Crustacean-acanthocephalan interaction and host cell-mediated immunity: parasite encapsulation and melanization

Host-parasite interactions of Pomphorhynchus laevis (Müller, 1776) in naturally infected amphipod, Echinogammarus stammeri (Karaman), from the Brenta River (northern Italy) are described. A fully developed acanthocephalan larva occupies a large portion of an amphipod's haemocoelic space; thus, the parasite frequently induces displacement of host digestive tract and other internal organs. However, no apparent damage to the host's internal structures was observed. Within the haemocoel of E. stammeri, each larva of P. laevis is surrounded with a membranous layer, formed by microvilli, which maintains intimate contact with the amphipod's internal organs and haemocytes. Three types of circulatory haemocytes were identified based upon their distinct appearance: hyaline cell, semi-granular cell and granular cell. Echinogammarus stammeri haemocytes surrounded acanthocephalan larvae and in some instances a partially and/or totally melanized P. laevis larva was noticed. Interestingly, no melanized larvae were found in E. stammeri parasitized with other acanthocephalans namely Echinorhynchus truttae (Schrank, 1788), Polymorphus minutus (Goeze, 1782) and Acanthocephalus clavula (Dujardin, 1845).     

New evidence on a cold case: trophic transmission, distribution and host-specificity in Hedruris spinigera (Nematoda: Hedruridae)

The life cycle of Hedruris spinigera Baylis, 1931 (Nematoda: Hedruridae) is determined here with the first formal identification of the parasite's intermediate host: the crustacean amphipod Paracorophium excavatum Thomson. Adult H. spinigera are redescribed from specimens collected from the stomach of fishes, Retropinna retropinna (Richardson) and Aldrichettaforsteri (Valenciennes), from Lake Waihola, New Zealand. Immature adults of the parasite collected from intermediate hosts (P. excavatum) are also described for the first time. The prevalence, abundance and intensity of infection of H. spinigera in several fish species are quantified along with the occurrence of P. excavatum, the parasite's intermediate host, in fish stomach contents. Although H. spinigera's transmission mode (trophic transmission) and fish diet potentially expose all fish species to infection, some level of host specificity must exist as parasite prevalence, abundance and intensity of infection vary greatly between potential definitive host species. We suggest here that the anatomy of the fish digestive tract and especially that of the stomach plays an important role in host suitability for H. spinigera. While P. excavatum is the only intermediate host in Lake Waihola, H. spinigera was found in six different fish species: Aldrichetta forsteri, Galaxias maculatus (Jenyns), Retropinna retropinna, Rhombosolea retiaria Hutton, Perca fluviatilis Linnaeus and Salmo trutta Linnaeus; although typical hedrurid attachment and mating positions were observed only in R. retropinna and A. forsteri. The limited distribution of H. spinigera is most likely due to that of its different host species (intermediate and definitive), all inhabitants of coastal fresh and brackish waters.     

Studies on transmission and life cycle of Enteromyxum scophthalmi (Myxozoa), an enteric parasite of turbot Scophthalmus maximus

In order to elucidate the transmission and dispersion routes used by the myxozoan parasite Enteromyxum scophthalmi Palenzuela, Redondo et Alvarez-Pellitero, 2002 within its host (Scophihalmus maximus L.), a detailed study of the course of natural and experimental infections was carried out. Purified stages obtained from infected fish were also used in in vitro assays with explants of uninfected intestinal epithelium. The parasites can contact and penetrate loci in the intestinal epithelium very quickly. From there, they proliferate and spread to the rest of the digestive system, generally in an antero-posterior pattern. The dispersion routes include both the detachment of epithelium containing proliferative stages to the intestinal lumen and the breaching of the subepithelial connective system and local capillary networks. The former mechanism is also responsible for the release of viable proliferative stages to the water, where they can reach new fish hosts. The finding of parasite stages in blood smears, haematopoietic organs, muscular tissue, heart and, less frequently, skin and gills, suggests the existence of additional infection routes in transmission, especially in spontaneous infections, and indicates the role of vascular system in parasite dispersion within the fish. The very high virulence of this species in turbot and the rare development of mature spores in this fish may suggest it is an accidental host for this parasite. This may also question the existence of a two-host life cycle involving an actinosporean stage in this species. Further studies are needed to clarify this open point of the life cycle.     

