Infection of its lepidopteran host by the Helicoverpa armigera stunt virus (Tetraviridae)

Techniques of microscopy and histopathology were employed to study the positive-sense, single-stranded RNA virus, the Helicoverpa armigera stunt virus (HaSV; omegatetravirus, Tetraviridae) infecting its caterpillar host. Infection of the virus per os during the first three instars of larval development is virulent and leads to rapid stunting and mortality. In contrast, no detectable symptoms occur in later larval development, signifying a high degree of developmental resistance. A quantitative study of cell populations in the host midgut during this time showed that increased cell numbers during development alone could not account for the increase in resistance. HaSV infection was restricted to the midgut and three of its four cell types. In younger larvae, the virus initiated its infection in closely situated foci that appeared to expand to link with others to cover larger areas of the midgut. The midgut cells of the infected larvae responded with an increased rate of sloughing to an extent rendering the midgut incapable of maintenance or recovery of normal function. In contrast, infection of older larvae by HaSV did not lead to overt pathology although foci of HaSV infection were detected in their midguts. However, the foci were more sparsely situated, failed to expand, and eventually disappeared, presumably due to cell sloughing. These observations indicate that cell sloughing is an immune response existing throughout larval development but midguts of older larvae have an additional mechanism to account for the increased resistance. This second mechanism results in midgut cells becoming more refractory to infection and, combined with cell sloughing, allows the midguts of older larvae to recover more readily from HaSV infection. These two mechanisms are similar to those seen with host responses to baculoviruses, which display developmental resistance to a lesser degree against more general infections. HaSV remaining in the midgut appears to amplify the degree of developmental resistance.     

Histopathology and ultrastructure of Platichthys flesus naturally infected with Anisakis simplex s.l. larvae (Nematoda: Anisakidae)

The histopathology, ultrastructure, and immunohistochemistry of the alimentary canal of flounder Platichthys flesus (L.), naturally infected with the nematode Anisakis simplex s.l. (Rudolphi 1809) from the River Forth (Scotland), were investigated and described. Eight of the 16 flounders were infected with A. simplex s.l. larvae (L3); parasites were encapsulated by serosa on the external surface of the host's digestive tract (intensity of infection 1-8 parasites per host), although nematode larvae were found encysted under the peritoneal visceral serosa of the host spleen and liver and, occasionally, in the liver parenchyma (intensity of infection 3-10 parasites per host). In all sites, different structural elements were recognized within the capsule surrounding larvae. Among the epithelial cells of the intestine of 5 flounders with larvae encysted on external surface of the gut, the presence of several rodlet cells (RCs) was observed. Furthermore, often the occurrence of macrophage aggregates (MAs) was noticed in infected liver and spleen, mainly around the parasite larvae. Eight neuropeptide antisera were tested with immunohistochemistry methods on gut sections of 4 P. flesus infected with extraintestinal nematodes. Sections from the gut of 5 uninfected flounder were used for comparative purposes. In the tunica mucosa of parasitized P. flesus, several endocrine epithelial cells were immunoreactive to anti-CCK-39 (cholecystokinin 39) and -NPY (neuropeptide Y) sera; furthermore, in the myenteric plexus, a high number of neurons were immunoreactive to antibombesin, -galanin, and several to -NPY and -PHI (peptide histidine isoleucine) sera.     

Tuberculosis (Mycobacterium microti) in wild field vole populations

Vole tuberculosis (TB; Mycobacterium microti) is an understudied endemic infection. Despite progressing slowly, it causes severe clinical pathology and overt symptoms in its rodent host. TB was monitored for 2 years in wild field voles in Kielder Forest, UK. The prevalence of characteristic cutaneous TB lesions was monitored longitudinally at 4 sites, with individuals live-trapped and repeatedly monitored. A prevalence of 5.2% of individuals with lesions was recorded (n=2791). In a cross-sectional study, 27 sites were monitored bi-annually, with TB assessed by post-mortem examination for macroscopic lesions, and by culture and histopathology. Seventy-nine voles (10.78%; n=733) were positive for mycobacteria, with the highest prevalence in spring (13.15%; n=327). TB prevalence varied, with between 0% and 50% of voles infected per site. Prevalence increased with age (mass), and apparent seasonality was due to a higher proportion of older animals in spring. Survival analysis supported this result, with cutaneous lesions only manifesting in the advanced stages of infection, and therefore only being found on older voles. The body condition of individuals with lesions declined at the time when the lesion was first recorded, when compared to individuals without lesions, suggesting there may be an acute phase of infection during its advanced stage. Although predicted survival following the appearance of a cutaneous lesion was lower than for uninfected individuals, this was not significant.     

