- Giai doan thur ba tir 35 ngay 63 hoac 70 ngay sau khi nhiem 6 giai doan nay, ca'u true va churc nang ciia cac tuyen bi mit di din dan.
E necatrix co sire gay ben h^ mivati co sCfc gay benh
2. DAC DIEM DICH t£ HOC BENH CAU TRUNG GA
2.1. Cac yeu td anh hudng tdi ty le nhi£m cau trung d ga
Co nhieu yeu td lien quan tdi benh cau trung d ga: thdi tiet khf hau, dieu kien chudng trai, cong tac quan ly va cham soc nudi dudng.... Cac yeu td tren deu cd anh hudng true tiep hoac gian tiep den tinh hinh nhidm va su lay lan benh.
- Didu kien chudng trai chan nudi ga
Chudng trai chan nudi la yeu td quan trong lien quan den dich td benh cau trung ga. Nudi ga trong long va nudi tren nen chudng co ty le nhidm cau trung khac nhau.
Hoang Thach (1996, 1997, 1998) da khao sat ty le nhidm cdu trung, thay: ty le nhidm cau trung d ga nuoi long la 0,37%, ga nudi trong chudng co dem lot la trau nhidm 22,49 - 57,38%. Nhu vay, ga nuoi trong long khdng tiep xuc vdi phan thi ty le nhidm cau trung giam rat thap.
- Tuoi ga cung la yeu td can chu y trong dac diem dich td cua benh.
Dao Huu Thanh va cs (1978) da nhan xet, benh cau trung ga cd ti'nh lay lan manh, dac biet d ga dudi 2 thang tuoi, dugc coi nhu mot benh truyen nhidm cua ga con 10 - 49 ngay tuoi. Theo Ho Thi Thuan (1985), ga nudi cdng nghiep d mot sd tinh phi'a Nam nhidm cdu trung chii yeu d giai doan 3 - 6 tuan tuoi.
Luong Td Thu va cs (1993), Le Van Nam (1995) cho bie't, ga nhidm cau trung nang nhat d giai doan 20 - 56 ngay tuoi, neu khong dugc didu tri kip thdi cd thd chet tdi
100%.
- Dieu kien thdi tiet am va am udt thuan lgi cho Oocyst cau trung phat tridn d ngoai canh, lam cho benh cau trung de lay lan.
Theo Pham Van Khue va cs (1996), Nguydn Thi Kim Lan va cs (1999), Ducmg Cong Thuan (2003), mdi trudng am udt va nhiet do on hoa la nhung dieu kien rat thuan lgi cho su phat trien cua cau trung. Vi vay, mua xuan va mua he ga bi nhidm cau triing nhidu va nang hon cac miia khac trong nam, viec phong benh cau triing cho ga d miia xuan va miia he cung can chu y hon.
- Nguon benh la nhung ga dm hoac khoi nhung van mang cau triing, hoac nhung ga ldn mang cau triing nhung khdng phat benh. O o cyst hang ngay dugc nhirng ga nay thai ra theo phan, phat tan tren nen chuong, dem lot, lan vao thuc an, nude udng, ga dd nuot vao va bi benh.
Le Van Nam (1995), Pham Van Khue va cs (1996), Nguydn Thi Kim Lan va cs
(1999) deu thong nhat rang, ga bi benh cau triing la nguon phat tan O o cyst cau triing. Ngoai ra, nhirng ga mang cau triing nhung khdng the’ hien trieu chung lam sang la nguon mang can benh nguy hiem, vi chung la ddi tugng ma ngudi chan nudi l't chu y (do khdng the hien trieu chung lam sang).
Mot so dong vat song trong chu6ng nudi ga hofic xung quanh chu6ng nudi c6 kha nang mang O ocyst ciu trung ga, nhu: ruoi, gian, kiefn, chudt. Chung mang O ocyst ciu trung d chan, tren long, da, canh...., trong khi di chuydn se truyen O ocyst c£u triing vao thurc an, nude uong cua ga, lam cho ga nhiem ciu trung.
Theo Pham Van Khue va cs (1996), khi O ocyst bi ru6i nu6t vao, trong dudng tieu hoa cua ruoi, chung van sdng va con kha nang g&y benh trong vong 24 gid.
Dung cu chan nudi cung la cac yeu td mang O ocyst c3u trung, gop phin gSy nhidm c£u trung cho ga.
Hoang Thach (1999) da khao sat cac mau thu thap tir dung cu chan nudi, ket qua tha'y, co 11,20% sd mau phat hien cd O ocyst, trong do ung bao hd cua cdng nhan chan nudi nhiem 5,60%, nhieu dung cu khac sir dung d£ chan nudi ga cung cd kha nang mang va truyen O ocyst ciu trung tir ga benh sang ga khoe.
2.2. A n h h u d n g cu a cac tac nh&n v a t ly , hoa hoc den su p h a t tr ie n Oocyst or ngoai
canh
2.2.1. A nh huong cua cdc tac nhdn vat ly
Nhiet do, am do va mdi trudng ndi chung deu tac dong vao O ocyst. Di£u nay cd y nghla trong nghien curu dich t i hoc va phuong phap phdng chdng benh cdu triing cho ga.
Theo Wamar (1937), O ocyst bam tr6n vo trung se chet khi a'p triing d 38 - 40°C, am do 40 - 70%. Ellis (1938) cho rang, nhiet do tdi uu cho su phat tri£n O ocyst c<iu triing E. tenella la 26,6 - 32,2°C. 6 nhiet do 48°C trong 15 phut, ddc luc cua O ocyst giam rd ret.
