A three-part epidemiological investigation was made on flaviviruses:
1. As a preliminary to tests on cattle sera from the field, the antigenic cross-reactivity of Wesselsbron, Spondweni, Usutu, Banzi, West Nile and yellow fever flaviviruses was studied in antisera prepared in guinea pigs. As described earlier for flaviviruses, sera were found to be highly cross-reactive in haemagglutination-inhibition (HAI) tests, less cross-reactive in complement-fixation (CF) tests and were virtually monospecific in microneutralization (NT) tests in Vero cell cultures.
2. Infection with Wesselsbron (WSL) virus produced mild febrile illness and viraemia in 5 out of 6 newborn calves, 3 out of 4 pregnant heifers and 3 out of 4 ewes. One heifer produced a weak calf which died soon after birth with WSL antibodies in its serum, indicating that infection had occurred in utero. The 3 other heifers produced healthy calves which lacked antibody in pre-colostral serum. Pathological changes occurred in the foetus in 2 out of 3 pregnant ewes and the ewe produced a healthy lamb which had antibodies to WSL virus in pre-colostral serum.
Unlike the situation in guinea pigs, cattle sera were monospecific for WSL virus in CF tests, but sheep sera cross-reacted with Banzi and yellow fever viruses. Re-infection of the cattle with Banzi, West Nile, Spondweni and Usutu viruses failed to induce marked antibody responses. The results suggest that antibodies to WSL virus in cattle sera from the field can be distinguished from those induced by other flaviviruses by quantitative serologial tests.
3. HAI antibodies to WSL virus were detected in 2648/14395 cattle sera tested over 11 years from 1967 to 1978 in the course of investigation of abortion, infertility and other diseases. Results of quantitative HAI, CF and NT tests with six flaviviruses on 409 selected sera confirmed that infection was due to WSL virus. Serological evidence failed to implicate WSL virus as a cause of abortion in cattle. In a prospective study, abortion occurred in only one out of 21 heifers observed to gain WSL infection during pregnancy in the field, but abortion also occurred in five out of 207 heifers which did not become infected with WSL. No histopathological lesions diagnostic of WSL disease were observed in 1998 specimens from cattle, sheep and goats examined over 44 months prior to October 1972, and WSL virus was isolated once, from the organs of a cow, out of 2106 specimens from cattle sheep and goats tested virologically over six years from October 1972 to September 1978. HAI antibodies to WSL virus were detected in one out of 374 sera from aborted cattle foetuses. It was concluded that WSL virus is not an important cause of disease in cattle, despite widespread occurrence of infection.