Saturday, October 18, 2014

CDC: Review of Human-to-Human Transmission of Ebola Virus




# 9211


My reasoning behind why I don’t believe that Ebola is an `airborne’ virus in the `classic sense’ (although short-distance droplet transmission appears possible) is pretty simple: 


If it was truly airborne, we’d be digging mass graves all around the globe by now.


Whether it could someday achieve airborne transmissibility is a debate I’ll leave to real scientists to wrangle over. While I suspect that it is probably a long shot, I try never to say `never’.


What we know about the transmissibility of Ebola come mostly from anecdotal reports from earlier outbreaks, along with some limited laboratory experimentation. There are certainly gaps in our knowledge, but thus far – as is stated in the following CDC detailed review  - Airborne transmission of EVD among humans has never been demonstrated.


The following is an excerpt from a much larger synopsis of the Ebola Virus, which was published yesterday.  The entire document is worth reviewing.

Review of Human-to-Human Transmission of Ebola Virus


This document is a concise summary of published information on the current science about human-to-human transmission of Ebola virus. It is developed for use by healthcare personnel and public health professionals to use. It is a complement to the many guidance documents that CDC has issued already online at .


Transmission Studies

Airborne transmission of Ebola virus has been hypothesized but not demonstrated in humans. While Ebola virus can be spread through airborne particles under experimental conditions in animals, this type of spread has not been documented during human EVD outbreaks in settings such as hospitals or households.1 In the laboratory setting, non-human primates with their heads placed in closed hoods have been exposed to and infected by nebulized aerosols of Ebola virus.28,29 In a different experiment, control monkeys were placed in cages 3 meters away from the cages of monkeys that were intramuscularly inoculated with Ebola virus.30 Control and inoculated monkeys both developed Ebola virus infection. The authors concluded that “fomite and contact droplet” transmission to the control monkeys was unlikely, and that airborne transmission was most likely,30 but they did not discuss the potential behaviors of caged non-human primates (e.g., spitting and throwing feces) that might have led to body fluid exposures.31 Similarly, an outbreak of Reston virus (Reston ebolavirus species, which does not cause EVD in humans) infection occurred in a quarantine facility housing non-human primates in separate cages and the transmission route could not be confirmed for all infected primates. Multiple animal handlers developed antibody responses to Reston virus suggesting asymptomatic infection was occurring in humans with direct animal contact and implicating animal handling practices in transmission between primates.32 In a different study, piglets that were oronasally inoculated with Ebola virus were able to transmit infection to caged non-human primates that were placed 20 cm from the piglets.33 The piglet and primate cubicle design did not permit the investigators to distinguish among aerosol, small or large droplet, or fomite transmission routes, and it was noted that pigs are capable of generating infectious short range aerosol droplets more efficiently than other species. A more recent experiment that was specifically designed to further evaluate the possibility of naturally-occurring airborne transmission of Ebola virus among non-human primates showed no transmission of Ebola virus from infected to control primates placed 0.3 meters apart in separate open-barred cages and ambient air conditions, but with a plexiglass divider that prevented direct contact between the animals.34

In outbreak investigations, some EVD patients have not reported contact with another EVD patient, leading to speculation regarding transmission via aerosolized virus particles. In the Kikwit outbreak, 12 (3.8%) of 316 EVD patients did not report high-risk contact with a known EVD patient.7 EVD was not laboratory-confirmed in any of these 12 patients, however, and exposure histories for 10 of the 12 patients were provided by surrogates (because the 10 patients died before they could be interviewed); direct contact with EVD patients could have been missed because of wording of the study instrument, and transmission via droplets or fomites were also not ruled out. All 74 patients with EVD confirmed by RT-PCR testing or an Ebola antibody or antigen detection assay in this outbreak had high-risk exposures to Ebola patients.7 Similarly, in the 2007-2008 Uganda outbreak, although some probable (not virologically-confirmed) cases did not have a reported contact exposure, all 42 laboratory-confirmed cases had contact with a known EVD case.13 Also, in a separate analysis of the Kikwit outbreak, the presence of cough (19% of primary cases within households) did not predict secondary spread of EVD.3

2014 EVD Outbreak

Most of the evidence regarding human-to-human transmission of Ebola virus is derived from investigations of previous Ebola outbreaks. Although the current EVD epidemic in West Africa is unprecedented in scale, the clinical course of infection (i.e., incubation period, duration of illness, case fatality rate) and the transmissibility of the virus (i.e., estimations of the basic reproductive number [R0]) are similar to those in earlier EVD outbreaks.2 In addition, genetic analyses of 99 Ebola virus genomes sequenced from 78 patients from the 2014 outbreak in Sierra Leone35 suggest that the 2014 EVD outbreak strains are very closely related to viral strains from the two most recent Ebola outbreaks in Central Africa.35 As has been observed in previous Ebola outbreaks, the genomic sequences from the 2014 EVD outbreak have a small number of distinct genetic changes, but it is not known if these changes have an impact on disease severity or transmissibility.35


EVD among healthcare personnel and other persons is associated with direct contact with infected persons (or the bodies of persons who have died from EVD) and direct contact with body fluids from EVD patients. CDC infection control recommendations for U.S. hospitals, including recommendations for standard, contact, and droplet precautions for general care, reflect the established routes for human-to-human transmission of EVD and are based on data collected from previous EVD outbreaks in Africa in addition to experimental data. Airborne transmission of EVD among humans has never been demonstrated in investigations that have described human-to-human transmission although hypothetical concerns about airborne transmission of EVD have been raised.3,10

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