What’s the risk that animals will spread the coronavirus?(附译文)
Researchers say there’s an urgent need to find out whether animals can catch the virus and pass it to people.
Smriti Mallapaty
Cats can be infected with the coronavirus.Credit: Carol Smiljan/NurPhoto/Getty
Soon after the new coronavirus started spreading around the globe, reports emerged of cases in animals — pet cats in Hong Kong, tigers in a New York City zoo and mink on farms in the Netherlands. Now researchers are urgently trying to discover which species can catch the virus, and whether they can pass it to people.
So far, there have been only two reported cases of animals — both mink — passing the virus SARS-CoV-2 to people. At the moment, the chance of contracting the disease from an infected animal is negligible compared to the risk of catching it from an ill person, say researchers.
But as the numbers of infected people fall and restrictions on movement ease, infected animals might have the potential to spark new outbreaks. Researchers are calling for extensive sampling of pets, livestock and wildlife to improve understanding of the risk.
The virus could be spreading undetected in some animals that we don’t know about, says Joanne Santini, a microbiologist at University College London. “We just don’t have enough data,” she says.
Several scientists worry that the virus could end up passing back and forth between animals and people. This could really frustrate efforts to control the pandemic, says Arjan Stegeman, a veterinary epidemiologist at Utrecht University in the Netherlands. “We need to take actions now to prevent that from happening,” he says.
At-risk animals
Roughly a dozen animals are known to be susceptible to the virus. Several species, including pet dogs and cats, captive lions and tigers, and farmed mink, almost certainly caught the virus from people. That probably means that related canids, felids and mustelids — the group that includes mink, weasels, badgers, martens and wolverines — could also be susceptible, but so far, no one has checked, says Jürgen Richt, a veterinary virologist at Kansas State University in Manhattan.
Hamsters, rabbits and common marmosets are also susceptible, according to laboratory experiments in which the animals were deliberately infected. Experiments in pigs, ducks and chickens show2 that they are not susceptible, but there have been no studies of other livestock animals, such as cows, sheep and horses. “If SARS-CoV-2 becomes established in wildlife or other species that have close contact with livestock, then this increases the possibility for interspecies transmission,” says Linda Saif, a virologist at the Ohio State University in Wooster.
More studies should assess the susceptibility of various species and whether they can infect other animals, says Richt. Cats, ferrets, hamsters1 and horseshoe bats were all able to pass the coronavirus to animals of the same species in the lab, and mink living in close quarters on Dutch farms have passed the infection between them.
But the fact that an animal can infect another of the same species doesn’t necessarily mean that it can infect people, says Saif. To assess this risk, researchers need a better understanding of the amount of virus to which a person needs to be exposed before they get infected, she says.
Animals that shed large amounts of virus and come into close contact with people should be watched closely, says Martin Beer, a virologist at the Federal Research Institute for Animal Health in Riems, Germany.
Mink farms
The infections at Dutch mink farms suggest that some animals can infect people. At least two dozen mink at four farms in the province of North Brabant have been infected with SARS-CoV-2, with some developing pneumonia and dying.
Looking at genomes from mink and people at two farms, Stegeman and his colleagues found that people working with the animals had probably passed the virus to some of them, which spread it to other mink. The results were posted on bioRxiv on 18 May3.
Further unpublished genomic analysis suggests that a person on one of the farms could have been infected by the mink, says Stegeman. That person seems to have gotten infected after starting to work with the animals, he says, so their infection probably came from the mink, rather than the other way around. The person’s viral genome was also more closely related to those found in the mink than to sequences from other infected people in the Netherlands, including those living near the farm.
But Saif, who hasn’t seen the genomes, says it is very difficult to prove direction of transmission. Finding closely related viruses, combined with other circumstantial evidence, such as the timeline of exposure and disease emergence can help, but “it will be challenging to directly prove animal-to-human,” she says.
Still, the possibility should not be excluded, says Soren Alexandersen, director of the Geelong Centre for Emerging Infectious Diseases in Australia. And the virus could be spreading undetected on other mink farms across Europe, North America and Asia, says Alexandersen.
