A. Infectious Bursal or Gumboro Disease

Most broiler parents will receive a single dose of live vaccine at about 6 weeks of age, the response to which will be checked at around 10 weeks of age. Clinical Gumboro disease is currently rare in this class of chicken. In order to provide high levels of maternal immunity in the progeny the chickens are vaccinated with an injectable oil-adjuvant vaccine, usually at 17-18 weeks of age.

Figure 3.1 shows the scatter graph of the mean response of 672 groups of sera (321 in rear, 351 in lay, 10860 individual sera). Between 10 and 18 weeks we would expect the response from the live vaccine to be falling off. The regression line for this period is quite flat suggesting that some field challenge may also be helping to boost immunity. The great majority of the data for birds in lay are generated by samples collected at 21-23 weeks (to monitor response to the oil vaccine) nevertheless the regression line suggests a gradual depletion of antibodies very similar to that described when oil-adjuvant IBD vaccines were first developed (Wyeth and Cullen, 1979). Note the wide range of response obtained both in rear and in lay.