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


*Snail families involved as intermediate hosts for trematodes (flukes) causing disease in humans or domestic animals. Only certain species within a family are intermediate hosts for a given parasite.* *Zoonotic Trematode Infections; Their Biology, Intermediate Hosts and Control DOI: http://dx.doi.org/10.5772/intechopen.102434*

#### **Figure 2.**

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

The species are divided into four groups, i.e., the *S. japonicum* group, the *Schistosoma mansoni* group, the *S. indicum* group and the *S. haematobium* group [8]. Some species primarily infect humans while others primarily infect non-human animals, but infections with the former group may be found in non-human vertebrates and infections with the latter group may be found in humans [9, 10].

Species within the group of *S. japonicum* use as intermediate hosts, species of the Pomatiopsidae. *Schistosoma japonicum* is the major species of medical importance in South-East Asia and the species has many definitive hosts [6]. *Schistosoma mekongi* is found in parts of the Mekong River basin region in Cambodia, Laos, and Thailand [11]. The other species of the *S. japonicum* group are mainly parasites of rats [12] although human infection by *Schistosoma malayanum* are found [11].

The group of *Schistosoma mansoni* has 2 species, *S. mansoni* and *Schistosoma rodhaini* both using *Biomphalaria* spp. (**Figure 3**) as an intermediate host. *S. mansoni* is found throughout sub-Saharan Africa, parts of the Arabian Peninsula and it is the only *Schistosoma* spp. found in South America [8]. The two species hybridize in the wild [12]. Species of the group *S. indicum* occur in the western and southern Asian regions and are commonly found in animals, i.e., ungulates, horses, pigs, and possibly dogs [8]. The species use *Indoplanorbis exustus* as intermediate host [8]. The group associated with *S. haematobium* includes nine species that are the most widespread, i.e., Africa, Indian Ocean Islands, Arabian Peninsula, and Mediterranean regions [8]. *Schistosoma haematobium* causes urogenital schistosomiasis, while other species, *S. intercalatum* and *S. guineensis* infecting humans in this group cause intestinal

#### **Figure 3.**

*Neotropical (1–5) and African (6–15) Biomphalaria species, B. glabrata (1), B. tenagophila (2), B. straminea (3), B. havanensis (4), B. helophila (5), B. alexandrina (6), B. angulosa (7), B. camerunensis (8), B. pfeifferi (9), B. salinarum (10), B. sudanica (11), B. choanomphala (12), B. rhodesiensis (13), B. smithi (14), B. stanleyi (15).*

schistosomiasis [8]. Natural interactions and introgressive hybridisation between these species are common [8, 13, 14].

Each of the species of schistosomes infecting humans has a characteristic and limited intermediate snail-host spectrum. The intermediate hosts of *S. mansoni*, *S. haematobium*, *S. intercalatum*, and *S. guineensis* belong to the family Planorbidae or Bulinidae (previously a subfamily of the Planorbidae) while those of *S. japonicum*, *S. mekongi*, and *S. malayensis* are caenogastropods that belong to the Pomatiopsidae [15]. Various species of *Biomphalaria* serve as intermediate hosts for *S. mansoni* (**Figure 3**) and certain *Bulinus* spp. (**Figure 4**) are intermediate hosts for *S. haematobium*, *S. guineensis*, and *S. intercalatum*. For *S. japonicum*, species of the genus *Oncomelania* are the intermediate hosts (**Figure 4**). Species-level relationships within *Oncomelania* are not fully resolved but there seems to be four species that currently transmit *S. japonicum*, i.e., subspecies of *O. hupensis* in eastern China, *O. robertsoni* in western China, *O. quadrasi* in the Philippines and *O. lindoensis* on Sulawesi [16]. For *S. mekongi* and *S. malayensis*, the intermediate host is from the subfamily Triculinae, tribe Pachydrobiini, i.e., *Neotricula aperta* and *Robertsiella kaporensis*, respectively [17, 18] (**Figure 4**). More species can

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

#### **Figure 4.**

*Representative species of Bulinus, Oncomelania, Robertsiella and Neotricula. The B. africanus group (a): Bulinus abyssinicus (1), B. africanus (2), B. nasutus (3), B. ugandae (4), B. jousseaumei (5), B. obtusus (6), B. obtusispira (7), B. umbilicatus (8), B. globosus (9), B. productus (10). B. truncatus/tropicus complex (b): Bulinus angolensis (11), B. liratus (12), B. natalensis (13), B. tropicus (14), B. nyassanus (15), B. succinoides (16), B. transversalis (17), B. trigonus (18), B. truncatus (19). The B. forskalii group (c): Bulinus bavayi (20), B. beccarii (21), B. canescens (22), B. cernicus (23), B. crystallinus (24), B. forskalii (25), B. scalaris (26), B. senegalensis (28). The B. reticulatus group (d): B. reticulatus (28), B. wrighti (29). Asian species (e): Oncomelania hupensis smooth (30) form and ribbed form (31), O. quadrasi (32), Robertsiella kaporensis (33), Neotricula aperta (34).*

be infected experimentally but they may not transmit the parasite in nature [19]. For example, there have been reports that *S. haematobium* was transmitted by *Ferrissia* in India; at least three endemic foci of human schistosomiasis have been described in India previously and sporadic autochthonous cases and cercarial dermatitis are also common (see references in [20]). Experimental exposure of *Ferrissia tenuis* to miracidia of *S. haematobium* showed that this snail could be infected and shed cercariae [21]. In the north-western extremity of Africa, *Planorbarius metidjensis* is common and although it is an experimental host for *S. mansoni* and *S. haematobium* [22, 23] there is no evidence that it is a natural host [23].

