As the COVID-19 pandemic has put both our health systems and our relationship with nature in the spotlight, Nina Seale investigates the relationship between health and biodiversity.
Let’s begin with the elephant in the room: diseases that have transferred from animals to humans. Not just COVID-19, but Severe Acute Respiratory Syndrome (SARS), HIV, bird flu (H5N1), Ebola, swine flu, malaria, Lyme disease and rabies are some of the most well-known infections humans have picked up from wildlife (known as zoonotic diseases).
Trade in wildlife for uses such as consumption and medicine are thought by many scientists to be responsible for the emergence of COVID-19 in 2019 (potentially from the Huanan Seafood Wholesale Market in Wuhan, which is thought to have been selling wildlife including snakes, porcupine and deer); SARS in 2002 (thought to have transferred to humans from masked palm civets and/or raccoon dogs which were being sold for consumption in Guangdong’s animal markets); Ebola (consumption of raw bushmeat (infected fruit bats, monkeys, apes, forest antelope or porcupines) in West and Central Africa); and HIV (HIV-1 is thought to have infected chimpanzee hunters for the bushmeat trade and HIV-2 from Sooty mangabey monkeys, which are hunted and kept as pets). Livestock has also been traced as a source in the case of the Middle East Respiratory Syndrome (MERS) coronavirus outbreak in 2012, which transferred to humans handling domesticated camels or possibly from consumption of raw camel products like unheated milk.
Why bats?
Bats are often ‘reservoir hosts’ for viruses, meaning they serve as a source of infection and potential reinfection of other species and as a means of sustaining a virus when it is not infecting others. Image: DMCA
With the exception of HIV, there is evidence that all of the above viruses originated in bats, and were then passed to intermediary animals (possibly pangolin for COVID-19, civets for SARS, camels for MERS etc.) before they infected humans. Bats are thought to be ripe for the rapid evolution of viruses into highly aggressive strains due to their fierce immune response (which possibly evolved as a result of bat flight, but that’s a fascinating conversation for another time). Viruses adapt to reproduce quickly, which bats can tolerate but these high viral loads then become extremely infectious when bats (or bat droppings) come into contact with other species. Therefore, in a species where the immune response is not nearly as high as a bat’s (like humans), it is extremely aggressive, leading to outbreaks of severe and highly infectious viruses like COVID-19.
There are conservation implications for bats as a result of being reservoir species for viruses. A predictable response is that human populations affected by a disease which originates in bats will call for large-scale culling of wild bat colonies. This happened when Hendra virus emerged in Australian flying fox populations and in Latin America, where vampire bats are culled as an attempt to control rabies. However, instead of dealing with the problem, this approach has been shown to have the opposite effect. For example, a study of the relationship between vampire bat culls and rabies transmission in Peru found that the size of a bat population barely affected rabies transmission. Plus the culling techniques disproportionately killed adult bats, who are less likely to transmit viruses than young bats, and this also caused greater movement between colonies – spreading viruses more widely.