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Müller, Ingetraud (2009): Studien zum Verlauf einer in ovo Infektion in Haushühnern (Gallus gallus) mit einem ausgewählten Aviären Orthoreovirusisolat. Dissertation, LMU München: Faculty of Veterinary Medicine
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Abstract

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.