C. Infectious Bronchitis

Vaccination against Infectious Bronchitis is carried out for the same reasons as is vaccination against ND, using a very similar programme to that described for ND. In addition to the normal live-primer followed by oil-adjuvant vaccine some companies will continue to vaccinate using a live vaccine during production. The objective of this is to maintain local immunity in the respiratory tract at a high level. The interpretation of the serological response to IB is complicated by the multiplicity of serotypes of varying relatedness (Cook, 1984, Cook and Huggins, 1986, King 1988). The HI test is much less serotype-specific than is serum-neutralization so this means HI is less precise for the characterization of new isolates (Cook and others, 1987). The implication of this is that even when we identify a marked increase in antibody using HI with a given IB serotype it cannot be assumed that the field challenge virus was of the same or a closely related serotype. An added complication is what has been termed "original antigenic sin" (Gelb and Killian 1987). When birds are immunized with one serotype and subsequently challenged with another serotype the antibody response tends to be much higher to the antigen used in the initial vaccination. Most vaccines currently used in the UK are standard Massachusetts strains. On this basis we can expect to detect sero-conversion using the M41 antigen when birds suffer challenge from a broad range of serotypes. The serological data are summarized as geometric mean titre for each of 949 house flocks (311 in rear, 638 in lay, 12498 individual sera) are shown in figure 3.4. These assays were all carried out using standard Massachusetts-type antigen (M41). The pattern of response is very different from that seen with IBD or ND. There is a very wide range of response in rear which substantially overlaps the level of response seen in lay. The response in lay remains high throughout the life of the birds. It seems likely that a substantial part of the high mean titres both in rear and during lay are due to challenge with a range of IB viruses. A large number of high mean titres are encountered even before the oil vaccine and also late in lay when we would have expected the titre to have fallen off. We do not believe that H120 vaccine, even when administered repeatedly during lay, can be the cause of these high titres.

As in the case of ND, a smaller number of tests (78 groups, 1567 sera) have been carried out using an Elisa test (IDEXX). Again, most of these were routine samples (not related to disease investigation). Mean titre groups were rather lower than the GMT's seen with the HI test (Figure 3.5).

A newly-identified variant of Infectious Bronchitis virus, designated 793B has been associated with quite severe signs typical of IB, but also with sudden death in apparently healthy birds, especially broiler parent chickens (Gough and others, 1992). In many cases these flocks show some birds serologically positive in a HI test to the 793B variant of Infectious Bronchitis. The HI antigen for this serotype is not widely available so the test tends to be performed only when there is good reason to believe that this strain is involved in a specific problem. In order to compare the response between conventional HI test (M41 antigen) and the 793B HI test the data-base was searched for groups of sera from broiler parents on which both assays were performed. This turned out to include 311 house-flocks (4014 M41 assays and 3391 793B assays).

Figure 3.6 shows the 793B HI mean titres according to the age of the birds. Mean titres of 4 or more can be considered to be indicative of a 793B challenge. The Central Veterinary Laboratory considers that for an individual serum to be considered positive it should show a HI titre of 1:64 or higher. The relatively low 793b titres late in lay should be interpreted with caution in view of the relatively small number of groups tested (only 53 of the groups were tested after 45 weeks of age).

In figure 3.7 we have plotted M41 mean titre for each flock group against the 793b mean titre for the same group. Approximately 50% of groups with an M41 mean titre of 8 or higher will have a 793b titre of 4 or higher. The 793b variant appears to be a common field challenge virus but it is far from being the only one.