Development of a vaccine against the poultry red mite (Dermanyssus gallinae)
The poultry red mite (PAM) (Dermanyssus gallinae) has been deemed the most deleterious ectoparasite in commercial housing systems for laying hens, affecting the cost of production, health and welfare. Present control of the PRM is hindered by the limited number of acaricides available, resistance by PRM to acaricides and the ability of the PAM to evade control. Therefore the aim of this thesis was to investigate the manipulation of the host immune system as a means of developing a vaccine to control the PAM. Two experiments were carried out to gain a better understanding of the relationships between natural PRM infestation, acaricide application, the immune response of laying hens and the consequences for egg production and welfare. A further two experiments were implemented to determine the effect of immunisation with PAM antigen extracts on the immune response of hens and subsequent survival and fecundity of PAM. Experiments 1 and 2 evaluated the effect of estimated PRM populations on production parameters and the immune response of poultry across commercial laying sites. Both experiments showed significant correlations between PAM population and production parameters, specifically building temperature and hen mortality in Experiment 1 and 2 respectively. A significant reduction in PAM population following acaricide application was also observed. However, no significant relationship was found between PAM populations and either IgY or cytokine levels, although a Significant negative relationship was observed between IgY and the cytokine IL-4. In both experiments, variability of data was high which may have contributed to the failure to show a relationship between PRM population and the immune response of poultry. Nevertheless, these experiments highlight the suitability of commercial egg laying systems for proliferation of PAM and the consequences for poultry. Experiment 3 assessed the effect of immunisation with PAM antigen extracts on IgY and cytokine responses of pullets, as well as PRM survival and fecundity. There were 2 treatments: an antigen treatment which received PRM antigen extracts in Complete Freund's adjuvant (CFA), followed by two immunisations of antigen with Incomplete Freund's adjuvant (IFA) and a control treatment in which the PRM antigen was substituted for saline. Significantly higher IgY levels were observed in the antigen treatment, although an increase in IgY levels in the control treatment was also seen, resulting from non-specific antibody binding which was confirmed by western blotting. PCR performed on PAM DNA revealed that non-specific binding was a likely effect of homology between Mycobacterium present in both PRM and in CFA. Significantly higher IL-10 levels were seen in the antigen treatment, which was in turn thought to be responsible for the significant inhibition observed in IL-4 and IL-S. Survival and fecundity of PRM was not significantly affected by treatment. Experiment 4 investigated the effect of immunisation of pullets with two different PRM antigen extracts, without the confounding effect of Mycobacterium in CFA. There were 2 control treatments receiving either PBS or IFA only, and two antigen treatments receiving IFA with either PBS or Urea-extracted PRM antigens. Levels of IgY and IgM were not significantly different between antigen treatments, but these in turn were significantly higher than the controls. Western blotting showed several bands in the antigen treatments, which were not seen in controls. An in vitro feeding system revealed no significant difference between treatments for survival or fecundity of PRM, confirming that immunisation with PRM antigens did not elicit a protective host immune response. In conclusion, this research programme has demonstrated that exposure to PRM antigens provoked an increase in immunoglobulin levels and changes to the relative expression of different cytokines in poultry. However, immunisation with PRM antigen extracts was not sufficient to cause a significant reduction in the survival or fecundity of PRM. Further research therefore is needed to identify a suitable antigen which elicits protection to laying hens from predation by the PRM.