Course:CONS200/2021/Deforestation Linked to Rising Infectious Diseases

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Introduction

Deforestation is a consequence of human-based systems and infrastructure. Systems including food, energy, or urbanization lead to infrastructure development that directly causes deforestation[1]. The impacts of deforestation on the landscape and humans are widespread, even influencing infectious diseases to the detriment of human health. Habitat loss and pathogen spillover, biodiversity loss, vector dynamics, climate change, and extreme weather events are all results of deforestation and increase infectious disease pathogen incidence and spread through various interactions[2][3][4]. For example, altered landscapes may create new opportunities or forced relocation of species, increasing the risk of transmission and the development of novel viruses[2]. In Brazil, malaria has increased due to mosquitoes thriving in newly deforested areas where there is more sunlight and water accumulation[4], while Severe Acute Respiratory Syndrome (SARS) may have resulted from a higher concentration of bat populations relocating near human dwellings[5]. Given the links between deforestation and infectious diseases, deforestation controls could play an important role in infectious disease prevention. The development of farming methods such as greenhouse gardens has reduced agricultural area size by increasing crop yield, and many countries have introduced an agroforestry system aiming to imitate natural forests by combining trees and crops[6]. Satellite-based monitoring systems for measuring forest degradation are also helping limit deforestation[7]. In many parts of the world, reforestation and afforestation are increasing forested land area[8].

What is Deforestation?

Deforestation is the complete and permanent removal of forest stands to convert land for non-forested use[9]. Globally 10 to 12 million hectares, roughly three times the size of Denmark, are deforested each year. Although the natural and replanted growth of forests offsets this deforestation by about 5 million hectares, the net result is an overall permanent loss of 5 to 7 million hectares per year[10][11]. While temperate regions like North America, Russia, China, and parts of Oceania account for 5% of deforestation, the majority occurs in tropical regions like Latin America, Southeast Asia, Africa, and parts of Oceania[10]. Brazil alone accounts for 33% of global deforestation, which amounts to 1.7 million hectares of forest lost each year—almost double the size of Lebanon[10][11][12]. Indonesia is the second-largest contributor to deforestation with 14%, meaning these two countries combined account for 47% of annual global deforestation. The leading cause of deforestation is agricultural expansion, followed by wood forest products and mining[13].

Indonesian lands being deforested in preparation for a palm oil plantation.

Though forests cover about 31% of land area globally, they are distributed unevenly around the world. Five countries hold slightly more than half of the world’s forests, around 54% of the global forest area: Russia, Brazil, Canada, the United States, and China[14]. While 66% of forests are found in ten countries, including those previously mentioned, the remaining 34% are distributed among the rest of the world. Only 71% of the planet's land area is habitable, 10.6 billion hectares of which 6 billion are forests, while the other 29% is covered by ice and glaciers or barren land like deserts, salt flats, and dunes[15]. Today there are 4 billion hectares left, meaning the world has lost one-third of its forest, an area almost twice the size of the United States[14][15]. To put this into perspective, just 10% of forests were lost in the first 5,000 years since the last ice age due to the very small world population. About half of total forest loss, roughly 1 billion hectares, occurred between 8,000 BC and 1900, while the other half occurred in the last century[15]. This drastic deforestation rate increase is largely due to the exponential growth of the human population and the resultant need for increased food production. Although the rate of deforestation has decreased in the past three decades, from 16 to 10 million hectares per year, an estimated 420 million hectares of forest have been lost since 1990[10][15].

