The history of foot-and-mouth disease virus serotype C: the first known extinct serotype?

Abstract

Foot-and-mouth disease (FMD) is a highly contagious animal disease caused by an RNA virus subdivided into seven serotypes that are unevenly distributed in Asia, Africa, and South America. Despite the challenges of controlling FMD, since 1996 there have been only two outbreaks attributed to serotype C, in Brazil and in Kenya, in 2004. This article describes the historical distribution and origins of serotype C and its disappearance. The serotype was first described in Europe in the 1920s, where it mainly affected pigs and cattle but as a less common cause of outbreaks than serotypes O and A. No serotype C outbreaks have been reported in Europe since vaccination stopped in 1990. FMD virus is presumed to have been introduced into South America from Europe in the nineteenth century, although whether serotype C evolved there or in Europe is not known. As in Europe, this serotype was less widely distributed and caused fewer outbreaks than serotypes O and A. Since 1994, serotype C had not been reported from South America until four small outbreaks were detected in the Amazon region in 2004. Elsewhere, serotype C was introduced to Asia, in the 1950s to the 1970s, persisting and evolving for several decades in the Indian subcontinent and for eighteen years in the Philippines. Serotype C virus also circulated in East Africa between 1957 and 2004. Many serotype C viruses from European and Kenyan outbreaks were closely related to vaccine strains, including the most recently recovered Kenyan isolate from 2004. International surveillance has not confirmed any serotype C cases, worldwide, for over 15 years, despite more than 2,000 clinical submissions per year to reference laboratories. Serology provides limited evidence for absence of this serotype, as unequivocal interpretation is hampered by incomplete intra-serotype specificity of immunoassays and the continued use of this serotype in vaccines. It is recommended to continue strengthening surveillance in regions of FMD endemicity, to stop vaccination against serotype C and to reduce working with the virus in laboratories, since inadvertent escape of virus during such activities is now the biggest risk for its reappearance in the field.

Keywords: extinction; foot-and-mouth; phylogeny; serotype C.

Figure 1.

Geographic and temporal distribution of historical FMDV serotype C events. The map provides the most likely timing and spatial directionality of the historical events attributed to serotype C by examining historical information on outbreaks, the genetic relationship between sampled isolates and the results generated by the phylogeographic analysis (Fig. 2, Table 1). Countries are coloured according to the last year when a serotype C case was reported. Arrows show the route of virus movements between continents for each of the topotypes, along with the suggested year of introduction. Geographic locations denoted in red are those of the 2004 outbreaks reported in Brazil and Kenya.

Figure 2.

Space-time evolution of the FMDV serotype C based on sequences of the VP1 capsid encoding region generated from 149 isolates. Time-calibrated genetic relationships between VP1 sequences, with the 95% Bayesian credible interval (BCI) region of node timing represented in light blue; branches and nodes are coloured according to the most probable ancestral continent inferred from the discrete phylogeography analysis. Topotypes with their subtypes are denoted (green panels). The viruses labelled as C/ETH/6/2005 and C/ETH/7/2005 were submitted to the WRLFMD in 2005 from samples collected in 1983.

Figure 3.

Evolutionary signature of the FMDV serotype C generated using 149 partial FMDV genome sequences encoding the VP1 capsid region. Left panel, maximum-likelihood topology where internal nodes with bootstrap values of ≥75 are coloured in red; right panel, matrix of pairwise evolutionary distances. Topotypes with their subtypes are denoted (green panels). The viruses labelled as C/ETH/6/2005 and C/ETH/7/2005 were submitted to the WRLFMD in 2005 from samples collected in 1983. There are two slightly different viruses called C₃/Indaial/Brazil/1971, which differ in cell culture passage history and by about 4% in VP1 (Ouldridge, July 1988, pers. comm.).