Untersuchungen zur antigenetischen Verwandtschaft aviärer Reoviren aus Broilern mit Malabsorptionssyndrom (MAS)

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.