Evidence of possible evasion of protective immunity by NAD-independent isolates of Haemophilus paragallinarum in poultry.

An indication of the ability of NAD-independent variants of Haemophilus paragallinarum to evade the immune system has been obtained from data obtained from several experiments. Firstly, it was noted that there was a difference in the serovar distribution between the NAD-dependent isolates in the 1990s and the NAD-independent isolates, as there was a significant decrease in the incidence of serogroup A NAD-dependent isolates. This can possibly be attributed to the extensive use of vaccines. On the other hand, most of the earlier NAD-independent isolates were serovar A. This is a possible indication of evasion of the protective immunity by the NAD-independent isolates. Further evidence of possible evasion of the protective immunity was obtained from results obtained when different isolates, both NAD dependent and NAD independent, were tested with a panel of monocional antibodies (Mabs). The V1 Mab reaction pattern was only seen in the reference strain 0083 among all of the NAD-dependent isolates tested in South Africa. This Mab was, however, found to react with some of the NAD-independent isolates. Furthermore, the isolation of NAD-dependent isolates in Australia which react with the V1 Mab also suggest possible evasion of the protective immunity by the NAD-independent isolates as no vaccines containing strain 0083 are used in Australia. In order to investigate the hypothesis of immune-evasion by NAD-independent H. paragallinarum, vaccinated and unvaccinated chickens were challenged with a NAD-independent serogroup C isolate. As a control, chickens were also challenged with NAD-dependent H. paragallinarum of the same serogroup. The results obtained indicate that there is no significnat difference in the disease profiles obtained in vaccinated and unvaccinated chickens challenged with the NAD-independent isolate, thus providing further evidence of evasion of the productivity immunity by the NAD-independent isolates. The ability of the NAD-independent isolates to evade the immune system suggests that a different vaccination strategy, or alternative control methods may be needed for the control of IC caused by these isolates.