Kinetics of primary and secondary infections with Taenia crassiceps metacestodes (Zeder, 1800) rudolphi, 1810 (cestoda: cyclophyllidea)

Siebert A. E. Jr., Good A. H. & Simmons J. E. 1978. Kinetics of primary and secondary infections with Taenia crassiceps metacestodes (Zeder, 1800) Rudolphi, 1810 (Cestoda: Cyclophyllidea). International journal for Parasitology8: 39–43. When three T. crassiceps metacestodes were inoculated intraperitoneally in mice as a primary infection, approximately 50% of the larvae recovered during the first 4 weeks after inoculation were found to be dead, while in mice primed by previous subcutaneous inoculation, about 85% of the larvae died. Larvae which survived the first 4 weeks following primary intraperitoneal inoculation reproduced asexually by exogenous budding and produced viable infections within the host mice. But larvae in secondary infections were encapsulated by host granulomata, failed to reproduce asexually, and did not produce viable infections. In mice given intraperitoneal inoculations of seven, ten and twenty metacestodes, fewer larvae were killed and little encapsulation response was noted, though host cells were common at the budding region of the larvae. Such a biphasic host-response to the infection has not previously been reported for larval cestode infections, and the reduction in host response associated with increased worm burdens may indicate possible depression of the host immune system.     

Infection by the microsporidium Vairimorpha necatrix (Microspora: Microsporidia) elevates juvenile hormone titres in larvae of the tomato moth, Lacanobia oleracea (Lepidoptera: Noctuidae)

The effects of infection by a microsporidium, Vairimorpha necatrix (Kramer), on the endogenous levels of juvenile hormones in tomato moth (Lacanobia oleracea L.) larvae were investigated. Levels of juvenile hormone II (JH II) were 10-fold greater in the infected larvae on day two of the sixth stadium but no significant difference was observed on day seven. Juvenile hormone I (JH I) was also detected in day two and day seven sixth instar infected larvae but was not detected in non-infected larvae. The duration of the fifth and sixth stadia was significantly longer for infected larvae when compared with non-infected larvae. No evidence was found to suggest that supernumerary moults are a feature of infection by V. necatrix in L. oleracea larvae. Experiments were performed to determine whether the elevation in JH levels, which probably prevents pupation, is an adaptive mechanism of the microsporidium for extending the growth phase of the host, thereby allowing increased spore production. A proportion of infected larvae were collected on days 9 and 24 of the sixth stadium and spore extracts prepared from each larva. These days represent the average duration of the sixth stadium required for uninfected larvae to reach pupation, and the average number of days that V. necatrix-infected larvae survive in the sixth stadium before dying from infection. The mean spore yields from infected larvae 24 days into the sixth stadium were significantly higher than the spore yields obtained from day nine sixth instar larvae. The hypothesis that V. necatrix manipulates host endocrinology (i.e. prolong the host larval state to maximise spore yield) is discussed in context with the results obtained.     

