Background: HSV-2 is the most prevalent sexually transmitted disease worldwide. In many risk groups in many geographic locations the HSV-2 and the HIV epidemics show substantial overlap. Many laboratory and epidemiological studies have shown that HSV-2 infected individuals are more susceptible to HIV infection. Results from other studies have suggested that individuals that are co-infected with both HSV-2 and HIV may be more capable of transmitting HIV than individuals infected with only HIV. However it is not clear what effects these biological changes in susceptibility and infectiousness will cause at the epidemic-level. Here we have developed a mathematical model of both epidemics; our model simultaneously tracks individuals infected with only HSV-2, only HIV and co-infected with both viruses. We analyze this model in order to determine the impact of HSV-2 epidemics on HIV transmission dynamics. 

Methods: We analyze our mathematical model: (i) to predict and (ii) to quantify how HSV-2 epidemics alter the speed and severity of HIV epidemics in both the short-term and the long-term. We quantify the impact of HSV-2 epidemics on: (i) the doubling time of the HIV epidemic, (ii) the increase in the basic reproduction number (R0) of HIV, and (iii) the PAF (Population Attributable Fraction; i.e., the number of HIV cases that are solely due to the HSV-2 epidemic).

Results: Our analyses reveal that HSV-2 epidemics significantly increase the speed and the severity of the HIV epidemic in both the short term and the long term. The degree of increase in both epidemic severity and speed is complex and changes over time, as it is a multifactorial process. Hence, we will show graphical temporal relationships that we have derived based upon our mathematical model. We will discuss the implications of HSV-2 induced HIV epidemic dynamic in increasing HIV epidemics.

Conclusion: HSV-2 epidemics can substantially increase the speed and severity of HIV epidemics.

Keywords: HSV-2; synergy; mathematical model