of the nucleic acids, proteins and amylopectin at the stages of endogenous development of E. akeriana has revealed a considerable similarity among the parasites of Eimeria though each type is characterized by some cytochemical peculiarities.     

Seasonal and demographic factors influencing gastrointestinal parasitism in ungulates of Etosha National Park

Host-parasite dynamics can be strongly affected by seasonality and age-related host immune responses. We investigated how observed variation in the prevalence and intensity of parasite egg or oocyst shedding in four co-occurring ungulate species may reflect underlying seasonal variation in transmission and host immunity. This study was conducted July 2005-October 2006 in Etosha National Park, Namibia, using indices of parasitism recorded from 1,022 fecal samples collected from plains zebra (Equus quagga), springbok (Antidorcas marsupialis), blue wildebeest (Connochaetes taurinus), and gemsbok (Oryx gazella). The presence and intensity of strongyle nematodes, Strongyloides spp. and Eimeria spp. parasites, were strongly seasonal for most host-parasite combinations, with more hosts infected in the wet season than the dry season. Strongyle intensity in zebra was significantly lower in juveniles than adults, and in springbok hosts, Eimeria spp. intensity was significantly greater in juveniles than adults. These results provide evidence that acquired immunity is less protective against strongyle nematodes than Eimeria spp. infections. The seasonal patterns in parasitism further indicate that the long dry season may limit development and survival of parasite stages in the environment and, as a result, host contact and parasite transmission.     

Some Problems of Host and Parasite Interactions in the Coccidia*

SYNOPSIS. The current status of some concepts of host and parasite interactions in the coccidia are discussed and evaluated. It is suggested that winter coccidiosis of cattle caused by Eimeria zuernii results from activation of arrested endogenous stages in the tissues of the host. A second aspect of clinical coccidiosis is that infections are seldom monospecific, but little work has been done on infections in animals with multiple species of coccidia. Information in the literature indicates that there are interactions between species of Eimeria in concurrent infections, and it is hoped that investigators will undertake studies to define more clearly what interactions there may be. Finally, the finding that there is a genetic basis for successful transmission of Eimeria separata from rats to mice provides a tool for studying the basis of host-specificity in the coccidia.     

Host defence versus intraspecific competition in the regulation of infrapopulations of the flea Xenopsylla conformis on its rodent host Meriones crassus

Mechanisms that regulate parasite populations may influence the evolution of hosts and parasites, as well as the stability of host-parasite dynamics but are still poorly understood. A manipulation experiment on the grooming ability of rodent hosts (Meriones crassus) and flea (Xenopsylla conformis) densities on these hosts successfully disentangled two possible regulating mechanisms: (i) behavioural defence of the host and (ii) intraspecific competition among parasites, and revealed their importance in suppressing the feeding of fleas. Moreover, the results suggest that flea competition is direct and is not mediated by host grooming, immune response, or parasite-induced damage to the host. These mechanisms, together with interspecific competition and density-dependent parasite-induced host damage, may limit the parasite burden on an individual host and may prevent parasites from overexploiting their host population.     

Effect of vasopressin and indomethacin on provoked diuresis in pregnant and non-pregnant Meriones crassus

The dynamics of induced diuresis was studied in Meriones crassus, a desert rodent species, during estrus and pregnancy. In response to hydration, pregnant Meriones crassus excrete excess water more rapidly than non pregnant females. Diuresis appears earlier in pregnant females indicating a more rapid inactivation of vasopressin. The possible interaction between vasopressin and prostaglandins, and its effect on renal excretion during pregnancy, was studied during arginine-vasopressin treatment and after inhibition of endogenous AVP and prostaglandins. Results show a decrease in renal response to endogenous vasopressin during pregnancy in Meriones crassus, which may be due to the increase in renal prostaglandin levels.     

