Survival and infectivity of Hypoderaeum conoideum and Euparyphium albuferensis cercariae under laboratory conditions.

The survival characteristics of the cercariae of Hypoderaeum conoideum and Euparyphium albuferensis (Trematoda: Echinostomatidae) at 20 degrees C and 30 degrees C are described, and the age dependency of their infectivity at 20 degrees C is studied to determine their respective transmission efficiencies. Cercarial survival was found to be age-dependent and was higher at 20 degrees C. For both cercariae, the maximum life-span was 26 h at 20 degrees C and 16 h at 30 degrees C, and their respective times to 50% mortality were similar at each temperature. Both cercariae seem to be well adapted to transmission in their natural habitat, though cercarial infectivity of H. conoideum was higher than that of E. albuferensis, this being correlated with their prevalences in nature. The age-dependency of cercarial survival may be related to steadily diminishing endogenous energy levels, though the delay in attaining maximum infectivity suggests that other factors not related to energy considerations are involved in the delimitation of the cercarial infective period. However, this latter observation may constitute an adaptative mechanism allowing cercarial dissemination.     

Influence of low temperatures on the cercarial shedding of Paramphistomum daubneyi from the snail Lymnaea truncatula

Lymnaeids in nature are subjected to temperature fluctuations that may have an influence on the shedding of cercariae. Thus, experimental infections of Lymnaea truncatula by Paramphistomum daubneyi were performed to determine whether a sudden fall in temperature--daily dipping of infected snails into spring water at 14-15 degrees C, or at 6-8 degrees C--followed by its increase at 20 degrees C in the subsequent hour, had an influence on the characteristics of snail infection and cercarial production. The immersion of infected snails in cold water during a short period delayed the first cercarial shedding, at day 66 on average, in the 6-8 degrees C group vs at day 57 in the 14-15 degrees C group, or at day 49 in the 20 degrees C group. The percentage of cercaria-shedding snails was greater in the 6-8 degrees C group than in the 14-15 degrees C and the 20 degrees C ones: 41.8% vs 17.3% and 7.1%, respectively. The total number of cercariae given by each infected snail was also higher in the 6-8 degrees C group. A fall in the temperature of water during daily change, followed by its increase at 20 degrees C within the subsequent hour, stimulated the cercarial shedding of P. daubneyi.     

Shedding patterns of Schistosoma mansoni and Ribeiroia marini cercariae from a mixed infection of Biomphalaria glabrata

When the snail Biomphalaria glabrata is doubly infected by two species of trematode, Schistosoma mansoni and Ribeiroia marini, each species keeps its own cercarial shedding pattern. Emergence of S. mansoni cercariae occurs during the photophase while the R. marini cercariae are shed during the night. These results demonstrate the absence of interference between the mechanisms responsible for the cercarial shedding of each species. The only difference noted concerns a two-hour shift in the peak emergence of S. mansoni as a consequence of the antagonism between the two parasites.     

Resistance to desiccation of Biomphalaria glabrata adults infested by various miracidia of Schistosoma mansoni

1200 adult Biomphalaria glabrata were submitted during 6 weeks to anhydrobiosis condition. Some snails were healthy, some were previously infected 3 days or 12 days ago with 8 +/- 2 miracidia of Schistosoma mansoni, others were shedding cercariae. The snails were put on soil or buried into hermetically closed, or ventilated, plastic boxes. There was no survival of snails kept in sealed boxes, or among positive snails, but 44% of control healthy snails and 40.6% of infected (for 3 or 12 days) snails in ventilated boxes were living at the term of the desiccation stage. Survival was better for "on soil" snails than for "buried" snails, but no difference was shown between 3-days and 12-days infection. The surviving desiccated B. glabrata had a lesser death rate and a lesser cercarial production than infected snails kept in water. An inferior production of male cercariae comparatively to female and to "mixed" cercariae was demonstrated by statistical analysis of the cercarial sheddings. In all positive snails, periodic variations of cercarial production was shown, whatever the sex of those cercariae. In addition many pauses of the sheddings were established by the authors.     

