A comparative study of mechanisms underlying digenean-snail specificity: in vitro interactions between hemocytes and digenean larvae

A panel of 4 digenetic trematode species (Echinostoma paraensei, E. trivolvis, Schistosoma mansoni, and Schistosomatium douthitti) and 5 snail species (Biomphalaria glabrata, Helisoma trivolvis, Lymnaea stagnalis, Stagnicola elodes, and Helix aspersa) was examined to determine if known patterns of host specificity could be explained by the tendency of digenean larvae to be bound by snail hemocytes, or by the ability of larvae to influence the spreading behavior of hemocytes. In short-term (1 hr) in vitro adherence assays, there was no overall pattern to suggest that sporocysts were more likely to be bound by hemocytes from incompatible than compatible snails. Compared with the other parasites, sporocysts of E. paraensei were less likely to be bound by hemocytes from any of the snail species tested. All rediae examined, including those of another species Echinoparyphium sp., were also remarkably refractory to binding by hemocytes from any of the snails. Of all the larvae examined, only sporocysts and young daughter rediae of E. paraensei caused hemocytes to round up in their presence. This was true for hemocytes from the compatible species B. glabrata and the incompatible lymnaeid species S. elodes and L. stagnalis. The patterns of host specificity shown by this particular panel of parasites and snails were not predicted by either the extent of hemocyte adherence to digenean larvae or by the ability of larvae to affect hemocyte spreading behavior. The results of this study suggest that a role for hemocytes, although likely, may require different assays, possibly of a more prolonged nature, for its detection. Also, different parasite species (notably E. paraensei) and intramolluscan stages have distinctive interactions with host hemocytes, suggesting that the determinants of specificity vary with the host-parasite combination, and with the parasite life cycle stage.     

Studies on resistance in snails: A specific tissue reaction to Echinostoma lindoense in Biomphalaria glabrata snails

In juvenile albino Biomphalaria glabrata snails exposed for the first time to Echinostoma lindoense miracidia, and observed to be resistant, the sporocysts migrated to the heart at the same speed as they did in susceptible snails. However, in resistant snails the sporocysts were soon destroyed in the heart by amebocyte clumps. When these snails were then re-exposed to miracidia of the same species of trematode, the sporocysts were quickly destroyed soon after miracidial penetration, chiefly in the head-foot region. This strongly accelerated tissue reaction appears to have been induced by the previous contact with the same parasite. The sensitization of the snail tissues was highly specific: the hosts remained susceptible to Schistosoma mansoni and Paryphostomum segregation (Echinostomatidae), although partial resistance was observed against Echinostoma paraensei and E. liei, which are closely related to E. lindoense.     

Intermediate host specificity in Schistosoma mansoni

Miracidia of Schistosoma mansoni penetrate into many kinds of snails, but development of normal sporocysts takes place only in certain species of Biomphalaria. Different populations of this snail vary greatly in laboratory infection rates with S. mansoni originating from diverse geographic localities. Cross-exposure experiments show that compatibility factors exist in both snails and parasites. Susceptibility of stocks of Biomphalaria to particular strains of S. mansoni is genetically determined and may be modified by selection in the laboratory. In a compatible snail, the sporocyst develops without host tissue reaction; in incompatible snails the early larvae are rapidly surrounded by amebocytes and fibroblasts, and destroyed. This reaction resembles the generalized host cellular response elicited by any foreign body. An individual snail exposed to many miracidia may have both developing and encapsulated sporocysts side by side within its tissues. The weight of current evidence suggests that elicitation or absence of this cellular response resides in the recognition or nonrecognition of the sporocyst as a foreign body. The sporocyst tegument surface, which forms within a few hours after miracidial penetration, may have a molecular conformation identical with that of the snail, or may be able to bind specific host molecules, so that detection and subsequent encapsulation by host cells are averted. Presuming genetic determination of the sporocyst surface structure and of the host cell detection capability, differing infection rates would result from the particular frequencies of relevant genes in the populations concerned.     

Observations on the host-parasite relations between Echinostoma revolutum and lymnaeid snails.