Causes of Deforestation:

Agricultural Expansion

Deforestation for agricultural expansion is caused by commercial and local agriculture. Agriculture is responsible for over 80% of deforestation worldwide, equivalent to about 4 million hectares of forest converted to pastures and croplands annually[16]. Large-scale commercial agriculture is responsible for 45% of total deforestation, including livestock, soy, palm, and many other crops, while another 33% is local subsistence agriculture, and 7% is dedicated to feeding crops for livestock[10][17]. Subsistence agriculture is the farming of crops or livestock for use by farmers and their families or the local community, leaving very little for sale or trade[18]. The majority of deforestation for cattle raising occurs in Latin America, specifically in Brazil where 1.2 million hectares of forest are converted to pastures each year[17]. Nearly 19% of global deforestation is from cropland expansion for the production of soybeans and palm oil, of which about 6.5% occurs in Indonesia[19]. It is often thought that soy products are for human consumption, however, only 6% is for humans while the rest is for livestock feed[13][14].

Wood Forest Products

Wood forestry products, including pulp for paper production, wood for fuel, and timber for infrastructure construction, are the second-largest driver of deforestation and cause 13-15% of deforestation[16][17]. The three biggest paper-producing countries are the United States, Canada, and China, all in the temperate forest region, but tropical countries, including Brazil and Indonesia, are the largest pulp-producing countries mostly due to their year-round growing season[19]. Fuelwood is often used for cooking or heating and accounts for about 80% of wood consumption in developing countries. Overall about one-third of wood extracted from forests is used for timber and any type of deforestation can produce timber[20]. Timber harvesting is high in Southeast Asia, particularly Indonesia, but the regions with the highest timber consumption are Europe, North America, and China, with a combined global consumption of over 75%[19].

Mining

Mining locations themselves only account for 1-5% of deforestation, however, infrastructures surrounding the mining industry, roads, buildings, and machinery, add another 1% of deforestation[21]. Over the past decade, with the rising prices of precious minerals and gems, both legal and illegal new mining operations have increased. Although mining operations are prominent in Africa, deforestation greatly impacts Brazil's rainforests[22]. Illegal mining is a large issue with health and safety impacts and causes deforestation without government knowledge. With roughly 90% of mining-related deforestation occurring outside of designated mine areas, deforestation from mining is estimated to be 12 times greater than expected[23]. An estimated 8 to 10 million hectares of forest have been lost to mining from 2005 to 2015[23].

Environmental Effects of Deforestation on Infectious Diseases

Habitat Loss and Pathogen Spillover

Deforestation leads to habitat loss and fragmentation, which increases human-wildlife interactions due to a rise in competing demand for space between animals and neighbouring human communities[2][3]. Pathogen spillover (a shift of pathogens from one host species to a new host species) can introduce new infectious diseases to humans if these increasingly frequent interactions result in pathogens spreading from wildlife to human hosts and then within the human population[2]. The genetics of pathogens and hosts, phylogenetic distance or time since the most recent common ancestor between hosts, interspecies contact intensity and frequency, and other factors are required for spillover to take place, but despite this complexity, most human infectious diseases stem from wildlife[2]. While contact intensity and frequency between species are only one of many factors required for pathogen spillover to occur, it represents a serious risk to human health and is a driving process of newly emerging infectious diseases[2][3].

Biodiversity Loss

Infectious diseases and biodiversity have an intricate relationship, but it is evident that loss of healthy and diverse ecosystems as a result of deforestation can increase infectious disease spread and human emergence[2][3]. This can occur through diminished ecological niches that normally balance pathogens, disease vectors that carry and transmit pathogens, and their predators. For example, when plant species loss results in habitat fragmentation and threatens predatory animal species which normally keep pathogen-carrying species populations in check[2]. Biodiversity loss can also result in alternative reservoir species (species acting as habitats in which infectious diseases can live, grow, and multiply[24]), vectors, and pathogens when new ecological niches are formed[2]. It is also important to note that the environmental degradation often associated with loss of biodiversity has social consequences (like malnutrition and stress) for human communities depending on these ecosystems which can increase their susceptibility to infectious diseases[2][3].

Ectothermic arthropods like mosquitoes are infectious disease vectors that are influenced by rising temperature, flooding, and rainfall changes, which can facilitate their reproduction, development, and movement into new territory where they may infect human hosts[2][3].