Effect of the organophosphorous insecticide, chlorpyrifos (Dursban), on growth, fecundity and mortality of Biomphalaria alexandrina and on the production of Schistosoma mansoni cercariae in the snail.

Exposure of Biomphalaria alexandrina to sublethal concentrations (0.125, 0.25 and 0.05 ppm) of the organophosphorous insecticide, chlorpyrifos (Dursban), induced a reduction in egg production and egg hatchability. Exposure of Schistosoma mansoni miracidia to the insecticide (60 min, 0.50 ppm) prior to infection of B. alexandrina did not affect the subsequent production of cercariae. However, exposure of S. mansoni-infected snails to the insecticide until day 55, from day 20 to day 62 and from day 35 to 62 following infection resulted in blockage of cercarial shedding. Cercarial shedding commenced in some snails when the treatment stopped. Exposure to the insecticide in concentrations of 0.125 and 0.25 ppm during the first 20 days following infection did not affect the subsequent production of cercariae, but exposure to 0.5 ppm during the first 20 days affected markedly the production of cercariae due to a high snail mortality. The findings indicate that the cercaria is the target stage for the activity of chlorpyrifos on the intramolluscan larval development. It is suggested that S. mansoni cercarial production in B. alexandrina may be a useful system for monitoring the effect of low concentrations of pesticides on the aquatic environment, and that the ability by chemical means to interrupt the cercarial production might be a useful tool in further analyses of important aspects of the snail/parasite relationship.     

Early and late shedding patterns of Schistosoma mansoni cercariae: ecological significance in transmission to human and murine hosts

A comparative analysis of the cercarial shedding of 2 Schistosoma mansoni populations originating from the same endemic area (Guadeloupe) allows us to distinguish an early (peak emergence at 1100 hr) and a late (peak at 1600 hr) shedding patterns of cercariae. This intraspecific variation in the chronobiology of S. mansoni cercariae may be related to the ecology in the transmission site. The early shedding pattern characterizes schistosome populations originated from urbanized foci where man plays the main role in the parasite transmission; the late shedding pattern characterizes schistosome populations originated from sylvatic focus where a rat (R. rattus) is the main host. The late shedding of cercariae is considered as an adaptation favoring transmission to a murine host whose behavior is preferentially crepuscular.     

The cercarial output from three Nigerian bulinids infected with two strains of Schistosoma haematobium

The daily cercarial output of two Nigerian strains of Schistosoma haematobium in sympatric Bulinus truncatus, B. globosus and B. senegalensis was measured at weekly intervals from the start of emission to the snails' death. In all cases cercariae were released throughout the life of the host, with no cases of "self cure". Patterns of output through the course of infections in B. truncatus and B. senegalensis were similar to those reported for S. haematobium by other workers, with daily production of cercariae rising to a peak within a few weeks of the onset of shedding, then declining until the host's death. In the longer lived B. globosus production was significantly higher, but declined to very low levels after the initial peak; in some individuals cercarial output remained very low, while others showed a second period of high cercarial emission. The relative compatibility of each host-parasite combination is discussed.     

Analysis and comparison of cercarial emergence rhythms of Schistosoma haematobium, S. intercalatum, S. bovis, and their hybrid progeny

In identical experimental conditions, the three schistosomes of the terminal spined egg group (S. haematobium, S. intercalatum and S. bovis) showed significant differences in their cercarial shedding patterns. The cercariae of the F1 hybrids, obtained by experimental crosses between these three species, showed the same circadian emergence rhythm (with one peak) as the cercariae of the parental species. However, the mean shedding time of these hybrid parasites (12:14 +/- 1 h 34 min for S. haematobium x S. intercalatum; 09:58 +/- 1 h 24 min for S. haematobium x S. bovis; 08:57 +/- 1 h 19 min for S. bovis x S. intercalatum) was always in advance to the one of their parental species (13:51 +/- 2 h 04 min for S. haematobium; 13:59 +/- 1 h 55 min for S. intercalatum; 10:09 +/- 2 h 04 min for S. bovis). These results are compared with those obtained from crosses between schistosomes with lateral spined eggs, and from crosses between intraspecific chronobiological variants of S. mansoni. They corroborate the genetic determinism of the cercarial emergence of schistosomes. Moreover, such significant differences between the hybrids and their parents in the times of cercarial emission may be of value in the epidemiological research and characterization of naturally occurring hybrids.     