Echinostoma revolutum from Taiwan was studied in lymneid snails at 29 +/- 0.5 degrees C. Given 3-5 miracidia, 95% of Lymnaea ollula and 40% of Lymnaea swinhoei became positive; the prepatent periods were 18 and 25 days, respectively. The following are based on the observations in Lymnaea ollula: The time required for miracidial penetration was about one hour. The sporocysts developed only in the ventricle of the snail but mother rediae developed in the heart and other organs. Mature daughter rediae were not found in the heart cavity. The sporocysts reached the ventricle within 3 days. Mother rediae were released after 6 days and daughter rediae after 8 days. Given 5 miracidia, 1-3 sporocysts reached the heart and 2-20 mother rediae were found per snail. The number of mature daughter rediae was usually 100-200 although more than a thousand may develop in a snail. The sporocysts and mother rediae attained maximal size 9 days postinfection and started degeneration 13 days postinfection. Daughter rediae were largest at the beginning of cercarial emergence and decreased in size thereafter. Simultaneous production of daughter rediae and cercariae by the mother redia was seen only once in this snail mature cercariae were obtained in 10 days postinfection. The cercariae emerged from a small area of mantle collar near the posterior corner of shell aperture. They were negatively phototactic and positively geotactic. An estimation showed that each snail shed about 350 cercariae a day. The cercariae reached the pericardial cavity of snail in one hour via the renal orifice and metacercariae were seen 4-5 hours after exposure. The infectivity of cercariae at various times after shedding, as expressed by cyst recovery rates, were: 51.6%, O-hr old; 76.1%, 2-hr; 68% 4-hr; 32%, 6-hr; 3%, 8 and 10-hr. Cyst recovery rates were not different within the dosage of 50-500 cercariae per snail. Most metacercariae recovered 1-2 days after cercarial exposure were viable; only 5 among 6,533 cysts were dead.     

The suitability of several aquatic snails as intermediate hosts for Angiostrongylus cantonensis.

Sixteen species of aquatic snails of four families were tested by quantitative technique under standardized conditions for their suitability as intermediate hosts for Angiostrongylus cantonensis. These species were the planorbid snails Biomphalaria glabrata, Biomphalaria alexandrina, Planorbis planorbis, Planorbis intermixtus, Bulinus truncatus, Bulinus contortus, Bulinus africanus, Bulinus tropicus and Helisoma sp.; the lymnaeid snails Lymnaea natalensis, Lymnaea tomentosa, Lymnaea stagnalis, and Stagnicola elodes; the physid snail Physa acuta (an Egyptian and a German strain) and the ampullariid snails Marisa cornuarietis and Lanistes carinatus. All these snail species proved to be susceptible to infection with A. cantonensis, and first stage larvae reached the infective third stage in all of them. However, the rate and intensity of infection varied with different species. B. glabrata was the most susceptible snail species with a 100% infection rate and an average percentage recovery of third stage larvae of 26.1. This was followed by S. elodes and B. africanus, with a 100% infection rate and an average percentage recovery of third stage larvae of 15.6 and 14.6 respectively. The rest of snail species proved to be less susceptible. For comparative evaluation of the suitability of the various snail species as intermediate hosts of A. cantonensis a "Capacity Index" was determined. This index should provide a useful method for the evaluation of the suitability of various snails as intermediate hosts of nematode parasites under standardized conditions in the laboratory.     

Larval development of Fasciola hepatica in experimental infections: variations with populations of Lymnaea truncatula

A retrospective study was undertaken on 70 French populations of Lymnaea truncatula experimentally infected with Fasciola hepatica to determine whether or not susceptibility of snails to infection influenced redial and cercarial production. Results were compared with those obtained from two control populations, known for prevalences higher than 60% when experimentally infected with F. hepatica. In the 70 other populations examined, the prevalences ranged from 2 to 75%. In 55 of these populations, where the prevalence was more than 20%, a high proportion (50.1-56.8%) of snails died after cercarial shedding, whereas in the other groups (non-shedding snails with the most differentiated larvae being free cercariae, rediae containing cercariae, immature rediae, or sporocysts, respectively), snail death was significantly less. In 11 populations, where the prevalence values were 5-19%, only 14% of snails died after cercarial shedding, whereas snails with free cercariae, rediae with cercariae, or immature rediae showed significant increases in snail mortality. In the remaining four snail populations, with prevalences of less than 5%, the most differentiated larval forms were only immature rediae and/or sporocysts. Overall, the number of rediae containing cercariae significantly decreased with decreasing prevalence values. The low prevalence of experimental infection in several populations of snails might be explained by the occurrence of natural infections with miracidia originating from a mammalian host other than cattle, and/or by genetic variability in the susceptibility of snails to infection.     

