Non-respiratory particles emitted by guinea pigs in airborne disease transmission experiments

Sima Asadi,Anthony S Wexler,Manilyn J Tupas,William D Ristenpart,Ramya S Barre,Nicole M Bouvier

doi:10.1038/s41598-021-96678-w

Abstract

Animal models are often used to assess the airborne transmissibility of various pathogens, which are typically assumed to be carried by expiratory droplets emitted directly from the respiratory tract of the infected animal. We recently established that influenza virus is also transmissible via “aerosolized fomites,” micron-scale dust particulates released from virus-contaminated surfaces (Asadi et al. in Nat Commun 11(1):4062, 2020). Here we expand on this observation, by counting and characterizing the particles emitted from guinea pig cages using an Aerodynamic Particle Sizer (APS) and an Interferometric Mie Imaging (IMI) system. Of over 9000 airborne particles emitted from guinea pig cages and directly imaged with IMI, none had an interference pattern indicative of a liquid droplet. Separate measurements of the particle count using the APS indicate that particle concentrations spike upwards immediately following animal motion, then decay exponentially with a time constant commensurate with the air exchange rate in the cage. Taken together, the results presented here raise the possibility that a non-negligible fraction of airborne influenza transmission events between guinea pigs occurs via aerosolized fomites rather than respiratory droplets, though the relative frequencies of these two routes have yet to be definitively determined.

Highlights

Animal models are often used to assess the airborne transmissibility of various pathogens, which are typically assumed to be carried by expiratory droplets emitted directly from the respiratory tract of the infected animal
Our preliminary experiments using both deionized water droplets generated by a humidifier, and respiratory droplets released from human speech, established that our IMI setup could successfully capture micron-scale liquid droplets (Fig. 1a)
We did not observe any airborne particles with clear fringes, which would be characteristic of a liquid droplet > 2 μm, nor any that were uniformly bright, suggesting a droplet < 2 μm

Summary

Introduction

Animal models are often used to assess the airborne transmissibility of various pathogens, which are typically assumed to be carried by expiratory droplets emitted directly from the respiratory tract of the infected animal. Many authors explicitly refer to the airborne pathogen transporters as “respiratory droplets”[9,11,19,20,21]; in most indoor human environments, water-laden droplets in the micron-scale range rapidly evaporate within seconds after exhalation to become “droplet nuclei,” the residual, non-volatile proteins, lipids, and salts in respiratory fluid[22,23]. It is these droplet nuclei, sometimes called “aerosols,” which may remain airborne for long periods of time before eventually being inhaled into the respiratory tract of a susceptible host. It establishes that a pathogen is airborne-transmissible, but does not establish the composition or origin of the airborne particles transmitting the pathogen

Methods

Results

Discussion

Conclusion