**2.4 Fish-borne zoonotic trematodes (Clonorchiasis, Opistorchiasis, and Heterophyidiasis)**

Fish-borne zoonotic trematodes utilize fish as their second intermediate host and comprise about 12 families, and five of these, Clinostomatidae, Echinostomatidae, Heterophyidae, Opisthorchiidae, and Troglotrematidae have been reported to infect humans. Among those, the opisthorchid flukes have the most public health importance [34]. It has been recognized as a Type I carcinogen, and chronic infection by this liver fluke leads to cholangiocarcinoma development. The heterophyid intestinal *Zoonotic Trematode Infections; Their Biology, Intermediate Hosts and Control DOI: http://dx.doi.org/10.5772/intechopen.102434*

#### **Figure 7.**

*Selected species of Pachychilidae (a), Heminiscidae (b), Paludomidae (c), Thiaridae (d) Potamididae (d), Melanopsidae (e) and other (d). Pachychilidae: Brotia sp. (1); Brotia costula (2); Brotia swinhoei (3); Sulcospira aubryana (4); S. housei (5); Potadoma moerchi (6); P. freethi (7); P. liricincta (8). Heminiscidae: Pachymelania aurita (9); Pachymelania fusca (10, 11). Paludomidae: Cleopatra bulimoides (12); C. nsedwensis (13), C. ferruginea (15) Pseudocleopatra togoensis (14). Thiaridae: Melanoides jugicosta (16), M. tuberculata (17), Sermyla riqueti (18), Thiara scabra (19), Tarebia granifera (22), Potamididae: Tympanotonus fuscatus (20), Cerithidea sp. (21). Melanopsidae: Melanopsis spp. (24). Other: Anentome helena (23).*

fluke sometimes coexists in the endemic region of the liver fluke and can cause confusion in diagnosis and prevalence since eggs of both the opisthorchid and heterophyid flukes are similar. An overview of the various species is given in Waikagul and Thaenkham [34] and Hung et al. [35].

Fully embryonated small eggs of *C. sinensis* are found in the stools of infected humans and other mammalian hosts (e.g., dogs and cats). These eggs do not hatch until they are eaten by the snail, the first intermediate host [36]. For *C. sinensis*, the snail hosts are species of caenogastropods of the family Bithyniidae (*Parafossarulus*, *Bithynia*) or Semisulcospiridae (*Semisulcospira*). In addition, species of the Thiaridae, especially *Melanoides tuberculata* have been reported as hosts for *C. sinensis* [37] but experimental infection of this species failed [38]. The miracidia hatch in the esophagus, intestine, or rectum of the snail, and then penetrate the wall of these organs to become sporocysts developing in the hemolymph spaces of the snail (**Figure 10**). Rediae develop and migrate to the digestive gland area and form cercariae, which soon leave the snail and swim in the adjacent water [36]. They then penetrate the skin of the fish second intermediate host and become encysted metacercariae in the muscles. The metacercariae become infective within a month, and when the infected fish is ingested raw, or slightly cooked or pickled, by man or eaten by other mammalian hosts, the metacercariae excyst and make their way to the liver, which they reach in about a day or less. After reaching the bile passages, the young worms mature. The

#### **Figure 8.**

*Selected species of the Truncatelloidea. Bithynia fuchsiana (1), Bithynia sp. (2), Digoniostoma siamensis (3), Alocinma longicornis (4), Parafossarulus manchouricus (7), Gabbiella stanleyi (5), Gabbiella senaariensis (6), Assiminea sp. (8), Hubendickia sp. (9), Pachydrobia spp. (10, 11), Fairbankia sp. (12), Julienia sp. (13), Stenothyra spp. (14), Neotricula aperta (15), Robertsiella kaporensis (16).*

first eggs are laid about 4 weeks after infection, but the worms continue to grow for some additional months. In addition to humans, dogs, cats, pigs, and rats have been found naturally infected and constitute effective reservoir hosts. In some regions where prevalence of infection among humans is very low or lacking, as in some parts of China and Vietnam, prevalence of infection is usually high among other mammals. Cats, rabbits, and guinea pigs are good experimental hosts.

