Aquatic stages of parasitic helminths

Transcription

1 Freshwater Aquatic stages Invertebrates of parasitic of helminths the Malaysian Region 81 Aquatic stages of parasitic helminths Praphathip Eamsobhana Department of Parasitology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok 10700, Thailand ( INTRODUCTION Helminths chiefly contain trematodes and cestodes of the phylum Platyhelminthes, and nematodes of the phylum Nematoda. The phylum Platyhelminthes (flatworms) is the more primitive group. The great majority of the worms belonging to this phylum are flattened dorsoventrally. The phylum is divided into three classes, the Trematoda (flukes), Cestoidea (tapeworms), and Turbellaria (turbellarians). All trematodes are parasitic; some externally on aquatic animals, others internally in aquatic and land animals. Most cestodes, in the adult stage, are parasites of the digestive tract of vertebrates. Their larval stages occur in a variety of invertebrate and vertebrate hosts on land, and a few live in fresh water. The turbellarians are mostly free-living worms in marine, freshwater and terrestrial environments. The phylum Nematoda (roundworms) includes worms of somewhat higher organization than that in flatworms. Most of them are elongated and cylindrical. The members of this phylum are ubiquitous, and most species are free-living. They are abundant and diverse in marine and freshwater habitats. Many species of this phylum, however, are parasites of vertebrates and invertebrates. The nematode parasites of human include several species that spend a period of their life cycle in an aquatic environment before they mature and become infective in humans. Humans unintentionally drink and eat a large number of worms in their lifetime. Several of them, however, cause diseases of great importance to humans. Many types of food ingested by man may carry a great variety of helminths. Such foods include those of aquatic animal origin such as freshwater fish, crabs, crayfish and freshwater snails. Various edible water plants may also contain aquatic stages of parasitic helminths. The helminths may be in or on these aquatic animals or plants as an obligate part of the parasitic life cycle, or their infective stages may be present as water contaminants. The helminth parasites of humans include trematodes (flukes), cestodes (tapeworms) and nematodes (roundworms). GENERAL BIOLOGY The trematodes, or flukes, are leaf-shaped or elongate, slender organisms, ranging in length from less than 1 mm to more than 8 cm. They possess attachment organs in the form of hooks or cup-shaped muscular depressions called suckers. The gut, with

2 82 Freshwater Invertebrates of the Malaysian Region only one opening, is branched and extends throughout the body, functioning in both digestion and transport of food. With the exception of the blood flukes, the schistosomes, all of the flukes parasitizing humans are hermaphroditic. Trematode life cycles involve the use of aquatic snails as first intermediate host, and there may be a second intermediate host, such as fish, crayfish, or crabs. In some species, the larval stage of the trematode may be deposited on aquatic plants. The final host of flukes is always a vertebrate. The adult fluke may live in blood vessels, in the liver, in the intestine, or in the lungs. The eggs laid in these sites pass out in urine, faeces, or sputum depending upon the location of the infection. Most eggs have an operculum that opens to allow the ciliated first-stage larva, or miracidium, to escape. In many trematodes the miracidium is free-swimming, moves in a spirally rotating manner, and actively penetrates the soft parts of the snail. The eggs of some flukes, however, will only hatch after ingestion by an appropriate snail. Generally, each species of trematode requires a particular species of snail in which to continue its development. Upon entering into the tissue of the snail, the miracidium loses its ciliated coat and reproduces enormously into other larval stages, a sporocyst and a redia. Regardless of the number of larval generations within the snail, the last stage produced is a cercaria. The cercariae leave the snail and pursue various courses depending upon the species involved. The Schistosoma cercariae actively penetrate the skin of final host. The cercariae of Fasciola and Fasciolopsis encyst on water vegetation; those of Opisthorchis in fish, and those of Paragonimus in crayfish and crabs, etc. The encysted metacercariae in edible water plants, fish, crayfish, or crabs enter man when he eats these foods insufficiently cooked or raw. The cestodes, or tapeworms, in the adult stage may range in size from several millimetres to many metres long. They have flat, segmented bodies consisting of a head (scolex), a neck region, and a series of segments (proglottids). Each proglottid possesses both male and female sex organs. Moving posteriorly along a tapeworm, the proglottids change from immature to mature, and finally to gravid, egg-laden segments. The scolex is the only point of attachment between the host and parasite. The cestodes lack digestive system, but attach themselves to the intestinal wall and absorb their food directly through their body walls. Most cestodes require one or more intermediate hosts for development. The larval stages occur in a variety of invertebrate and vertebrate hosts. Humans may be host to either the adult or larval stage, depending on the species of cestode. Eggs or gravid proglottids are passed in the faeces of the final host. The generative stages, eggs or proglottids, of some species reach water and continue development there. A few species are able to complete the life cycle in a single host. The common species of cestodes that infect man are from the orders Pseudophyllidea and Cyclophyllidea. The life cycle of Pseudophyllidea normally involves three larval stages and two

