The life cycle and biology

At least eight genera of cercariae of non-human schistosomes causing swimmers' itch have been recorded (see Table 22.1) (Farley 1971; Blair and Islam 1983; Kolárová 2000), most of them from freshwater snails but several also from marine snails (Penner 1950; Chu and Cutress 1954; Bearup and Langsford 1966). Much of the scientific work carried through in order to clarify the biology and epidemiology of these parasites has focused on Trichobilharzia spp, the species, together with Bilharziellapolonica, considered to be most frequently involved

Figure 22.1 (1) Infectious cercaria giving rise to swimmers' itch. (2) Typical severe cercarial dermatitis with maculo-papular rash caused by Trichobilharzia spp. swimmers' itch of 'accidental host' and the definite host for the parasite, a seabird. (3) Adult Trichobilharzia spp. in seabird. (4) Egg of Trichobilharzia spp. which gives rise to the miracidial stage which infects the intermediate host, a Lymnaea snail. (5) The infectious cercaria (1) arise from sporocysts developing in the snail. (See Colour Plate IV.)

Figure 22.1 (1) Infectious cercaria giving rise to swimmers' itch. (2) Typical severe cercarial dermatitis with maculo-papular rash caused by Trichobilharzia spp. swimmers' itch of 'accidental host' and the definite host for the parasite, a seabird. (3) Adult Trichobilharzia spp. in seabird. (4) Egg of Trichobilharzia spp. which gives rise to the miracidial stage which infects the intermediate host, a Lymnaea snail. (5) The infectious cercaria (1) arise from sporocysts developing in the snail. (See Colour Plate IV.)

in outbreaks of swimmers' itch in Europe and North America. Apparently the basic biology and life cycles of other species of avian/mammalian schistosomes are rather similar although the species of intermediate and final hosts are different (McMullen and Beaver 1945; Hoeffler 1974; Goff and Ronald 1981; Wojcinski et al. 1987; McKown et al. 1991; Loken et al. 1995). Various water fowls (usually mallards, Anasplatyrhinchos) are the final hosts of the genus Trichobilharzia, while snails of the genera Lymnaea, Stagnicola, Radix, Planorbis, and Planorbarius, are the most common intermediate hosts (Pirila and Wikgren 1957; Beer and German 1994; Kolarova et al. 1997). The adult worms are located in the vessels of the gut and mesenteries of the final host and produce eggs, which are transported to the intestinal lumen and are

Table 22.1 The family Schistosomatidae [from Farley (1971) and Kolârovâ (2000)]

Subfamily

Genus

Final Agent of host SI

References

Schistosomatinae

Austrobilharziaa (Microbilharzia)

Birds Yes

(Stunkard and Hinchcliffe 1952; Appleton and Lethbridge 1979)

Bivitellobilharzia

Elephants

(Vogel and Mmnmg 1940)

Heterobilharzia

Mammals Yes

(Malek and Armstrong 1967; Goff and Ronald 1981; McKown et al. 1991)

Macrobilharzia

Birds

(Kohn 1964)

Orientobilharzia

Mammals Yes

(Sahba and Malek 1979)

Ornithobilharziaa

Birds

(Witenberg and Lengy 1967; Morales et al. 1971)

Schistosoma

Mammals Yes

(Kullavanijaya and Wongwaisayawan 1993)

Schistosomatium

Mammals Yes

(Swartz 1966; Malek 1977; Loker 1983)

Bilharziellinae

Bilharziella

Birds Yes

(Beer et al. 1995)

Trichobilharzia

Birds Yes

(Matheson 1930; Müller and Kimmig 1994; Pilz et al. 1995)

GigantobilharziinaeDendritobilharzia

Birds

(Vande Vusse, 1980; Canaris et al.

1981; Wojcinski et al. 1987)

Gigantobilharzid0

Birds Yes

(von Dönges 1965; Matsumura et al. 1984; Appleton and Randall 1986)

Griphobilharziinae Griphobilharzia

Crocodiles

Notes a Cercariae developing in marine water snails. b Cercariae developing in marine and freshwater snails.

Notes a Cercariae developing in marine water snails. b Cercariae developing in marine and freshwater snails.

shed with the faeces into the water. Some species seem to be located in the nasal blood vessels of the host and accordingly eggs may be shed in the nasal secretions (Fain 1955; Palmer and Ossent 1984; Horak et al. 1998). The ciliated miracidium, after hatching from the egg, penetrates into the snail (Figure 22.2). A complicated process of asexual replication in the snail tissues, through sporocyst and daughter sporocyst generations, generates the characteristic free-swimming, brevifurcate cercaria larvae, infective for the final host (Islam 1986). The cercariae are phototactic, swimming close to the water surface and when finding a suitable host bird penetrate the skin of the feet and migrate via the heart and lungs to their final localization, the tissues of the gut or the mesenteric veins (Neuhaus 1952; Bourns et al. 1973; Ellis et al. 1975).

Shallow, warm coastal waters are the biotopes preferred by the intermediate hosts as well as by the final hosts. Such biotopes are usually also much frequented by humans for recreational purposes. Although humans are not appropriate hosts for further development of the parasite, cercariae, coming in contact with human skin, attach to and penetrate the skin (Haas and van de Roemer 1998). The mechanisms for attachment to and penetration of the human skin are essentially analogous to those of the tropical schistosomes capable of utilizing man as final hosts. The bird schistosomes, however, are usually unable to penetrate deeper into the human tissues and are destroyed in the skin (von Dönges 1964). However, some authors have shown that cercariae of the genera Trichobilharzia and Bilharziella are able to migrate to the lungs in mammals (Oliver 1953; Appleton and Brock 1986; Haas and Pietsch 1991; Horak and Kolarova 2000). Recently Horak and Kolarova (2001) reported

Figure 22.2 Serum antibodies from patient with cercarial dermatitis strains Trichobilharzia cercariae in infected Lymnaea stagnalis snail (A) and delicate structures connecting subtegumental cells with surface of male schistosomes in

Figure 22.2 Serum antibodies from patient with cercarial dermatitis strains Trichobilharzia cercariae in infected Lymnaea stagnalis snail (A) and delicate structures connecting subtegumental cells with surface of male schistosomes in

central nervous system (CNS) injury in ducklings and mice caused by the nasal schistosome Trichobilharzia regenti. The swimmers' itch is the immune response involving an allergic reaction induced by antigenic components released from the penetrating larvae or, more importantly, from dead, decomposing cercariae in the human skin (Macfarlane, 1949).

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