Development of Fasciola hepatica in Lymnaea columella infected with miracidia derived from cattle and marmoset infections

The development of Fasciola hepatica from two species of definitive hosts, i.e. cattle (Bos taurus) and a marmoset (Callithrix penicillata) in the snail Lymnaea columella was determined based on the production of rediae and cercariae and snail survival rate. More rediae and cercariae at 60-74 days post-infection were produced by snails infected by cattle-derived miracidia (cattle group) than by those infected by marmoset-derived miracidia (marmoset group). Among the L. columella parasitized by the marmoset group, the survival rate and the percentage of positive snails were higher than among those parasitized by the cattle group. Eggs of F. hepatica released in cattle faeces were significantly bigger than those released in marmoset faeces. Miracidia originating from parasites that completed their development in cattle were more efficient in infecting the intermediate host. These results suggest that vertebrate-host origin influences the eggs produced by the parasite and the infection rates in the snail host L. columella.     

Schistosoma mansoni: influence of Biomphalaria glabrata size on susceptibility to infection and resultant cercarial production

Biomphalaria glabrata snails of the same age, but different sizes, were used to determine size-related susceptibility to Schistosoma mansoni miracidial infection and the influence of snail size on total cercarial production. Snails with shell diameters from less than 5 to greater than 17 mm were individually exposed to one or several miracidia, depending on the experiment. In snails exposed to multiple numbers of miracidia, the percentage of snails which developed patent infections was lower in snails with larger shell sizes. This was also reflected by fewer primary sporocysts per infected snail found in tissues of the larger snails. Upon determining cercarial production in these groups over a 1-month period there were no statistical differences between any groups in the numbers of cercariae produced per snail. However, upon determining the number of successful primary sporocysts found in cohort snails of each size group, cercarial production increased as a function of the number of successful primary sporocysts. This was verified by examining cercarial production in various size snails with known monomiracidial infections. Our data therefore confirm and extend earlier work using snails infected with unknown numbers of miracidia and clearly show that total S. mansoni cercarial development and decreased susceptibility of snails is a direct reflection of snail size and not necessarily age of the snail.     

Host choice and penetration by Schistosoma haematobium miracidia

Schistosome parasites commonly show specificity to their intermediate mollusc hosts and the degree of specificity can vary between parasite strains and geographical location. Here the role of miracidial behaviour in host specificity of Schistosoma haematobium on the islands of Zanzibar is investigated. In choice-chamber experiments, S. haematobium miracidia moved towards Bulinus globosus snail hosts in preference to empty chambers. In addition, miracidia preferred uninfected over patent B. globosus. This preference should benefit the parasite as patent snails are likely to have mounted an immune response to S. haematobium as well as providing poorer resources than uninfected snails. Miracidia also discriminated between the host B. globosus and the sympatric, non-host species Cleopatra ferruginea. In contrast, S. haematobium did not discriminate against the allopatric Bulinus nasutus. Penetration of the host by miracidia was investigated by screening snails 24 h after exposure using polymerase chain reaction (PCR) with S. haematobium specific DraI repeat primers. There was no difference in the frequency of penetration of B. globosus versus B. nasutus. These responses to different snail species may reflect selection pressure to avoid sympatric non-hosts which represent a transmission dead end. The distribution of B. nasutus on Unguja is outside the endemic zone and so there is less chance of exposure to S. haematobium, hence there will be little selection pressure to avoid this non-host snail.     

