Phenotypic plasticity in the cercarial indices of human and rodent samples of Schistosoma mansoni (Digenea: Schistosomatidae).

In the epidemiology of schistosomiasis, the cercarial index (CI) has been used for several purposes, including the identification of the species of vertebrate host involved in transmission in a particular setting. The CI of samples of Schistosoma mansoni of human (BH) and rodent (CMO) origin have now been compared, in an attempt to identify morphological markers on cercariae that would indicate whether the eggs from which the cercariae have developed were produced in humans or in rodents. After silver-nitrate impregnation of the cercariae, the CI (and their reaction norms) for the antero-dorsal (AD), antero-lateral (AL) and tail-stem (U) sensory receptors were investigated. The data analysis was based on linear-regression methods, Levene's test for the equality of variances, pooled-variance independent t-tests, and discriminant analysis. The results indicated that CI varied with vertebrate-host origin, that the AD reaction norms were more consistent and plastic than the other reaction norms investigated, that there appeared to be no simple allometric relationship between any of the CI investigated and cercarial body length, and that the BH sample gave larger standard deviations and a higher incidence of mis-classification than the CMO, indicating some developmental instability in the sample of human origin. The relative instability of the BH sample was, however, possibly the result of the experimental procedures. As the parasites used to produce the BH sample had been isolated from humans fairly recently and then routinely maintained in mice, host-induced selection could have affected the sample, altering its CI and their reaction norms. The results of the discriminant analysis indicated that, of the indices investigated, AD was the best morphological marker for distinguishing human and rodent samples of S. mansoni.

Chaetotaxy and ultrastructure of sensory receptors in the cercaria of a species of Allassogonoporus Olivier, 1938. (Digenea:Lecithdendriidae).

A standard procedure that combines chaetotaxic, ultrastructural and neuromorphological observations has recently provided a new perspective to the study of cercarial sensory systems. In the present work, we aimed to extend the use of this combination of techniques to investigate the chaetotaxy of Allassogonoporus sp. in conjunction with the ultrastructure of sensory receptors and neuromorphology. Five nerve regions were distinguished. A conspicuous bilobed cerebral ganglion was observed at the level of the pharynx. The chaetotaxic pattern was generally consistent with that of other lecithodendriids. Four types of receptors were distinguished with scanning electron microscopy. These types differed in cilium length (short, moderately long or long) and tegumentary collar length (moderately low or high). Internal ultrastructure of receptor type IIAL revealed an unsheathed cilium, a closed basal body, septate extracellular junctional complexes and thickened nerve collars. Some receptor types were site-specific. Long uniciliated receptors were found mainly on the dorsal surface, whereas short uniciliated receptors were widespread across the tegument. Ultrastructure and site-specificity observations suggest that most sensory receptors are mechanoreceptors, probably reflecting the important role mechanoreception plays in host finding.

Ultrastructure and chaetotaxy of sensory receptors in the cercariae of a species of Crepidostomum Braun, 1900 and Bunodera Railliet, 1896 (Digenea: Allocreadiidae).

Previous investigations of cercarial sensory systems have focused on chaetotaxy and ultrastructure of sensory receptors and have revealed chaetotaxic patterns within families, genera, and species as well as different types of sensory receptors. However, chaetotaxic and ultrastructural observations have rarely been combined. We investigated the ultrastructure of cercarial sensory receptors in conjunction with the chaetotaxy and neuromorphology in 2 allocreadiid species belonging to the genera Crepidostomum and Bunodera. Cercariae were treated with acetylthiocholine iodide and silver nitrate, and for scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Similar cholinergic nerve networks were revealed. Chaetotaxy was consistent with that of other allocreadiids. Seven and 6 types of receptors were distinguished with SEM in Crepidostomum sp. and Bunodera sp., respectively. Types differed in number of cilia (1 or 2), cilium length (short, moderately long, or long), presence or absence of a tegumentary collar and a domelike base, and tegumentary collar length (low, moderately low, or high), TEM of some types revealed unsheathed cilia, basal body, and thickened nerve collars. Some receptor types were site specific. Thus, long uniciliated receptors were concentrated on the dorsal surface. Other types, such as short uniciliated receptors, were widespread throughout most regions. Ultrastructure and site-specificity observations suggest that most receptors are mechanoreceptors.

Ultrastructure and chaetotaxy of sensory receptors in the cercaria of a species of Allopodocotyle Pritchard, 1966 (Digenea: Opecoelidae).

Previous investigations of sensory systems in opecoelid cercariae have focused on chaetotaxy and ultrastructure of sensory receptors. They revealed chaetotaxic patterns within family, genus, and species as well as different receptors. Chaetotaxic and ultrastructural observations have rarely been combined. We investigated the ultrastructure of cercarial sensory receptors in conjunction with chaetotaxy and neuromorphology in a species of Allopodocotyle. Cercariae were treated with acetylthiocholine iodide and silver nitrate, and some were processed for light, scanning (SEM), and transmission (TEM) electron microscopy. Five nerve regions were distinguished. Chaetotaxy was consistent with that of other opecoelids. Five types of receptors were distinguished with SEM. Types differed in number of cilium-like structures (one or more), length of cilium-like structure (short, moderately long, or long), presence or absence of a tegumentary collar, and length of tegumentary collar (low, moderately low, or very high). Internal ultrastructure of some types revealed unsheathed cilium-like structures, basal body, and thickened nerve collars. Possible subtegumentary and sheathed receptors are introduced. Some receptor types were site-specific. For example, receptors with multiple cilium-like structures were concentrated on cephalic region whereas receptors with short cilium-like structure were widespread throughout most regions. Ultrastructure and site-specificity observations suggest that most receptors are mechanoreceptors.

Ultrastructure and chaetotaxy of sensory receptors in the cercaria of a species of Allopodocotyle Pritchard, 1966 (Digenea: Opecoelidae)

Previous investigations of sensory systems in opecoelid cercariae have focused on chaetotaxy and ultrastructure of sensory receptors. They revealed chaetotaxic patterns within family, genus, and species as well as different receptors. Chaetotaxic and ultrastructural observations have rarely been combined. We investigated the ultrastructure of cercarial sensory receptors in conjunction with chaetotaxy and neuromorphology in a species of Allopodocotyle. Cercariae were treated with acetylthiocholine iodide and silver nitrate, and some were processed for light, scanning (SEM), and transmission (TEM) electron microscopy. Five nerve regions were distinguished. Chaetotaxy was consistent with that of other opecoelids. Five types of receptors were distinguished with SEM. Types differed in number of cilium-like structures (one or more), length of cilium-like structure (short, moderately long, or long), presence or absence of a tegumentary collar, and length of tegumentary collar (low, moderately low, or very high). Internal ultrastructure of some types revealed unsheathed cilium-like structures, basal body, and thickened nerve collars. Possible subtegumentary and sheathed receptors are introduced. Some receptor types were site-specific. For example, receptors with multiple cilium-like structures were concentrated on cephalic region whereas receptors with short cilium-like structure were widespread throughout most regions. Ultrastructure and site-specificity observations suggest that most receptors are mechanoreceptors

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 Gerais) 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. glabrata 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.