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) virus can be differentiated from those that have been vaccinated on the basis of the detection of antibody to one or more of the non-structural (NS) proteins of the virus. Cattle which have been protected by vaccination can become persistently infected with FMD virus (FMDV) without ever showing clinical signs. Vaccinated, protected cattle which are persistently infected cannot be distinguished from animals that merely have been vaccinated on the basis of serological tests for antibody to the structural proteins of FMDV. Sera were collected from groups of cattle for varying periods after exposure to infection under experimental conditions. On the basis of isolation of virus from probang samples collected during the course of the experiments it was possible to classify the cattle according to the following criteria; naive, infected and eliminated the virus (convalescent), infected and persistently infected with FMDV (carriers), vaccinated alone, vaccinated and either convalescent or carrier. Sera were examined for antibody to the NS proteins Lb, 2C, 3A, 3D, and 3ABC by an indirect profiling ELISA using E. coli-expressed fusion proteins as antigens. Considerable variation was observed in the antibody response to NS proteins of both naive and vaccinated animals following infection. The extent of individual variation was so great that convalescent animals could not be differentiated from carrier animals on the basis of their antibody response to any of the NS proteins examined. The majority of vaccinated, protected animals showed an antibody response to NS proteins, particularly 3ABC, following 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 to NS proteins can therefore be used on a group, or herd, basis to detect circulation of virus in a vaccinated population but further investigations in the field are required to determine the sampling level necessary for statistical acceptance. On an individual animal basis, however, 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.