Possible transmission of COVID‐19 epidemic by a dog as a passive mechanical carrier of SARS‐CoV‐2, Chongqing, China, 2022

Abstract

An outbreak of coronavirus disease 2019 (COVID‐19) was reported in Yongchuan district of Chongqing, China in March 2022, while the source was unknown. We aimed to investigate the origin and transmission route of the virus in the outbreak. We conducted field investigations for all cases and collected their epidemiological and clinical data. We performed gene sequencing and phylogenetic analysis for the cases, and draw the epidemic curve and the case relationship chart to analyze interactions and possible transmission mode of the outbreak. A total of 11 cases of COVID‐19, including 5 patients and 6 asymptomatic cases were laboratory‐confirmed in the outbreak. The branch of the virus was Omicron BA.2 which was introduced into Yongchuan district by a traveler in early March. Patient F and asymptomatic case G had never contact with other positive‐infected individuals, but close contact with their pet dog that sniffed the discarded cigarette butts and stepped on the sputum of patient B. Laboratory test results showed that the dog hair and kennel were positive for severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2), and the 10 isolates were highly homologous to an epidemic strain in a province of China. The investigation suggested that the contaminated dog by SARS‐CoV‐2 can act as a passive mechanical carrier of the virus and might transmit the virus to humans through close contact. Our findings suggest that during the COVID‐19 pandemic, increasing hygiene measures and hand washing after close contact with pets is essential to minimize the risk of community spread of the virus.

1. INTRODUCTION

Since late December 2019, the outbreak of the coronavirus disease 2019 (COVID‐19), caused by severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2), has raised global concerns and had an unexpected impact on the lives and livelihoods of people around the world. ¹ , ² COVID‐19 is suspected to be a zoonosis because of its association with a seafood and live animal market in Wuhan, China, ³ and several countries have reported the occurrence in various animal species such as dogs, cats, lions, minks, and tigers. ⁴ As a respiratory virus, SARS‐CoV‐2 is mainly transmitted through respiratory droplets when an infected person coughs, sneezes, or just talks close with others, ⁵ while the detection of SARS‐CoV‐2 in pets raised questions about the possible zoonotic transmission from animals in close contact with humans. ⁶ However, most of these infections primarily originated from COVID‐19‐positive owners, ⁷ , ⁸ , ⁹ and there was rare evidence of animal‐to‐human transmission. Therefore, some scholars believed that even if pets were infected, they would not play an epidemiological role in the transmission of SARS‐CoV‐2 to humans. ⁶ A recent report described a suspected zoonotic SARS‐CoV‐2 transmission from a cat to a human in Thailand. ⁴ Oude M. et al. reported a possible animal‐to‐human transmission of SARS‐CoV‐2 within mink farms in the Netherlands. ¹⁰ In addition, contact with contaminated surfaces has also been considered a potential form of transmission of SARS‐CoV‐2. ¹¹ Therefore, owners may get infected with the virus through close contact with the hair, paws, or pads of contaminated pets, such as hugging or kissing them. Consequently, there is a possibility of spreading SARS‐CoV‐2 through domestic pets, ¹² but reliable epidemiological evidence remains scarce.

In this study, we investigated an outbreak of COVID‐19 in Yongchuan district of Chongqing, in Southwest China. Field epidemiological investigation, laboratory tests, and analysis of the transmission chain were conducted to identify the cause of the outbreak and investigate the potential routes of the virus transmission between humans.

2. MATERIALS AND METHODS

2.1. Case report

Since March 9, 2022, a 20‐year‐old female nurse (case E) in Yongchuan district of Chongqing sought treatment for asthenia at a private clinic where she worked, but with no obvious improvement in symptoms. On March 14, she was confirmed positive for SARS‐CoV‐2 infection by reverse‐transcription PCR (RT‐PCR), and information was reported to National Infectious Disease Information System. Chongqing Center for Disease Control and Prevention (CQCDC) and Yongchuan District Center for Disease Control and Prevention (YCCDC) traced and tested her contacts.

