Effect of chloroquine on African swine fever virus infection.

When present during the whole infective cycle, the lysosomotropic drug, chloroquine, inhibited cytopathic changes and production of African swine fever virus (ASFV) in Vero cells. This inhibition decreased when the drug was added from 1 h to 4 h after infection. Chloroquine had no effect on the virus nor on viral adsorption and internalization. Electron microscopy showed that, in the presence of the drug, the virions were retained in large vacuoles having a lysosomal appearance. This inhibition was fully reversible, even when the drug was removed as late as 72 h after infection. The results support the hypothesis that ASFV enters the cells by adsorptive endocytosis and not by fusion with the plasma membrane.