Abstract
Newcastle disease is an economically important disease of poultry for which vaccination is applied as a preventive measure in many countries. Nevertheless, outbreaks have been reported in vaccinated populations. This suggests that either the vaccination coverage level is too low or that vaccination does not provide perfect immunity, allowing the virus to spread in partially vaccinated populations. Here we study the requirements of an epidemiologically effective vaccination program against Newcastle disease in poultry, based on data from experimental transmission studies. The transmission studies indicate that vaccinated birds with low or undetectable antibody titres may be protected against disease and mortality but that infection and transmission may still occur. In fact, our quantitative analyses show that Newcastle disease virus is highly transmissible in poultry with low antibody titres. As a consequence, herd immunity can only be achieved if a high proportion of birds (>85%) have a high antibody titre (log2 haemagglutination inhibition titre ≥3) after vaccination. We discuss the implications for the control of Newcastle disease in poultry by vaccination.
Highlights
Newcastle disease (ND) is a highly contagious viral disease affecting wild and domestic avian species (Seal et al, 2000; Alexander, 2003)
All three challenge viruses used in this study are highly virulent as measured by the Intracerebral Pathogenicity Index (ICPI >0.7), and are able to kill experimentally-infected, susceptible chickens within two days
In this paper we have presented quantitative analyses of Newcastle disease virus (NDV) transmission in vaccinated chickens
Summary
Newcastle disease (ND) is a highly contagious viral disease affecting wild and domestic avian species (Seal et al, 2000; Alexander, 2003). There has been a trend to use ever less virulent strains as the seed viruses for vaccine production This strategy has reduced the disease rates after vaccination, it may have contributed to the fact that current vaccines and vaccination campaigns are not maximally effective in preventing infection and transmission (Burridge et al, 1975; Voeten et al, 1987; Alexander, 2003; Senne et al, 2004; Kapczynski and King, 2005). It is not clear whether the ultimate goal of prevention of major outbreaks after primary virus introductions can be achieved with current vaccines and vaccination programs
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