Microbiology Nuts & Bolts
  • Home
  • Basic Concepts
    • What is infection?
    • Normal flora
    • Diagnosing infection
  • Microbiology
    • Basic bacterial identification
    • Interpreting bacteriology results
    • Interpreting serology results
  • Infection Control
    • What is infection control?
    • Universal precautions
    • MRSA
    • Clostridium difficile
  • Clinical Scenarios
    • Respiratory infections
    • Urinary infections
    • GI infections >
      • D&V
      • CDAD
    • CNS infections
    • Skin & bone infections
    • Sepsis
  • Antibiotics
    • Antimicrobial Stewardship
    • How antibiotics work
    • How to choose an antibiotic?
    • Reviewing antibiotics
    • Antibiotic resistance
    • Testing antibiotic resistance
    • Penicillin allergy
    • Theraputic Drug Monitoring
  • Guidelines
  • Lectures & Lecture Notes
    • Medical Students
    • Curriculum for the Foundation Program
    • Foundation Year 1
    • Foundation Year 2
    • Other Lectures
  • The Bug Blog
  • Buy the book...
  • NEW Edition Updates
  • Peer Reviews
  • Our Facebook page
  • Want to know more?
  • Contact

Don’t blame the bats!

2/7/2020

 
Bats are getting a hard time at the moment. They seem to be being blamed for all of the infections threatening our species, from Ebola, Marburg, Rabies, Nipah, Hendra and now SARS Cov2. The name for these animal-related human infections is zoonoses. I can just imagine the bats cringing every time the news comes on wondering what they’re going to be blamed for next.
 
You have to admit, it’s not great PR to be associated with the scariest viruses known to cause deadly human infections. But is this fair? Are bats really MORE likely to be the source of infections in humans? Do bats harbour more nasty viruses than other animals? Or are they just getting a bad press and we should cut them some slack?
​There is no denying that bats do carry some pretty nasty viruses but they are not affected by these virus, they are just a host. It is thought that the bats immune system has evolved to the “immunological environment”; responding to these viruses that infect them in such a way that they don’t develop infections with these microorganisms. This is just the same way as we have evolved with the different organisms of our own normal flora which don’t cause us any problems.
 
And yet there appear to be a lot of bat viruses that cause disease in humans…
 
Why are there a lot of bat viruses affecting humans?
The reason that there appear to be a lot of bat viruses affecting humans is that there are a lot of different bat species; lots of different types of bats means lots of different viruses.
 
Research has shown that when you look at the extent of variation within bird and mammal species there is no single species that is more likely to cause zoonoses than any other. In fact the relationship is linear as shown in the graph below (Mollentzea and Streickera); the more subspecies there are within an animal group the more viral zoonoses there are. Bats cause a lot of zoonoses because there are lots of different subspecies of bats and therefore there are lots of different bat viruses.
rate of zoonoses from animals
Click for larger image
Mollentzea & Streickera found that from a total of 673 virus species, 415 species from 30 families had compelling evidence for transmission by one or more of the 11 taxonomic orders of non-human reservoirs (three avian, eight mammalian). And medicine has only named a handful of zoonotic infections out of these 415 species… yikes! Cetartiodactyls e.g. whales, dolphins and even-toed ungulates… (just so you know there are odd-toed ungulates too?!) as well as rodents were the most common reservoirs in their dataset, together accounting for half of the included viruses (50.6%). Anseriforms (e.g. ducks, geese, swans) on the other hand were the most poorly represented, with only 1.4% of viruses. Their work showed no support for the “special reservoir hypothesis”, which predicted that viruses associated with certain species groups, would be more likely to be zoonotic. They conclude that virus transmission to zoonotic infection is a feature of viral clades (common ancestors) rather than host; the more species-rich animals maintain more viruses and hence transmit more zoonoses. Added to this is their observation that the risk of zoonotic emergence is higher in regions with species-rich habitats; more species, more virus species and therefore more zoonoses (damn there goes my trip to the DR Congo jungle) … 

So it isn’t the species itself that’s the risk factor for zoonotic infection but rather the biodiversity within a habitat; more animals = more microorganisms = more zoonoses.
 
