Why do bats carry so many diseases?
Nipah, Hendra, Ebola, Marburg, SARS. These are some of the world’s scariest viruses. Hemorrhagic fevers like Ebola are extremely fatal they kill up to 90 percent of people infected while SARS, a coronavirus, has a lower mortality rate but spreads incredibly rapidly. All of these nasty pathogens have surfaced in humans in just the last 50 years, and they are all carried by bats. Which to be clear, really isn’t bat’s fault. The recent rise in outbreaks is likely due to humans and our animals creeping ever-farther into bat’s territory, especially in the tropics.
In Malaysia, for example, the spread of commercial farms into bat-inhabited forests led to the first human outbreak- via pigs of Nipah. And in Australia, human Hendra cases are cropping us as destruction native forests force fruit bats to feed in suburban gardens. But still bats do appear to carry more human-killing diseases than pretty much any other animal.
One big reason is that, with a few notable exceptions, bats love the company. Different kinds of bats often roost together in huge numbers and close quarters, which helps viruses spread not just between individuals, but also between species. What’s more, most infected bats don’t die. They live pretty normal bat lives flapping around and giving the viruses time to spread. The flight may be the reason bats are so resilient to infection. As a rule, mammals can’t produce the immense amount of energy needed for flight without also producing a lot of reactive waste products that damage our DNA. So when our bat cousins took to the air, they to leveled up their in-flight DNA damage repair kits and other defenses, including specialized cells that keep viral invaders in check. So bats can survive the deadly viruses but what may matter even more for humans anyway, is how the viruses survive the bats.
Nasty as they are, most viruses are also extremely finicky- to thrive, they require the perfectly controlled climate inside a normal, resting, on-the-ground mammal. But when bats take to the air, their internal temperatures cruise to around 40°C. Those frequent in-flight saunas are far too toasty for your average virus, but a few hardy viruses have evolved to tolerate the heat. Which incidentally, means they can weather a meager human fever. Essentially, the flight may have helped bats gain virtual immunity to viruses and trained viruses to be virtually immune to us. Stupid flying. So, what should we land-lubbers do? We need bats for pollination and a whole bunch of other things- maybe we could even learn some immune tricks from them- like how to be good at not getting cancer! Plus, bats aren’t the biggest carriers of human disease. Humans are, just do the math.
How do Viruses jump from animals to humans?
More often than not, sick animals don’t infect humans. But when they do, these cross-species infections or viral host jumps, have the potential to produce deadly epidemics. So how can pathogens from one species infect another, and what makes the host jumps so dangerous? Viruses are a type of organic parasite infecting nearly all forms of life.
To survive and reproduce, they must move through three stages: contact with a susceptible host, infection and replication, and transmission to other individuals. As an example, let’s look at human influenza. First, the flu virus encounters a new host and makes its way into their respiratory tract. This isn’t so difficult, but to survive in this new body, the virus must mount a successful infection before it’s caught and broken down by an immune response. To accomplish this task, viruses have evolved specific interactions with their host species. Human flu viruses are covered in proteins adapted to bind with matching receptors are on human respiratory cells. Once inside a cell, the virus employs additional adaptation to hijack the host cell’s reproductive machinery and replicate its genetic material.
Now the virus only needs to suppress or evade the host’s immune system long enough to replicate to sufficient levels and infect more cells. At this point, the flu can be passed on to its next victim via any transmission of infected bodily fluid. However, this simple sneeze also brings the virus in contact with pets, plants, or even your lunch. Viruses are constantly encountering new species and attempting to infect them.
More often then not, this fails. In most cases, the genetic dissimilarity between the two hosts is too great. For a virus adapted to infect humans, a lettuce cell would be a foreign and inhospitable landscape. But there is a staggering number of viruses circulating in the environment, all with the potential to encounter new hosts and because viruses rapidly reproduce by the millions, they can quickly develop random mutations. Most mutations will not affect, or even prove detrimental, but a small proportion may enable the pathogen to better infect a new species. The odds of winning this destructive genetic lottery increase over time, or if the new species is closely related to the virus’s usual host. For a virus adapted to another mammal, infecting a human might just take a few lucky mutations. And a virus adapted to chimpanzees, one of our closest genetic relatives, might barely require any changes at all. It takes more than time and genetic similarity for a host jump to be successful. Some viruses come equipped to easily infect a new host’s cells but are then enable to evade an immune response. Others might have a difficult time transmitting to new to hosts.
For example, they might make the host’s blood contagious, but not their saliva. However, once a host jump reaches the transmission stage, the virus becomes much more dangerous. Now gestating within two hosts, the pathogen has twice the odds of mutating into a more successful virus. And each new host increases the potential for a full-blown epidemic.
Virologists are constantly looking for mutations that might make viruses such as influenza more likely to jump. There’s a huge diversity of viruses that we’re only just beginning to uncover. Researches are tirelessly studying the biology of these pathogens. And bye monitoring populations to quickly identify new outbreaks, they can develop vaccine containment protocols to stop these deadly diseases.
In the end, we will leave with a warning. The new outbreak is not as far we think. We might see a new outbreak next year. So, if we want to survive, we should act and reduce animal humans interaction as much as we can. And we failed, I don’t think humanity will survive the new virus.