The incidence and clinical significance of herpes simplex virus type 1 (HSV-1) acyclovir resistance were determined in patients with recurrent herpetic keratitis (RHK). Sequential corneal isolates (n = 39) from 15 immunocompetent patients with RHK were assayed for acyclovir susceptibility and genotyped by analyzing the hypervariable regions of the HSV-1 genes US1 and US12. The thymidine kinase (TK) gene of each isolate was sequenced, and the proportion of acyclovir-resistant viruses within isolates was determined. Uniform acyclovir-resistant or acyclovir-sensitive sequential isolates were identified in 4 and 2 patients, respectively. Notably, the acyclovir susceptibility of sequential isolates changed from acyclovir sensitive to acyclovir resistant (5 patients) or from acyclovir resistant to acyclovir sensitive (3 patients). The acyclovir-resistant phenotype of the isolates correlated with the patient's unresponsiveness to acyclovir therapy. Combined analyses of the TK gene and genotype of sequential isolates showed that acyclovir-sensitive isolates contained multiple acyclovir-resistant variants of the same virus and that an identical acyclovir-resistant HSV-1 strain reappeared in the patient's cornea during RHK episodes. Corneal HSV-1 isolates are mixtures of acyclovir-sensitive and acyclovir-resistant viruses that share the same genotype but have different TK sequences. Recovery of the same acyclovir-resistant virus during consecutive herpetic keratitis episodes suggests that acyclovir-resistant HSV-1 establishes latency and reactivates intermittently to cause acyclovir-refractory RHK.
