![]() In one outbreak of SUDV, however, 14 (64%) of 22 health care workers became infected after barrier precautions were established this led to reinforcement of infection control practices ( 16). Use of barrier protection and surgical masks apparently was adequate to halt most nosocomial transmission during past outbreaks of EBOV and SUDV ( 6, 7, 11, 13, – 15). In addition, investigators have shown that health care workers are at particularly high risk ( 6, 7, 11, 12). Early outbreak investigations demonstrated the importance of parenteral transmission via nonsterile needles, although this has not been noted more recently ( 6, 11). Spread within hospitals has been documented repeatedly, and outbreak amplification has occurred in health care settings for both EBOV and SUDV ( 6, 7, 11). Past outbreaks provide opportunities to examine human-to-human transmission of Ebola viruses. WHAT WE KNOW ABOUT EBOLA VIRUS TRANSMISSION IN HUMANS In this review, we explore what we know-and what we do not know-about Ebola virus transmission. Furthermore, information about Ebola virus transmission in humans remains incomplete, given the relatively small number of outbreak investigations and cases recognized before 2013 as a result, additional questions remain ( 10). Limited data are available, however, regarding virus genomics (affecting phenotype/pathotype), patient viral loads, and certain epidemiological features for this unique EBOV strain. Many experts have concluded that the extensive transmission documented in the 2013–2015 West Africa epidemic is due to societal factors (poverty, urban density, population migration patterns, and poor health care and public health infrastructure) rather than unique biological characteristics of the agent ( 8, 9). Investigators recorded four generations of spread during the EBOV outbreak in Kikwit, DRC (315 cases) ( 7). One report estimated 15 generations of viral transmission during a 1976 SUDV outbreak (284 cases), which was the most that were identified ( 6). The maximum number of generations of human-to-human transmission for these outbreaks is unknown but is likely relatively low. Only seven outbreaks involved more than 100 reported cases. Outbreaks caused by Reston virus (RESTV) have occurred in nonhuman primates and pigs, with associated asymptomatic human infections ( 5). Most pre-2013 outbreaks were caused by Zaire Ebola virus (EBOV) (14 outbreaks) or Sudan virus (SUDV) (7 outbreaks) Bundibugyo virus (BDBV) caused two outbreaks, and Tai Forest virus (TAFV) was identified in a single case from Côte d’Ivoire ( 1). To date, five species of Ebola viruses have been identified four from Africa (Zaire, Sudan, Bundibugyo, and Tai Forest) and one from the Philippines (Reston) ( 1, 3, 4). One additional outbreak involving 69 cases occurred in the Democratic Republic of the Congo (DRC) between July and October 2014 ( 2). We also hypothesize that Ebola viruses have the potential to be respiratory pathogens with primary respiratory spread.īetween the first recognized outbreak of Ebola virus disease (EVD) in 1976 and the onset of the 2013–2015 Ebola epidemic in West Africa, 24 outbreaks of EVD involving approximately 2,400 reported cases had been recognized by the World Health Organization (WHO) ( 1). In this review, we address what we know and what we do not know about Ebola virus transmission. Key areas requiring further study include (i) the role of aerosol transmission (either via large droplets or small particles in the vicinity of source patients), (ii) the role of environmental contamination and fomite transmission, (iii) the degree to which minimally or mildly ill persons transmit infection, (iv) how long clinically relevant infectiousness persists, (v) the role that “superspreading events” may play in driving transmission dynamics, (vi) whether strain differences or repeated serial passage in outbreak settings can impact virus transmission, and (vii) what role sylvatic or domestic animals could play in outbreak propagation, particularly during major epidemics such as the 2013–2015 West Africa situation. ![]() ![]() Available evidence demonstrates that direct patient contact and contact with infectious body fluids are the primary modes for Ebola virus transmission, but this is based on a limited number of studies.
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