Bats and the origin of outbreaks

Scientists have long suspected that the rate of new infectious diseases could accelerate, especially in developing countries where human and animal interaction is increasing.

Changes in the environment are driving displaced species of animals into new habitats, allowing them to mix with other species or potential hosts.

Those shifts, combined with greater human interaction with animals as people move deeper into forests, increases the chances of a virulent virus jumping species.

This kind of spillover, when a pathogen in one species could start circulating in another and potentially create a new disease – is what appears to have happened in China with the virus that causes COVID-19. Like many infectious viruses introduced this way, the outbreak is believed to have started with bats.

Data shows that the closest known relative of the novel coronavirus is a virus discovered in horseshoe bats in southwest China.

Zoonotic diseases, those caused by pathogens that spread between animals and people, can be problematic because the human immune system has not evolved to fight against this type of invasion.

The reservoir host species often displays no symptoms despite carrying the pathogen, as the hosts and germs are often well-adapted to each other. However, when these pathogens - viruses, bacteria or other disease-causing microorganisms - jump from animals to humans, the effects can be devastating.

Bats were again thrust into the spotlight as they were thought to be the original host of the SARS-CoV-2 virus that causes COVID-19. Molecular studies have demonstrated that bats are natural reservoirs to many other viruses, some of which have already led to disease outbreaks.

Many deadly viruses in the past have originated from bats including the deadly Ebola outbreaks in Western Africa. Nipah, also carried by bats, has already caused human outbreaks across South and South East Asia and has “serious epidemic potential”, according to global health and infectious disease specialists.

The coronavirus family of viruses also includes diseases such as Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS). Scientists have discovered that SARS and MERS were caused by viruses that originated in bats, with other animals serving as intermediate hosts.

Why bats?

Bats are a group of flying mammals, with more than 1,300 species in 20 families, according to IUCN. They make up approximately 20% of all mammal species and are found all over the world except for the Arctic, the Antarctic and a few oceanic islands.

Bats first appear in the fossil record roughly 50 million years ago and represent the third group of flying vertebrates in Earth’s history, after the flying reptiles called pterosaurs and birds.

The only other group coming close to harbouring as many viruses are rodents, the most diverse group of mammals. There are approximately 2,300 species of rodents in 33 families, making up about 40% of all mammals. Rodents are believed to harbour more viruses as a group but bats harbour more viruses per species.

The diversity of both groups has been seen by scientists as a possible mechanism for driving virus diversity, as the greater number of species can create more potential niches for viruses.

Some bats roost in forests while others in caves. Most bats eat insects such as beetles, moths and mosquitoes. Some bats eat fruit, nectar, seeds and pollen from flowers while others eat small animals such as birds, fish, frogs and lizards. A small number of bats consume blood.

The smallest bats are the Kitti’s hog-nosed bat at about 3cm (1.2 inches) and the biggest, known as a flying fox, can grow to around 45cm (18 inches).

Studies have shown that bats are unique when it comes to hosting zoonotic viruses even when compared to rodents, as bats host more zoonotic viruses per species than rodents do. Here are some factors that could potentially aid this.

Longevity

Apart from diversity, other traits that make bats suitable as virus hosts include their size and longevity.

Bats have relatively long life spans for their body size, which can make it easier for viruses to persist as chronic infections are more common.

Sympatry & hibernation

When bat species with large ranges migrate or use seasonal roosting sites for hibernation, this increases potential exposure to pathogens. Moreover, members of some bat species live together in large communities with close-quarter colonies in locations such as caves.

Flight

Bats are the only mammals capable of powered flight. There is a high energy and metabolic demand for flight, leading to elevated body temperatures in bats which is similar to the effects of human fever that occurs during immune response. This means some viruses they carry have adapted to be more tolerant to higher temperatures, potentially bad news for other animals if infected.

Why bats are important

Bats provide many beneficial ecosystem services. Some bats play important roles such as plant pollination and seed dispersal. Bats can pollinate more than 500 species of plants including avocados, bananas, dates and mangoes.

In Southeast Asia, durian, a highly valued crop, can be effectively pollinated by the Dawn Bat. In this sense, bats are important economically for people. Some bats can also play critical roles in spreading seeds and regrowing forests.

The insect-eating bats also serve as natural biological controls of insects, consuming millions of them at night, including some major crop pests.

Conservation

More than 200 bat species (~15%) in 60 countries are considered threatened with extinction and more than 20 are critically endangered. In addition, eight bat species have been documented as having gone extinct in the recent past. Bat population decline is not a regional issue but rather a global one.

There are nearly 250 bat species classified as Data Deficient (~19%), a relatively high proportion when compared to other mammals in general (~13%) or birds (~1%), showing that for many bat species not enough is known to even assess their status.

Threats such as habitat loss, climate change and the wildlife trade are global phenomena leading to mass biodiversity loss. For example, tens of thousands of flying foxes in Australia and South Asia have died because of extreme heat waves.

Bats are also exploited for food or traditional medicine, as around 170 of bat species have been recorded to be hunted. Given their larger size, Old World fruit bats are disproportionately affected by hunting, with roughly half of the species in that family hunted.