Bats, Encroachment into Habitat, and New Pandemics. Part 1
The new coronavirus pandemic is commonly believed to have started from contact with infected animals in a market in Wuhan. But such markets are by far not the only place where carriers, or hosts, of dangerous viruses come close together with people and other animals, not immune to such infections. People’s contacts with new viruses and, correspondingly, outbreaks of diseases are associated, in part, with the “human encroachment into wild habitat”. But what exactly is behind these words? How does human intervention in nature affect the emergence of new pandemics?
We will discuss this on the example of epidemics that have spread from bats. For the review, we ran a systematic literature search in the scientific publication database Scopus and found the publications devoted to bats as carriers of viruses. (For a description of the data and visualization of the science map, see page 3 of the review.)
The search results are split into clusters that correspond to the main groups of viruses transmitted by bats. In addition to rabies and influenza viruses, these are coronaviruses that cause acute respiratory syndromes; filoviruses, such as Ebola and Marburg, causing hemorrhagic fever; and henipaviruses, such as Hendra and Nipah, leading to dangerous encephalitis.
It appears that the impact of humans on nature and the subsequent spread of new viruses has long been discussed in the scientific literature – on the case of henipaviruses.
This is not surprising since the outbreaks of diseases caused by henipaviruses occurred earlier than the famous epidemics of coronaviruses. So, the Hendra henipavirus was first seen in Australia in 1995, while the noticeable Nipah henipavirus epidemic unfolded in Malaysia in 1998-9 (Mackenzie et al. 2001).
In terms of virus transmission, an important difference is that coronaviruses live in microbats, while henipaviruses are spread by megabats, or flying foxes (Figure 1). These are two different suborders of the bats order. They differ, in particular, in size and diet. Microbats are small and mostly insectivorous, although there are also predators and vampires among them. Flying foxes reach 1.5 m in the wingspan, and feed on fruits, nectar, pollen, and sometimes insects. (Remember the differences in diet, as food is an important infection pathway.)
In the review, we will use the conventional term “bats” for both types of mammals. When it comes to flying foxes or a certain species of microbats, we will note this explicitly.
We will mainly discuss the cases of henipaviruses spread by flying foxes. Where appropriate, we will also draw on examples of filoviruses and coronaviruses. All of them have pandemic potential (Luby 2013; Simons et al. 2014). The diseases they cause are characterized by high mortality: for encephalitis from the Nipah virus it is 40–75% (Singh et al. 2019), for Ebola fever it is 50% on average and has been up to 90% before (Ebola Virus Disease).
This review is divided into two parts. Today we will discuss whether bats are “especially” active hosts of the virus, and how environmental changes affect their activity.
Why are all eyes on bats?
There is a debate in science about whether bats are “special” as hosts of viruses. Some argue that humans most often get infected from a narrow range of animal groups, including bats (Luis et al. 2013). In other words, a bat species hosts relatively more zoonotic infections than a species of any other animals.
Opponents of this hypothesis believe that all animals spread viruses equally actively. It is the species diversity’ that differs, so the diversity of the transmitted viruses varies accordingly. The more species of an animal there are and, consequently, the more different viruses this group of the animal carries, the greater the likelihood that some of the viruses from this group of animals will spill over into humans.
This second stance is supported by the April publication by Mollentze and Streicker (2020). The paper is based on the most (to date) comprehensive dataset on the relations between viruses and hosts. According to the study, bats mostly do not differ from other animals in the frequency with which they transmit viruses to people (except for the rabies virus). The danger posed by these animal hosts follows a statistical pattern:
The more species of an animal there exist, the more different viruses this animal group hosts, and, accordingly, the more viruses are transmitted to people. Bats are no exception.
Figure 2 shows that this pattern holds for many types of animals. The most diverse species are among rodents, and they are also the most active carriers of viruses to humans. There are about half as many species of bats; therefore, they host proportionally fewer viruses, and fewer diseases spill over into humans from them.
The line shows a partial effect in the model The line shows the linear regression fit
(R2 = 0.88, P < 0.001)
X-axis: (1) the number of animal species (log); (2) the number of virus species. Y-axis: (1) the effect on the number of virus species transmitted to humans; (2) the number of virus species transmitted to humans. The lines of a partial effect/regression are shown and the 95% confidence interval is indicated by the shading.
We can see that the point corresponding to bats on both graphs is included in the confidence interval or is located close to it. If bats transmitted a disproportionate amount of viruses to humans, this point would be much higher on the graph, indicating a significant deviation from the general pattern.
Source: Mollentze and Streicker (2020). Click on the pictures to see the full resolution (opens in the same tab)
Although bats seem to be not “special” in the sense of transmitting viruses to humans, from a physiological and environmental point of view, they are unusually predisposed to host infections.
Bats have a very strong immune system, presumably associated with their, unique among mammals, ability for sustained flight (O’Shea et al. 2014). Therefore, viruses that enter their bodies usually do not cause disease, and henipaviruses probably hardly even replicate – that is, they rarely reproduce, infecting new cells (Halpin et al. 2011). So viruses remain in the body without manifesting themselves.
Besides, bats sleep in caves, where sometimes thousands of individuals of different species gather, and if one of them is sick, then many bats can become infected. Moreover, bats live very densely in caves, hanging over each other and thus spraying each other with infected biological fluids. In such an unsafe environment, every single contact with the virus rarely leads to infection, but when many individuals spread it, the chance of getting infected increases manyfold (Plowright et al. 2015).
Viruses, in turn, in the course of evolution have adapted to the strong immune defenses of bats (see a review in Calisher et al. 2006). There is a hypothesis, although not verified experimentally, that due to this adaptation, the infections are very serious and even fatal when transmitted from bats to other hosts including humans (Luis et al. 2013).
Please proceed to page 2 to read why bats, being such good virus hosts, do not transmit it to humans too often.