Individual and population level effects of partner notification for Chlamydia trachomatis

Abstract

Partner notification (PN or contact tracing) is an important aspect of treating bacterial sexually transmitted infections (STIs), such as Chlamydia trachomatis. It facilitates the identification of new infected cases that can be treated through individual case management. PN also acts indirectly by limiting onward transmission in the general population. However, the impact of PN, both at the level of individuals and the population, remains unclear. Since it is difficult to study the effects of PN empirically, mathematical and computational models are useful tools for investigating its potential as a public health intervention. To this end, we developed an individual-based modeling framework called Rstisim. It allows the implementation of different models of STI transmission with various levels of complexity and the reconstruction of the complete dynamic sexual partnership network over any time period. A key feature of this framework is that we can trace an individual's partnership history in detail and investigate the outcome of different PN strategies for C. trachomatis. For individual case management, the results suggest that notifying three or more partners from the preceding 18 months yields substantial numbers of new cases. In contrast, the successful treatment of current partners is most important for preventing re-infection of index cases and reducing further transmission of C. trachomatis at the population level. The findings of this study demonstrate the difference between individual and population level outcomes of public health interventions for STIs.

Figure 1. Sexual partnership networks from the three individual-based models.

The ‘instantaneous contact’ model has no connectivity at cross-section but exhibits variation and connected components of different size within a period of one year. The ‘pair model’ and ‘triple model’ illustrate the connectivity at cross-section and larger connected components during a period of one year. Note that in all three models, the average number of new partnerships formed within one year is equal. For illustrative purposes, the population size was limited to 100, resulting in higher connected networks compared to larger population sizes. Different sexes are indicated by filled and empty circles.

Figure 2. Partner notification strategies.

An illustrative example of an individual’s history of contacts or partnerships is shown for each model. One strategy is to notify partners of an index case in order of their recency (time since their partnership ended). Another strategy is to notify all partners from a certain time period.

Figure 3. Simulated and empirical data of C. trachomatis-positivity in partners of index cases.

Black crosses correspond to published data of the proportion of positive partners out of those with a positive test results together with the 95% CI . The other symbols represent simulated data for each of the three different models. In the simulations, it is assumed that the steady-state prevalence of C. trachomatis is 3%. Means of 100 simulation runs are shown. Standard errors are small and omitted for better visibility.

Figure 4. Proportion of C. trachomatis-positive partners of index cases.

The proportion of partners of an index case who are infected with C. trachomatis is shown at a steady-state prevalence of 3% (dashed line). (A) The proportion of infected partners in order of their recency. (B) The proportion of infected partners in order of their breakup date. For each strategy, means of 100 simulation runs are shown. Standard errors are small and omitted for better visibility.

Figure 5. Representative time plots of the prevalence of C. trachomatis after the start of a screening intervention.

The dots indicate the prevalence at the beginning (dotted line) and after 5 years of the screening intervention. The dashed line indicates the steady-state prevalence of 3% in absence of screening. Every individual receives screening at a rate of 0.1 per year and no partner notification is performed. The colored lines represent five individual simulation runs.

Figure 6. Population level effect of partner notification.

The reduction in the prevalence of C. trachomatis is given after screening the population for 5 years at a rate of 0.1 per year. (A) The prevalence of C. trachomatis for increasing numbers of notified partners, in order of their recency. (B) The prevalence of C. trachomatis for different partner notification periods. There is a 50% probability that each notified partner will be tested and successfully treated. For each strategy, means of 100 simulation runs are shown. Standard errors are small and omitted for better visibility.

Figure 7. Schematic depiction of the formation and dissolution of a triple.

Top: An infected single (unconnected white circle) forms a new pair with a susceptible of the opposite sex (black circle in existing pair) which results in a triple indicating that a susceptible of sex 1 is involved in two pairs. Mid: Transmission can now occur through sexual contacts between the newly formed pair, rendering the triple into . Bottom: The triple can break up through dissolution of one () or the other pair (). Note that pair formation between one single and one pair results in two pairs and one triple (and vice versa for the pair dissolution). The mathematical representation of each structure is given in rectangles. Subscript 0 denotes white circles (e.g. females) and subscript 1 denotes black circles (e.g. males).