Acyclovir resistance in herpes simplex viruses: Prevalence and therapeutic alternatives

Herpes simplex viruses (HSV), the causative agents of recurrent orofacial and anogenital infections, can cause significant morbidity and mortality in both immunocompetent and immunocompromised individuals. In immunocompromised patients, HSV tends to be more persistent with chance of dissemination. The nucleoside analogue acyclovir has drastically improved the management of HSV infections although acyclovir resistant strains have been reported in the clinic. We performed a systematic search to summarize the prevalence data reported in both the immunocompetent and immunocompromised populations. Defining the global prevalence of acyclovir resistance in HSV infections is hampered by the high variability in methodology, patient selection, study design, and treatment history among the studies. Acyclovir resistant HSV is infrequent in the immunocompetent population (generally below 1%), irrespective of treatment history. Exceptions are infections at immune-privileged sites such as the cornea, where frequent recurrences and extensive acyclovir therapy favor the emergence of acyclovir resistance. Higher frequencies of acyclovir resistant HSV infections are reported among immunocompromised individuals, with the highest prevalence seen among hematopoietic stem cell transplant recipients. All antivirals approved for the treatment of HSV infections have the same target, i.e. the viral DNA polymerase, and cross-resistance to different antivirals has been described, complicating therapy of acyclovir resistant strains. In this review we will discuss acyclovir mode of action, mechanisms of resistance, prevalence of resistance, and alternative antiviral treatments for acyclovir resistant HSV infections.