Health effects

Public health context

The degradation and loss of forests disrupts nature's balance. Indeed, deforestation eliminates a great number of species of plants and animals which also often results in an increase in disease, and exposure of people to zoonotic diseases. Deforestation can also create a path for non-native species to flourish such as certain types of snails, which have been correlated with an increase in schistosomiasis cases.

Forest-associated diseases include malaria, Chagas disease (also known as American trypanosomiasis), African trypanosomiasis (sleeping sickness), leishmaniasis, Lyme disease, HIV and Ebola. The majority of new infectious diseases affecting humans, including the SARS-CoV2 virus that caused the current COVID-19 pandemic, are zoonotic and their emergence may be linked to habitat loss due to forest area change and the expansion of human populations into forest areas, which both increase human exposure to wildlife.

Deforestation is occurring all over the world and has been coupled with an increase in the occurrence of disease outbreaks. In Malaysia, thousands of acres of forest have been cleared for pig farms. This has resulted in an increase in the zoonosis the Nipah virus. In Kenya, deforestation has led to an increase in malaria cases which is now the leading cause of morbidity and mortality the country. A 2017 study in the American Economic Review found that deforestation substantially increased the incidence of malaria in Nigeria.

Another pathway through which deforestation affects disease is the relocation and dispersion of disease-carrying hosts. This disease emergence pathway can be called "range expansion", whereby the host's range (and thereby the range of pathogens) expands to new geographic areas. Through deforestation, hosts and reservoir species are forced into neighboring habitats. Accompanying the reservoir species are pathogens that have the ability to find new hosts in previously unexposed regions. As these pathogens and species come into closer contact with humans, they are infected both directly and indirectly.

A catastrophic example of range expansion is the 1998 outbreak of Nipah virus in Malaysia. For a number of years, deforestation, drought, and subsequent fires led to a dramatic geographic shift and density of fruit bats, a reservoir for Nipah virus. Deforestation reduced the available fruiting trees in the bats' habitat, and they encroached on surrounding orchards which also happened to be the location of a large number of pigsties. The bats, through proximity spread the Nipah to pigs. While the virus infected the pigs, mortality was much lower than among humans, making the pigs a virulent host leading to the transmission of the virus to humans. This resulted in 265 reported cases of encephalitis, of which 105 resulted in death. This example provides an important lesson for the impact deforestation can have on human health.

Another example of range expansion due to deforestation and other anthropogenic habitat impacts includes the Capybara rodent in Paraguay. This rodent is the host of a number of zoonotic diseases and, while there has not yet been a human-borne outbreak due to the movement of this rodent into new regions, it offers an example of how habitat destruction through deforestation and subsequent movements of species is occurring regularly.

A now well-developed theory is that the spread of HIV it is at least partially due deforestation. Rising populations created a food demand and with deforestation opening up new areas of the forest the hunters harvested a great deal of primate bushmeat, which is believed to be the origin of HIV.

General Overview

According to the World Economic Forum, 31% of emerging diseases are linked to deforestation.

According to the US Center for Disease Control and Prevention (CDC), 75% of emerging diseases in humans came from animals. The rising number of outbreaks is probably linked to habitat and biodiversity loss. In response, scientists created a new discipline: Planetary health, which says that the health of the ecosystems and the health of humans are linked. In 2015, the Rockefeller Foundation and The Lancet launched the concept as the Rockefeller Foundation–Lancet Commission on Planetary Health.

Since the 80's every decade the number of new diseases has increased more than 3 times. According to a major study by American and Australian scientists degradation of ecosystems increases the risk of new outbreaks. The diseases that passed to humans in this way in the latest decades include HIV, Ebola, Avian flu, Swine Flu, and likely the COVID-19 pandemic.

In 2016 the United Nations Environment Programme published the: "UNEP Frontiers 2016 Report". In this report, the second chapter was dedicated to Zoonotic diseases, that is diseases that pass from animals to humans. This chapter stated that deforestation, climate change, and livestock agriculture are among the main causes that increase the risk of such diseases. It mentioned that every 4 months a new disease is discovered in humans. It is said that outbreaks that already happened (as of 2016) led to loss of lives and financial losses of billions dollars and if future diseases become pandemics it will cost trillions of dollars.

The report presents the causes of the emerging diseases, large part of them environmental:

Cause	Part of emerging diseases caused by it (%)
Land-use change	31%
Agricultural industry changes	15%
International travel and commerce	13%
Medical industry changes	11%
War and Famine	7%
Climate and Weather	6%
Human demography and behavior	4%
Breakdown of public health	3%
Bushmeat	3%
Food industry change	2%
Other	4%

On page 23 of the report are presented some of the latest emerging diseases and the definite environmental cause of them:

Disease	Environmental cause
Rabies	Forest activities in South America
Bat accosiated viruses	Deforestation and Agricultural expansion
Lyme disease	Forest fragmentation in North America
Nipah virus infection	Pig farming and intensification of fruit production in Malaysia
Japanese encephalitis virus	irrigated rice production and pig farming in Southeast Asia
Ebola virus disease	Forest losses
Avian influenza	Intensive Poultry farming
SARS virus	contact with civet cats either in the wild or in live animal markets

