Not just the lungs
Which organs are affected by the coronavirus
Virologists say that coronaviruses cause 15-20% of all colds. However, SARS-CoV-2 affects the human body in a special way and is able to have a devastating effect on different systems in our body. Scientists still do not know the exact mechanisms of infection development, but they got to know it better in a year. Pharmacologist Yuri Kiselyov told post-science about how the coronavirus spreads through the body, affects the lungs, heart, intestines, kidneys, liver and even the brain.
Most patients carry COVID-19 easily and do not have severe consequences. At the same time, there is no unique symptom that clearly indicates a coronavirus infection.
Coronavirus and lungs
The entry point into the human body for SARS-CoV-2 is mucous membranes. So the coronavirus enters the epithelium of the upper respiratory tract, which includes the nasal cavity, as well as the nasal and oral parts of the pharynx, and begins to multiply in them. The concentration of viral particles in the upper respiratory tract increases already during the first day – this can be determined, including with the help of a smear from the nasopharynx (PCR analysis) - and the active reproduction of the virus in the upper respiratory tract lasts about 5-10 days. At the same time, the first symptoms of the disease may appear only after a few days, when the virus is already actively multiplying in the body. For this reason, some reputable medical organizations, such as the Centers for Disease Control and Prevention of the United States (CDC), recommend that 10 days after the first symptoms appear, patients with a mild form of COVID-19 should no longer be tested using PCR. With a high degree of probability, by this time people are no longer contagious and can end quarantine if they do not have a fever.
The next point of infection is the lower respiratory tract: trachea, bronchi and the lungs themselves. Gradually, the amount of virus in the lower respiratory tract becomes greater than in the upper. Therefore, at a late stage of the disease, even when the PCR test shows a negative result, the virus can still actively reproduce in the body. At the same time, a person will no longer be able to infect others.
There are several mechanisms that lead to lung damage in coronavirus infection. First, the virus itself has a direct damaging effect on the cells, which is why they die. The cells affected by the virus are attacked by the immune system of the body itself. T-lymphocytes begin to destroy infected cells, as a result of which the tissues become inflamed, and neutrophils, in particular, enter them from the bloodstream – a group of leukocytes that normally fight bacterial infection. These white blood cells secrete molecules that also provoke cell death. All this leads to a vicious circle of inflammation: the tissues are damaged again, and a large number of neutrophils reappear in the inflamed place.
In rare cases, the reaction to infection with the virus from the immune system develops into a cytokine storm. It can be defined as an increased release of cytokines into the bloodstream and tissues, that is, molecules that activate the immune response. It was such a storm that was talked about a lot at the beginning of 2020. However, according to the results of studies, the concentration of the main cytokines in severe COVID-19 is significantly lower than, for example, in bacterial pneumonia and sepsis. So the hyperimmune response in coronavirus infection still cannot be called a classic cytokine storm.
Lung damage does not always lead to respiratory failure that occurs with COVID-19. Therefore, it is difficult to unambiguously predict the further course of the disease based on the results of computed tomography (CT). In some people, even with a 25% lesion, shortness of breath appears and saturation decreases, that is, the degree of oxygen saturation of hemoglobin in the blood. Other patients cope with
with a 70% lesion, while they have normal saturation and almost no symptoms of the disease, except weakness. The fact is that we are not talking about necrosis, irreversible death of lung tissue, but about changes: inflammatory edema appears in it, visible in the picture. Such edema to a certain extent disrupts the gas exchange function of the lungs. However, the human body is well able to compensate for damage, and gas exchange (oxygen – carbon dioxide) can continue quite effectively even with changes in a large area of the lungs. Therefore, doctors monitor the saturation of patients: according to this indicator, it is possible to objectively assess how successfully a person's lungs cope with their function.
All these pathological processes, from the moment the coronavirus appears in the lungs, take, according to various sources, from 3 to 12 days. As a rule, if a person does not have severe symptoms in two weeks, then, most likely, he will not get worse. There are cases when the disease progresses rapidly, but this is rather an exception. In 80% of cases, coronavirus infection passes in mild form and does not go further than the upper respiratory tract. But in the remaining 20%, the course of the disease may be more severe and in some cases lead to complications associated with the spread of SARS-CoV-2 with blood flow through the body, possible thrombosis or an excessive immune response.
