With the Ebola virus all over the news these days, it can be hard not to think about it. I remember reading the book The Hot Zone when it first came out, and being surprised when the threat seemed to simply evaporate.
Since it has returned to the center of the global stage I find myself pondering the question “Why?” Looking at the history of Ebola outbreaks, a pattern emerges: A brief, intense period of activity followed by a period of no activity. Why does the virus seem to disappear for sometimes years at a time? What are the conditions or lack thereof that stop the spread of infection? And, can we identify and reproduce these conditions?
According to the CDC, that pattern looks something like this:
- 1976-1979: First recorded case of Ebola virus in humans–638 infected in Zaire, Sudan, and England (laboratory-infected)/454 deaths (71% mortality rate)
- 1980-1994: No documented cases of Ebola in humans*
- 1194-1997: Ebola resurfaces–468 infected in Gabon, Côte d’Ivoire, Democratic Republic of the Congo (formerly Zaire), South Africa (aid worker and nurse), and Russia (laboratory-infected)/349 deaths (75% mortality rate)
- 1998-1999: No documented cases of Ebola in humans*
- 2000-2004: Ebola resurfaces–743 infected in Uganda, Gabon, Republic of Congo, Sudan, Russia (laboratory-infected)/485 deaths (65% mortality rate)
- 2005-2006: No documented cases of Ebola in humans*
- 2007-2009: Ebola resurfaces–445 infected in Democratic Republic of Congo and Uganda/239deaths (53% mortality rate)
- 2010: No documented cases of Ebola in humans*
- 2011: 1 infected in Uganda/1 death (100% mortality rate)
- 2012-2013: 53 infected in Uganda and Democratic Republic of Congo/20 deaths (37% mortality rate)
- 2014: ~4655 infected in outbreaks across Guinea, Liberia, and Sierra Leone, with limited travel-associated and/or localized infection in Nigeria, Spain, USA, and Senegal/2431 deaths (52% mortality rate)
4 years active/15 years silent
4 years active/2 years silent
5 years active/2 years silent
3 years active/1 year silent
4 years (so far) active…
Perhaps we are due for another period of silence, soon.
A few things to note:
- I do understand that the scope of this most recent outbreak–100 times greater than the mid-90s event that sparked the book The Hot Zone–is unlike anything we’ve seen in the past and that is likely to extend the active dates well beyond the previous pattern.
- All of the numbers of infected and deaths are reported numbers only. Many additional cases may have occurred that were not tracked.
- The CDC website tracks Ebola cases on an outbreak basis. I have combined both locations and numbers here to simplifying the math in my search for an over-arching trend. Looking at each outbreak and each location individually would yield different patterns.
- I am not a researcher, a statistician, or any kind of health care professional, and all of my musings should be taken, at best, as conjecture. I am simply a curious onlooker playing with numbers and asking questions.
* Reston virus, a variant of Ebola found in animal populations in the Philippines, did occur during otherwise Ebola-free years as follows:
- 1989-1992: Outbreak in Philippine monkey populations–7 laboratory workers in the USA, Philippines, and Italy exposed, all developed antibodies but no symptoms
- 1996: Outbreak in Philippine monkey populations and virus identified in USA-based lab–0 human exposure
- 2008: First known cases of virus in pigs–6 farm and slaughterhouse workers in the Philippines developed antibodies but no symptoms