Possible Seasonal Pattern of Ebola Virus Disease in the Democratic Republic of the Congo

Based on available documents from the direction responsible of outbreaks in DR Congo, the country experienced since 1976 seven recognized Ebola outbreaks, 3 out of 7 occurred in the North of Equator (Yambuku (Bumba), Isiro and Djera) and 4 out of 7 occurred in the South of Equator (Kikwit, Cuvette Ouest, Mweka 1 and 2 and Luebo).

The 1^st outbreak (Yambuku, Bumba) from September 1976 to October 1976 coincide with the rainy season.

The 2^nd outbreak (Kikwit) occurred over decade from the first outbreak, this deadly outbreak occurred from January 1995-July 1995 which matches with both seasons.

Author emphasized that the outbreak started during the rainy season then continued in dry season, with a slight different in terms of mortality (higher during rainy season compared to the dry season) (Figure 1).

Figure 1: Democratic Republic of the Congo map of Koppen climate classification.

The 3^rd outbreak (Cuvette Ouest), from December 2002 to December 2003 is the longest outbreak observed in DR Congo but with less lethality compared to the Kikwit episode. Like the second outbreak, it’s covered both seasons.

The 4^th outbreak (Mweka 1 and 2), from May 2007 to October 2007. The first outbreak (Mweka 1) occurred during the dry season and last until the rainy season. From November 2008 to January 2009, Ebola resurfaced in the same region (Mweka 2), as we can see it occurred during the rainy season.

The 5^th outbreak (Luebo), occurred during the same period with the Mweka 2, both outbreaks are linked since the distance between Mweka and Luebo is 36 miles.

The 6^th outbreak (Isiro) from August 2012 to October 2012 matches the period starting from the minor dry season to the rainy season.

The 7^th outbreak (Djera), the last one, from August 2014 to November occurred during the minor dry season then continued during the rainy season prior ending the major dry season (Figure 2).

Figure 2: Possible seasonal pattern of Ebola virus disease.

The onset of these outbreaks appeared to be linked with higher absolute humidity and lower temperature (23 degree Celsius for DRC) if we consider the parameter of time [1].

Seasons have possibly an impact in the trend of outbreaks related to the EVD, in the north of Equator, 2 out 3 outbreaks started during the dry season, except from the first outbreak (Yambuku in 1976) which occurred during the rainy season; and in the south of Equator, 4 out 4 outbreaks started during the rainy season (except the outbreak of Mweka 1 who started during the dry season).

Even though, the occurrence of Ebola Virus Disease is closely bind to ecological factors, to date there is no clear evidence of Ebola seasonality in DRC, even if it is clear that seasons have an impact on our behaviors, further studies will be required to determine the association between seasons and Ebola Virus Disease in DRC.

1. Rosello A, Mossoko M, Flasche S, Hoek AJV, Mbala P (2015) Ebola virus disease in the Democratic Republic of the Congo, 1976-2014. eLife digest 4: e09015.