Multiple hosts
It is not uncommon for pathogens to jump between species, making it difficult to control their spread. SARS-CoV-2 most likely originated in bats, but researchers do not know whether other animals were involved in its journey to people. The 2009 pandemic H1N1 influenza virus originated in pigs, jumped to people, spread worldwide and then passed back to pigs. The virus continues to circulate in the animals, where it has combined with other flu viruses to create new variants that have crossed over to people, says Stegeman.
Several scientists also worry that SARS-CoV-2 could jump back and forth between cats and people, because the animals often roam between households. Although cats can infect other felines, so far there have been no reports of cats infecting people.
Asisa Volz, a veterinary virologist at the University of Veterinary Medicine Hannover in Germany, plans to investigate whether cats spread the virus in a retirement home in Bavaria where residents separated from infected individuals still became ill. A cat there was found to have traces of RNA from the coronavirus, which suggests it could have been shedding virus as it roamed the facility. Volz and Beer will test the facility’s cats for antibodies against the virus and study the chronology of events to see whether the felines were a source of infection.
Stegeman also plans to test cats living with people who have had COVID-19 in the Netherlands. If it turns out that cats can pass the virus to people, he says, it would become even more difficult to control the spread.
“The establishment of a pandemic virus in animal populations can be critical and should be always taken into consideration,” says Beer.
doi: 10.1038/d41586-020-01574-4
References
- 1.
Chan, J. F.-W. et al. Clin. Infect. Dis. https://doi.org/10.1093/cid/ciaa325 (2020).
- 2.
Shi, J. et al. Science 368, 1016–1020 (2020).
- 3.
Oreshkova, N. et al. Preprint available at bioRxiv https://doi.org/10.1101/2020.05.18.101493 (2020).
-
- 4.
Alekseev, K. P. et al. J. Virol. 82, 12422–12431 (2020).
中译文:
动物传播新冠病毒的风险有几分？
原文作者：Smriti Mallapaty
研究人员认为目前迫切需要弄清楚动物是否可能感染新冠病毒并将其传染给人。
猫会感染新冠病毒。来源：Carol Smiljan/NurPhoto/Getty
新冠病毒开始在全球范围内传播后不久，便陆续有动物感染的报道——香港的宠物猫、纽约市动物园的老虎和荷兰养殖场上的水貂。现在，研究人员正在迫切地探索哪些物种会感染新冠病毒，以及它们是否会将病毒传染给人。
