engleski

Broilers with No or Low Mortality but with Reduced Body Masses and Immunosuppression

Highly virulent infectious bursal disease (IBD) struck the Croatian poultry industry in mid 1990&#8217; s and has been a major poultry viral disease for a decade. As noted in other countries, mild live IBD vaccines, which were widely used in Croatia, could not protect chickens against the disease. Also, beside the usual high mortality due to highly virulent IBD (hvIBD), which ranged up to 25% in broilers and 60% in layers, cases with no or very low mortality, accompanied with reduced body mass and increased feed conversion ratio (FCR), could be seen in commercial broilers vaccinated with either mild or intermediate live IBD vaccines. Thirty-day-old vaccinated and unvaccinated broilers were exposed to contact infection with the hvIBD virus isolate under controlled conditions to find out whether this unusual form of hvIBD was an outcome of possibly partial protection by vaccination or it could occur even in fully susceptible broilers. Each group comprised 20 birds. Vaccinated broilers were given an intermediate live IBD vaccine, strain VMG 91, at ages of 8 and 15 days. Beside IBD antibody responses and body masses, immune status of the chickens was assessed by abundance of IFN-&#945; and IFN-&#947; mRNA in blood before and 2 h after challenge with inactivated Newcastle disease virus on days 7 and 14 following contact exposure. No mortality was recorded throughout the experiment (to age of 44 days), whereas body masses were significantly lower in unvaccinated broilers from day 37 onward. At the end of the experiment, vaccinated and unvaccinated broilers gained average body masses of 2.29 kg &plusmn ; 0.23 and 1.78 &plusmn ; 0.22, respectively (p < 0.05). IBD antibody titres, measured by IDEXX ELISA, increased slightly in vaccinated broilers from 2934 &plusmn ; 863 at 30 days to 3421 &plusmn ; 1226 at 44 days of age. The unvaccinated broilers were free from IBD antibodies when they where exposed, and afterwards titres increased greatly from 134 &plusmn ; 72 at age of 30 days to 3553 &plusmn ; 567 at age of 44 days. Induced transcription of IFN-&#945; and IFN-&#947; mRNA in unvaccinated broilers was significantly less than in vaccinated birds on days 7 and 14 after hvIBD exposure. In conclusion, hvIBD viruses occasionally cause a disease with no or very low mortality even in fully susceptible broilers, but the disease is accompanied with immunosuppression, significantly reduced body mass gain and increased FCR. However, the disease can be successfully controlled with intermediate live IBD vaccines.

highly virulent IBD; broilers; reduced body mass; immunosuppression; vaccination

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