Oral vaccination in chickens against Newcastle disease and some factors affecting vaccination in developing countries

Abstract

The studies described in this thesis were undertaken in two countries and involved different elements of the problem of Newcastle disease. In Pakistan virulent strains of Newcastle disease virus were available, and Newcastle disease was a serious problem to local farmers. The candidate’s laboratory was deeply involved in the practical problems of vaccine production and application in the field. In Australia, Newcastle disease did not occur and only the avirulent V4 strain of Newcastle disease virus was available. The host laboratory played a leading role in an international effort to evolve an effective system for oral vaccination of village chickens in developing countries. This vaccine utilised a heat-resistant variant of the V4 virus, and the candidate contributed to these studies. In Pakistan laying flocks are revaccinated with the mesogenic Mukteswar strain of Newcastle disease virus before they come into lay. There is no precise knowledge of the levels of passive antibody that this procedure endows to chicks, nor of most efficient methods of subsequently vaccinating the chicks that are produced. Eggs were obtained from a laying flock 24 weeks after revaccination with Mukteswar vaccine and chicks were hatched. Haemagglutination-inhibition antibody titres to Newcastle disease virus declined from a geometric mean titre of 25 3 on the day of hatching to 2°3 at 25 days of age, with a half-life of about 5 days. Chickens were challenged with a local, virulent strain of Newcastle disease virus at intervals, and mortality ranged from 18% at one day of age to 90% at 25 days of age, increasing with waning levels of immunity. No chicken with a titre of 25 3 at challenge succumbed. Mortalities in other chickens varied from 88% (no detectable antibody) to 27% (geometric mean titre of 24 3). There was a strong correlation (r = 0.96) between the level of passively acquired antibody and survival after challenge. Levels of neutralizing antibody were also assessed and these correlated strongly with levels of haemagglutination-inhibition antibody. It seemed likely that very young chicks would not respond to vaccination because of the high levels of maternal antibody, while delaying vaccination for 3 weeks would involve unacceptably high losses if natural challenge occurred. Similar chicks were reared and vaccinated at 12 days of age. Three strains of vaccine (Mukteswar and lentogenic strains La Sota and F) and 2 routes of vaccination (eyedrop and drinking water) were used.