ANTIBODY TO THE NONSTRUCTURAL PROTEINS OF FOOT-AND-MOUTH-DISEASE VIRUS IN VACCINATED ANIMALS EXPOSED TO INFECTION

Cattle which have been infected with foot-and-mouth disease (FMD) viru s can be differentiated from those that have been vaccinated on the ba sis of the detection of antibody to one or more of the non-structural (NS) proteins of the virus. Cattle which have been protected by vaccin ation can become persistently infected with FMD virus (FMDV) without e ver showing clinical signs. Vaccinated, protected cattle which are per sistently infected cannot be distinguished from animals that merely ha ve been vaccinated on the basis of serological tests for antibody to t he structural proteins of FMDV. Sera were collected from groups of cat tle for varying periods after exposure to infection under experimental conditions. On the basis of isolation of virus from probang samples c ollected during the course of the experiments it was possible to class ify the cattle according to the following criteria; naive, infected an d eliminated the virus (convalescent), infected and persistently infec ted with FMDV (carriers), vaccinated alone, vaccinated and either conv alescent or carrier. Sera were examined for antibody to the NS protein s Lb, 2C, 3A, 3D, and 3ABC by an indirect profiling ELISA using E. col i-expressed fusion proteins as antigens. Considerable variation was ob served in the antibody response to NS proteins of both naive and vacci nated animals following infection. The extent of individual variation was so great that convalescent animals could not be differentiated fro m carrier animals on the basis of their antibody response to any of th e NS proteins examined. The majority of vaccinated, protected animals showed an antibody response to NS proteins, particularly 3ABC, followi ng exposure to infection. However, the carrier state was demonstrated in some vaccinated, protected animals in which no antibody response to any of the NS proteins could be detected. The detection of antibody t o NS proteins can therefore be used on a group, or herd, basis to dete ct circulation of virus in a vaccinated population but further investi gations in the field are required to determine the sampling level nece ssary for statistical acceptance. On an individual animal basis, howev er, freedom from antibody to NS proteins in a vaccinated animal, or an animal of unknown history, does not necessarily imply that the animal is free from infection with FMD virus and, furthermore, the titre of antibody to NS proteins is not a useful predictive measure of whether or not an infected animal has successfully eliminated the virus.