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Poultry red mite (Dermanyssus gallinae) infestation has high animal welfare relevance and leads to remarkable economic losses particularly in layers. Due to developments of resistances, residue problems, and the current legal regulations, control options are very limited in the EU, especially for organic layer farms. An alternative to chemical acaricides are silica products, which are based on silicon dioxide. In this study the twelve silica products were characterized with regard to water absorption capacity (WAC), silicon dioxide content, cation exchange capacity (CEC) and specific BET-surface. Furthermore, an electron microscopy was conducted. Moreover acaricidal efficacy of these twelve products, nine in powder form and three for liquid application, was tested under laboratory conditions.
Mite mortality was measured over time and the mean lethal time (LT50) was determined by Probit analysis. The LT50 values of powdery formulations ranged from 5.1 to 18.7 and of the fluid ones from 5.5 to 12.7 hours. Significant differences between the products were detected by Oneway ANOVA and Tukey´s HSD Test (p < 0.05). The ranges of LT50 of fluid and powdery formulations overlapped. Backward stepwise regression analysis was conducted in order to analyze the impact of the measured parameters on acaricidal efficacy. Silicon dioxide content had no significant impact on efficacy, while specific surface and CEC were positively and WAC negatively related to efficacy. Influence of these parameters on acaricidal efficacy was significant according to the results of a stepwise regression analysis (p < 0.01).
No direct relation was found between the analyzed parameters and the application form (powdery or liquid). To test the safety of the treatment procedure, hens were exposed to two fluid products. The products were sprayed in presence of the hens. One week after the initial exposure, the hens were slaughtered and trachea, lungs and air sacs were inspected by histology and compared to the not exposed control group. No alterations of high degree were found in any of the hens. Dust was found in very few lungs of the silica exposed hens as well as in lungs of the control hens. Application of the silica in presence of the hens did not lead to acute alterations in the hens respiratory tract.
Consequences of long term exposure were examined in ten hens which came from an organic layer farm and were silica exposed for a period of one year. Foreign matter, supposed to be dust, was found in the lungs of all hens and in the airsacs of three hens.
There is no evidence for silica exposure as origin of the dust found in lungs and airsacs of the hens. A further study was conducted to survey mite infestation and different schedules for control of D. gallinae on 17 organic layer farms.
Both mite infestation and control strategies differed concerning the use of prophylactic silica application, curative use of acaricides and in the monitoring of the mite infestation by the farmer. Within the scope of this project a monitoring method modified at the Institute of Animal Welfare and Animal Husbandry, Celle (FLI) was validated. Monitoring and prophylactic silica application are important components of successful mite control. Farms with curative use of acaricides did not show lower mite burden than those without. The reason for this might be that highly infested farms rather use curative control than those that do not suffer from mite infestation. It was found that on highly infested farms with curative mite control mite infestation did not increase.