Immunological types of horsesickness virus and their significance in immunization

Abstract

1. A brief review is given of the epizootiology of horse sickness, the antigenic plurality of the virus strains, and the history of the control of the disease by immunization. 2. It is often not possible to isolate virus directly in mice from cases of horse sickness in immunized horses. 3. The usefulness of the ferret, and possibly the dog, for the isolation of virus from such horses was demonstrated. 4. Immunological studies on 42 mouse-adapted horsesickness virus strains were conducted. 5. For these studies an intracerebral neutralization test in mice was used. Hyperimmune sera from rabbits were mainly used in the tests. 6. Cross-neutralization tests with rabbit antisera were carried out on the eight virus strains included in the present vaccine issued from Onderstepoort and 16 strains recently isolated from cases of horsesickness in immunized horses. 7. This series of tests showed that the virus strains could be grouped into seven immunological types. It was also evident that some immunized horses become infected with strains of the same immunological type as the vaccine strains. 8. Using type rabbit antisera a further series of neutralization tests was conducted on 18 other horsesickness virus strains. These tests showed that these strains also could be grouped within the same immunological types. 9. It was shown that four recently isolated strains belonging to the same immunological type were not represented in the present Onderstepoort vaccine. 10. A limited number of neutralization tests with ferret antisera supported the antigenic grouping as revealed by the tests with rabbit antisera. 11. A serological study of sera obtained from eight immunized horses reacting to horsesickness was made. 12. Viruses isolated from each of these horses were included in the cross-neutralization tests which showed that seven of these viruses are of the same immunological type as the vaccine strains. 13. High level antibody against all the vaccine strains was shown to be present in sera from seven horses. 14. It was concluded that these particular failures in immunity were not due to inadequate immunization but were apparently the result of slight antigenic differences between the infecting virus and the vaccine strains. 15. The significance of the antigenic grouping based on the mouse neutralization test to immunity in equidae is discussed. 16. It is believed that this antigenic grouping has a definite relationship to immunity in these animals.