2. The infected organs and transmission in COVID-19 patients
Pathology and clinical symptoms indicated that SARS-CoV-2 can do harm to multiple organs. Therefore, it is important to understand the possible infected organs and route of transmission of SARS-CoV-2. Report showed that SARS-CoV-2 has the higher aerosol and surface stability than SARS-CoV(van Doremalen et al. , 2020). The human-to-human viral transitions of the SARS-CoV-2, reported by the previous studies, occurs due to close contact with an infected person, exposed to coughing, sneezing, respiratory droplets or aerosols. These aerosols can penetrate the human body (lungs) via inhalation through the nose or mouth. Therefore, the World Health Organization (WHO) has suggested that the personal protective equipment and infection control guidelines should be based on the assumption that the primary mechanism of transmission is direct and indirect droplet spread.
Direct droplet spread is said to occur when respiratory particles greater than 5um in diameter make contact with the mucosal surface of a recipient(Wilson et al. , 2020). Direct droplets can enter the human respiratory system through air(Jiang et al. , 2020), which causes respiratory symptoms. COVID-19 patients with respiratory distress could produce high levels of aerosols secondary to cough, high airway pressures, minute volumes, altered secretions and basal collapse. When infected patients expel respiratory particles and someone would inhale them, and subsequently the SARS-CoV-2 would find a welcome home in the nose(Sungnak et al. , 2020). High-flow nasal oxygen would disperse a concentrated jet of aerosols, potentially spreading them over a further distance or into the alveoli. These particles could gain direct access to alveolar surface ACE2 receptors under suitable biological, physical and environmental conditions. If the immune system does not beat back SARS-CoV-2 during this initial phase, the virus then marches down the windpipe to attack the lungs, where it can turn deadly. Several reports have reported that occur within families(Chan et al. , 2020). After the SARS-CoV-2 enters into the blood, it would invade different organs by binding to ACE2 receptor. Although the lungs are ground zero, the SARS-CoV-2 would reach to many organs including the heart, kidneys, gut, brain, blood vessels and reproductive system. Clinical trials have demonstrated that the infected patients not only have respiratory symptoms, but have kidney, brain, reproductive system, gut and liver injury. SARS-CoV-2 may enter different organs through the hematogenous transmission. Coronavirus invades the central nervous system (CNS) through the olfactory bulb(Bohmwald et al. , 2018). The retrospective, observational case series have reported the COVID-19 patients may appear neurologic manifestations(Chen et al. , 2020; Mao et al. , 2020), including central nervous system manifestations (dizziness, headache, seizure and so on), and peripheral nervous system manifestations (nerve pain, taste impairment, smell impairment and so on). Although the clinical manifestation of COVID-19 is dominated by respiratory symptoms, some patients have severe cardiovascular damage(Huang et al. , 2020; Wang et al. , 2020), which may be related to the distribution of ACE2 in heart tissue. At the same time, acute kidney injury could also be induced by the virus(Guan et al. , 2020; Huang et al. , 2020; Wang et al. , 2020), which may be related to cytokine storm resulting from respiratory symptoms. Moreover, the liver injury should not be ignored because abnormal liver enzymes in serum was found in the hospitalized patients(Liu et al. , 2020; Weber et al. , 2020; Xu et al. , 2020). Furthermore, the glycemic variability was associated with composite adverse outcomes and death in COVID-19 patients(Zhu et al. , 2020). Moreover, impairment of reproductive health induced by SARS-CoV-2 may be caused by high levels of cytokines resulted from respiratory symptoms(Wang et al. , 2020). These complications in various organ mainly demonstrated the transmission of virus in blood.
The respiratory droplets transmission is the main route of infection, while there is indirect droplet spread, including contact transmission and Fecal-Oral transmission. Although there is little reported about the contact transmission, it still an important way in infection. The contact transmission is mainly touched by a hand, which then contacts mucosal surfaces. The SARS-CoV-2 patients have found the presence of virus in tears and conjunctival secretions(Xia et al. , 2020), while a single-center cross-sectional study found the risk of infection on ocular surface is low(Zhang et al. , 2020). Meanwhile, the SARS-CoV-2 could be found in the nasal cavity. The Fecal-Oral transmission is also possible in COVID-19, as virus was noted in stools(Lin et al. , 2020; Tang et al. , 2020; Zhang et al. , 2020). At the same time, some patients appeared gastrointestinal symptoms including diarrhea, vomiting and abdominal pain during course of the disease(Jin et al. , 2020; Young et al. , 2020). This may be related to the virus that could be detected in Oral or saliva, subsequently infected along the digestive tract(Azzi et al. , 2020). Altogether, the transmission and the infected organs were displayed in Figure 1 and Table 1 .