- Xam nhap qua ducmg bao thai: hien nay, ngudi ta da chumg minh duoc rang, nhi£u vat ky sinh cd kha nang cam nhiem qua dudng bao thai Fromann (1963) da xac dinh
3. KHU HE KY SINH TRUNG PHU THU0CVA OS l/ DI CUCUA VAT CHU
Mot trong nhung yeu td co anh hudng ldn den su bien doi cua khu he ky sinh triing la hien tuong di cu cua vat chu.
Vi du ro nhat la d ca di cu. Ca hoi con sdng d song nude ngot phuong Bac, dinh dudng kern va phat trien cham. Sau do, chung ra bien va do thuc an day du nen sinh trudng rat nhanh. Sau 2 - 3 nam sdng d bien, ca trd ve song de de lan dau tien thudng vai thang, doi khi ca nam, trong thdi gian nay ca khong an, mac dii mat rat nhieu nang luong de’ boi nguoc dong song (doi khi la hang ngan km) vao trong co the’ con phat trien cac san pham sinh due. Sau khi de chung lai boi ra bien, d day 1 - 2 nam sau mdi quay lai song.
Cac ket qua nghien cuu cho thay, d bien ca hoi giai phong hoan toan ky sinh triing ma chung da nhidm d vung nude ngot va nhidm mdi ky sinh triing co nguon goc bien. Khi nhung ky sinh triing bien ciing ca ve song, chung cung lan luot bi dao thai, trudc tidn la ddi vdi ngoai ky sinh trung, sau do la ky sinh triing dudng ruot, su dao thai phu thuoc vao thdi gian sdng cua chung d song.
Mot vi du nua la khu he ky sinh triing d chim di cu. Mua am, chim sdng d phuong Bac, khi trdi lanh (mua dong) chung di cu ve phuong Nam tranh ret. Khi trd ve phuong Bac, nhung chim nay mang theo khu he ky sinh triing da nhidm tir phuong Nam.
Co the chia khu he ky sinh triing do chim di cu mang tir phuong Nam vd thanh 3 nhom:
- Nhom 1: la nhung ngoai ky sinh trung (chu yeu la ve). Nhom nay ky sinh d chim khi chim sdng d phuong Bac, khi chim di cu ve phuong Nam van mang theo nhom ky sinh triing nay (nhom nay khdng chi ky sinh d chim bd me ma cdn lan truydn sang chim non).
- Nhom 2: gom mot so ngoai ky sinh triing khac. Nhung ky sinh triing nav bi dao thai dan d chim trudng thanh va khdng tim thay d chim trong mua he (khi chim da quay vd phuong Bac).
- Nhom 3: gom cac noi ky sinh triing, chung ky sinh d chim trudng thanh trong sudt mua he (khdng lan truyen sang chim non).
D6ng thdi, ngoai cac ky sinh trung mang tir phuong Nam, d noi lam 16, khu hd ky sinh trung nay cang phong phu them do nhidm hang loat cac dang mdi. Trudc tien la ngoai ky sinh trung, vi du: mo, mot sd loai ru6i hut mau.... rdi ddn ndi ky sinh trung d phuong Bac. Sau do chung bi chet trudc khi chim bay vd phuong Nam. Cudi cung la ndi ky sinh trung ma chim nhidm d phuong Nam se bi thai ra khoi co th£ chim trudc khi chim bay vd phuong Bac lam t6.
Trdn co sd cac du lieu neu tren va dua vao nguon gdc cua vat ky sinh ngudi ta chia khu hd ky sinh trung d chim di cu thanh 4 nhom:
1. Nhom ky sinh phan bd rong (thudng gap): tha'y d vat chu quanh nam, khong phan biet d phuong Nam hay phuong Bac.
2. Nhom phuong Nam: nhidm d noi tru dong. 3. Nhom phuong Bac: nhidm d noi lam to. 4. Nhom ky sinh nhidm trdn dudng di di cu.
Nhu vay, su bien dong cua khu he ky sinh trung d chim di cu ra't phuc tap do tac dong cua nhieu yeu td khac nhau: loai chim di cu, mua, dac tmh thurc an, chu ky sdng cua vat chu trung gian, phuong thurc sdng cua vat chu cudi cung v.v....
4. KHU HE K f SINH TRUNG PHU TH U 0C VAO DCfl SONG XA H 0 I CUA
VAT CHU
Thuc te' cho tha'y, khu he ky sinh trung phu thuoc rd ret vao ddi sdng xa hoi cua vat chu. Neu cac ca thd vat chu ft gap nhau thi khu he ky sinh trung d chung se ngheo hon, cudng do nhidm thap hon va kha nang truyen tu ca the vat chu nay sang ca thd vat chu khac se kern hon so vdi dong vat co ddi sdng xa hoi. Dieu do cung giai thfch tai sao cac dong vat phan bd d vung bidn cua vung phan bd ft co co hoi gap nhau hon la dong vat d ben trong vung phan bd, va ta't nhidn se dua den khu he ky sinh triing d chung ngheo nan hon.
