Wednesday, October 15, 2014

Controlling the Spread of Ebola

An interesting commentary by the Centre for Infectious Disease Research and Policy in mid-September may help explain how Ebola really spreads.

The authors, Lisa M. Brosseau and Rachel Jones note that it is critical that health care workers in outbreaks wear the correct type of personal protective equipment (PPE) to prevent the further spread of the virus.  Given that the virus seems to be spreading far more widely than historically normal, the authors state that it is critical that more conservative measures be employed.

They state that workers should use respiratory protection that is designed for a pathogen that has:

1.) No proven pre- or post-exposure treatments.

2.) A high fatality rate.

3.) Unclear modes of transmission.

I found the "unclear modes of transmission particularly interesting, given the 2012 study by Canadian scientists that I posted here.

The authors go on to state that they believe that:

"...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."

The authors recommend that the minimum protection should be a respirator with a protection factor greater than 10, the protection factor of a dust mask.  A powered air-purifying respirator (PAPR) with a hood or helmet is preferred over a filtering face piece; in general, these have a protection factor of 25.  Here is are two charts from the Occupational Health and Safety Association showing the protection factors of major types of respirators: 

From the MSA website, here is what a PAPR looks like:

You can see that while the PAPR is quite bulky, it offers far greater protection than a traditional surgical filtering face mask.

I'd like to quote further from the commentary regarding the reasoning behind why, until now, scientists have concluded that Ebola can only be spread through contact with contaminated bodily fluids:

"There has been a lot of on-line and published controversy about whether Ebola virus can be transmitted via aerosols. Most scientific and medical personnel, along with public health organizations, have been unequivocal in their statements that Ebola can be transmitted only by direct contact with virus-laden fluids, and that the only modes of transmission we should be concerned with are those termed "droplet" and "contact."

These statements are based on two lines of reasoning. The first is that no one located at a distance from an infected individual has contracted the disease, or the converse, every person infected has had (or must have had) "direct" contact with the body fluids of an infected person.

This reflects an incorrect and outmoded understanding of infectious aerosols, which has been institutionalized in policies, language, culture, and approaches to infection control. We will address this below. Briefly, however, the important points are that virus-laden bodily fluids may be aerosolized and inhaled while a person is in proximity to an infectious person and that a wide range of particle sizes can be inhaled and deposited throughout the respiratory tract." (my bold)

Modern science has shown that microscopic aerosol droplets can be inhaled through the nostrils and that many bodily fluids, including saliva, vomit, blood and diarrhea are capable of creating aerosols that can be inhaled.  Vomiting produces an aerosol and has been implicated in airborne transmission of gastrointestinal viruses.  Diarrhea, even when contained by toilets, emits a pathogen-laden aerosol that disperses in the air when the toilet is flushed.

The authors recommend the following protocols:

"Caring for a patient in the early stages of disease (no bleeding, vomiting, diarrhea, coughing, sneezing, etc). In this case, the generation rate is 1. For any level of control (less than 3 to more than 12 ACH), the control banding wheel indicates a respirator protection level of 1 (APF of 10), which corresponds to an air purifying (negative pressure) half-facepiece respirator such as an N95 filtering facepiece respirator. This type of respirator requires fit testing.

Caring for a patient in the later stages of disease (bleeding, vomiting, diarrhea, etc).If we assume the highest generation rate (4) and a standard patient room (control level = 2, 3-6 ACH), a respirator with an APF of at least 50 is needed. In the United States, this would be equivalent to either a full-facepiece air-purifying (negative-pressure) respirator or a half-facepiece PAPR (positive pressure), but standards differ in other countries. Fit testing is required for these types of respirators.

The control level (room ventilation) can have a big effect on respirator selection. For the same patient housed in a negative-pressure airborne infection isolation room (6-12 ACH), a respirator with an assigned protection factor of 25 is required. This would correspond in the United States to a PAPR with a loose-fitting facepiece or with a helmet or hood. This type of respirator does not need fit testing."

It is becoming increasingly obvious that the traditional infectious control methods used by health care workers when dealing with Ebola are insufficient and are putting health care workers, their families at perhaps the wider public at greater risk of infection.

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