Exhaled Aerosol Transmission of Pandemic and Seasonal H1N1 Influenza Viruses in the Ferret

Abstract

Person-to-person transmission of influenza viruses occurs by contact (direct and fomites) and non-contact (droplet and small particle aerosol) routes, but the quantitative dynamics and relative contributions of these routes are incompletely understood. The transmissibility of influenza strains estimated from secondary attack rates in closed human populations is confounded by large variations in population susceptibilities. An experimental method to phenotype strains for transmissibility in an animal model could provide relative efficiencies of transmission. We developed an experimental method to detect exhaled viral aerosol transmission between unanesthetized infected and susceptible ferrets, measured aerosol particle size and number, and quantified the viral genomic RNA in the exhaled aerosol. During brief 3-hour exposures to exhaled viral aerosols in airflow-controlled chambers, three strains of pandemic 2009 H1N1 strains were frequently transmitted to susceptible ferrets. In contrast one seasonal H1N1 strain was not transmitted in spite of higher levels of viral RNA in the exhaled aerosol. Among three pandemic strains, the two strains causing weight loss and illness in the intranasally infected ‘donor’ ferrets were transmitted less efficiently from the donor than the strain causing no detectable illness, suggesting that the mucosal inflammatory response may attenuate viable exhaled virus. Although exhaled viral RNA remained constant, transmission efficiency diminished from day 1 to day 5 after donor infection. Thus, aerosol transmission between ferrets may be dependent on at least four characteristics of virus-host relationships including the level of exhaled virus, infectious particle size, mucosal inflammation, and viral replication efficiency in susceptible mucosa.

Comparison of the Levels of Infectious Virus in Respirable Aerosols Exhaled by Ferrets Infected with Influenza Viruses Exhibiting Diverse Transmissibility Phenotypes

“…The detection of infectious influenza virus in respirable aerosols highlights the potential for a role of respirable aerosols in transmission events occurring during RDT experiments. Others have shown that influenza virus present in particles Ͻ15 m is responsible for transmission of virus between ferrets, although only viral RNA was detected in these studies (13,35). It is also plausible that aerosols play a role in transmission experiments in which animals are housed in direct contact; the contribution of close-range aerosol transmission cannot be ruled out in these scenarios (36).…”

“…The majority of the aerosols measured in the current study during normal breathing were in the respirable size range, but when the volumes were compared, a greater difference between the two groups of infected animals was revealed at the submicrometer particle size range (Ͻ1 m), albeit with substantial variability. Others have measured the size distribution of aerosols in air exhausted from cages housing ferrets infected by influenza virus and have reported the greatest concentrations of particles in the submicrometer size range (13). Submicrometer aerosols may remain suspended in still air for 12 hours or more, increasing the opportunity for aerosols of this size to be inhaled (27).…”

“…We recently evaluated differences in influenza virus infections in ferrets inoculated intranasally (IN) or by aerosol (AR) inhalation and found that AR inoculations can result in disease that more closely resembles naturally acquired infections (12); however, questions remain about how these differences may affect virus transmission. Due to the limited information that has been reported on the aerosol shedding patterns of influenza virusinfected ferrets (12,13) and the need for improved techniques of sampling infectious influenza virus in aerosols, we set out to analyze the aerosol shedding patterns of ferrets inoculated IN or by AR and determine the level of infectious virus present in respirable aerosols (Ͻ5 m). By comparing these parameters among a panel of human and avian influenza viruses exhibiting diverse transmissibility efficiencies in ferrets, we found that animals infected by highly transmissible viruses exhale more aerosol particles and more infectious virus within aerosol particles than those infected by influenza viruses that do not readily transmit among ferrets.…”

Complexities in Ferret Influenza Virus Pathogenesis and Transmission Models

“…Furthermore, once contact is established, the animals typically remain in continuous exposure for the duration of the experiment. However, several studies have modulated this parameter, creating contact pairs for short periods of time during the acute phase of infection to better understand the temporal dynamics of virus transmission in the ferret model (49,70) or establishing contact at different times throughout the acute phase of infection to determine links between virus shedding in inoculated ferrets and virus transmissibility (71). Donor chains, where virus is serially transmitted or passaged from ferret to ferret, have also been employed to model the selective pressure that may occur by sustained transmission of viruses through a population (72)(73)(74).…”

“…The distance between these paired cages is typically proximal (separated by only 3 mm to 10 cm) (38,77), depending on the construction of the cages and the experimental design, though transmission from inoculated to susceptible ferrets has been reported at distances up to 5 feet (63). Furthermore, airflow in transmission caging can vary between laboratories, with examples of directional flow from the inoculated ferret to the contact ferret (70,75,77) or top-to-bottom (31,38), bottom-to-top (73), front-to-back (76), or unspecified (14) directionality. Modulation of temperature and relative humidity, the study of which was previously limited to the guinea pig transmission model (78,79), has also recently been shown to affect virus transmissibility in ferrets (80).…”