Studien zum Verlauf einer in ovo Infektion in Haushühnern (Gallus gallus) mit einem ausgewählten Aviären Orthoreovirusisolat

The aim of the present study was to evaluate the relevance of an
Avian Orthoreovirus (ARV) isolated from broiler chicken suffering
from infectious runting and stunting syndrome (RSS) after simulated
vertical infection of chicken. The influence of the age at
infection on the progression of ARV infection is of particular
importance. At first, the consequences of an early in ovo
inoculation with the ARV-isolate 8474DP4-6 in specific pathogen
free (spf) embryonated chicken eggs were evaluated. Infected
embryos that survived until the 19th day of incubation were
observed regularly after inoculation into the allantoic cavity of
embryonated chicken eggs with varying doses of the ARV-isolate
8474DP4-6 on the ninth day of incubation. The inoculation into the
yolk sac of embryonated chicken eggs on the sixth day of incubation
with the same virus resulted in surviving infected embryos only in
singular cases. On the other hand, infected embryos died after in
ovo inoculation with the ARV vaccine strain S1133 into the
allantoic cavity or the yolk sac on the corresponding days of
hatch. The mean death time for the infected embryos displayed a
high variability. The mean death time after in ovo inoculation with
the ARV strain S1133 was observed between 100 and 168 hours while
embryos infected with the ARV-isolate died between 60 and 120 hours
post inoculation. For a subsequent animal experiment embryonated
chicken eggs (Lohmann Selected White Leghorn) were inoculated with
the ARV-strain 8474DP4-6 into the allantoic cavity on the 14th day
of incubation and the hatched chickens were raised until the age of
36 days. Inoculated birds were kept together with non-inoculated
sentinels while a mock-infected group was raised separately. The in
ovo inoculation resulted in the hatch of ARV-infected chicks. The
hatchability was reduced in the infected group (56%) as well as in
the mock-infected control group (46%). ARV was reisolated regularly
from organs of infected chicks until the 12th day post hatch and
sporadically until day 36 post hatch. Several organs (liver,
duodenum and pancreas [pooled], proventriculus, jejunoileum and
bursa of Fabricius) were sampled in order to evaluate the
organotropism and the progression of infection in inoculated
chicks. ARV was reisolated from all kinds of examined organs,
whereas the isolation from intestinal organs occurred more
frequently. The transmission to non-inoculated sentinels was
demonstrated by reisolation of ARV. The inoculated as well as the
sentinel chicks developed antibodies against ARV. The results
demonstrate the possibility of a vertical and likewise a horizontal
transmission for the ARV-strain 8474DP4-6. An increased mortality
became evident within the infected group. From the day of hatch and
the seventh day of life 52,2% of the animals died, until the 12th
day of life the mortality peaked to 56,5%. Within the control group
a mortality of 6 and 12% respectively was observed during the
corresponding period. Inoculated as well as control animals were of
poor condition and suffered from diarrhea. Not inoculated control
birds recovered quickly after an antibiotic treatment while the
inoculated chicken did not respond well to the therapy. The gross
pathology did not reveal any alterations in neither of the groups.
Histopathological changes included myocarditis, pancreatic
degeneration and lymphocyte depletion in the spleen. Within the
bacteriological examination various ubiquitous bacterial species
including Staphylococcus sp., Streptococcus sp., Enterococcus sp.,
Bacillus sp., Escherichia coli, Citrobacter freundii and
Enterobacter cloacae were cultivated from organ swabs from animals
of both groups. Body weight and weight gain were significantly
decreased in inoculated chicks from day seven post hatch on. In ovo
infection of embryonated eggs from ARV-vaccinated broiler breeders
with the ARV-isolate 8474DP4-6 resulted in a hundredfold lower egg
infectious dose 50 compared to embryonated spf chicken eggs.
Furthermore, a larger proportion of infected embryos survived until
the 19th day of incubation. In conclusion, the in ovo inoculation
of embryonated eggs with the RSS-associated ARV strain resulted in
the impairment of hatched chicks. The bacteriological findings
indicate an increased susceptibility of the ARV-infected chicks to
bacterial overgrowth. Future experiments evaluating the effects of
early ARV-infections with the focus on alterations of the
immune-response will help to elucidate the role of early
ARV-infections in chicks.