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Three native bacterins against Pasteurella multodda (PM) were tested for potency in chicks and ducklings. The sensitivity in duckling challenged by live PM was the highest. followed by chick and then mouse. The 50% lethal dose (LO 50) of the group challenged with standard strain of PM were 10 -8- 2 4 (duckling), 10 - 7•42 (Chick) and 10 -6,7 5 (mouse); and that of the group challrnged with field strain of PM was 10- 8•6 4 (duckling). 10- 7•68 (chick) and 10 - 7•1 6 (mouse), respectively. Above result shows that the toxicity of field strain of PM is slightly higher than standard strain of PM.
After the trials of three commercial bacterins, it seemed that the mouse was the best sutiable among 3 experimental animals in its stability and cost for the potency test. The protective index of mice should be larger than 0.8 by using the protective index to substitute the current standard of inspection. The effetive storage time of above bacterins is less than 6 months. So. if we want to prolong the espir-ation time, the best is to improve the quality of the bacterin itself.
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