Significance of resistance of herpes simplex virus to acyclovir

The genome of herpes simplex virus codes for several enzymes, including viral thymidine kinase and viral deoxyribonucleic acid (DNA) polymerase. When viral resistance develops, it does so by changes in these two enzymes. Three possible mechanisms of viral resistance to acyclovir include (1) selection of viral mutants that make little or no thymidine kinase and do not phosphorylate acyclovir adequately, (2) selection of mutants that can phosphorylate thymidine but cannot phosphorylate acyclovir (i.e., these viruses have thymidine kinases with altered substrate specificity), and (3) selection of viruses that have altered DNA polymerases that replicate viral DNA in the presence of acyclovir triphosphate. Thymidine kinase-deficient virus has been isolated from clinical isolates frequently, but few strains appear to be virulent for animals or humans and only a few seem to have caused clinical disease. Viruses with altered substrate specificity have been reported but viruses with an altered DNA polymerase have not occurred in clinical practice. Antiviral drugs should be used only when necessary to minimize the appearance of resistant strains of virus.