Immunity assays on sheep sera using passive mouse protection tests showed that vaccines containing more than 4 strains of Pasteurella multocida did not give a good immunity. The immune response was not enhanced by the use of an oil adjuvant, and high concentrations of bacteria had only a partial positive effect.
Attempts to extract selectively the protection-inducing antigen(s) from P. multocida by veronal, phenol or potassium thiocyanate extraction were unsuccessful. Furthermore, it was found that sheep antisera to the recognized type strains of P. multocida afforded only limited protection against a number of field strains. We concluded from this that successful immunization against ovine pasteurellosis will depend on either the identification of a strain of P. multocida that gives a wide spectrum of immunity or the discovery of a live mutant suitable for vaccine production and the definition of cultural conditions that promote the expression of a common immunizing antigen.
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