Long-term Studies of the Immunity in East African Cattle Following Inoculation with Rinderpest Culture Vaccine

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Virus-neutralising antibody was investigated over periods of 24 to 50 months in East African cattle which had been inoculated with culture-attenuated rinderpest virus of 91 or more passages in calf kidney monolayers. The cattle were of 3 types, viz: Friesian or fersey grades, Boran (shorthorn zebu) and Ankole (longhorn). Grade cattle showed a peak mean antibody titre of 102.17 at 4 weeks, but a considerable decline occurred between 12 and 18 months after inoculation; in the following 3 years little additional change took place. A few animals had very low titre antibody after 6 to 12 months, but this was probably associated with the presence of minimal quantities of passively-acquired antibody at the time of vaccination. When 3 such cattle were challenged after 27 and 37 months, 2 developed inapparent reinfection, with a delayed serological response, beginning on the fifth day. Two other animals showed a rapid anamnestic response reaching a peak within 48 hr.; one of the latter probably injected arinderpest-susceptible ox which was housed with it during the post-challenge period. Four grade cattle which were challenged after 50 months showed no clinical or serological reaction. Boran cattle produced a high mean antibody level 0f about 102.0 which persisted until the end of the second year, while any fall thereafter was very slow. Of the individual animals, only 26% showed a decline > 1 log10 unit in the first 3 to 4 years, compared with 46% among the grades. Not one of 56 vaccinated animals was serologically negative at the end of 3 1/2 years. In the Ankoles, no significant decline in mean antibody titre occurred during the first 2 years of the experiment but, as in other groups, there was great variation in the peak titres attained in different individuals (100·6 to 102·8). The rate of decline of antibody and the resistance to challenge of animals with very low titres makes it probable that rinderpest culture vaccine confers a lifelong immunity. Nevertheless, "immune" cattle with very low levels of circulating antibody may contract inapparent reinfection, thus becoming transitory "carriers" of virulent rinderpest virus.