Evaluation of Diagnostic Tools for Epidemiological Purposes — Application To FMD

Abstract

Since the implementation in 1988 of the foot-and-mouth disease (FMD) eradication programme in South America (PHEFA), significant advances have been made in the control of the disease. Particularly relevant has been the progress attained in one of the three subregions, the Southern Cone, where during the late 1990s most areas were recognized as FMD-free by OIE, with or without vaccination. To achieve PHEFA goals, development and implementation of diagnostic tools to trace for potential sources of infection was of utmost importance. Accordingly, priority was given to the identification of genetic links among circulating strains and, in relevant areas, to the systematic sero-epidemiological monitoring of persistent viral activity, regardless of vaccination condition. A genetic database of representative strains from South America, constructed at the Pan American Foot-and-Mouth Disease Center, constituted the basis for the phylogenetic analysis of viruses types O and A recorded recently in endemic regions, and during the emergencies in FMD-free areas of the Southern Cone. The genetic data placed all variants from the Southern Cone in a single lineage, which clearly differed from the evolutionary nodes that included the isolates in the Andean subregion, reflecting two independent production systems and livestock trade circuits. To complement the search for potential sources of infection, serosurveillance became an important adjunct. To this end, new tools to infer viral circulation within and between herds, irrespective of vaccination, were developed and validated. The diagnostic approach, based on an immunoenzymatic system that detects antibodies against non-capsid proteins, was particularly useful in support of epidemiological investigations after episodes and to confirm absence of viral activity in regions to be recognized as FMD-free, being especially important prior to the suspension of vaccination. Results of samplings in the Southern Cone during the 1990s strongly suggested that emergencies in this subregion did not originate from residual persistent activity in the areas declared FMD-free, but most probably from trade activities involving "hot spots" within the subregion.