Cercarial shedding of Echinostoma cinetorchis and experimental infection of the cercariae to several kinds of snails

The development of Echinostoma cinetorchis in several snail species reared in laboratory aquaria was observed. The eggs from adult flukes collected from the intestine of rats were cultivated to miracidia, and exposed to Hippeutis sp. snails. Observations were made for cercarial shedding from the exposed snails. The cercariae shed from the snails were again exposed to several species of fresh water snails in order to observe metacercarial formation in the snails and their infectivity to final hosts. The results obtained in this study were as follows: 1. Twenty miracidia were exposed to each snail of Hippeutis sp. About 58.3% of the above snails (7 out of 12) were dead before shedding the cercariae, and the remainder shed the cercariae for a period of 7 to 9 days before death. 2. Cercarial shedding from the infected snails started from the 25th day after the exposure to miracidia, and the total number of cercariae shed per snail was 684 in average (range; 482-904). 3. The size of rediae developed in the infected Hippeutis sp. snails was 1,242 x 214 microns in average, and the number of rediae per snail was 350 in average (range; 120-510). 4. About 40 to 50 cercariae shed from the Hippeutis sp. snails were each exposed to several species of snails reared in the laboratory. The metacercarial formation was confirmed by dissecting the infected snails, 12 to 16 days after the infection.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of Plagiorchis elegans (Digenea: Plagiorchiidae) infection of Biomphalaria glabrata (Pulmonata: Planorbidae) on a challenge infection with Schistosoma mansoni (Digenea: Schistosomatidae)

Prior exposure of Biomphalaria glabrata to the eggs of an incompatible digenean, Plagiorchis elegans, rendered this snail host less suitable to a compatible species, Schistosoma mansoni. Although P. elegans failed to develop patent infections in B. glabrata, it reduced the production of S. mansoni cercariae by 88%. Concomitantly, host attributes such as reproduction, growth, and survival were compromised. The effect of P. elegans infection was most severe among snails that, in addition, had developed patent schistosome infections. Although few S. mansoni cercariae were produced, egg production by B. glabrata was only 4% of control values. Furthermore, no doubly infected snails survived for more than 3 wk after patency, whereas controls experienced no mortality during the same time period. The above effects were attributable to the establishment and persistence of P. elegans sporocysts in the tissues of the incompatible snail host. Their indirect antagonistic interaction with thelarval stages of S. mansoni may be mediated, in part, through their long-term stimulation of the host's internal defense mechanisms. These findings are discussed with a view to use P. elegans and other plagiorchiid digeneans as agents in the biological control of snails and snail-borne diseases.     

Austropeplea ollula (Pulmonata: Lymnaeidae): a new molluscan intermediate host of a human intestinal fluke, Echinostoma cinetorchis (Trematoda: Echinostomatidae) in Korea

Three freshwater snail species of the family Lymnaeidae have been reported from Korea, Radix auricularia coreana, Austropeplea ollula and Fossaria truncatula. Out of 3 lymnaeid snail species, A. ollula was naturally infected with the Echinostoma cinetorchis cercariae (infection rate = 0.7%). In the experiments with the laboratory-bred snails, F. truncatula as well as A. ollula was also susceptible to the E. cinetorchis miracidia with infection rates of 25% and 40%, respectively. All of three lymnaeid snail species exposed to the E. cinetorchis cercariae were infected with the E. cinetorchis metacercariae. It is evident that A. ollula acts as the first molluscan intermediate host of E. cinetorchis in Korea, and F. truncatula may be a possible candidate for the first intermediate host of this intestinal fluke. Also, three lymnaeid snail species targeted were experimentally infected with E. cinetorchis metacercariae.     

Experimental life history of Echinostoma cinetorchis

The life history of Echinostoma cinetorchis was completed in the laboratory using Hippeutis cantori as the first and second intermediate host. The incubation for maturation of eggs and hatching of miracidia took 24 days at 28 degrees C. On the 66th day after miracidial challenge, 16 snails were crushed and examined for the presence of E. cinetorchis larvae. The metacercariae were detected in all of the snails, and from three of them were found the rediae and cercariae. The morphological characteristics of the larvae and the experimentally obtained adults were compatible with that of E. cinetorchis.     

