Development of an improved haemorrhagic septicaemia vaccine

• Veterinary Research Institute, Malaysia

Haemorrhagic septicaemia (H.S.) - an infectious disease of cattle and buffalo caused by a particular serotype of Pasteurella multocida (type 2 or 6:B) - has great economic significance to southern Asia. Its incidence varies from high in some countries to moderate in many and sporadic in a few.

Although the currently used vaccines, developed nearly 40 years ago, have been of great benefit, alarming epizootics of H.S. still regularly and insidiously destroy a sizeable proportion of the buffalo and cattle populations in enzootic areas, with massive epizootics occurring elsewhere. Three types of H.S. vaccines, all chemically inactivated, are used depending on the circumstances: plain bacterin; alum-precipitated; oil adjuvant. The first two are used quite widely because they are easy to inject, but confer immunity for only 4-6 weeks (plain bacterin) or 4-6 months (for the alum-precipitated type). The oil-adjuvant vaccines last longer (100% protection for up to 6 months, 50% by 9 months and none at all 12 months after vaccination) but their high viscosity makes them very hard to administer.

In addition, a proportion of cattle can die from post-vaccination shock unless treated immediately with antihistamines and adrenalin, and this problem is apparently associated with a structural component of P. multocida type 6:B. Also, breakdowns in immunity have been reported, even from areas where vaccination is regularly practised.

The major difficulty underlying these problems is poor understanding of the nature of the most potent immunising or protective antigens. The present project seeks to improve that understanding and apply it to the search for suitable newer-generation adjuvant formulations.

Scientists in Australia will isolate and characterise the most potent immunising or protective antigens, and manipulate capsular and LPS-associated polysaccharide antigens immunochemically to improve their vaccine potential. Development of enzyme-linked immunosorbent assays will allow them to estimate protective-antigen-specific antibody titres and isotypes and to quantify protective antigens in culture. The team will also evaluate different adjuvant formulations for their capacity to stimulate sustained antibody responses in dairy calves, design a suitable nutritional medium permitting maximal expression of the most potent protective antigens and then produce monoclonal antibodies to these.

In Malaysia, scientists will produce Malaysian strains of P. multocida type 6:B (grown in vivo and in culture) for use in the Australian experiments. After screening crude and purified antigen preparations in mice, using P. multocida type 6:B as the challenge organism, they will evaluate the protective efficacy of individual and combined most protective antigens - both in aqueous solution and with oil adjuvant - by active immunisation of buffalo calves and in passive mouse protection tests. They will determine the buffalo antibody isotype(s) responsible for protection against H.S., establish the correlation between that protection and the quantity of protective antigens in the vaccine and evaluate serological assays as indirect indicators of the immune status of the vaccinated buffalo. Finally, they will evaluate protection and persistence of antibody response to the combined most potent protective antigens vaccine in buffalo calves following presentation in the most promising adjuvant formulations.

It is hoped that this work will lead to the development of a highly efficacious haemorrhagic septicaemia vaccine, which would be of inestimable value to many developing countries, particularly in Southeast Asia. Moreover, the project may have potential spillover into the study of another important tropical disease - melioidosis - as the gram-negative organisms involved in the two diseases have similar lipo-polysaccharide coats. The newer biochemical and immunological techniques used offer a high probability of success.