HIV/AIDS

AIDS is probably linked to deforestation. The virus firstly circulated among monkeys and when the humans came and destroyed the forest and most of the primates, the virus needed a new host to survive and jumped to humans. The virus, which killed more than 25 million people, is believed to have come from the consumption of bushmeat, namely that of primates, and most likely chimpanzees in the Congo

Malaria

Malaria, which killed 405,000 people in 2018, is probably linked to deforestation. When humans change dramatically the ecological system the diversity in mosquito species is reduced and: ""The species that survive and become dominant, for reasons that are not well understood, almost always transmit malaria better than the species that had been most abundant in the intact forests", write Eric Chivian and Aaron Bernstein, public health experts at Harvard Medical School, in their book How Our Health Depends on Biodiversity. "This has been observed essentially everywhere malaria occurs".

Some of the reasons for this connection, found by scientists in the latest years:

• When there is less shadow of the trees, the temperature of the water is higher which benefits mosquitos.
• When the trees don't consume water, there is more water on the ground, which also benefits mosquitos.
• Low lying vegetation is better for the species of mosquitos that transmit the disease.
• When there is no forest there is less tanin in water. Than the water is less acidic and more turbid, what is better for some species of mosquitos.
• The mosquitos that live in deforested areas are better at carrying malaria.
• Another reason is that when a large part of a forest is destroyed, the animals are crowded in the remaining fragments in higher density, which facilitate the spread of the virus between them. This leads to a bigger number of cases between animals which increase the likelihood of transmission to humans.

Consequently, the same type of mosquito bites 278 times more often in deforested areas. According to one study in Brazil, cutting of 4% of the forest, leaded to a 50% increase in Malaria cases. In one region in Peru the number of cases per year, jumped from 600 to 120,000 after people begun to cut forests.

Coronavirus disease 2019

According to the United Nations, World Health Organization and World Wildlife Foundation the Coronavirus pandemic is linked to the destruction of nature, especially to deforestation, habitat loss in general and wildlife trade.

In April 2020, United Nations Environment Programme published 2 short videos explaining the link between nature destruction, wildlife trade and the COVID-19 pandemic and created a section on its site dedicated to the issue.

The World Economic Forum published a call to involve nature recovery in the recovery efforts from the COVID-19 pandemic saying that this outbreak is linked to the destruction of the natural world.

In May 2020, a group of experts from the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services published an article saying that humans are the species responsible for the COVID-19 pandemic because it is linked to nature destruction and more severe epidemics might occur if humanity will not change direction. It calls to "strengthen environmental regulations; adopt a 'One Health' approach to decision-making that recognizes complex interconnections among the health of people, animals, plants, and our shared environment; and prop up health care systems in the most vulnerable countries where resources are strained and underfunded", which can prevent future epidemics and therefore is in the interest of all. The call was published on the site of the World Economic Forum.

According to the United Nations Environment Programme the Coronavirus disease 2019 is zoonotic, e.g., the virus passed from animals to humans. Such diseases are occurring more frequently in the latest decades, due to a number of factors, a large part of them environmental. One of the factors is deforestation because it reduce the space reserved for animals and destroys natural barriers between animals and humans. Another cause is climate change. Too fast changes in temperature and humidity facilitate the spread of diseases. The United Nations Environment Programme concludes that: "The most fundamental way to protect ourselves from zoonotic diseases is to prevent destruction of nature. Where ecosystems are healthy and biodiverse, they are resilient, adaptable and help to regulate diseases.

In June 2020, a scientific unit of Greenpeace with University of the West of England (UWE) published a report saying that the rise of zoonotic diseases, including coronavirus is directly linked to deforestation because it change the interaction between people and animals and reduce the amount of water necessary for hygiene and diseases treatment.

Experts say that anthropogenic deforestation, habitat loss and destruction of biodiversity may be linked to outbreaks like the COVID-19 pandemic in several ways:

• Bringing people and domestic animals in contact with a species of animals and plants that did not contacted by them before. Kate Jones, chair of ecology and biodiversity at University College London, says the disruption of pristine forests, driven by logging, mining, road building through remote places, rapid urbanisation and population growth is bringing people into closer contact with animal species they may never have been near before, resulting in transmission of new zoonotic diseases from wildlife to humans.
• Creating degraded habitats. Such habitats with a few species are more likely to cause a transmission of zoonotic viruses to humans.
• Creating more crowded habitats, with more dense population.
• Habitat loss prompts animals to search for a new one, which often results in mixing with humans and other animals.
• Disruption of ecosystems can increase the number of animals that carry many viruses, like bats and rodents. It can increase the number of mice and rats by reducing the populations of predators. Deforestation in the Amazon rainforest increase the likelihood of malaria because the deforested area is ideal for mosquitoes.
• Animal trade, by killing and transporting live and dead animals very long distances. According to American science journalist David Quammen, "We cut the trees; we kill the animals or cage them and send them to markets. We disrupt ecosystems, and we shake viruses loose from their natural hosts. When that happens, they need a new host. Often, we are it."

When climate change or deforestation causes a virus to pass to another host it became more dangerous. This is because viruses generally learn to coexist with their host and became virulent when they pass to another.