E. S.enteritidis

S.enteritidis infection in both broiler parents and layer chickens has been a significant issue in recent years. In both classes of chicken there are public health implications, in addition infected broiler parents can produce progeny with clinical salmonellosis. Bacteriological screening of parent flocks is required in the UK under the Breeding Flocks and Hatcheries Order. In addition a substantial amount of serological screening is carried out. All assay results summarized here are based on single tests of individual sera using a commercially available Elisa assay (Guildhay). Generally between 20 and 60 samples were examined per house or air-space. \par \par To demonstrate the effect of field challenge on S.e. serology we have extracted the data for two contrasting companies. Company A had a history of repeated outbreaks of S.e. infections in broiler progeny while company B had no isolates in routine hatchery monitoring and no evidence of a problem in the progeny. Sera were routinely collected at approximately 6 weeks intervals during production, alternating between 60 and 20 samples per air-space. The data for a period of approximately 18 months have been extracted. Data from S.e. vaccinated flocks were excluded. A total of 22120 assays representing 652 house/flock groups were included, with 10356 sera (317 groups) from company A, and 11764 (335 groups) from company B. We normally report the results of the Elisa testing by indicating the number of sera in each group in each of 13 Serum-to-Positive Ratio Bands. Band 0 sera are negative, 1 suspect, and bands 2 through 12 are associated with increasingly intense reactions. Table 5 shows the total % of samples received from each company in each non-negative reporting band. This clearly indicates that low grade reactions in reporting bands 1 to 6 are poorly predictive of a history of, or an ongoing, S.e. problem. Many of these reactions became less intense when the same samples were repeat-tested. Generally if only low-grade reactions were present their intensity is reduced when the flock are re-sampled after 2 weeks. Reactions in bands higher than 6 were, on the other hand, very rare in company B, and were relatively common in company A, especially band 12. The great majority of samples in these bands occurred in a few flocks which underwent a distinct sero-conversion with a high percentage of birds becoming sero-positive.

Our standard serology reports include a mean value for each group of sera which is the simple arithmetic mean of the band values for each of the sera. Figures 3.10 and 3.11 below show the contrasting distribution of means for the two companies. Mean values above 2 occurred only in company A. These high mean values usually reflect a broad range of reactions with a substantial number of samples falling in the lower bands.}

Turning to the response to vaccination, the mean S/P band for 156 groups of vaccinated birds is plotted in Figure 3.12. At the time of writing only a single aluminium-hydroxide adjuvanted inactivated vaccine is available in the U.K. The vaccine is applied as 2 doses, at 10-12 weeks and then 15-18 weeks of age. It must be kept in mind that about 30% of the groups shown in the figure refer to individual pens of vaccinated birds in controlled trials and these included only a small number of samples per group (generally 4-5 sera). The great majority of the groups of sera from vaccinated birds had a mean band in excess of 2. Lower values were more common at around 14 weeks of age. This may be due to too short an interval since the first application of vaccine. The vaccinal responses seen at 22 weeks includes a large number of birds vaccinated under normal commercial conditions. The broad range of response seen must be a cause for concern even if we do not know that high levels of circulating antibody are essential for protection. Two regression lines have been added to the chart. One covers the period up to 23 weeks and represents the mean results during the period of response to vaccination. The second line is based on all data from 21 weeks to 55 weeks of age. This is intended to estimate the depletion of mean antibody response with age.}

It seems likely that the birds which have low levels of antibody or no detectable antibody response to vaccination will show sero-conversion when field challenge occurs with a virulent strain of S.e.. This remains to be investigated.