Larval productivity of Fasciola gigantica in two lymnaeid snails

Two groups of Galba truncatula and two groups of Lymnaea natalensis were experimentally infected with Fasciola gigantica to determine if snail species had an influence on the redial burden and cercarial shedding of this trematode when snails of both species were infected with the same isolate of miracidia. In the two groups used for the study of redial burden, the total number of free rediae was significantly higher at day 49 post-exposure in L. natalensis than in G. truncatula. In the groups used for cercarial shedding, the life-span of cercaria-shedding snails and those of infected snails which died without cercarial emission, and the duration of the prepatent period were significantly longer in L. natalensis than those noted in G. truncatula. However, the mean numbers of shed cercariae did not significantly differ and showed no differences in their daily distribution throughout the shedding period. These results demonstrate that G. truncatula might be the principal intermediate host of F. gigantica in Egypt, at least in the areas where this lymnaeid species lives.     

Effect of miracidial dose on adoptively transferred resistance to Schistosoma mansoni in the snail intermediate host, Biomphalaria glabrata

Adoptively transferred resistance to Schistosoma mansoni in the snail intermediate host Biomphalaria glabrata was measured as a function of miracidial challenge dose. Schistosome-susceptible snails implanted with the amebocyte-producing organ (APO) from resistant donors showed 29 and 39% prevalences of infection after challenge with 5 and 10 miracidia, respectively, but 68-83% prevalences when exposed to 25-200 miracidia. Prevalences in control (untampered) susceptible snails ranged from 97 to 100% at the different miracidial doses. Higher infection prevalences at elevated doses suggest that a range of transferred resistance occurs and possibly that low levels of APO-derived plasma factors or hemocytes in some recipients can be overwhelmed by larger numbers of parasites.     

Double infection experiments with echinostomatids (Trematoda) in Lymnaea stagnalis by implantation of rediae and exposure to miracidia

1972. Double infection experiments with echinostomatids (Trematoda) in Lymnaea stagnalis by implantation of rediae and exposure to miracidia. International Journal for Parasitology, 2: 409–423. Echinostomatid species parasitizing Lymnaea stagnalis as first intermediate hosts in a South German Lake have been found present in natural double infections, but at frequencies lower than expected. Simultaneous double infection and superinfection experiments in Lymnaea stagnalis with Isthmiophora melis, Echinoparyphium aconiatum and Echinostoma revolutum were performed by redial implantation and by exposure to miracidia. All three combinations possible of these echinostomatids proved to be unstable, one species being eliminated by another ‘stronger’ one after an invariable suppression order. The degree of vigour of Isthmiophora melis in this suppression order is greater if mother rediae (macropharyngeate) are present, i.e. after miracidial invasion instead of daughter redial implantation. Snails parasitized by rediae of a ‘weak’ type could be superinfected by implantation of rediae of a ‘strong’ type, but not if the first (‘weak’) infection had reached the stage of shedding cercariae. Superinfection by implantation of Echinoparyphium aconiatum rediae (‘strong’ type) was not successful when the first infection consisted of sporocysts of plagiorchiids, or of Apatemon sp. (Strigeidae) that had reached the stage of shedding cercariae.     

A potential vector of Schistosoma mansoni in Uruguay

Susceptibility experiments were carried out with a Biomphalaria straminea-like planorbid snail (Biomphalaria aff. straminea, species inquirenda) from Espinillar, near Salto (Uruguay), in the area of the Salto Grande reservoir, exposed individually to 5 miracidia of Schistosoma mansoni (SJ2 and BH2 strains). Of 130 snails exposed to the SJ2 strain, originally infective to Biomphalaria tenagophila, 30 became infected (23%). The prepatent (precercarial) period ranged from 35 to 65 days. The cercarial output was irregular, following no definite pattern, varying from 138 to 76,075 per snail (daily average 4.3 to 447.5) and ending up with death. Three specimens that died, without having shed cercariae, on days 69 (2) and 80 after exposure to miracidia, had developing secondary sporocysts in their tissues, justifying the prospect of a longer precercarial period in these cases. In a control group of 120 B. tenagophila, exposed to the SJ2 strain, 40 became infected, showing an infection rate (33.3%) not significantly different from that of the Espinillar snail (chi 2 = 3.26). No cercariae were produced by any of the Espinillar snails exposed to miracidia of the BH2 strain, originally infective to Biomphalaria glabrata. Four specimens showed each a primary sporocyst in one tentacle, which disappeared between 15 and 25 days post-exposure, and two others died with immature, very slender sporocysts in their tissues on days 36 and 54. In a control group of 100 B. glabrata exposed to BH2 miracidia, 94 shed cercariae (94%) and 6 remained negative. Calculation of Frandsen's (1979a, b) TCP/100 index shows that "Espinillar Biomphalaria-SJ2 S. mansoni" is a vector-parasite "compatible" combination.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interspecific Interactions Among Larval Trematode Parasites of Freshwater and Marine Snails

