First published on SciLogs on July 29, 2014
Author’s note: this post was updated on October 19th, 2014, to reflect more current news and statistics.
The internet is awash with news that Ebola has spread from West African countries to Europe and the United states. History’s deadliest outbreak of Ebola is currently sweeping across Sierra Leone, Guinea, and Liberia, having claimed more than 4,000 lives already. With a mortality rate of anywhere between 40% and 90% (this epidemic’s is around 65%), what is it that makes Ebola one of the most lethal diseases in the pantheon of pathogens? Why is there no cure or vaccine? And why is so difficult to control?
The Basics: Ebola’s full name is Ebola hemorrhagic fever, a rather gruesome title (the details of which we’ll address shortly). It’s caused by the Ebola virus (no surprises there), which is a member of the Filoviridae family. For context, viruses can be 50-100 times smaller than bacteria, and come in a variety of shapes. Ebola virus, along with its sister, Marburg virus, is thread-like in appearance. Ebola is also a zoonotic disease, which means it is transferrable from non-human animals to humans (and vice versa). In Ebola’s case, humans can catch it from non-human primates such as chimpanzees and gorillas, as well as from rodents, bats, and a few ungulates (hooved mammals such as antelope). Transfer of the virus occurs through close contact with infected bodily fluids such as blood, vomit, and feces, all of which usually abound after Ebola has done its damage.
A Little Background: The family of zoonotic diseases includes Rabies, Hanta virus, Anthrax, Lyme disease, the ever-infamous Plague, Toxoplasmosis, and many more, all of which have fascinating life cycles. They also present a unique challenge to communities that live in close contact with the animals that carry these diseases. Animals can often act as a reservoir, meaning they carry the bacteria or virus but present no symptoms themselves. Alternately, the disease can decimate populations of livestock and wildlife upon which a community might depend, contributing to a lack of resources and increased pressure on the human population. Identifying the natural reservoir of a disease is important in containing outbreaks–if you can identify the little bugger that carries the disease without showing it, you can minimize contact with the organism or take steps to control those populations. It’s widely believed that the fruit bat is the main reservoir for Ebola. This is an issue because many communities in Ebola-prone areas consume the fruit that bats snack on (providing easy host-to-host transfer for the virus via bat saliva). Homes are also often located near fruit bat colonies. Some outbreaks may also begin when an individual consumes bush meat, a dead animal carcass found in the vegetation surrounding the community. Consumption of bush meat is traditional in many parts of West Africa, and can sometimes be the only protein that is readily available. If the animal has been killed by Ebola or another zoonotic disease, the individual (and anyone to whom he feeds or sells the meat) can become infected. This ‘human-consumption-of-bushmeat’ infection path is the also leading theory for how the first human became infected with AIDS.
How it Works: Ebola. The nasty, worm-looking virus that’s causing damage in West Africa. There are five subtypes known so far, all named after their place of origin—the one that’s in the headlines right now is closely related to the Zaire Ebola virus, the strain with highest mortality rate. (As of now, no one knows what makes it more deadly than the other strains). So how does it kill you? When your body produces any kind of fever, that’s the immune system’s response to an infection—inflammation can help kill whatever’s bothering your body. Ebola essentially sends your body’s immune system into overdrive. The virus infects the immune cells that are your first line of defense–monocytes and macrophages. This means your body is incapable of killing the virus when you first become infected. The virus then gets to replicate as much as it wants, using your own primary immune cells as hosts until the infection has grown so large that the rest of your immune system can’t help but notice. In a valiant effort to kill the virus, your body produces a fever so high that you go into shock, your tissues start to degrade and your blood pressure drops, and eventually your organs start to fail.
The virus acts quickly. Initial symptoms of Ebola are varied and often resemble the symptoms of malaria, typhoid fever, influenza or various bacterial infections, all of which are more common than Ebola. This misdirection can mean that it takes too long for Ebola to be diagnosed, and by then the secondary symptoms may have set in: diarrhea, red eyes, vomiting blood, gastrointestinal bleeding from the mouth or rectum, hemorrhaging from the nose and mouth, and bleeding in the brain that may lead to seizures and delirium. After the presentation of these symptoms and a miserable week or so, patients either recover or die from systemic multi-organ failure. While the visible hemorrhaging is what gives the virus its name, it actually presents in less than half of the patients infected with Ebola. Unfortunately for the current outbreak, the Zaire strain is the form of the virus that causes the most severe symptoms.
Why There’s No Cure: Viruses are too small to reproduce on their own, so they have to inject their genome into a host cell. This means that when the host cell replicates, so does the virus. In order for the virus to be able to do its injecting, ‘do all the hard work for me’ trick, it needs to be able to attach to a molecule on the surface of the host cell—this molecule is called a receptor. If the virus doesn’t have the equipment to attach to a certain kind of cell’s receptor, it can’t invade that cell. In fighting viruses, it’s important to identify the virus’ corresponding receptor so you can prevent it from binding and thus prevent it from replicating. With Ebola, this is the missing piece of the puzzle—we don’t know what the receptor is, so we don’t have a way to stop the virus from proliferating. In humans, Ebola is capable of invading many different kinds of host cells, so it’s possible that it is capable of binding to more than one receptor, making it a dangerously versatile aggressor. Ebola is also notoriously hazardous to study, although many research universities and institutions are working on unraveling its mysteries.
What’s Happening This Time: The current outbreak is the first time that Ebola has rapidly spread to urban areas. With previous incidents of the virus, responders were able to contain the infection within small, rural populations by identifying modes of transmission, points of origin, and mitigating the damage. With large, closely packed populations, however, the virus spreads faster. It’s much harder to identify who has it, who’s given it to whom, and it’s difficult to stop people from leaving the area. The West African nations suffering from this outbreak also lack adequate medical facilities and personnel. Without isolation wards and advanced labs, the virus is extremely hard to contain. The infection of the physicians on the ground is a result of the sheer number of patients in their charge. They wore all the proper protective gear, but somewhere in the melee they came into direct contact with infectious fluid. Additionally, some traditional rural practices have accompanied recent transplants to the cities–it is customary in some of the cultures affected by the virus to wash a dead relative’s body by hand before the burial. This is obviously an issue when it comes to preventing disease transmission. To tackle the epidemic without losing the trust and cooperation of those being treated, it’s necessary to implement informed, culturally appropriate health care and education.
David Quammen has written a fantastic book, Spillover, about the histories, cultural contexts, and life cycles of the world’s zoonotic diseases. His research makes a convincing case that the next human pandemic could be of zoonotic origin. Will it be Ebola? Probably not. In Q&A with National Geographic, W. Ian Lipkin, of the Mailman School of Public Health at Columbia University addresses the fear that Ebola will spread worldwide. He posits that with the health care systems available in high-income countries like the US and the UK, “it’s unlikely that we would have widespread disease as a result. We would be on top of it, and we would be able to contain it. Our health care system affords people access to gloves and gowns and personal protective equipment. I don’t think there’s reason for panic that we’re going to be hit with an outbreak of Ebola”. The classism and injustice of who gets proper care and aid in times like these is a hard issue to resolve, and stems from deep worldwide inequality of resources, education, and healthcare. All we can do is wait and see, stay updated on developments, and support WHO and Médecins Sans Frontières in their efforts to combat the spread of the disease.
Questions about Ebola or about viruses in general? Comments? Additional resources? Let me know in the comments below or on Twitter @thatlunchbxgirl