But it can’t just be about the number of species can it?
Okay, so there might be other factors at play as to why bats get such a bad press.
 
One reason is that these “bat viruses” are particularly nasty. Let’s consider the mortality from the best known:
  • Rabies 100%
  • Marburg 90%
  • Ebola 70%
  • Nipah 70%
  • Hendra 50%
  • Middle Eastern Respiratory Syndrome (MERS) 35%
  • Severe Acute Respiratory Syndrome (SARS) 10%
 
The mortality from these diseases is very high, “Killer Disease” headlines, and that doesn’t even take into account the widespread chaos that can result; just look at the global reaction to recent Ebola outbreaks or the financial crisis of Covid-19. But this isn’t a “feature” of the bats but rather the “viruses” themselves. They are just bad viruses for humans, which are just some of the bats’ “normal flora”. These “viruses” are highly virulent and transmissible but that’s not the bats fault!
 
There are plenty of other bad zoonoses which have close to 100% mortality if untreated including African trypanosomiasis (sleeping sickness), visceral leishmanisis and pulmonary anthrax (which can all be transmitted either via tsetse fly, sand fly or directly from infected domestic and wild animals) and yet these “Killer Diseases” don’t have the same bad reputation of the bat zoonoses (oh! was it cos you hadn’t heard of these ones?!! ARGH, panic!!)
 
Are we prejudiced?
I think part of the reason that societies centre the blame on bats is due to the fact that lots of people find bats creepy and unpleasant. Historically they have been the “bad guys” in popular culture e.g. often seen as a ghost, shadow of darkness or a separate soul, vampire bats were used in folklore to discourage people staying out late, seeing one meant death or destruction was sure to happen in their near future, and then there’s the blood drinking villain in Dracula who was able to turn into a bat to evade capture. The Chinese on the other hand believe bats are associated with happiness and longevity. Then there’s Count Duckula…who didn’t love him?! And Batman’s a superhero…
 
Human nature appears to apportion blame where we can and bats make an easy target. Generally they aren’t a cuddly cute looking animal either, they only come out at night and they live in caves and other dark places… OK I get the concerns.
 
Should we be batty about bats?
There are some of us who actually think they do possess some cuteness, are thrilled to see them take flight in mass at dusk, are amazed that they can hunt in the dark using echolocation and are excited to see them when exploring the places where they live. Personally I think they’re amazing and think we should care for them more! They are a protected species here in the United Kingdom.
 
Did you know there are over 1,400 different species of bats worldwide; making up about 20% of all mammal species? And within the bat species there is a huge amount of diversity and they are the only mammal which can fly. At one end of the scale we have flying foxes with a wingspan of up to 2m weighing 1.5kg, and at the other end there are bumblebee bats the size of bees weighing just 2g! Bats have different dining habits too, from blood to insects, to fruit and pollen. They are truly an amazing group of mammals!
A Bumblebee bat?!
A Bumblebee bat?!
A Bumblebee bat
A real Bumblebee bat!
​Another amazing fact is that bats are more closely related to us humans than they are to rodents, even though they look more like mice. But then again ECIC has just discovered that humans share 41% of their DNA with bananas! (from the animated Genome by National Human Genome Research Institute)… Keep on topic ECIC!!!
Humans have more in common with bats
Blame the humans
There is another school of thought as to why bat viruses transfer into humans and that is to do with the environment, or more specifically our constant destruction and invasion of the different habitats.
 
For a bat virus to cross into a human there usually has to be close proximity between bats and humans. This tends to happen when humans push into the bats habitat, trade in wildlife, cut down the forests and destroy bat roosting places. This forces the bats to use human dwellings as roosts, and forces bats to use farms and plantations as food sources, which all bring bats into closer contact with humans.
 
This doesn’t just apply to bats. If we push into new habitats we will eventually become exposed to new microorganisms that we, as a species, haven’t been exposed to before. This is exactly what has happened with the coronaviruses SARS Virus, MERS Virus and SARS Cov2. These viruses were probably not “new” but we humans hadn’t come into contact with them before; when we did we had no immunity to them and we became sick.
 
