Speech May Spread More COVID-19 Than Feces

Laird Harrison

May 19, 2020

Editor's note: Find the latest COVID-19 news and guidance in Medscape's  Coronavirus Resource Center.

Normal human speech emits droplets capable of carrying the novel coronavirus (SARS-CoV-2) that causes COVID-19, researchers say. The droplets lingered in the air for up to 14 minutes in a recent study, which could help explain why the disease has spread so rapidly.

On the other hand, a separate study turned up no infectious particles of SARS-CoV-2 in stool samples.

The findings could provide clues about how best to limit the disease's spread. "Fecal–oral is not a happening thing," said Andrew Noymer, PhD, MSc, associate professor of population health and disease prevention at the University of California, Irvine. "I think this reinforces the importance of masking."

SARS-CoV-2 floats in particles in the air, and public health officials believe this is the primary route of transmission between people. Less clear is whether someone has to cough or sneeze to expel enough droplets to infect another person with COVID-19, or whether talking and other activities also spread the disease.

"Early on, people asked me how are they spreading it if they're not coughing and sneezing," Noymer told Medscape Medical News. "It was a bit of a conundrum for a while. One thing I speculated was the occasional sneeze that everyone does every now and then. But it could just be speech is spreading it, because of the droplets that come out when we're talking."

Previous research showed these droplets can contain respiratory pathogens such as Mycobacterium tuberculosis, influenza viruses, and measles virus. Could they also carry SARS-CoV-2?

Valentyn Stadnytskyi, from the National Institute of Diabetes and Digestive and Kidney Diseases, and colleagues put a piece into that puzzle for a study published online May 13 in PNAS.

To measure the size of droplets emitted through speech, they used an intense sheet of laser to visualize bursts of speech droplets produced when subjects said the words "stay healthy."

The method is particularly sensitive in detecting speech droplets with diameters of less than 30 μm, which could remain airborne for longer than the larger droplets that have typically been the subject of research, they say.

Stadnytskyi and colleagues estimated that one minute of loud speaking generates at least 1000 droplets capable of carrying virions — units of the virus that can cause infection. The droplets remained airborne for 8 to 14 minutes, long enough for someone to inhale them and become infected.

They say droplet nuclei averaged 12 to 21 μm in diameter when first emitted and 4 μm after drying in the air, and that a 10-μm droplet has a 0.37% probability of containing at least one virion.

The study supports the conclusions of a National Academies of Sciences committee, as reported by Medscape Medical News. But it falls short of proving speech is spreading COVID-19, said Joshua L. Santarpia, PhD, MS, associate professor of pathology and microbiology at the University of Nebraska in Omaha. "They didn't do any SARS-CoV-2 studies directly," he told Medscape Medical News.

The study does build on previous research, notably a study of hospitals in Wuhan, China, that showed RNA from the virus could be found in droplets as small as 0.25 μm. Yuan Liu, from State Key Laboratory of Virology, Wuhan University, China, and colleagues published their findings online April 27 in the journal Nature. They also did not measure virions.

On the other hand, a laboratory study led by the National Institute of Allergy and Infectious Diseases (NIAID) showed that virions persisted in aerosols created by artificial means for hours, with a median half-life of 1.1 to 1.2 hours. NIAID researchers Neeltje van Doremalen, PhD, and Trenton Bushmaker, BSc, along with colleagues published their findings online April 19 in the New England Journal of Medicine. Their study did not measure the size of droplet needed to carry a virion.

Santarpia and colleagues did conduct these measurements and are now analyzing their data. "I would hope as we learn more about what size particles we need to be concerned about, and how those particles behave, we can look at what kind of precautions we need to take — whether it's ventilation systems or some other means — to try to remove these particles," he said.

About using a casual bandanna or improvised mask in place of a properly fitted medical mask, he said, "The value of cloth masks may be more questionable."

Santarpia cited a study by Chicago researchers finding that tightly woven cloth masks — cotton, natural silk and chiffon — can filter more than half of particles ranging from 10 nm to 6.0 μm if they fit well. But a poor fit can decrease their efficiency by more than 60%. A bad mask might be worse than none at all if it gives the wearer false confidence, he said. Abhiteja Konda, from the Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois, and colleagues published their findings online April 24 in ACS Nano.

UC Irvine's Noymer disagreed, saying that even a mask that provides partial protection can significantly reduce exposure. "It's really beside the point to say it doesn't block all the particles," he said. "The higher the dose the worse the case. We all want to get the kind of SARS-CoV-2 that causes mild symptoms."

Oral–Fecal Transmission Less Likely

Less attention has focused on the possibility of oral–fecal transmission.

People with COVID-19 often have gastrointestinal (GI) symptoms such as diarrhea, and a lot of SARS-CoV-2 RNA has been detected in their stool  specimens. However, previous attempts to isolate virions in such samples have yielded conflicting results.

Ruochen Zang, MD, from Washington University, St. Louis, Missouri, and colleagues assayed fecal samples from 10 people with COVID-19, and examined the virus in simulated human GI fluids. They published their findings online May 13 in Science Immunology.

High levels of viral RNA were found in three of the samples, but no virions were seen in any of them. Investigating further, they found that SARS-CoV-2 can infect human enterocytes in a laboratory model of the human gut, and they identified proteases that facilitated the virus' invasion into the cells.

Finally, they bathed the virus in various simulated human gastric and intestinal fluids. In contrast to the rotavirus, which is transmitted by the oral–fecal route, SARS-CoV-2 "lost infectivity" in the low pH simulated gastric fluid within 10 minutes. It was "inactivated" by components of human colonic fluids, decreasing by fivefold within an hour.

Putting these results together, Zang and colleagues theorized that SARS-CoV-2 replicates in human intestinal enterocytes, but then is inactivated in the lumen of the colon. "So we're not going to get COVID spread the way Montezuma's Revenge [traveler's diarrhea] is spread," said Noymer.

The finding means that toileting is not a particular concern for this virus, he said. It remains important to wash hands because the virus may be transmitted when infected people touch their noses or mouths, then touch a surface that is touched by someone else.

But the most important point from the new studies is to control the spread of the disease through the air. "Don't be such a big mouth, first of all, and second wear masks," Noymer said.

Noymer and Santarpia have disclosed no relevant financial relationships. Stadnytskyi and colleagues have disclosed no relevant financial relationships. Zang and colleagues have disclosed relationships with Sanofi, Pfizer, FluGen, Aridis, Inbios, Eli Lilly, Vir Biotechnology, NGM Biopharmaceuticals, Emergent BioSolutions, and Moderna.

Proc Natl Acad Sci. (Stadnytskyi et al) Published online ahead of print
May 13, 2020. Full text
Sci Immunol. Published online May 13, 2020. Full text

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