Engineers at Stanford describe potential transmission pathways of COVID-19 and their implications.
Much remains unknown about how SARS-CoV-2, the virus that causes COVID-19, spreads through the environment. A major reason for this is that the behaviors and traits of viruses are highly variable – some spread more easily through water, others through air; some are wrapped in layers of fatty molecules that help them avoid their host’s immune system, while others are “naked.”
This makes it urgent for environmental engineers and scientists to collaborate on pinpointing viral and environmental characteristics that affect transmission via surfaces, the air and fecal matter, according to Alexandria Boehm, a Stanford professor of civil and environmental engineering, and Krista Wigginton, the Shimizu Visiting Professor in Stanford’s department of civil and environmental engineering and an associate professor at the University of Michigan.
Boehm and Wigginton co-authored a recently published viewpoint in Environmental Science & Technology calling for a broader, long-term and more quantitative approach to understanding viruses, such as SARS-CoV-2, that are spread through the environment.
They are also principal investigators on a recently announced National Science Foundation-funded project to study the transfer of coronaviruses between skin and other materials, the effect of UV and sunlight on the coronaviruses, and the connection between disease outbreaks and virus concentrations in wastewater.
Scientists and medical experts don’t have a good understanding of what virus characteristics and environmental factors control virus persistence in the environment – for example, in aerosols and droplets, on surfaces including skin and in water including seawater, according to Boehm and Wigginton.
“When a new virus emerges and poses a risk to human health, we don’t have a good way of predicting how it will behave in the environment,” Boehm said.
Part of the problem is historically there has been limited funding for this sort of work. The National Institutes of Health historically hasn’t funded work on pathogens in the environment, and funding at the National Science Foundation for this work is limited.
Also, coronaviruses and most of the emerging viruses that have caught the world’s attention over the last decade are enveloped viruses that are wrapped in an outer layer of fatty lipid molecules that they’ve stolen from their hosts.
Proteins on the surface of the envelopes can help these viruses evade the immune systems of the organisms they are infecting.
“There has been much more work on the fate of non-enveloped or naked viruses because most intestinal pathogens in excrement are nonenveloped viruses – like norovirus and rotavirus,” said Wigginton.
In their paper, Boem and Wigginton address potential threats that viruses such as SARS-CoV-2 pose to water sources.
We usually only worry about viruses in water if they are excreted by humans in their feces and urine. Most enveloped viruses aren’t excreted in feces or urine, so they aren’t usually on our minds when it comes to our water sources.
There is increasing evidence that the SARS-CoV-2 viruses, or at least their genomes, are excreted in feces.
If infective viruses are excreted, then fecal exposure could be a route of transmission, according to Boehm, who added, “It’s unlikely this could be a major transmission route, but a person could potentially be exposed by interacting with water contaminated with untreated fecal matter.”
Drinking water treatment systems have numerous treatment barriers to remove the most prevalent viruses and the most difficult-to-remove viruses, according to the engineers.
Research on viruses similar to the SARS-CoV-2 virus suggests they are susceptible to these treatments.
“In terms of virus concentration and persistence, this isn’t a worst-case scenario,” Wigginton said.
Broadly, Wigginton and Boehm write, we tend to study viruses very intensely when there is an outbreak, but the results from one virus aren’t easy to extrapolate to other viruses that emerge years later.
“If we took a broader approach to studying many kinds of viruses, we could better understand the characteristics driving their environmental fate,” Wigginton said.
The two researchers call for experts across various fields – including medicine and engineering and – to work together to move methods forward faster, make discoveries and formulate strategies that wouldn’t be possible independently.
The Coronaviridae have been recognized for many years as a cause of common-cold-like, self-limiting respiratory infections (Monto, 1998), but the 2003 emergence of Severe Acute Respiratory Syndrome (SARS) brought new recognition that coronavirus infection could result in serious, even fatal, disease.
The etiologic agent of SARS was quickly identified as a previously unknown coronavirus (Drosten et al., 2003).
Emerging in an age of global travel, large healthcare facilities, and high-density housing developments, SARS coronavirus (SARS-CoV) was not only a novel human pathogen, but one that spread by novel routes.
