Zoonotic diseases (zoonoses) are diseases that can transmit between species and from non-humans to the human animal (WHO, 2020). Zoonotic infections have been noted for as long as humans have been documenting their past. In fact, 5 out of the 10 plagues mentioned in the Old Testament are likely to have been zoonotic (Will, 1994), and the familiar story of the “Black Death” caused by Yersinia pestis (the bacteria that causes bubonic plague) is noted in written records from the 1300s (British Library, 2023).
Infectious agents that spread from animals to humans have also helped us combat some of the dangerous diseases infecting human populations. The most famous example is the case of Edward Jenner using discharge from cowpox (Orthopoxvirus) pustules on the hands of milkmaids as the first vaccine against smallpox (variola) in the 1790s (WHO, 2023).
Government agencies keep a close eye on the presence and epidemiology of zoonotic diseases, maintaining lists of pathogens to help with surveillance (PHE, 2019). This is because zoonoses cause damage to livestock production and agricultural losses, as well as their impact on human health – something fresh in everyone’s minds post-COVID.
Disease as population regulation
In the wild, disease plays a natural role in population regulation. Healthy, self-sustaining populations should not be greatly impacted by disease as it is a natural controller of population growth, balanced by other environmental factors. Ecosystem services also control exposure to disease, for example the filtration of water by the roots of reeds (He et al., 2020) and the consumption of dead bodies by scavenging species such as vultures (Gyps spp.) (Plaza et al., 2020).
Healthy, self-sustaining populations should not be greatly impacted by disease as it is a natural controller of population growth, balanced by other environmental factors
Anthropogenic influences such as the disruption of ecosystems, climate change, introduction of alien species, extirpation of species from native ranges and misuse of chemicals leaching into the environment cause disease to spread more quickly across susceptible hosts. Any regulating influence disease may have on a population is, therefore, lost (Figure 1).
How does disease impact conservation?
Conservation programmes for free-living and captive threatened species must keep a close eye on the presence, spread and occurrence of zoonotic diseases. This is because these infectious agents pose a risk to the growth, recovery and stability of the populations of species of conservation concern as well as being a health risk to those working directly with these species.
Many recent cases of zoonoses impacting endangered species recovery have been caused by human encroachment into wild places, the trade of exotic species and the movement of animals to new parts of the world. These factors increase the chances of a pathogen “jumping” between species and causing a new form of infection in a novel host (Magouras et al., 2020).
Urbanisation and the increase in population sizes of species that are able to cope with anthropogenic changes to the environment (eg the replacement of wild habitats with towns and cities) also accelerate the spread of zoonoses. Research on feral pigeons (Columba livia domestica) in southern Spain identified the species as a key vector of disease in captive pigeons, non-pigeon birds and some mammals at zoos in the same location as the feral pigeons (Cano-Terriza et al., 2015). This research isolated pigeon-borne flaviviruses in zoo birds and mammals, suggesting that feral pigeons play a significant role in transmitting disease to a wide variety of captive exotic species (Figure 2).
Feral pigeon populations have markedly increased across Europe in the 21st century, with larger population densities noted in cities compared to farmland (Sacchi et al., 2002). This closer proximity of individual birds (to each other, to human populations and to populations of other species) increases their role as vectors of infectious agents in non-pigeon populations.
What can be done?
Providing opportunities for other species that prey on feral pigeons – peregrine falcons (Falco peregrinus), for example – to exist in urban environments to naturally control pigeon numbers makes all species’ populations healthier.
Zoos and other conservation centres should be mindful of the presence of feral pigeons in and around the housing and enclosures used for captive species
Zoos and other conservation centres should be mindful of the presence of feral pigeons in and around the housing and enclosures used for captive species. Good biosecurity should also be maintained to keep living collections healthy and disease-free.
What can veterinary professionals do?
Veterinary surgeons can have a role to play in disease surveillance and pandemic control as they often see wildlife casualties brought in by members of the public (Trocini et al., 2008).
Feral pigeons are common wildlife casualties that may be admitted to general practices for emergency first aid. Vets will, therefore, gain experience of signs and symptoms of common diseases and the risk they pose to other species (both wild and captive, common and rare). However, good biosecurity is required to reduce contact between admitted wildlife cases and the pets or companion species treated in the veterinary hospital.
Reporting of cases of disease in feral birds (and indeed other wildlife species) helps track the prevalence and spread of specific diseases at a local and national level.
Examples of disease impacting conservation
Current examples of zoonoses that have become a problem for the conservation of wild species include:
- African wild dogs (Lycaon pictus) and rabies infections from feral dogs (Vial et al., 2006)
- Highly pathogenic avian influenza infection in an endangered Cape cormorant (Phalacrocorax capensis) population (Molini et al., 2023)
- Mountain gorilla (Gorilla beringei beringei) populations being exposed to a range of zoonoses caused by close contact with people (Cranfield and Minnis, 2007)
IUCN Red List Assessments will identify the key threats to species, including any impacts of disease on population number, and a review of the Red List assessments shows us that disease is a current and important factor still causing population declines for the wild dog (Woodroffe and Sillero-Zubiri, 2020), cormorant (BirdLife International, 2018) and gorilla (Hickey et al., 2020).
Conservation to combat disease
Ultimately, species conservation action aims to restore wild populations after any threats causing their reduction or extirpation have been neutralised. Adding a species back to an ecosystem increases the health of that ecosystem and improves ecosystem services. Rewilding rivers by creating bends and “wiggles” in the course of the river, for example, alters water flow, thus improving plant diversity and the growth of different species (Gill and Woodcock, 2023). In turn, this introduction of plants and species naturally cleans the river, improving water quality and oxygen content, further increasing the diversity and abundance of fish.
Rather than reactive treatment of disease cases when they appear, proactive restoration of landscapes and environments reduces the opportunities for zoonoses to occur
Initially, this does not appear to have much to do with veterinary medicine, but it is preventative care. Rather than reactive treatment of disease cases when they appear, proactive restoration of landscapes and environments reduces the opportunities for zoonoses to occur (Reaser et al., 2021), keeping animals and humans healthy in a more cost-effective and environmentally grounded manner.
Final thoughts
Maintaining the biological diversity of plants and animals in natural habitats may be an excellent way of ensuring natural defences against disease agents (Keesing et al., 2010). A mix of species can prevent one specific disease agent from dominating, as the chances of all species in the habitat being a compatible host for the disease are slim. Therefore, both the population of hosts and the population of the disease organism is naturally regulated.
We can learn a lot from how disease spreads quickly due to the monoculture of plants and trees; uniform planting of the same species of trees in cities or for forestry allows disease to spread quickly between susceptible hosts (Balogh, 2021; Jackson et al., 2023). There is no natural “firebreak” caused by species diversity and the presence of unsuitable hosts. We must use biodiversity to help naturally manage disease organisms and outbreaks. After all, a decline at the producer level of our food chains will spell catastrophe for species, including humans, higher up the many ecological webs.
