In the current health and scientific community, mathematical models can be useful to understanding the different mechanisms at play, as well as making predictions about the global dynamics of epidemics. Infectious disease outbreaks not only pose a threat to human health, but also to wildlife populations, especially those whose conservation state is already threatened by other factors. This is the case for native wild camelid populations in Northwestern Argentina, where a recent sarcoptic mange outbreak has driven the two wild species, Vicugna vicugna and Lama guanicoe, to local extinction.

Even though many diseases are well understood, in some cases the exact infection mechanisms that led to a particular outcome (such as in the camelid case) remain unclear. Taking these points into consideration, it is essential to study different potential scenarios that may explain these mechanisms and contribute to a deeper understanding of the outbreak dynamics.

Some infectious diseases, such as sarcoptic mange, can be transmitted through direct contact (between individuals) or indirect contact, which occurs when a susceptible individual comes into contact with a reservoir of mites and larvae, commonly referred to as a fomite.

We adopted a mean field approach to analyze the specific dynamics of a disease with indirect transmission and to understand how it alters the long-term stable states of an epidemic of a lethal disease, considering the possibility that fomites may act as outbreak triggers [1].

Our model incorporates indirect transmission through fomites into a compartmental mean-field framework. We identify a novel regime where a disease can persist solely through fomites, without direct transmission. This suggests a potential evolutionary pathway for the parasite. We also establish an epidemiological threshold to predict disease persistence and assess the effectiveness of various intervention measures. This approach aims to enhance the understanding of infectious disease outbreaks.

[1] González, T. I., Laguna, M. F., Abramson, G. (2024). A mean field analysis of the role of indirect transmission in emergent infection events. Physica A: Statistical Mechanics and its Applications, https://www.sciencedirect.com/science/article/abs/pii/S0378437124004424