Quantitative analysis of foot-and-mouth disease virus RNA loads in bovine tissues: implications for the site of viral persistence

To understand better the pathogenesis of foot-and-mouth disease (FMD), the levels of viral RNA in various bovine tissues during the acute and persistent stages of FMD virus (FMDV) infection were investigated by using quantitative RT-PCR. The viral RNA levels in the tissues examined had peaked by day 1 post-infection (p.i.) and were markedly different among the tissues examined. The epithelium collected from sites of lesion development, i.e. the interdigital area and coronary band on the feet, and the tongue, contained the highest level of viral RNA, indicating the predominant tissue sites of viral infection and amplification during the acute stage of infection. Clearance of viral RNA from most of the tissues occurred relatively rapidly and the rate of clearance was largely independent of the level of viral RNA. The viral RNA load in most of the tissues declined slower than in serum, in which viral clearance is rapid. Beyond 28 days p.i., a proportion of pharyngeal region tissues (soft palate, pharynx, tonsil and mandibular lymph node) from infected animals still contained a detectable level of viral RNA, while viral RNA in non-pharyngeal region tissues was generally only detectable for variable periods ranging from 4 to 14 days p.i. The presence of viral RNA in dorsal soft palate tissue had a good correlation with the presence of infectious virus in oesophageal-pharyngeal fluid (OP-fluid) samples, a finding indicative of the specific tissue sites of FMDV persistence.