Some pathogens can stay in the body for a long time and take advantage of weaknesses in the immune system to make us sick again. But what are they? And can the new coronavirus belong to this group?
If viruses could be defined by a single characteristic, objectivity would be a good option. After all, its purpose is quite simple: to invade the cells of a living being and use them to create new copies of itself, which will repeat this process.
Usually, the rite of invasion and replication lasts a few days, and the immune system is able to deal with the problem or the condition progresses to a more serious and life-threatening situation.
But there is a group of viruses that go even further. Shortly after the initial infection, they manage to hide in a corner of the body.
This phase can last for months, years or even decades, and only ends when the defense cells stop working as expected. Then the infection reappears and again causes health problems.
And this group has several well-known representatives, ranging from HIV, the cause of AIDS, to herpes simplex 1 and 2, which cause sores in the corner of the mouth and in the genital area.
But how do they go unnoticed? And how do they reappear after so long? Could this be the case of Sars-CoV-2, the coronavirus responsible for covid-19?
An appendix (or an edition) in the book of life
Basically, there are four ways for a virus to hide in the body.
The first of these is frequently used by the herpes family, which in addition to herpes simplex viruses types 1 and 2 includes varicella, which causes chickenpox, Epstein-Barr, which is the origin of the “kissing disease”, among others.
“They have DNA as their genetic material and manage to stay inside the nucleus of cells, as an appendix to our own genetic code,” explains infectious disease specialist Décio Diament, from Israelita Albert Einstein Hospital, in Sao Paulo.
Remember here that DNA is the set of letters (known in science as nitrogenous bases) that make up the genome. They are “aligned” on two strands, usually in the classic double helix format.
“These viruses from the herpes family remain dormant for a long time, without replicating very intensely. They manage to inhibit the internal defenses of cells and become ‘invisible’ to the immune system”, adds the doctor, also a consultant for the Brazilian Society of Infectology.
The second mechanism is commonly used by another family: retroviruses such as HIV and HTLV.
It is important to mention that this duo does not have DNA, but RNA. In other words, their genetic information is organized in a simpler way, in a single strand of nitrogenous base sequences.
Retroviruses are able to fuse with our genetic code. This integration most often occurs in T cells and macrophages, two important parts of the immune system.
But how do they achieve this feat, if we are talking about RNA viruses and relying on DNA?
Both HIV and HTLV have an enzyme called reverse transcriptase. In short, it can convert the genetic code of these viruses from RNA to DNA. Thanks to this, they are able to integrate into the human genome and remain hidden for a long time.
“This represents a huge difficulty, because we cannot eliminate them even with the available drugs”, contextualizes Doctor Estevão Portela Nunes, Deputy Director of Clinical Services at the National Institute of Infectious Diseases of the Oswaldo Cruz Foundation (INI-FioCruz ), in Rio de Janeiro in January.
In other words, the antiretroviral cocktail used in HIV-infected patients may even inhibit viral replication. However, if the person stops taking the drugs, there is a risk that the HIV will become active again.
Sanctuaries and other mysteries
The third masking option for some viruses is so-called immunoprivileged sites.
These are areas of the body that the immune system cannot access as easily, such as the testicles, eyes, and central nervous system (spinal cord and brain).
The action of defense cells is limited to these places to prevent the inflammatory process, which occurs when they fight an infection, from damaging more sensitive structures, such as nerves and the reproductive system.
If on the one hand this represents a form of protection for the body itself, on the other hand it creates a kind of “sanctuary” for certain viruses to develop for a little longer.
Work published in recent years has, for example, found Zika and Ebola in the semen of patients.
Immunologist Daniel Mucida, a professor at Rockefeller University in the United States, considers that the fact that the virus is in semen or other parts of the body does not necessarily mean that it is active and can cause problems future.
“The impact of this viral persistence is still unclear,” he says.
