It may be no surprise that zoonoses — diseases that spread between humans and non-human animals — threaten the health of both the public and animal populations. It’s estimated that about 60% of diseases infectious to humans are zoonotic. Zoonoses are also well known for contributing to population decline in both farmed and wild animals. For example, one strain of avian flu now impacts birds in 30 countries and has even contributed to shrinking sea lion populations.

However, less is known about how zoonoses affect wild animal welfare. Beyond just the risk of an early death, diseases can impact an animal’s body systems and behavior, including their ability to reproduce. For this reason, understanding how animals experience infection may help refine management strategies in a way that death counts alone can’t. By examining over 2,500 relationships between wild animal hosts and bacterial pathogens, two researchers measured disease impacts on welfare to offer insights for management and conservation.

Creating And Applying A Severity Index

Using an existing database, the authors identified 2,588 pairings of host species and pathogens that had information available on signs of infection. For each of these relationships, they calculated a score designed to represent how severely the pathogen impacts the host. Their formula incorporated three factors:

1. Symptom severity: How intensely the disease causes the animal to suffer, calculated as an average of all symptoms. The researchers ranked each symptom on a scale of one to three based on its impact on normal behaviors and likelihood of mortality, with three representing the highest severity.
2. Body system count: How many body systems the pathogen interferes with out of a total of 11 (e.g., respiratory, muscular, reproductive).
3. Animal welfare count: How many animal welfare domains the pathogen directly or indirectly affects out of a total of four (i.e., nutrition, health, behavior, and mental state). The researchers’ formula accounted for how the effects on welfare compound with each additional domain affected.

As an example, the authors found a score of 969.48 for one pathogen’s impact on Arabian sand gazelles, as the average symptom severity is 1.86 (causing symptoms including fever, lack of appetite, and weight loss), and the pathogen impacts three body systems and all four welfare domains.

The authors then explored the scores’ application to wild animal management and conservation. First, they examined whether there was a connection between severity and the host’s bacterial richness — the number of unique bacterial pathogens a host species has been known to carry. They also looked into whether severity differed between species of different conservation statuses (based on the International Union for Conservation of Nature’s Red List of Threatened Species). Overall, they wanted to see if bacterial zoonoses impact certain groups of animals more than others.

Findings And Implications For Wild Animal Management And Conservation

The minimum severity score was 1, which applied to over 1,000 examples of asymptomatic infections, and the maximum score was 324,645.7 (for a harbor porpoise infected with Staphylococcus aureus). The average score was 2,259.854.

Overall, the data showed that higher bacterial richness was associated with lower severity, and vice versa. However, the authors don’t know for sure if this means that species that host more pathogens experience less severe impacts of infection.

Severity scores weren’t significantly different between species of lower and higher conservation concern. But when the authors looked at the relationship between bacterial richness and severity scores for the two conservation statuses separately, they saw severity decreased with a higher bacterial richness only for species of least concern. In their view, this suggests that some species of conservation concern are less resilient to severe infection. One possible explanation is that these species’ smaller population sizes lead individuals to have lower genetic diversity, which can make them more susceptible to disease.

Study Limitations

The authors acknowledge that their findings, especially those on species of conservation concern, could’ve been biased because of how much of their data came from animals in captivity. Of the species studied, nearly half (47%) of the conservation concern species were based on observations in captivity compared to only 15% of the least concern species. The authors emphasize that captivity raises the risk of infection, can worsen existing signs of disease, and provides more opportunities for people to monitor, record, and treat disease progression. The trade-off is that the observations based on animals found in the wild are more likely to have captured only a snapshot of infection rather than symptoms throughout the course of the disease.

Further, this analysis only looked at bacterial pathogens and hosts who were mammals, birds, or reptiles. Therefore, animals such as amphibians and fishes and other types of pathogens like viruses, fungi, and parasites weren’t considered. Also, because this analysis focused on zoonoses, it didn’t examine diseases that occur only in animals but may also impact wild animal welfare and populations.

Wild Animals Deserve To Thrive, Not Just Survive

There’s a host (pun intended!) of reasons to measure disease severity in wild animals through a welfare lens that considers how animals experience infection — not just whether they survive it.

Understanding how different pathogens impact different species can help in tailoring management strategies to specific diseases and animal populations. It can also help inform the design of conservation programs like translocation and reintroduction, which can cause animals stress and raise their risk of getting infections that harm their ability to thrive in the wild.

Thus, advocates can emphasize the importance of prioritizing welfare — an animal’s physical and mental well-being — in conversations, policies, and programs that aim to advance wild animal management and conservation. Ultimately, expanding our perspective this way offers value across scales, from the protection of individual animals to the safeguarding of Earth’s biodiversity as a whole.