  We developed a mathematical model to investigate the role of indirect
transmission in the spread of infectious diseases, using the illustrative
example of sarcoptic mange as a case study. This disease can be transmitted
through direct contact between an infected host and a susceptible one, or
indirectly when potential hosts encounter infectious mites and larvae deposited
in the environment, commonly referred to as fomites. Our focus is on exploring
the potential of these infectious reservoirs as triggers for emerging infection
events and as stable reservoirs of the disease. To achieve this, our mean field
compartmental model incorporates the epidemiological dynamics driven by
indirect transmission via fomites. We identify different types of dynamics that
the system can go into, controlled by different levels of direct and indirect
transmission. Among these, we find a new regime where the disease can emerge
and persist over time solely through fomites, without the necessity for direct
transmission. This possibility of the system reveals an evolutionary pathway
that could enable the parasite to enhance its fitness beyond host co-evolution.
We also define a new threshold based on an effective reproductive number, that
enables us to predict the conditions for disease persistence. Our model allows
us to assess the potential effectiveness of various disease intervention
measures by incorporating a feature observed in real systems. We hope this
contributes to a better understanding of infectious disease outbreaks.