Cercarial shedding patterns of Schistosoma bovis and S. haematobium from single and mixed infections of Bulinus truncatus

The cercarial shedding of Schistosoma bovis and S. haematobium were studied in single and mixed infections in the snail host Bulinus truncatus. The two species displayed a distinctive diurnal cercarial emergence with an earlier shedding pattern for S. bovis than S. haematobium (the average emergence peaks were respectively at 0800 h and 1200 h). In mixed infections, each species kept its own cercarial shedding rhythm with no marked alterations. The cercarial emergence pattern is proposed as a new method to identify natural mixed infections in the snail intermediate hosts. The interactions between the two parasites are discussed.     

A stock of infected snails for the mass breeding of cercariae and schistosomules

A routine to maintain infected snails Biomphalaria glabrata able to produce 1-2 million cercariae per week and a mean shedding of about 3,500 cercariae per snail was quantitatively described. To a monthly production of about 5.3 and 6.5 million cercariae during the years of 1982 and 1983 it was necessary to expose an average of 1,557 and 1,957 snails, respectively. The efficiency described was maintained by infecting 2,000 snails per month (infectivity index of about 60%). A maximal production of 6,000 cercariae/snail was obtained in the period between 56-70 days after the snail infection by miracidia. By this period, however, 90% of the infected snails were dead. In the summer of 1982-1983 it was observed an impairment on the daily cercarial harvests which was related to ambient temperature increase, and the presence of rotifers in the aquaria water. When the cercariae were transformed into schistosomula in vitro, a yield of 55% schistosomula was obtained, with 7% of tail contamination. Lyophilization of cercariae produced 50.9 +/- 63 g dry weight per 1,000 cercariae.     

Circadian Pattern of Cercarial Emergence in Schistosoma Mansoni(Platyhelminthes: Digenea) from Isolated Biomphalaria Glabrata

The present work aimed to compare the acrophases (peak hours) of emergence of Schistosoma mansoni cercariae among isolated individuals of the snail Biomphalaria glabrata. Laboratory stocks of melanic B. glabrata from the same biotope as the S. mansoni strain (Belo Horizonte, Minas Ger-ais) were used. Twenty-two snails individually exposed to five miracidia were tested. Chronobiological trials were performed outdoors after an acclimation period of at least a week. Three groups of snails were tested between November 1989 and April 1991. Cercarial emergence from individual isolated snails was quantified every 3 h for 3 consecutive days. In all trials, most cercariae were found to emerge during daytime (94.9%). Time series and chronograms showed recurrent peaks during the daytime. The periodogram suggested that 24 h was the period that best fitted cercarial emergence data in 90.9% of the snails. The single cosinor analysis confirmed 24-h rhythms in 95.5% of the snails. Acrophases of cercarial emergence among individual snails occurred between 14:15 and 17:02. They did not differ significantly. The population cosinor analysis indicated greater homogeneity in the 24-h rhythms of cercarial emergence than in the snail groups of each chronobiological trial. Acrophases of cercarial emergence occurred between 14:53 and 15:27 and did not differ significantly among all trials. Data from the three trials were pooled and analyzed using the population cosinor. This statistical method indicated a homogeneity in the 24-h rhythms of cercarial emergence from all snails, with acrophase occurring around 15:00. Results showed that the acrophases of cercarial emergence of S. mansoni are similar among isolated B. glubrura specimens. Data support the hypothesis of a “gate” rhythm in the dynamics of cercarial production and emergence. It is suggested that the adaptive importance of the “gate” mechanism is associated with the concentration of cercariae in the water at times when the vertebrate is present, optimizing the contact between the parasite and the host. The emergence of some cercariae at night (5.1% of the total number of emerged cercariae) suggests a flexible “gate” that could be associated with a residual light effect or with experimental procedures in the laboratory.     