Again this isn’t the fault of the rodents, pigs, birds or bats; if anyone is to blame it’s us, humans. Our increasing population, our need for more agricultural land to feed this growing “meat-eating” population and the demand for products (soy, beef, palm oil), that come from this land means we are “evicting” more and more animals. We are forcing the animal contact and increasing the likelihood of zoonotic exposure to new microorganisms.
 
What does this all mean?
Current policy with regards to pandemic awareness and tracking of potential zoonotic agents is currently targeted at high risk animals or “special reservoirs” such as bats, birds, pigs and rodents but this probably isn’t really the right approach? The “special reservoir hypothesis” is that physiological or ecological traits of these animals make them more likely to maintain zoonotic viruses or transmit them to humans. Whereas the “reservoir richness hypothesis” believes host species maintain a similar number of viruses with a similar per-virus risk of zoonotic transmission. Variation in the number of zoonoses among animal groups e.g. rodents and bats arises as a consequence of their species richness, rather than the species alone.

​It would be wrong to ignore animals we know carry zoonotic diseases but what some scientists are proposing is that we should be looking at highly biodiverse places where there is close contact between humans and nature. On these margins where humans and lots of animals meet, like jungles and forests, we need to invest in research and make sure we look at the human pathogenic potential of the microorganisms we find. Globally we should see the investment as an insurance policy; if we invest the millions now we won’t have to pay the trillions later… if nothing else Covid-19 should have taught us this lesson.
 
DO NOT HANDLE BATS… it’s not good for you but more importantly it is not good for the bat! It also illegal to do so without a license in the UK.
References
  • Bat Conservation Trust  https://www.bats.org.uk/
  • Viral zoonotic risk is homogenous among taxonomic orders of mammalian and avian reservoir hosts. Nardus Mollentzea and Daniel G. Streickera Proceedings of the National Academy of Sciences of the United States of America (PNAS) 2020; 117 (17): 9423-9430 

Comments are closed.

    RSS Feed

    Blog Author:

    David Garner
    Consultant Microbiologist
    Surrey, UK

    Please DO NOT advertise products and conferences on our website or blog

    Categories

    All
    Antibiotic Resistance
    Antibiotics
    Basic Concepts
    Clinical Scenarios
    Guidelines
    Infection Control
    In The News
    Microbiology

    Archives

    August 2021
    July 2021
    June 2021
    May 2021
    April 2021
    March 2021
    February 2021
    January 2021
    December 2020
    November 2020
    October 2020
    September 2020
    August 2020
    July 2020
    June 2020
    May 2020
    April 2020
    March 2020
    February 2020
    January 2020
    December 2019
    November 2019
    October 2019
    September 2019
    August 2019
    July 2019
    June 2019
    May 2019
    April 2019
    March 2019
    February 2019
    January 2019
    December 2018
    November 2018
    October 2018
    September 2018
    August 2018
    July 2018
    June 2018
    May 2018
    April 2018
    March 2018
    February 2018
    January 2018
    December 2017
    November 2017
    October 2017
    September 2017
    August 2017
    July 2017
    June 2017
    May 2017
    March 2017
    February 2017
    January 2017
    December 2016
    November 2016
    October 2016
    September 2016
    August 2016
    July 2016
    June 2016
    May 2016
    April 2016
    March 2016
    February 2016
    January 2016
    December 2015
    November 2015
    October 2015
    September 2015
    August 2015
    July 2015
    June 2015
    May 2015
    April 2015
    March 2015
    February 2015
    January 2015
    December 2014
    November 2014
    October 2014
    September 2014
    August 2014
    July 2014
    June 2014
    May 2014
    April 2014
    March 2014
    February 2014
    January 2014
    December 2013
    October 2013
    September 2013
    August 2013
    July 2013

    Categories

    All
    Antibiotic Resistance
    Antibiotics
    Basic Concepts
    Clinical Scenarios
    Guidelines
    Infection Control
    In The News
    Microbiology

    RSS Feed

Powered by Create your own unique website with customizable templates.