球孢白僵菌不同感染方式侵染棉铃虫幼虫的毒性比较及组织病理变化

为明确球孢白僵菌在不同感染方式下对棉铃虫的侵染能力, 采用饲喂法和浸渍法测定了球孢白僵菌HFW-05对棉铃虫Helicoverpa armigera (Hübner) 2龄幼虫的致病力, 并通过组织切片显微技术、 扫描电镜技术观察了球孢白僵菌HFW-05对棉铃虫的致病方式。结果表明: 白僵菌HFW-05可通过消化道（饲喂法）成功侵染2龄棉铃虫, 接种感染6 d后的校正死亡率为75.8%。经由体表（浸渍法）接种白僵菌HFW-05的试虫, 试验中体重的变化和取食量与对照相近（6 d校正死亡率仅为17.3%, 不能通过体表达到致病效果）。组织病理学变化表明: 26±1℃条件下, HFW-05菌株对棉铃虫以消化道侵染为主, 侵染后可导致寄主中肠微绒毛脱落严重, 肠壁组织溶解并最终只剩余底膜; 马氏管变形萎缩, 边缘向外突出隆起, 管径变大; 脂肪体萎缩解体, 结构松散; 表皮下的细胞被菌丝侵染破坏。浸渍法接种的试虫, 切片观察处理6 d后试虫, 体内未发现菌丝, 肠壁组织正常完整。扫描电镜观察, 浸渍法接种的分生孢子未能穿透棉铃虫表皮, 而是贴于寄主表皮表面生长, 在湿度合适的条件下, 菌丝生长到一定时间后断裂成为芽生孢子。白僵菌HFW-05可经由消化道对棉铃虫达到较高的致病效果, 在一定程度上弥补外界环境对白僵菌侵染的不利影响, 对今后应用白僵菌进行生物防治具有重要意义。     

Virulence of Corynosoma constrictum (Acanthocephala: Polymorphidae) in Hyalella azteca (Amphipoda) throughout parasite ontogeny

Development and growth of parasites depend on resources provided by the host and the parasite's ability to use them. Identifying specific costs incurred by the host provides insight for assessment of parasite energy budgets, which differ among taxa and ontogenetic stages. Data from this study were analyzed using an accelerated failure-time model with intensity as a covariate. Results indicated significantly reduced survival of amphipods, Hyalella azteca, infected with the acanthocephalan Corynosoma constrictum compared with uninfected controls. Male and female amphipod survivorship and infection intensity did not differ; however, amphipods with high-intensity infections (> 16 larvae) died earlier compared with amphipods with low-intensity infections (< 3 larvae). The majority of infected amphipods died between 12 and 24 days postexposure, a period of rapid larval development. It is hypothesized that host death may be due either to an increase in overall larval nutritional demands or to parasite-mediated depletion of a specific host substance. Results from this study suggest that developing C. constrictum satisfies energy requirements by depriving amphipod hosts of resources normally used for somatic growth and maintenance.     

Hatching strategies in monogenean (platyhelminth) parasites that facilitate host infection

In parasites, environmental cues may influence hatching of eggs and enhance the success of infections. The two major endoparasitic groups of parasitic platyhelminths, cestodes (tapeworms) and digeneans (flukes), typically have high fecundity, infect more than one host species, and transmit trophically. Monogeneans are parasitic flatworms that are among the most host specific of all parasites. Most are ectoparasites with relatively low fecundity and direct life cycles tied to water. They infect a single host species, usually a fish, although some are endoparasites of amphibians and aquatic chelonian reptiles. Monogenean eggs have strong shells and mostly release ciliated larvae, which, against all odds, must find, identify, and infect a suitable specific host. Some monogeneans increase their chances of finding a host by greatly extending the hatching period (possible bet-hedging). Others respond to cues for hatching such as shadows, chemicals, mechanical disturbance, and osmotic changes, most of which may be generated by the host. Hatching may be rhythmical, larvae emerging at times when the host is more vulnerable to invasion, and this may be combined with responses to other environmental cues. Different monogenean species that infect the same host species may adopt different strategies of hatching, indicating that tactics may be more complex than first thought. Control of egg assembly and egg-laying, possibly by host hormones, has permitted colonization of frogs and toads by polystomatid monogeneans. Some monogeneans further improve the chances of infection by attaching eggs to the host or by retaining eggs on, or in, the body of the parasite. The latter adaptation has led ultimately to viviparity in gyrodactylid monogeneans.     