A respiratory agent transmitted from person-to-person by droplets and aerosols, SARS-CoV spread from passenger to passenger on an airplane (Olsen et al., 2003) and from patients to healthcare workers and visitors in hospitals (Seto et al., 2003, Varia et al., 2003, Chen et al., 2004).
As efforts increased to halt further person-to-person spread of the disease from travelers and in healthcare facilities, an outbreak of SARS in a high-density Hong Kong housing complex led to the discovery of a new route of transmission, previously unknown for a respiratory virus.
In this outbreak, SARS-CoV shed in the feces of an infected building visitor may have spread disease to other occupants of the building via droplets and aerosols of virus-contaminated commode water, which entered multiple apartments through faulty toilet plumbing and floor drains (McKinney et al., 2006).
This outbreak scenario suggests that if SARS were to reemerge in the future, water contaminated with the fecal waste of infected individuals could be a vehicle for transmission.
This unique fecal droplet–respiratory route is potentially important, but aspects of it remain poorly understood. One such aspect is the role of viral stability: if SARS-CoV is capable of surviving for relatively long periods of time in water, exposure and transmission via fecally contaminated droplets of water may be more likely.
In order to better assess the risks posed by this novel exposure pathway, data are needed on the survival and persistence of SARS-CoV in water and sewage.
Because working with SARS requires specially trained personnel working in BSL-3 laboratory containment, there are significant challenges involved in studying the survival of this virus, and very few data are currently available.
The use of surrogate viruses to overcome these challenges and expand the available data on coronavirus survival and persistence in water was the focus of this study.
Other members of the Coronaviridae may be appropriate surrogates for SARS-CoV, providing representative survival data that can be used to conduct risk assessments of SARS transmission via water-related pathways.
Choosing a surrogate virus most similar to SARS-CoV is challenging because there is still disagreement about the exact placement of this virus within the Coronaviridae. The family is divided into three groups:
1 and 2 include human and other mammalian coronaviruses, and
Group 3 consists of avian viruses. SARS is thought to be related to the Group 2 coronaviruses (Jackwood, 2006), and phylogenetic analyses have indicated it may be closely related to mouse hepatitis virus (MHV) (Lio and Goldman, 2004).
However, its exact relationship to the other coronaviruses is still unclear (Gorbalenya et al., 2004).
Therefore, two potential surrogates were evaluated in this study, representing both groups of mammalian coronaviruses. The two viruses chosen for study were transmissible gastroenteritis virus (TGEV), a diarrheal pathogen of swine and a member of the Group 1 coronaviruses, and mouse hepatitis virus (MHV), a respiratory and enteric pathogen of laboratory mice and a member of the Group 2 coronaviruses (Jackwood, 2006).
The survival and persistence of these viruses was observed in reagent-grade water, lake water, and settled human sewage at two temperatures over a period of weeks to provide estimates of how long members of the coronavirus family, as potential surrogates for SARS-CoV, can remain infectious in these waters.
In the SARS outbreak of 2002–03, 16–73% of patients with SARS had diarrhoea during the course of the disease, usually within the first week of illness.3 SARS-CoV RNA was only detected in stools from the fifth day of illness onwards, and the proportion of stool specimens positive for viral RNA progressively increased and peaked at day 11 of the illness, with viral RNA still present in the faeces of a small proportion of patients even after 30 days of illness.4
The mechanism for gastrointestinal tract infection of SARS-CoV is proposed to be the angiotensin-converting enzyme 2 (ACE2) cell receptor.2
In the initial MERS-CoV outbreak in 2012, a quarter of patients with MERS-CoV reported gastrointestinal symptoms such as diarrhoea or abdominal pain at presentation.5
Some patients initially presented with both fever and gastrointestinal symptoms before subsequent manifestation of more severe respiratory symptoms.6
Corman and colleagues7 found MERS-CoV RNA in 14·6% of stool samples from patients with MERS-CoV. In-vitro studies have shown that MERS-CoV can infect and replicate in human primary intestinal epithelial cells, potentially via the dipeptidyl peptidase 4 receptor.8
In-vivo studies showed inflammation and epithelial degeneration in the small intestines, with subsequent development of pneumonia and brain infection.8
These results suggest that MERS-CoV pulmonary infection was secondary to the intestinal infection.