Clonorchiasis is caused by the fluke *Clonorchis sinensis* (China, Japan, Korea, Russia, Thailand, and North Vietnam), and human opisthorciasis is caused by primarily *Opisthorchis viverrini* (Cambodia, Lao PDR, Thailand, Central and South Vietnam) or *O. felineus* (The Baltic States, eastern Germany, Italy, Kazakhstan, Poland, Russia, Eastern Siberia, and Ukraine) and is contracted through eating raw infected fish [34]. These trematodes have very similar life cycles.

Heterophyidae comprises several genera and species of trematodes of almost worldwide distribution. More than 25 species have been found parasitizing humans around the World [34, 35]. The heterophyid is a small-sized fluke, about 1 mm in length, and is parasitic mostly in the small intestine of birds and mammals and rarely in fish and reptiles.

The worms are usually found lodging in intestinal mucosa between villi, however, they have invaded the submucosal level in experimental immunosuppressive mice. Within a week after the metacercaria is ingested by the definitive host, metacercaria develop to mature adults in the intestine. Heterophyid adults have a short life; the reported life spans varied among different host species [34, 39].

Fish-borne zoonotic trematodes (FZT) are an important problem and fish produced in aquaculture may present a food safety risk in some areas of *Zoonotic Trematode Infections; Their Biology, Intermediate Hosts and Control DOI: http://dx.doi.org/10.5772/intechopen.102434*

**Figure 9.**

*Life cycle of Paragonimus westermani (source: Alexander J. da Silva & Melanie Moser, public health image library (PHIL), Centers for Disease Control and Prevention).*

Southeast Asia where aquaculture is very important [36]. In at least parts of Vietnam, however, transmission of *C. sinensis* is not common in aquaculture ponds and usually occurs in natural habitats [40]. On the other hand, heterophyid trematodes are very common in aquaculture ponds. The fish culture industry may have aggravated the situation in some areas, although *C. sinensis* and *O. viverrini* mainly are transmitted in natural waterbodies. Large and small lakes and ponds are used for culturing fish, and these bodies of water are usually polluted with human and animal excreta containing eggs of FZT. Because raw fish are mainly eaten by adults, infections with clonorchiasis are, in general, more prevalent in the higher age groups (30–50 years); this is usually consistent in all endemic areas.

#### **2.5 Echinostomatidiasis**

The superfamily Echinostomatoidea is a large, cosmopolitan group of digeneans currently including nine families and 105 genera, with the vast majority parasitic, as adults, in birds with relatively few taxa parasitizing mammals, reptiles, and exceptionally, fishes [41]. Recent studies on the phylogeny of the group combining morphology and molecular data have resulted in several changes [41].

**Figure 10.** *Life cycle of fish-borne zoonotic trematodes (Opistchorchidae and Heterophyidae) (source Clausen et al. [36]).*

Echinostomatidiasis is caused by a number of fluke species, belonging to the Echinostomatidae, which share certain morphological features, among which are the presence of a head collar surrounding the oral sucker, provided with a single or double crown of large spines which are larger than those covering the body surface. They are usually stout, fleshy, medium-sized flukes parasitizing birds and mammals in various parts of the world [42]. Several birds, during their migration, carry the infection with several echinostome species along their migratory routes. Various life cycle patterns are exhibited by echinostomes. Usually they are less specific than schistosomes as to their first or second intermediate hosts or their definitive hosts. The first intermediate hosts are several species of aquatic Hygrophila or Caenogastropods and the second intermediate hosts are the same or other species of snails, bivalves, tadpoles, or fish. The cercariae of certain species do not require a second intermediate host but, instead, encyst in the open.

Echinostomes are usually harmless flukes in the intestine of their hosts. Certain species, however, and heavy infections of the harmless species, produce some pathology and pronounced symptoms in poultry and small mammals. They are, therefore, of significance in veterinary medicine.

Transmission of the echinostome to humans is either through eating raw or undercooked fish, snails, or amphibians. Human cases have been reported mostly in Asia. Duodenum mucosal bleeding and ulceration are the main clinical findings due to mechanical damages caused by the worms. The common symptoms are abdominal pain and diarrhea followed by weakness and weight loss [42].

