Understanding how deforestation and species loss lead to new diseases
Those involved in the ecology of diseases have more and more scientific evidence to point out that deforestation, habitat fragmentation and loss of diversity increase the presence of emerging pathogens, which cause major public health problems.
Hantavirus Pulmonary Syndrome is a disease caused by a virus that was initially only found in some species of rodents, but then passed to humans. In the early 1990s, this transfer from animals to humans was discovered after cases were reported in the United States.
Mexican researcher Gerardo Suzan Azpiri, who received his doctorate in biology from the University of New Mexico, recalls this chapter of science to illustrate how the foundations of what is now known as disease ecology were laid.
This discipline, among other things, seeks to use the tools of various scientific specialties to identify what triggers the spread of pathogens, especially viruses and bacteria. "We see disease as a natural phenomenon that has an ecological explanation and an evolutionary explanation," explains Suzan, in statements to Mongabay Latam.
Scientists working on the ecology of diseases have more and more scientific evidence to point out that deforestation, habitat fragmentation and loss of diversity increase the presence of emerging pathogens, which cause major public health problems.
"We are seeing that the dynamics of many pathogens are increasingly related to the drastic changes we are making to the environment, such as deforestation, contamination, invasion of natural areas or loss of diversity," explained Gerardo Suzán Azpiri, a researcher at the Laboratory of Disease Ecology at the UNAM School of Veterinary Medicine and Animal Science, and who was president of the Wildlife Disease Association (WDA) for Latin America between 2017 and 2019, in an interview.
Dilution effect, a concept that is gaining strength
Since the mid-1990s, the drivers of disease ecology have warned that the fragmentation of ecosystems and the loss of species diversity increase health risks not only to wildlife, but also to humans.
Even at that time, several scientists - mainly biologists - began to talk about a concept now known as the "dilution effect".
Suzán explains that when there is an undisturbed ecosystem, where the local fauna and flora coexist in a balanced way, the pathogens are diluted and, therefore, do not have conditions to generate disease outbreaks.
But when an ecosystem is deforested and fragmented, there are species - together with their pathogens - that begin to dominate; this is where a risk area for a disease outbreak is created.
The Mexican researcher, who is a member of Conabio's Scientific Committee on Invasive Species, mentions one of the examples that have already been documented: where there is ecosystem fragmentation and deforestation, the presence of certain rodent species, carriers of hantavirus, increases. "The more diverse the rodents in an ecosystem, the lower the prevalence of the virus. The same diversity of species," says Suzán, "means that certain strains of the virus are regulated. When the diversity of species is lost, the infections are triggered".
The loss of diversity has not only been associated with hantavirus outbreaks, but also with other conditions such as Lyme disease, which is caused by bacteria acquired through the bite of an infected tick and affects humans, mice, and deer.
Rodrigo Medellín, a researcher at the UNAM Institute of Ecology, is one of the scientific voices who believes that "the dilution effect has shown us that it is more universal than we would have thought.
The dilution effect - says Medellín - can not only be applied to explain the higher prevalence of viruses, but it has also been documented with parasites that cause leishmaniasis or Chagas' disease.
Conserving species in their habitats
At a time when the world is facing the VIDOC-19 pandemic, where it is still not clear how the SARS-CoV-2 virus originated, scientists dedicated to studying the ecology of diseases, but also those focused on biodiversity conservation, agree on the need to reflect on how human health depends, to a large extent, on the health of animals and ecosystems.
Scientists point out that in recent years there has been an increase in the occurrence of zoonoses, diseases that are transmitted from animals to humans.
Scientists studying zoonoses and disease ecology have long warned of the importance of combating wildlife trafficking, loss of diversity and habitat fragmentation. "We had said this when there were outbreaks of diseases - such as SARS, hantavirus, dengue or zygai - in certain regions," Suzan recalls.
Selene Zarate, a member of the Mexican Society of Virology and a specialist in the evolution, diversity, and dynamics of viruses, also agrees that there is sufficient evidence to show that deforestation influences the expansion of outbreaks of various diseases, including those transmitted by insects.
Global warming, Zárate points out, is another factor contributing to increased public health problems. "The presence of mosquitoes -which transmit dengue fever or other diseases- has been documented in places where they were not previously found.
Like other scientists, Luz Dary Acevedo, a veterinarian, and researcher with the Wildlife Conservation Society (WCS-Colombia), points out that there is scientific evidence showing the link between deforestation and activities that change land use with the increase of diseases such as malaria, leishmaniasis or Chagas' disease.
"We are destroying the habitat and populations of wild species that are reservoirs of various diseases," says Acevedo, who also mentions that it has been shown that when a species is extracted from the wild, some of its parasites - with which it was in balance - "take advantage".
The WCS researcher gives the example of a species she has studied in recent years: the gray marmoset (Saguinus leucopus), a primate endemic to Colombia that faces fragmentation of its habitat and is extracted from the wild to be illegally traded as a pet.
In one study, Acevedo compared the health conditions of marmosets in the wild with those in zoos and rescue centers. He wanted to know if, for example, parasites that caused their death in captivity also affected them in the wild.
The researcher found that some parasites present on marmosets in captivity are also present on marmosets in the wild. However, when animals are in captivity, their vulnerability to these pathogens increases, due to stress and adaptation to a new diet.
"In the wild," Acevedo explains, "the parasites evolve with the species and are in balance. When they are in captivity, the animals are stressed and their immune system is weak, so the parasites take advantage".
The researcher has also found how marmosets living in fragmented habitats have different physical and physiological conditions compared to those where the animals have better habitat conditions.
"Viruses, and infectious agents in general," Acevedo insists, "evolve with their hosts, interact with them, stay in balance with them, and exist in a way that is harmless to humans. But we have interrupted those natural cycles and have had direct access to those agents. And as we continue to access and transform these habitats, that risk increases.
Promoting "one health"
In order to prevent the continued creation of "risk zones" for the outbreak of new diseases, specialists agree that it is necessary to take into account that changes in land use in areas with high biodiversity, in addition to contributing to the climate crisis and the loss of species, also generate major public health problems.
For Selene Zárate, who studies the evolution of viruses, in order to have better health for humans, it is necessary to conserve the planet's biodiversity.
With the COVID-19 pandemic that we are experiencing," Suzán points out, "it is time to recognize that "prevention is not just about washing your hands. Prevention, she stresses, is also about conserving species, having healthy ecosystems and respecting natural barriers. "It means having conservation, sustainability and low environmental impact development policies.
To achieve a balance between human health, animal health, and ecosystem health, the planet's biodiversity must be preserved.
At present, Suzán points out, only less than 1% of the diversity of viruses is known. "We still see them as enemies and not as part of the diversity. We need to make inventories, map them, know which are the families of viruses that exist, where they are, how they have evolved; it is necessary to integrate them into biological thinking".
Studying viruses in greater depth would also allow us to have more information about the evolution of living beings, including humans. For example, Selene Zarate explains that in mammals it has been found that the protein that allows the union between the placenta and the uterus has a viral origin.
Valeria Souza, Ph.D. in ecology, who directs the Laboratory of Evolutionary and Experimental Ecology at the Institute of Ecology at UNAM, agrees with Dr. Suzán: the world crisis experienced by the COVID-19 pandemic should trigger a reflection on the role of human beings on the planet. We are," he says, "one more organism that is subject to the laws of ecology and evolution. We are part of nature and, therefore, we cannot abuse it. We are part of a network of interactions, part of a whole. And it is the disdain for nature that has led us to these disasters.