* Vat ky sinh khdng xen ke the he vd mot lan thay doi vat chu. Vi du, san day
Taenia, Taeniarhynchus.... hoac giun tron dai dien cua bo Spirurida (giun chi, giun bao d ngudi), giun dau gai va nhieu vat ky sinh thuoc lop giap xac Copepoda, Isopoda. Doi voi Trichinella, vat chu co xuong song vira giu vai trd vat chu cuoi cung, vua la vat chu trung gian. Khi nhidm benh, dong vat trd thanh vat chu cuoi cung. Trichinella sinh san huu tinh, con cai de ra au trung, au trung nay theo cac mao mach xam nhap vao co tao thanh cac nang, luc nay vat chu trd thanh vat chu trung gian.
* Vat ky sinh khdng xen ke the he vd hai lan thay ddi vat chu. Kieu chu ky nay gap d mot sd san day. Vi du, Diphyllobothrium ky sinh d ngudi, cho, meo co vat chu trung gian la Cyclops va vat chu trung gian bd sung la ca.
* Vat ky sinh cd xen ke the he va cd thay ddi vat chu
- Thay doi vat chii mot ldn: Gap d san day Multi ceps, Echinococus, san la Fasciola
(hinh 33), Schistosoma. 6 vat chii trung gian, au trung sinh san vd tinh bang cach don tinh sinh.
Hinh 33. Cac giai doan phai trien cua san la gan -
Fasciola hepatica
A - San tri/dng thanh; B - TrOng.C - Miracidium; D - Sporocyst C - Miracidium; D - Sporocyst
non; E - Sporocyst gia; G - Redia chtfa Cercaria; G - Cercaria;
H - Adolescaria
- Thay doi hai lan vat chu: gap a san la phoi giong Paragonimus ky sinh a ngucri va gia sue (hinh 34). Vat chu trung gian thur nhat la oc, a oc xay ra qua trinh sinh san don tfnh. Sau do, Cercaria xam nhap vao vat chu trung gian thur hai la cua suoi hoac tom, phat trien trong mang, co, gan cua vat chu trung gian thur hai tao thanh Metacercaria. Cac ho san la Opithorchidae (hinh 35), Echinostomatidae.... cung tha'y tuong tu.
Hinh 34. Cac giai doan phat trien cua
A - San trudng thanh; B - Redia gia; C - Cercaria thoat ra khoi oc;D - Metacercaria trong cua; E - G - Cac loai vat chu trung gian thtf hai; D - Metacercaria trong cua; E - G - Cac loai vat chu trung gian thtf hai;
Hinh 35. Cac giai doan phat trien ciia san la gan Opisthorchis felineus
A - San tri/dng thanh B - Miracidium B - Miracidium