*Zoonotic Trematode Infections; Their Biology, Intermediate Hosts and Control DOI: http://dx.doi.org/10.5772/intechopen.102434*

#### **2.6 Fascioliasis**

Fascioliasis, a disease caused by the liver flukes *Fasciola hepatica* and *Fasciola gigantica*, is cosmopolitan in distribution and occurs in sheep- and cattle-raising countries of the world, parasitizing these animals and other herbivores on almost every continent and on several islands. In Europe, diseases of sheep and cattle are found in every country, and they manifest in the form of epidemics among humans, especially in England, France, and Italy. Human fascioliasis has also been reported from Mexico, South and Central America, Asia, Africa, and Australia and is believed to be more common than has been thought. This could be ascertained if there were more accurate diagnostic methods and better reporting [43].

Fascioliasis due to *F. hepatica* generally occurs in temperate climates, and thus the disease is prevalent in Europe, North America, northern Asia, Australia, and northern Africa [44]. It is also present in the highlands of Kenya, in South Africa, and in Central and South America. *F. gigantica*, on the other hand, is the common liver fluke in widespread areas of Africa and Asia. In some parts of the world, the geographical distribution of *F. gigantica* overlaps that of *F. hepatica*.

The life cycle of *F. hepatica* is illustrated in **Figure 11**. Adults reside in the intrahepatic biliary ducts of the mammalian host. The eggs are laid in the bile

#### **Figure 11.**

*Life cycle of Fasciola hepatica (source: Alexander J. da Silva & Melanie Moser, public health image library (PHIL), Centers for Disease Control and Prevention).*

ducts, proceed to the intestine with the bile, and are evacuated with the feces. The development of the eggs in the water takes from 10 to 15 days at an optimal temperature of 23–25°C. The fully developed miracidium escapes from the egg. The miracidium swims actively in the water and tries to invade various snail species of the Lymnaeidae (**Figure 5**) in different geographic areas of the world. The sporocyst developing from each miracidium near the site of penetration, usually in the mantle edge, the kidney and the esophageal area, produces rediae in about 2–3 weeks. The latter move actively and migrate to the distal area of the snail, containing mainly the digestive gland. Within the body of rediae, cercariae are produced which exit the snail and actively swim in the water. Development of *Fasciola* in the snail host is affected by the environment, in particular temperature. Cercariae encyst rapidly on aquatic vegetation, grass, bark, or any floating debris, or encyst free in the water, and may either float or settle to the bottom of the water. The metacercarial cysts are resistant and remain viable for a long period but are killed by excessive heat and dryness.

Mammalian hosts, including humans, consuming aquatic vegetation with metacercariae or drinking water from contaminated snail habitats containing the metacercariae, contract the infection. The metacercariae, soon after ingestion, excyst in the small intestine. After excystment, they penetrate the wall of the small intestine to the abdominal cavity. They have been found in the latter cavity 1–3 days from the time that they have been ingested, depending on the species of the host. They wander around in the viscera and may settle and become established in ectopic sites other than the liver.

#### **2.7 Paramphistomatidiasis**

The paramphistome flukes are represented by many species throughout the world, and they are parasites of the alimentary tract (stomach and intestine) of humans, nonhuman primates, ruminants, equines, and other herbivores; only about two species occur in birds [45]. These flukes are large fleshy parasites, measuring up to 20 mm in length and 15 mm in width. Some of these flukes cause gastrodisciasis or paramphistomiasis. Whereas gastrodisciasis is restricted to Africa and Asia, paramphistomiasis occurs throughout the world [46].

Three important intestinal parasites cause gastrodisciasis: *Gastrodiscoides hominis*; *Gastrodiscus aegyptiacus*, and *Gastrodiscus secundus*. High prevalence rates have been reported from humans in various parts of India, but the parasite was also found in the Philippines, Kazakhstan, Vietnam, and from Indian immigrants in Guyana. In some countries, *G. hominis* infecting humans is a different strain from that infecting pig. *Gastrodiscus aegyptiacus* and *G. secundus* are common parasites of equines in Africa and Asia, and several pathological cases are on record in horses in Africa.

Infections with all the paramphistomatids (including the gastrodiscids) are acquired from the same habitats where the animals also contract fascioliasis, bovine schistosomiasis, and others, where various species of snails live together. The life cycle, though differing in minute details, is similar to that of *Fasciola* spp.

Like the fasciolid flukes, the paramphistomatids utilize freshwater pulmonate snails as intermediate hosts. Whereas *Fasciola* spp. utilize only lymnaeid snails throughout their wide geographical area, some rumen paramphistomes, such as those in the U.S., use lymnaeid snails, while other paramphistomes, and also gastrodiscids, utilize planorbid snails in other parts of the world.
