Epidemiology and Management of the 2013-16 West African Ebola Outbreak

Abstract

The 2013-16 West African Ebola outbreak is the largest, most geographically dispersed, and deadliest on record, with 28,616 suspected cases and 11,310 deaths recorded to date in Guinea, Liberia, and Sierra Leone. We provide a review of the epidemiology and management of the 2013-16 Ebola outbreak in West Africa aimed at stimulating reflection on lessons learned that may improve the response to the next international health crisis caused by a pathogen that emerges in a region of the world with a severely limited health care infrastructure. Surveillance efforts employing rapid and effective point-of-care diagnostics designed for environments that lack advanced laboratory infrastructure will greatly aid in early detection and containment efforts during future outbreaks. Introduction of effective therapeutics and vaccines against Ebola into the public health system and the biodefense armamentarium is of the highest priority if future outbreaks are to be adequately managed and contained in a timely manner.

Keywords: Ebola virus; epidemiology; filoviruses; outbreak management; point-of-care testing; viral diagnostics; viral genetics; viral therapeutics; viral vaccines.

Figure 1

Transmission of Ebola virus (EBOV). (a) The natural reservoir(s) of EBOV is unknown. There is evidence that fruit bats may serve this role. A number of animals, including apes, may be infected with EBOV and serve as intermediate hosts by transmitting the virus to humans. Humans may be infected by exposure to EBOV-contaminated surfaces or food, such as fruit, or by consuming an EBOV-infected animal. Sequencing studies have suggested that the West African Ebola virus disease outbreak was likely initiated by a single animal-to-human transmission event. (b) Human-to-human transmission of EBOV sustained the outbreak. Humans can be infected by direct contact with cases, because the virus can be found in sweat and saliva. Bodily fluids, such as blood, or diarrhea fluids can also serve as a source of infection and may be stable on surfaces for up to one week. EBOV on surfaces is inactivated by exposure to sunlight or disinfectants (such as bleach) or by drying. Transmission of EBOV from humans via suspended small aerosol droplets (<5 μm) is not known to occur, but large droplets (>5 μm) potentially transmit EBOV at a distance of up to 2 m (126). (c) EBOV can persist in immunologically privileged sites such as the central nervous system, eyes, or testes. Reactivation in or from these sites coupled with evasion of immune responses can lead to inflammation and other sequelae. EBOV in semen can result in sexual transmission from males. It is unclear whether sexual transmission from female Ebola virus disease survivors occurs.

Figure 2

Past outbreaks of Ebola virus and related filoviruses. The 2013–16 outbreak is the first filovirus outbreak to have occurred in West Africa, except for the lone human case of Taï Forest virus infection in 1994. Outbreaks of filovirus disease in Africa have involved representatives of four species: Ebola virus (yellow), Sudan virus (red), Bundibugyo virus (blue), and Taï Forest virus (orange). The size of each outbreak is approximated by the size of the corresponding circle. Some outbreaks involving single cases or limited importation from endemic regions are not depicted. Also delineated (dashed line) is the approximate combined range of several prominent species of fruit bats that may serve as reservoirs for Ebola virus. Each virus outbreak is listed with its year(s), the number of deaths over the total number of cases, and the case fatality rate. The map showing African rainforests is reproduced from Mayaux et al. (127), with permission.

Figure 3

Four phases of the 2013–16 West African Ebola virus disease outbreak. Weekly numbers of confirmed Ebola virus disease cases in Guinea, Liberia, and Sierra Leone are depicted. Although no cases were confirmed in 2013, investigations suggest that Phase 1 of outbreak began with limited local transmission in November or December of that year. Phase 2 of the outbreak, marked by widespread transmission, began May 25, 2014, when the first case of Ebola virus disease was diagnosed by Augustine Goba, director of the Kenema Government Hospital viral hemorrhagic fever laboratory. Phase 3 involved sporadic transmission chains that were blunted by the international and community responses begun in January 2015. Phase 4 of the outbreak is driven at least in part by sexual transmissions from survivors and began in August 2015.

Figure 4

Spread of major Ebola virus Makona variants during the four phases of the West African outbreak. Phase 1 of the outbreak involved limited local transmission of the Guinea 1 variant (GN-1) in the tricountry area. There were limited incursions of GN-1 to Conakry, Guinea, and to southern Liberia. Phase 2 began with Sierra Leone 1 (SL1) and Sierra Leone 2 (SL2) variants, which appear to have arisen from GN-1 in the tricountry area. SL2 was largely replaced by the SL3 variant, which spread via major roads in Sierra Leone from east to west. SL2 and the genetically related sublineages GN-2 and Liberia variants LB1–LB8 were dominant in Phase 2. Sporadic transmission chains involving a mixture of GN-1 and SL3 variants occurred in western Sierra Leone and the Conakry area of Guinea during Phase 3. Viruses related to those that were prominent during Phases 2 and 3 have been isolated from a limited number of cases in Phase 4. Because these Phase 4 viruses show limited genetic change, they appear to have come from sites of limited replication in Ebola virus disease survivors.

Figure 5

Comparison of Ebola virus diagnosis using quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) and rapid immunodiagnostics. (a) Management of the Ebola virus disease outbreak in West Africa relied on diagnosis of the disease in central laboratories by qRT-PCR. (b) An alternative strategy involving the use of antigen capture lateral flow immunodiagnostic assays followed by qRT-PCR confirmation could improve case identification and triage of Ebola virus disease patients. Some images have been digitally modified from unattributed photographs.