Author: Lesley van Helden
Where do new diseases come from?
The majority of human pathogens originate from other species of animals. Many important drivers of the emergence of infectious diseases, therefore, exist at the interface between people and animals. It is believed that some of the most important human diseases in our history, like measles, mumps, rubella, smallpox and many strains of influenza, came to humans from domestic animals when we transitioned from hunting and gathering to keeping livestock for food production.
Approximately 70% of recently emerged diseases, however, originate from wildlife. A pathogen that exists in animals needs to evolve through several stages before it can become capable of infecting humans and spreading between them. The likelihood of this occurring increases the more opportunities there are for contact between people and other animals. Areas that are at high risk for emerging infectious diseases are therefore those in which:
There is a tropical or subtropical climate that supports the existence of diverse wildlife
There is an interface between natural habitats and human activity, for instance where deforestation is occurring for mining or agricultural purposes
Practices exist where wild animals are hunted, traded or kept for food production or as pets
There is a dense human population to allow contact between people or there are well-connected populations with travel and trade between regions
The coronaviruses that have emerged this century, SARS and MERS, most likely originated from bats, as certain bats have been found to carry closely related viruses. However, these diseases probably did not transition directly from bats to humans, as humans rarely have close and frequent contact with bats. Evidence points towards an intermediate species in each case: the palm civet for SARS and the domestic camel for MERS. Cases such as these illustrate that practices that bring multiple species of animals into close contact with each other provide opportunities for transfer of pathogens. Some examples of this may include keeping wildlife species closely confined in wet markets or keeping high concentrations of domestic animals in a recently deforested area.
In the case of the coronavirus causing the current pandemic, related viruses have been found in bats in China, but the origin of the virus remains unclear. Many of the first reported cases of COVID-19 occurred in people who had been to a wet market in Wuhan selling several species of live wild animals. However, the earliest recorded case occurred in a person who had not been to the market, meaning that the virus may have originated elsewhere and simply spread to many people from another infected person at the market. It is possible that the virus may have been circulating in the human population for some time before it became capable of spreading widely and causing severe symptoms.
Could this happen in South Africa?
As in most other parts of the world, several risk factors for emergence of infectious diseases exist in South Africa. More risk factors exist in the more sub-tropical and densely-populated eastern part of the country than the drier western parts, but there is no area where there is an absence of risk. A new pathogen could technically emerge anywhere in the world.
Live animal markets are common in South Africa as many South Africans prefer to buy their meat this way, either because of cultural preferences, because they do not have access to refrigeration or supermarkets close by, or because they perceive supermarket produce to be of inferior quality. The majority of animals sold this way are farmed domestic animals, e.g. chickens and sheep.
Hunting of wildlife also takes place in many areas, either to provide people with food, to collect products used in traditional medicine or for the hunters to earn an income from selling wildlife and wildlife products. The most obvious example of the latter in South Africa is the poaching of rhinos to sell rhino horn to consumers in other countries that place a high value on it. This type of unsustainable hunting and trade both brings humans into close contact with wild animals and disrupts ecosystems.
Areas of wildlife habitat in South Africa are under further pressure in many areas where they are seen to compete with human land-use needs, such as for agriculture, mining or the expansion of urban areas.
What can be done to reduce the risk of a new disease emerging?
Preventing close contact between people and animal species that are potential hosts of new diseases is possibly the most important step in preventing the emergence of a new disease. However, how this is achieved is as important. Attempting to remove or kill wildlife species perceived as threats will have the opposite effect because functioning natural environments are stable systems that provide protection against disease emergence. Maintaining a balance of biodiversity as close as possible to a natural habitat generally has a synergistic effect on health for all players in the system. In the scientific community, this concept that the health of humans, animals and the environment is interconnected is known as ‘one health’.
Why would a healthy ecosystem be less likely to be the source of a new disease? Amongst many other factors, changes in resource availability in habitats lead to changes in the population numbers and behaviour of the species living there. This can cause increased interactions between species than would have occurred before, which can provide opportunities for pathogens to spread from one species to another. Furthermore, when animals experience chronic physiological stress, this affects their immune systems in a way that could make them more likely to be infected by and shed any potentially dangerous pathogens in the environment.
Practical steps for protecting the health of ecosystems should, therefore, consist of measures to protect the wellbeing of not only animals but also people and the environment around them. These include:
Good agricultural practice in farming: Producers should be educated about and encouraged to follow the animal welfare standards set by the World Organisation for Animal Health (OIE) and the good agricultural practices recommended by the Food and Agricultural Organisation of the United Nations (FAO). Encouragement to follow these standards can come from both regulations and from consumer pressure. The more educated consumers are about the conditions under which their food and other products have been produced, the more of a positive impact they can have on ensuring the systems they support are those that cause minimal harm or present minimal risk.
Environmental impact assessments: These should be done for any development projects and should thoroughly examine the potential effects on the ecosystem in that area.
Food security: All people should be ensured access to fresh and nutritious food to reduce the need for unregulated animal slaughter and hunting of wildlife.
Public health: Measures should be put in place to ensure safe and sanitary living conditions for all communities to reduce contact with rodents and water-borne pathogens.
Controlling illegal wildlife trade: This can be achieved in the short term by thorough law enforcement and in the long term by culturally-sensitive education of the consumers of wildlife products to reduce demand.
Conservation of natural habitats: Lastly, and most importantly, we need to look critically at how wild land and animals are valued in society. If the perceived value of the conserved land is not high enough, this land is likely to be converted into something that will be considered more valuable, such as a farm, a housing complex or a golf course. This type of land-use change is the biggest disrupter of natural ecosystems.
As humans, we often regard ourselves as separate from nature. However, as this pandemic has made clear, we are very much a part of and subject to forces of nature just like every other living thing that shares our planet. By embracing the principle of ‘one health’ we can see that degradation of ecosystems through human activity has the potential to result in the degradation of our own quality of life. Let us think carefully about the choices we make and how they impact other species because, eventually, these choices may come to impact us in a similar way.
Lesley van Helden is a veterinary epidemiologist with an MSc in International Animal Health. She currently works as a state veterinarian at the Western Cape Department of Agriculture and has a special interest in the relationship between environmental, animal and human health.