Cell: sporozoite interactions and invasion by apicomplexan parasites of the genus Eimeria

The site specificity that avian Eimeria sporozoites and, to a more limited degree, other apicomplexan parasites exhibit for invasion in vivo suggests that specific interactions between the sporozoites and the target host cells may mediate the invasion process. Although sporozoite motility and structural and secreted antigens appear to provide the mechanisms for propelling the sporozoite into the host cell,there is a growing body of evidence that the host cell provides characteristics by which the sporozoites recognise and interact with the host cell as a prelude to invasion. Molecules on the surface of cells in the intestinal epithelium, that act as receptor or recognition sites for sporozoite invasion, may be included among these characteristics. The existence of receptor molecules for invasion by apicomplexan parasites was suggested by in vitro studies in which parasite invasion was inhibited in cultured cells that were treated with a variety of substances designed to selectively alter the host cell membrane. These substance included cationic compounds or molecules, enzymes that cleave specific linkages, protease inhibitors, monoclonal antibodies, etc. More specific evidence for the presence of receptors was provided by the binding of parasite antigens to specific host cell surface molecules.Analyses of host cells have implicated 22, 31, and 37 kDa antigens, surface membrane glycoconjugates,conserved epitopes of host cells and sporozoites, etc., but no treatment that perturbs these putative receptors has completely inhibited invasion of the cells by parasites. Regardless of the mechanism,sporozoites of the avian Eimeria also invade the same specific sites in foreign host birds that they invade in the natural host. Thus, site specificity for invasion may be a response to characteristics of the intestine that are shared by a number of hosts rather than to a unique trait of the natural host. Protective immunity elicited against avian Eimeria species is not manifested in a total blockade of parasite invasion. In fact, the effect of immunity on invasion differs according to the eliciting species and depends upon the area of the intestine that is invaded. Immunity produced against caecal species of avian Eimeria, for example Eimeria tenella and Eimeria adenoeides, inhibits subsequent invasion by homologous or heterologous challenge species, regardless of the area of the intestine that the challenge species invade. Conversely, in birds immunised with upper intestinal species, Eimeria acervulina and Eimeria meleagrimitis, invasion by challenge species is not decreased and often is significantly increased.     

Molecular characterization of isosporoid coccidia (Isospora and Atoxoplasma spp.) in passerine birds

Prevalence and disease caused by isosporoid coccidia in passerine birds are well recognized, but confusion about the life cycles of the parasites has led to taxonomic inconsistencies. In this study, we characterized segments of the chromosomal small and large-subunit ribosomal RNA (rRNA) genes of coccidial parasites from 23 species of passerine birds, as well as heat shock protein 70, apicoplast rRNA, and chromosomal 5.8s rRNA genes from a subgroup of these animals, and we correlated genetic data with morphologic findings for different parasite developmental stages, host phylogeny, and overall taxonomic relations within the phylum Apicomplexa. Our findings indicate that isosporoid coccidia of passerine birds are monophyletic but exhibit substantial diversity, with most avian species having one or several unique parasite lineages that underwent synchronous speciation with their hosts, interrupted by sporadic episodes of lateral transmission across species and families. Molecular analyses support a homoxenous life cycle, with sexual forms occurring chiefly in the intestines and asexual merozoites present systemically. Rarely, extraintestinal sexual stages can occur. The passerine coccidia are genetically most closely related to species of Eimeria rather than Isospora. We suggest that these parasites, whether identified from blood merozoite stages or fecal oocysts, be provisionally grouped as a homogeneous clade of individual species in a single taxon and formally named when reliable criteria allowing reclassification of related genera in the suborder Eimeriina are clarified.     

Failure of highly immunogenic filarial proteins to provide host-protective immunity

In areas that are endemic for lymphatic filariasis, there are individuals who are parasite free and who appear not to have experienced symptoms attributable to filarial infection. These "putatively immune" individuals may recognize immunogens that could be important in host protection. We have immunoscreened expression libraries expressing epitopes encoded by filarial open reading frames and have identified three antigens that are differentially recognized by the two polar clinical groups-endemic normals and asymptomatic microfilaremics. Pre-immunization of susceptible hosts (Meriones unguiculatus) with these antigens revealed that none was able to elicit consistent host protective immunity. Our data are consistent with Waksman's conjecture that highly immunogenic antigens of parasite origin may be inappropriate candidates for prophylactic immunization.     

