Bovine ephemeral fever in Taiwan

Bovine ephemeral fever (BEF) is a vector-borne disease of cattle, spanning tropical and subtropical zones of Asia, Australia, and Africa, caused by Ep hemerovirus of the Rhabdoviridae. Taiwan has had 3 BEF epizootics, occurrin g in 1989, 1996, and 1999, since the vaccination regimen was initiated in 1 984, given once a year in the spring with a single-dose formaldehyde-inacti vated vaccine using the 1983 isolate as the seed virus. This study evaluate d the 1999 population immunity against BEF virus in Taiwanese dairy cows wi th a neutralization test and whether the recent BEF virus isolates have mut ated significantly from the vaccine virus. In March 1999, before vaccinatio n, 94% of the animals studied were already seropositive, suggestive of an e ndemic or persistent infection from the previous year. By June 1999, when 5 1% of herds had been vaccinated, the antibody level rose, and by September 1999, the serum-neutralizing antibody (SNA) level fell to a minimum, preced ing the outbreak of BEF in October 1999, during which the antibody levels o f vaccinated cows continued to decline while those of unvaccinated cows ros e sharply. The results suggest that, in 1999, vaccine-induced immunity was partially protective against BEE Because the current single-dose vaccinatio n regimen resulted in minimal population immunity by September, a booster v accination given in late summer may be advisable for future disease control . Analysis of the glycoprotein gene of Taiwanese isolates between 1983 and 1999 showed a 97.4-99.6% homology, with an alteration of 4 amino acids in a ntigenic sites Gl, G3b, and G3c. Phylogenetic analysis of Taiwanese isolate s revealed at least 2 distinct clusters: the 1983-1989 isolates and the 199 6-1999 isolates. Both were distinct from 2 Japanese strains and the Austral ian BB7721 strain. Thus, at least 2 distinct BEF viruses, which had diverge d before 1983, existed in Taiwanese dairy cows.