Across Africa, millions of men and women working on agricultural plantations represent a unique, understudied, and potentially vulnerable population. The parent study was conducted at one such plantation located in northern Tanzania, which employs about 3,800 people. One author (AHN) conducted an observational study in 2004 to assess how the specific context of life on an agricultural plantation influences sexual behavior and risks for several sexually transmitted infections (STIs), including HIV, syphilis and HSV-2 . The parent study enrolled agricultural plantation residents aged 18 years or older, who were able to give consent, and were randomly selected or volunteered in person at the study site . The team used a mobile research unit to administer a questionnaire and conduct STI/HIV testing. A total of 556 participants completed detailed questionaires; of these, 505 had complete data on number of sexual partners; 513 provided biological specimens for HSV-2 testing; and 410 had both complete questionnaire and HSV-2 data.
The survey was administered in Swahili. Many sensitive topics were addressed, and thus ACASI was utilized in an attempt to increase honesty in participant responses. Each participant heard the questions in a gender-matched voice. Most participants (82.0%) completed the ACASI survey independently, although individuals with limited literacy or inability to use the computer (16.0%) were assisted by a gender-matched interviewer who entered participants’ reponses on the computer. Mode of administration was missing for 2.0%; these respondents are excluded from multivariate models.
Correct information on participants’ lifetime number of sexual partners was thought to be of central importance to the study. Thus, in addition to the efforts described above to increase honest self-report, we asked about lifetime number of sexual partners two times, with the second question following immediately after the first question, prefaced with an explanation about the importance of the participants’ honest response. The survey included the following two questions, modified so women were asked about male partners and men about female partners:
Q1 Please, can you tell me how many women/men you have had sex with in your life? Include your current partner and all sexual partners you have had in your life, even if you were forced. (If you aren’t sure, please estimate).
Q2 Because this question is very important, I am going to ask you again. Please tell me honestly, without hiding anything, to the best of your ability. In your life, how many women/men have you had sex with, whether you agreed or you were forced? Include the person you had sex with first, your current partner, and all other sexual partners from your whole life.
Respondents who answered Q1 and then selected “not applicable” for Q2 (n = 78) were assumed to have already provided their most valid response to Q1, thus the analyzed value of Q2 for these individuals was the same as Q1. Number of partners was coded as an ordinal categorical variable, with values 0–9 corresponding to the reported number of partners, 10 corresponding to 10–14 partners, 11 corresponding to 15–19 partners, and 12 corresponding to ≥20 partners. (Categories 10, 11 and 12 were assigned to divergent groups based on the frequencies of the raw data, justified by relative sparsity in the higher categories).
HSV-2 testing was conducted on serum using HerpeSelect 2 ELISA IgG test (Focus Technologies, Cypress, CA).
Data analysis was performed using R (2.10.0 through 2.14.0). We used frequency statistics to evaluate whether respondents changed responses and in which direction (increased or decreased), and whether any variable (sex, age, mode of survey administration, HSV-2 status) was associated with changing responses. We used Fisher’s exact test because of small expected cell sizes. Statistical signficance was set at α = 0.05.
We used separate multivariable logistic regression models to examine the association between self-reported number of partners and HSV-2: Model A used Q1 as the independent variable, Model B used Q2, and Model C used the maximum of Q1 and Q2. Number of partners satisfied the assumption of linearity in the log odds of HSV-2 positivity. Because of biologically plausible and statistically meaningful (based on Akaike information criterion, see below) differences between men and women, for each model we included a product-interaction term between sex and number of partners. We first computed unadjusted odds ratios (ORs) and 95% confidence intervals (CI) for the effect of increasing lifetime partners on HSV-2 seropositivity, seperately for men and women. We then computed estimates adjusting for respondent age and mode of survey administration. Precision was assessed using the confidence limit ratio (CLR: ratio of upper to lower bounds of the 95% CI). Models were compared using the Akaike information criterion (AIC) and area under the receiver operating characterictic curve (AUC).