Differential host utilisation by different life history stages of the fish ectoparasite Argulus foliaceus (Crustacea: Branchiura)

In this study we examine differences in the occurrence of life history stages of the destructive fish ectoparasite Argulus foliaceus (L., 1758) on eight fish species (stickleback, rudd, roach, gudgeon, bream, tench, crucian carp and common carp) sampled from a mixed-species recreational fishing lake on nine occasions during late spring and summer. Total numbers ofA. foliaceus, as well as the number of larval, juvenile and adult parasite stages, from each fish were recorded along with the fish species. Lice generally exhibited an aggregated distribution approximating a negative binomial distribution. Significant differences in the prevalence, intensity and intensity frequency distribution were observed between life history stages and between host species. In general, all life history stages of A. foliaceus exhibited an over-dispersed distribution. However, larval lice did show some degree of aggregation particularly within the stickleback samples. Infection data for parasite larval stages suggested that sticklebacks are more likely to be infected than other host species. For adult lice, however, carp appeared to be the main host. We propose that A. foliaceus infection characteristics are predominantly determined by the level of host exposure to the parasite and its life history stages (larval, juvenile and adult) rather than by an innate difference in host susceptibility related to individual host factors such as immune responses. We conclude that host exposure is determined by the parasite-host behavioural interplay related to species-specific ecology and behavioural traits such as microhabitat preference and normal swimming speed.     

Effects of temperature on the in vivo and in vitro multiplication of Trypanosoma danilewskyi Laveran et Mesnil.

Temperature directly affected the in vivo and in vitro multiplication of Trypanosoma danilewskyi Laveran et Mesnil. The parasitemia in experimentally infected goldfish (Carassius auratus L.) increased rapidly at 20 degrees C and reached its peak in 3 to 5 weeks post-infection. The multiplication was slower at 10 degrees C. The parasitemia declined after the peak at 20 degrees C. Lowering of temperature of the infected goldfish from 20 degrees C to 10 degrees C slowed multiplication of the parasite. Similarly, raising of temperature of infected fish from 10 degrees C to 20 degrees C resulted in rapid multiplication of the parasite. These were confirmed in the in vitro studies. It is suggested that lower temperature slows the multiplication of the parasites because T. danilewskyi is a parasite of a warm water host (Carassius auratus gibelio Bloch).     

Semiochemical strategies for sea louse control: host location cues

The development of behavioural bioassays and electrophysiological recording techniques has enabled the role of semiochemicals to be investigated for the first time in Lepeophtheirus salmonis (Kr?yer). Adult male sea lice L salmonis were activated by, and attracted to, salmon-conditioned water (SCW) and SCW extract. Non-host fish odours, turbot-conditioned water (TCW) and turbot-conditioned water extract (TCW extract), elicited activation but no attraction in the lice. Solid-phase extraction techniques were developed to extract low molecular weight components of fish odours, and biological activity was shown to be confined to these. Electrophysiological activity from aesthetascs in the first antennae was found and neural responses to odours were recorded from the antennal nerves. Preliminary experiments on individual chemicals (isophorone, 1-octen-3-ol) linked to salmon revealed behavioural activation and electrophysiological responses in adult male L salmonis. Isophorone was shown to be significantly attractive to sea lice when placed in a slow-release system in a perfused tank of seawater in a choice situation. Proof of concept has been established for successfully extracting odour cues from seawater, analysing their biological activity and applying these to slow-release technologies for field trapping of lice. Future work involving linked GC-MS techniques using behaviour, electrophysiological responses, and organolepsis will establish further host location cues specific to Atlantic salmon.     

