In vitro susceptibility study of herpes simplex virus to acyclovir and foscarnet. Are routine susceptibility studies necessary?

The objective of this study was to investigate the prevalence of resistance of herpes simplex virus to acyclovir and foscarnet. An in vitro susceptibility study of HSV strains isolated from HIV-infected and non-infected (control group) patients was conducted by means of qualitative screening. When the screening results were positive, the method for reducing cytopathic effect was utilized for calculating ID50. An ID50 < 1 microgram/ml indicated susceptibility to acyclovir, ID50 1-2 microgram/ml was intermediate susceptibility to acyclovir and a value of ID50 >/= 2 microgram/ml denoted resistance. Resistance to foscarnet was considered at ID50 >/= 100 microgram/ml. The study involved investigating 84 HSV strains, 49 HIV-infected patients, and 19 control patients. In the control group, no strains resistant to acyclovir were present and infection recurred in only one patient. In patients with HIV infection, one acyclovir resistant strain was detected and one moderately resistant to acyclovir, with good response to acyclovir treatment. In this group, 24.4% of patients presented recurrent infection. No resistance to foscarnet was detected. Percentage of HSV strains resistant to acyclovir is very low and resistance to foscarnet was not detected. These data suggest that routine in vitro susceptibility testing of antiviral drugs against HSV does not seem to be necessary.