Snails may be widely distributed in an area, but there is a tendency for infected snails with *Schistosoma* spp. to be focally distributed in particular areas where infected people contaminate the water with their wastes. In some cases, human infections may be facilitated by prior contamination of habitats by reservoir hosts especially for *S. japonicum*, but also possibly for *Schistosoma mansoni* and *S. haematobium*. Snail populations undergo great seasonal variations in density and infection rates. Rainfall and/or temperature are the main causative factors. This results in the pattern of transmission commonly being of a focal and seasonal nature. Although prevalence of infection in the intermediate hosts may be low, this can result in a high percentage of human infections, e.g., in Lake Malawi [24].

#### **2.2 Avian schistosome**

Swimmer's itch or cercarial dermatitis is a short-term immune reaction occurring in the skin of humans that have been penetrated by cercariae of schistosomes (Schistosomatidae) that normally develop in birds or in mammalian hosts other than humans. Genera often associated with swimmer's itch in humans are *Trichobilharzia* and *Gigantobilharzia*, but species such as *Schistosomatium douthitti*, a parasite of rodents, may also cause swimmer's itch. In marine habitats, especially along the coasts, swimmer's itch can occur as well. Symptoms, which include itchy, raised papules, commonly occur within hours of infection and do not generally last more than a week. It is common in freshwater, brackish, and marine habitats worldwide and application of molecular diagnostic techniques has begun to unravel the many schistosome species that can be responsible [25]. Various species of Lymnaeidae, Physidae, Planorbidae, Bulinidae (see some species in **Figures 5** and **6**) and other taxa are intermediate hosts [25–28].

In Thailand, *Indoplanorbis exustus* together with *Lymnaea rubiginosa* are referred to as the "itchy snail" by rural people [29]. These snail species are hosts for a number of schistosomes including *Schistosoma spindale*, *S. indicum*, and *S. nasale*. Though these species do not develop to maturity in humans, they may cause cercarial dermatitis.

#### **2.3 Paragonimus**

Paragonimiasis, also known as pulmonary distomiasis, is a parasitic disease of humans and animals in various parts of the world, but principally in the Orient (Far East). Its etiological agents are species of the trematode genus *Paragonimus* which utilize caenogastropods (specifically superfamilies Cerithiodea and Truncatelloidea) as first intermediate hosts (see examples in **Figures 7** and **8**) and decapod crustaceans, primarily freshwater crabs and crayfish, as second intermediate hosts.

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

#### **Figure 5.**

*Some species of the Lymnaeidae, Physidae and Bulinidae. Lymnaeidae: Radix natalensis (1), Pseudosuccinea columella (2), Radix auricularia (3), Galba truncatula (4), Austropeplea viridis (6). Bulinidae: Indoplanorbis exustus (6). Physidae: Aplexa waterloti (7), Physa acuta (8).*

The genus *Paragonimus* contains several species that infect a variety of mammals and birds and some of these species affect other vertebrates. Lung fluke infections are distributed in certain parts of the World where food habits include eating raw crabs, and crayfish, which contain the infective metacercarial cysts. Several million people are thought to be infected with paragonomiasis and almost 300 million are at risk of infection [30, 31]. In Africa, *Paragonimus africanus* is important [32], while in Mexico and Central America *Paragonimus mexicanus* causes occasional human infections [30]. Similarly, in the Midwest states of the USA, a number of human infections with *Paragonimus kellicotti* have been reported. The main endemic areas are in the Orient and Southeast Asia. In these areas, the etiological agent in humans is mainly *Paragonimus westermani* [31].

The cercariae penetrate the soft body parts of the crustacean host and then invade the viscera and muscles of this host, where they usually become encysted in specific organs depending on the species of lung fluke and the species of the crustacean host (**Figure 9**). When the mammalian host, human or reservoir host ingests infected crab or crayfish meat or viscera (raw, soaked in rice wine, or salted), the metacercaria excyst in the duodenum and migrates through the intestinal wall in about an hour, reaching the abdominal cavity in 3–6 h. The larvae of various lung flukes enter and remain in the abdominal wall for several days (up to 3 weeks), then migrate through the diaphragm to the pleural cavity, where they penetrate the serosal layers of the lungs. Finally, they arrive near the bronchioles, where they develop to adult worms in pairs, and exist in tissue capsules laid down by the host, about 6–8 weeks after ingestion of the parasitized crustacean host. The lung capsules containing the worms connect with the respiratory passages of the lung, and the eggs of the parasite are moved along with lung exudates [33].

#### **Figure 6.**

*Species of the Planorbidae (a and c) and Burnupiidae (b). Planorbidae: Planorbella (Helisoma) duryi (1), Planorbarius metidjensis (2), Planorbis planorbis (3), Africanogyrus coretus (5), Ceratophallus natalensis (6), Drepanotrema sp. (7), Gyraulus spp. (8), Polypylis hemisphaerula (9), Segmentorbis spp. (10), Ferrissia sp. (11). Burnupidae: Burnupia sp. (4). The scale shown in part c is also used for part b.*