Coronavirus and the cardiovascular system
The circulatory system is next after the lungs, which suffers from SARS-CoV-2 in the first place. In severe patients, blood clots may occur – blood clots that interfere with the free passage of blood in the arteries. There are three main causes of this condition. First, there is a direct viral lesion of the endothelium - the inner wall of the vessels. Secondly, hypercoagulation, that is, excessive blood clotting. The third factor is the so–called stasis – slowing down of blood flow due to blood thickening.
With coronavirus infection, blood clots can occur both in the lungs (for example, pulmonary embolism) and in the vessels feeding the heart muscles, intestines or brain. But in general, this is not a widespread phenomenon.
The tendency to form blood clots increases the risk of stroke – an acute violation of cerebral circulation. Strokes can be ischemic, that is, associated with arterial thrombosis, and hemorrhagic – caused by bleeding into the brain tissue. Patients with coronavirus infection most often have strokes of the first type. In addition, COVID-19 patients sometimes suffer from heart attacks, but coronavirus infection is very rarely their only cause.
In 2020, it was reported that myocarditis, inflammation of the muscular layer of the heart (myocardium), occur in almost a third of COVID-19 patients. However, today it becomes clear that this condition is less common – only in a few percent of patients. Some people have cardiac arrhythmias. There is also a lesion of the pericardium – the heart sac. All these conditions can be caused both by the direct impact of the virus on the cells, and by an overly strong immune response from the body.
Coronavirus and the brain
There is no unique symptom that clearly indicates a coronavirus infection. Even the loss of taste and sense of smell, which often leads to suspicions about COVID-19, is also found in other infections affecting the upper respiratory tract and central nervous system. However, with ordinary acute respiratory infections, changes in taste and smell occur much less frequently.
SARS-CoV-2 is capable of affecting the cells of the olfactory, that is, olfactory, epithelium in the nasal cavity. But it can also damage nerve fibers that pass through the latticed bone and go into the olfactory bulbs of the anterior parts of the brain. This may indicate the neurotropicity of SARS-CoV-2, that is, its ability to penetrate into the nervous tissue and multiply in it.
It should be noted that from 20 to 80% of the population, in principle, have reduced olfactory sensitivity, and a recent study of the symptoms of coronavirus infection showed that in 20% of patients who reported a loss of sense of smell, a PCR test did not detect an infection at all.
About a third of people who have had a coronavirus infection talk about certain symptoms from the central nervous system. With COVID-19, depressive states, anxiety, difficulty concentrating, drowsiness, headaches and fatigue can be observed. In rare cases, hallucinations and confusion may occur. But scientists are still not completely sure about the origin of these nonspecific symptoms. It is not necessary that this is a consequence of the direct action of the virus on nerve cells. For example, it is known that in elderly patients, especially those weakened by other diseases, any feverish condition, severe acute respiratory viral infection or influenza can lead to confusion, delirium, loss of orientation in time and space. The brain is very sensitive, and it is he who primarily suffers from a decrease in the concentration of oxygen in the blood.
In a large study by scientists from In China, in which about 1,700 subjects participated, symptoms of depression were noted in about 30-40% of those who had a coronavirus infection. But in the vast majority, the severity of these symptoms was minimal. It cannot be excluded that the general stress due to the spread of the coronavirus, possible lockdown and the general level of anxiety affect the psyche of almost all people, not just those who have suffered COVID-19.
In rare cases, patients with coronavirus infection were diagnosed with meningoencephalitis – inflammation of the membranes and tissues of the brain. For example, in one of the Scandinavian capitals, a case was recorded when a woman in her 50s and with almost no risk factors fell ill with COVID-19. For three days she had fever, weakness, slight shortness of breath. And on the fourth day, she suddenly died. At the autopsy, it turned out that the victim had moderate pathological changes in the lungs, but her brain was much more affected. Moreover, the virus found in it differed from what was found in the lungs by four mutations. It is assumed that it has changed right inside the woman's body, and perhaps this allowed it to spread so quickly through the central nervous system and lead to meningoencephalitis.
Coronavirus and gastrointestinal tract
Some symptoms from the gastrointestinal tract are noted in 17% of patients. However, this number includes such banal manifestations as loss of appetite.
