Seasonal epidemics and sporadic pandemics of influenza cause a large public health burden. Airborne influenza viruses can be transmitted through direct contact and indirect contact of contaminated fomites. Sunlight has been shown to inactivate enveloped viruses in water but its effect on viruses on surfaces remains unexplored. This study investigated the sunlight inactivation of bacteriophage MS2, bacteriophage Phi6, and influenza A virus H1N1 (IAV) on plastic surfaces, as well as viral genome damage during sunlight inactivation and the impact of saliva on viral inactivation. Our findings suggest that sunlight is a highly effective natural disinfectant for viruses on nonporous surfaces. The time required for 99% of inactivation of MS2, Phi6, and IAV was 2.2-4.2 h, 0.7-0.8 h, and 0.2-0.3 h, respectively. Sunlight exposure resulted in significantly faster decay for MS2 in PBS (k=2.07-2.25 hour-1) compared to human saliva (k=1.09-1.21 hour-1) (p<0.05). The protective effect of saliva on MS2 against sunlight inactivation underscores the role of biological fluids in modulating virus stability under environmental stressors. Droplet size had no significant impact on sunlight inactivation of all viruses on plastic surfaces (p>0.05), which suggested that the modeling of viral inactivation in sunlight-rich environments could be simplified. Our study supports the potential use of sunlight as a natural disinfection strategy, especially in outdoor environments, and emphasizes the need for further research into factors and mechanisms affecting virus survival on surfaces, like virus type, viral matrices, and environmental factors.

Collaborators: Catherine Blish, Stanford University School of Medicine

Influenza A viruses present important public health risks, with recent outbreaks of highly pathogenic avian influenza (HPAI) H5N1 in dairy cattle raising concerns about potential transmission through raw milk consumption. This study investigated the persistence of influenza A virus H1N1 PR8 (IAV PR8) in raw cow milk at 4 °C. We found a first-order decay rate constant of −2.05 day–1 equivalent to a T99 of 2.3 days. Viral RNA remained detectable for at least 57 days with no degradation. Pasteurization (63 °C for 30 min) reduced infectious virus to undetectable levels and reduced viral RNA concentrations, but reduction was less than 1 log10. These findings highlight the potential risk of zoonotic virus transmission through raw milk consumption and underscore the importance of milk pasteurization. The prolonged persistence of viral RNA in both raw and pasteurized milk has implications for food safety assessments and environmental monitoring, particularly in the context of the environmental surveillance of influenza viruses.

Publication: Infectivity and Persistence of Influenza A Virus in Raw Milk

Collaborators: Catherine Blish, Stanford University School of Medicine

Wastewater-based epidemiology has emerged as a valuable tool for monitoring respiratory viral diseases within communities by analyzing concentrations of viral nucleic-acids in wastewater. However, little is known about the fate of respiratory virus nucleic-acids in wastewater. Two important fate processes that may modulate their concentrations in wastewater as they move from household drains to the point of collection include sorption or partitioning to wastewater solids and degradation. This study investigated the decay kinetics of genomic nucleic-acids of seven human respiratory viruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), respiratory syncytial virus (RSV), human coronavirus (HCoV)-OC43, HCoV-229E, HCoV-NL63, human rhinovirus (HRV), and influenza A virus (IAV), as well as pepper mild mottle virus (PMMoV) in wastewater solids. Viruses (except for PMMoV) were spiked into wastewater solids and their concentrations were followed for 50 days at three different temperatures (4°C, 22°C, and 37°C). Viral genomic RNA decayed following first-order kinetics with decay rate constants k from 0 to 0.219 per day. Decay rate constants k were not different from 0 for all targets in solids incubated at 4°C; k values were largest at 37°C and at this temperature, k values were similar across nucleic-acid targets. Regardless of temperature, there was limited viral RNA decay, with an estimated 0% to 20% reduction, over the typical residence times of sewage in the piped systems between input and collection point (<1 day). The k values reported herein can be used directly in fate and transport models to inform the interpretation of measurements made during wastewater surveillance.

Understanding whether or not the RNA targets quantified for wastewater-based epidemiology (WBE) efforts decay during transport between drains and the point of sample collection is critical for data interpretation. Here we show limited decay of viral RNA targets typically measured for respiratory disease WBE.

Publication: Persistence of human respiratory viral RNA in wastewater-settled solids

The coronavirus disease 2019 pandemic illustrates the importance of understanding the behavior and control of human pathogenic viruses in the environment. Exposure via water (drinking, bathing, and recreation) is a known route of transmission of viruses to humans, but the literature is relatively void of studies on the persistence of many viruses, especially coronaviruses, in water and their susceptibility to chlorine disinfection. To fill that knowledge gap, we evaluated the persistence and free chlorine disinfection of human coronavirus OC43 (HCoV-OC43) and its surrogates, murine hepatitis virus (MHV) and porcine transmissible gastroenteritis virus (TGEV), in drinking water and laboratory buffer using cell culture methods. The decay rate constants of human coronavirus and its surrogates in water varied, depending on virus and water matrix. In drinking water without disinfectant addition, MHV showed the largest decay rate constant (estimate ± standard error, 2.25 ± 0.09 day-1) followed by HCoV-OC43 (0.99 ± 0.12 day-1) and TGEV (0.65 ± 0.06 day-1), while in phosphate buffer without disinfectant addition, HCoV-OC43 (0.51 ± 0.10 day-1) had a larger decay rate constant than MHV (0.28 ± 0.03 day-1) and TGEV (0.24 ± 0.02 day-1). Upon free chlorine disinfection, the inactivation rates of coronaviruses were independent of free chlorine concentration and were not affected by water matrix, though they still varied between viruses. TGEV showed the highest susceptibility to free chlorine disinfection with the inactivation rate constant of 113.50 ± 7.50 mg-1 min-1 L, followed by MHV (81.33 ± 4.90 mg-1 min-1 L) and HCoV-OC43 (59.42 ± 4.41 mg-1 min-1 L).

This study addresses an important knowledge gap on enveloped virus persistence and disinfection in water. Results have immediate practical applications for shaping evidence-based water policies, particularly in the development of disinfection strategies for pathogenic virus control.

Publication: Persistence and free chlorine disinfection of human coronaviruses and their surrogates in water

Collaborators: Catherine Blish, Stanford University School of Medicine

    Bill Mitch, Environmental Engineering at Stanford University