Is it a coincidence that some of the dreadful viral disease outbreaks in the last 20 years such as SARS, MERS, Ebola, Marburg, Nipah, Hendra including the latest COVID-19 are all attributed to the bats??
It is not a random fact that the bats are hypothesized to be the natural reservoir of the above mentioned deadly viruses and are responsible for severe disease outbreaks in humans and other animals.
What is special about bats in hosting the viruses??
Bats possess several unique features that might help to explain the supposedly high viral load.
?1. Bats love company
Image source Canva
Bats constitute the second largest order of mammals next to rodents. There are over 1200 bat species worldwide, which represents more than 20% of all the classified mammalian species. The bats of different species roost together in huge numbers and close proximity on trees, cave, mines and barns which helps viruses to spread not just between individuals but also between the species.
2. The only flying mammal
Bats are the only flying mammals and some bat species can migrate hundreds of miles providing an opportunity to have direct or indirect contact with other animal and plant species at different geographical locations compared to the terrestrial mammals, thereby increasing the convenience for virus transmission between interspecies.
3. Long life-span
Some bat species exhibit exceptionally long life-spans of 25?35 years (long life compared to the size) facilitating the transmission of the virus to animals and humans. They fly around shedding viruses.
4. Co-evolved with the virus
Bats and viruses could have evolved together and may have contributed to each other’s evolution by maintaining an ecological balance. One such indication of this relationship is the persistence of bats in the absence of clinical signs of disease with bat viruses. Infection and transmission in the absence of disease favor both pathogens and hosts.
However bats don?t get sick with viruses
Are you imagining a sick bat with several viral infections???? No, they don?t get sick with the virus.
The flight may be a unique biological characteristic of bats harboring a large number of viruses without clinical signs of disease.
It is the most distinguishing feature of bats amongst mammals. Flight consumes a large amount of energy and a higher rate of metabolism. The high metabolic rates increase the oxidative damage to the DNA and other cellular structure. Bats possess multiple mechanisms in resisting oxidative damage and some of these mechanisms keep the viral invaders on the check.
Moreover, viruses survive the bats too. The internal temperature of the bat cruises to 40? C during the flight and only some hardy viruses are evolved to tolerate this and viruses can easily withstand human fever.
In Short, the flight may have helped bats to survive the virus and trained the virus to be virtually immune to us.
Furthermore, bats have some biological advantages by harboring well-adapted viruses.
Persistent infections with one pathogen likely enhance the host innate immunity to protect others. Though such mechanisms would not be able to protect every individual in a population, it would be effective in preserving the species.
There could be both extrinsic and/or intrinsic factors contributing to the spillover of the bat-borne virus.
Environmental stresses such as climatic events (typhoons, cyclones, drought, etc.) that destroy bat habitat and their food resources are hypothesized to have an impact on the bat health and bat viral load.
Human interference in the bat environment by artificially increasing the domestic animal densities increases the contact between humans, domesticated animals, and bats. Domesticated animals are found to be the amplifying hosts in the case of Hendra virus, Nipah virus and Menangle virus.
Another important factor is deforestation, which could lead to habitat change in bats and roosting sites which in turn affects their population density, migratory patterns, etc. this imbalance in their equilibrium impacts our equilibrium by the easy spillover of zoonotic viruses.
In the case of the Nipah virus outbreak in Malaysia, agricultural intensification and the co-location of pig farms and fruit orchards were considered as the primary reason for the outbreak. The fruiting trees attracted the bats (flying foxes) that were overhanging the pig farms leading to the spillover of the Nipah virus to the pigs and then to humans.
It is also been hypothesized that there were multiple introductions of the Nipah virus into the pig farm which in turn resulted in the outbreak. The host shift in the virus (from bats to pigs) allowed for high viral load shedding and persistence within a new host. Relocation and trading of the pigs increased the further transmission of the virus and the spread of the outbreak.
The Nipah outbreak in Bangladesh was different from Malaysia. The consumption and trade of date palm sap have allowed for the transmission of NiV from bats to humans.
Another human activity such as cultural traditions of eating wild animal meat in Africa and Asia has led to outbreaks of EBOV and SARS-CoV.
EBOV transmission has been further amplified by the lack of barrier nursing and the use of traditional burial practices.
The desire for wild animal meat including bats and trade in live wet markets caused the susceptible animals to come into contact with bats permitting further transmission of SARS-CoV into humans.
The viral outbreaks are also impacted by the intrinsic factors which are the virus and host factors.
The intrinsic factors affecting the host (bats) are age, body condition, reproductive status, sex, and social status.
Transmission of viruses could also happen during the changes such as changes in the hierarchy of the colony that leads to fighting for dominance, and mating.
In the case of EBOV, it is believed that starvation made the fruit bats come into close proximity during their quest for food, facilitating spillover.
Bats live on average 3.5 times longer than a mammal of similar size, hence long life span in bats promotes the persistence of viruses in the reservoir host, while the ability to fly allows long-distance dispersal of the infectious agent.
But we cannot eradicate all the bats, as they serve us many purposes from the ecological point of view, such as insect control and pollination. We even could learn (research) some immune traits from the bats to survive the viral infection.
So what we can do is leave the bats alone for our better!