In early reports from Wuhan, 2–10% of patients with COVID-19 had gastrointestinal symptoms such as diarrhoea, abdominal pain, and vomiting.9, 10
Abdominal pain was reported more frequently in patients admitted to the intensive care unit than in individuals who did not require intensive care unit care, and 10% of patients presented with diarrhoea and nausea 1–2 days before the development of fever and respiratory symptoms.9 SARS-CoV-2 RNA has been detected in the stool of a patient in the USA.11
The binding affinity of ACE2 receptors is one of the most important determinants of infectivity, and structural analyses predict that SARS-CoV-2 not only uses ACE2 as its host receptor, but uses human ACE2 more efficiently than the 2003 strain of SARS-CoV (although less efficiently than the 2002 strain).2
Data exist to support the notion that SARS-CoV and MERS-CoV are viable in environmental conditions that could facilitate faecal–oral transmission. SARS-CoV RNA was found in the sewage water of two hospitals in Beijing treating patients with SARS.12
When SARS-CoV was seeded into sewage water obtained from the hospitals in a separate experiment, the virus was found to remain infectious for 14 days at 4°C, but for only 2 days at 20°C.12
SARS-CoV can survive for up to 2 weeks after drying, remaining viable for up to 5 days at temperatures of 22–25°C and 40–50% relative humidity, with a gradual decline in virus infectivity thereafter.13
Viability of the SARS-CoV virus decreased after 24 h at 38°C and 80–90% relative humidity.13 MERS-CoV is viable in low temperature, low humidity conditions.
The virus was viable on different surfaces for 48 h at 20°C and 40% relative humidity, although viability decreased to 8 h at 30°C and 80% relative humidity conditions.14 At present, no viability data are available for SARS-CoV-2.
The viability of SARS-CoV and MERS-CoV under various conditions and their prolonged presence in the environment suggest the potential for coronaviruses to be transmitted via contact or fomites. SARS-CoV and MERS-CoV are both viable in conditions with low temperatures and humidity.12, 13, 14
Although direct droplet transmission is an important route of transmission, faecal excretion, environmental contamination, and fomites might contribute to viral transmission.
Considering the evidence of faecal excretion for both SARS-CoV and MERS-CoV, and their ability to remain viable in conditions that could facilitate faecal–oral transmission, it is possible that SARS-CoV-2 could also be transmitted via this route.
The possibility of faecal–oral transmission of SARS-CoV-2 has implications, especially in areas with poor sanitation.
Keeping water supplies safe
The COVID-19 virus has not been detected in drinking-water supplies, and based on current
evidence, the risk to water supplies is low (22). Laboratory studies of surrogate coronaviruses that took place in well-controlled environments indicated that the virus could remain infectious in water contaminated with faeces for days to weeks (20).
A number of measures can be taken to improve water safety, starting with protecting the source water; treating water at the point of distribution, collection or consumption; and ensuring that treated water is safely stored at home in regularly cleaned and covered containers.
Conventional, centralized water treatment methods that utilize filtration and disinfection should inactivate the COVID-19 virus. Other human coronaviruses have been shown to be sensitive to chlorination and disinfection with ultraviolet (UV) light (23).
As enveloped viruses are surrounded by a lipid host cell membrane, which is not robust, the COVID-19 virus is likely to be more sensitive to chlorine and other oxidant disinfection processes than many other viruses, such as coxsackieviruses, which have a protein coat.
For effective centralized disinfection, there should be a residual concentration of free chlorine of ≥0.5 mg/L after at least 30 minutes of contact time at pH < 8.0 (22).
A chlorine residual should be maintained throughout the distribution system.
In places where centralized water treatment and safe piped water supplies are not available, a number of household water treatment technologies are effective in removing or destroying viruses, including boiling or using high-performing ultrafiltration or nanomembrane filters, solar irradiation and, in non-turbid waters, UV irradiation and appropriately dosed free chlorine.24
Safely managing wastewater and faecal waste
There is no evidence to date that the COVID-19 virus has been transmitted via sewerage systems with or without wastewater treatment.
