Despite the presence of vaccines, Newcastle disease (ND) and Infectious Bronchitis (IB) outbreaks are not uncommon in endemic countries. Therefore, the current study was conducted to investigate the efficacy of commercially available vaccines against each virus alone as well as co-infections with NDV and IBV in commercial poultry. For assessment of vaccine efficacy, commercial chickens were vaccinated with La Sota (against NDV) and Massachusetts (against IBV) vaccine preparations. After screening the protective titers in birds at 19 days post inoculation three week-old broilers were challenged with 10-5.76 and 10-6.03 EID50 dose of NDV and IBV, respectively. The hosts were divided into six groups of 35 birds each. These were groups A (NDV-challenged), B (IBV- challenged), C (NDV+IBV-challenged), D (IBV+NDV challenged), E (NDV and IBV- challenged) and F (negative control). Mild to moderate clinical presentations were observed in most of the co-infected birds of groups C, D and E. Tissue samples collected at 2nd, 4th and 6th days of post infection (dpi) showed histopathological signs of clinical infection. Overall, the severity of infection was higher in the co-infected birds as compared to host challenged with either of the viruses alone. Notably, moderate clinical infections raised concerns vaccine efficacies. The current study concluded that the commercially available vaccines may not provide enough protection particularly in the case of co-infection with NDV and IBV. Therefore, here is need to conduct further studies on production of vaccines with current prevailing strains in disease endemic countries such as Pakistan.
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