Standard requirements for vaccines against infectious bursal disease.

A variety of criteria must be considered when examining the suitability of preparations of live infectious bursal disease (IBD) virus for use as vaccines: identity and purity of the vaccine strain, lack of spread, or of tendency to revert to virulence, lack of immunosuppressive effect, safety and potency. Arguments as to th identity of IBD virus are well documented, and after ensuring that the virus strain is in fact IBD virus, it must be ascertained that it is free from contaminating agents. If the vaccine strain spreads, lack of reversion to virulence must be determined by several direct chick to chick passages: histological observations of bursae from first and final passages must be made to ensure that no change has occurred. In order to formulate proposals for standard tests of safety, lack of immunosuppressive effect and potency for IBD vaccines, comparative studies were made of products of different origins. Safety of the vaccines was assessed by observing young chicks after administration and measuring loss in weight gain. The effect on the bursa was judged by weighing and making microscopic observations at intervals. The immunosuppressive effect of the vaccines was assessed by comparing the serological and protective responses to subsequent Newcastle disease (ND) vaccination with the response in chicks given no IBD vaccine. Potency of the vaccines was determined by challenge of vaccinated chicks with virulent IBD virus. Various properties were found in the products examined. Some were concluded to be insufficiently attenuated because of the effect on the bird or upon its bursa and the consequent effect on the bird's immune competence. The ability to confer protection was not correlated with the degree of damage caused in the bursa. Vaccines which had no adverse effect on the health of the bird, caused little or no damage to the bursa and conferred adequate protection against challenge were considered to be of a satisfactory standard.