In the gastrointestinal tract, SARS-CoV-2 may be the result of so-called auto-infection. Sputum or nasal fluid is swallowed when coughing, and a certain amount of viral particles passes through the stomach, despite the acid barrier. In another variant of infection, viral particles that fall into a person's hands end up on his lips, along with saliva enter the gastrointestinal tract, pass the acid barrier of the stomach and affect the intestinal epithelium. In this case, the coronavirus first appears in the gastrointestinal tract, and then with the blood flow it comes to the lungs. Being in the gastrointestinal tract, SARS-CoV-2 can cause diarrhea or nonspecific abdominal pain in a person. Severe gastrointestinal lesions are a rare phenomenon.
Most often, the virus acts cytopathically on the stomach and intestines, that is, it directly causes degenerative changes in cells. But some secondary mechanisms of its effect on the gastrointestinal tract are also described. For example, there are cases when, due to the formation of blood clots in the vessels feeding the intestines, people experienced acute abdominal pain that was not relieved by analgesics. Such intestinal ischemia with necrosis is a life–threatening condition, but it still cannot be called a typical complication of a coronavirus infection.
Coronavirus and kidneys, liver, pancreas
Damage to the liver, pancreas and kidneys in coronavirus can also occur due to direct damage to SARS-CoV-2 cells or occur due to microthrombosis. A number of scientific publications say that coronavirus can potentially trigger autoimmune processes, which, in particular, lead to damage to the beta cells of the pancreas that secrete insulin. Due to coronavirus infection, the filtration capacity of the kidneys may decrease. If COVID-19 occurs in a person in severe form, then due to liver damage, including medications, levels of liver enzymes may rise, pancreatitis occurs. And due to the defeat of the pancreas, diabetes mellitus of the first type can develop, insulin resistance occurs – an element of diabetes mellitus of the second type. However, today we have no evidence that diabetes is a frequent consequence of coronavirus infection.
In some patients with coronavirus infection, kidney damage occurs (among hospitalized patients, the frequency can reach 17%), but this is a rare phenomenon. In general, persistent lesions of the kidneys, liver and pancreas, although they sometimes occur with COVID-19, are the exception rather than the rule. The vast majority of people who have had a coronavirus infection recover without getting complications like kidney failure or diabetes.
Atypical manifestations of coronavirus
In order to multiply in the body, the virus needs to enter the cell, but at the same time leave it alive. Therefore, SARS-CoV-2 does not make "holes" in the cell membrane, but acts cunningly: it uses receptors for the APF2 protein (ACE2) located on the cell surface to enter. It so happened that many cells of different tissues of the body have APF2, so the virus can cause complications in the most unexpected places.
Eyes
During the severe course of COVID-19, the virus can even affect the eyes. A person with a coronavirus infection often has symptoms of conjunctivitis, and in rare cases, even lesions of the retina of the eye can be observed.
Skin
Various types of rashes that can occur with coronavirus infection are described: a blister-like rash, as with urticaria, rashes in the form of small red dots and the so-called macular rash, when large pink spots appear on the skin. Often this is not a narrowly localized, but a diffuse skin lesion, the rash can spread over large areas of the body. The so-called covid fingers are also described: on the hands, fingers or feet there is a skin lesion that looks like a consequence of frostbite: massive edema, impaired blood flow, inflammation, secondary infection, tissue rejection. Perhaps this condition is caused by thrombosis of small vessels, which leads to ischemia and tissue necrosis.
Coronavirus and pregnancy
During pregnancy, the mother transmits antibodies to coronavirus infection to the child in utero. But reports of transplacental transmission of the virus itself are extremely rare.
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In general, COVID-19 is characterized by a number of unusual, previously unknown manifestations. First of all, this is due to the fact that our body has not encountered this coronavirus before. And the immune response to the infection caused by it is also probably not formed in a completely standard way.
COVID-19 research is still limited. For example, they can be performed in a cardiology clinic, where patients with symptoms from the cardiovascular system are admitted. In such a situation, it is easy to make the so-called selection error: it is clear that a patient with cardiological symptoms will be taken there, and doctors in that clinic will think that all patients with coronavirus infection have such symptoms. If we take it more broadly, then even if a third of COVID-19 patients experience certain symptoms in the hospital, we still do not know how often they occur in patients treated at home. To make more accurate conclusions about the frequency of various manifestations of coronavirus infection, scientists will have to collect a large amount of data from different countries and hospitals.
About the author: Yuri Kiselyov – Ph.D., Associate Professor at Oslo Metropolitan University.
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