Effect of light intensity on Opisthorchis viverrini cercarial shedding levels from Bithynia snails--a preliminary study

Opisthorchis viverrini requires Bithynia snails as the first intermediate host and cyprinid fish as the second intermediate host. Very low natural infection rates have been reported in Bithynia snails, but very high rates have been found in cyprinid fish in the same endemic region. This study investigated the effect of light intensity, the most important stimulus, on the quantity of O. viverrini cercariae shed from naturally infected Bithynia (Digoniostoma) siamensis goniomphalos snails. Snails were evaluated for cercariae output every hour after exposure to various light intensities for a total period of 7 h. The same infected snail was tested under different intensities of light: in the dark, and at 1000, 3000 and 5000 lx. The data showed that under exposure to 1000 and 3000 lx of light, the average percentage and number of cercariae released were higher than that exposed to 5000 lx during the first 2 h of the experiment. In contrast, under higher illumination (5000 lx) a longer time (6 h) was required to stimulate the peak emergence of cercariae. Darkness was not able to induce O. viverrini cercariae emergence. Among the three intensities of light, exposure at 1000 lx induced the highest average number of released cercariae per snail and the highest percentage of cercarial emergence within the first 2 h (125, 54.86%), followed by exposure at 3000 lx (69, 25.58%) and 5000 lx (12, 7.78%). The results suggest that the light intensity of 1000 lx for 2 h would be optimal for O. viverrini cercarial shedding from naturally infected B. (D.) siamensis goniomphalos snails.     

Schistosoma bovis: patterns of cercarial emergence from snails of the genera Bulinus and Planorbarius

A comparative experimental study of the rhythmic shedding of two geographic strains of Schistosoma bovis cercariae by Bulinus truncatus and Planorbarius metidjensis did not show any differences in emergence patterns. In our opinion, the results support the hypothesis that cercarial emission rhythms are determined primarily by the parasite in these snail/parasite associations.     

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.     

The effect of photoperiod inversion upon Schistosoma mansoni cercarial emergence from Biomphalaria glabrata

A delayed pattern of Schistosoma mansoni cercarial emergence from Biomphalaria glabrata is presented, in which cercariae did not emerge from the snail under a diurnal photoperiod until after 12 noon. Even with a reversed (nocturnal) photoperiod, delayed cercarial emergence still persisted.     

Comparative analysis of the Schistosoma mansoni and Schistosoma bovis cercarial production under the influence of praziquantel

Snails infected with Schistosoma mansoni and S. bovis and fed with a food-Praziquantel mixture stop shedding cercariae for several days. The cercarial production restarts at different levels after stopping treatment: for some Biomphalaria glabrata, the restarting phase of production reaches a high level whereas for other B. glabrata and all of the P. metidjensis, the level of production remains low. Histological studies revealed that at the exact moment of this treatment, there is a total destruction of many nearly mature cercariae whereas the young cercarial embryos and the tegument of the sporocyst remain unharmed. When treatment stops, there is a pronounced proliferation of third-generation sporocysts (Sp III) which invade the available space of the snails' digestive gland.     

