Vaccination can be a very effective measure to control the spread of foot-and-mouth disease (FMD) and to reduce reliance on preventive culling. However, vaccination may mask infection without completely preventing it and this leads to (i) a requirement for post-vaccination testing to demonstrate freedom from infection, and (ii) a prolonged waiting period before the FMD-free status can be regained for international trade purposes.
FMD vaccination elicits antibodies mainly against structural proteins whereas infection produces antibodies against both structural and non-structural proteins (NSP). Detection of anti-NSP antibodies therefore provides a basis to identify infected animals in vaccinated populations. We have validated available NSP ELISAs, finding that their sensitivity and specificity are insufficient to detect low levels of infected animals with a high degree of confidence (Paton et al., 2006). This is a problem, considering that recent modelling (Arnold et al., in press) has shown that the expected prevalence of carrier containing herds after reactive vaccination is indeed likely to be very low, ~0.2%, with only a small number of carriers, most likely one, in positive herds. Therefore, there is a need to develop and validate more sensitive tests or test combinations for detecting infection in vaccinated animals. Sensitivity for carrier detection can be optimised by adopting an individual-based testing regime in which all animals in all vaccinated herds are tested and positive animals rather than herds are culled (Arnold et al., in press). This may be achieved by simultaneously measuring the antibody response to a collection of different NSPs rather than a single NSP as is used in available tests.
To design testing strategies to confirm whether or not infection is present, further studies are also needed to improve our understanding of virus circulation and persistence following application of emergency vaccination