Antiviral drug resistance of herpes simplex virus

Abstract

Infections with herpes simplex virus (HSV) usually have an asymptomatic or benign course. However, severe infections do occur, particularly in HIV/AIDS patients or transplant recipients, and may be life-threatening unless adequate antiviral therapy is given. Since its introduction in the early 1980s, aciclovir (ACV) is the drug of choice for treatment of severe HSV infections and it has been increasingly used also for suppressive therapy and for prophylaxis in patients at risk for frequent and/or severe HSV infections. Nowadays, ACV use is even more widespread due to its 'over the counter' availability. Soon after its introduction, emergence of resistance of HSV to ACV was reported. In the past decades it has been shown that ACV-resistant HSV occur relatively frequently especially in immunocompromised patients and can be associated with serious disease. In contrast, ACV-resistant HSV isolates have been reported infrequently in immunocompetent subjects. However, the prevalence in immunocompetent population is less clear. Due to the effective control of HSV infections by host immune system the extensive spread of resistant virus in the population could occur before becoming apparent. In this thesis, antiviral drug resistance of HSV was investigated from a virological and clinical perspective. Focus was directed at the detection, prevalence and characterization of drug-resistant HSV. A real-time PCR was developed and validated for the detection of HSV-1 or HSV-2 infections. The assay was also applied as a sensitive and objective read-out system for HSV phenotypic antiviral susceptibility assay. A phenotypic assay (ELVIRA HSV) was developed for rapid screening of HSV clinical specimens for decreased susceptibility to ACV and it was successfully used in a large-scale survey of the prevalence of ACV-resistant HSV. A survey of resistance to ACV was performed on HSV strains isolated in The Netherlands between 1999 and 2002. A total of 542 isolates from 496 patients were examined; of these, 128 patients were immunocompromised. The prevalence of ACV resistance in the population at large was 0.27%, and thus did not differ from the level of naturally occurring resistance as observed in the era before introduction of ACV. In contrast, the prevalence of ACV-resistant HSV in immunocompromised patients was 7.0%. Our results indicate that 20 years of ACV usage, including over the counter sales, has not resulted in a measurable increase of HSV antiviral resistance in the Dutch population. However, the prevalence of 7.0% in immunocompromised patients indicates that antiviral resistance has become a considerable problem in this patient group. The ACV-resistant HSV strains were examined to determine the molecular changes underlying the emergence of antiviral resistance. A total of 26 HSV isolates were studied, and yielded 10 new mutations conferring resistance against ACV. This information is of crucial importance for our understanding of the emergence of antiviral resistance, for the development of rapid, sensitive genotypic methods for determination of antiviral resistance directly in patients' specimens and for the development of new antiviral drugs.