2.2. Epidemiological investigation and management

After the first positive case was identified, the local health administrative department launched a public health emergency response immediately. The patient was sent to the designated hospital and received treatment. Based on Prevention and Control Protocol for Novel Coronavirus Pneumonia (version 8), the response team consisted of CDC officers, technicians of the big data center, clinicians and community workers, conducted epidemiological investigation, aggressive case finding and contact tracing, isolation of patients and asymptomatic persons, quarantining of close contacts, and thorough environmental testing and disinfection.

A patient was defined as a symptomatic person with positive RT‐PCR for SARS‐CoV‐2. An asymptomatic case was defined as a person whose RT‐PCR result was positive without any symptoms. We defined the index case as the person with the earliest laboratory‐confirmed COVID‐19 in Yongchuan district. A person who had unprotected close contact with a case was defined as close contact, such as studying, living, working together, or taking the same vehicle at the same time. The focuses of the survey were exposure history and close contacts of the case. Investigators conducted telephone interviews or face‐to‐face interviews with a case to identify the close contacts in the shortest time. In addition, technical means such as electronic records (payment records and check‐in hotel records) and surveillance video were used to assist in sorting out the activity traces of cases.

All close contacts were quarantined in the local hotels for 14 days, under medical observation including checking of temperature and clinical symptoms, and the nucleic acid test for SARS‐CoV‐2 regularly. Community workers provided them with water, food, and other necessities. Once the close contact was confirmed positive for COVID‐19, she/he would be transferred to the hospital for treatment. The epidemic would be over until the last contact was released from medical observation.

2.3. Data collection

All information of traceability investigation including demographic characteristics, clinical symptoms, and activities, from 14 days preceding symptom onset or positive nucleic acid test until isolation in the hospital were collected using big data tools and reported by CDC based on the “questionnaire on the individual case of COVID‐19”. ¹³ , ¹⁴

2.4. Laboratory test

The pharyngeal swabs and environmental samples were collected by healthcare workers and sent to YCCDC, hospitals, or qualified testing institutions. The RT‐PCR assay was performed to confirm the infection caused by SARS‐CoV‐2, and all positive samples were retested by YCCDC. All positive samples were sent to CQCDC for further virus isolation and sequencing. The specimens were considered positive if the cycle threshold (C _t) value was 40.0 or lower.

The extracted RNA was subjected to reverse transcription and amplification using a ULSEN COVID‐19 Whole Genome Capture Kit (MicroFuture), and the sequencing library was constructed by using IIIumina DNA Prep (M) and Nextera^TM DNA CD Indexes Kit (IIIumina, Inc.). Whole Genome sequencing was performed on MiniSeq, Miseq, and NextSep2000 platforms (Illumina, Inc.).

2.5. Statistical analysis

Information including questionnaires, clinic and laboratory results were entered and checked in the excel database. The incubation period was defined as the duration between estimated dates of exposure and reported symptom onset or detection positive. Epidemiological characteristics were descriptively analyzed, and the epidemic curve was drawn based on the date of case onset for the patient or of sampling for the asymptomatic case. The case relationship chart was designed to analyze the transmission chain. The sequences of different lineages and clades of SARS‐CoV‐2 were downloaded from National Center for Biotechnology Information, and together with the sequences obtained in this study, they were aligned using Mafft 7.22 software. Phylogenetic analyses were performed with the Maximum‐Likelihood (ML) method implemented in MEGA software vision 6.06, with a bootstrap value of 1000.

3. RESULTS

3.1. Clinical and epidemiological characteristics of this outbreak

As of March 21, 2022, a total of 11 cases related to this outbreak were laboratory‐confirmed with a COVID‐19 infection in Yongchuan district of Chongqing, China, with a male‐to‐female ratio of 0.57. The median age was 27 years old (range: 16–79). The clinical symptoms of cases were relatively mild, and the most common symptoms at the illness onset were asthenia (3/5) and cough (3/5). No severe or critically ill patients appeared and six cases were asymptomatic. The vast majority of cases (9/11) received the full of domestically made COVID‐19 vaccine (Table 1). Totally, 2558 close contacts were quarantined in this outbreak.

Table 1.