Coevolution of the Monogenoidea (Platyhelminthes) based on a revised hypothesis of parasite phylogeny

A revised hypothesis for the phylogeny of the Subclass Polyonchoinea (Monogenoidea) was contructed employing phylogenetic systematics. The Acanthocotylidae (formerly of the Order Capsalidea) is transferred to the Order Gyrodactylidea based on this analysis. The new phylogeny is used to determine coevolutionary relationships of the familial taxa of Monogenoidea with their hosts. The coevolutionary analysis suggests that the Monogenoidea apparently underwent sympatric speciation or dispersal while parasitic on ancestral Guathostomata, resulting in two primary clades: the Polyonchoinea and the Oligonchoinea + Polystomatoinea. The two parasite clades apparently cospeciated independently with divergence of the Chondrichthyes and Osteichthyes. In the Polyonchoinea, the clade associated with Chondrichthyes experienced primary extiaction within the Holocephala, but coevolved into the Loimoidae and Monocotylidae in the Galeomorphii and Squalea (Elasmobranchii), respectively. Within the Osteichthyes, polyonchoineans experienced primary extinction with the divergence of Sarcopterygii, Polypteriformes and Acipenseriformes. They demonstrate primary dispersal from the Neopterygii into the Squalea (as Amphibdellatinea), Actinistia (as Neodactylodiscinea) and Urodela (as Lagarocotylidea). Secondary dispersals of polyonchoineans occurred in the Gyrodactylidae to the Polypteriformes, Urodela and Anura; in the Acanthocotylidae to the Myxinoidea and Squalea; in the Capsalidae to the Acipenseriformes and Elasmobranchii; and in the Monocotylidae to the Helocephala. The Oligonchoinea and Polystomatoinea developed upon divergence of the Chondrichthyes and Osteichthyes. Oligonchoineans cospeciated within the Chondrichthyes, with the Chimaericolidea developing within the Helocephala and the ancestor of the Diclybothriidea + Mazocraeidea within the Elasmobranchii. Two cases of primary dispersal occurred within this clade: the Diclybothriidae to the Acipenseriformes and the ancestor of mazocracidean families to the Neopterygii (both Osteichthyes). Secondary dispersal within the Oligonchoinea includes host switching of the common ancestor of Callorhynchocotyle (Hexabothriidae) to the Holocephala. Polystomatoineans coevolved within the Osteichthyes, but experienced primary extinctions in the Actinopterygii, Actinistia, Dipnoi and Amniota. Coevolution of the Sphyranuridae and Polystomatidae occurred with divergence of the Urodela and Anura, respectively. Secondary dispersal of polystomatids to the Urodela, Dipnoi and Amniota is suggested. A preliminary phylogenetic analysis of the Polystomatoinea suggests that primary extinction with secondary dispersal of polystomatids to the Dipnoi may not be necessary to explain extant parasite distributions, since Concinnocotyla (Concinnocotylinae) appears to represent the sister taxon of the remaining Polystomatidae + Sphyranuridae.     

Host-parasite relationships: occurrence and effect of the parasitic isopod Mothocya epimerica on sand smelt Atherina boyeri in the Mesolongi and Etolikon Lagoons (W. Greece)

The parasite-host relationship between Mothocya epimerica, Costa 1851 (Isopoda: Flabellifera: Cymothoidae) and sand smelt Atherina boyeri (Osteichthyes: Atherinidae) fish populations were studied in the Mesolongi and Etolikon Lagoons (W. Greece). Prevalence varied during the year from 12.5% in November to 52.5% in September; overall mean prevalence was 41.9%. Parasite size increased with host size. Infections did not have a significant effect on the host's body condition, such as length-weight relationship, gonadosomatic index, hepatosomatic index and relative condition factor. Histological damage to the host was observed on the gills (especially in the second and third and arches upon which the female parasite rests her abdomen). The physiological cost resulting from this infection seems to be little and probably does not constitute a serious threat for individual host survival.     

