Design and Validation with Influenza A Virus of an Aerosol Transmission Chamber for Ferrets

Abstract

Background: The importance of aerosols in the spread of viruses like influenza is still a subject of debate. Indeed, most viruses can also be transmitted through direct contact and droplets. Therefore, the importance of the airborne route in a clinical context is difficult to determine. The aim of this study was to design a chamber system to study the airborne transmission of viruses between ferrets. Methods: A system composed of three chambers connected in series, each one housing one ferret and preventing direct contact, was designed. The chambers were designed to house the ferrets for several days and to study the transmission of viruses from an infected (index) ferret to two naïve ferrets via aerosols and droplets or aerosols only. A particle separator was designed that can be used to modulate the size of the particles traveling between the chambers. The chamber system was validated using standard dust as well as with ferrets infected with influenza A virus. Conclusions: The 50% efficiency cut-off of the separator could be modulated between a 5-µm and an 8-µm aerodynamic diameter. In the described setup, influenza A virus was transmitted through the aerosol route in two out of three experiments, and through aerosols and droplets in all three experiments.

Keywords: aerosol chamber; bioaerosols; ferret animal model; influenza virus.

Figure 1

System overview. (1) Cage number one, (2) cage number two, (3) cage number three, (d) high-efficiency particulate air (HEPA) filter inlet air, (e) HEPA filter exhaust air (not visible in the picture), (f) particle separator, (g) muffler, (h) airflow adjustment valve, (i) control panel, (j) pump installed in an insulated box, (k) support table with rails.

Figure 2

Side view of a cage. (a) Perforated grates on each side of the cage, (b) excreta pan with a perforated lid, (c) sampling ports, (d) rubber seal between cages.

Figure 3

Cage door components. (a) Perforated stainless steel door and transparent polycarbonate door. (b) Rubber seal of the polycarbonate door.

Figure 4

Feeder and water bottle. (a) Outside view of the feeder with butterfly valve and water bottle installed on a cage, (b) cage inside view with feeder, animal water supply and excreta pan.

Figure 5

Particles separator. (a) Assembly of stainless steel plate with four rows of 40 orifices. On the picture, orifices are covered with impaction plates located 5 mm from the orifice’s outlets. (b) Schematic representation of the particle separator principle and design.

Figure 6

Particle separator D₅₀ measurement as a function of airflow and the number of separator orifices used. Comparison of particle distribution in cage three with and without a particle separator, as measured with an aerodynamic particle sizer (APS) located at 2″ from the particle separator.

Figure 7

Influenza genome per cubic meter of air, and influenza virus titer in nasal washes of ferrets hosted in cage system for 7 or 12 days from three experiments. The index ferret (cage one) was infected on day 0. Air samples were collected using National Institute for Occupational Safety and Health (NIOSH) two-stage bioaerosol cyclone samplers. Genome concentrations found with the NIOSH first stage, second stage and backup filter are superimposed.