Characteristics of cases with COVID‐19 in this outbreak

Case	Sex	Age	Occupation	Clinical symptoms	Underlying diseases	Vaccinated	Travel outside in the past 14 days
A	Female	16	Unemployed	Asymptomatic	No	No	Yes
B	Male	21	Freelancer	Asthenia, sore throat	No	Yesa	No
C	Male	21	Freelancer	Fever	No	Yes a	No
D	Female	50	Cleaner	Asymptomatic	No	Yesb	No
E	Female	20	Nurse	Asthenia, cough	No	Yes b	No
F	Female	27	Nurse	Asthenia, fever, sore throat, cough	No	Yes a	No
G	Female	19	Student	Asymptomatic	No	Yes b	No
H	Male	50	Unemployed	Asymptomatic	No	Yes b	No
I	Female	45	Doctor	Asymptomatic	No	Yes b	No
J	Male	79	Retiree	Asymptomatic	Hypertension, heart disease	Yes a	No
K	Female	73	Retiree	Cough, headache	Breast cancer	No	No

Abbreviation: COVID‐19, coronavirus disease 2019.

^a Did not take booster vaccination.

^b Complete vaccination.

3.2. Time distribution

The date of onset for the patients and of sampling for asymptomatic cases was from March 8 to 21 (Figure 1), and there was no obvious peak in the epidemic curve. Table 2 listed the characteristics of cases in the COVID‐19 outbreak. The first COVID‐19 patient (case E) of this outbreak was reported on March 14, while she fell ill on March 9. The onset date of the first two cases (patients B and C) was March 8, diagnosis on March 15. Case A, an asymptomatic case, returned from an area with a higher risk of COVID‐19 infection on March 5.

Figure 1.

Temporal distribution of coronavirus disease 2019 (COVID‐19) cases in this outbreak. Epidemic curves of cases were drawn according to the date of onset for the patients and of sampling for asymptomatic cases.

Table 2.

Time distribution characteristics of cases in the COVID‐19 outbreak

Case	Date of onset	Date of infection	Incubation period, days	Date of confirmation
A	NAa	March 4	NA	March 15
B	March 8	March 5–7	1–3	March 15
C	March 8	March 5–7	1–3	March 15
D	NA	March 7–15	NA	March 19
E	March 9	March 8–9	1	March 14
F	March 11	March 9	2	March 17
G	NA	March 9	NA	March 17
H	NA	March 7–15	NA	March 19
I	NA	March 9–16	NA	March 17
J	NA	March 9–17	NA	March 21
K	March 19	March 9–17	2–10	March 21

Abbreviation: COVID‐19, coronavirus disease 2019.

^a Asymptomatic case with no date of onset.

3.3. Transmission chain

Based on the epidemiological investigation, this outbreak was caused by an asymptomatic case (case A) who returned from another province of China, where there had been a continuous COVID‐19 epidemic reported from February to March 2022, while there had been no local COVID‐19 cases reported for more than 150 days in Yongchuan district of Chongqing.

In the transmission chain (Figure 2), case E was the index patient, asymptomatic case A was the real first case of the outbreak, and the second generation comprised patients B and C who were flatmates. Patient C was the boyfriend of asymptomatic case A. Through living together and close contact, case A transmitted the virus to patients B and C. Then, there were two parts of the transmission chain from the same source of infection (case A).

Figure 2.

The transmission process of the entire coronavirus disease 2019 (COVID‐19)‐infected cluster

In the lower part of the transmission, the third generation comprised asymptomatic case D and patient E. Asymptomatic case D, a cleaner, has been cleaning near the offices of patients B and C for a long time, existing possibility of exposure. At the same time, patients B and C both visited the private clinic where patient E worked. Asymptomatic case H was the fourth generation, and he was the husband of the asymptomatic case D.

In the upper part, the possibility of direct contact between cases F–G and other infected persons was ruled out according to the epidemiological investigation and alignment of activity traces. The video surveillance showed that at 17:17 on March 9, patient B spat and littered cigarette butts around. About 20 min later, the boyfriend of case F walked over to the area with his pet dog following him. The dog sniffed the discarded cigarette butts and stepped on the sputum. Subsequently, case F and her sister, case G, met with her boyfriend and hugged the dog immediately, which vomited during that time. In the following days, cases F and G had close contact with the pet dog, including hugging and petting, and both of them were licked by the dog on the backs and palms of their hands. They did not wash or sanitize after close contact with the dog. Laboratory test results showed that the samples of dog hair and kennel were positive for SARS‐CoV‐2, while the throat swab test of the dog was negative. The fourth generation was the asymptomatic case I, who was the colleague of patient F, and they worked at a local hospital. The fifth generation comprised asymptomatic case J and patient K, who were the parents of case I.