3 Aquatic stages of parasitic helminths 83 intermediate hosts. The egg is usually operculate and gives rise to a ciliated oncosphere known as a coracidium. The other two larval stages are the procercoid in copepod crustaceans and the plerocercoid in freshwater fish. Adults are mainly parasitic in vertebrates. The life cycle of Cyclophyllidea involves only one intermediate host, usually an arthropod or vertebrate. Adults are parasitic in mammals. The nematodes, or roundworms, are bilaterally symmetrical, cylindrical, unsegmented organisms of minute to large size. They have separate sexes, and females of a species are usually larger than their male counterparts. The typical nematode, both larva and adult, is surrounded by a durable outer coat, the cuticle, that is resistant to chemicals. The digestive system of nematodes is divided into three major regions the oral cavity and oesophagus, the midgut and hindgut, and the anus. Reproductive organs in the female consist of one or two ovaries that lead to the vagina by a tubular oviduct, uterus, and a seminal receptacle. The male has a testis connected to the vas deferens, seminal vesicle, ejaculatory duct, and cloaca. Nematode life cycles may be direct or may require aquatic or terrestrial animals as intermediate hosts. Stages in the life cycle of nematodes include the egg, larvae, which undergo several moults, and adult. Infective stages of many parasitic nematodes may be present as contaminants in the aquatic environments. REGIONAL TAXA Generally, helminths that need specific intermediate hosts have restricted distributions, whereas helminths for which this is unnecessary are likely to be more widely dispersed. As any helminth must grow according to its developmental requirements, places where the life cycle can be easily carried out may have a diversity and abundance of helminths. The distribution and intensity of worms are also strongly influenced by geographic, climatic, and economic factors. Places that meet most of the various conditions demanded for growth and transmission are tropical areas with constantly high temperature, humidity, rainfall, and poor environmental sanitation. In such areas, including most of the Southeast Asian countries, various intermediate hosts are easily found, and also environmental factors are most favourable for development of worms. Their eggs and larvae do not easily perish after being egested outside the hosts, so that they may become an infectious source to other animals and humans. NOTES ON TAXA Schistosoma species Phylum Platyhelminthes; Class Trematoda; Order Strigeiformes; Family Schistosomatidae For humans, one of the most important flukes that has its aquatic infective stage in fresh water is the blood fluke in the genus Schistosoma. Three species of schistosomes are of vast medical importance: S. japonicum, S. mansoni and S. haematobium. The distribution of S. japonicum is limited to Japan, China, Taiwan and Southeast Asia.

4 84 Freshwater Invertebrates of the Malaysian Region Schistosoma haematobium and S. mansoni are widely distributed in Africa. The blood flukes unlike most flukes are unisexual. Adult female (12 28 by 0.3 mm) and male (12 20 by 0.5 mm) of S. japonicum live in the veins of the small intestine where the female deposits eggs. The tiny spherical yellow egg ( µm by µm), with a rudimentary lateral spine, leaves the human body through the faeces. The miracidium, which is completely formed within the egg, hatches in fresh water. It swims rapidly by the use of cilia on its surface. When the miracidium reaches its freshwater snail host, it develops into a sporocyst. The sporocyst divides asexually to give rise to daughter sporocysts, and the daughter sporocysts in turn give rise to cercariae. A cercaria is about 0.4 mm in length with a tail. The terminal one-third of the tail is bifurcated. The two branches are closed in forward swimming and opened in backward motion. A single infected snail may release 100,000 cercariae during its lifetime of several months. The cercariae burrow into human skin. They ultimately reach the bloodstream and are swept along to the lungs. Subsequently they re-enter the circulatory system and migrate to the hepatic and portal veins, that supply the liver. The worms become sexually mature and pair on their journey to the destination, where they mate and then migrate joined together to the veins that supply the upper intestine. In these sites, the adult worms of S. japonicum copulate and shed from 1500 to 3500 eggs per day continuously. The very large number of eggs that these worms produce is the primary cause of the inflammation, and sometimes death associated with other complications. Figure 1 illustrates the life cycle of S. japonicum. The morphology of stages in the life cycle of S. japonicum is shown in Figure 2. Human schistosomiasis occurs widely in various countries of Southeast Asia. However, there is enough genetic variation in S. japonicum strains isolated from different areas of endemic infection that they are separate species (Woodruff et al. 1987). The blood fluke S. japonicum, which was once believed to occur in Malaysia, has been identified as S. malayensis (Greer et al. 1980; Yong et al. 1985; Shekhar 1987; Shekhar and Pathmanathan 1987; Greer et al. 1988). The species of blood fluke found in Thailand, Laos, and Cambodia has also been recognized as S. mekongi (Voge et al. 1978; Sornmani et al. 1980). The demonstration of characteristic eggs in stool is the most straightforward mode of diagnosis. Proper sanitation is important in controlling this disease, since it can continue only if faeces reach bodies of fresh water in which snails are living. It can also be controlled by elimination of the snail populations or reducing them to very low levels. In endemic areas, swimming in rivers should be prohibited. Various schistosome cercariae whose final hosts are mammals or birds are capable of readily penetrating the skin of humans. Non-human schistosomes as a rule do not survive or develop in humans. Several hours after exposure to these schistosome cercariae one may experience intense itching in the areas where the parasites have

5 Aquatic stages of parasitic helminths 85 entered the skin and cercarial dermatitis (swimmers itch) results from a sensitivity reaction. The reaction may become more severe with repeated exposures to schistosome cercariae. Consequently sensitive persons should be warned not to swim or wade in fresh water suspected of harbouring snails from which fork-tailed schistosome cercariae are emerging. Flukes causing this disease include S. spindale, which uses a mammal such as cow, sheep, goat, horse, or buffalo, as the final host, but it Figure 1. Life cycle of Schistosoma japonicum. testis oral sucker ventral sucker rudimentary spine miracidium intestinal caecum uterus Egg Miracidium ovary vitelline gland Male Female Figure 2. Schistosoma japonicum. Cercaria

6 86 Freshwater Invertebrates of the Malaysian Region cannot mature in humans and causes only acute dermatitis. Schistosoma spindale is widely distributed in Malaysia, Indonesia, Thailand, Vietnam, Laos, and India (Miyazaki 1991; Krishnasamy et al. 2001). Opisthorchis viverrini Phylum Platyhelminthes; Class Trematoda; Order Opisthorchiformes; Family Opisthorchiidae Opisthorchis viverrini, Opisthorchis felineus, and Clonorchis sinensis, are members of a large group of trematodes called liver flukes that live in the bile duct of humans. Only O. viverrini is widespread in Southeast Asia, especially in the Mekong River Valley in Thailand and Laos (Bunnag et al. 1991). Adult O. viverrini is spindleshaped with a slightly pointed anterior end, reddish-brown with a smooth outer surface. It measures about by 2 5 mm. Eggs entering the small intestine with the bile are excreted in the faeces. The egg is yellow-brown and small, measuring about by µm. A small operculum is seen at the narrower end. Each egg contains a fully developed miracidium. When eggs enter water, they are ingested by certain species of freshwater snails. The miracidia hatch and develop into sporocysts which produce rediae and finally cercariae. The cercariae leave the snail and swim in the water for a few hours before they penetrate the scales of a suitable fish host and encyst as metacercariae in the muscles. The metacercaria is a short ellipse within a thin cyst; the size is about 0.15 by 0.1 mm. When the metacercaria is ingested by humans, it excysts in the intestine and enters the common bile duct to become an adult worm, where it causes Figure 3. Life cycle of Opisthorchis viverrini.

