Researchers from the University of Florida have highlighted a growing concern: reverse zoonosis, where infections move from humans to animals. Their work appears in the journal Zoonosis: Infections Affecting Humans and Animals (ZIAHA).
Differences in biology between species often block the easy transfer of pathogens. For viruses, success hinges on binding to specific receptors on host cells to replicate and complete their life cycle. The feasibility of cross-species infection, however, is influenced by the remarkable diversity of viral populations. Some strains show greater ability to overcome host defenses and immune barriers, enabling spillover in unlikely hosts (Source: University of Florida study).
There is also the risk that a virus could acquire a random mutation that allows it to fit a new cellular receptor, effectively breaching the species barrier. In practical terms, this means occasional events where a virus adapts just enough to infect a new animal host (Source: ZIAHA findings).
In practical terms, animals most commonly become ill after exposure to human infections such as swine flu, norovirus, dengue fever, and COVID-19—that is, along with other, less widely known viral, fungal, bacterial, and parasitic diseases (Researchers’ observations).
While dogs and cats are the species most frequently implicated in these transmissions, there have also been reported cases in horses, ferrets, and parrots, suggesting that reverse zoonosis can affect a range of companion animals and, in turn, create broader health implications (Epidemiology notes).
Beyond the immediate risk to beloved pets, reverse zoonosis poses a potential pathway for human illness as well. When viruses that typically spread among humans pass into animals, they can re-enter the human population in altered forms, potentially intensifying transmission or altering virulence. Seasonal influenza, for example, binds to receptors that are present in humans and pigs, with birds showing a different receptor configuration. This receptor arrangement can allow pigs to be co-infected with human and avian influenza strains, sometimes producing novel viruses that circulate more broadly and pose pandemic risks (Virology assessments).
Experts advise that anyone currently infected with an illness should minimize contact with animals to reduce the chance of transmission. Even people without symptoms are encouraged to avoid overly affectionate interactions with pets during active infection. On a broader scale, advancing research at both animal and human health levels is essential. This means gathering comprehensive data across species to understand disease dynamics, not just focusing on human cases. Monitoring animal and bird health alongside human health can help identify patterns, detect emerging threats early, and guide prevention strategies (Public health guidance).
Earlier researchers have posed questions about what illnesses might be transmitted from poultry, turtles, and even goldfish, underscoring a spectrum of potential cross-species risks and the need for ongoing surveillance and responsible pet care practices (Historical inquiries). [citation: University of Florida study; ZIAHA journal]