Natural outbreaks of the Ebola virus, while severe, are typically isolated and usually affect no more than a few hundred people at a time. However, from 2014-2016, infections from this deadly virus caused more than 11,000 deaths in West Africa. During this time, several cases of Ebola virus disease were also diagnosed in other countries, including the United States, due to infected travelers from West Africa that had unknowingly harbored and incubated the virus while en-route to their respective destinations.
By the time a person infected with Ebola virus becomes symptomatic, they are typically starting to shed the virus. During an outbreak, health care workers and family members of patients are often the first to respond and provide care. In this role, they are at a high risk of becoming infected with the virus as well. Therefore, it is important to understand how we can best prevent transmission in both clinical and home settings.
Human to human transmission of the Ebola virus occurs primarily through direct contact and exposure to the blood or other bodily fluids of infected patients. However, there have been new infections that occurred without documented contact between a patient and health care provider or family member. While these cases are uncommon, it is possible that a small fraction of the cases of Ebola virus infection may be the result of exposure to small droplets or aerosols containing the virus.
Laboratory studies have shown that the Ebola virus can remain infectious outside of the body for long periods of time. The virus can survive in blood samples on various surfaces for several days, even in hot and humid conditions that would typically kill most other viruses and bacteria. In an aerosolized form, the Ebola virus can survive for over an hour. Additionally, laboratory experiments have demonstrated that inhaling small amounts of Ebola virus can be fatal , and there are examples of Ebola virus disease being transmitted between individuals in close proximity, even though they had never been in direct contact with each other .
However, assessing the risk of transmission via droplets or aerosols is a complicated process. While the minimum amount of virus required to cause infection is thought to be very low, so far it has not been possible to determine a definitive value. This is because commonly used testing methods are often not sensitive enough to detect or measure the amount of infectious virus in the air. To address these challenges, researchers at the Department of Homeland Security (DHS) Science and Technology Directorate’s (S&T) National Biodefense Analysis and Countermeasures Center (NBACC) designed and conducted a study to optimize methods for collecting and measuring very small amounts of Ebola virus in the air.
“Ebola can be both a national security and public health concern,” explained Lloyd Hough, who leads S&T’s Hazard Awareness and Characterization Technology Center. “We’re looking forward to applying these methods to better characterize the risks associated with Ebola virus, and are hopeful that others can benefit from these techniques as well.”
NBACC researchers assessed and compared multiple devices designed for collecting microorganisms from the air. They concluded that filters made of gelatin were the best for collecting infectious Ebola virus from the air and were also the easiest and safest type of sampling device to use. The researchers utilized a cell line, developed by the Centers for Disease Control and Prevention, in an assay designed to measure the amount of infectious Ebola virus present in a sample. The cell line glows when infected by the virus, which enabled the researchers to differentiate Ebola virus infection from other causes of cell death.
After testing various sampling and assay methodologies, the NBACC researchers found that the combination of the gelatin filter samplers and improved assay was easier to use, more reliable, and nearly ten times more sensitive than the previous methods utilized to measure the amount of infectious Ebola virus in air samples. The results of these experiments have been published in two peer-reviewed journal articles [3,4], which will enable researchers at other institutions to understand and utilize these newly developed sampling and assay methodologies.
“Our ability to detect Ebola virus in air samples at levels that are ten-fold lower than what was possible with previous methods, will enable us to provide a better understanding of the aerosol hazard posed by this virus,” said Mike Schuit, an investigator in the Aerobiology group at NBACC.
The new sampling and assay methodologies developed by NBACC researchers, as well as the data from associated studies, will be useful in a number of ways. For example, these methods are currently being employed in a study in partnership with the National Institute of Allergy and Infectious Diseases (NIAID) to determine the minimum amount of Ebola virus that needs to be inhaled in order to cause infection or death in an animal model of Ebola virus disease. This study will also determine whether infected animals produce aerosols containing Ebola virus when they breathe, which will help scientists to better understand the potential for natural aerosol transmission of the virus. Various DHS components and partners, along with the Department of Defense and the Department of Health and Human Services, will use these data to conduct hazard modeling and gain a better understanding of the potential risk for airborne transmission of Ebola virus and how it may impact health workers.
Taking lessons learned from the response to Ebola virus, the NBACC researchers are conducting similar studies with SARS-CoV-2, the virus that causes COVID-19. This includes studying the performance of aerosol samplers with SARS-CoV-2, and optimization of methodologies to detect small quantities of SARS-CoV-2 in the air. Furthermore, the NIAID partnership has been extended to examine how much virus it actually takes to start a new infection when aerosol particles containing SARS-CoV-2 are inhaled. As with the Ebola study, this study will also measure whether infectious virus is present in the exhaled breath of infected animals to better understand how COVID-19 spreads in human populations and inform strategies to prevent its continued spread.