Serology

Twenty sera which had been randomly-collected in each of the three houses at 22 weeks of age and again at 34 weeks of age, were tested using a commercially available Elisa kit for antibodies to Pasteurella multocida (IDEXX). The results, expressed as both a serum to positive ratio for each sample, and classified as to "Titre Group" are shown in Table 3. Not a single positive sample was encountered in the birds at 22 weeks, in fact most of the samples reacted more weakly than the negative control sera provided in the kit (and, hence had negative S/P ratios). In contrast the samples at 34 weeks were much more reactive. Even though only 15 of the 60 samples examined came within the recognized positive range for the test (and these had vastly differing degrees of reaction), nearly all of the samples had positive S/P ratios and the mean S/P values increased by between 0.32 and 0.54 depending on house. These results help provide further confirmation of the diagnosis. Unfortunately the samples taken at 23 weeks were no longer available and no samples were taken between then and 34 weeks. We might well have encountered positive reactions earlier if these samples had been available. However the total lack of reaction at 22 weeks of age (when birds were already dying) indicates that lack of positive sera must be interpreted with care, especially early in an outbreak.

Table 3. Summary of results of an Elisa test for Pasteurella multocida, carried out on 20 randomly selected serum samples collected at 22 and 34 weeks of age.