Orobanche crenata resistance and avoidance in pea (Pisum spp.) operate at different developmental stages of the parasite

Orobanche crenata (broomrape) is an important constraint to pea (Pisum sativum) cultivation in the Mediterranean area, because little resistance is available in commercial crop varieties. Field experiments have demonstrated that some resistance is present in a number of P. sativum and P. fulvum accessions. The goal of this work was to characterize such resistance. The Pisum–O. crenata interaction and the resistance symptoms were studied under controlled conditions by using Petri dish and polyethylene bag assays. The content of phenolics and peroxidase activity in host tissue from infected and non-infected plants were also measured. Resistance and avoidance mechanisms, acting at different developmental stages of the parasite, have been identified, including low stimulation of O. crenata seed germination, unsuccessful penetration of host roots, delay in post-attachment tubercle development and necrosis of the attached tubercles. Infection caused an increase in the content of total soluble phenolics in some Pisum genotypes. Peroxidase activity was higher in resistant than in susceptible accessions. Results obtained with different Pisum genotypes showed that resistance is the result of several mechanisms acting at different stages of the infection process. Resistance is also related to increased levels of peroxidase activity in host roots.     

Infection dynamics of the monogenean parasite Gyrodactylus gasterostei on sympatric and allopatric populations of the three-spined stickleback Gasterosteus aculeatus

Parasites with high host specificity maximally depend on their hosts, which should increase the likelihood of coevolution. However, coevolution requires reciprocal selection exerted by the host and the parasite, and thus a considerable level of parasite virulence. In species of the monogenean ectoparasite genus Gyrodactylus consecutive generations are confronted with a single host, which may constrain the evolution of virulence. Transmission, which is often important in the ecology of Gyrodactylus species, may have the opposite effect, but may also lead to the avoidance of coevolutionary arms races. We investigated the potential outcome of coevolution between Gyrodactylus gasterostei Gl?ser, 1974 and its host, the three-spined stickleback (Gasterosteus aculeatus L.) by determining the strength of genotype by genotype (GxG) interactions on two levels: within and between sympatric and allopatric host populations. To do so, we compared the parasite's infection dynamics on laboratory-reared sympatric (Belgian) and allopatric (German) hosts. We found that a parasite line successfully infected a range of sympatric host genotypes (represented by families), while it failed to establish on allopatric hosts. Phylogeographic studies suggest that neutral genetic divergence between the host populations cannot explain this dramatic difference. Provided that this result can be generalised towards other parasite lines, we conclude that coevolution in this host-parasite system is more likely to lead to local adaptation on the population level than to GxG interactions within populations.     

Uptake and translocation of imazethapyr in peas as affected by parasitism of Orobanche crenata and herbicide application methods

[¹⁴C]-Imazethapyr was applied as a seed treatment and at plant pre-emergence and post-emergence to peas ( Pisum sativum L.) parasitized by Orobanche crenata Forsk. Herbicide uptake increased with time regardless of the application method. Uptake reached about 98%, 89%, 81% and 94% of the total herbicide applied for the seed coating, seed soaking, pre-emergence and post-emergence treatments respectively. Herbicide translocation within the host plants consistently differed between O. crenata -infected and non-infected plants. High levels of ¹⁴C activity were accumulated by parasitic plants from the host. In non-infected pea plants, pods were stronger sinks for imazethapyr than the other parts of the plant, regardless of the application method. The herbicide distribution in the pea plant: O. crenata complex showed the same pattern regardless of the application methods. However, accumulation of radioactivity in the parasite was lower with pre-emergence and post-emergence application than with the seed treatments. In addition, radioactivity concentration in O. crenata plants was slightly higher when [¹⁴C]-imazethapyr was applied to pea seeds by coating than by soaking.     

