Reflections on the cross species transmission of virus

Incubation period, subclinical infection, case detection and detection of virus

Source: STD / AIDS prevention and control center of China Center for Disease Control and Prevention

Novel coronavirus (2019-nCoV) has been identified as a highly infectious disease in Wuhan in late 2019. Now we have clearly realized that this is a highly contagious disease caused by a new coronavirus (2019-nCoV). In just 2 months, the number of infected persons has reached more than 70 thousand, and more than 2000 people have died.

This has caused great impact on China's economic and social life. Under the strong leadership of the Party Central Committee, the people of the whole country have joined forces to fight against the epidemic, and the number of newly confirmed cases has gradually declined in recent days. We have seen the dawn of fighting against the epidemic.

38 years ago, in 1981, we found a disease among gay men in the United States, which is now known as AIDS caused by human immunodeficiency virus. Now let's understand from the perspective of virology that these two kinds of serious infectious diseases are caused by viruses. They have different transmission ways and control strategies.

Reflections on the cross species transmission of virus

2019-ncov is the seventh known coronavirus that can infect human beings. In addition to SARS CoV, which causes SARS, and mers CoV, which causes Middle East respiratory syndrome, several other coronaviruses (HCoV-NL63, hcov-229e, hcov-oc43, hcov-hku1) only cause mild cold symptoms. Previous studies have shown that bats and mice have unique immune systems that can co-exist with many viruses in the absence of disease.

Some studies have found that the coronavirus detected by bats in China is the most similar to the 2019-ncov epidemic, and the similarity of the whole gene sequence is more than 95%, so bats may be the natural reservoir of 2019-ncov virus. However, bats usually do not directly transmit coronavirus to humans, because there are at least 1000 bases or hundreds of amino acids between the two, and it will take a long time for them to cross the species barrier. So there's an intermediate host from bats to humans.

SARS CoV in 2003 and mers cov in 2012 were able to infect and spread human through the evolution and adaptation of the intermediate host in civet cat and camel, respectively, until there are only a few base differences between them. Although the novel coronavirus intermediate host is not clear at present, the current investigation and study suggest that it is likely that it is a wild animal in Southern China seafood market or other markets. The latest research shows that there are a large number of coronaviruses in pangolin population with higher similarity to 2019 ncov, which is likely to be a potential intermediate host of 2019 ncov.

To find and confirm the intermediate host of 2019-ncov will be the focus of the traceability research in the future. In order to find out the origin of this epidemic and prevent the coronavirus from infecting the intermediate host of human beings, we can not only find out the origin of this epidemic, but also prevent the coronavirus from infecting human beings.

(quoted from Nature Reviews Microbiology 2019, such as Jie Cui, etc., modified and adjusted by the author)

We know that HIV was transmitted to humans from the SIV of primates through cross species transmission about 100 years ago. More detailed traceability analysis shows that the HIV-1 m and N viruses that cause widespread transmission in humans are derived from the SIV virus of chimpanzees. HIV-1 group O and P viruses and HIV-2 viruses are derived from SIV viruses of gorillas, African langurs and other primates. That is to say, AIDS is also a viral infectious disease of human to human transmission caused by multiple cross species transmission of SIV.

Phylogenetic relationship between human coronaviruses and animal coronaviruses (website of China CDC)

The phylogenetic relationship between HIV and SIV in different primates (fields virology, 6th Edition)

Both coronavirus and HIV traceability show that there are a large number of different kinds of viruses in the natural host of the virus, and the virus has broken through the species barrier more than once to spread to humans. But before these viruses break through the species barrier to infect humans, we knew little about these viruses.

Although it is a relatively small probability event that the virus crosses the species barrier and spreads among human beings, in the past one or two hundred years, human society has developed unprecedentedly. In the process of developing and using natural resources, it is inevitable to contact more and more wild animals, which leads to similar events repeatedly. It also reminds us to keep awe in the process of contacting nature and obtaining natural resources.

At the same time, more efforts should be devoted to basic research to better understand the virus infection of wild animals as natural and intermediate hosts of viruses, so as to make full preparations for the prevention and control of possible virus cross species human to human transmission in the future.

Incubation period, subclinical infection, case detection and detection of virus

According to the length of incubation period, infectious diseases are divided into acute and chronic infectious diseases. The incubation period of acute infectious diseases is calculated by days and weeks, while that of chronic infectious diseases is calculated by months or even years. Generally speaking, the longer the incubation period, the slower and harder the detection of infectious diseases, and the shorter the incubation period, the faster and easier the detection of infectious diseases.

The later it is found, the more difficult it is to control it, because the disease has spread far and many people in silence. The more difficult it is to trace the origin of the disease, or the mission that is impossible to accomplish. It is estimated that AIDS, with a incubation period of 7-10 years, may have infected hundreds of thousands of people around the world when it was first found in the United States in 1981. The data of patient No. 0 traced back to the United States, which was updated decades later, proved to be completely wrong.