Vector Dynamics

Mosquitoes, ticks, and other arthropods are primary vectors of disease that transmit pathogens like malaria, dengue, and other viral diseases within a species and between species[25][26]. As vector-borne diseases represent 17% of all infectious diseases worldwide[26], environmental effects on the populations of these primary vectors are significant[25]. Deforestation can directly affect vector dynamics by increasing infectious disease vector habitat[2][4]. For example, Anopheles darlingi mosquito populations thrive better in deforested areas as there is more sunlight and pH levels are less acidic compared to forested environments (see Malaria in Mâncio Lima County, Brazil below for more information)[4]. However, vector dynamics are also shaped indirectly by deforestation through its effect on climate change and extreme weather events—in particular, temperature and humidity—which can increase vector species development and survival rates, but also their advance into new habitat[2][25].

Forest fires, facilitated by deforestation and climate change, damage ecosystems and increase community exposure to particulate matter which can result in a greater risk of health conditions that weaken the immune system and contribute to a rise in infectious diseases[2].

Climate Change and Extreme Weather Events

Deforestation contributes to increasing average regional and global temperatures as well as the recurrence of extreme weather events, from cold waves, varied rain patterns, flooding, and severe storms, to drought, heatwaves, and forest fires[2]. These changes in climate and extreme weather events can advance parasitic, fungal, viral, and bacterial infectious diseases in humans by:

1) moving organisms that cause infection away from their natural hosts and environments[2];

2) increasing pathogen reproduction/replication rates, survival, and ability to infect human hosts[2][25];

3) affecting vector dynamics and capacity through increasing presence, spread, and proliferation of agents that carry and transmit infectious diseases[2][4]; and

4) increasing susceptibility of human communities to infectious diseases through effects on social and environmental conditions that heighten the risk of pathogen exposure, reduce the ability to fight infection, and/or increase illness severity once infected[2][3].

Human Diseases Emerging from Deforestation

Malaria in Mâncio Lima County, Brazil

Malaria is an infectious disease caused by parasites that are transmitted to people through the bites of infected mosquitoes[27]. It is not transmissible directly from person to person through physical contact, but it is possible to transmit malaria through blood transfusions or organ transplants[27]. Symptoms may include flu-like illness such as fever, shaking and chills, headache, nausea, and vomiting. In more extreme conditions, it may cause anemia and jaundice (yellowing of the skin and eyes) in individuals with low blood cell counts[28].

Brazil had about 500,000 confirmed cases annually from 1997 to 2006[4] with the majority occurring in the Amazon Basin. One study demonstrated that the incidence of malaria in various health districts in Mâncio Lima County, Acre, Brazil in 2006, was positively correlated with the increase in percentage of cumulative deforestation within each health district[4].

Anopheles darlingi mosquitoes are the main vectors of malaria transmission in the Amazon[4]. They search for larval habitat in sunlit areas, where the water is clear with neutral pH, and are found notably in altered landscapes. A. darlingi are rarely observed in undisturbed forests because the water is shaded and soils are more acidic[4]. The mosquito biting rate and larval count increase with more deforestation. The average biting rate in areas with more than 80% deforestation was 8.33 per night compared with 0.03 per night for areas with less than 30% deforestation[4]. Human-altered landscapes including dams, vehicle ruts, road ditches, and mining pits provide suitable larval habitats for A. darlingi mosquitoes.

Another concern potentially related to health district deforestation patterns is the emerging local aquaculture industry. Ponds with a circumference of more than 50m significantly increase the number of A. darlingi larvae[4]. Fish farms are often located in degraded lands and may lead to more mosquito larval habitats and higher malaria incidence[4].

The epidemic in Mâncio Lima County, Brazil is the result of increased deforestation and aquaculture, creating landscape conditions more suitable to A. darlingi mosquitoes.

