Abstract
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.
Highlights
Immunity by NAD-independent isolates Haemophilus paragallinarum in poultry ried on a transferable plasmid and they succeeded in transforming reference strains of NAD-dependent H. paragallinarum into NAD-independent isolates
The data obtained revealed that there is a significant difference (P = 0.000358) between the disease profiles obtained for the vaccinated and unvaccinated birds. These results indicate that the vaccine used in this experiment provided protection when the birds were challenged with the NAD-dependent serovar C-3 isolate
These findings strongly suggest that the NAD-independent isolate was capable of evading the protective immunity induced in the vaccinated birds and support the hypothesis of immune-evasion by NAD-independent strains which was postulated from results obtained from previously described work
Summary
Bragg, Coetzee & Verschoor (1996) established that there has been a significant change in the serovar distribution of NAD-dependent H. paragallinarum isolates since the 1970s. Miflin, Horner, Blackall, Chen, Bishop, Morrow, Yamaguchi & Iritani (1994) reported that all of the NAD-independent South African isolates which they had received were serogroup A isolates. These isolates were, collected from a localized area of South Africa. Bragg, Greyling & Verschoor (1997) reported on a much wider collection of NAD-independent isolates and they isolated serogroup C NAD-independent isolates for the first time, and they indicated that the incidence of serogroup A NAD-independent isolates was 58 %
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