Comparing the effects direct and indirect pathogen transmission have on epidemic time series

Background/Question/Methods

Infectious diseases and other pathogens can be transmitted to uninfected hosts directly via contact with infected individuals or indirectly through contact with free-living pathogens that were excreted into the environment. While most models of disease transmission ignore indirect routes, here we consider whether and when such a choice may influence epidemiological characteristics of the disease. In particular, using models of ordinary differential equations, we explore under what biological conditions direct and indirect transmission can be distinguished when given only time series for the prevalence of the pathogen within the host population (e.g., when direct measurements of environmental pathogen load are costly or difficult).  We also explore the conditions under which data on the environmental pathogen load can help distinguish the two modes of transmission.

Results/Conclusions

Our analysis shows that while models with indirect transmission can capture the conventional SIR dynamics of directly transmitted pathogens, the reverse is not true. Thus, in some biological systems, the two modes of transmission will not have different effects on epidemic time series, but in other systems, the mode of transmission can be determined from prevalence data alone.  Similarly, environmental pathogen load data can sometimes help identify the mode of transmission.  We present under what conditions the mode of transmission has an effect on the epidemiological time series and what features of the time series allow one to identify when indirect transmission is present and important.