Theo Glullough (1952), O ocyst bi diet d 40°C sau 96 gid, 45°C sau 3 gid va 50°C sau 30 phut. 6 nhiet do 12 - 20°C, O ocyst co sure gay benh ton tai duoc 14 ngay, nhung
O ocyst chua cd sure gay benh chi tdn tai trong 56 gid.
Long P. L. (1952) cho rang, O ocyst cua loai cau triing E. tenella co the sdng qua mua ddng lanh gia, nhung khdng chiu duoc dieu kien nhiet do cao va anh ndng chieu true tiep, song, co dai da che phu va bao ve O ocyst. Theo Smith (1963), cau triing ga co the ton tai den 14 tuan d san nudi ngoai trdi.
Kay M. W. (1976) cho biet, O ocyst loai E. tenella va E. m axima khdng chiu tac dong ciia qua trinh len men chat don chudng.
2.2.2. A n h huong cua tac nhdn hod hoc
O ocyst cau triing ga cd sire de khang vdi mot sd hoa chat khir triing, tay ue chuong trai. Day la va'n de can chu y khi nghien cuu ve dich t6 hoc cua benh cau triing ga. Penard (1925) va Iakimoff (1927) cho biet, O ocyst loai E. tenella cd sue de khang kha tot vdi dung dich mudi, axit, baza d nong do tuang ddi cao, cdn dung dich formol, H2S 0 4, NH4OH, crezol gay anh hudng den su hinh thanh bao tir. Tuy nhien, mot sd hoa
chat co tac dung diet Oocyst nen da duoc su dung tieu doc chuong trai, vf du nhu dung dich amoniac (NH3) 10% (Smith, 1940), methyl bromid (Andrew va Taylor, 1943).
Williams (1997) da nghien ciru tac dung cua dung dich NH3 10% toi Oocyst va cho biet, sau 12 gid, 100% Oocyst khong sinh bao tu, nen co the dung dd tieu doc chuong nuoi ga.
3. B$NH L t VA LAM SANG BENH CAU TRUNG GA
Didn bien cua benh gan lidn vdi qua trinh sinh san noi sinh cua cau trung. Williams va cs (1996) da theo ddi tac dong gay benh cua cau trung, tha'y rang: sau khi ga nhidm
E. tenella 3 ngay, niem mac manh trang da phu ne, xung huyet. Tac gia cho biet, ngay
thur nha't sau khi nhidm, trong ruot chura nhieu bao tu con (Sporozoit) duoc giai phong ra
tir Oocyst gay benh. Cac Sporozoit xam nhap te bao bieu mo ruot, lap tuc sinh san vo
tfnh nhieu dot, tao ra cac Schizont the he I, tu do tao ra vo so' Merozoit. Ngay thur 2, cac
Merozoit lai tiep tuc xam nhap te bao bieu mo ruot lanh, lai sinh san vo tfnh tao ra cac
Schizont the he II, tir do tao ra hang loat cac Merozoit the he tiep theo. Sau vai ngay, cac
Merozoit the he cuoi cung cua qua trinh sinh san vo tfnh se phat trien trong te bao bieu
mo thanh cac Schizont va biet hoa thanh cac tieu phoi tir (Microgamete) va cac dai phoi tir (Macrogamete). Tieu phdi tu va dai phdi tu giai phong ra khoi te bao bieu mo se ket hop vdi nhau thanh hop tu.
Qua trinh sinh san vd tfnh va huu tmh trong te bao bieu md ruot thoat dau gay hien lugng xung huyet, sau do la hoai tu va xuat huyet niem mac ruot. Su pha vd hang loat te bao niem mac ruot gay xuat huye't lan tran, te bao bieu mo bong troc. 6 cac giai doan phat trien tiep theo, cau trung xam nhap sau vao vach ruot gay hoai tur, xuat huyet ca ldp te bao ha niem mac va tuyen ruot. Ngay thu 6 bat dau thay xuat hien Oocyst trong phan (Larry, 1991;Williams, 1991).
Trieu chung benh cau trung noi chung ddu the: hien hau qua ciia qua trinh pha huy niem mac dudng tieu hoa cua cau trung. Theo Trinh Van Thinh (1963), Pham Van Khue va cs (1996), Nguydn Thi Kim Lan va cs (1999), Duong Cong Thuan (2003), ga bi benh cau trung thudng ld dd, cham chap, an kem hoac bo an, long xu, canh xa, phan loang lan mau tuoi, khat nude, udng nhieu nude, dd chet do mat nude va mat mau.
S '
Hinh 120. Gc, b, bail, cdn irimg co b,e\, /2 7 . Phan gd co man
Mo kham ga benh, tha'y benh tich ro nha't la hidn tuong xuat huydt va hoai tu nidm mac ruot. Neu bi loai E. tenella ky sinh, manh trang ga sung ra't to, nhin bdn ngoai tha'y mau nau den hoac den, cat ra tha'y trong long manh trang chura day mau tuoi lin trong chat chura mau den, mau co the d6ng thanh nhung cuc l6n nh6n. Hau m6n con vat udt, I6ng bd't, xung quanh co vong hau mon co nhung di&n xua't huydt. Hidn tumig phu nd the’ hien ro d cac co quan va mo bao.
Hinh 128. Xiiat huye't vd hoai tu d manh trang
H inh 129. Xua't huye't vd h o a i tit a ru o t
1 2
H inh 130. Benh tich v i the d ru o t ga b i benh can trim g