Gill parasites of Cephalopholis argus (Teleostei: Serranidae) from Moorea (French Polynesia): site selection and coexistence

The distribution and coexistence of gill ectoparasites of 121 specimens of Cephalopholis argus Bloch et Schneider, caught between October 1994 and October 1995, were investigated. Adults of the monogenean Benedenia sp. and copepod Hatschekia sp., the larval caligid copepod Caligus sp. (copepodite and chalimus stages), and praniza larvae of the isopod Gnathia sp. were found. All species were aggregated within the host population. Infracommunities were poor, with only 40.5% of fish infected by two parasite species. Only two individual fish harboured all the parasite species observed at the component community level. Prevalences were less than 50% and mean intensities were low (less than 6 parasites/host). No dominant parasite species were observed in the host population. The spatial distribution of each parasite species was studied on different partitions of the gill arches. Adult parasite stages that are mobile showed much overlap in their distribution, whereas temporarily attached larvae of Caligidae were more site specific. Copepodite and chalimus larvae showed niche restriction that is probably due to gregarious behaviour. Positive associations between caligid larvae reflected intraspecific interaction for site and/or resources. Each of the Caligus sp. larval stages prefers specific sites, as do the adults, which occur exclusively in the buccal cavity of the host. Infracommunities were too poor and too few to induce processes of interspecific competition.     

Observations on the development of Rhabdochona phoxini Moravec, 1968 (Nematoda: Rhabdochonidae).

The life-cycle of the nematode Rhabdochona phoxini, an intestinal parasite of the fish Phoxinus phoxinus, has been studied experimentally. The mayfly nymphs, Habrophlebia lauta Eaton and H. fusca (Curtis) were found to serve as experimental intermediate hosts. After the eggs of R. phoxini have been swallowed by the mayfly nymph, the toothed first-stage larvae of this parasite are released and penetrate through the intestinal wall into t-e body cavity of the mayfly. Before reaching the infective third stage, the larvae moult twice in the body of the invertebrate host (2 to 16 and20 to 36 days after infection at water temperatures of 13--15 degrees C). However, the development of the encysted infective larvae does not cease in the intermediate host and the larvae may undergo one more moult. One of two moults then occur in the intestine of the definitive host (Phoxinus phoxinus) (depending on the fact whether it was infected with third- or fourth-stage larvae) before the larvae attain the maturity. The development of R. phoxini in the definitive host lasts about two months.     

Studies on the development of the cestode Proteocephalus neglectus La Rue, 1911 (Cestoda: Proteocephalidae) under experimental conditions

A part of the life cycle of Proteocephalus neglectus La Rue, 1911, a parasite of trout, starting from release of eggs from mature parasites into water, to the early phase of development in the definitive host, was studied under experimental conditions. Special regard has been paid to development in the intermediate host, copepod Cyclops strenuus. Some oncospheres in eggs kept in water at 5 and 10 degrees C remained infective for 20-25 days. The percentage of infected copepods depended on the length of their contact with parasite eggs. Cestode larvae (cercoscoleces) were formed in the intermediate host on days 8-10 p.i. at the temperatures of 21-22 degrees C, on days 18-21 at 15 degrees C, on days 24-28 at 10 degrees C, and on days 59-65 at 6 degrees C. Most larvae, including infective cercoscoleces, were localized in the cephalothorax of the intermediate host, particularly in its first segment. This localization did not change during their development. The infectivity of cercoscoleces was verified by experimental infection of Salmo gairdneri fry. The development of the cestode in this definitive host was observed for 17 days after infection at 10 degrees C. The finding of P. neglectus cereoscoleces in fish of the families Cottidae and Cyprinidae on day 2 after experimental infection indicates that these larvae can survive for a short time in atypical fish hosts.     

