Ebola Update

ebl2The West African Ebola epidemic is winding down with only one case confirmed in Liberia last month. In Guinea new cases continue to emerge, but at a reduced rate compared to last year’s peak. The exponential increase in Ebola cases seen last summer is over – the rate of transmission from person to person, the “r” factor, is now less than one. We all owe a debt of gratitude to individual healthcare workers and charitable organizations who fought the West African Ebola epidemic on-site. We should be proud as well that our American military has helped by constructing Ebola treatment facilities, and happy that pharmaceutical companies are currently developing effective treatments and vaccines.

What have we learned from this sad episode? Over 25,000 Ebola cases occurred with over 10,000 deaths, including almost 500 physicians, nurses and other healthcare workers, by far making this the worst Ebola outbreak to date. We are also now more aware of our national vulnerability to an Ebola epidemic since we have over 300 million un-vaccinated Americans, and as yet no widely available cure. We have also learned an especially important piece of science regarding the transmission of Ebola. Leaders in the field of virology and epidemiology have determined with a high degree of probability that the Ebola virus is spread by airborne means in certain circumstances, particularly in the confined spaces of clinic and hospital settings where Ebola patients are bleeding, vomiting, having diarrhea, coughing and sneezing, and where aerosol-generating medical procedures are performed on them. Under these conditions microscopic contaminated droplet nuclei, which cannot be seen and which can travel across a room with air currents, can remain infectious for about an hour and a half as noted by the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota.

“The potential for transmission via inhalation of aerosols, therefore, cannot be ruled out by the observed risk factors or our knowledge of the infection process. Many body fluids, such as vomit, diarrhea, blood, and saliva, are capable of creating inhalable aerosol particles in the immediate vicinity of an infected person. Cough was identified among some cases in a 1995 outbreak in Kikwit, Democratic Republic of the Congo, and coughs are known to emit viruses in respirable particles. The act of vomiting produces an aerosol and has been implicated in airborne transmission of gastrointestinal viruses. Regarding diarrhea, even when contained by toilets, toilet flushing emits a pathogen-laden aerosol that disperses in the air… Aerosolized (1-3 mcm) Marburg, Ebola, and Reston viruses, at 50% to 55% relative humidity and 72°F, had biological decay rates of 3.04%, 3.06%. and 1.55% per minute, respectively. These rates indicate that 99% loss in aerosol infectivity would occur in 93, 104, and 162 minutes, respectively.Center for Infectious Disease Research and Policy

The term “airborne transmission” is now somewhat out of favor for Ebola; the term “aerosol transmission” is now preferred as explained in a recent study of Ebola transmission by the American Society for Microbiology. These medical scientists also concluded that aerosol transmission of Ebola likely did occur during the West African outbreak.

“Both traditional models of droplet transmission and airborne transmission can fit under the broader category of aerosol transmission. Aerosols can be generated…via forceful emission of body fluids such as vomitus or diarrhea, or from aerosol-generating procedures… Breathing, talking, coughing, and sneezing generate particles suspended in air (aerosols), which range in diameter from less than 1 µm to greater than 100 µm. These particles traditionally have been divided into two categories: droplet and airborne. Droplets have been thought to range from 5 µm to 100 µm in diameter, to travel no more than a few feet, and to be projected onto surfaces, skin, or mucous membranes. Airborne particles (often referred to as “droplet nuclei”) have been thought to be less than 5 µm, to remain suspended in the air for a period of time, and to transmit infection by inhalation only at some distance from the source patient. In the traditional infection control paradigm, transmission via particles can only occur in two ways: (i) by large droplets directly impacting skin or mucous membranes and (ii) by inhalation of small airborne particles at a distance from the source. Larger particles also may settle on objects, allowing fomite transmission. This paradigm fails to recognize that infectious aerosols include suspended particles in a wide range of particle sizes (small to large droplets) that are easily inhaled by someone standing near the point of generation. Thus, aerosol inhalation can occur both near and far from an infectious source… It is very likely that at least some degree of Ebola virus transmission currently occurs via infectious aerosols generated from the gastrointestinal tract, the respiratory tract, or medical procedures…” American Society for Microbiology

During the height of the West African Ebola outbreak CIDRAP became very concerned that aerosol transmission of Ebola was occurring, and they wasted no time in recommending a higher level of respiratory protection for health care workers in Africa and elsewhere.

