Ebola Epidemiology in a Nutshell
The reservoir for the Ebola virus is in African wildlife. Its principal host is the fruitbat; it also can infect great apes and some species of antelope. Humans may come in contact with the virus when they forage for bushmeat, butchering these animals for food. Although the virus appears not to make fruitbats ill, when it infects humans it causes a serious, relentlessly progressive infection with high fatality rates. Fortunately, introductions into human populations are uncommon and, in the past, have occurred in quite isolated small rural communities. This relative remoteness has assisted in their containment.
The current outbreak of Ebola in West Africa was facilitated by an unfortunate confluence of circumstances. First, Ebola had not been seen in that part of the continent previously, so the initial diagnosis was delayed. The initial cases occurred in a location where the borders of the three affected countries joined, thus introducing the infection into three administratively distinct entities. The three countries (Liberia, Sierra Leone and Guinea) all were poor with poorly developed medical care and public health resources. Further, recent political turbulence had made many in the population skeptical of governmental actions. Lastly, Ebola soon was introduced into crowded urban areas, making the tracing of contacts much more challenging than it had been in previous Ebola outbreaks in isolated rural villages.
Ebola infection is not transmitted easily in human populations, except under specific circumstances. After the Ebola virus is introduced into a person, it takes a period of time before it makes the new host sick. This “incubation period” can range from 2 to 21 days; most persons become ill after 5-10 days. The virus cannot be transmitted to others during this incubation period. It is only after a person becomes ill that they become hazardous to others, and then only if others are exposed to the body fluids (for example, vomitus, diarrhea or blood) of the infected person. A further important nuance: when persons first become ill (fever, headache, feeling poorly, loss of appetite), the amount of virus in their bodies still is quite small. Indeed, blood tests for Ebola may not be positive during the first 72 hours of illness, testifying to the low “viral load” at that time. As illness progresses, the virus spreads throughout the body, and, as a result, the patient becomes more hazardous to others. Thus, patients are cared for using elaborate personal protective equipment. The patient remains highly infectious even after death because the Ebola virus has been able to infect skin cells. Thus, bathing the corpse as well as touching or kissing it has been shown to be highly hazardous.
It follows from the above that persons infected with the Ebola virus, but still within their incubation period, cannot transmit virus; the risk of transmission begins with the onset of illness and, even then, is not great until the illness has progressed and the patient starts to vomit and has diarrhea.
William Schaffner is a Professor of Preventive Medicine and Medicine, specializing in Infectious Diseases at Vanderbilt University School of Medicine (VUMC).