Epizootiology of Hyalinocysta chapmani (Microsporidia: Thelohaniidae) infections in field populations of Culiseta melanura (Diptera: Culicidae) and Orthocyclops modestus (Copepoda: Cyclopidae): a three-year investigation

The epizootiology, transmission dynamics and survival strategies employed by the microsporidium Hyalinocysta chapmani were examined in field populations of its primary mosquito host, Culiseta melanura and its intermediate copepod host, Orthocyclops modestus over a three-year period in an aquatic subterranean habitat. H. chapmani was enzootic and was maintained in a continuous cycle of horizontal transmission between each host. There were three distinct periods during the summer and fall when developing mosquito larvae acquired infections; each was preceded by or coincident with the detection of infected copepods. Results were corroborated in laboratory bioassays, wherein transmission was achieved in mosquito larvae that were reared in water and sediment samples taken from the site during the same time periods. The highest infection rates, ranging from 60% to 48%, were repeatedly observed during the first six weeks of larval development. These were coincident with the most sustained collections of infected copepods obtained during the year and highest levels of infection achieved in the laboratory transmission studies. The high prevalence rates of lethal infection observed in larval populations of C. melanura at this site are among the highest recorded for any mosquito-parasitic microsporidium and clearly suggest that H. chapmani is an important natural enemy of C. melanura. H. chapmani appears to overwinter in diapausing mosquito larvae but may also persist in copepods. The absence of vertical transmission in the life cycle of H. chapmani and the sole reliance on horizontal transmission via an intermediate host are unique survival strategies not seen among other mosquito-parasitic microsporidia. The epizootiological data suggest that this transmission strategy is a function of the biological attributes of the hosts and the comparatively stable environment in which they inhabit. The subterranean habitat is inundated with water throughout the year; copepods are omnipresent and C. melanura has overlapping broods. The spatial and temporal overlap of both hosts affords abundant opportunity for continuous horizontal transmission and increases the likelihood that H. chapmani will find a target host. It is hypothesized that natural selection has favored the production of meiospores in female host mosquitoes rather than congenital transfer of infection to progeny via ovarian infection as a strategy for achieving greater transmission success.     

A Dermocystidium infection in Trichomycterus sp. (Osteichthyes, Trichomycteridae)

Two whitish elongate cysts in the left pectoral fin of Trichomycterus sp. (Osteichthyes, Trichomycteridae) were packed with Dermocystidium sp. spores. The spores were spherical and had a large PAS positive central refractile body, the cytoplasm being restricted to a narrow peripheral layer containing the nucleus. The cysts were surrounded by a thin homogeneous wall of parasite origin, and there was no encapsulation of the cysts by host tissue.     

Phenoloxidase in larvae of Plodia interpunctella (Lepidoptera: Pyralidae): molecular cloning of the proenzyme cDNA and enzyme activity in larvae paralyzed and parasitized by Habrobracon hebetor (Hymenoptera: Braconidae)

Phenoloxidase (PO) is a major component of the insect immune system. The enzyme is involved in encapsulation and melanization processes as well as wound healing and cuticle sclerotization. PO is present as an inactive proenzyme, prophenoloxidase (PPO), which is activated via a protease cascade. In this study, we have cloned a full-length PPO1 cDNA and a partial PPO2 cDNA from the Indianmeal moth, Plodia interpunctella (Hubner) (Lepidoptera: Pyralidae) and documented changes in PO activity in larvae paralyzed and parasitized by the ectoparasitoid Habrobracon hebetor (Say) (Hymenoptera: Braconidae). The cDNA for PPO1 is 2,748 bp and encodes a protein of 681 amino acids with a calculated molecular weight of 78,328 and pI of 6.41 containing a conserved proteolytic cleavage site found in other PPOs. P. interpunctella PPO1 ranges from 71-78% identical to other known lepidopteran PPO-1 sequences. Percent identity decreases as comparisons are made to PPO-1 of more divergent species in the orders Diptera (Aa-48; As-49; and Sb-60%) and Coleoptera (Tm-58; Hd-50%). Paralyzation of host larvae of P. interpunctella by the idiobiont H. hebetor results in an increase in phenoloxidase activity in host hemolymph, a process that may protect the host from microbial infection during self-provisioning by this wasp. Subsequent parasitization by H. hebetor larvae causes a decrease in hemolymph PO activity, which suggests that the larval parasitoid may be secreting an immunosuppressant into the host larva during feeding.     

