Growing up, my old man always told me, “if you leave them alone, they’ll leave you alone.” That was the word in the house pretty much any time a wasp would fly inside or a creepy spider was in the basement. Just let it be. It’s more scared of you than you are of it. That rule extended out to when I was in the woods or swimming in the ocean as well. Don’t fuck with anything. Leave it as you came. I grew up with a kind of fear that if I were to truly fuck with anything in nature, it would come back to bite me in the ass. As it turns out, I was right. We are currently in the midst of the largest Ebola outbreak the world has ever seen. As humans encroach on animal habitat, we will only see more and more of this. The HIV pandemic is a direct result of this. The bushmeat trade itself has grown in large part due to overfishing in the Atlantic off of West-Central Africa. As fish stocks were depleted, locals turned to an alternate source of protein from the forests. Bats are an abundant and valued source of meat in West-Central Africa. They are also riddled with viruses. Which of course brings us to why, why is it that bats of all things seem to carry all these deadly viruses like Ebola so often? How do they not die from it?

Bats and super-immunity 

Bats are very, very important pollinators. They eat a variety of pest insects like mosquitoes. They are the second largest order of mammals, under rodents. They are everywhere. They also happen to carry an exceptional number of viruses with them. They are known to carry rabies, histoplasmosis, several hemorrhagic fevers like Marburg and Ebola and encephalitic viruses like Nipah and Hendra.

These things don’t kill the bats. So bats act as a natural reservoir for the viruses. Nobody knows for certain why bats can survive with something like Ebola virus living inside of it. Though scientists have a pretty good idea.

When you have a fever, your body is raising its temperature in order to kill whatever pathogen is inside of you. When bats fly, their internal temperatures kick up to around 104 degrees F. A temperature like that will kill off a lot of pathogens and this is just normal, every day stuff for a bat. That isn’t all though. Flight boosts metabolic rate way up. This can generate stress and damage cells and DNA if it isn’t detected and repaired in a timely manner. So bats have evolved to have highly efficient damage repair proteins in order to combat the damage done during flight. Bats have a kind of super immune system.

Since these viruses have evolved to live in the bat’s incredibly effective immune system, it’s no wonder why they are so lethal when they finally spill out into other species. Add to this the fact that bats live in huge colonies and are in very close contact with one another very often, able to communicate all kinds of weird diseases to each other, you can see how they are a perfect reservoir.

Deforestation means emerging disease

Bats have been living like this with these pathogens inside of them for a very long time. This is nothing new. What is new however, is the destruction of their habitat and the appearance of humans where there previously had not been. So events like what happened with Nipah virus in Malaysia start to occur.

In 1997, an area of tropical rain forest in Malaysia larger than the size of New Jersey was slashed and burned for industrial livestock farming, namely the farming of pigs. The burning created a massive noxious haze. Then with it came a drought. The fruit trees the local bats lived on were all gone and so the bats moved on to orchards near the pig farms. These bats happened to be carrying Nipah virus. The pigs ate fruit with bat urine and saliva on it and in doing so, transmitted the virus to the livestock.

Nipah causes respiratory and neurological illness. These pig farms are awful places. They are tightly packed, so the virus was spread from animal to animal easily and then on to farm workers. In humans, Nipah will cause fever, nausea, vomiting, headache, and dizziness,  reduced consciousness, flaccid muscles, and spasms. The virus spread and wound up killing 105 people in Malaysia. This is only one example, but I feel it is classic.

Diminishing diversity means more competent vectors

It isn’t only bats that act as vectors, plenty of invertebrates are capable of that as well. Deforestation diminishes mosquito diversity and the species of mosquito that survive and become dominant after these events almost always transmit malaria better than the species whose numbers went down. We don’t know why that’s true, but it always is. This has been observed everywhere malaria occurs.

Anopheles darlingi, a mosquito species from the Amazon, is really good at transmitting malaria, and it has replaced twenty other less effective Anopheles species that were around before the forests were destroyed.

Snails act as a vector for disease as well. Snails that survive these deforestation events tend to be snails that are more adept to open, sunny areas. In turn, they are generally also better able to serve as intermediate hosts for the parasitic flatworms that cause schistosomiasis in humans.

There also tends to be a “dilution effect” in normal, healthy forests that isn’t present in fragmented forest. Greater animal diversity in a particular area means a greater proportion of  hosts that don’t properly carry the pathogen. These are called “incompetent hosts”. When all is normal and right in a forest ecosystem, a pathogen is “diluted” when an incompetent host is poorly able to pass it on to new vectors. This interrupts the infection cycle. It also reduces the chances of a person becoming infected with whatever is out there.

The dilution effect was first discovered with Lyme disease. Mice are highly competent hosts for Lyme. Mice tend to thrive in fragmented forest. So in turn, people run a greater risk for Lyme if they live in or near fragmented forest. The mice carry ticks and ticks are the vectors for Lyme. Large, intact forests naturally have a greater diversity of vertebrates, this means more incompetent hosts, fewer ticks with Lyme. You see how this goes.

Education is key

We are going to see this happen more and more if serious steps are not taken to ensure that our growth is checked. If we continue to disrupt habitat, we are going to continue to see new disease epidemics. Contact with wild animals is something that should be avoided, every step should be taken to do so. Education is very important here. The situation is dire. Individuals and communities should be educated on how diseases are transmitted and what needs to be done about the problem of wild animals used as food. This is far more easily said than done, seeing as many of these regions are extremely poor and war torn, but we ignore it at our peril. After all, “if you leave them alone, they’ll leave you alone” is a great philosophy when it comes to dealing with wild animals. It’s not a good philosophy when it comes to tackling big problems.

Sources:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1247383/
http://wwwnc.cdc.gov/eid/article/11/12/04-0789_article

http://www.pnas.org/content/109/11/4269.abstract
http://thebulletin.org/deforestation-and-emerging-diseases