Role of viruses in causing diseases

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The features of a virus affect its ability to spread. Some viruses can remain active on an object for some time. If a person with the virus on their hands touches an item, the next person can pick up that virus by touching the same object. The object is known as a fomite. Viruses often change over time. Some of these changes are very small and do not cause concern, but others can be more significant. Significant changes could make a virus more transmissible, as has been the case with the B. They may also help the virus evade the immune system or existing treatments.

For example, doctors use several drugs in combination to treat HIV so that it is harder for the virus to develop resistance to treatment. Influenza viruses can also do so-called antigenic shift. This can happen if a host cell has become infected with two different types of influenza virus.

For instance, pigs can often serve as a mixing vessel for avian and human influenza viruses. Some viruses, such as HPV, can lead to cancer.

The full impact of a virus can take time to appear, and sometimes there may be a secondary effect. For example, the herpes zoster virus can cause chickenpox. The person recovers, but the virus may stay in the body. Years later, it may cause shingles in the same individual. Coronaviruses are a large family of viruses and include viruses that cause the common cold. However, it has changed many times since scientists first identified it in China. By September , scientists had logged over 12, mutations, and the development continues.

Some variants are more transmissible and more likely to cause severe illness than others. The main concern with new variants is the unpredictability of their impact. The main symptoms of COVID are dry cough , fatigue , and fever, but there are many possible symptoms.

Anyone who has symptoms should seek a test. It is also important to self-isolate until 10 days after symptoms appear and when no fever has been present for 24 hours. If a person has difficulty breathing , they should seek emergency medical attention. However, special T cells, known as cytotoxic T cells, can recognize cells that contain viruses, and release substances that kill those cells.

Some viruses can escape detection by cytotoxic T cells, but other immune cells — natural killer cells — can cause the cell containing the virus to die. In addition, body cells that contain a virus emit proteins called interferons, which warn other cells that a virus is present. This gives healthy cells a chance to defend themselves by changing the molecular makeup of their surface.

Antibodies can also help fight a virus before it enters a cell. They do this by neutralizing or damaging the virus or by changing its features so that it can no longer enter healthy cells. Antibiotics treat bacterial infections , but they cannot treat a viral infection.

People will need either a vaccination to prevent infection, or antiviral drugs to treat any symptoms. Sometimes, the only option is symptom relief. In recent decades, scientists have developed antiviral drugs, largely in response to the AIDS pandemic. These drugs do not destroy the virus, but they slow or prevent its development.

With antiviral treatment for HIV, for example, the level of virus in the body can become so low that tests cannot detect it. At this point, it becomes untransmittable, which means that a person cannot pass the virus on to another person. Herpesviruses and papillomaviruses are among the viruses that presumably use this strategy.

Obviously, replication in oral tissue places the oral cavity more at risk for clinical symptoms. Herpesviruses have a double-stranded DNA genome and are among the largest and most complex human viruses. There are eight members of the human herpesvirus HHV family Table 2. The more common, as to oral health problems, are the two herpes simplex viruses HSV-1 and -2 , which cause recurrent herpetiform ulcerations referred to as cold sores.

These ulcers typically occur on the lips, but the viruses can also cause similar lesions in the mucosa, such as in the case of gingivostomatitis.

The mucosal affection is normally associated with a primary herpes infection in children, and is accompanied by bodily symptoms such as fever and malaise. Both HSV-1 and -2 may be involved in oral manifestations, although the latter is primarily associated with the genitals. Epstein-Barr virus EBV and cytomegaloviruses CMV are present in the vast majority of adults, but in most cases probably without ever causing any overt disease. Both can, however, cause mononucleosis; EBV being responsible for most of the cases.

The condition is common at puberty, and is considered a consequence of the host not having been in contact with the virus earlier in life. EBV's potential for affecting the mouth is further underlined by oral hairy leukoplakia, i.

Varicella-Zoster virus VZV is associated with chicken pox, as a primary infection, and with herpes zoster if reactivated later in life. The vesicular rash formed occurs primarily in the skin, but may also affect the mucosa.

The three remaining human herpesviruses HHV-6, -7 and -8 rarely cause serious disease, but the former two are responsible for a particular type of rash roseola with associated fever in infants.

HHV-8 is presumably responsible for Kaposi's sarcoma, a rare form of skin cancer seen in immunosuppressed patients. The herpesviruses typically form chronic infections where the virus remains with its host till death do them apart; much of the time in latency, but with occasional bursts of activity. As these viruses are contact transmitted, either by means of virus production in the skin accompanied by rashes and blister or viral presence in sputum, it is not surprising to find viral activity in the oral cavity.

In addition to the traditional clinical picture referred to above, two of them in particular, EBV and CMV have been associated with periodontitis recently reviewed in Reference 6. Several laboratories have demonstrated that these viruses are found significantly more frequently in samples taken from affected pockets compared to healthy pockets 7 — 10 , however, this association does not necessarily mean that they are involved in the pathology.

Some authors, e. Slots et al. Periodontitic lesions may, in part, be the transmission strategy for these viruses, particularly CMV, as replication in the lesions allows viruses to reach the saliva and thus potentially infecting other individuals One should be careful when evaluating the presence of low levels of viral genomes in clinical samples as an indication of a viral role in pathogenesis.