3.4. Gene sequencing

We obtained the whole genome sequence of SARS‐CoV‐2 in 11 samples except case H (the viral concentration in the sample of case H was too low to meet the requirements of the library construction reagents). The sequence indicated that all 10 isolated viruses of this outbreak belonged to the variant of concern (VOC)/Omicron (branch BA.2), and 9 of them shared the same nucleotide sequence, while the virus collected from case G had an additional mutation (C1938T) (Figure 3). The sequencing showed that they were highly homologous to an epidemic strain in a province of China during the same period.

Figure 3.

Neighbor‐joining phylogenetic tree based on the whole genome sequences of coronavirus disease 2019 (COVID‐19) representative strains. The 10 cases of this outbreak were indicated by red dots while the PANGOLIN lineages were marked on the right. The phylogenetic tree was rooted by using Wuhan‐Hu‐1 (NC_045512.2).

4. DISCUSSION

In this study, we focused on investigating the possibility and mode of COVID‐19 transmission by the dog. After a detailed epidemiological investigation and genetic analysis, we found that there were two transmission branches in the outbreak, and the most likely epidemiological connection was a pet dog, and the gene sequences of the virus in the two branches were identical. The results suggested that the pet dog may mechanically transmit SARS‐CoV‐2 to the owners through close contact. These findings may contribute to confirming that direct contact with contaminated pets represents a potential route for the transmission of SARS‐CoV‐2 and play a crucial role in outbreak investigations.

Since health authorities in South Africa reported the emergence of a new SARS‐CoV‐2 variant, Omicron, in November 2021, it has spread rapidly in 149 countries ¹⁵ and became the dominant strain worldwide. Several outbreaks of COVID‐19 of Omicron BA.2 emerged in many provinces and cities of China, including Jilin Province, Hong Kong, and Shanghai City since January 2022. ¹⁶ , ¹⁷ , ¹⁸ This investigation conducted in Chongqing identified that the virus strain of the outbreak involving 11 COVID‐19 cases was Omicron BA.2 which had a higher fusogenicity, effective reproduction number, and pathogenic potential than BA.1, ¹⁹ while less pathogenicity compared with early SARS‐CoV‐2 strains. ²⁰ However, as similar viral loads were reported in patients and asymptomatic cases, ²¹ the asymptomatic infection can increase the difficulty of disease control. In this outbreak, the first case was identified later than the index case because he had no symptoms, which led to a lag in outbreak detection. Since asymptomatic cases are the common sources of infection, strict monitoring and extensive laboratory testing of contacts are essential to containing early outbreaks. In addition, our investigation suggested that vaccine breakthrough infection was common for BA.2 because 81.8% of the cases were vaccinated. It was consistent with previous research that compared with the ancestral virus, the neutralizing activity for the Omicron variant was much reduced in humans. ²² , ²³ , ²⁴

