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 .