EBV and CMV are known to be occasionally found in any mouth, as long as a relevant sample of sufficient size is analysed by a sensitive technique. EBV in particular is known to be cyclically active in the body and periodically present in sputum.

Thus, the mere presence of viruses in the absence of signs of local viral activity, either in the form of high viral titres or detection of viral RNA or proteins, probably do not constitute any appreciable impact on the aetiology. The initial viral load in this patient was considerably higher than what was observed in any of the other patients tested. Both the clinical condition and the viral load remained stable, and close to the detection level, during a one-year follow-up period after the antiviral treatment.

Although the amount of sample obtained from the various teeth examined was not standardised, and is expected to be smaller when obtained from healthy pockets, the sampling differences can neither explain the three log increase in initial viral load compared to other patients, nor the five to six log drop in viral load observed after treatment.

More cases utilising similar antiviral treatments are necessary to provide statistical significance to implicate a potential role of viruses in periodontitis. Moreover, if successful, it may save the patients from considerable pain and agony.

If the tests find appreciable amounts of virus, or other signs of local viral activity, antiviral treatment should be considered as an adjunct to conventional periodontal therapy. It should be noted that the latter form of therapy has also been reported to reduce the viral load Herpesviruses are well known for their capacity to manipulate the immune system. Although the obvious purpose of manipulation is to boost viral replication, it is easy to envision that a down-regulation of immunological surveillance may also benefit other agents present, such as bacteria.

The issue, in regard to periodontitis, has been recently reviewed by Slots Briefly, viral activity in periodontal tissues may impact the local immune response in a way that benefits opportunistic bacteria, and thus leads to aggravated symptoms. For example, the viruses produce cytokine mimics designed to modulate the host's immune defence.

It should be noted that the microbial activity can also induce viral replication, as has been shown recently in the case of EBV and malaria If the impact of viral replication on the bacterial environment is real, then it might be expected that the bacterial profiles would differ between sites with or without virus. Such correlations have been previously reported Parts of the viral genome are occasionally integrated in the DNA of the host cells, and some of the genes are assumed to have a malignant potential, as reviewed in Reference The role of HPV in cervical cancer is well accepted and has led to the widespread use of papillomavirus vaccines for young women; related carcinomas occur in the mouth cavity as well as in the oropharyngeal area, however, the role of HPV is less obvious in these cases Based on their putative role in cervical carcinoma, the viruses are classified as having either high primarily 16 and 18 or low primarily 6 and 11 oncogenic potential.

Although HPVs are also found in biopsies from healthy mouths, their prevalence is typically reported to be higher in biopsies from oral lesions such as leukoplakia or cancers. In the former case, the association with oncogenic HPVs is less obvious; while in the case of malignant cancers most laboratories find a definite overrepresentation of the more malignant HPVs 19 — The observed prevalence of oral cancers are, however, considerably lower than those reported for cervical cancers.

Still the case favouring a role of these viruses is reasonably strong. In future, as those who receive papillomavirus vaccines grow up, it will be interesting to see whether the prevalence of oral carcinomas declines along with the expected decline in cervical cancer. In fact, it has been argued that the vaccines should also be offered to men, partly because they too are at risk for genital cancer, and partly due to the assumed connection with oral cancer The main argument against vaccinating both sexes is that these forms of cancer have a considerably lower prevalence in males compared to cervical cancer in females.

As the virus forms chronic infections, vaccination of individuals who already contain the potentially malign subtypes is considered less useful. Nevertheless, it seems reasonable, however, to make the vaccine available at an early age to both sexes. The enteroviruses belong to the family of picornaviruses. Enteroviruses have a single-stranded RNA genome and are classified into five species with all together more than a hundred subtypes.

Although the majority of human enterovirus HEV infections are asymptomatic, they can cause upper respiratory illness, febrile rash, aseptic meningitis, pleurodynia, encephalitis, acute flaccid paralysis and neonatal sepsis-like disease As to oral affection, the enteroviruses are primarily associated with hand, foot and mouth disease. This is a febrile illness with tender papulovesicular lesions of both the hand, feet and oral mucosa 25 , It occurs mostly in children, but can also affect adults.

The association with enteroviruses primarily concerns the type A viruses, e. Coxsackie virus A16 and enterovirus, but other enteroviruses may also be involved. Herpangina is a related condition where the clinical manifestation is primarily in the oral cavity in the form of ulceration and blisters. Again, the condition is rare and restricted to children. Viral diagnostics have become more relevant in clinical dentistry.

This is partly because of an increased awareness that viruses are possible aetiological agents, and partly because the methods of viral detection have become considerably easier. The preferred methods are based on variants of real-time PCR, which not only offer a test for the viral presence, but also yield quantitative data.

The latter point is particularly relevant as several of the viruses in the oral cavity may be prevalent even in healthy mouths. A high viral load in a sample taken from affected tissues may, however, as a rule of thumb, suggest direct involvement in the underlying condition. One problem is that several viruses that are chronically present in the body can replicate in leukocytes e. As the typical clinical samples will stem from inflamed tissues, such as periodontal pockets or ulcerations, one would expect a presence of these viruses; if for no other reason due to the accumulation of leukocytes; a point that has been demonstrated at least in the case of CMV and periodontitis Again, a clinical role is suspected if the titre is particularly high, and even more so if the condition improves upon antiviral treatment.