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ZOOLOGY Biology of Parasitism Morphology, Life Cycles, Mode Of Paper No. : 08 Biology of Parasitism Module : 21 Morphology, Life cycles, Mode of entry of Schistosoma Development Team Principal Investigator: Prof. Neeta Sehgal Head, Department of Zoology, University of Delhi Co-Principal Investigator: Prof. D.K. Singh Department of Zoology, University of Delhi Paper Coordinator: Dr. Pawan Malhotra ICGEB, New Delhi Content Writer: Dr. Haren Ram Chiary and Dr. Kapinder Kirori Mal College, University of Delhi Content Reviewer: Prof. Rajgopal Raman Department of Zoology, University of Delhi 1 Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma Description of Module Subject Name ZOOLOGY Paper Name Biology of Parasitism: Zool 008 Module Name/Title Protozoan Module Id M21 Morphology, Life cycles, Mode of entry of Schistosoma Keywords Schistosoma, cercaria, miracidium, sporocysts, gynaecophoric canal Contents 1. Learning Objectives 2. Introduction 3. Schistosoma mansoni 4. Characteristic features 4.1 Habit and habitat 4.2 Morphological features 5. Modes of transmission 6. Life cycle and development 7. Other genus of Schistosoma 7.1 S. haematobium 7.2 S. japonicum 7.3 S. intercalatum 7.4 S. mekongi 8. Summary 2 Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma 1. Learning Outcomes After studying this module, you shall be able to: General aspects of Schistosoma Recognise the digenetic nature of parasite and its habit, habitat and other characteristics. Species affecting human population Identify the morphology of different parasitic forms Life cycle of Schistosoma and other species infecting human hosts. 2. Introduction The parasites belong to class Trematoda of Phylum Platyhelminthes. The hierarchy of its systematic position is as underneath: Phylum: Platyhelminthes Class: Trematoda Order: Prosostomata Family: Schistosomatidae Genus: Schistosoma “Schistosome” stands for split body that defines the appearance of the couple. Schistosome are dioeciuos i.e. separate sexes, blood trematodes/flukes that belongs to Schistosomatidae family and Platyhelminthes Phylum. The species possess non-operculate egg and lack muscular pharynx. The species of the family mainly inhabit caval venous and mesenteric- portal blood vessels and within their life cycle lack encysted metacercarial stage. They develop into a fork-tailed cercariae possess organs of penetration in their pre-oral sucker for lytical and mechanical functioning. The species of the family are digenetic parasites that pass their life cycle in two distinct hosts, one intermediate and one definite host. Skin is the portal of entry into their definite host and vesical plexus of veins is the site of localization. Ranges of hosts are parasitized by this group of parasites including mammals, birds, turtles and fishes. Some species are potential parasites of man while others are agents of diseases in humans. S. haematibium, S. japonium, S. makongi, S. intercalatum and S. mansoni are the important agents of disease in man. Other less epidemiologically important species are S. bovis, S. malayani, S. curossoni and S. mattheei. The species cause Schistosomiasis in man. In the endemic infection areas it is a significant cause of disease affecting nearly 77 countries globally. Schistosoma species can infect cattle, sheep, horses and humans. Not all schistosoma infections cause diseases in humans. 3 Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma 3. Schistosomatidae species: S. mansoni S. mansoni It is a species of Schistosomatidae family which enter their definite host i.e. man, hamsters and mice through skin and gets localized in the mesenteric veins. The developmental stage of S.mansoni called miracidium is an infectious larval form infecting an intermediate snail host, found in the freshwater bodies in shallow coastal regions. In the head foot of the snail the miracidium larvae transform into sporocyst and then migrate to the gonads or digestive glands. The sporocyst containing cercariae releases the parasitic form (cercariae) which moves through the snail tissues and ultimately released into the freshwater to further invade the definitive human host. Biomphalaria species is the molluscan host of this parasite. S. mansoni have synonyms like Afrobilharzia mansoni, Schistosomum americanum and Distoma haematobium. The parasite causes Manson’s intestinal schistosomiasis or Schistosomiasis mansoni, commonly called as bilharziasis. In the earliest times, the existence of schistosomiasis was reported by the studies of mummies of ancient Egypt. In the twentieth dysentery African Schistosomiasis was one of the earliest reported diseases noted by Theodor Bilharz. 