Transmission of influenza virus via aerosols and fomites in the guinea pig model

Abstract

Limited data on the relative contributions of different routes of transmission for influenza virus are available. Person-to-person transmission is central to seasonal and pandemic spread; nevertheless, the modes of spread are a matter of ongoing debate. Resolution of this discussion is paramount to the development of effective control measures in health care and community settings. Using the guinea pig model, we demonstrated that transmission of influenza A/Panama/2007/1999 (H3N2) virus through the air is efficient, compared with spread through contaminated environmental surfaces (fomites). We also examined the aerosol transmission efficiencies of 2 human influenza virus A strains and found that A/Panama/2007/1999 influenza virus transmitted more efficiently than A/Texas/36/1991 (H1N1) virus in our model. The data provide new and much-needed insights into the modes of influenza virus spread and strain-specific differences in the efficiency of transmission.

Figure 1

Long-range transmission of influenza A/Panama/2007/1999 (Pan99) virus, suggesting that airborne transmission occurs. A Experiment at 80 cm. Four Hartley guinea pigs were intranasally inoculated with 1×10³ pfu of Pan99 and placed on the bottom 2 shelves of an environmental chamber that maintained constant temperature and relative humidity. Twenty-four hours later, 4 uninoculated animals were placed on the upper 2 shelves of the chamber. Temperature (T) and relative humidity (RH) in the chamber are indicated. B Nasal wash titers. Nasal washes were performed on alternate days, and viral titers were determined by plaque assay on MDCK cells. C Experiment at 107 cm. Inoculated animals were placed in the same large cage, and a distance of >1 m separated 4 inoculated and 4 exposed animals. Nasal wash titers are shown in panel D, and the experiment was reproduced, with results shown in panel E

Figure 2

Short-range spread of influenza A/Panama/2007/1999 (Pan99) virus through the air from droplet- and aerosol-infected strain 13 guinea pigs. A Shedding from the upper respiratory tract of groups of 3 Hartley guinea pigs inoculated by aerosol with 1×10², 1×10⁴, 1×10⁶, or 1×10⁸ pfu of Pan99, as described in Methods. B Cage arrangement for intranasally and aerosol-inoculated guinea pigs and exposed counterparts, indicating which naive animals became infected (asterisks). C Short-range aerosol transmission from 3 strain 13 guinea pigs intranasally inoculated with 1×10³ pfu of Pan99 to 2 of 3 exposed strain 13 guinea pigs. D Short-range aerosol transmission from 3 strain 13 guinea pigs inoculated by aerosol with 1×10⁶ pfu of Pan99 virus to 3 exposed strain 13 guinea pigs

Figure 3

Inefficiency of transmission of influenza A/Panama/2007/1999 (Pan99) virus via fomites in Hartley and strain 13 guinea pigs. Arrows indicate the time point for each experiment when intranasally inoculated animals were removed from the cages and replaced with naive guinea pigs. White circles represent nasal wash viral titers for inoculated animals on day 2 after inoculation, and black triangles represent nasal wash viral titers for exposed animals on days 3, 5, 7, and 9 after inoculation. In panel A, 8 Hartley guinea pigs were intranasally inoculated with Pan99 and then replaced with 8 uninfected Hartley guinea pigs (^a7 of 8 exposed animals did not become infected). In panel B, the experiment was repeated using 4 inoculated and 4 exposed Hartley guinea pigs (^b3 of 4 exposed animals did not become infected). In panel C, the same experimental approach was used with 4 Pan99-inoculated and 4 exposed strain 13 guinea pigs (^c3 of 4 exposed animals did not become infected)

Figure 4

Transmission efficiency and recovery from aerosols of influenza A/Panama/2007/1999 (H3N2) virus (Pan99) and influenza A/Texas/36/1991 (H1N1) virus (Tx99). Short-range transmission experiments were conducted with 4 inoculated and 4 exposed guinea pigs, as described in Methods. A Recombinant Pan (rPan99) virus. B Tx/91 virus. C Nasal wash titers for 8 of 16 guinea pigs intranasally inoculated with Pan99 or Tx/91 influenza viruses. D Titers in air of Pan99 or Tx91 from aerosols generated by inoculated guinea pigs and sampled using a glass liquid impinger, as described in Methods. Data shown in panels C and D are compiled from 4 independent experiments; data in panel D are means ± SDs