7 Aquatic stages of parasitic helminths 87 Adult oral sucker intestinal caecum ventral sucker operculum miracidium uterus vitelline gland ovary Egg Cercaria Miracidium testis Metacercaria Figure 4. Opisthorchis viverrini. mechanical irritation and releases toxic substances. Figure 3 illustrates the life cycle of O. viverrini. The morphology of stages in the life cycle is shown in Figure 4. Humans acquire the infection when they eat the metacercariae in improperly cooked, or raw fish. Human infection with O. viverrini is common in Thailand and Laos; the infection also occurs in Malaysia and India (Bunnag et al.1991; Miyazaki 1991; Pungpak et al. 1994). Infection with O. felineus is more endemic in Europe and Siberia. Infection in humans with C. sinensis is found in most parts of China and also in Japan, northern Vietnam, Taiwan and Korea. Diagnosis can be confirmed by detecting the characteristic Opisthorchis eggs in the faeces and duodenal fluid. Prevention of Opisthorchis infection is by avoiding eating raw, undercooked freshwater fish, which may contain metacercariae. Health education is another important measure to control this infection. Paragonimus species Phylum Platyhelminthes; Class Trematoda; Order Plagiorchiformes; Family Troglotrematidae The genus Paragonimus contains a number of species of which P. westermani is the most widespread human species in Asia. Paragonimus westermani is widely distributed in the Far East, parts of Southeast Asia including Malaysia, Thailand, Indonesia and some Pacific Islands. Paragonimus heterotremus, the other less widely distributed species, is thought to be the main cause of human paragonimiasis in Thailand (Goldsmith et al. 1991; Miyazaki 1991). Mature Paragonimus flukes are reddishbrown, small and oval, measuring mm by 3 4 mm long and mm thick. They are covered with short spines. The adult flukes live in the lungs. The egg is immature when oviposited and is passed up the bronchioles, expectorated with the sputum from the body or is swallowed and voided in the faeces. It measures µm by µm, and is golden-brown in colour and broadly oval with a flattened operculum. Eggs passed in water require several days to embryonate. The

8 88 Freshwater Invertebrates of the Malaysian Region emerging miracidia swim in the water, penetrate various species of freshwater snails and develop into sporocysts, rediae and cercariae. The cercariae emerge from the snail to penetrate the soft parts of the exoskeleton of freshwater crabs and crayfish. There they encyst in various parts of the crustacean, and develop into metacercariae. Several species of crustaceans serve as second intermediate hosts in endemic areas. Johora johorensis, Stoliczia cognatum, and Parathelphusa maculata are known to serve as hosts in Malaysia, and Potamon smithianum is well known as the host of P. heterotremus and P. westermani in Thailand (Goldsmith et al. 1991; Miyazaki 1991). Figure 5. Life cycle of Paragonimus species. oral sucker operculum intestinal caecum ovary ventral sucker Egg Miracidium vitelline gland testis Adult Cercaria Figure 6. Paragonimus species. Metacercaria

9 Aquatic stages of parasitic helminths 89 Humans acquire the Paragonimus infection by eating the infected crustaceans raw or insufficiently cooked. The liberated metacercariae are released in the intestine and begin their migration to the lungs. As the worms develop in the lung parenchyma, a chronic pulmonary disease is seen. In cases involving a large number of worms, severe and sometimes fatal disease occurs. Figure 5 illustrates the life cycle of Paragonimus fluke. The morphology of stages in the life cycle is shown in Figure 6. The diagnosis is based upon finding of the characteristic eggs in sputum or stool. Prevention of infection is by avoiding eating raw, undercooked or pickled freshwater crabs or crayfish which may contain infective metacercariae. Fasciola species Phylum Platyhelminthes; Class Trematoda; Order Echinostomatiformes; Family Fasciolidae Although Fasciola hepatica and Fasciola gigantica have been known as parasites of sheep and cattle in many parts of the world, human infections occur only occasionally. Fasciola hepatica is found worldwide, especially in temperate countries, while F. gigantica is distributed mainly in tropical Africa and various parts of Asia including Southeast Asia. The adult Fasciola is flat and leaf-like with a thick conic structure at the anterior end. Fasciola gigantica measures by 12 mm, while F. hepatica is small and more triangular in shape, up to 30 by 12 mm. The mature fluke lives in the bile duct, where it produces inflammatory and fibrotic changes in the biliary epithelium. The egg is light yellow in colour with a small operculum at one end. It contains an unsegmented ovum surrounded by yolk cells. The size of the egg is about 140 by 85 µm. Eggs released by the worms are passed out in faeces. They enter fresh water, and then egg cell cleavage occurs. After several days of embryonation, the miracidium hatching from the egg penetrates certain species of aquatic snails. In the snail, the miracidia develop into sporocysts, which produce generations of rediae and finally cercariae. The cercariae get into water and then attach to the surface of water plants, where they encyst to become the metacercariae. The final host acquires the fluke when plants such as grass and various weeds are ingested raw. Following ingestion, the metacercariae excyst in the duodenum and the young flukes migrate through the intestinal wall into the peritoneal cavity. They reach the bile ducts by penetrating through the liver capsule. Figure 7 illustrates the life cycle of Fasciola. The morphology of stages in the life cycle of Fasciola is shown in Figure 8. Humans usually acquire the infection from watercress, or other water vegetation used in the diet, or when water, containing metacercariae is drunk. The diagnosis will be confirmed by finding the Fasciola eggs in the stool or duodenal fluid. Human infections have been recorded from Europe, Central and South America, the Middle East, parts of Asia, and Australia (Miyazaki 1991). Prevention involves not eating raw plants that are likely to be the source of infection. Although it is basically important to eradicate the snail hosts, it is not easy.

