Maternal immunity may affect vaccination success in young lambs. This study was aimed to determine the duration of the maternal immunity in lambs born from vaccinated ewe against Sheep pox. The level of neutralizing antibody against Sheeppox virus (SPPV) was measured in 14 lambs that were fed colostrum of their immunized mother (MCF lambs; n = 14) or reared with colostrum of ewes that were not immunized against Sheep pox, as the control (C lambs; n = 7), using virus neutralization index. The lambs were randomly divided into four experimental groups and after blood sampling, they were challenged by a virulent SPPV at different postbirth days, including 15 (2 CF and 1 C lambs), 30 (4 CF and 2 C lambs), 60 (4 CF and 2 C lambs), and 90 (4 CF and 2 C lambs) days of age. All the C lambs had no antibody titers against SPPV. Results of the challenge test showed that, up to 60 days of age, the MCF lambs had only localized scabs at the inoculation site and did not show any secondary pock lesions associated with Sheep pox; while all the C lambs were infected (p < 0.05). However, the percentage of lambs with Sheep pox signs was not different between C and MCF lambs after the challenge test at 90 days old lambs (p > 0.05), indicating decrease of maternal immunity. These findings revealed that the duration of maternal immunity against SPPV is ∼60 days, and vaccination is better to be performed at later ages.
Keywords: antibody titer; challenge; maternal immunity; vaccination.