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SUMMARYA highly reproducible monoclonal antibody (Mab) blocking ELISA (B-ELISA) has been developed and evaluated for the detection of NDV-specific antibodies. The Mab utilised is specific for a conserved PMV-1 serotype-specific epitope, as demonstrated by the indirect immunoperoxidase test. It reacted with all strains representing different serogroups within the PMV-1 serotype, but not with any strain belonging to other PMV serotypes.Sensitivity and specificity of the B-ELISA were compared with the haemagglutination inhibition test (HI). Blocking and HI antibodies were detected in sera of chickens 8 days post-experimental infection. The B-ELISA proved consistently more sensitive than the HI test. In another survey, 62 sera from experimentally vaccinated chickens were tested; 95.2% proved positive by B-ELISA, 85.5% by indirect ELISA and 74% by HI test. When 504 field sera from vaccinated chickens and turkeys were tested, 98% were positive by B-ELJSA, and 69% by HI. The specificity was evaluated by testing 1066 samples from NDV-free flocks, all of which proved negative by both methods. Other advantages of the B-ELISA include easy standardization and quality control, and ability to test sera from any species (including exotic or wild birds as well as mammals). The use of low dilution serum or egg-yolk samples makes the test quick and easy to perform and suitable for large-scale screening.

Sera from 211 ostriches were tested for the presence of Newcastle disease virus (NDv)-specific antibodies by the virus neutralisation test, the haemagglutination inhibition (HI) test and a recently developed avian paramyxovirus serotype 1 (PMv-1) specific monoclonal antibody blocking ELISA (b-ELISA). The virus neutralisation test was used as the reference for the estimation of the sensitivity and specificity of the b-ELISA and HI tests. Of the 211 sera, 140 contained NDv-specific neutralising antibodies, 130 were positive by the HI test and 122 by the b-ELISA. The sensitivity, specificity and predictive accuracy of the HI and b-ELISA tests relative to the virus neutralisation test were similar. The good agreement between the HI and b-ELISA test (ic = 0*85) suggested that the two methods are interchangeable.THE breeding and trading of ratites (ostriches, emus and rheas) has expanded considerably all over the world in recent years. They are susceptible to several diseases of domestic fowl, including Newcastle disease (Samberg and others 1989). The characterisation of 14 Newcastle disease virus (NDv) isolates from ostriches in South Africa showed that they carried vaccine strains (like La Sota) and a number of velogenic strains (Manvell and others 1996). Ostriches may therefore carry and spread Newcastle disease and imported birds should be tested serologically, like poultry flocks, and positive results should be confirmed by virus isolation.The haemagglutination inhibition (HI) test is the accepted method for detecting NDv-specific antibodies in poultry sera (Alexander 1991). However, sera from other species often induce non-specific reactions in the HI test and, in order to avoid these reactions, sera have to be adsorbed with chicken red blood cells (RBcs) before they are tested.Allwright (1996) reported that the HI test gave poor results when ostrich sera were tested, although chicken RBcs were replaced by ostrich red blood cells. Inactivation and kaolin treatment (King and Hopkins 1983) reduced the number of false positive reactions but false negative results still remained a serious problem. The results obtained with a modified indirect ELISA, using coated plates from a commercial Newcastle disease kit and a biotinylated rabbit anti-ostrich-IgG conjugate, correlated very well with the results of the microneutralisation test. Recently, a PMV-1 specific monoclonal antibody blocking ELISA (b-ELISA) has been described by Czifra and others (1996). The specificity of this test is based on a reaction between a monoclonal antibody (mAb) and its well conserved PMV-i specific binding site (epitope). The mAb reacted with all strains representing different serogroups within the PMV-1 serotype, but not with any strain belonging to other PMV serotypes. Consequently, sera from any PMV-1 virusinfected or vaccinated animal 'block' the binding of the mAb. It was therefore decided to test ostrich sera with the b-ELISA and collect data on the prevalence of NDv-specific blocking antibodies in ostriches.This paper describes the ...