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Sahm, Isabel (2009): Untersuchungen zur antigenetischen Verwandtschaft aviärer Reoviren aus Broilern mit Malabsorptionssyndrom (MAS). Dissertation, LMU München: Faculty of Veterinary Medicine
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Abstract

Avian reoviruses (ARV) are supposed to play an important role in development of malabsorption syndrome (MAS) in broiler flocks. By seriously affecting weight gain, considerable economic losses occur. The present study deals with ARV which had been isolated from broilers with MAS despite vaccination of parental flocks. Thus, the question arose if the isolates were antigenic variants which could bypass protection by vertically transmitted maternal antibodies. For examination, cross neutralization tests were chosen. To gain repeatable results for all ARV isolates, a plaque reduction assay including an immunohistochemical staining method was adapted to ARV. Thereby, different ARV strains induced three distinguishable plaque types on CEL and CEF cells. The examinations on CEL cells included hyperimmune sera raised against vaccine strain S1133, against a flock-specific vaccine strain and against four MAS-strains isolated in the "Klinik für Vögel". Additionally, four pooled serum samples from parental flocks were tested. Besides the vaccine strain S1133, eleven ARV strains/ isolates were examined. The neutralization capacity of hyperimmune sera differed considerably. Sera which were raised against the MAS-isolates showed a clearly reduced neutralization capacity compared to serum a-S1133. ARV strains displayed a considerable antigenic heterogeneity with clear differences to the reference strain S1133. Concurrently, an extended cross reactivity occurred. Remarkably, vaccine strain S1133 was neutralized best by every used serum, whereas the MAS-isolates resisted neutralization to a noticeable extent. These results led to the consideration as to whether the used testing system purely reflects antigenic relatedness or if it is fundamentally affected by the host cell used. Therefore, neutralization examinations with three selected strains which were obtained from four different tissues followed: CEL and CEF cells, embryonic liver and embryonic gut. Additionally, resulting virus stocks of one MAS isolate were neutralized comparing the reaction pattern on CEL and CEF cells. Thereby, the reaction pattern of strain S1133 stayed constant, whereas one MAS isolate was neutralized to a greater extent when it was obtained from embryonic instead of cellular origin. Hence, the formerly noted dominance of the S1133 strain was overcome when a CEL cell system was used. On a CEF cell system, these differences did not occur. In order to determine antigenic relatedness of MAS isolates, R-values were calculated and interpreted by a definition given for foot-and-mouth-disease . Thus, MAS isolates were defined as minor and major subtypes of strain S1133 and were all closely related. For strategies of controlling early ARV infections, especially the partial resistance to neutralization and the lesser immunogenicity of the existing MAS isolates are of great importance. It can be assumed that the inclusion of novel ARV isolates in vaccination protocols will not guarantee complete protection since ARV will retain the ability to escape the systemic humoral immune response. Possibly, stimulation of the cytotoxic immune response provides some potential for controlling early ARV infections. Nevertheless, thorough hygiene protocols remain indispensable and should include regular checking of disinfectants, particularly their capacity to degerminate existing ARV variants.