The distribution and prevalence of SARS-CoV-2 in the Huanan seafood market

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In a recent study published in the Nature Journal, researchers surveyed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at the Huanan seafood market (HSM).

Study: Surveillance of SARS-CoV-2 at the Huanan Seafood Market. Image Credit: VladimirKrupenkin/Shutterstock.com

Study: Surveillance of SARS-CoV-2 at the Huanan Seafood Market. Image Credit: VladimirKrupenkin/Shutterstock.com

Background

Coronavirus disease 2019 (COVID-19) cases were first reported in Wuhan, China in December 2019 as pneumonia of unknown etiology (PUE).

The early cases were mostly associated with the HSM in Wuhan, which sold a variety of animal meats, live animals, and exotic seafood.

The HSM is suspected to be the origin of the COVID-19 outbreak. Some scientists suggest that bats from Asian countries such as China, Japan, Laos, Thailand, and Cambodia may be the original SARS-CoV-2 source due to their high similarity with certain coronaviruses found in these bats. It is currently unclear if another animal played a role in transmitting the virus from bats to humans.

About the study

In the present study, researchers assessed the findings of SARS-CoV-2 surveillance in the HSM market, Wuhan.

The HSM is situated in Wuhan's downtown area, in the Jianghan District. The market covers an area of over 50,000 square meters and has 678 closely situated stalls in a highly congested environment.

The market is divided into two areas: the West and East Zones. The West Zone is where seafood and animals are primarily sold, while the East Zone is where livestock meat is sold.

Ten stalls selling domesticated wildlife were noted among the 678 stalls located in the market. These stalls were situated in the West Zone's southwestern side and the East Zone's northwestern region.

In late December 2019, 10 animal stalls sold animals or animal products. The traded animals included avian species, snakes, badgers, Sika deer, bamboo rats, rabbits, hedgehogs, porcupines, giant salamanders, Siamese crocodiles, and bay crocodiles.

Live snakes, crocodiles, and salamanders were among the animals traded. Between January 1, 2020, and March 2, 2020, a comprehensive collection of 923 environmental samples from various regions in and around the HSM market, and 457 animal samples, such as dead animals in freezers and refrigerators, stray animals and their excrement, were gathered. A few stray animals were sampled till March 30.

The examination of the overground drainage pathway found in the HSM revealed that the wastewater from the overground drainage flowed into the underground drainage within the market and eventually into the wells located on the market's edge.

Human upper respiratory excreta and animal waste would be combined in the overground drainage. The study aimed to test for live viruses in the HSM by inoculating 27 SARS-CoV-2-positive environmental samples obtained on January 1, 2020, into cell lines such as Vero E6 and Huh7.5 cells.

Results

923 environmental samples were collected within and around the HSM, with 73 testing positive for SARS-CoV-2 using real-time polymerase chain reactions (RT-PCR). This resulted in a positivity rate of 7.9%. The RT-PCR cycle threshold (CT) values varied between 23.9 and 41.7. Out of 828 samples from the HSM, 64 tested positive.

The majority of SARS-CoV-2-positive specimens collected in the HSM were found in the West Zone, specifically on streets numbered one to eight, where 71.4% of positive specimens were detected. Five out of 14 samples from HSM-related warehouses tested positive. This could indicate the presence of SARS-CoV-2 in the market in the early period of the COVID-19 outbreak.

Three out of 51 sewerage well samples from the surrounding regions outside the HSM tested positive. On January 22, 2020, 30 environmental samples were obtained from Dongxihu Market in Wuhan. Among them, a floor surface swab termed Env_0601 tested positive.

Almost 24 out of 110 samples obtained from market sewers or sewerage wells tested positive for the  SARS-CoV-2 nucleic acid. The market's four sewerage wells also tested positive. The data indicated that the virus might have been transmitted within the market through contaminated sewage from infected people or animals.

After three days of inoculation, sample Env_0313 induced cytopathic effects (CPE) on Vero E6 cells. CPE was observed on Huh7.5 cells five days after inoculation. Virus particles were observed in both the supernatant as well as cells of Vero E6 five days after inoculation. Coronavirus morphology was observed in negative-stained viral particles and ultra-thin cultured cell sections.

Samples Env_0354 and Env_0313 were obtained from stalls housing COVID-19-positive patients. Live SARS-CoV-2 consisting of high titers were found in the environment of the HSM through successful virus isolation from original samples having low CT values.

Conclusion

The study findings showed that SARS-CoV-2 nucleic acid and live virus were found in environmental samples obtained from the HSM West Zone.

The researchers believe that the market's high proportion of visitors may have amplified the transmission of the virus, leading to several COVID-19 infection clusters within the early stages of the SARS-CoV-2 outbreak.

Journal reference:
Bhavana Kunkalikar

Written by

Bhavana Kunkalikar

Bhavana Kunkalikar is a medical writer based in Goa, India. Her academic background is in Pharmaceutical sciences and she holds a Bachelor's degree in Pharmacy. Her educational background allowed her to foster an interest in anatomical and physiological sciences. Her college project work based on ‘The manifestations and causes of sickle cell anemia’ formed the stepping stone to a life-long fascination with human pathophysiology.

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