Cung vdi ly do nhu vay ma su nhidm benh d dong vat sdng don le kho khan hon rat nhieu so vdi dong vat co ddi sdng xa hoi. Vf du dien hinh nhat la su phong phu ciia don bao Infusoria sdng d da day hoac ruot gia cua dong vat cd sung (trau, bd, dd, ciru....). Cd trdn 200 loai Infusoria: d trau, bd 70 loai, ciru 35 loai, dd 20 loai.... Rd rang la do sd luong ca thd nhidu va su tiep xuc thudng xuyen giua cac ca the vat chii vdi nhau ma ky sinh triing dd dang truyen tir cac thd vat chu nay sang ca the vat chii khac, va so luong chiing loai ky sinh triing cung trd nen phong phu hon.
Moi loai dong vat cd ddi song xa hoi, d chung ngudi ta cung tim thay khu he ddng vat don bao ky sinh rat phong phu (trdn 200 loai Infusoria). Nguoc lai, d kien cung cd ddi song xa hoi, tuy nhidn khu he ky sinh triing lai ngheo nan, cd le vi d chung cd axil focmic lam chet nhieu vat ky sinh, dac biet la don bao ky sinh.
Ddi sdng xa hdi cua loai ngudi cung lam cho loai ngudi trd thanh vat chu cua khu he ky sinh trung ra't phong phu va benh ky sinh trung dd lan truyen tir vung nay sang vung khac. Dong thdi do su tiep xuc cua ngudi vdi cac loai dong vat khac ma co nhung benh ky sinh trung lan truydn tir ngudi sang dong vat va tu dong vat sang ngudi. Nhieu loai ky sinh trimg hien nay tim tha'y d ngudi co nguon goc tir dong vat nhu san day Dipylidium caninum d cho; Echinococcus a Ion, ngua, bo la do ngudi ngau nhien nuot phai trung san day. Hoac Ascaris suum d Ion chuyen thanh A. lumbricoides d ngudi, hoac
Trichinella spiralis a Ion chuydn sang ngudi do ngudi an thit xong khoi hoac thit nau chua chin ky; Opisthorchis felineus d meo chuyen sang ngudi (do ngudi an ca sdng). Co trudng hop vat ky sinh thay doi vat chii ma khong thay doi ch6 d. Mot sd trudng hop khac co thay doi vat chii va ca ch6 d. VI du, giun dua Ion Ascaris suum tir Ioti lay nhidm cho ngudi van ky sinh d ruot non ngudi; song mot sd loai don bao sdng d ruot gia ngua khi lay nhidm sang bo lai sdng d da day bo.
5. KHU HE KY SINH TRUNG PHU T H U0C VAO VUNG DIA L t
Su phan bd giun san d gia sue, gia cam chiu anh hudng cua yeu td vung.
Ky sinh triing noi chung phan bd rat rong. Theo thong ke, gidi dong vat gom 20 ldp thi ldp nao cung cd ky sinh triing. Rieng giun, san da phat hien hon 3.00 loai
Trematoda, hon 1.500 loai Cestoda, hon 3.000 loai Nematoda, hon 400 loai
Acanthocephala.
Nude Viet Nam d day dat ven ddng cua ban dao Ddng Duong, la tidn don tan ciing phia Ddng Nam cua luc dia Au - A rong ldn, phan bo lot han trong mien nhiet ddi. Lanh dia Viet Nam dai hon 1.600 km (keo dai tir Bac xuong Nam), tir 23,22° v! Bac (chd giap vdi nude Cong hoa nhan dan Trung Hoa) den 8,30° (mui Ca Mau).
Su phan bo cua ky sinh triing phu thuoc vao cac dieu kien khi hau, nhat la nhiet do va am do. Su phat trien ciia ky sinh triing va cac ky chii trung gian ciia chung (nhuydn thd, ca, cdn trimg) doi hoi nhung didu kien nhiet do va am do nhat dinh. O ciing mot khu vuc khi hau thi do nhiet phu thuoc mot phan vao do cao. Su phat trien ciia ky sinh triing cdn phu thuoc vao khu he dong vat va thuc vat dia phuong, su di chuydn ciia cac loai dong vat va tap quan sinh hoat ciia con ngudi. Vi vay, cd loai ky sinh triing tha'y quanh nam, cd loai chi xua't hien vao miia nhat dinh.
La'y su phan bo ciia giun san theo vung dia ly lam vi du. Giun san chia thanh hai nhom: mot nhom goi la giun san sinh hoc (biohelminth), phai qua ky chii trung gian mdi phat tridn duoc, nhom nav phan bo khdng rong, cd nguon dich tu nhidn nhat dinh ciia chung; mot nhom goi la giun san dia hoc (geohelminth), nhom nay khdng can ky chii trung gian va phan bo kha rdng. Do dd, mot so loai giun san cd the gap d khap noi trdn the gidi, trong khi mot sd loai chi gidi han d nhung vung nhat dinh.