10 90 Freshwater Invertebrates of the Malaysian Region Figure 7. Life cycle of Fasciola species. Genital Organs Digestive Organs uterus ovary oral sucker ventral sucker intestinal caecum operculum testis vitelline gland Egg Adult Miracidium Cercaria Figure 8. Fasciola species. Metacercaria

11 Aquatic stages of parasitic helminths 91 Fasciolopsis buski Phylum Platyhelminthes; Class Trematoda; Order Echinostomatiformes; Family Fasciolidae The giant intestinal fluke, Fasciolopsis buski, is widely distributed in Asia, especially in China, Vietnam, Cambodia, Laos, Thailand, Malaysia, Indonesia, Myanmar, and India. The adult is large, being 2 4 by 1 cm or larger. The body is thick with small spines on the body surface. The adult flukes live in the duodenum and jejunum attached by the ventral and oral suckers that can cause ulceration of the mucosa at the point of attachment. A large number of the flukes can interfere with absorption and passage of food in the intestine. Fasciolopsis buski has a life cycle similar to that of Fasciola hepatica and Fasciola gigantica. Although the egg is somewhat similar to that of Fasciola hepatica, the length is shorter and has a slightly lighter colour. Eggs liberated in the faeces of humans or pigs hatch, and the life cycle is completed in the presence of the aquatic snail intermediate host and the cultivated aquatic plants. Humans become infected through eating water plants that harbour encysted metacercariae. These include water bamboo, water chestnut, water caltrop, and lotus plant roots. Parasites are often transmitted when the outer skin of the freshwater chestnut or water caltrop, containing the encysted metacercariae, is bitten into and peeled off with the teeth before eating. The metacercariae excyst in the small intestine, and then become parasitic adults mainly in the duodenum. Figure 9 illustrates the life cycle of F. buski. The morphology of stages in the life cycle is shown in Figure 10. Figure 9. Life cycle of Fasiolopsis buski.

12 92 Freshwater Invertebrates of the Malaysian Region ventral sucker oral sucker operculum vitelline gland uterus Egg Miracidium ovary intestinal caecum testis Cercaria Adult Metacercaria Figure 10. Fasiolopsis buski. Human infections are confined to China, Thailand, Malaysia, Indonesia, Taiwan, Vietnam, and India (Sadun and Maiphoon 1953; Cross 1969; Harinasuta et al. 1987; Miyazaki 1991). The diagnosis is based upon finding the egg in faeces. It is important to differentiate it from that of F. hepatica and F. gigantica. To prevent infection, people must be reminded not to eat raw water plants, which potentially serve as a source of infection. Diphyllobothrium latum Phylum Platyhelminthes; Class Cestoidea; Order Pseudophyllidea; Family Diphyllobothriidae The fish tapeworm, Diphyllobothrium latum, is ubiquitous but spottily distributed. It has been reported from parts of Europe, Asia, and North and South America. Humans are infected by ingesting raw or insufficiently cooked freshwater fish (Tanowitz and Wittner 1991). The adult cestode lives in the small intestine of humans. Worms may grow as long as 3 10 m with segments. Eggs, which form in proglottids, are released through uterine pores and are passed in faeces. The egg is pale and oval in shape, measuring by µm, and has an operculum at one end. The egg develops in water and the coracidium or ciliated embryo is formed. The coracidium escapes through the operculum of the egg as a free-swimming organism. The life cycle continues when the coracidium is eaten by any of several species of copepods in the genera Cyclops and Diaptomus. Each coracidium develops into an elongated larva called a procercoid in the body cavity of the copepod. When an infected copepod is eaten by certain species of freshwater fish, the larva develops into the plerocercoid or sparganum form, which migrates to the muscles and connective tissue of the fish. When the final host (humans or other suitable vertebrate host) eats this fish, the plerocercoid proceeds to develop into a hermaphroditic adult in the intestine and starts laying eggs.

13 Aquatic stages of parasitic helminths 93 Diphyllobothrium latum infections in humans are either asymptomatic or produce nausea, vomiting, weakness, dizziness, and diarrhoea. Confirmation of D. latum infection is by finding the egg in faeces. The infection in humans can be controlled by thorough cooking of fish that are caught in endemic areas. Figure 11 illustrates the life cycle of D. latum. The morphology of stages in the life cycle is shown in Figure 12. Accidental infection with the larval stages of other diphyllobothriid tapeworms that are not normally parasitic in humans causes sparganosis (Mueller 1974). Sparganosis occurs mainly in the Far East and Southeast Asia. It is an infectious disease of humans caused by migrating larvae of tapeworms belonging to the genus Spirometra (Phylum Platyhelminthes; Class Cestoidea; Order Pseudophyllidea; Family Diphyllobothriidae). Figure 11. Life cycle of Diphyllobothrium latum. scolex Adult operculum proglottid Egg Coracidium Procercoid Plerocercoid Figure 12. Diphyllobothrium latum.

