The Four Waves of Ebola 四波伊波拉

Ebolavirus was first identified in 1976 in Africa.  Over the nearly forty years of history, it has been found to cause acute viral illness with sudden onset of fever, intense weakness, muscle pain, headache, and sorethroat. Shortly after there will be gastrointestinal symptoms (vomiting and diarrhoea), rash, organ damage and various bleeding disorders.  There will be decreased white cells and platelets as well as abnormal liver enzymes.

As regards its transmission, it is through contact of body fluids containing the virus.  There is an incubation period of 2 to 21 days.  People are infectious as long as their blood and secretions contain the virus - can last much longer after their own recovery.

Ebolavirus belongs to the family of Filoviridae (the filovirus).  There are two other genus in this family, the Marburgvirus, and Cuevavirus.

Genus Ebolavirus comprises 5 distinct species. Zaire ebolavirus (EBOV) seems to be causing the worst and deadliest outbreaks and Sudan ebolavirus (SUDV) and Bundibugyo ebolaviruses (BDBV) can also cause highly fatal diseases. The Reston ebolavirus (RESTV) is found in the Phillipines and Mainland China.   can infect humans but no known human illness or deaths have been reported.  The Taï Forest ebolavirus (TAFV) has been reported causing a non-fatal case in Cote d'Ivoire in 1994.

A summary of major Ebola outbreaks from Bloomberg (http://www.bloomberg.com/news/2014-08-18/ebola-spurs-nigeria-surveillance-regional-border-limits.html)

The above graph, published at Bloomberg in an article on the subject by Zoker and Chen, has nicely shown the geographical relationships of the major outbreaks across the African continents over the past years.  When we look further into the species of the ebola virus causing these outbreaks, we can actually identify three different patterns caused by three out of the five species of Ebola virus.

The Four Waves of Spreading Zaire ebolavirus (EBOV)

This Zaire ebolavirus seems to be the most deadly species of the genus.  It was first detected (1976) in an outbreak in 1976 in the then Zaire, which subsequently changed to the present Democratic Republic of Congo, in Central Africa.  The species causing this outbreak was identified as the Zaire Ebola virus.  It caused 391 reported cases with 281 deaths, with a case-mortality rate of 88%.  It was placed under the Genus Ebola virus because cases were initially found in northern Zaire near the River Ebola.

The second wave (1994 to 1996) of Zaire Ebolavirus was detected in 1994 in Gabon - after an apparent 18-year quiescent period after the 1976 outbreak.  In 1995, an outbreak of the same virus also occurred in Zaire resulting in 315 cases with 254 deaths, with mortality again high at 81%.  The outbreak in Gabon continued till 1996 with 143 cases and 97 deaths and mortality at 68%.  One fatal case, presumably an imported case from Gabon, was detected in South Africa in 1996.

The third wave (2001 to 2008) of Zaire Ebolavirus outbreak occurred fiver years later in 2001 in Gabon again, and also newly found in Congo, the country located between Gabon and Zaire.  Zaire has undergo political change and had been renamed as the Democratic Republic of Congo since 1997, had its third Zaire Ebolavirus outbreak in 2007 to 2008.  The case numbers in Gabon, Congo and the Democratic Republic of Congo were 65, 249, and 296 with case-mortality rate of 82%, 85% and 68% respectively.

Ebolavirus Outbreak Dec 2013 to Aug 2014 in West Africa (WHO) http://who.int/csr/disease/ebola/geographic-map-29-aug-2014.png?ua=1)

As we can see now, the current outbreak occurring in West Africa since December 2013 is basically the fourth wave (2013 to current) of the Zaire Ebolavirus outbreak.  Over the past five years of silent period, the virus has travelled through more than 3,000 km west to Guinea, with subsequent detection in Sierra Leone, Liberia, as well as Nigeria.  Based on the WHO statistics, as of 11 August 2014, the number of cases in these countries are:

• Guinea - 510 cases, 377 deaths
• Liberia - 670 cases, 355 deaths,
• Sierra Leone - 783 cases, 334 deaths, and
• Nigeria - 12 cases, 3 deaths.

Sudan ebolavirus (SUDV) primarily in Sudan and Uganda

Around the time of the first Zaire ebolavirus outbreak, a similar outbreak was also detected in a neighbouring country, Sudan (north to Zaire), due another species, named Sudan Ebolavirus.  The worst first outbreak in 1976 resulted in 284 cases with 151 deaths, with a case-fatality rate of 53%; followed by two smaller outbreaks in 1979 (34 cases and 65% deaths) and in 2004 (17 cases and 41% deaths).

Sudan ebolavirus was detected outside Sudan in Uganda, a country south to Sudan in Central Africa, in 2000 and again in 2011 to 2012.  In the first outbreak, there were 425 cases with 224 deaths (case fatality rate 53%).  The second one was smaller causing 32 cases and 22 deaths (69% deaths).  

Budibugyo eboavirus (BDBV) detected in 2007 and 2012

There is a third species, the Budibugyo ebolavirus, found to be able to cause human outbreaks and deaths.  It was first detected in Uganda in 2007 with 149 cases and 27 deaths, with a case fatality rate of 25%.  It was detected in Democratic Republic of Congo (the previous Zaire) in 2012 resulting in 57 cases including 29 deaths.

Reference: 

Ebolavirus outbreaks updates at World Health Organization

Elise Zoker and Caroline Chen.  Ebola Spurs Nigeria Surveillance, Regional Border Limits at Bloomberg access http://www.bloomberg.com/news/2014-08-18/ebola-spurs-nigeria-surveillance-regional-border-limits.html.