Freshwater and marine snails serve as intermediate hosts fornumerous species of larval trematodes. Any particular populationof snails may be infected by several species. It is commonlyobserved that mixed species infections are less frequent thanexpected by change in collections of host snails from naturalpopulations. While several mechanisms might generate such negativeassociations, laboratory studies of freshwater snail-trematodeassociations have demonstrated the presence of strong antagonisticinteractions between intramolluscan larval stages (rediae andsporocysts) of species that infect the same host individual.Both predatory and non-predatory antagonism has been observed,the former taking the form of predation by large, dominant redialforms on the sporocysts and rediae of subordinate species. Theseinteractions are largely hierarchical, although in some systemspriority effects have been observed, and in one case a sporocystspecies replaced a redial species by strong non-predatory antagonism.Several instances of positive association between larval trematodespecies have also been observed. In such cases, interferencewith host defense mechanisms by the first parasite appears toenhance superinfection by the second. My own study of the larvaltrematode guild that infects the salt marsh snail, Cerithideacalifornica, has revealed patterns of association and interactionthat are very similar to those demonstrated by laboratory studiesof freshwater systems. Ultimately, the frequency of interactionsamong larval trematodes depends on the availability, relativeto the numbers of susceptible snails, of infective eggs andmiracidial larvae transmitted from definitive hosts.     

Capacity of irradiated echinostome sporocysts to protect Schistosoma mansoni in resistant Biomphalaria glabrata

Three closely related species of Echinostoma flukes each has a distinctive pattern of protection of Schistosoma mansoni in schistosome-resistant Biomphalaria glabrata host snails. Protection of developing S. mansoni by irradiated E. paraensei sporocysts in the schistosome-resistant snail host was strong; protection induced by irradiated E. lindoense and E. liei sporocysts was weak or not measurable. The capacity of irradiated E. paraensei sporocysts to interfere with the host's innate anti-schistosome response also differed between strains of B. glabrata. Protection of S. mansoni strain Lc-1 was greater in B. glabrata strain 10-R2 than it was in strain M-RLc snails. Irradiated E. paraensei sporocysts also induced a different response to the two schistosome strains in a single host strain. Irradiated E. paraensei sporocysts induced in B. glabrata 10-R2 snails a stronger protection of S. mansoni strain PR-1 than of strain Lc-1. Exposure of each snail to the irradiated E. paraensei miracidia usually protected the following challenge schistosome infection better when 30 rather than 10 irradiated echinostome miracidia were used.     

In vitro culture of marine trematodes from their snail first intermediate host

The ability to culture parasites outside their host (i.e. in vitro) is essential for several aspects of parasitological research. Here, a culture medium for marine trematode parthenitae was optimized using Philophthalmus sp. rediae from the intermediate snail host, Zeacumantus subcarinatus. The medium was optimized by sequentially testing the suitability of different levels of osmolality, different commercially available media, and different concentrations of supplemented chicken serum, while controlling for genetic variation among cultures. Philophthalmus sp. rediae survived up to 56 days in cultures of the best tested medium, remaining active and continuously shedding cercariae. The broader suitability of the culture medium was tested using five other trematode species from different families (using either the same or other marine snails as first intermediate hosts): Galactosomum sp., Acanthoparyphium sp., Maritrema novaezealandensis, Curtuteria australis, and an undescribed species of the family Opecoelidae. Survivorship of rediae and sporocysts from these species ranged from eight days to 42 days. The culture procedures developed here can therefore be used in the future as a system under which to culture marine trematode parthenitae for experimental studies.