And, as far as we know, the permanence of viruses in such sanctuaries does not last so long. Within months, even with the most limited access, the immune system is finally able to eliminate the invaders.
There is still a fourth group of viruses capable of prolonging their stay in the body, even outside immuno-privileged sites.
“This is the case of the respiratory syncytial virus, which can persist in the lungs and is associated with chronic inflammation, especially in children, and the chikungunya virus, which remains in the muscles and joints”, illustrates the immunologist Carolina Lucas, a researcher at Yale University School of Medicine in the United States.
Scientists are still investigating why this happens in some patients (and not others).
While it may seem that all of these viruses are dormant and no longer of concern, some of them “wake up” after a few years (or decades).
“When the immune system makes a mistake for whatever reason, these pathogens can resurface and cause problems,” says Diament.
This “stupid” varies from infection to infection. In the case of herpes simplex, if the infected person spends long hours in the sun or is in a very stressful period of life, it may be enough for the lesions to grow back.
In other situations, the natural aging of the body allows certain infectious agents to resume work.
The classic example is varicella zoster, a virus that causes chickenpox (usually in childhood) and then spends decades hiding in the body.
Later, after the age of 50 or 60, this pathogen can reappear and cause a condition called shingles or shingles, marked by very painful blistering lesions on a strip of the body (usually on the trunk or belly).
Nowadays, there is even a vaccine indicated for these elderly people. In Brazil, it is only available on the private network.
And there are, of course, a number of other conditions that impair the action of the immune system and can provide an opportunity for viruses.
“This includes serious accidents and trauma, major surgeries, transplants, tumors, drug treatments and other serious infections,” Diament lists.
In such cases, doctors are already monitoring them and can suggest treatments to minimize the damage.
Finally, it should also be remembered that some pathogens are linked to diseases that do not always have to do with the initial manifestations of infection.
This is the case of certain hepatitis viruses, which can cause liver cancer, HPV, which is the cause of several types of tumors, and Epstein-Barr, which has recently been linked to multiple sclerosis.
And the coronavirus?
Faced with such a wide diversity of viruses, could Sars-CoV-2, the cause of covid-19, also persist after the initial infection?
Experts consulted by BBC News Brasil consider it unlikely to have or develop this capability.
“Sars-CoV-2 is an RNA virus that does not have this reverse transcriptase enzyme, like HIV. Therefore, it cannot integrate into our genome”, teaches molecular biologist Carlos Menck, from the Department of Microbiology from the Institute of Biomedical Sciences at the Institute. University of São Paulo (ICB-USP).
It also does not have DNA in its constitution, as with the viruses of the herpes family, which prevents its prolonged persistence in the cell.
“And the cases that we see now, of people who get covid again, happen because they were infected with the coronavirus for the second time, not because it was hidden in their body for years. months”, explains the specialist.
“If we see something different with Sars-CoV-2, it will be a very big surprise for us.”
But how to explain the cases of long covid, in which individuals present with discomfort for months, even after recovering from the first discomfort?
Diament specifies that this phenomenon seems to be more related to the response of the immune system to the invasion of the coronavirus.
“In some patients, the covid causes real damage that can last for months and manifest itself in fatigue, difficulty concentrating, loss of smell…”
“It seems to be a consequence of the inflammatory process that occurs during the first days of the infection. In some cases, the immune system reacts violently, and the effects can be prolonged”, explains the doctor.
Lucas and Mucida say, however, that the persistence of certain viral components, such as bits of protein and RNA, in the body cannot be ruled out in these cases.
“There is evidence that goes both ways, including the observation of viral RNA in regions such as the gut for a prolonged period,” they point out.
It remains to be seen whether these small bits of virus would be able to keep the immune system in a dormant state and cause damage for weeks or months or whether they are just a discovery with no practical effect on people’s health. people.
But, as Portela Nunes, from FioCruz, points out, in the case of covid-19, we already have at least one good news: “Vaccination seems to protect or minimize these long-term inconveniences”.
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