1. An attenuated Pasteurella multocida (P. multocida) bacterium wherein the P. multocida bacterium is attenuated by comprising a mutated hyaC gene that encodes a non-functional hyaC polypeptide having the amino acid sequence of SEQ ID NO:11 or encodes a polypeptide having an amino acid sequence at least 80% identical to the amino acid sequence of SEQ ID NO: 11 over the length of SEQ ID NO:11 that is in-common with said polypeptide; or comprising a mutated nanP gene that encodes a polypeptide unable to take-up sialic acid from growth media having the amino acid sequence of SEQ ID NO:4, or a polypeptide having an amino acid sequence at least 80% identical to the amino acid sequence of SEQ ID NO:4 over the length of SEQ ID NO:4 that is in-common with said polypeptide.
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