Scale-Free Networks and Sexually Transmitted Diseases: A Description of Observed Patterns of Sexual Contacts in Britain and Zimbabwe.

Sexually Transmitted Diseases. 31(6):380-387, June 2004.

Background: Sexually transmitted infections spread through a network of contacts created by the formation of sexual partnerships. In physics, networks have been characterized as "scale-free" if they follow a power law with an exponent between 2 and 3.

Objective: The objective of this study was to test statistically whether distributions of numbers of sexual partners reported from different populations are well described by power laws.

Study Design: Power laws and an exponential null model are fitted by maximum likelihood techniques to reported distributions of numbers of partners. Data are taken from 4 population-based surveys, 3 from Britain and 1 from rural Zimbabwe.

Results: The networks can be described by power laws over a number of orders of magnitude. In addition, the derived exponents differ significantly and meaningfully, with an "accelerating network" formed between men who have sex with men (MSM).

Conclusions: A scale-free network approach provides a reasonable description of distributions of reported numbers of sexual partners.

Clients of Female Sex Workers in Lima, Peru: A Bridge Population for Sexually Transmitted Disease/HIV Transmission?.

Sexually Transmitted Diseases. 31(6):337-342, June 2004.

Objectives: The objectives of this study were to determine the prevalence of risk behaviors, gonorrhea, and chlamydia in clients of female sex workers, and to compare them with men selected from the general population.

Study Design: We conducted a cross-sectional study of men recruited from commercial sex venues in Lima, Peru from January to February 2002. Subjects answered a survey and provided a urine sample.

Results: Men reported that in the recently concluded commercial sex encounter, 95.8% used condoms, and 85.8% always used condoms with female sex workers. Only 16% reported always using condoms with their stable partners; 50.8% always used condoms with casual, noncommercial partners; and 59.6% always used condoms for homosexual anal sex. There were 8 (2%) cases of chlamydia and no cases of gonorrhea.

Conclusions: Clients of female sex workers report high rates of condom use with sex workers and a low prevalence of chlamydia and gonorrhea. It is unlikely that they constitute a bridging population.