Epidemiology of Ebolaviruses from an Etiological Perspective

Abstract

Since the inception of the ebolavirus in 1976, 32 outbreaks have resulted in nearly 15,350 deaths in more than ten countries of the African continent. In the last decade, the largest (2013-2016) and second largest (2018-2020) ebolavirus outbreaks have occurred in West Africa (mainly Guinea, Liberia, and Sierra Leone) and the Democratic Republic of the Congo, respectively. The 2013-2016 outbreak indicated an alarming geographical spread of the virus and was the first to qualify as an epidemic. Hence, it is imperative to halt ebolavirus progression and develop effective countermeasures. Despite several research efforts, ebolaviruses' natural hosts and secondary reservoirs still elude the scientific world. The primary source responsible for infecting the index case is also unknown for most outbreaks. In this review, we summarize the history of ebolavirus outbreaks with a focus on etiology, natural hosts, zoonotic reservoirs, and transmission mechanisms. We also discuss the reasons why the African continent is the most affected region and identify steps to contain this virus.

Keywords: ebolavirus in Africa; ebolavirus outbreaks; ebolavirus reservoirs; ebolavirus transmission.

Figure 1

All ebolavirus outbreaks registered to date. The size of the circle represents the number of cases, while the color of the circle represents the ebolavirus type responsible for the outbreak. The squares indicate the chronological order of the outbreak, while the color of the squares corresponds to the ebolavirus type.

Figure 2

A map presenting African countries affected by specific ebolaviruses. The date of an outbreak, number of cases, deaths, and CFR per country are presented. (a–d) represent areas affected by SUDV, EBOV, TAFV, and BDBV, respectively. We present the number of cases and deaths recorded by the CDC where possible. However, it should be noted that the actual number of confirmed cases might vary, as suggested by the ambiguity in various reports and published literature. This might affect the CFR mentioned in the figure.

Figure 3

A Venn diagram presenting the bat species found to be positive for ebolavirus antibodies. Eidolon helvum and Rousettus leschenaultii species have been detected to possess EBOV, SUDV, and RESTV antibodies.

Figure 4

Plausible ebolavirus hosts and reservoirs, as well as modes of virus transmission. Bats are considered the primary carriers of ebolaviruses. Some virus transmission to bats may occur from arthropods. Bats could transmit the virus directly to humans or to other bats and NHPs, which serve as amplifying hosts before transmitting to humans. In addition, consuming fruits contaminated with bat saliva or droppings is a plausible mode of virus transmission. Direct infection from NHPs is also suggested. Human-to-human transmission could happen via three mechanisms: (a) contact with body fluids or secretions from a victim; (b) hospital settings, wherein the use of unsterilized needles or syringes and the non-availability of proper PPE equipment results in a brisk viral spread; and (c) traditional burial practices, which can involve close contact with a dead patient.

Figure 5

The ten tacks to thwart the global spread of ebolaviruses. The dotted circles represent communication and coordination channels at every level.