Changes in parasite transmission stage excretion after pheasant release

The production of parasite transmission stages was investigated in the faeces of 77 farm-bred ring-necked pheasants (Phasianus colchicus). Coccidian oocysts (Eimeria sp.), and nematode eggs (Heterakis sp., and Capillaria-like eggs) were recovered before and after release but all birds were treated prior to release. Treatment with fenbendazole significantly reduced the abundance of transmission-stage excretion for all parasites, and reduced the prevalence in the case of Eimeria sp. and Heterakis sp. Nonetheless, a significant increase in the excretion abundance for all parasites and in the prevalence of Eimeria sp. and Heterakis sp. was found after release. Eggs of Ascaridia sp. were found only after releasing, suggesting infection ocurred in the wild. A negative relationship was found between the pheasant body condition and Heterakis excretion abundance and a higher abundance of Capillaria sp. eggs in female birds. No significant relationship was found between parasite excretion abundance and pheasant survival. Despite this, results suggest that an increase in the excretion of parasite transmission stages follows the release of captive pheasants into the wild. This can in part explain restocking failures, but also means that autochtonous free-living birds may become exposed to new and potentially harmful pathogens. To avoid these risks it is proposed that improved prophylactic measures should be taken.     

Experimental infection and adaptation of Rodentolepis nana to the Mongolian jird Meriones unguiculatus

A mouse-derived strain of Rodentolepis ( = Hymenolepis) nana was transferred to the Mongolian jird Meriones unguiculatus. It was found that M. unguiculatus has low susceptibility to R. nana mouse isolates. Likewise, adaptation of the parasite to jird hosts, in the absence of dexamethasone treatment, was not demonstrable, at least during ten-passage trials. Nevertheless, the parasite was able to establish, grow and develop to gravid adults in M. unguiculatus treated daily with dexamethasone. Prepatent periods in dexamethasone-treated jirds in ten-passage series ranged from 10 to 17 days post-infection (DPI), the average being 12 days, and the patent periods lasted from 18 to 30 DPI, with an average of 25 days. The population pattern of faecal egg output in immunosuppressed jirds suggests that under a daily dexamethasone treatment protocol, the parasite is able to maintain egg production as long as treatment is sustained.     

Be in motion . .

Most Apicomplexan are obligate intracellular parasites and at different steps of their life cycle they invade host cells. The invasive forms are generally called zoites and the majority of them largely depend on a unique form of gliding motility to invade cells. Although the parasite intracellular motor complex that drives gliding motility and/or invasion is shared across different parasite stages and species, the extracellular transmembrane adhesins required to recognize and bind host molecules are not only species‐ but also stage‐specific (even if homologues). This is not such a surprise as different parasite stages interact with different hosts or distinct host cells. In this issue, Siden‐Kiamos et al. shows that specificity extends into the parasite cell, affecting how motility is regulated. Why is specificity occurring at this level? And how important is it? These are critical issues that will be hopefully addressed in the near future.     

Eimeriidae of the Herbivorous Mole-Rat, Spalax ehrenbergi Nehring

SYNOPSIS. A survey of 41 herbivorous mole-rats, Spalax ehrenbergi Nehring, in Urfa, Adiyaman, and Maras provinces of Turkey revealed 7 new species of Eimeria in addition to previously described Eimeriidae. The shape, average dimensions (in μm) of their oocysts, and the numbers of hosts from which the new species were isolated were as follows: Eimeria urfensis sp. n., ellipsoidal (33 × 21), from 8 hosts; Eimeria adiyamanensis sp. n., ovoid to ellipsoidal (33 × 18), from 6 hosts; Eimeria haranica sp. n., elongate ovoid (37 × 20), from 22 rats; Eimeria marasensis sp. n., ellipsoidal (36 × 18), from 2 rats; Eimeria oytuni sp. n., pear-shaped (24 × 17), from 2 hosts; Eimeria celebii sp. n., ellipsoidal (16 × 9), from 1 rat; and Eimeria torosicum sp. n., spherical to subspherical (11 × 10), from 2 animals.     

Parasitophorous vacuole: morphofunctional diversity in different coccidian genera (a short insight into the problem)

We present a short insight into the problem of parasitophorous vacuole (PV) formation as a most peculiar kind of cell vacuolization occurring in the course of intracellular development of coccidian pathogens of the genera Eimeria, Isospora, Toxoplasma, Sarcocystis, Cryptosporidium, Epieimeria, and Karyolysus. The review focuses on the morpho-functional diversity of PVs in these parasites. By the present time, the PVs containing different parasite genera and species have been examined to different extent. The membrane of the PV (PVM) obviously derives from the host cell plasmalemma. But soon after parasite penetration, the morphofunctional organization and biochemical composition of the PVM drastically changes: its proteins are selectively excluded and those of the parasite are incorporated. As the result, the PV becomes not fusigenic for lysosomes or any other vacuoles or vesicles, because host cell surface markers necessary for membrane fusion are eliminated from the PVM during parasite invasion.The pattern of the PVs is parasite specific and demonstrates a broad diversity within the same genera and species and even at different stages of the endogenous development. The PV is far from being an indifferent membrane vesicle containing the parasite. Instead, it represents a dynamic system that reflects the innermost events of host-parasite relationships, thus promoting the accomplishing of the parasite life cycle, which, in its turn, is a necessary prerequisite of the parasite eventual survival as a species.     

