Health and biodiversity Part I: Wild diseases

Jan van der Ploeg/CIFOR

By |2022-04-28T06:25:07+00:00May 5th, 2020|Biodiversity, Conservation, Essay, Evidence, Health, Illegal Wildlife Trade, In-Depth|Comments Off on Health and biodiversity Part I: Wild diseases

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.

A better response could be mediated using conservation techniques such as community outreach about the importance of bats as pollinators, seed dispersers and pest controllers, therefore the value of protecting them and their habitat, as well as the dangers of bat capture and consumption – and the danger of any supposed bat control techniques that in fact makes these animals move around more, thus increasing the chances of them spreading disease. Habitat conservation is also extremely important; in Central and South America, the conversion of forested habitats into pastures shifted the dominant food source for vampire bats from native vertebrates to livestock. This increased rabies transmission from vampire bats to livestock and domestic animals, and consequently humans.

Conservation solutions

Wildlife conservation encapsulates many different disciplines, from forest restoration (as shown here by the Mpingo Conservation and Development Initiative (MCDI) in Tanzania), creating protected areas, interventions for endangered species, community outreach, lobbying against land use change and much more. Image © International Tree Foundation

The current pandemic has brought conversations with conservationists about disease control into mainstream media. If more than 70% of all emerging diseases affecting people have originated in wildlife and domesticated animals, could conservation provide solutions to preventing future outbreaks?

The short answer is yes. Conservation preserves biodiversity, and biodiversity itself shields us from the spread of diseases through a mechanism called the dilution effect. In a richly diverse ecosystem, intermediary species which are not very infectious (so are less likely to pass on a zoonotic infection such as a coronavirus) are likely to outnumber intermediary species and individuals which are highly infectious, diluting the rate of infection. There are lots of other aspects to this, such as an intact food chain in a healthy, diverse ecosystem where predators will control the numbers of more infectious animals (such as rodents, ticks, etc.).

In addition to the diluting effect of biodiversity, many of the factors which have led to the transmission of zoonotic diseases from wildlife to humans (including COVID-19) are being tackled by conservationists: habitat loss and degradation, industrial agriculture, unsustainable, illegal and/or unregulated wildlife trade, and the vulnerability of communities living alongside wildlife, to name just a few.

Habitat loss and degradation

Habitat loss is a key driver of the spread of zoonotic diseases. The more fragmented the habitat, the longer the ‘edge’ where crossover between wildlife and domesticated animals or humans is more likely. Image © Chris Scarffe

Habitat loss and degradation (when a habitat is degraded so far as to lose most of its biodiversity due to human activities such as pollution, development, agriculture, livestock farming, war etc.) causes animals to live in smaller and smaller spaces. This makes it easier for diseases to spread through populations as well as causing them stress, which can compromise their immune systems.

A lack of habitat also means a lack of food, which can cause animals to leave their home ranges to look for alternatives, and when there is a lack of connecting habitat, this often forces them to enter human-inhabited areas. This increases the likelihood of wildlife-human disease transmission, but also wildlife-intermediary-human transmission through domesticated animals such as pets (free-ranging dogs are a well-known intermediary host for zoonotic diseases) and livestock. The conversion of natural habitats to land for human uses also rarely takes into account the ‘edge’ effect – where fragmenting natural habitats into unconnected ‘pieces’ spread across a landscape increases the amount of edges there are which form ‘contact zones’ between humans and wildlife where disease transmission may occur. Another angle to the correlation between habitat loss and increased disease transmission is that the species which are more likely to survive disruption (for example, rats and some bats) are also the ones which are most likely to promote transmission. By losing other species in the ecosystem, natural control systems like predation and competition are lost and these species thrive.

But by conserving habitats and restoring degraded areas, we can reduce this risk. Conservation NGOs, local communities and governments have already implemented habitat protection models such as protected areas, national parks, community conservancies and indigenous-managed conservation areas – they just need to be supported and scaled up. These also have the additional benefits of cleaner air and water, less human-wildlife conflict (especially when protected areas are part of greater connected networks) and recognising and compensating communities and indigenous peoples for their role.

The expansion of industrial agriculture