Studies on non-bovine hosts for bovine ephemeral fever virus

Abstract

The ability of mouse-adapted strains of bovine ephemeral fever (BEF) virus to replicate in chicken embryos after intravenous inoculation was confirmed. Chick embryos died late during the incubation period or hatched with signs of neurological damage. BEF virus was demonstrated in the blood and brain of infected embryos and in the brain of chickens hatching from infected eggs. Unadapted BEF virus from cattle blood was also pathogenic for chicken embryos. Intravenous inoculation of BEF virus resulted in viraemia in one day-old chickens but not in 4 week-old or adult chickens. Chickens of the 3 age groups developed specific neutralising antibodies. Duck embryos were also susceptible to intravenous inoculation with BEF virus, and ducklings and ducks developed neutralising antibody in the absence of detectable viraemia. Strains of BEF virus adapted to chicken embryos resembled cell culture-adapted virus in physico-chemical properties. BEF virus present in the blood of an infected calf was used to inoculate chick embryos. The blood and brain of the embryos were infectious for experimental calves which developed typical BEF after intravenous inoculation. The pathogenicity of the virus for cattle had not been altered by adaptation to chicken embryos. The role of birds as hosts for BEF virus in nature was suggested as a topic worthy of study. Nineteen of 21 primary or secondary cell cultures failed to support the growth of BEF virus. Replication of the virus, without cytopathic changes, occurred in foetal rat fibroblasts and foetal rat skin. Foetal mouse fibroblasts were refractory. The cell lines Vero and BHK21 supported the growth of the virus, and cytopathic changes were produced. Organ cultures from chick embryos did not support the growth of the virus. Factors influencing the plaquing behaviour of BEF in Vero cells were studied. Plaque numbers were increased with increasing incubation time and increasing absorbtion time. Slight increases in plaque count were achieved under alkaline (pH 8.5) overlay medium, or medium containing gum tragacanth. The inclusion of DEAE-dextran in the overlay increased plaque size but not plaque count. The ultrastructure of BEF virus was studied in mouse brain and in Vero cell cultures. Most of the particles were cone-shaped and approximately 155 x 65 nm. Cross striation and surface spikes were demonstrated. Truncated particles were also observed.