到目前为止，只有两例动物将新冠病毒SARS-CoV-2传染给人的报道——均为水貂。研究人员表示，当前从受感染动物身上感染新冠病毒的可能性与从确诊人类身上感染的可能性相比，前者是微不足道的。
但是，随着人类感染数量的下降，以及人口流动限制的放松，受感染动物可能会引发新一轮疫情。研究人员呼吁对宠物、牲畜和野生动物进行大范围的采样，以增进对潜在风险的理解。
伦敦大学学院的微生物学家Joanne Santini说，新冠病毒可能在某些我们还不知道的动物中间传播而未被发觉，“我们的数据还不够。”
多名科学家担心新冠病毒最终会在动物和人类之间来回传播。荷兰乌得勒支大学的兽医流行病学家Arjan Stegeman表示，这着实会破坏防疫工作，“我们需要立即采取措施，防止那种情况发生。”
存在风险的动物
已知易感新冠病毒的动物大约有十余种。包括宠物猫狗、圈养的老虎狮子和养殖场饲养的水貂在内的若干物种，几乎都是从人类身上感染了该病毒。这也许意味着近亲的犬科、猫科和鼬科动物（包括水貂、黄鼠狼、獾、貂鼠和狼獾）也易感，但是到目前为止还没有检测到一例，堪萨斯州立大学的兽医病毒学家Jürgen Richt说。
有实验室在实验中故意使仓鼠、兔子和普通狨猴感染新冠病毒，结果显示它们对该病毒也易感。针对猪、鸭和鸡的实验显示[2]，它们不易感，不过目前尚无关于其他牲畜的研究，比如牛羊马。“如果SARS-CoV-2能在一些和牲畜有密切接触的野生动物或其他物种中间传播，这就会增加种间传播的可能性。”俄亥俄州立大学的病毒学家Linda Saif说。
Richt说，应开展更多的研究来评估不同物种的易感性，以及它们是否能够感染其他动物。在实验室中，猫、雪貂、仓鼠[1]和菊头蝠都能够将新冠病毒传染给同类，荷兰养殖场上生活相距不远的水貂之间也发生了病毒传播。
但是事实上，一种动物可以感染同类并不一定意味着它可以感染人，Saif表示。为了评估这种风险，研究人员需要进一步了解一个人需要暴露于多少量的病毒才能发生感染，她说。
德国联邦动物卫生研究所（Federal Research Institute for Animal Health）的病毒学家Martin Beer表示，应该密切监测会排出大量病毒并且与人类密切接触的动物。
水貂养殖场
荷兰水貂养殖场发生的感染表明，一些动物能感染人类。在荷兰北布拉班特省的四个养殖场，至少有24只水貂感染了SARS-CoV-2，其中一些患上了肺炎，生命垂危。
Stegeman及其同事研究了其中两个养殖场上的水貂和人的基因组，发现可能是一些职工将新冠病毒传染给了水貂，继而在水貂之间发生传播。该研究结果于5月18日发布在bioRxiv上[3]。
Stegeman说，尚未发表的进一步的基因组分析显示，其中一个养殖场上的一名职工可能被水貂感染了。他说这名职工似乎是在工作中接触了水貂之后才感染的，因此可能是水貂感染了人，而不是人感染水貂。这名感染者的病毒基因组也与水貂的病毒基因组关系更近，而不是其他感染者的基因组，包括居住在养殖场附近的感染者。
Saif还没有看到基因组数据，但是她认为病毒传播的方向是很难证明的。找到亲缘相近的病毒，再加上其他旁证，比如病毒暴露和发病的时间线，可能会有帮助，但是“要直接证明动物到人的病毒传播非常难”，她说。
尽管如此，澳大利亚吉朗新发传染病中心（Geelong Centre for Emerging Infectious Diseases）主任Soren Alexandersen认为不能排除这种可能性。他说新冠病毒可能在欧洲、北美和亚洲的其他水貂养殖场上悄悄地传播。
多重宿主
病原体在不同物种之间传播并不稀奇，这也加大了控制传播的难度。SARS-CoV-2最有可能起源于蝙蝠，但是研究人员并不知道中间是否经过了其他动物才传染给人类。2009年的H1N1流感病毒起源于猪，传播到人类身上后在全球范围内蔓延，最后又传回到猪身上。该病毒继续在猪中间传播，并且与其他流感病毒结合形成了新的变体，再传播到了人类身上，Stegeman说。
一些科学家还担心SARS-CoV-2会在猫和人类之间来回传播，因为猫经常在人类家庭之间活动。虽然猫可以感染其他猫科动物，但是目前还没有猫传染人的报道。
德国汉诺威兽医学院的兽医病毒学家Asisa Volz计划调查猫是否在巴伐利亚的一家养老院里传播了新冠病毒；在这家养老院里，一些和感染者隔离的人也被感染了。一只猫被发现带有新冠病毒RNA痕迹，这意味着这只猫可能在院内活动的时候排出了病毒。Volz和Beer将检验这家养老院内的猫是否有新冠病毒抗体，并梳理事件发生的时间线，以了解猫是否是一个感染源。
Stegeman还计划在荷兰对那些与COVID-19确诊者共同生活的猫进行检验。他说如果发现猫可以将新冠病毒传染给人，控制病毒传播就会变得更难了。
“流行病毒在动物种群中形成传播链绝不是小事，应把这种可能性始终考虑在内。”Beer说。
参考文献：
1.Chan, J. F.-W. et al. Clin. Infect. Dis. https://doi.org/10.1093/cid/ciaa325 (2020).
2.Shi, J. et al. Science 368, 1016–1020 (2020).
3.Oreshkova, N. et al. Preprint available at bioRxiv https://doi.org/10.1101/2020.05.18.101493 (2020).
4.Alekseev, K. P. et al. J. Virol. 82, 12422–12431 (2020).