Antibody Responses to Foot-and-Mouth Disease Virus VIA Antigen Monitored During a Field Vaccination Trial

Sera was collected from village livestock during the vaccination trial in 21 northern Thai villages. Animals from the project database were divided into two groups. Those with 10glO serum neutralisation test (SNT) titres of < 1.2 to all three serotypes (A, 0, and Asia I) were classified as seronegative and those with a higher titre to at least one were classed as seropositive. At the start of the trial there were no rea ... 'tors among the seronegative animals and there was an increase to about 10070 one month after the first vaccination. Most reactions disappeared by six months after vaccination but returned after revaccination. Vaccination with the new production batch of the same vaccine provoked a substantial reactor prevalence. The seropositive group reactor rate rose from 30070 to 70070 after one vaccination, and many animals were still seropositive six months after vaccination. THE antibody response to the foot-and-mouth disease (FMD) virus non-structural protein called the virus infection-associated (VIA) antigen has been used to detect recent infections with FMD virus (McVicar and Sutmoller 1970). Although it was initially thought this antigen would prove to be a very useful diagnostic tool to discriminate immune responses induced by vaccination from those induced by infection, animals that have received multiple vaccinations respond serologically to VIA antigen present in inactivated vaccines (Dawe and Pinto 1978; Pinto and Garland 1979). The VIA agar gel diffusion test (AGDT) is still used to gather epidemiological information about virus activity in the field, although recently a specific and sensitive liquid phase blocking enzyme-linked immunosorbent assay (ELlSA) has been described (Alonso et al. 1990). While VIA AGDT status is very useful in situations where vaccine is not used, the results must be interpreted with caution where vaccine is in use. Widespread application of a new trivalent vaccine produced by the Department of Livestock Development is a key strategy of a program to control and * Northern Veterinary Research and Diagnostic Center, Hang Chat, Lampang 52190, Thailand. t CSIRO Australian Animal Health Laboratory, PO Bag 24, Geelong, Victoria 3220, Australia. 45 eradicate FMD in Thailand. This paper briefly describes the VIA AGDT reactor prevalence profile of a group of animals monitored during a longitudinal study of the responses of village livestock in Northern Thailand to the trivalent vaccine. Materials and Methods