The most important finding of this study was that we provided novel epidemiological evidence that contaminated pets may transmit SARS‐CoV‐2 to humans through passively carrying the virus. First, the two sisters (cases F and G) did not travel outside somewhere in the preceding 14 days and did not contact with any other local cases. Second, the viral genomes of the two sisters were identical to that obtained from previous cases, suggesting the correlation of virus transmission between cases. Third, the dog was the most likely epidemiological association of the two sisters with other cases. The video surveillance showed that the two sisters immediately hugged the dog and caressed it without any personal protection or disinfection after the dog was exposed to the contaminants of Case B. Based on the above evidence, the contaminated pet dog may mechanically carry the SARS‐CoV‐2 and result in virus transmission. As a zoonotic pathogen, SARS‐CoV‐2 is thought to have originated in bats, but the intermediate host is still unclear. ²⁵ Animals such as turtles and snakes are suggested as potential intermediate hosts, while pangolin is the highly suspected candidate as its intermediate. ²⁶ A variety of species including wild and domestic animals have been reported to be susceptible to SARS‐CoV‐2 infection, indicating that the virus crossed the species barrier. ⁷ , ²⁷ To date, sporadic cases of SARS‐CoV‐2 infection have been confirmed in dogs. Two dogs living in Hong Kong and close contact with SARS‐CoV‐2‐infected human cases were reported as the first animal case in 2020. ²⁸ In the Netherlands and United States, three pet dogs tested positive for anti‐SARS‐CoV‐2 antibodies, with COVID‐19‐positive owners. ²⁹ Dogs were found to have angiotensin‐converting enzyme 2 (ACE2), similar to human ACE2, which raised the possibility that dogs might be a potential intermediate host of SARS‐CoV‐2. ³⁰ However, all these infections primarily originated human‐to‐dog, and the transmission of pet‐to‐human was rarely reported. Limited evidence involving animal‐to‐human transmission of SARS‐CoV‐2 was only found in farm minks in the Netherlands and a cat in Thailand. ⁴ , ¹⁰ Inconsistent with previous studies, the possible dog‐to‐human transmission of Omicron BA.2 in this outbreak was passive and mechanical, because the nasopharyngeal swab specimens from the dog were not detectable SARS‐CoV‐2, while the dog's hair and kennel showed positive. Strictly speaking, it was more similar to the transmission of contact with the surface contaminated with the virus, which belonged to indirect transmission. Italian scholars collected and tested the cutaneous and interdigital swabs from dogs and cats owned by COVID‐19 patients, and all samples showed negative for SARS‐CoV‐2, which suggested that pets will not passively carry the virus on their hair and pads. ⁶ However, it should be noted that there has been ample evidence that SARS‐CoV‐2 can survive on the surface of objects for a short period of time and rapidly spread from via fomites. ¹⁴ , ³¹ Compared with contacting contaminated surfaces of inanimate objects, two important differences were that the interaction between owners and pets was frequent, and the way of contact with pets was closer, such as kiss and caress. In this scenario, pets including dogs and cats were not only act as victims of a human‐to‐pet but also may be as vectors and sources of infection. Although the incidence of this transmission method is relatively rare, we advise caution in contacting with pets during COVID‐19 surges, increasing hygiene measures and hand washing after close contact with pets. In addition, regular cleaning of pets is also necessary.

The limitation of this study was that we did not sequence the relevant samples from the dog because the viral concentrations of the samples were too low to meet the requirements of the library construction reagents, which restricted the strength of the evidence. In addition, we were unable to obtain the samples of the dog on March 9, and the viral RNA on the dog more likely originated from individuals F and G, who were in close contact with the dog while sick. Therefore, the causal link between the dog and the cases is circumstantial. Extensive epidemiological investigations in pets living in areas with a high prevalence of COVID‐19 in humans and further animal experiments can definitively clarify their role in the transmission of SARS‐CoV‐2. ³²

5. CONCLUSIONS

In summary, we provided new evidence that contaminated dogs may act as passive mechanical carriers of SARS‐CoV‐2 when they live in close contact with humans. Despite the incidence of this transmission method being relatively uncommon, persons should complete the standard personal protection and refrain from contact with their contaminated pets. Surveillance to track virus prevalence in household pets is urgently needed in the epidemic areas. Washing pets and pet supplies regularly during the epidemic period is also essential.

AUTHOR CONTRIBUTION

Chunbei Zhou, Ailin Wu, Kun Su, Qin Li, and Shibin Xu contributed to the literature search, study design, data collection, data analysis, and writing of the report. Sheng Ye and Shuang Chen contributed to the laboratory testing and analysis. Zongliang Zhou, Hongjun Zhang, Xiyou Zhao, Ya Wang, Huan Wu, Dandan Ruan, and Wenge Tang contributed to the epidemiological data collection and analysis. All authors read and approved the final report.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

ETHICS STATEMENT

Data collection from COVID‐19 cases and their close contacts were part of the continuing public health investigation of an emerging outbreak which was exempt from institutional review board assessment. Even so, we obtained the oral informed consent of all participants before conducting the interviews.

Zhou C, Wu A, Ye S, et al. Possible transmission of COVID‐19 epidemic by a dog as a passive mechanical carrier of SARS‐CoV‐2, Chongqing, China, 2022. J Med Virol. 2022;95:e28408. 10.1002/jmv.28408

DATA AVAILABILITY STATEMENT

The data analyzed are not publicly available as they contain personal data.