 “Being at first skeptical that Ebola virus could be an aerosol-transmissible disease, we are now persuaded by a review of experimental and epidemiologic data that this might be an important feature of disease transmission, particularly in healthcare settings… We believe there is scientific and epidemiologic evidence that Ebola virus has the potential to be transmitted via infectious aerosol particles both near and at a distance from infected patients, which means that healthcare workers should be wearing respirators, not facemasks… We strongly urge the US Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) to seek funds for the purchase and transport of PAPRs [powered air-purifying respirators] to all healthcare workers currently fighting the battle against Ebola throughout Africa – and beyond.” Center for Infectious Disease Research and Policy

The CIDRAP airway protection recommendation for Ebola care-givers was divided into two levels depending primarily on the severity of illness of the Ebola victim. For patients not yet experiencing bleeding, vomiting, diarrhea, coughing or sneezing, and for those not undergoing aerosol-generating medical procedures, they recommended an assigned protection factor (APF) of 10, which corresponds to a negative pressure disposable N95 half-face respirator mask. For sicker patients experiencing those aerosol-generating symptoms or medical procedures CIDRAP recommended an APF of 50 which corresponds to a negative pressure full-face respirator or a positive pressure air-purifying respirator (PAPR). To achieve an APF of 50 these respirators would require a P100 filter, otherwise known as a HEPA filter.

After CIDRAP made their recommendations the CDC did in fact revise their airway protection guidance for Ebola care-givers, yet the CDC website still states “Ebola is not spread through the air.” The CDC now recommends PAPRs or disposable half-face N95 respirator masks, but they did not specify under which circumstances the choice should be made. The CDC prefers disposable respirators due to the potential risk to individuals reprocessing reusable respirators, such as PAPR components and full-face respirators, but they failed to recommend the use of disposable half-face P100 respirator masks for the sicker Ebola patients or for aerosol generating medical procedures, so hospitals and clinics are likely to err on the side of risk rather than on the side of safety by selecting the N95 mask over the P100 in those circumstances. The P100 standard filters out 99.97% of inhalable particles (such as the Ebola virus) while the N95 standard only filters out 95% of those particles, thus the P100 filter is 167 times better than the N95 filter, and the P standard is resistant to biological lipid aerosols whereas the N standard is not. In the study linked above The American Society of Microbiology reports that the outer surfaces of 3 out of 16 masks worn by West African health workers were Ebola-positive, and were not visibly dirty or bloody. It takes only 1 to 10 inhaled Ebola viruses to transmit the disease which has a death rate of 50% or higher. For now I believe it is advisable to follow the general CIDRAP recommendation which requires a PAPR fitted with a P100 (HEPA) filter, a full-face respirator with P100 filters, or a disposable half-face P100 respirator mask (along with eye protection) for Ebola patients having bleeding, vomiting, diarrhea, coughing or sneezing, and during aerosol-generating medical procedures such as endoscopy, intubation, mechanical ventilation, BiPap operation, or during surgical operations. Ebola patients without aerosol-generating symptoms, i.e.: those earlier in the course of the disease with few or no symptoms, and those not undergoing aerosol-generating medical procedures, would only require health workers to use the disposable half-face N95 respirator mask (along with eye protection). The CDC’s guidelines for the remainder of personal protective equipment remain intact and require strict adherence.

Ebola and other dangerous microbial epidemics will occur again – we should be better prepared next time.

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