Studies on potential fowl cholera vaccine

The avian disease fowl cholera has consistently plagued the poultry industry for many years and has led to excessive monetary losses. Although better management of layer birds and use of inactivated and live vaccines have been in place, fowl cholera remains an inadequately controlled problem. In fact, the use of some of the current vaccines has resulted in fowl cholera outbreaks within the flock. Thus, the poultry industry has a need for a safe, rationally attenuated vaccine against fowl cholera. Therefore a study was initiated with the main objective, in the short-term, to further the development of a rationally attenuated live vaccine against fowl cholera using the P. multocida X-73 mutant in which the genes pnhA and pnhB have been inactivated.
Preliminary studies suggest that X-73 mutant was attenuated for virulence. The recombinant pnhA protein from P. multocida (strain X-73) was successfully isolated and purified. Biochemical and enzymatic studies of the purified pnhA confirmed that this protein was a Nudix hydrolase, more specifically, classified as a dinucleoside oligophosphate pyrophosphatase.
pnhA has properties very similar to other dinucleoside oligophosphate pyrophosphatase, and the preferred substrate target for the enzyme is diadenosine pentaphosphate.
Further studies of the complemented X-73 mutant using the chicken embryo-lethatlity assay showed an incomplete restoration of virulence. The complemented X-73 mutant contains a functional pnhA gene, but an inactivated pnhB gene. From our studies it was assumed that a functional pnhB gene was necessary to restore virulence within the mutant and that the function of pnhA in pathogenesis requires a functional pnhB protein.
Due to unforeseen problems with the stability of the X-73 mutant, bird trials and thus vaccine assessment was not performed in this study.
The pnhA protein is the first Nudix hydrolase identified within the Pasteurellaceae family. Nudix hydrolases have been shown to play a role in the pathogenesis of other bacterial pathogens. The X-73 mutant has potential as a trial vaccine, which may control fowl cholera, but further work beyond the extent of this project is needed to further develop the X-73 mutant.
Vaccine development for this disease is important because the disease has been controlled by the use of antibiotics which are costly and P. multocida has the potential to become resistant to the antibiotics in use.

Source: Carmel Ruffolo, Ph.D., Department of Biological Science, University of Wisconsin-Parkside, Kenosha, WI, USA.