14 94 Freshwater Invertebrates of the Malaysian Region When unsuitable hosts certain amphibians, reptiles, and mammals (including humans) eat the procercoid-infected copepods, the plerocercoid stage develops in their tissues. Infection of the final host, cats and dogs, comes from eating a second intermediate host containing the viable plerocercoids. The plerocercoid fixes itself to the wall of the small intestine and matures. Sparganosis in humans may result from drinking water containing procercoid-infected cyclops or by eating amphibians, reptiles, and mammals containing viable plerocercoids, and by applying plerocercoid-infected flesh of frogs, snakes and possibly mammals to wounds or inflamed eyes as a poultice. Prevention of the infection is through boiling or filtering drinking water that may contain infected copepods. Folk medicine such as applying fresh animal meat to a diseased part, which is a common practice in the Orient, should be avoided. Capillaria philippinensis Phylum Nematoda; Class Enoplea; Order Trichurida; Family Capillariidae Freshwater fish in many parts of the Southeast Asian countries are infected with larval stages of Capillaria philippinensis. Human infection with C. philippinensis is sporadic, but epidemics are reported. The disease is endemic in the Philippines and Thailand (Cross 1990). Adult C. philippinensis is a parasite of the small intestine and causes a severe enteropathy and at times death in humans. Humans become infected when eating raw or improperly cooked infected small freshwater and brackish-water fish. Figure 13. Life cycle of Capillaria philippinensis.

15 Aquatic stages of parasitic helminths 95 Adult worms are slender. The male worm measures mm in length, and the female mm. They live in the mucosa of the small intestine and the female deposits larvae there. These larvae are infectious within the host. As the infection progresses, the female worms deposit eggs and the host begins to pass the unembryonated eggs in the stools. The finding of larval stages, and of oviparous as well as larviparous females in the bowel, suggests that the parasite multiplies in the intestine and that overwhelming infections are the result of autoinfection. Eggs passed in the faeces by infected persons, are thick-shelled and peanut-shaped with bipolar plugs, and measure by µm. Eggs develop in water and are ingested by freshwater fish, in which the larval stages have been found. The natural life cycle of C. philippinensis is not known. Experimentally eggs hatch and develop into larvae when fed to several species of freshwater and brackish-water fish, and to the adult stage if the infected fish is fed to monkeys. Various freshwater fishes are eaten raw in the endemic areas, and humans become infected by this means. Figure 13 illustrates the life cycle of C. philippinensis and Figure 14 shows stages in the life cycle. Male spicule with sheath esophagus flattened plug Egg Female uterus vulva Figure 14. Capillaria philippinensis.

16 96 Freshwater Invertebrates of the Malaysian Region Laboratory confirmation of C. philippinensis infection is by finding eggs, larvae or adults in faeces of infected patients. Human infections can be prevented through cooking freshwater or brackish-water fish. Control measures in endemic areas should include treatment, education, and improved sanitation. Gnathostoma spinigerum Phylum Nematoda; Class Rhabditea; Order Spirurida; Family Gnathostomatidae Although many species of Gnathostoma have been reported worldwide, only G. spinigerum, G. hispidum, G. doloresi and G. nipponicum have been reported as human parasites, G. spinigerum being especially important (Miyazaki 1991). Gnathostoma malaysiae was discovered from rats caught in Malaysia and Thailand (Daengswang 1980; Kamiya et al. 1987; Miyazaki 1991). The worm might infect humans, but this has not yet been proven. Gnathostoma spinigerum is widely distributed in parts of Asia, including Malaysia, Thailand, Indonesia and the Philippines. The adult G. spinigerum live in gastric tumours of dogs and cats, which are the normal hosts. The adult has a characteristic head-bulb with about eight concentric lines of hooks, and a thick, short body. The female measures mm and the male measures mm. The anterior half of the worm is covered with leaf-like spines. Figure 15. Life cycle of Gnathostoma spinigerum.

17 Aquatic stages of parasitic helminths 97 The egg is by µm in size, and has a thin bulge at one pole. Fertilized eggs released by the female are passed in the faeces into fresh water. After an embryonation period of about one week, an active free-swimming larva measuring 0.3 mm or less in length, hatches from the egg and is ingested by a copepod, several species of which are first intermediate host. There it grows into an early third-stage larva. The larva is about 0.5 mm in length. The infected copepods are eaten in turn by any of several species of freshwater fish, amphibians, reptiles, birds, and mammals that serve as second intermediate host and paratenic host. The larva leaves the intestinal tract and enters the muscle to become an advanced third-stage larva, 3 4 mm in body length. When the advanced third-stage larva is eaten by the final host, the parasite becomes mature in the stomach. When humans inadvertently ingest third-stage larvae in raw or undercooked freshwater fish, and meat of various animals, the larva migrates to various parts of the body and evokes a variety of symptoms. Evidence of this migration appears more commonly as migratory swelling with inflammation, redness and pain. Humans might become infected through drinking non-processed water containing infected copepods. Definitive diagnosis depends on worm recovery and identification. Figure 15 illustrates the life cycle of G. spinigerum. The morphology of stages in the life cycle is shown in Figure 16. The prevention of this infection to humans is by eating well-cooked flesh of the second intermediate host or paratenic host. esophagus head bulb (8 rows of hooks) head bulb (4 rows of hooks) intestine Egg esophagus testis ovary intestine Male Female Third-Stage Larva Figure 16. Gnathostoma spinigerum.

18 98 Freshwater Invertebrates of the Malaysian Region Angiostrongylus cantonensis Phylum Nematoda; Class Rhabditea; Order Strongylida; Family Metastrongylidae Angiostrongylus cantonensis, the rat lungworm, has been found as a naturally occurring parasite of rodents in Asia and a number of other Pacific islands (Cross 1987; Alicata 1991). The adult A. cantonensis is thin and long, the female measuring 2.5 to 4 cm and the male measuring about 2 cm. It lives in the pulmonary artery of rodents and here the female worm lays eggs. Eggs develop into larvae, hatch and migrate from the alveoli to the digestive tract and are passed outside with the stool. After being eaten by an intermediate host (slugs or freshwater as well as terrestrial snails), the first-stage larvae develop into the third-stage larvae, mm long. Frogs, prawns, crabs and others can serve as paratenic hosts. When rats eat either the intermediate or paratenic hosts, the larvae migrate to the meninges and develop in the brain for about a month. Young adults then migrate to the pulmonary artery, where they attain maturity and begin egg deposition. Humans acquire A. cantonensis by eating raw, infected molluscs. A wide variety of freshwater snails and prawns as well as terrestrial snails, slugs, land crabs and planarians are naturally infected with A. cantonensis throughout the endemic Figure 17. Life cycle of Angiostrongylus cantonensis.