4. Characteristic features 4.1 Habit and habitat S. mansoni is a digenetic protozoan parasite. The life cycle completes in two hosts: the primary host is human and the secondary host (vector) is snail. Food Habits The host completely fills the dietary requirements of the parasite, Schistosoma mansoni. The blood feeding parasite as the name suggests gains nourishment from the host blood components, mainly monosachharides and proteins of the blood. Precisely, the parasite devours a lot of glucose to generate energy for its reproduction and other activities. It has been reported that the malnutrition status of host can negatively impact the parasite development. Habitat The parasite S. mansoni is commonly found in freshwater ecosystem. According to the time of the year and region, the distribution and location of parasite may vary. The reason that they are predominately found in the freshwater environment is that the environment remains comparatively stable in such type of ecosystem. Attributing to their parasitic nature, they can 4 Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma infect other species present in the water body. Most commonly they infect intermediate snail hosts of the family Planorbidae, genus Biomphlaria. Other organism by ingesting the infected snails can infect themselves such as rats, monkeys and other rodents. On the flip side, the definite human host not becomes infected by consuming infected snails but through contact with contaminated water. Species used as Host 1. Humans 2. Rodents 3. Primates 4. Snails, Biomphlaria Home Range The adult form of parasite survives within the host body. The other forms of worm such as cercariae and miracidia are free-swimming but limiting to range of areas with potential hosts. Behavior S. mansoni has both a definitive as well as intermediate host. In most cases, the definitive host is a man and the intermediate host is a snail from the genus Biophlaria. The parasitic form commonly infect human host by invading the skin and flow into the circulation. The larval form of parasite called miracidium can swim and its motile nature provides them the ability to search for their intermediate host i.e. freshwater snail. Another form of parasite called ceracriae is motile and swims from intermediate host to definitive human host. Adult forms of parasite remain within the host forming a mating pair (male and female together) where male has residing female within its groove called gynecophoric canal. 4.2 Morphological forms Schistosoma have two separate sexes (male and female) and thus called as dioecious. The adult form is long, whitish and slim where females are longer than males whereas males are stouter than females. Living a life span of 20 to 30 years, these are long-lived worms. The adult worm of the three species: S. mansoni, S. haematobium and S. japonicum closely resemble each other. Males are flat and the lateral margins of the body are folded ventrally to form a female receiving gynecophoric canal. A syncytium of cells is contained in the tegument like surface of the worm. Many prominent tuberculations are present in the tegument of male whereas female worm surface is a tegument that is devoid of tegumentations. The worms maintain their attachment in the mesenteric blood vessels with the help of pediculated sucker that follows a second type of funnel-shaped sucker called oral sucker at the anterior end of the body. The buttons around the suckers as well as in the inner parts of the ventral and oral suckers, small thorn are present. 5 Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma Males Males of S. mansoni species possess 8 to 9 testes arranged in a zig-zag row, situated dorsally with efferent ducts connected to form a single vas deferences reaching to seminal vesicles, near starting of gynacophoric canal. The cutica is grossly tuberculated. Adult male can reach the size of 1 cm long and 1mm wide. Just posterior to ventral sucker lie the genital pore that have a non-muscular cirrus tube opening of seminal vesicles, the vesicle formed by the swelling of vas deferences. The males have diploid number of chromosome (16). Behind the ventral sucker, the body of the adult male worm is flattened, characteristically forming gynaecophoric canal and within this grove the female worm is held (fig.1). The caecal region and large intestine is surrounded by the small inferior mesenteric blood vessels which are prominent sites of male parasitic worm localization. A short esophagus is followed by the intestine that bifurcates into two branches at the ventral sucker level. These two branches reunite to form a closed caecum at the posterior end. The gut of the adult parasites ends close to the posterior tip of the body. Numerous small tubercles cover the body wall of the worm. The membranocalyx is a bilayered external part of the tegument which is renewed and shed continuously. The bottom of the oral sucker is the anterior extremity of the body from where the digestive tube begins and shows a bifurcated esophagus that reunites to form a single blind caecum. The intestine end blindly. Figure 1: Schistosoma male Females Females have cylindrical body held within the gynecophoric canal held within the gynecophoric canal of the male. The size of the female worm parasite can reach upto 1.4 cm in length and 0.25 mm in width, which can digest ten times more RBC’s in comparison to 6 Biology of Parasitism ZOOLOGY Morphology, Life cycles, Mode of entry of Schistosoma male parasite.
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