Kyasanur Forest Disease Virus (KFDV) in the Western Ghats States, India

KFDV is transmitted to humans by various tick species and is most commonly spread by Haemaphysalis spinigera due to its relatively larger populations and greater viral infection rate to humans and non-human organisms[29]. These ticks can contract KFDV at any life cycle stage as they feed off the blood of other organisms from their larval stage. This increases transmission risk as the ticks not only infect others, but can also infect each other by feeding off of already infected individuals and continuing the spreading cycle of KFDV. As a result, higher population densities of H. spinigera pose a greater risk of exposure to the ticks and infection at any life stage[29].

There have been 47 outbreaks of KFDV identified along the Western Ghats states in India from January 1, 2012, to June 30, 2019[29]. KFDV symptoms begin around an incubation period of 3-8 days in which individuals will experience chills, fever, and headaches[30]. Following the initial symptoms, within 3-4 days, muscle pain, vomiting, bleeding, and gastrointestinal symptoms can occur. This will cause individuals to experience low blood pressure and have low platelet, red blood cell, and white blood cell counts. Generally, individuals will recover after 1-2 weeks, but an estimated 10-20% continue to experience another onset of symptoms. These symptoms may include fever, headaches, mental disturbances, tremors, and vision deficits[30]. Overall, the fatality rate of KFDV is between 3-5%, however, infection rates have been increasing since notable temperature and precipitation changes measured between 1950-2000[29].

KFDV is influenced by dry seasons as that is when ticks look for hosts to feed on. Over time, deforestation in these developing regions in India has influenced the region’s climate by disrupting water and air movement, delaying the rain season, and by increasing temperatures as the land is no longer protected by vegetation[31]. Under these growing dry conditions, the ticks now have longer feeding seasons, which has led to an 83% increase in KFDV outbreak risks[29]. As deforestation in these regions is for agriculture, introducing more humans to the former habitats of the ticks gives them more opportunity to prey on humans and further increases the risks of transmission[29].

Ebola-Zaire Virus (ZEBOV) in Western Africa

The Ebola-Zaire virus, or Ebola virus, is transmitted to humans through direct contact with reservoirs or indirectly through wildlife infection. Duikers, apes, and insectivorous and frugivorous bats are examples of wildlife that can transmit the virus to humans[32]. Unfortunately, outbreaks are an ongoing issue for developing countries like Guinea and the Republic of Congo where challenging political and socio-economic settings have resulted in deteriorating infrastructure and societal function leaving people unable to access proper resources, land, and food[33]. In these countries, poverty is a major driver of human migration, agriculture, mining, and foraging into forested areas[33]. Land-use change has caused habitat fragmentation, forest destruction, and changing forest boundaries as humans encroach into and disturb forested areas where they are exposed to many possible zoonotic infections, like the Ebola-Zaire virus[32].

Symptoms arise in people between 2-21 days after contact with the virus. Individuals may experience fever, aches and pains, weakness and fatigue, gastrointestinal symptoms, abdominal pain, and unexplained hemorrhaging, bleeding, or bruising[34]. If individuals experience later symptoms then that may include red eyes, skin rashes, and hiccups[34].

The first outbreak of the Ebola-Zaire species was in 1976 in the Republic of Congo with more than 300 cases and a mortality rate of 88%, and outbreaks continued until the unprecedented 2014-2016 epidemic[35]. Habitat destruction resulted in land degradation and altered climate which led to further loss of biodiversity. When losing biodiversity, it is said that the chances of developing zoonotic infections increase among some species as there is growing pressure on the resources and land[32]. Therefore, with the addition of biodiversity loss and altered climate, the influx of human populations living in previously forested areas results in a higher likelihood of humans contracting the Ebola-Zaire virus[32].