19 Aquatic stages of parasitic helminths 99 areas. Humans also acquire the infection by ingesting the parasites as contaminants of vegetables (Heyneman and Lim 1967). Angiostrongylus does not complete its developmental cycle in humans. When the third-stage larvae are ingested, they penetrate into blood vessels in the intestinal tract and are carried to the meninges but are unable to migrate to the lungs, as they do in rats. Human infections with this worm present in the brain may involve the eye (Alicata 1991). Angiostrongylus cantonensis causes eosinophilic meningoencephalitis. The life cycle of A. cantonensis is shown in Figure 17 and the morphology of stages in the life cycle is illustrated in Figure 18. Human infections that cause eosinophilic meningitis in Malaysia and Thailand appear to be associated with eating uncooked freshwater molluscs such as Pila snails, either mixed with vegetables or as a form of medication (Cross 1987; Alicata 1991). Definitive diagnosis is by recovery of larvae or young adult worms in the spinal fluid or eyeballs. Two other species of Angiostrongylus are possible agents of human eosinophilic meningitis A. mackerrasae in Australia, and A. malaysiensis in Malaysia, Thailand and Indonesia (Lim and Ramachandran 1979; Cross 1987). For prevention of infection, intermediate and paratenic hosts (slugs, frogs, prawns and crabs) in the endemic areas should not be eaten raw or insufficiently cooked. Raw vegetables must be washed well to remove contaminating infective larvae before eating. esophagus uterus Female intestine esophagus esophagus intestine intestine Third-Stage Larva Male bursa First-Stage Larva Figure 18. Angiostrongylus cantonensis.

20 100 Freshwater Invertebrates of the Malaysian Region INFECTIVE STAGES OF OTHER PARASITIC HELMINTHS FROM FRESH WATER Although the helminths mentioned above have part of their life cycle involving the aquatic environment and aquatic animals, other cestodes and nematodes are also important from a public health or medical viewpoint as water-borne parasites. Their infective eggs are often found in contaminated water. Ascaris lumbricoides and Trichuris trichiura in Malaysia, Indonesia and Thailand are likewise transmitted through faecally polluted water or foods (Muennoo et al. 1992; Hadju et al. 1995; Uga et al. 1995; Hidayah et al. 1997). The eggs of Taenia solium, A. lumbricoides and T. trichiura are very resistant to environmental changes. Taenia solium Phylum Platyhelminthes; Class Cestoidea; Order Cyclophyllidea; Family Taeniidae Taenia solium (pork tapeworm) is found worldwide and is especially common in Southeast Asia. Human infection with T. solium normally results from eating raw or insufficiently cooked pork. The adult worm may be several feet long. It is composed of proglottids and lives in the small intestine of humans. When the terminal proglottids become gravid, they break off and rupture to release the eggs, which are passed in faeces. The egg is spherical and covered by an embryophore of about 29 to 34 µm in diameter. Figure 19 shows the detailed structure of a Taenia egg. Taenia eggs are very resistant to environmental changes and may be found in water. Pigs are the most important intermediate host. When a pig takes in eggs, the onchospheres hatch in the intestine, penetrate the intestinal wall and are carried to all parts of the body via the circulating system. Many make their way to muscles and mature to bladder worms (cysticerci). Humans can also be infected by ingesting water or food contaminated with human excrement containing eggs. As in the case of pigs, the larval worms become lodged in muscles or other vital organs of humans, and mature to bladderworms, resulting in cysticercosis. Figure 19. Egg of Taenia solium.

21 Aquatic stages of parasitic helminths 101 Ascaris lumbricoides Phylum Nematoda; Class Rhabditea; Order Ascaridida; Family Ascarididae Ascaris lumbricoides, the large intestinal roundworm, is found throughout the temperate and tropical areas of the world, under conditions of poor sanitation. Female worms range from 20 to 35 cm in length, while males are smaller and are cm long. Humans and swine are final host, and the adult worms live in the small intestine. Mature female worms may produce an average of 200,000 eggs daily. There are fertilized and unfertilized eggs. The fertilized egg of Ascaris is broadly oval, measuring µm by µm. The outer covering is a protein coat, and when seen in the fresh stool it is usually stained a golden brown by bile. The outer coat is irregular and wrinkled, and lies directly on top of a thick, smooth shell, which is usually not easily distinquished. Some eggs will be seen to have lost the outer protein coat, and in these the thick yellowish inner shell is obvious. The unfertilized egg is longer and narrower. It measures by µm, and the inner and outer coat may be thin. Figure 20 illustrates the morphology of fertilized and unfertilized eggs of A. lumbricoides. Ascaris eggs are unsegmented when passed; under favourable conditions the fertilized eggs develop to the infective stage. When fully embryonated eggs are swallowed, the larvae hatch and migrate in their normal life cycle. The adult parasites, after larval migration through the lung, live in the small intestine of humans. Ascaris infection can cause clinical illness and induces pathological processes. Large numbers of adult worms can cause intestinal blockage. Ascariasis, while cosmopolitan in distribution, is more prevalent in areas of poor sanitation and where human faeces are used for fertilizer. As the Ascaris egg cell is well protected by its thick coating, it is able to survive for some months in faecal matter and sewage. Humans may become infected when the fully embryonated eggs in contaminated water and food are swallowed. Fertilized Egg Unfertilized Egg Decorticated Egg Figure 20. Eggs of Ascaris lumbricoides.

