Viruses like the influenza virus and even the novel coronavirus are capable of spreading from person to person via airborne droplets as well as dust, fibers, and other surfaces. Now, a new study provides evidence of airborne virus transport on microscopic particles called “aerosolized fomites.”
The study from the University of California, Davis, and the Icahn School of Medicine at Mount Sinai titled, “Influenza A virus is transmissible via aerosolized fomites,” is published this week in the latest issue of Nature Communications. The study was funded by the National Institute of Allergy and Infectious Diseases of the National Institutes of Health.
What was this study about?
The researchers wrote that there may be several ways in which the influenza virus can spread from one person to another. These have been speculations with no concrete evidence. Every season of influenza, there are thousands of deaths due to the disease around the globe, they wrote. The exact way the infection spreads is thus an essential piece of the puzzle that must be understood, they wrote.
Direct and indirect contact
They wrote that it had been agreed that direct contact needs “a susceptible person to self-inoculate by, for instance, touching one’s nose with a virus-contaminated hand.” On the other hand, indirect contact helps the transfer of the virus between an infected person and a non-infected person is via “a fomite, which is an object like a doorknob or toy that has been contaminated with infectious virus.”
Transmission via air can occur in two ways, they write. This could be via “sprays of virus-laden respiratory droplets, such as from a cough or sneeze, impacting immediately onto the respiratory mucosa of a susceptible individual.” It could also be via “inhalation of droplet nuclei, microscopic aerosol particles consisting of the residual solid cores of evaporated respiratory droplets.”
What was done?
For this study, a sample strain of Influenza A/Panama/2007/1999 (H3N2) virus (Pan99) was used. The animals used for this experiment were six-week-old female Hartley strain guinea pigs. The test animals were infected with the Pan99 strain of the virus.
Four pairs of experimental animals were used. One of them was infected with the virus, and the other was virus naive. They were kept in custom made cages joined together by a stainless-steel air conduit to allow airflow. Donor guinea pigs had been infected with the virus 6 weeks prior to the experiment. Transmission pairs were kept together for a total of 7 days. Nasal washings were taken from the naive recipients on days 2, 4, and 6 of exposure.
In the second part of the experiment, the infection carrying guinea pigs were dead while the recipients were alive. Next, they painted virus filled liquid onto the furs of influenza immune guinea pigs and placed them in an adjacent cage with non-infected guinea pigs. The spread of infection from dead infected guinea pigs as well as from virus painted guinea pigs to uninfected animals was noted at the end of the experiment.
To show fomite borne transmission, the team then applied the Pan99 virus in liquid solution on different paper tissues and towels and allowed them to dry in a biosafety cabinet. Then if these tissues were crumpled or folded by hands or rubbed, they were found to release up to 900 particles of the virus per second. This release was similar to that of what infected guinea pigs release into the air.
What was found?
The spread of the infection from the infected guinea pig to the uninfected one was clearly seen in the experiment. The experiment also showed that the infected animals had contaminated fur, which spread the virus to their surrounding environment. These dust particles could become laden with the virus and spread act as “aerosolized fomites” that could be inhaled by the susceptible guinea pigs. The virus could be spread via rubbing of a virus contaminated tissue paper as well.
The team writes, “...we find that an uninfected, virus-immune guinea pig whose body is contaminated with the influenza virus can transmit the virus through the air to a susceptible partner in a separate cage”.
Dr. Nicole Bouvier, an infectious disease physician at Mount Sinai, said, “The particles were definitely coming from the animal even when it wasn’t breathing.” The virus was spreading from the animal even when it was not alive, she said. “That was just very surprising,” she said.
Professor William Ristenpart of the UC Davis Department of Chemical Engineering, one of the lead researchers said, “It’s really shocking to most virologists and epidemiologists that airborne dust, rather than expiratory droplets, can carry influenza virus capable of infecting animals. The implicit assumption is always that airborne transmission occurs because of respiratory droplets emitted by coughing, sneezing, or talking. Transmission via dust opens up whole new areas of investigation and has profound implications for how we interpret laboratory experiments as well as epidemiological investigations of outbreaks.”
Conclusions and implications
The team wrote, “aerosolized fomites may contribute to influenza virus transmission in animal models of human influenza, if not among humans themselves, with important but understudied implications for public health.”
Ristenpart said, “Our experiments very clearly show that when guinea pigs move around, they stir up dust. And if that dust is contaminated with the virus, then it can transmit that virus through the air to another animal in a separate cage.” He added, “When you rub your face or brush your shirt or crumple a piece of tissue paper, you’re aerosolizing micron-scale particulates. And if that surface had been previously contacted by virus-containing mucus, then you’re also aerosolizing the virus that other people can inhale.”