The novel coronavirus control and test also tested our ability to control and research. Chinese scientists identified novel coronavirus as novel coronavirus in the early stage of the discovery, and determined and published its complete genome sequence and shared it with the world. Nucleic acid diagnostic reagents were quickly developed and distributed throughout the country; several teams began to develop new coronavirus vaccines; the most stringent control measures were taken, and medical personnel and control personnel from all over the country rushed to the emergency.

Transfer to the most serious epidemic in Hubei Province to treat patients, Wuhan as the center to carry out national epidemic prevention and control. Novel coronavirus has been fully recognized and praised by the WHO. China's effective prevention and control provides a valuable time window for the world to fight new coronavirus.

However, the new coronavirus in Wuhan, Hubei Province and even the whole country, has brought serious threat to people's life and health, social and economic life has been seriously affected. From a scientific point of view, we will explore why the new coronavirus and HIV can cause such a wide spread and huge damage.

We know that the mortality of SARS is about 10%, and that of mers is about 40%. Up to now, novel coronavirus pneumonia (COVID-19) has a mortality rate of 3-4%, and the mortality rate outside Hubei is less than 1% (0.7%). At present, the novel coronavirus is found to have a latent period of 1-14 days, mostly for 3-7 days. There is a period of asymptomatic symptoms or only mild symptoms before the onset of the disease. Many epidemiological investigations have observed that some infected persons with mild or asymptomatic symptoms may transmit the virus to others without knowing it.

The human to human transmission chain of SARS CoV and mers cov is very clear, and retrospective investigation has almost restored all the communication relationships. However, at present, the transmission chain of new coronavirus is relatively complex, which is not as clear as SARS CoV and mers cov. It is very difficult to fully understand.

If the virus can be detected as early as possible, the infected person can be identified as early as possible, which will bring great help to the prevention and treatment. But unfortunately, after a period of practice, we found that at present, the throat swab test will show false negative test results of infected people, and some infected people only show nucleic acid positive after multiple nucleic acid tests, and some patients who have not been able to confirm nucleic acid positive all the time, but the chest CT shows obvious signs of infection.

Therefore, more sensitive and accurate detection methods are urgently needed, or some auxiliary means are used to help diagnosis. In the latest novel coronavirus pneumonia diagnosis and treatment manual, CT results have been used as auxiliary diagnostic criteria.

The characteristics of long incubation period and difficult to detect after virus infection will cause many infected people can not be diagnosed, admitted and isolated in time, which is very easy to cause large-scale transmission. From this point of view, the new coronavirus is more difficult to control than SARS CoV and mers cov.

Since 2004, we have not found any SARS CoV infectors or potential intermediate host viruses. It can be said that China's very successful prevention and control has completely eliminated SARS CoV. Since mers cov has been infected with intermediate host camels for decades, a large number of camels in Africa and Central Asia have been infected by mers CoV, so it is difficult to remove all the infected camels. Now, there are still sporadic cases of mers. This novel coronavirus epidemic is the best result of its eradication. Otherwise, if human fails to completely eliminate the transmission of 2019 ncov, it will become a resident virus like HIV or a seasonal epidemic like influenza virus.

Compared with the novel coronavirus, HIV has become the permanent virus of human beings for nearly a hundred years. One hundred years ago, we did not have the current scientific level and consciousness. We can not take timely and strict measures to prevent the virus from spreading continuously at the beginning of the outbreak. Moreover, although HIV diagnostic reagents have been repeatedly updated, with high sensitivity and accuracy, but the incubation period of HIV asymptomatic is longer, making it difficult to early diagnosis and early detection, which is a major problem in AIDS prevention and treatment.

Despite the continuous development of antiviral drugs for the treatment of AIDS, there are more and more kinds, which can effectively prevent virus replication and prolong the lives of infected people, but there is still no drug or method to completely cure AIDS. If the treatment fails, the infected person will still face the threat of morbidity and death. By the end of 2018, more than 70 million people had been infected with HIV in the world, resulting in more than 30 million deaths.

In 2016, member states of the United Nations adopted a political declaration promising to end the HIV epidemic by 2030. According to the latest data of the United Nations AIDS Programme (UNAIDS), the global AIDS prevention and control work has achieved remarkable results, and new infections have declined. However, there are still 1.7 million new infections in 2018. To achieve the end of the AIDS epidemic in 2030, the prevention and control work still has a long way to go.

We hope that through our efforts, the new coronavirus will become a history just like SARS. We also hope that through the continuous struggle with HIV, we can finally achieve the grand goal of ending the AIDS epidemic.

In order to achieve the goal of ending the AIDS epidemic, we must follow the prevention and control law of infectious diseases, constantly improve the public awareness of prevention and control, adopt new technical means to carry out precise prevention and intervention, try to find out early diagnosis and treatment as soon as possible, find new and rapidly growing virus transmission clusters and networks in time, and look forward to major breakthroughs in cure methods and vaccines. (official account of AI)

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