22 102 Freshwater Invertebrates of the Malaysian Region Trichuris trichiura Phylum Nematoda; Class Enoplea; Order Trichurida; Family Trichuridae Trichuris trichiura, popularly known as whipworm, has a worldwide distribution but is more common in tropical areas and in regions where sanitation is poor. The adult female is 3.4 to 5 cm long, while the male is 3 to 4.5 cm long. The body can be divided into two parts, the thin anterior and thick posterior parts. They inhabit the caecal area of large bowel of humans. A female worm produces eggs per day. The egg is characteristically barrel-shaped with refractile polar plugs at both ends (Fig. 21). It measures by µm. Fertilized eggs are passed in faeces and deposited in the soil, where they embryonate before becoming infectious. Human infection is acquired by ingestion of the fully embryonated eggs. The larva hatches in the small intestine and moults four times to become mature adult in the large intestine. Children usually harbour a larger number of whipworms in the intestinal tract than do adults. Heavily infected children often go on to develop colitis and growth stunting. Deaths have been also reported in children. The whipworm infection is most common in tropical areas. Warm, moist soil in tropical and subtropical regions favours the infection. The eggs are relatively resistant to chemical disinfectants. Humans may get infected by ingesting fully embronated eggs from soil and contaminated water. Figure 21. Egg of Trichuris trichiura. OTHER FREE-LIVING HELMINTHS IN FRESH WATER Of the three classes of flatworms, the turbellarians (Class Turbellaria) are free living. One of the most familiar is the genus Dugesia, the common freshwater planarian (Fig. 22). Dugesia is easily recognized by its tripartite intestine. The worms move intestinal diverticulum intestine eye spot opening to pharynx protruding pharynx Figure 22. Dugesia, a common freshwater planarian.

23 Aquatic stages of parasitic helminths 103 from place to place by means of their ciliated epithelial cells, which are particularly concentrated on their lower surfaces. They have eyespots on their heads, which are pigmented cups containing light-sensitive cells, connected with the nervous system. These eyespots enable the worms to move away from strong light. They use sensory pits or tentacles along the sides of their heads to detect food, chemicals, or movements of the fluid in which they are moving. They eat various small animals and organic debris. Turbellarians are hermaphroditic and their fertilized eggs are laid in cocoons strung in ribbons and hatch into juveniles. [Refer to Cannon (2004) and Kawakatsu and Mitchell (2004) for further information.] REFERENCES Alicata J.E. (1991) The discovery of Angiostrongylus cantonensis as a cause of human eosinophilic meningitis. Parasitology Today 7: Alicata J.E. and Jindrak K. (1970) Angiostrongylosis in the Pacific and Southeast Asia. Charles C Thomas. Springfield. Ash L.R. and Orihel T.C. (1977) Atlas of Human Parasitology. 4 th Ed. ASCP Press, Chicago. Beaver P.C., Jung R.C. and Cupp E.W. (1984) Clinical Parasitology. 9 th Ed. Lea and Febiger. Philadelphia. Brauce J.I. and Sornmani S. (Eds) (1980) The Mekong schistosome. Malacological Reviews Suppl 2. Bundy D.A.P. and Cooper E.S. (1989) Trichuris and trichuriasis in humans. In Baker J.R. and Muller R. (Eds) Advances in Parasitology. Pp Academic Press, London. Bunnag D., Bunnag T. and Goldsmith R. (1991) Liver fluke infections. In Strickland G.T. (Ed.) Hunter s Tropical Medicine. 7 th Ed. Pp W.B. Saunders Co. Philadelphia. Cannon L.R.G. (2004) Platyhelminthes: Microturbellarians. In Yule C.M. and Yong H.S. (Eds) Freshwater Invertebrates of the Malaysian Region. Pp Academy of Sciences Malaysia and Monash University Malaysia, Kuala Lumpur. Cook G.C. (1996) Manson s Tropical Diseases. 20 th Ed. W.B. Saunders Co. Philadelphia Crompton D.W.T., Nesheim M.C. and Pawlowski Z.S., (Eds) (1985) Ascariasis and its public health significance. Taylor and Francis. London. Crompton D.W.T., Nesheim M.C. and Pawlowski Z.S. (Eds) (1989) Ascariasis and its prevention and control. Taylor and Francis. London. Cross J.H. (1969) Fasciolopsiasis in Southeast Asia and the Far East: A Review. In Harinasuta C. (Ed.) Proceedings of the 4 th Southeast Asian Seminar on Parasitology and Tropical Medicine, Schistosomiasis and other Snail transmitted Helminthiasis. Pp Cross J.H. (1987) Public health importance of Angiostrongylus cantonensis and its relatives. Parasitology Today 3: Cross J.H. (1990) Intestinal capillariasis. Parasitology Today 6: Daengsvang S. (1980) A monograph on the genus Gnathostoma and gnathostomiasis in Thailand. Southeast Asian Medical Information Foundation Center, Tokyo. Greer G.J., Ow-Yang C.K. and Yong H.S. (1988) Schistosoma malayensis n. sp.: A Schistosoma japonicum-complex schistosome from Peninsular Malaysia. Journal of Parasitology 74: Greer G.J., Ow-Yang C.K., Singh K.I. and Lim H.K. (1980) Discovery of a site of transmission and hosts of a Schistosoma japonicum like schistosome in Peninsular Malaysia. Transactions of the Royal Society of Tropical Medicine and Hygiene 74: 425. Goldsmith R.D., Bunnag D. and Bunnag T. (1991) Lung fluke infections: paragonimiasis. In Strickland G.T. (Ed.) Hunter s Tropical Medicine. 7 th Ed. Pp W.B. Saunders Co. Philadelphia.

