Zoonotic pathogens spread from animals to humans, and sometimes from humans back to animals. Mapping out the step-by-step pathway different zoonotic pathogens follow could help with surveillance efforts designed to prevent outbreaks. Raina Plowright of Montana State University is working with infectious disease experts, ecologists, and a range of other scientists to develop a general framework of the factors that influence how infectious agents jump from one species to another.

In the framework, Plowright and her colleagues identify three main areas that affect spillover: the physiology and ecology the reservoir species, how infected animals come in contact with humans, and what happens to humans after exposure to infection. Within those three broad categories are more detailed factors that affect whether the pathogen can jump from animals to people and whether it will spark an epidemic in the human population. Because different zoonotic pathogens travel different pathways from reservoir animal...

Reservoir Species

Zoonotic pathogens need a host species to survive. The prevalence and intensity of pathogenic infection in the reservoir animal population can influence the chances of spillover, as can the density of the reservoir species and its proximity to human populations.

Jumping Species

How humans come into contact with animals infected with zoonotic pathogens is another critical factor in spillover. Most commonly, people are exposed via the host animals’ excrement, during slaughter of livestock, and from bites, including from mosquitoes and other arthropods. The pathogen’s hardiness and ability to infect different species comes into play in all three scenarios.

Human-to-human Transmission

Finally, once the pathogen moves to a human, it has to battle our immune system and coopt cells in our body to replicate. Only then can the pathogen jump to other humans, replicate, spread, and cause an epidemicof disastrous proportions.

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