Experimental infection of vaccinated slaughter ostriches in a natural, open-air feedlot facility with virulent Newcastle disease virus

The presence of virulent Newcastle disease virus (NDV) since the 1993-94 ep idemic in southern Africa holds major implications for the export of ostric h products from this region. A challenge experiment with this field strain was conducted in open-air feedlot facilities under strict: biosecurity meas ures. The experiment was designed to follow vaccination and preslaughter qu arantine regulations currently enforced in South African export ostrich fac ilities in order to determine the viremia period and immune response under these specific circumstances. One hundred forty-three slaughter ostriches w ere allocated into three test groups, according to the time period between pretrial vaccination and challenge (1-2 mo, 2-4 mo, 4-6 mo), and an unchall enged control group. All birds in the test groups were challenged by oral, tracheal, and ocular routes with a field isolate of NDV. They were slaughte red over the next 4 wk on nine separate occasions and bled on 12 occasions. Virus isolation was attempted from seven sets of pooled samples from each bird to determine the viremia period and the serum antibody concentrations were measured by hemagglutination inhibition (HI) and enzyme-linked immunos orbent assay (ELISA) methods to establish an immune response curve. NDV cou ld be back-isolated only up to day 9 postinfection and from only six ostric hes with poor immune response titers and corresponding to a rise in antibod y levels above an indirect ELISA optical density reading of 0.33. Virus cou ld be recovered only from brain and respiratory tract tissue. The HI test w as less sensitive than the ELISA. Immune response curves did not differ sig nificantly between the groups and peaked on day 14 post-infection. From the se data, ELISA titers would appear to be a good indicator of the probabilit y that an ostrich will be clinically infected after velogenic NDV challenge . These results also suggest that the current vaccination schedule enforced by the South African Veterinary Authorities results in protective immunity in up to 95% of slaughter ostriches from export approved facilities. The s tandard 30-day preslaughter quarantine period introduced as part of Crimean -Congo hemorrhagic fever virus control measures also appears sufficient to encompass the determined NDV viremia period of 9-11 days in slaughter ostri ches.