Mdt dieu ro rang la, giun san ph6 bien va co nhi^u chung loai hon d xur nong, d vung £m tha'p, lay loi va vao mua nong im . Didu kien nong, Im la nhung dieu kien can thi£t cho su phat trien cua giun san, cua trumg va a'u trung giun san. Triing giun san chi phat tridn thuan loi d nhiet do 15 - 30°C. Dieu kien thi£u khdng khi ( 0 2) va nhiet do than thd cua dong vat thudng cao (tren 37°C) lam trung giun san khdng the phat tridn thanh phdi thai trong dudng tieu hoa ky chu. Co nhung loai trutng chi nd dupe khi roi vao mdi trudng nude (trimg san la); cd loai chi thfch hop vdi dd 3m trung binh, chd cd reu, bim, phan sue vat.
Ddi vdi cac ky chu trung gian va cac nguon dich tu nhien cua giun san sinh hoc, hoc thuyet ve nguon dich tu nhien cua Pavlovski E. N. yeu cdu phai nghien curu ve dong vat cd xuong sdng da sinh, cdn triing ky sinh va nhung ddng vat khdng xuong sdng khac co thd lam mdi gidi truydn benh, cac da thu, chim trdi va cac loai gam nha'm thudng cd thd la ky chu mang triing cua nhieu loai giun san nhu: Fasciola, Dicrocoelium, Echinococcus, Taeniahyrnchus, Trichostrongylidae, Dictyocaulus, Prosthogonimus, Raillietina, Echinostoma....
Su phan bd theo vung dia ly cua ky sinh trung rong hay hep phu thudc vao su cung tdn tai trong mdt khu vuc hai hay ba sinh vat khac nhau, c ln thiet cho chu ky sinh hoc cua ky sinh trimg gay benh. Cd thd la'y vf du ve mot sd benh do san la gay ra. Benh san la d gia sue nhai lai (trau, bd, de, ciru) do Fasciola hepatica tha'y ph6 bien d khap noi tren the' gidi, bdi vi oc nude ngot - ky chu trung gian cua san la nay phan bd rdng khap the' gidi. Trai lai, benh san la do Fasciola gigantica tha'y pho bien hon d cac nude phfa Nam Chau A. Theo Trinh Van Thinh (1978), trong mot khu vuc tuong ddi rong la vien ddng, san la Fasciolopsis buski gay benh san la ruot d Ion va ngudi. Trudng hop phan bd dia ly hep hon nua la san la d voi Fasciola jacksoni hau nhu chi gap d Ddng Nam A. Tuy nhien, do viec tang cudng van chuydn sue vat ma tinh trang phan bo hep theo viing dia ly cua mdt so loai ky sinh trung se khdng tdn tai nua. Nhu vay, nhung dieu kien sinh thai cua cac viing tu nhien va cac khu vuc chan nuoi da quyet dinh tinh trang phan bd theo viing dia ly cua khu he ky sinh triing dac trung cho timg viing.
7 ,
O nude ta, ty le nhidm ky sinh triing d gia sue, gia c^m kha cao. Ngoai canh bi o nhidm nang, nhat la d nhung vung cd benh, thudng xuyen hang nam benh phat timg dot (nhu vung nghe ia cut trang do giun dua d midn nui, benh san la gan trau, bd d viing ruong nude, benh giun dua va san la ruot d mot so trai chan nuoi Ion tap trung, benh gao Ion d vung nui nuoi Ion tha rdng.... ). 6 nhung viing nay, m£m benh ky sinh triing dupe phat hien vdi ty le cao d bai chan, nen chuong va xung quang chudng nuoi, ao va vudn trong cay thuc an cho gia sue.
Su phan bo ky sinh trung va benh ky sinh triing cd lien quan mat thiet vdi su phan bo cac dong vat, thuc vat la ky chii trung gian hoac vat mdi gidi truyen benh (oc doi vdi
cac loai Trematoda, giun dat ddi vdi giun phoi Metastrongylus.... ). Co the nhan xet chung la, khu he san la (Trematoda) d vung dong bang phong phu hon mien nui; giun than va giun phoi Ion tha'y nhieu d vung nui. Tuy nhien, phai chu y la tinh hinh nhiem nang hay nhe ddi vdi mot loai giun san co quan he vdi dieu kien dia hinh, khu he sinh vat va tap quan, ky thuat canh tac, phuong thiic chan nuoi cua tung vung (vung trong lua hay trong mau, dieu kien nuoi tap trung hay phan tan, nuoi nhot hay tha rong, dieu kien ve sinh chuong trai, bien phap xu ly phan va chat thai trong chan nuoi, thuc an chin hay sdng, thieu hay du v.v....