CHARACTERIZATION OF NIGERIAN STRAINS OF NEWCASTLE DISEASE VIRUS

Abstract

Natural outbreaks of Newcastle disease were investigated in different parts of Nigeria and samples for virus isolations were collected from vaccinated and unvaccinated commercial farms and indigenous chickens. Eleven NDV isolates were recovered from a total of 37 samples collected from chickens and one isolate was recovered from a parrot. All the 12 isolates were characterized as velogenic strains. Two types of clones -small and large were isolated from the velogenic strains. Results of the pathogenecity characterization of the clones showed a significant difference in the degree of virulence between the small and large clones. A clone of low virulence UI-104-s similar to a lentogenic strain was successfully isolated from a velogenic strain. A further pathogenecity test on this clone is suggested with the aim of developing an attenuated live vaccine from it. Eight of the isolated clones were antigenically compared with the 2 vaccine strains in use in Nigeria by the cross haemagglutination test, plaque reductionneutralization test, polyacrylamide gel electrophoresis and cross protection test. All the in vitro tests showed significant antigenic differences between the clones derived from field samples and the vaccine strains. The in vitro cross protection test did not however reveal any significant antigenic diversity between the strains. The susceptibility of chickens vaccinated with the three live vaccines in use in Nigeria to challenge by Nigerian velogenic strains was studied. Between 0 to 30% of birds vaccinated with the NDV-i/o vaccine died after challenge with the field virus. While no mortality was recorded in bird vaccinated with NDV i/o and boosted with NDVLasota, 40 out of 90 birds showed mild nervous signs. Birds vaccinated with NDV i/o Lasota and Komarov vaccines resisted challenge and showed no nervous signs. This result showed that the lentogenic vaccines in use in Nigeria are not immunogenic enough to confer the degree of immunity required to protect chickens against the prevalent virulent viscerotropic velogenic strains. There¬fore it is recommended that the Vom recommendation on vaccination programme be strictly adhered to. However because of the prevalence of the extremely virulent velogenic viscerotropic strains of the virus a reappraisal of the existing vaccine and vaccination procedure is advocated. It is therefore recommended that the use of inactivated oil elision vaccine be introduced in Nigeria. Four experimental inactivated oil emulsion vaccines were prepared from two selected clones, ui-104-I and UI-1O4-S derived from Nigerian velogenic field strain vaccine strains currently in use in Nigeria. Result of immunogenecity and challenge tests showed that the oil emulsion vaccines prepared from the Nigerian strain ui-104-L and the vaccine strain Komarov stimulated the highest antibody response in chickens. Birds vaccinated with these two vaccines resisted challenge at 10 and 20 weeks after challenge. Results obtained from the study with the live and inactivated vaccines showed a similarity in their response and suggest that vaccines prepared from the moderately virulent and virulent strains are more effective than those prepared from the less virulent strains. Based on this a cooperative study of the duration of immunity of vaccinated chickens using both the live vaccines and inactivated oil emulsion vaccines is suggested for further study.