Microsimulation study of the release of pneumonic plague and smallpox on a synthetic civilian population

The confrontation in Syria is a cause for concern over the use of chemical and biological weapons. There have been reports of the use of chemical weapons including Sarin. While both sides accuse each other, there is very little ground evidence at this time (August 2013) to substantiate claims or to determine who is involved. Their use would demonstrate a total disregard for civilian casualties and UN conventions. Syria is thought to have stockpiles of a number of agents including anthrax, plague, tularaemia, botulinium, smallpox and cholera (Gordon, 2007). Some groups sympathetic to Al Qaeda might also have access to some of these through their terrorist networks. If chemical weapons have been used by either side, then the potential use of biological weapons cannot be disregarded. Of concern is whether the response to an attack involving a single agent would be the same as when more than one agent is used. While there are a number of papers on the management of both smallpox and plague (Halloran, (2002), Rani et al, (2004)), there are few, if any, which discuss joint infection or the likely confounding factors that will affect outcomes in their post attack management. In this paper we explore the application of microsimulation modelling of a joint attack on a civilian population using plague and smallpox as an example. The literature suggests that plague affects the innate immune system by suppressing cytokine responses while smallpox activates the cytokine response. The possible interaction between the two diseases in people who are infected with both diseases is examined and its effect on the spread of disease and number of deaths. The simulations involve a population of 1250 people based on NSW statistics for households and work. The structure of a community model of social mixing is briefly discussed, over which a multi-infection model is imposed that accounts for varying infectivity in different stages of each disease as well as confinement to home as each disease progresses. A number of simulations were run, assuming 10% immunity to both diseases, to establish a baseline for each disease in the community. Further simulations where both are released together with delay of the introduction of plague compared to smallpox of 0 and 35 days respectively. The strength of the interaction by smallpox on plague deaths was also investigated. The outcome is complex as the number of deaths is dependent on the delay in the release of plague and varies according to the number of people progressing through the active release locations that can infect them. Additionally confinement to their household during the later stages of both diseases reduces the number of susceptible people in the general community although it still allows for spread to other household members. It was found that the number of joint infections was about 10% of the total number infected and that the reduction in deaths from plague where the two infections were present also depended on how many smallpox cases were at the correct stage of development compared to the plague infection. We believe that the use of microsimulation has a benefit in that many of the human factors that affect delay in control and hence the size of any epidemic can be easily incorporated into the simulation without affecting the other components of the simulation. For example, vaccine and drug supply can be added as additional peeps that move to hospitals or doctors\u27 surgeries and then interact with the human population through creation of additional states based on these locations. Further work is planned to assess the impact of these human factors on preventing epidemic spread as well as assessing current WHO guidelines for each disease. The intention is to develop well understood protocols for application both within and outside of conflict areas. Progress on this will be reported at the conference