Component population study of Acanthocephalus tumescens (Acanthocephala) in fishes from Lake Moreno,Argentina

Seasonal samples of all fish species from Lake Moreno were taken in order to determine the presence of paratenia, to evaluate the status of the hosts and to characterise the transmission of Acanthocephalus tumescens (von Linstow, 1896) at the component population level. Prevalence, mean abundance, mean intensity, numbers of gravid females, relative abundance of the different fish species, relative output of eggs and relative flow rates for each host species were computed. Acanthocephalus tumescens showed low host specificity, successfully parasitizing six out of eight fish species present in the lake. No paratenic infection was registered. If prevalence, mean abundance, and number of gravid females are considered, host species can be placed in a continuum from the most to least suitable as follows: Galaxias platei Steindachner, Diplomystes viedmensis (Mac Donagh), Oncorhynchus mykiss (Walbaum), Salvelinus fontinalis (Mitchill), Percichthys trucha (Cuvier et Valenciennes) and Galaxias maculatus (Jenyns). However, when parasite flow rates and egg output were calculated, including relative abundance of each fish species, the continuum was rearranged as follows: P. trucha, O. mykiss, G. platei / G. maculatus, S. fontinalis and D. viedmensis. The first four species would be the main contributors to the population of A. tumescens in this lake, P. trucha being the major one. Different regulatory and non-regulatory mechanisms are suggested.     

Distribution of Hatschekia pagellibogneravei (Copepoda: Hatschekiidae) on the gills of Pagellus bogaraveo (Teleostei: Sparidae) from Madeira, Portugal

A population of the gill parasite Hatschekia pagellibogneravei (Hesse, 1878) was studied on one of its sparid fish hosts, the blackspot seabream, Pagellus bogaraveo (Brünnich), off the coast of Madeira Island, Portugal, northeast Atlantic. Very high infection levels of this copepod were detected, with no significant seasonal differences. Abundance was negatively correlated with fish size. There were significant differences in the distribution of this copepod among the gill arches of the host, which seem to be best explained by differences in water flow within the gill habitat.     

Colonisation and extinction in relation to competition and resource partitioning in acanthocephalans of freshwater fishes of the British Isles

This paper challenges two paradigms long held in relation to the ecology of parasites in freshwater systems: (1) autogenic species are poorer colonisers than allogenic ones; and (2) parasites with direct life cycles are more successful colonisers than those with complex life cycles. Using new and existing data for Acanthocephala in freshwater fish from the British Isles, it is suggested that all six species present have been able to colonise and persist successfully, in spite of the supposed limitations of their autogenic life-style. It is proposed that these parasites have overcome these limitations by a variety of means, which apply equally to all species considered. Foremost among these is the utilisation of a migratory fish host as either a preferred or a suitable host in their life cycle, allowing colonisation of new areas and rescue effects in established areas, whilst equally important is the use of a common and widespread crustacean as the intermediate host. In addition, all six species appear to exhibit resource partitioning by host at either or both the larval and adult stages, thus reducing the potential for competition and further facilitating colonisation and survival. This hypothesis is supported by data from previous studies both on acanthocephalans from Europe and North America and on other autogenic parasites. It also provides an explanation for the apparently atypical host utilisation patterns of some acanthocephalan species in areas on the edge of their distributions, notably in Ireland.     

Morphometrics and seasonal occurrence of metacestodes of Neogryporhynchus cheilancristrotus (Cyclophyllidea: Dilepididae) in the blue bream (Abramis ballerus) from the Oder River (Germany/Poland)

From May 1993 to April 1995, the seasonal occurrence of metacestodes of Neogryporhynchus cheilancristrotus (Wedl, 1855) (Cyclophyllidea: Dilepididae) in its second intermediate host, the blue bream Abramis ballerus (L.) was studied monthly in the Oder River on the borders of Germany and Poland. Based on the parasite specimens found, detailed data on their morphometrics are presented. The metacestodes occurred in the blue bream intestine throughout the year (overall prevalence 27% and intensity 1-56 (mean 4.8) metacestodes per infected fish). Increased prevalences and mean intensities of infection were noted from March to June and November to December indicating that spring, late autumn and early winter are the main periods of new infections.     

