Increase in rates of herpes simplex virus type 1 as a cause of anogenital herpes in western Sydney, Australia, between 1979 and 2003

Backgound/objective: Recent studies suggest that herpes simplex virus type 1 (HSV-1) is becoming more common as a cause for genital herpes, relative to HSV-2. We aimed to calculate trends in HSV type from isolates and serology samples sent to a reference virology laboratory in New South Wales (NSW), Australia. Methods: We compared the proportions of HSV-1 and HSV-2 positive samples, adjusting for age and sex of source patient, in three datasets: anogenital isolates from 1979 to 1988; anogenital isolates from 1989 to 2003; and HSV type specific IgM seropositivity from 1994 to 2003. Results: The number of specimens in each analysis was 17 512, 4359, and 497, respectively. There was a progressive rise in the proportions of typed specimens being HSV-1 in all analyses. The proportion of isolates that were HSV-1 ranged from 3% in 1980 to 41% in 2001. Female sex and age under 25 were associated with a greater proportion of HSV-1 isolates in both time periods. In the period 1979 - 88, comparing the proportions of HSV-1 and HSV-2 gave an odds ratio (OR) per additional year of 1.24 (95% confidence interval (CI) 1.20 to 1.27; p, 0.005) after adjustment for age and sex. In the period 1989 - 2003 there was a steeper rise in the proportion of isolates that were HSV-1 in samples from younger individuals (OR per year 1.17, 1.12 to 1.22) compared to those over 25 (OR per year 1.06, 1.03 to 1.08). The rise in the proportion of IgM seropositive results reactive for HSV-1 compared to HSV-2 gave an OR of 1.36 per year (1.26 to 1.47; p, 0.005). Conclusions: These data suggest that HSV-1 has become more common as a cause of anogenital herpes in NSW.