Ebola. For those who are familiar with it, that word strikes fear into the heart. It is the name of a tropical disease, which has been back in the news recently due to an outbreak in several West African Countries. Ebola is a disease in a class known as hemorrhagic fevers, which are endemic to Africa. It has a high mortality rate, usually killing over half of those who get it ̵ sometimes as many as 90% of patients may die. Because of its fairly rapid progression and gruesome symptoms (uncontrollable internal and external bleeding), it has become a staple for thriller writers and conspiracy theorists, many of whom imply that it could become a weapon for terrorists or the next great global pandemic.
Ebola was first identified in Zaire, in Central Africa, in 1976. While there were only a few hundred cases over the course of the epidemic, the mortality rate was horrific – 88% of those who were diagnosed with the disease died of it. It even wiped out 11 of 17 staff members at a hospital where many of the patients were treated. Thankfully, after a couple of months, the disease vanished as silently as it came. Subsequently, the disease has reoccurred periodically, always in Africa, infecting a few hundred unfortunates at most.
What is Ebola?
Ebola is caused by a virus. A virus is an infectious particle, consisting of genetic material enclosed in a protein coat, which is sometimes surrounded with a membrane. There has been an ongoing debate among scientists as to whether viruses are even alive. It is important to understand that viruses are not bacteria, which are cells. Unlike most bacteria, viruses cannot propagate on their own because they do not contain the cellular machinery necessary to replicate themselves. In order to reproduce, they must infect a host cell, where they hijack the host’s reproductive mechanisms, often killing the host cell in the process.
Ebola is an example of a zoonosis, which is a disease that is transmissible between humans and some animal species. Often, zoonotic viruses are less deadly to their animal hosts than to humans, which means that the virus can be constantly maintained in the animal population. Recurring epidemics are a characteristic of zoonotic diseases, as they are periodically transmitted from the animal host to the human population. It is often difficult to identify the natural reservoir for a zoonosis. Since we know that Ebola also occurs in anthropoid apes, it is strongly suspected that they are the reservoir for the disease.
At onset, most viral diseases present similar signs and symptoms, making them difficult to diagnose until the secondary symptoms appear. Early Ebola symptoms are flu-like, including tiredness, fever and chills, a sore throat, headache, and general body aches and pains. Unlike some other viruses, an Ebola infection is usually systemic, meaning that multiple bodily systems are affected, including the respiratory system, the circulatory system and the nervous system. One of Ebola’s more frightening symptoms is internal and external hemorrhaging, which can present as black marks under the skin or bleeding from bodily orifices. However, death from Ebola is rarely due to blood loss – rather, it is caused by multiple organ failure brought on by internal clotting and disruption of the body’s fluid balance.
The incubation period for Ebola is about a week or two, meaning that the patient is infected but shows no signs or symptoms during that time. The disease typically runs its course in two to three weeks. It is largely unknown why some people survive infection, and others do not.
There is no specific treatment for Ebola disease. Since it is cause by a virus and not a bacteria, antibiotics are not effective. There has been some work with antiviral drugs, but human tests have yet to be done. Currently, treatment consists of supportive care. It is possible to develop an effective vaccine against Ebola, but such a vaccine is likely years away because of economic considerations, as well as the periodic nature of the outbreaks.
Is Ebola the Next Global Pandemic?
While Ebola is of grave concern in the regions in which it is endemic, it is not a good candidate for the next global pandemic, for several reasons.
One measure of the ease of transmission of a communicable disease is the basic reproduction rate, known as R0. R0 indicates how many secondary cases of a disease develop from each primary case. So an R0=2 means that an infected person is likely to infect two other people. For a disease to spread throughout a population, R0 must be greater than 1. For example, the R0 for measles, a highly communicable disease, is 12 -16. R0 is lower for other communicable diseases such as diphtheria and smallpox. The R0 for Ebola is estimated as 1 – 4. This value is low, likely because Ebola is principally transmitted by direct contact with the blood and body fluids from infected patients, and not by casual contact. This implies that Ebola is not transmitted during the asymptomatic incubation phase. Further evidence for the poor transmissibility is that Ebola epidemics die out rather quickly. For comparison, The Black Death (bubonic plague) in Europe lasted some seven years, from 1346 – 1353, and the 1918 Spanish Flu epidemic lasted two years. The lifetime of an Ebola outbreak is usually measured in months, not years.
The devastating symptoms of active Ebola also disqualify it as a good candidate for a pandemic or a weapon. Patients are likely to be hospitalized and quarantined, not walking around among the population like a flu patient in the early stages would be. The fact that Ebola kills rather quickly also works against it becoming widespread in a population.
The persistence of Ebola in Africa is due to the fact that it is a zoonosis, with a natural reservoir in the primate population. These host animals are not found in other locations. The virus would have to mutate to become suitable for a different host in the wild in order to become established in a different area. Compare this with another zoonosis, bubonic plague. The natural reservoir for that disease was a flea that lived on a rat that was found virtually worldwide.
Finally, medical care and public health policies in other parts of the world are superior to those in the poor countries where Ebola now rages. It is likely that the disease would exhibit both lower mortality rates and transmissibility in more developed countries. It would come under control and die out sooner.
Global air travel and perfunctory inspections when crossing national borders make the possibility of a global pandemic very real, but thankfully, it does not appear that Ebola will be the disease that causes it.
Photo credit: CDC/Cynthia Goldsmith – Public Health Image Library, #10816 This media comes from the Centers for Disease Control and Prevention‘s Public Health Image Library (PHIL), with identification number #1081