Furthermore, there is no evidence that sewage or wastewater treatment workers contracted severe acute respiratory syndrome (SARS), which is caused by another type of coronavirus that caused a large outbreak of acute respiratory illness in 2003.
As part of an integrated public health policy, wastewater carried in sewerage systems should be treated in well-designed and well-managed centralized wastewater treatment works.
Each stage of treatment (as well as retention time and dilution) results in a further reduction of the potential risk. A waste stabilization pond (that is, an oxidation pond or lagoon) is generally considered to be a practical and simple wastewater treatment technology that is particularly well suited to destroying pathogens, as relatively long retention times (that is, 20 days or longer) combined with sunlight, elevated pH levels, biological activity and other factors serve to accelerate pathogen destruction.
A final disinfection step may be considered if existing wastewater treatment plants are not optimized to remove viruses.
Best practices for protecting the health of workers at sanitation treatment facilities should be followed.
Workers should wear appropriate personal protective equipment (PPE), which includes protective outerwear, gloves, boots, goggles or a face shield, and a mask; they should perform hand hygiene frequently; and they should avoid touching eyes, nose and mouth with unwashed hands.
This study observed the stability of coronaviruses in water and sewage over long periods of time, and quantified the kinetics of viral inactivation in these media. The coronaviruses studied were capable of remaining infectious in reagent-grade waters, natural environmental waters, and waters contaminated with human fecal waste (sewage) for periods of weeks.
This long-term survival was seen at both low (4 °C) and ambient (25 °C) temperatures. In all water types, the titer of infectious virus declined more rapidly at 25 °C than at 4 °C. Infectivity titer reductions over about 6 weeks ranged from none, to slight (<1 log10) to modest (1–2 log10) at 4 °C, depending on water quality and virus type. Virus inactivation was more rapid in settled sewage than reagent-grade water.
Some comparisons can be made with the limited data available on the extent of SARS-CoV survival in water, sewage and other aqueous media. Rabenau et al. (2005) found that the titer of SARS-CoV declines approximately 0.5 log10 over 9 days in serum-free cell culture medium at room temperature.
This is a slower rate of inactivation than was observed for TGEV and MHV in reagent-grade water and pasteurized settled sewage, and may be due to protective effects of the buffers, salts and organic nutrients found in sterile cell culture medium as compared to non-sterile water or sewage.
Longer virus survival in the presence of protective buffers and salts in a sterile aqueous medium is supported by data from other investigators, who found that SARS-CoV survived longer in PBS (14 days) than in dechlorinated tap water or domestic sewage (2 days) at 20 °C (Wang et al., 2005).
The survival times observed by Wang et al. (2005) in tap water and sewage are shorter than those demonstrated for TGEV and MHV. Because the authors did not report the actual change in virus titer or detection limit of the assays performed, however, a quantitative comparison of viral inactivation rates between their study and other studies is not possible.
Over the course of the present study, the titer of infectious TGEV and MHV remained relatively stable in all test water types at 4 °C. This is consistent with other investigations that found SARS-CoV persisted at least 14 days at 4 °C in domestic sewage and dechlorinated tap water. Again, direct quantitative comparisons of inactivation rates are difficult, because the actual changes in viral titers over time were not reported by Wang et al. (2005).
Although coronavirus inactivation rates are difficult to compare between studies, one finding this study shares with previous work is that temperature is an important factor influencing viral survival.
Temperature and incubation time were significant predictors of viral reduction in this study, which is consistent with previous findings on viral survival in water (Yates et al., 1985, Hurst et al., 1989, Enriquez et al., 1995).
Water type was also a significant predictor of the rate of viral reduction, with greater reduction in pasteurized settled sewage as compared to reagent-grade water. Factors that have been suggested as contributors to greater virus reduction in more contaminated water include pH extremes, the presence of other microorganisms, and certain chemical constituents, such as proteolytic enzymes (Ward et al., 1986).
