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Challenge is usually carried out 21 days after vaccination, either by injecting the tongue of the animal or by placing it in contact with other infected animals. In the needle challenge method, an animal is scored as protected if it does not develop any lesions on the feet. Mouth lesions are permitted, although high quality vaccines are even capable of preventing significant replication at the challenge site.
The potency of a vaccine may be described either in terms of the percentage of animals protected with a field dose or the effective dilution factor of vaccine which confers 50% protection (PD50 value, sometimes PB50). The OIE Manual of Recommended Diagnostic Techniques and Requirements for Biological Products indicates a minimum value of 3 PD50
Calculation of the PD50 value is made by a suitable statistical method such as probits or Spearman-Karber. While the Reed and Muench method has been used by some authorities, it is unsuitable for most data generated by potency tests because of its dependence on symmetrical data bounded by 0 and 100% responses.
No official test is prescribed for sheep, goats and pigs.
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