Non-sibling parasites (Strepsiptera) develop together in the same paper wasp

Host discrimination by immature host-seeking endoparasites is a complex and somewhat unexplored topic. In the case of multiple infections, conflicts among conspecifics may occur to monopolize space and resources in the same host. Two or more 1st instar larvae of Xenos vesparum (Strepsiptera, Stylopidae) may enter into a Polistes dominulus (Hymenoptera, Vespidae) larva and develop together until the adult stage of both parasite and host. We carried out a screening of mitochondrial haplotypes in X. vesparum individuals extracted from superparasitized wasps taken in 5 naturally infected nests from different areas of Tuscany (Italy), to assess whether non-sibling parasites may infect the same colony and host. In total, we obtained 12 different haplotypes out of 122 genotyped individuals of both sexes: 17 of 34 superparasitized wasps hosted parasites that originated from females differing in their haplotypes. To date, this is the first described case of superparasitism with non-sibling host-seeking larvae infecting a single individual hymenopteran host. In addition, at least in heavily infected colonies, there is evidence of a male-biased sex-ratio and synchronous development of the parasites, regardless of their haplotypes. Finally, the distribution of haplotypes per nest is consistent with either phoretic infection or larvipositing on nests by means of superparasitized wasps.     

Plagiorchis elegans (Digenea: Plagiorchiidae) infections in Stagnicola elodes (Pulmonata: Lymnaeidae): host susceptibility, growth, reproduction, mortality, and cercarial production.

Eggs of Plagiorchis elegans were readily ingested by Stagnicola elodes of all ages, but were more infective to immature than mature snails. Infection enhanced the growth of the host in a dose-dependent manner. The number of cercariae released by immature snails increased with the age of the snail host; mature snails yielded fewer cercariae. Heavily infected snails tended to die prematurely, thereby reducing their total production of cercariae to levels below those of more lightly infected individuals. Even light infections castrated the snail host. Snails that acquired the infection as juveniles never produced eggs. Actively reproducing snails ceased egg laying within days of infection and never recovered. All parasite effects on the growth and reproduction of the snail host first manifested themselves during the early stages of infection, long before the development of daughter sporocysts and cercariae, and are therefore attributable to the effects of mother sporocysts. The study provides insight into how this natural enemy of mosquito larvae may be established in natural snail populations by means of strategically timed introductions of parasite eggs, with a goal of maximizing cercarial production for the biological control of sympatric mosquito larvae.     

The host specificity of Lepeophtheirus pectoralis (Müller, 1776) (Copepoda: Caligidae)

The host specificity of Lepeophtheirus pectoralis (Müller) was examined experimentally by exposing different fish species to infection by artificially reared copepodid larvae. Copepodids which were hatched from eggs of adults parasitic on plaice ( platessae copepodids) preferred plaice to all other fishes tested, whereas copepodids which were hatched from eggs of adults parasitic on flounder ( flesi copepodids) preferred flounder to all other fish species. These behavioural differences suggest that two strains of L. pectoralis exist, one ( platessae ) adapted to plaice as its host and the other ( flesi ) to flounder. Comparison of an experimentally derived order of host preference with a table of occurrence obtained from the literature, suggests that a third strain of L. pectoralis , adapted to dab as its host, might also occur.
The process of infection by L. pectoralis copepodids is also described. It comprises a host location phase, during which the copepodid enters the habitat of its flatfish hosts and locates a host individual, and an attachment phase. The host location phase appears to be governed by changes in the activity of the copepodid and by its positively rheotactic response to water currents produced by the host. The attachment phase is probably based on the response of the copepodid to chemical factors produced by the host.     

