- Volume 83, Issue 3, 2002

The competence of a Frankliniella occidentalis and a Thrips tabaci population to transmit Tomato spotted wilt virus (TSWV) was analysed. Adults of the F. occidentalis population transmitted this virus efficiently, whereas those of the thelytokous T. tabaci population failed to transmit. TSWV replicated in the midgut of the larvae of both populations after ingestion of virus; however, lower amounts accumulated in T. tabaci larvae than in F. occidentalis larvae. The virus was almost undetectable in T. tabaci adults, whereas high titres were readily detected in the F. occidentalis adults. The first infections in F. occidentalis larvae were detected by immunocytochemical studies in midgut epithelial and subsequently in midgut muscle cells, the ligaments, and finally in the salivary glands. The infections were weaker in the midgut epithelial and muscle cells of T. tabaci larvae, followed by an almost complete absence of any infection in the ligaments, and a complete absence in the salivary glands. Studies by electron microscopy revealed the budding of some virus particles from the basal membrane of midgut epithelial cells of F. occidentalis larvae into the extracellular space of the basal labyrinth. Enveloped virus particles were also seen in midgut muscle cells of F. occidentalis larvae. They were not discerned in epithelial and muscle cells of T. tabaci larvae and adults. This study showed that the rate of virus replication in the midgut and the extent of virus migration from the midgut to the visceral muscle cells and the salivary glands are probably crucial factors in the determination of vector competence.

Scrapie was transmitted to mice from ten sheep, collected in the UK between 1985 and 1994. As in previous natural scrapie transmissions, the results varied between scrapie sources in terms of the incidence of disease, incubation periods and neuropathology in challenged mice. This contrasted with the uniformity seen in transmissions of BSE to mice. The scrapie and BSE isolates were characterized further by serial passage in mice. Different TSE strains were isolated from each source according to the Sinc or PrP genotype of the mouse used for passage. The same two mouse-passaged strains, 301C and 301V, were isolated from each of three BSE sources. Despite the variation seen in the primary transmissions of scrapie, relatively few mouse-passaged scrapie strains were isolated and these were distinct from the BSE-derived strains. The ME7 scrapie strain, which has often been isolated from independent sheep sources in the past, was identified in isolates from four of the sheep. However, a new distinct strain, 221C, was derived from a further four scrapie sheep. These results suggest that there is agent strain variation in natural scrapie in sheep and that the spectrum of strains present may have changed over the last 20 years. The tested sample is too small to come to any conclusions about whether the BSE strain is present in sheep, but the study provides a framework for further more extensive studies.