Experimental infection of three Nigerian breeds of sheep with the Zinga strain of the Rift Valley Fever virus

Abstract

Experimental infection of three indigenous breeds of sheep in Nigeria, namely the West African Dwarf (WAD), Yankasa and Ouda resulted in fatal disease with the Zinga Rift Valley Fever virus. Infected sheep of the three breeds responded by pyrexia within 24 h of infection, that lasted 6 to 7 days, but peaked between day 2 and 4 post-infection. Viraemia coincided with pyrexia and peaked (109 PFU/ml) 3 days p.i. in Yankasa and WAD sheep, but with highest titre (107.5 PFU/ml) in Ouda sheep. Zinga Rift Valley Fever virus infection of sheep was characterised by hyperactivity, watery and mucoid nasal discharges, projectiles and bloody diarrhoea, external haemorrhage and clinical manifestations of nervous disorders. Viraemia was followed by low level of antibody development in all the infected sheep. Haemotological changes included a Sharp fall in the PCV, Hb concentration and total RBC Count during the course of the disease. These changes were most severe in the Yankasa, followed by WAD and Ouda breeds. There were thrombocytopaenia, prolongation of prothrombin and clotting times in all the infected sheep. There was also progressive leucopaenia associated with lymphopaenia. The total protein and albumin levels were depressed, but the globulin level rose from day 5 p.i. The changes in the serum biochemical constituents included Sharp and progressive increase in the level of alanine aminotransferase and aspartate aminotransferase. The sodium level decreased gradually while that of potassium was initially stable but later increased until the infected animals died. There was a significant increase in the level of blood urea nitrogen from day 3 p.i. that continued until the infected animals died. Cross and microscopic examinations of the carcasses of the infected sheep showed significant lesions in many organs, including disseminated intravascular coagulation.