Denggi
Hari ini kita dihantui dengan masalah Denggi yang meruncing, ketika saat kritikal barulah pengumuman dibuat tanpa pencegahan yang lebih baik terlebih dahulu. Sedar-sedar sahaja, angka kematian sudah melepasi dua orang berbanding tempoh yang sama tahun lepas. Kita melihat bagaimana perkembangan nyamuk juga semakin proaktif dan kebal terhadap racun semburan asap, sehingga pengarah farmasi di Pusat Racun Negara mengumumkan racun biologi larvicide bti juga dikenali sebagai Bacillus thuringiensis israelensis (BTI). Nyamuk juga berevolusi untuk sehingga membentuk antibodi yang kuat , dan menjadi lali dengan racun konvensional yang bertindak secara kimia. Setakat artikel ini ditulis sebanyak 60 kes kematian akibat denggi.
BTI adalah racun biologi berupa spora yang disemburkan dan menunjukkan tindak balas tentu kepada nyamuk sahaja. Haiwan-haiwan lain tidak terjejas dengannya. Menarik juga? Sebenarnya bacillus bermakna struktur virus (pathogen) yang melawan nyamuk berbentuk rod, dan berpintal-pintal. Apabila nyamuk hinggap di tempat yang disemburkan BTI, spora itu akan melekat pada kakinya dan meresap masuk sebagai pathogen di dalam badan nyamuk. Hal ini akan menyebabkan berlaku mutasi kepada sel-sel nyamuk, yang akan memusnahkan larva dan nyamuk itu sendiri. Ia adalah peperangan biologi di antara nyamuk dan BTI. Betapa hebatnya racun biologi ini, siapa tahu, pada masa hadapan nyamuk yang mutan akan menjadi lebih ganas berbanding ketika ini? Siapa tahu?
Biological control with B.t.i.
While numerous biological control agents can kill mosquitoes, only one biological agent, Bacillus thuringiensis subsp. israelensis (B.t.i.), has seen widespread operational use for the control of mosquito larvae. B.t.i., a spore forming Bacili, was discovered in ‘a stagnant pond located in the Nahal Besor Desert river basin near Kibbutz Zeelim in the north-western Negev Desert of Israel’ (Margalit, 1990) in 1976 by Goldberg and Margalit (1977). de Barjac (in Nugud and White, 1982) identified the new bacterial strain and designated it B. thuringiensis subsp. israelensis serotype H14. Many studies, such as those by Lacey and Lacey (1981), Nugud and White (1982), Lacey and Inman (1985), Kramer et al. (1988), Tietze et al. (1994), and Chui et al. (1995), have evaluated and confirmed the larvicidal capacity of B.t.i. for many mosquito species in different parts of the world.
Thomas and Ellar (1983) describe the mode of action of B.t.i. endotoxins. B. thuringiensis ‘produces a proteinaceous parasporal crystalline inclusion during sporulation.’ When this is ingested by an insect, it solubilizes in the midgut and releases proteins called d-endotoxins. These proteins disrupt internal membranes and result in death. In addition to B.t.i.’s effectiveness against arthropods, the World Health Organization (1995) reports an excellent safety record and a very low mammalian toxicity: ‘LD50 values for both oral and dermal toxicity are more than 30,000mg/kg’.
Most studies on Bacillus thuringiensis resistance development have been done with Lepidopteron populations and considerably fewer have been done on mosquitoes (Tabashnick, 1994). Such studies were undertaken because of the fear that resistance to B.t.i. would develop quickly as it had to other insecticides. In an effort to examine the possibility of resistance occurring and the speed at which it would occur in mosquito populations, Goldman et al. (1986) attempted to artificially select wild and laboratory populations of Ae. aegypti for resistance to B.t.i.. After 14 generations, one wild strain from Rio de Janeiro, Brazil had developed a slight, but significant twofold increase in resistance. A study by Becker and Ludwig (1993) on B.t.i. resistance in Ae. vexans (Meigen) field populations that had sustained 10 years of B.t.i. control, revealed change in susceptibility. The future for B.t.i. is promising since the development of resistance to B.t.i. in mosquitoes is by no means comparable to the speedy and encompassing growth of resistance to organochorine, organophosphate, carbamate and synthetic pyrethroid insecticides.
BTI adalah racun biologi berupa spora yang disemburkan dan menunjukkan tindak balas tentu kepada nyamuk sahaja. Haiwan-haiwan lain tidak terjejas dengannya. Menarik juga? Sebenarnya bacillus bermakna struktur virus (pathogen) yang melawan nyamuk berbentuk rod, dan berpintal-pintal. Apabila nyamuk hinggap di tempat yang disemburkan BTI, spora itu akan melekat pada kakinya dan meresap masuk sebagai pathogen di dalam badan nyamuk. Hal ini akan menyebabkan berlaku mutasi kepada sel-sel nyamuk, yang akan memusnahkan larva dan nyamuk itu sendiri. Ia adalah peperangan biologi di antara nyamuk dan BTI. Betapa hebatnya racun biologi ini, siapa tahu, pada masa hadapan nyamuk yang mutan akan menjadi lebih ganas berbanding ketika ini? Siapa tahu?
Biological control with B.t.i.
While numerous biological control agents can kill mosquitoes, only one biological agent, Bacillus thuringiensis subsp. israelensis (B.t.i.), has seen widespread operational use for the control of mosquito larvae. B.t.i., a spore forming Bacili, was discovered in ‘a stagnant pond located in the Nahal Besor Desert river basin near Kibbutz Zeelim in the north-western Negev Desert of Israel’ (Margalit, 1990) in 1976 by Goldberg and Margalit (1977). de Barjac (in Nugud and White, 1982) identified the new bacterial strain and designated it B. thuringiensis subsp. israelensis serotype H14. Many studies, such as those by Lacey and Lacey (1981), Nugud and White (1982), Lacey and Inman (1985), Kramer et al. (1988), Tietze et al. (1994), and Chui et al. (1995), have evaluated and confirmed the larvicidal capacity of B.t.i. for many mosquito species in different parts of the world.
Thomas and Ellar (1983) describe the mode of action of B.t.i. endotoxins. B. thuringiensis ‘produces a proteinaceous parasporal crystalline inclusion during sporulation.’ When this is ingested by an insect, it solubilizes in the midgut and releases proteins called d-endotoxins. These proteins disrupt internal membranes and result in death. In addition to B.t.i.’s effectiveness against arthropods, the World Health Organization (1995) reports an excellent safety record and a very low mammalian toxicity: ‘LD50 values for both oral and dermal toxicity are more than 30,000mg/kg’.
Most studies on Bacillus thuringiensis resistance development have been done with Lepidopteron populations and considerably fewer have been done on mosquitoes (Tabashnick, 1994). Such studies were undertaken because of the fear that resistance to B.t.i. would develop quickly as it had to other insecticides. In an effort to examine the possibility of resistance occurring and the speed at which it would occur in mosquito populations, Goldman et al. (1986) attempted to artificially select wild and laboratory populations of Ae. aegypti for resistance to B.t.i.. After 14 generations, one wild strain from Rio de Janeiro, Brazil had developed a slight, but significant twofold increase in resistance. A study by Becker and Ludwig (1993) on B.t.i. resistance in Ae. vexans (Meigen) field populations that had sustained 10 years of B.t.i. control, revealed change in susceptibility. The future for B.t.i. is promising since the development of resistance to B.t.i. in mosquitoes is by no means comparable to the speedy and encompassing growth of resistance to organochorine, organophosphate, carbamate and synthetic pyrethroid insecticides.
posted by Naim @ 2:28 PTG
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