Schistosoma mansoni: Long-term maintenance of clones by microsurgical transplantation of sporocysts

Schistosoma mansoni sporocysts originally derived from monomiracidially infected Biomphalaria glabrata snails were serially transplanted into the cephalopedal sinus of anesthetized snails by the microsurgical implantation of fragments of parasitized hepato-pancreas and ovotestis. Three to six passages each of five male and five female clones were maintained for as long as 2.0 years. Of the recipient snails which survived surgery, 87% released cercariae, usually beginning 5–7 weeks after surgery. The percentage of snails which released cercariae increased with successive passages. The mean survival time of surgically infected snails after cercarial emergence began was 9.2 ± 0.5 weeks, nearly the same as that of miracidially infected snails. Longevities of snails infected with male or female clones were similar. Recipient snail size and age did not influence cloning success. Beginning 5 weeks from the onset of cercarial emergence large numbers of cercariae (a mean of 3900/snail from male clones and 1300/snail from females) were obtained during each shedding period. These results clearly demonstrate that the microsurgical transplantation of sporocysts is a practical means of maintaining and expanding populations of genetically homogeneous schistosomes (clones).     

Emergence of cercariae of Echinostoma caproni and Schistosoma mansoni from Biomphalaria glabrata under different laboratory conditions

Release of Echinostoma caproni cercariae and Schistosoma mansoni from experimentally infected Biomphalaria glabrata snails maintained under different laboratory conditions was studied. Infected snails were isolated individually for 1 h in Stender dishes containing 5 ml of artificial spring water and the number of cercariae released during this time was recorded. Of numerous conditions tested, the addition of lettuce, the use of water conditioned by B. glabrata snails and a temperature of 35 degrees C significantly increased the release of E. caproni cercariae. A significant increase in cercarial release of S. mansoni was seen only in cultures fed lettuce. A temperature of 12 degrees C caused a significant decrease in cercarial release of both E. caproni and S. mansoni. Increased snail activity associated with feeding behaviour was probably responsible for the enhanced cercarial sheds observed in this study.     

The daily cercarial emission rhythm of Schistosoma margrebowiei with particular reference to dark period stimuli

The cercariae of Schistosoma margrebowiei showed two peaks of emergence from Bulinus natalensis in a 12 h light/dark cycle. Peak emission occurred at 0700 h (one hour after the onset of light) and at 1900 h (one hour after the onset of darkness). Both peaks were of equal magnitude and were maintained during constant illumination indicating that the rhythm is innate. Delaying or advancing the timing of the dark period did not affect the timing of these two peaks. Following a five minute dark treatment elevation in cercarial output resulted irrespective of when the treatment was applied. Subjecting snails to various intensities of light only resulted in an elevation in output when a sudden change in intensity from 0 to 360 Lux or the reverse was applied. No response was seen to a gradual change in light intensity although the parasite could detect a change in light from 1 to 0 Lux. These responses appear to optimize the chances of host parasite contact.     

Patterns and processes of larval emergence in an estuarine parasite system

Trematode parasites in intertidal estuaries experience constantly varying conditions, with the presence or absence of water potentially limiting larval transport between hosts. Given the short life spans (< or =24 h) of cercariae, emergence timing should be optimized to enhance the probability of successful transmission. In the present study, field measurements and laboratory experiments identified processes that regulate the emergence of cercariae from their first intermediate snail hosts in an intertidal marsh. Larvae emerged over species-specific temperature ranges, exclusively during daylight hours, and only when snails were submerged. The three factors operate over different temporal scales: temperature monthly, light diurnally (24-h period), and water depth tidally (12-h period). Each stimulus creates a necessary condition for the next, forming a hierarchy of environmental cues. Emergence as the tide floods would favor transport within the estuary, and light may trigger direct (downward or upward) swimming toward host habitats. Abbreviated dispersal would retain asexually reproduced cercariae within the marsh, and local mixing would diversify the gene pool of larvae encysting on subsequent hosts. In contrast to the timing of cercarial release, emergence duration was under endogenous control. Duration of emergence decreased from sunrise to sunset, perhaps in response to the diminishing lighted interval as the day progresses. Circadian rhythms that control cercarial emergence of freshwater species (including schistosomes) are often set by the activity patterns of subsequent hosts. In this estuary, however, the synchronizing agent is the tides. Together, exogenous and endogenous factors control emergence of trematode cercariae, mitigating the vagaries of an intertidal environment.