Introduction

The Aphthovirus genus, family Picornaviridae, is composed of seven distinct serotypes which cause foot-and-mouth disease (FMD), a severe disease of cloven-hoofed animals. FMD has serious economic consequences (Pereira, 1981) and is enzootic in most South American and African countries, as well as in regions of Asia and the Middle East. Foot-and-mouth disease virus (FMDV) usually causes a systemic acute infection with high morbidity and low mortality (Shanan, 1962). In ruminants, the virus may also produce an asymptomatic, persistent infection that involves limited viral amplification (Van Bekkum et al., 1959). This type of infection has been proposed as an epidemiologically important reservoir of FMDV (Hedger & Condy, 1985).

The general structure and molecular features of FMDV are, in general, similar to those of other picornaviruses (Domingo et al., 1990; Stanway, 1990). The capsid is composed of 4 proteins (VP1-4) and includes an RNA molecule of about 8500 nucleotides in length which encodes the structural proteins and at least 11 different, mature, non-structural polypeptides. The antigenic structure of FMDV includes continuous and discontinuous neutralizing epitopes located in exposed regions of the viral capsid, in which one or more of the capsid proteins, particularly VP1, are involved (Domingo et al., 1990).

Aphthoviruses show considerable antigenic diversity; 7 serotypes, more than 65 subtypes and a multitude of variants have been identified mainly by in vitro cross-serum neutralization (Pereira, 1981) and, more recently, by the use of monoclonal antibodies (MAbs) (Domingo et al., 1990). Immunization with viruses of one type does not confer protection against viruses of other serotypes, whereas cross-protection within serotypes is not always complete (Kitching et al., 1989).