By Natalie Taylor
The coronavirus pandemic, suspected of originating in bats and pangolins, has brought into focus the risk of viruses that jump from wildlife to humans. This is called a zoonotic spillover, and frighteningly, this is more common than you might think. Six out of 10 diseases in humans are zoonotic. Some are benign; others, like COVID-19 can be deadly.
For a spillover to occur, a number of things have to line up. First, the pathogen (virus, bacteria, fungus) needs to find a way to jump from animal to human. It may be eaten by a hunter. It could be picked up on the ground after being nibbled or handled by an infected animal. It may be passed through a mosquito bite. Or, it can be inhaled. Once the pathogen arrives, it attempts to stick to where it has landed, evading the body’s defenses long enough to set roots. All of these events happen with greater ease when humans are close to animals and there is interaction between them. Proximity is opportunity.
In 1997, a huge rainforest area in Malaysia was burned to make way for agriculture. With trees and their fruits gone, fruit bats went looking for food in nearby orchards. Soon after, pigs and pig farmers began to fall sick probably after eating fruits the (disease-carrying) bats had nibbled on. The Nipah virus caused deadly outbreaks across Southeast Asia and has recurred since.
In 1969 in Liberia, forest clearings brought hordes of mice to plantations and settlements, passing on the Lassa virus. Virus-carrying rodents have appeared in deforested areas in Panama, Bolivia, and Brazil. Spillovers of infectious diseases to people is more common in the tropics because overall wildlife and pathogen diversity is higher, but scientists have seen a connection between deforestation and Lyme disease in the northeastern U.S.
Yellow fever, malaria, Venezuelan equine encephalitis, Ebola, SARS, and now COVID-19 have all spilled over from one species to another. And they have spilled over as forests are destroyed. The more we clear, the more we come into contact with wildlife that carries deadly microbes. Many viruses exist harmlessly within host forest animals because the animals have co-evolved with them. But not humans. So it’s a numbers game. The more we degrade forest habitats, the more likely these epidemics are to occur.
Disturbing a forest upsets nature; it breaks up the balance between pathogens and people. Scientists have long warned that reshaping Earth’s landscapes will greatly affect climate and biodiversity. Now, evidence shows that forest loss can increase the risk of animal-borne infectious diseases, similar to the pandemic we are living with today.
Health experts believe that the novel coronavirus, the cause of COVID-19, most likely originated in bats and spread to humans in China. Interaction between wildlife and humans is definitely a contributing factor to pandemics, including our current one. Unfortunately, poaching and deforestation have increased during the lockdown. Instead of mitigating the problem, humans seem bent on intensifying it.
This is the quandary for humanity. We’ll address the potential solutions next week.