Abstract
Despite the intensive vaccination programs used for controlling Newcastle disease (ND) in the Iranian poultry industry, outbreaks of ND have been reported in poultry farms. This study was conducted to evaluate the effectiveness of vaccines for the protection against ND infection and virus-shedding period of velogenic Newcastle disease virus (vNDV) field strain after different immunization schemes. Eight groups of commercial broiler chickens were used. Six groups were vaccinated with different vaccination programs using commercial live and inactivated ND vaccines. All groups, except for group 8, were challenged with a virulent field isolate (104EID50/bird) at 28 days of age. Clinical signs, mortality rate and gross lesions were investigated. Antibody titers were assayed by hemagglutination inhibition test and fecal virus shedding was determined for 14 days post challenge (dpc) with 3-day intervals by the RT-PCR method. All unvaccinated-challenged control birds died. Vaccination with these ND vaccines protected chickens from clinical disease. The mortality rate in the vaccinated groups was significantly lower than in the positive control group. However, vaccinated chickens shed the challenge virus in fecal samples. Although the different vaccination regimens displayed close degrees of protection against the disease, the best protection was observed in broilers primed with the live B1 vaccine via eye drop simultaneously with inactivated vaccine at 8 days of age and boosted with B1 or LaSota via drinking water on day 18. In conclusion, the currently used vaccines with different vaccination schemes can protect chickens against the disease in areas where ND is endemic, while the spread of the field virus to other flocks cannot be prevented.
Highlights
Newcastle disease, which is caused by Newcastle disease virus (NDV), is one of the most serious infectious diseases affecting birds, poultry, and can lead to serious economic losses (Aldous et al, 2003)
Chickens in the vaccinated-challenged groups showed significantly weaker clinical signs compared with the positive control group
In Iran, the velogenic ND has been is enzootic in commercial poultry for years, despite the intensive vaccination programs applied
Summary
Newcastle disease, which is caused by Newcastle disease virus (NDV), is one of the most serious infectious diseases affecting birds, poultry, and can lead to serious economic losses (Aldous et al, 2003). NDV is classified as a member in the Avulavirus genus, within the Paramyxoviridae family. NDV isolates are classified as lentogenic (low), mesogenic (intermediate), or velogenic (highly virulent), depending on the severity of the disease produced by the isolate in chickens (Alexander, 1997). NDV has been the cause of significant outbreaks in poultry (Alexander et al, 2012). The virus is highly transmissible in poultry with low antibody titers (Boven et al, 2008). There have been several panzootics of this serious disease in poultry during the last four decades (Alexander, 2011) and the velogenic strains are endemic in the commercial poultry farms of many countries (Boney et al, 1975, Giambrone & Closser, 1990), including Iran
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