Effect of vaccination on transmission characteristics of highly virulent Newcastle disease virus in experimentally infected chickens

Abstract

An experimental study was conducted to evaluate the effect of vaccines produced in Ethiopia from vaccine strains used worldwide on the transmission characteristics of velogenic Newcastle disease virus field strain after different vaccination schemes. Chickens were vaccinated with Hitchner B1, La Sota or I-2 via the intraocular and intranasal routes. Vaccine and challenge viruses induced high antibody levels, both in inoculated and contact birds. Prime-boost vaccination protected birds against morbidity and mortality and significantly reduced the incidence of viral shedding from chickens compared with single vaccinated and unvaccinated birds. Protection from disease and mortality was correlated with the presence of positive antibody titres (>4 log2) at day of challenge. Most of the unvaccinated and in-contact birds excreted the virus and showed a high level of antibody titres, indicating the high infectivity of the challenge virus. The detection of the challenge virus in most of vaccinated birds demonstrated that the tested vaccination protocols cannot fully protect birds from viral infection, replication and shedding, and vaccinated-infected birds can act as a source of infection for susceptible flocks. The high mortality observed in unvaccinated birds and their contacts confirmed the virulence of the challenge virus and indicated that this field virus strain can easily spread in an unvaccinated poultry population and cause major outbreaks. Progressive vaccinations supported by biosecurity measures should therefore be implemented to control the disease and introduction of the virus to the poultry farms.

Comparative Evaluation of the Protective Efficacy of Different Vaccination Programs Against a Virulent Field Strain of the Newcastle Disease Virus in Broilers

“…Our findings are consistent with the reports of Fentie et al (2014), Boven et al (2008), and Kapczynski & King (2005). The chickens in all vaccinated-challenged groups shed the challenge virus from day 3 dpc.…”

“…As we know, over time, maternal antibody titer wanes and eventually cannot protect against infections with wild viruses. Similar findings were reported by Fentie et al (2014) and Kapczynski & King (2005). This confirms that circulating vNDV strains are capable of causing high mortality in unvaccinated susceptible flocks, which is consistent with the suggestions of Alexander et al (2004) and Alders & Spradbrow (2001).…”

“…Vaccination programs used in groups 1 and 4 shortened the duration of virus shedding (until day 6 pc), while few of the challenged birds in the other vaccinated groups continued to shed the virulent virus in the feces until the end of the experiment. Similar findings were reported by Fentie et al (2014) and Kapczynski & King (2005). The high antibody levels in the vaccinated challenged groups may have been responsible for the reduced virulent virus shedding.…”

“…In addition, necropsy findings revealed that lesions were less severe in all vaccinated groups as compared with positive control group. Protection against clinical signs and mortality following infection with vNDV in vaccinated birds has also been reported by other researchers (Boven et al, 2008, Fentie et al, 2014, Kapczynski & King, 2005, Degefa et al, 2004.…”

“…This confirms that circulating vNDV strains are capable of causing high mortality in unvaccinated susceptible flocks, which is consistent with the suggestions of Alexander et al (2004) and Alders & Spradbrow (2001). Despite the expectation that mean antibody titers increased a week after challenge (Fentie et al, 2014;Alexander et al, 2004), substantial changes were not observed in the HI antibody titers in the sera of the positive-control group. This may be explained by the small serum sample size (only one bird of the positive control group was alive on 10 dpc).…”