However, the pasteurization process used to inactivate vegetative bacteria in the pasteurized settled sewage in these experiments may have reduced proteolytic activity in the test water.
The virus inactivation kinetics observed in this study may differ from those that would be seen in raw sewage, which retains the natural proteolytic activity of vegetative bacteria, and may increase rates of viral inactivation compared to pasteurized sewage.
In pasteurized settled sewage spiked with MHV, pH declined over a period of weeks (data not shown), but remained within the range of stability for these viruses, suggesting that it may not have been a significant factor in declining viral infectivity.
The lack of pH effect on virus survival is consistent with previous studies (Yates et al., 1985). Chemical constituents found in sewage may have antiviral activity (Sobsey et al., 1980), and previous investigations have found that virus survival in water is influenced by high molecular weight dissolved matter (Noble and Fuhrman, 1997), which is present at higher concentrations in sewage.
It has been established with other human pathogens that formation of droplets and aerosols from water contaminated with microorganisms can serve as a vehicle for transmission.
Examples include Legionella, a respiratory pathogen acquired when contaminated water droplets are inhaled (Butler and Breiman, 1998), and Cryptosporidium, an enteric pathogen acquired via ingestion of contaminated droplets (CDC, 1998).
Desiccation and aerosolization of body fluids and fecal matter, resulting in ingestion or inhalation of dried particles, can also serve as a source of pathogens such as norovirus (Marks et al., 2003) and hantavirus (LeDuc, 1998). SARS was spread when water contaminated with fecally shed virus was inhaled, causing respiratory infection.
This person-to-person fecal droplet–respiratory transmission route was observed in the Amoy Gardens apartment building outbreak in Hong Kong, the largest point-source outbreak attributable to this type of transmission pathway.
When an individual shedding infectious virus in feces used the toilet facilities in a building, a combination of faulty drain traps and powerful exhaust fans in residential units resulted in virus-laden liquid droplets being drawn from the waste system into living spaces via floor drains.
The droplets were inhaled by occupants and carried on air currents to other areas of the building, resulting in a large number of SARS cases (WHO, 2003, McKinney et al., 2006). More data are needed on the survival of SARS-CoV in fecal droplets and aerosols to assess this new risk pathway in the event that SARS reemerges.
The results of this study suggest that coronaviruses can survive long enough in water and sewage for these vehicles to serve as a source of exposure. The potential for long-term survival, along with the airborne fecal droplet transmission model, suggests that fecally contaminated aqueous media could pose a health risk in future outbreaks.
If water or sewage contaminated with SARS-CoV becomes aerosolized, it could potentially expose large numbers of people to infection. This could create an ongoing risk during an outbreak, even with quarantine measures to isolate infected individuals.
Commercial, residential, and hospital water or sewer systems contaminated with persistent infectious SARS-CoV might defeat quarantine measures by continuing to spread virus even after infected individuals have been removed from the area.
The persistence of coronaviruses in water and sewage in this study suggests that quarantine measures, which proved effective in containing the last SARS outbreak, could be seriously undermined unless adequate attention is paid to the safety and security of building plumbing systems.
For assessment of these risks, further work is necessary to better define the kinetics of SARS-CoV survival and inactivation in water, sewage, and other aqueous media. The survival and persistence data presented here show that TGEV and MHV may serve as conservative indicators of the survival of SARS-CoV in water and sewage, providing a starting point for risk assessments of water and sewage as vehicles for SARS transmission.
- The coronaviruses TGEV and MHV survived and remained infectious for long periods in different water types, including reagent-grade water, surface water, and pasteurized settled sewage.
- Both viruses survived and remained infectious at both low (4 °C) and ambient (25 °C) temperatures.
- In all water types tested (reagent-grade water, lake water and settled sewage), the titer of infectious virus declined more rapidly at 25 °C than at 4 °C.
- Water type, incubation time, and temperature were significant predictors of log10 viral reduction kinetics.
- The persistence of coronaviruses in water observed in this study suggests that if SARS-CoV should reemerge in human populations, water contaminated with these viruses may continue to pose an exposure risk even after infected individuals are no longer present.
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