Zoonotic diseases: why environmental and social justice is at the heart of tackling this complex threat
28 May 2021
To prevent the emergence of disease spread from animals, we must first look at the environmental and societal causes of such outbreaks
Human health cannot be viewed in isolation from the health of the animals with whom we share our planet. The COVID-19 pandemic, which many experts believe was caused by a virus spread to humans from bats and possibly through pangolins, has alerted people beyond the world of epidemiology to the danger of diseases that can transfer from animals to humans. These are known as zoonotic diseases.
How big is the threat of zoonotic diseases to humans?
Zoonotic disease transmission (also called zoonosis) has long threatened human health. In fact, the World Health Organization estimates that over 60% of all infectious diseases that affect humans have come from other animals. This proportion is even larger when we look at new and emerging infectious diseases. Over 70% of infectious diseases that have been newly identified in humans since 2000 are thought to be of animal origin.
Many of the most deadly diseases, including rabies, tuberculosis and African trypanosomiasis, are carried and transmitted by animals, particularly in the world’s poorest places. Infection occurs following contact with an infected animal or animal product, such as meat, milk or eggs. Contact with animal bodily fluids and drinking water can also lead to infection. Diseases such as malaria and dengue fever (known as vector-borne diseases) are also spread from species to species, but are carried between different organisms by a host animal, such as mosquitoes and ticks.
Over 70% of the infectious diseases that have been newly identified in humans since 2000 are thought to have come from animals.
Zoonosis was at the root of a number of outbreaks that have caused acute crises in recent years, with effects felt around the world:
- Severe acute respiratory syndrome (SARS), first identified in 2003. Experts believe the virus responsible for SARS probably mutated from a form found in small mammals and may have spread to humans through civet cats.
- Middle East respiratory syndrome (MERS-CoV), first identified in 2006. While it has not been proven, evidence suggests that this virus spread to humans from dromedary camels.
- Variants of influenza have spread to humans from pigs and domestic birds, including chickens and ducks.
- The Zika virus, spread by Aedes mosquitoes. It was linked to outbreaks of illness and birth defects in the Americas, Africa and beyond in 2015–16.
- Ebola virus disease, first identified in humans in 1976. It is transmitted to humans from a number of wild animal species, including fruit bats, porcupines and non-human primates. A major Ebola outbreak occurred in the west African countries of Guinea, Sierra Leone and Liberia in 2013–16, prompting a global response.
Zoonotic transmission also occurs between different animal species, threatening the health and livelihoods of people who rely on livestock, as well as putting already endangered species at further risk.
The destruction of wild animals’ habitats brings them into closer contact with both humans and domesticated animals, while deforestation in particular reduces biodiversity. This leads to an increase in species that can act as disease carriers, such as rats and mosquitos. Environmental changes can have dire consequences for wildlife, livestock and people. In east and central Africa, for example, gorilla populations have been decimated by Ebola.
Research increasingly shows that global trends as diverse as deforestation, international air travel, abattoir safety, and antibiotic usage in both humans and animals are all leading to the emergence of zoonotic diseases.
Controlling zoonotic diseases depends on a delicate human–animal balance
Recognition of the delicate balance between human, animal and environmental health has led to the development of the One Health concept. Proponents of One Health encourage veterinarians, healthcare professionals and environmental experts to come together to tackle the threat of zoonotic diseases and stifle outbreaks. I spent several years researching zoonotic disease control, and I was consistently struck by the role that economic and social conditions play in the emergence and spread of these diseases.
People who live with and depend on animals are frequently blamed for the spread of zoonotic diseases, when they fail to maintain certain hygiene standards or are reluctant to report infections to relevant authorities. To improve this situation, it is important to understand the relationships that exist between humans and the animals on whom their livelihoods depend. Anyone who remembers the bovine spongiform encephalopathy (BSE) or foot and mouth disease crises in the UK will recall the emotional and economic devastation faced by farmers who had to slaughter their animals to control the outbreak. Now imagine the same situation in countries where farmers have no insurance and limited social security.
Egypt, for example, was hit by a major bird flu outbreak in 2006. Farmers were so concerned about losing their livelihoods that, rather than allow their potentially infected ducks and chickens to be culled, they would hide them from public health authorities. When I was researching how small-scale cattle farmers view measures to control bovine tuberculosis in Ethiopia, they told me that they would never slaughter cows infected with the disease, as they were both highly valuable and seen as family members. If farmers were truly concerned that their animal was infectious, they might sell it, but without informing the buyer that it had a disease.
Such behaviours might appear irresponsible to those on the outside, but for many people whose lives depend on these animals, poverty is a far more terrifying prospect than the spread of an illness.
Addressing the social factors behind the spread of zoonotic diseases
There is an unfortunate tendency to blame zoonotic disease transmission largely on the actions of those living in the place where the disease first starts affecting humans. The consumption of meat from wild animals (known as bushmeat) was implicated in the west African Ebola outbreak in 2013, for example. But it is unfair to attribute zoonotic diseases to certain practices more common in low- and middle-income countries, especially when the cases mentioned in Egypt, Ethiopia and Sierra Leone were all driven by precarious food security and a very real fear of poverty.
Research increasingly shows that global trends as diverse as deforestation, international air travel, abattoir safety, and antibiotic usage in both humans and animals are all contributing to the emergence of zoonotic diseases. As the COVID-19 pandemic has demonstrated, this has a huge impact on health and way of life for us all. The decisions that we make about how we eat, travel and live all contribute to the dynamics of zoonosis. It is time to stop blaming people we have never met, who live in places we have never been to. Instead, we must call on governments worldwide to learn from past outbreaks, and address the underlying social conditions of those living and working with animals when formulating policies to prevent future pandemics.