Observations on antibody levels associated with active and passive immunity to African horse sickness.

Tests for neutralising (NT) antibodies to the nine serotypes of African horse sickness (AHS) virus on the sera of three groups of horses confirmed that an increasing number of immunisations with vaccine containing attenuated strains of serotypes 1 to 6 of the virus, leads to broader response to the various serotypes and to higher individual titres. Nevertheless some horses failed to respond to one or more serotypes despite receiving numerous immunisations and it was clear that vaccine containing only serotypes 1 to 6 could not be relied upon to induce adequate cross-immunity to serotypes 7 to 9 of the virus. Highest antibody titres and broadest cross-reactivity were recorded in a fourth group of horses which had apparently suffered natural infection recently. The levels of antibody acquired from colostrum by seven foals generally correlated well with the levels of antibody in the sera of their dams and the rate of decline of passively acquired antibody was proportional to initial titre. Antibodies to individual serotypes of virus declined to undetectable levels in two to four months from birth in some instances implying that susceptibility to infection could occur well before the age of six to nine months which is commonly recommended for initial immunisation. Vaccination of eight foals at three to four months of age resulted in weak antibody response but did not adversely affect pre-existing low levels of maternal antibody so that early immunisation could be recommended as a means for attempting to control the losses of foals experienced in Zimbabwe.