The present invention pertains to the use of Haemophilus parasuis bacteria of serotype 5 expressing an iron-restriction protein visible on a Western-blot when reacted with serum of a convalescent animal which has recovered from an infection by Haemophilus parasuis bacteria of serotype 4, the said protein not being visible on a Western-blot when Haemophilus parasuis bacteria of serotype 5 grown under iron-replete conditions, are reacted under the same conditions, in a vaccine to protect a subject animal against a disorder arising from Haemophilus parasuis serotype 4 bacteria.
