Zoonotic spillover: Understanding basic aspects for better prevention

Abstract

The transmission of pathogens from wild animals to humans is called "zoonotic spillover". Most human infectious diseases (60-75%) are derived from pathogens that originally circulated in non-human animal species. This demonstrates that spillover has a fundamental role in the emergence of new human infectious diseases. Understanding the factors that facilitate the transmission of pathogens from wild animals to humans is essential to establish strategies focused on the reduction of the frequency of spillover events. In this context, this article describes the basic aspects of zoonotic spillover and the main factors involved in spillover events, considering the role of the inter-species interactions, phylogenetic distance between host species, environmental drivers, and specific characteristics of the pathogens, animals, and humans. As an example, the factors involved in the emergence of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) pandemic are discussed, indicating what can be learned from this public health emergency, and what can be applied to the Brazilian scenario. Finally, this article discusses actions to prevent or reduce the frequency of zoonotic spillover events.

Figure 1 -. Factors between wildlife, livestock animals and humans. The intensity and frequency of interactions between species, the phylogenetic distance between host species, characteristics of the pathogens, source host-associated factors, recipient host-associated factors, environmental factors, and intermediate host-associated factors (when this particular host is present) can hinder or facilitate the transmission of pathogens from wild animals/livestock to the human population. The zoonotic spillover occurs when these factors are favorable to the pathogen, allowing the crossing of barriers between species. This figure was created using Mind the Graph illustrations (available at www.mindthegraph.com).

Figure 2 -. Models of zoonotic spillover. Pathogens can overcome the barriers between species in different ways. A: the pathogen can be transmitted directly from the source host to the recipient host (e.g., a virus is transmitted from a bat to a human, via a bat bite). B: the pathogen can be transmitted from the source host to an intermediate vertebrate host and then be transmitted to the recipient host (e.g., a protozoan is transmitted from a wild species to a domestic dog, which then transmits the parasite to a human host). C: the pathogen can be transmitted from the source host to an intermediate invertebrate host and then be transmitted to the recipient host (e.g., an arbovirus is transmitted from a wild primate to a mosquito, which then transmits the virus to a human host). D: the pathogen can be transmitted from the source host to the environment and then be transmitted to the recipient host (e.g., a swine species releases an enterovirus into the environment through feces, which subsequently infects a human who has come into contact with the animal’s feces present in the environment).

Figure 3 -. Intermediate host as a “mixing vessel” for the emergence of new viral strains. When an intermediate vertebrate host is infected by different viral strains, recombination or reassortment can occur between viral species, generating new viral strains. The intermediate host can function as a mixing vessel of new viral trains in two ways. A: a viral strain from a source host exchanges genetic information with a viral strain originally present in the intermediate host, producing a new viral strain. B: the intermediate host is infected with viral strain from two (or more) source hosts, allowing the exchange of genetic information between the viral species, producing a new viral strain. Hypothetical viral recombination in an intermediate host is exemplified in this figure, but the same processes can be applied to viral reassortment.

Figure 4 -. Outcomes after a spillover event. A spillover event can have two basic outcomes. A: the pathogen that crossed the barriers between species can spread among the population (through human-to-human transmission or a highly competent vector). This outcome will occur if the social, biological, and environmental conditions are favorable for the adaptation of the pathogen in the human host and its transmission among the new population. B: the pathogen is transmitted to a human, but without spreading among the human population due to the absence of social, biological, or environmental conditions favorable to the adaptation of the pathogen to the human population or its dissemination. This condition is called dead-end spillover.