Arch . Inst . Razi , 1966 , 18 . 101 - 111 MONOV ALENT LIVE . . VIRUS HORSE - SICKNESS VACCINE ( * )

Since ALEXANDER (1) reported the development of polyvalent mouse neu· rotropic horse-sickness vaccine, it has been widely used for many years in Africa and Asia. Vaccine prepared from 6 or 7 different African types of horse-sickness virus have been used in the Middle East and Asia since 1960. These vaccines provided protection for equines against the local strains, serologically resembling type 6 but later classified as a separate type (No. 9). There were two main reasons for using polyvalent vaccines in the Middle East and Asia during epizootics since 1959. Firstly no single attenuated strain capable of immunizing equines against local strains was available when severe outbreaks occurred. Secondly it was not known how many different types of horse-sickness virus were involved. However, recent investigations by the Authors (3) and SHAH (19) showed that ail the strains isolated from natural cases of horse-sickness in the Middle East and Asia were serologically idential with type 9 virus. SeriaI intracerebral passages of an lranian strain of horse-sickness virus in Swiss albino mice were carried out at the Razi lnstitute. After 100 passages, this strain, S2, was found to be fully attenuated for equines while maintaining its immunizing properties (2). The S2 strain was successfully adapted to monkey kidney stable (MS) ceIls by OZAWA and HAZRATI (13) and standard procedures for preparing horsesickness tissue-culture vaccines have been established (14, 15). The immunizing properties of tissue-culture adapted viruses have been confirmed by MIRCHAMSY and TASLIMI (8,9) and by OZAWA and al. (15). This paper extends previous observations (15) and presents the results of stabilit y tests of these vaccines under various conditions and results of comparative