During the 1974/75 lambing season numerous reports were received from various parts of the Republic of South Africa and South West Africa of severe abdominal distension in ewes after vaccination with the attenuated
Rift Valley fever and/or attenuated Wesselsbron disease
vaccine. The ewes were vaccinated at different stages of gestation in spite of recommendations to the contrary, the syndrome being especially obvious in ewes immunized with one or both of these
vaccines during the first trimester of pregnancy. In some of the flocks
hydrops amnii was recorded in as many as 15% of the ewes. Many of the ewes so affected showed a prolonged gestation of up to 6-7 months and, towards the end of gestation, were unable to rise or walk. They eventually died of
ketosis, hypostatic
pneumonia and complications due to
dystocia. The foetuses examined were malformed and larger than normal with a mass of 3,6-6,7 kg. They usually showed
arthrogryposis, brachygnathy inferior,
hydranencephaly, hypoplasia or segmental aplasia of the spinal cord and
neurogenic muscular atrophy. The amnion contained 8,0-18,0 1 of amniotic fluid, the endometrium was oedematous, and cystic tube-like dilatations, 1-10 mm in diameter, filled with a clear fluid, were scattered in the endometrium. No definite conclusions as to the aetiology of the syndrome could be drawn from serological tests performed on the ewes, lambs or foetuses. Preliminary experimental work confirmed previous observations that the attenuated Wesselsbron disease
vaccine virus is responsible for this syndrome and that the wild-type virus is also implicated. In addition, the attenuated
Rift Valley fever vaccine virus was found to the responsible for
arthrogryposis and
hydranencephaly without
hydrops amnii and for micrencephaly and
arthrogryposis associated with
hydrops amnii in the ewe.