*3.2.2 Ampullariidae*

Ampullariidae (**Figure 12**) are predominately distributed in humid tropical and subtropical habitats in Africa, South and Central America, and Asia. The family

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

includes 186 recent species with the majority in the three genera *Pomacea* (96 species), *Lanistes* (43 species), and *Pila* (29 species) [52]. These are large or very large species (some species can reach shell height or diameter of about 10 cm). The animal has a short rostrum that carries a tentacle-like process (pseudopodia) on either side. Tentacles are very long and thin, the eyes are placed on separate stalks beside the tentacles. The mantle cavity is separated into two parts by a septum; the right side contains the gill, the left side serves as a lung. The male has a copulatory organ formed by part of the mantle edge. Some species may harbor metacercariae of trematode species, i.e., *Echinostoma* spp. and it may also serve as first intermediate host for some avian schistosomes or echinostome species. *Pila* spp. deposit egg masses in damp soil, *Pomacea* deposit their egg masses on various objects above the water while *Lanistes* spp. deposit gelatinous egg masses in water. Only one genus, *Pila*, is represented in Asia but *Pomacea* spp., which were introduced into Asia for commercial purposes have become a very prominent element of the gastropod fauna in Asia. *Pomacea* spp. were originally introduced to Taiwan and later to other Asian countries and has become pests of wetland rice and other crops causing massive economic losses. Especially, *P. canaliculata*, the Golden Apple Snail, is effective in spreading and has become a major pest in the area. Apple snails have highly diverse feeding mechanisms (shredding, scraping and collecting) to exploit diverse food sources. *M. cornuarietis*, which has a discoid shell (**Figure 12**), has potential in biological control of schistosome intermediate hosts and it has been introduced in Africa [53–55]. Due to the unforeseeable potential impact of such introductions, further introduction or propagation of the species outside its original area of distribution should be discouraged.

#### *3.2.3 Superfamily: Cerithioidea*

The Cerithioidea (**Figure 7**) is a superfamily within the Sorbeoconcha and comprised of marine, brackish water, and freshwater gastropods containing more than 200 genera. The freshwater species are found on all continents, except Antarctica. They are dominant members of mangrove forests, estuarine mudflats, fast-flowing rivers, and placid lakes. The shell is generally turreted, sometimes ovoidal-conic, rarely subglobose. It can be smooth or with spiral and/or axial sculpture, sometimes with spiral microsculpture. The operculum is corneous, generally spiral, rarely concentric; it is retractable into the shell. The male reproductive organs are without a verge. Female reproductive organs often have a brood pouch, generally with an egg transfer groove. Many species seem to be parthenogenetic.

The superfamily contains the Hemisinidae [56], Melanopsidae [57], Pachychilidae [58], Paludomidae [59], Pleuroceridae [60], Semisulcospiridae [61], and Thiaridae [62]. Only some of these families are described further below. Some of these species are important as intermediate hosts for medically important trematodes, e.g., Semisulcospiridae is an important host for *Paragonimus westermani* and some species for *C. sinensis*.

#### *3.2.4 Potamididae*

The family has a circumtropical, distribution but is also found in moderate climates. The Potamididae (mudwhelks or mud creepers) are small to large brackish water snails that live on mud flats, mangroves, and similar habitats. The trees provide the snails with shelter, protection from predators, a solid substrate, and sometimes food [63]. Some species are intermediate hosts for some fish-borne zoonotic trematodes.

#### *3.2.5 Pachychilidae*

Pachychilidae are a group of freshwater gastropods only recently recognized as an independent freshwater radiation within the diverse and predominantly marine gastropod superfamily Cerithioidea [58]. Pachychilids were previously assigned to other cerithioidean freshwater families, such as Thiaridae or Pleuroceridae. Pachychilidae has a circumtropical distribution with the freshwater inhabiting *Pachychilus* and *Doryssa* from South and Central America, *Potadoma*, from tropical western Africa and the Congo River drainage system, and *Madagasikara* from Madagascar [58]. Asian taxa include *Paracrostoma*, *Sulcospira* and *Brotia* from Southeast Asia [58].

Pachychilid gastropods are a conspicuous element of the freshwater macroinvertebrate fauna of Southeast Asia. In this region, three spatially separated groups of pachychilids can be differentiated mostly by means of their brooding strategy [64]. Pachychilids have rather heavy, thick shells and are not eaten by molluscivores in experimental studies [65]. They often occur at very high density [66]. Some species have rather specialized habitat requirements, and this may make them more vulnerable to habitat degradation, modification, and pollution [67].

#### *3.2.6 Thiaridae*

The Thiaridae form a monophyletic group with its constituent species being probably autochthonous in Southeast and South Asia, Australia, and some Pacific Islands, as well as sub-Saharan Africa, both in lotic and lentic freshwater environments, with some species also tolerating brackish conditions in the lower courses and estuaries of rivers [62]. Some species, such as *Melanoides tuberculata*, have an extraordinarily high invasive potential and today have an almost circumglobal distribution in tropical and subtropical biomes [62]. This spread apparently has assisted in the elimination of schistosomiasis from the Caribbean [68]. The females are ovoviviparous and the young are brooded within a non-uterine subhaemocoelic brood pouch situated in the right headfoot and extending deep into the neck region above the columellar muscle. Functional males have been found in a few species.

Some populations of *M. tuberculata* are comprised of only females reproducing parthenogenetically, while males may appear only during periods of environmental stress, for example high level of infection. Relatively few studies have been done on population dynamics of thiarid snails. Dudgeon [69] found that there was a single peak in juvenile recruitment coinciding with the warmer months; hatchlings grew quickly and were sexually mature before the next breeding season. Thiarid snail species, but in particular *M. tuberculata*, are very abundant in fishponds in Vietnam and Thailand [70, 71].

The family is very important as intermediate hosts for heterophyid intestinal trematodes and possibly *Paragonimus westermani*. Some papers mention that selected species also are intermediate hosts for liver flukes [37], but this has been questioned by others [70].

#### *3.2.7 Paludomidae*

The genera and species suggested to be included in the Paludomidae have hitherto been classified as Thiaridae, especially the endemic thalassoid species from Lake Tanganyika [59]. Generic diversity of African paludomids is concentrated in the Lake Tanganyika basin and adjacent water bodies, with only two genera, *Cleopatra*

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

and *Pseudocleopatra* recorded from outside of this region. The genus *Cleopatra* is widespread in rivers, lakes, and even temporary water bodies of sub-Saharan Africa reaching North Africa through the Nile River system and in Madagascar, while *Pseudocleopatra* is reported from Ghana and the Congo River basin [59].
