RIFT VALLEY FEVER VIRUS HEPATITIS LIGHT AND ELECTRON MICROSCOPIC STUDIES IN THE MOUSE

Rift Valley fever virus (RVFV) infects a wide range of species in- cluding man.'Total hepatic necrosis is seen in the highly susceptible species, e.g., the lamb, mouse and hamster.2Under natural conditions or when these animals are inoculated with minimal doses, the lesions are initially focal, gradually enlarge and increase in number, and finally, be- tween 6o and go hours, all of the hepatic parenchymal cells become ne- crotic in a precipitous manner.Mims showed that by intravenous inocu- lation with very large doses of RVFV in mice a synchronous cycle of infection and necrosis occurred within io hours.3We chose to study RVFV in a similar system in order to minimize sampling errors and to determine whether the cells actually were infected, where the virus was produced and the significance of the nuclear inclusion. MATERIAL AND METHODSThe source, production and assay of RVFV were as previously described.4Thirty- SiX IO-to I2-gm.Swiss-Webster mice were given intravenous inoculations of o.5 ml. of lamb serum containing I X 09.7 mouse intraperitoneal median lethal doses (MIPLD50) of RVFV.Fourteen mice were inoculated with o.s ml. of normal lamb serum.Two of the infected and one of the control mice were killed at time zero and then at hourly intervals for 12 hours.The remaining infected mice were observed until they died.Samples of blood, liver and lung from each mouse were assayed for virus.Portions of the liver were frozen, and the remainder was fixed in cold buffered formalin, Carnoy's fixative (6:3: i), absolute methanol, acetone or i per cent osmium tetroxide in WVhite's saline at pH 7.2.The frozen and acetone-fixed tissues were used in fluorescent antibody studies.The osmium-fixed tissues were dehydrated in ethanol and propylene oxide, and embedded in Epon 8I2.They were then cut with glass knives on a Porter-Blum microtome, stained with uranyl acetate and examined with an RCA EMU-3F microscope at 5o KV.The remaining tissues were processed through paraffin, cut at 4 ,u and stained with hematoxylin and eosin, methyl green pyronine, and with the periodic acid-Schiff method with and without prior diastase digestion, the Feulgen method, and the method for demonstrating the Millon re-