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Bats, Encroachment into Habitat, and New Pandemics. Part 2

In Part 1 of this review, we have examined whether bats are “special” as virus hosts, and how environmental changes affect their ability to infect other animals and humans. It turns out that bats mostly pose danger when their immunity is weakened due to stress. We have also seen how the environmental conditions make the potential receivers especially vulnerable to the virus. 

Now it is time to discuss how human activities can exacerbate the new epidemics, where these epidemics are likely to emerge, and what are the action strategies against their spread.

During the past 40 years, about one-third of Southeast Asian forests have disappeared. Rainforests were cut down for wood, because of agricultural development, and the uncontrolled expansion of cities (Afelt, Frutos, and Devaux 2018). The forests were replaced by houses, barns, vegetable gardens, farms, orchards, and woods. Sometimes orchards are planted next to a livestock farm, as fruit bring extra income to farmers, and trees provide additional shade.

What remains after deforestation is called a fragmented forest, that is, divided into relatively small isolated areas. One type of fragmentation is the so-called forest perforation (Figure 1).

Figure 1. Perforated forest in New England, USA. Source

The forests of Upper Guinea, West Africa, have also declined by a third from 1975 to 2013, with 84% of their former area being lost before 1975. On the place of the rainforest, there appear monocultural oil palm plantations. Amazonian forests are cut down a similar way, for the oil palm and sugarcane. When the new plantations do not require cutting the forest, most likely they are relocated to the place of farms, while these farms move further in the forests.

Recall now that the Nipah henipavirus, which causes dangerous encephalitis in humans, as well as the coronaviruses SARS-CoV and SARS-CoV-2, the current pandemic virus, have jumped to humans from animals in Asia. Ebola filovirus, which causes hemorrhagic fever and is fatal in half the cases, has spread to humans in West Africa (Figure 2). And all these viruses are hosted by bats. Studies suggest that this is not just a coincidence.

Figure 2. Forest fragmentation in Central (a, b) and West Africa (c, d)
The top panels show the situation as of 2000, and the bottom ones – as of 2014. Dark green color indicates untouched forests. Yellow color on the map indicates the edge of the forest. Orange color (especially visible in image b) corresponds to zones of perforated forest.
Yellow triangles on the map represent the first detected cases of human infection with the Ebola virus, after which outbreaks of the virus started from 2004 to 2014. These first cases are caused by the transmission of the virus from animals to humans, and most of them occurred in areas of growing forest fragmentation.
Source: Rulli et al. (2017). Click on the picture to see the full resolution (opens in a new tab)

When a forest is cut down, the bats’ habitat is depleted. Their immunity is weakened due to a lack of food and the need to look extensively for nutrients. As we have seen, it is in this state that bats begin to spread infections.

The zones inside and adjacent to the forest, shaped by humans, attract a variety of bats. In orchards and palm plantations, fruit-eating flying foxes get their food. Insects flock to the light of dwellings, attracting insectivorous bats, and bats that are used to sleeping in caves move to abandoned houses and barns (Plowright et al. 2015; Afelt et al. 2018).

We usually think that some animal species simply die out because of deforestation. It is not always so. Bats, devoid of habitat and food source, look for it everywhere and also near people. The diverse landscapes of the former forest territories only contribute to the multitude of viruses close to humans.

Now, imagine a farm in Southeast Asia. There, pigs are bred and mango trees grow nearby, tree branches hanging over the piggery to provide an extra shadow. At night, hungry flying foxes – hosts of the virus – fly to the farm and eat the fruit. Half-eaten fruit falls to the ground, with the infected saliva and excreta left on it. The next day, it is eaten by pigs that are not immune to the virus. After some time, an outbreak of the disease occurs in pigs, and then in farmers who contact with the swine. Before this, some infected piglets have already been sold to other regions of the country, and other people became infected from them. This is the story of the first major outbreak of the Nipah virus in Malaysia in 1998-99 (Pulliam et al. 2012).

There is another version of why it was in those years that the Nipah virus was introduced to farms. Then, due to the slash-and-burn method of deforestation, Southeast Asia was covered with haze. At the same time, there was a drought caused by the temperature anomaly El Niño. As a result, the remaining trees bore very little fruit. Therefore, in search of food, the species of flying foxes from other places migrated to Malaysia and infected the fruit trees of the farms (Chua, Chua, and Wang 2002). However, the recent evidence shows that cases of infection had been observed even before the haze and drought occurred (Pulliam et al. 2012). Thus, we can assume that the virus did not spread from migrating flying foxes, but from the local ones, and El Niño only exacerbated the emerging epidemic, caused by deforestation and lack of nutrition in bats.

Figure 3 shows the aforementioned pathways of the Nipah virus transmission:

  1. Flying foxes – the natural hosts of the virus – drink the date palm sap and leave drops of biological fluids in it.
  2. The palm sap is sold or left to ferment – but not subjected to disinfecting heat treatment.
  3. Traditionally, the sap is drunk in the first few hours after collection. One way or another, in a sugar-rich environment, the virus survives for a long time and is transmitted to humans.
  4. Flying foxes come to fruit trees located next to pig farms. They eat fruit, leaving biological fluids on them.
  5. Half-eaten fruit falls to the ground, where pigs and other animals pick them up and get infected.
  6. Infected pigs are slaughtered and/or sold.
  7. Humans eat infected pork.
  8. In close contact, the Nipah virus can be transmitted from person to person. (There is a hypothesis that not all strains of the virus can be transmitted this way. However, recent epidemics in Bangladesh and India have witnessed some infections from sick people. See Singh et al. 2019.)
Figure 3. Nipah virus transmission routes
Source: Singh et al. (2019). Click on the image to see the full resolution (opens in a new tab)

The most important consequence of deforestation is the increased contact of bats with domestic animals and people.

In the case of the MERS-CoV coronavirus and the epidemic of the Middle East respiratory syndrome caused by it, the first transmission of the virus to humans did not occur in the tropical zone of a fragmented forest, but in contact with camels (and the camels probably picked the infection up from vespertilionid bats). However, the virus was also found in another species of bats, Taphozous, which lived in the ruins of houses. Other pets in contact with bats could spread the infection, too (Afelt, Frutos, and Devaux 2018).

Proceed to page 2 to see what forecasts the researchers make about the next transmission of the virus from bats to humans.

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