Myxobolus buckei sp. n. (Myxozoa), a new pathogenic parasite from the spinal column of three cyprinid fishes from the United Kingdom

Myxobolus buckei sp. n. is described from the spinal column of Leuciscus cephalus (L.), Rutilus rutilus (L.) and Abramis brama (L.) from freshwater rivers in the North of England. The plasmodia develop within the remnants of the embryonic notochord in the intervertebral spaces. The spores are large, measuring (in microm) 14.0 +/- 0.7 x 11.5 +/- 0.6 (mean +/- SD), smooth, round to ellipsoid in valvular view with several sutural edge markings. The polar capsules are pyriform and of equal size, measuring 7.5 +/- 0.5 x 4.2 +/- 0.2 (mean +/- SD), with 11-12 turns of the polar filament arranged perpendicularly to the longitudinal axis of the polar capsule. The parasite has a large intercapsular appendix and large iodinophorous vacuole. The parasite can be differentiated from all known species of Myxobolus Bütschli, 1882 by a combination of the morphological characters defined. Infected fish show marked longitudinal compression of the body compared to uninfected individuals of the same year class, a feature which is pathognomonic for the disease. Histologically, host responses ranged from mild hypertrophy of the zygapophyseal process and expansion of the intervertebral membrane to complete hypertrophy and fusion of the vertebrae. Prominent notochord is present in the intervertebral spaces of infected fish and sporogony of the parasite leads to a vigorous focal inflammatory response involving proliferating fibroblast and osteogenic cells. The parasite causes a radial expansion of the centra and extensive dorsal and ventral outgrowths of the vertebrae leading to compression of the spinal cord and blood vessels running through the neural and haemal spines respectively. The parasite is considered highly pathogenic to juvenile cyprinids.     

Intestinal helminths from grey mullets, Mugil capito Cuv. and M. cephalus L., of Lake Borullus, A.R.E.

Helminthological examination of grey mullets, Mugil capito Cuv. and M. cephalus L. from brackish Lake Borullus in the Nile Delta in the Lower Egypt revealed that these fishes are infected with two species of digenetic trematodes, Haplosplanchnus pachystoma (Eysenhardt, 1829) and Saccocoelium obesum Loos, 1902 and one acanthocephalan species, Neoechinorhynchus agilis (Rudolphi, 1819). On comparing the incidence and intensity of infection of individual fish samples it was found that after migration from the sea to the lake the grey mullets lose gradually their intestinal trematodes.     

Post-embryonic development of Camallanus cotti (Nematoda: Camallanidae), with emphasis on growth of some taxonomically important somatic characters

In this paper, the quantitative post-embryonic development of the Asian freshwater fish nematode Camallanus cotti Fujita, 1927, is described. Larval and adult morphometrics were obtained by following the parasite's life cycle experimentally using copepods Macrocyclops albidus (Jurine) as intermediate host and guppies Poecilia reticulata (Peters), southern platyfish Xiphophorus maculatus (Günther) and paradise fish Macropodus opercularis (L.) as definitive host. Additionally, adult worms were obtained from heavily infected paradise fish imported from Singapore. It is suggested that the gradual change in proportions of the worm's somatic body parts reflects the specific ecological role of each developmental stage. The free-living infective first-stage larva seems to be adapted for transmission, as indicated by its relatively long tail, designed to generate host-attracting movements, and its non-functional intestine. The second- and third-stage larvae from the copepod intermediate host seem mainly to invest in trophic functionality, i.e., the development of the buccal capsule and the oesophagus, which are crucial structures for the worm's successful establishment in the definitive fish host. Once in the fish intestine, the larvae enter a period of considerable growth. After the fourth (i.e., last) moult, a 72% increase in average female body length occurs. This is accompanied by doubling the average vulva-tail tip distance and the average tail length. The length of the female hind body expands in an accelerating allometric fashion, and seems to be closely linked to the posterior-wards expansion of the uterus. In the males however, growth seems to cease after the final moult. We conclude that female post-maturational body size, but especially the length of the hind body and the tail, are closely related to reproductive state, i.e., the developmental stage of the offspring in the uterus, and, probably, the worms' age. Any future taxonomical studies of camallanids in general, and C. cotti in particular, should thus be aware of the reproductive state of the females used.