Factors influencing the infectivity of a Canadian isolate of Steinernema kraussei (Nematoda: Steinernematidae) at low temperature

The effectiveness of Canadian isolate 76 of Steinernema kraussei, at 10 degrees C, in penetrating Galleria mellonella larvae (percentage parasitism and number of IJs developed to adult nematodes) was measured at different host densities (differing number of larvae and size of experimental arena) and for different durations of exposure. The greater the size of the inoculum of infective juvenile nematodes per unit area and the longer the duration of exposure, the greater the number of larvae that were killed and the larger the number of mature nematodes in the larval host. The infection rate (alpha) and the adjusted infection rate (beta) were determined using the modified Anderson model. This model successfully described the behavior of the S. kraussei-G. mellonella interaction.     

Seasonal population dynamics of the nematode Cystidicoloides tenuissima (Zeder) from the River Swincombe, England

The seasonal population dynamics of adult and larval Cystidicoloides tenuissima were studied in its definitive hosts brown trout, Salmo trutta and juvenile Atlantic salmon, S. salar , and mayfly intermediate host, Leptophlebia marginata , from the River Swincombe, Dartmoor National Park, Devon, U.K. Infective larvae were present in each mayfly generation for almost its entire duration in the steam benthos. The infection parameters (prevalence and mean intensity) and maturation in the fish indicated C. tenuissima was an annual parasite exhibiting a seasonal periodicity and also systematic variation with the host age. Maturation was correlated to river water temperature. Infection parameters increased from September to May, then declined in June and July and remained relatively constant for the rest of the summer. Variation in the fish infection parameters over time, site, and host species appeared to be controlled by transmission related events; the availability of infective larvae, host feeding behaviour and water temperature. The availability of infective larvae and host diet controlled the rate at which parasites were added to the parasite population, but the pattern of gains and losses was determined by a temperature dependent rejection response.     

Infection patterns in white sea blue mussels Mytilus edulis of different age and size with metacercariae of Himasthla elongata (Echinostomatidae) and Cercaria parvicaudata (Renicolidae)

Infection of mussels Mytilus edulis L. by 2 trematode species was studied in a natural intertidal population in the Chupa inlet of the White Sea. The prevalence of metacercariae of Himasthla elongata (Mehlis, 1831) and Cercaria parvicaudata (Stunkard & Shaw, 1931) in mussels reached 100% in 3 to 4 yr old molluscs and remained at this level in older individuals. Infection intensity increased evenly with the age of the molluscan host, showing a tendency to decrease only in the oldest (9 yr old) mussels. These patterns of age dynamics of prevalence and infection intensity were associated with accumulation of trematode larvae in the course of the molluscs' lives. Ability of metacercariae to exist in mussels for long periods (at least 2.5 yr) was verified in the course of an experiment, during which infected molluscs were kept in a subtidal net cage. Decrease of infection intensity in the oldest individuals may reflect selective mortality of the most severely infected molluscs. Among mussels of the same age, higher infection intensity values occurred in larger individuals. This may be due to an enhanced pumping rate in large molluscs, which increases the probability of cercariae, free-living trematode larvae, infecting them via water currents.     

Pathogenicity of the gill artery worm Philometra obturans (Nematoda) in northern pike (Esox lucius) in southwest Finland

The pathogenicity of female Philometra obturans (Prenant, 1886) was investigated. Fish samples were taken in May and June 2001 from Lake Littoistenj?rvi, in southwest Finland. The intensity of infection varied between 1 and 4 worms, these being found in the afferent artery of the 2 outermost gill arches. The prevalence was 33.9% (n = 59). Host condition and tissue reactions were investigated through dissection and analysis of histopathology. The parasites obstructed the gill arteries, and the arterial wall was stretched around intact worms. Hypertrophy of the arterial wall was observed, especially when only disintegrating remains of the parasites were present. Infected pike were in a poorer condition after catch, transportation, and holding in aquaria and thus less tolerant to stress than uninfected pike. This was correlated with the number of infected gill arteries (P < 0.05). The results show that infection with this nematode has a detrimental effect on the host when combined with environmental or physiological stress and may result in host mortality.