24 104 Freshwater Invertebrates of the Malaysian Region Hadju V., Abadi K., Stephenson L.S., Noor N.N., Mohammed H.O. and Browman D.D. (1995) Intestinal helminthiasis, nutritional status and their relationship: a cross-sectional study in urban slum school children in Indonesia. Southeast Asian Journal of Tropical Medicine and Public Health 26: Harinasuta T., Bunnag D. and Radomyos P. (1987) Intestinal fluke infections. Bailliere s Clinical Tropical Medicine and Communicable Diseases 2: Hidayah N.I., Teoh S.T. and Hillman E. (1997) Socio-environmental predictors of soil-transmitted helminthiasis in a rural community in Malaysia. Southeast Asian Journal of Tropical Medicine and Public Health 28: Heyneman D. and Lim B.L. (1967) Angiostrongylus cantonensis : proof of direct transmission with its epidemiological implications. Science 158: Ito A. (1992) Cysticercosis in Asia-Pacific regions. Parasitology Today 8: Kamiya H., Kamiya M., Ohbayashi M., Klongkamnuankarn K. and Vajrasthira S. (1987) Gnathostoma malaysiae Miyazaki and Dunn, 1965 from Rattus surifer in Thailand. Southeast Asian Journal of Tropical Medicine and Public Health 18: Kawakatsu M. and Mitchell R.W. (2004) Platyhelminthes: Dugesiidae and Dimarcusidae. In Yule C.M. and Yong H.S. (Eds) Freshwater Invertebrates of the Malaysian Region. Pp Academy of Sciences Malaysia and Monash University Malaysia, Kuala Lumpur. Khalil L.F. and Jones A. (1994) Keys to the cestode parasites of vertebrates. CAB International. Wallingford, Oxon, England. Krishnasamy M., Chong N.L., Ambu S., Jeffery J., Oothuman P. and Eduriah Abu Bakar (2001) Schistosomiasis in Malaysia, with special reference to Schistosoma spindale, the causative agent of cercarial dermatitis. Tropical Biomedicine 18: Lim B.L. and Ramachandran C.P. (1979) Ecological studies of Angiostrongylus malaysiensis (Nematoda: Metastrongylidae) in Malaysia. In Cross J.H. (Ed.) Studies on Angiostrongyliasis in Eastern Asia and Australia. Pp NAMRU 2. Special Publication No. 44, Taipei, Taiwan. Loker E.S. (1983) A comparative study on the life histories of mammalian schistosomes. Parasitology 87: McGreevy P.B. and Nelson G.S. (1991) Larval cestode infections: paragonimiasis. In Strickland G.T. (Ed.) Hunter s Tropical Medicine. 7 th Ed., Pp W.B. Saunders Co. Philadelphia. Miyazaki I. (1991) An illustrated book of helminthic zoonoses. SEAMIC Publication No. 62. International Medical Foundation of Japan, Tokyo. Mueller J.F. (1974) The biology of Spirometra. Journal of Parasitology 60: Muennoo C., Chiamratana B., Sa-nguankiat S., Yaemput S., Waikagul J. and Charoenlarp P. (1992) Study on prevalence and intensity of soil-transmitted helminths in primary school children, Nakhon Si Thammarat province. Journal of Medical Parasitology 15: Popiel I. (1986) The reproductive biology of schistosomes. Parasitology Today 2: Pungpak S., Chalermrut K., Harinasuta T.,Viravan C., Schelp P.F., Hempfling A., Schlattmann P. and Bunnag D. (1994) Opisthorchis viverrini infection in Thailand: symptoms and signs of infection a population-based study. Transactions of the Royal Society of Tropical Medicine and Hygiene 88: Roberts L.S. and Janovy J. (1996) Foundations of Parasitology, 5 th Ed. W. C. Brown Publishers, McGraw-Hill Co. Sadun E.H. and Maiphoon C. (1953) Studies on the epidemiology of the human intestinal fluke, Fasciolopsis buski (Lankester) in central Thailand. American Journal of Tropical Medicine and Hygiene 2: Schmidt G.D. and Roberts L.S. (1989) Foundations of Parasitology. 4 th Ed. Times Mirror/Mosby College Publishing. St. Louis. Shekhar K.C. and Pathmanathan R. (1987) Schistosomiasis in Malaysia A review. Reviews of Infectious Diseases 9:

25 Aquatic stages of parasitic helminths 105 Shekhar K.C. (1987) A comparative parasitological study of Malaysian schistosomes (Koyan and Baling strain) with Schistosoma japonicum (Philippine strain) and S. mekongi (Thai strain) in various laboratory animals. Tropical Biomedicine 4: Tanowitz H.B. and Wittner M. (1991) Tape worm infections. In Strickland G.T. (Ed.) Hunter s Tropical Medicine. 7 th Ed., Pp W.B. Saunders Co. Philadelphia. Ubelaker J.E. (1983) The morphology, development and evolution of tapeworm larvae. In Arme C. and Pappas P.W. (Eds) Biology of the Eucestoda 1. Academic Press. London. Uga S., Ono N., Nataoka N., Safrioh A., Tantular I.S., Dechlan Y.P. and Ranuh I.G. (1995) Contamination of soil with parasite eggs in Surabaya, Indonesia. Southeast Asian Journal of Tropical Medicine and Public Health 26: Voge M., Bruckner D. and Bruce J.I. (1978) Schistosoma mekongi sp. n. from man and animals, compared with four geographic strains of Schistosoma japonicum. Journal of Parasitology 64: Woodruff D.S., Merenlender A.M., Upatham E.S. and Viyanant V. (1987) Genetic variation and differentiation of three Schistosoma species from the Philippines, Laos, and Peninsular Malaysia. American Journal of Tropical Medicine and Hygiene 36: World Health Organization (1995) Control of food borne trematode infections. World Health Organization Technical Reports Series no Yong H.S., Greer G.J. and Ow-Yang C.K. (1985) Genetic diversity and differentiation of four taxa of Asiatic blood flukes (Trematoda, Schistosomatidae). Tropical Biomedicine 2: Note: Figures in this chapter are adapted from various sources.