A Model for Taxonomic Work on Homoxenous Coccidia: Redescription, Host Specificity, and Molecular Phylogeny of Eimeria ranae Dobell, 1909, with a Review of Anuran-Host Eimeria (Apicomplexa: Eimeriorina)

ABSTRACT. We attempt to extend knowledge of anuran Eimeria , and to provide a model for a complex approach to studies on coccidia. New host and geographic records of coccidia in European Anura are provided. In the second part, Eimeria ranae Dobell, 1909 is redescribed from European terrestrial frogs of the genus Rana based on light microscopic and ultrastructural data on both exogenous and endogenous developmental stages, host specificity, and molecular phylogenetic data. Results of experimental transmissions show for the first time that the host specificity of E. ranae is restricted to the genus Rana and that isolates from tadpoles and adults are conspecific. Disappearance of infection during metamorphosis was confirmed experimentally, suggesting that infections in adults result from reinfections. Poikilotherm-host Eimeria species possessing a Stieda body (SB) are for the first time included in a molecular phylogenetic analysis. Eimeria ranae and Eimeria arnyi from a colubrid snake form together a well-supported clade, basal to other SB-bearing coccidia. The other analysed reptile–host eimerians, Eimeria tropidura and Choleoeimeria sp., which possess bivalved sporocysts and lack a SB, represent a distinct basal lineage of the eimeriid clade. The third part of the article reviews anuran-host Eimeria . Three distinct oocyst morphotypes, apparently correlating with the character of endogenous development, are recognized and characterized among anuran eimeriids.     

The dynamics of infection of Tribolium confusum by Hymenolepis diminuta: the influence of infective-stage density and spatial distribution

The mean parasite burden of a population of Tribolium confusum is shown to rise to a plateau as the exposure density of infective eggs of Hymenolepis diminuta increases. The level of this plateau is shown to be dependent on the nutritional status of the host population, being depressed from approximately 18 cysticeroids/beetle in hosts which have been starved prior to experimentation, to approximately 2 cysticercoids/beetle in satiated hosts. A simple model is used to describe the shape of this infection functional response in terms of the predator-prey interaction between hosts (T. confusum) and parasite infective stages (H. diminuta eggs). The distribution of successful infections/host is shown to be over-dispersed, even when hosts are exposed to infective stages arranged in a uniform spatial pattern. The over-dispersion of parasite numbers/host is shown to become more severe as the spatial pattern of infective stages changes from under-dispersed, through random, to over-dispersed. Experimental results are discussed in relation to the dynamics of parasite-host interactions, in which infection takes place by host ingestion of a free-living infective stage.     

The Living, Endogenous Stages of the Rat Coccidium, Eimeria nieschulzi

SYNOPSIS. The living, endogenous stages of Eimeria nieschulzi Dieben, 1924 (Landers isolate) were studied under the phase contrast microscope. Active sporozoites were found as early as 2.5 hours after exposure and as late as 48 hours after exposure. The first generation schizont was recognized by the presence of a refractile globule remaining from the sporozoite. First and second generation merozoites were only weakly motile and had small paired organelles. Third generation merozoites were seen 48–120 hours after exposure and were strongly motile from 72 hours after exposure onward. The paired organelle consisted of 2 intertwining portions, one 5.5 μ long, the other tapering to a slender filament and continuing to about the posterior quarter of the parasite. The fourth generation merozoites were short, curved, and weakly motile. A paired organelle about 3 μ long was seen. Gametocytes and gametes were seen 144–192 hours after exposure. Macrogametes appeared to elaborate refractile granules in the vicinity of the nucleus. No motility of any type was seen in the macrogametes. Microgametocytes were recognized when nuclear material moved to the periphery of the parasite for the formation of microgametes. Observations on living organisms agreed generally with those made on fixed and stained organisms with the exception that the living merozoites were about 20% larger.