Severe Acute Respiratory Syndrome - Coronavirus (SARS-CoV) in China

In 2002, China experienced an outbreak of SARS-CoV, more commonly known as SARS, that spread to over 29 countries[5]. It was caused by a variant of the coronaviruses (CoV), which are predominantly spread by bats as these viruses make up 31% of all viruses found in or on bats. Because bats have a higher resistance to viruses than most other species, when they carry a virus, their low mortality rate raises the likelihood for infection and mutation that can allow the viruses to be transmitted to more individuals and species[5]. Yet bats are very important and beneficial for the environment as many of them assist agriculture by pollinating crops and controlling insect populations. They are also very important to sustaining low-income populations as over 56 species of bat are hunted for human consumption[5].

SARS-CoV that drove the 2002 outbreak in China, causes symptoms like fever, dry cough, and shortness of breath[36]. Like most coronaviruses, it is transmitted from animals, and in this case from bats to humans[5]. Once humans are infected, SARS-CoV is transmitted from one person to another through air droplets when an individual coughs, sneezes, or talks, close personal contact, and coming in contact with contaminated objects[36]. Though the symptoms do not necessarily sound serious, severe illness and infection can lead to death as there is a 10% fatality rate[5].

Unfortunately, deforestation and anthropization have led to increased human migration into natural environments. This has forced many animal species to find shelter in human dwellings which creates a higher concentration of wildlife populations in close contact with humans[5]. Especially among lower-income populations, consumption and unhygienic cohabitation with animals, like the bats that have been the source of virus outbreaks, have increased the risk of transmission and mutation of animal-derived viruses that affect humans. This can occur from direct contact, domestic animal infection, and contamination by urine or feces[5].

Deforestation Controls

New Farming Methods

Agroforestry systems in British Columbia, Canada. Integrating agriculture, conservation, and silviculture practices in the same land-use system[37].

Farming methods, such as greenhouse gardens, have been developed in order to cultivate more intensively and reduce agricultural areas. Greenhouse gardens can increase yield by providing optimal growing conditions for the crop and protecting the crops from diseases, harsh weather, and common field pests. However, some scientists claim that the temperature control system for a greenhouse garden and irrigation management are generally supported by fossil fuel energy such as oil and gas, thereby increasing greenhouse gas emissions[38]. A new method called Agroforestry is designed to mimic natural forests by combining trees and shrubs with crops or forage. This system has low dependence on fossil fuels and chemicals, is highly self-maintaining, and has a strong positive impact on water quality and soil[6].

Monitoring Deforestation

There are many suitable and reliable methods to reduce and monitor deforestation. One common method is to monitor satellite images, which is labour-intensive but does not require advanced training in computer image processing or extensive computational resources[39]. Another method is hot spot analysis, which uses coarse resolution satellite data to identify rapidly changing locations, and then uses high-resolution satellite images to conduct detailed digital analysis[39]. From an environmental point of view, it is more important to quantify the damage and its possible consequences, while conservation work focuses more on forested land protection and the development of alternative land use to avoid continuous deforestation[39].

Reforestation/Afforestation

Reforestation refers to the re-establishment of forests lost naturally or by anthropogenic means, while afforestation is the building of a new forest on lands where there were no forests[40]. In many parts of the world, reforestation and afforestation have increased forest coverage[40]. Reforestation on at least 43% of reforestable areas, about 369 million hectares, would likely benefit threatened vertebrates and contribute to species conservation[8]. In the tropics, particularly Indonesia and Brazil, conserving threatened vertebrate species is needed the most. Afforestation has been shown to have overall positive effects on plant species richness and carbon storage, but could lead to a decrease in plant species richness in certain locations[41]. Therefore, spatial planning should avoid afforestation in these places. Reforestation and afforestation can greatly contribute to the mitigation of climate change and provide a way to achieve many sustainability commitments simultaneously[41]. However, all countries must work to overcome key barriers such as unclear revenue streams and high transaction costs to invest in reforestation[8].

A satellite-based system called DEGRAD, developed by INPE (the National Institute for Space Research) for measuring forest degradation in the Amazon rainforest[7].

References

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