Viral Infections

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton  

Introduction

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Viral infections are among the most common afflictions of man. It has been estimated that children experience two to seven respiratory infections each year; adults are afflicted with one to three such episodes[1]. Image shows cell with relative size of virus (14)

  • Viruses cause familiar infectious diseases such as the common cold, flu and warts. They also cause severe illnesses such as HIV/AIDS, Ebola, influenza and COVID-19.

Viral infections occur due to infection with a virus.

  • Millions of different viruses may exist, but researchers have only identified about 5,000 types to date.
  • Viruses contain a small piece of genetic code, and a coat of protein and lipid (fat) molecules protects them.

Viruses invade a host and attach themselves to a cell. As they enter the cell, they release their genetic material. This material forces the cell to replicate the virus, and the virus multiplies. The cell may then:

  • Die and it releases replicates of the virus, which infect new cells.
  • Change the function of the host cell. eg. Some viruses like the human papillomavirus (HPV) and Epstein-Barr virus (EBV), can lead to cancer by forcing cells to replicate in an uncontrolled way.

For most viral infections, treatments can only help with symptoms while you wait for your immune system to fight off the virus. There are antiviral medicines to treat some viral infections (however dramatic progress in antiviral therapeutics is occurring[2]). Vaccines can help in prevention of many viral diseases.[3]

  • Viruses may remain dormant for a period before multiplying again. The person with the virus can appear to have fully recovered, but they may get sick again when the virus reactivates[4].

Viral Infections

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A few notable examples that have garnered the attention of the public health community and the population at large include: COVID 19, Ebola, SARS, Influenza, Zika, Yellow fever, Human immunodeficiency virus (HIV / AIDS), Human papillomavirus (HPV), Viral gastroenteritis, Varicella, and Viral hepatitis[5].

Image at R: A simplified diagram of the Hepatitis C virus replication cycle.

Respiratory Infections

A variety of viruses cause different types of respiratory infections.

  • Rhinovirus, coronavirus and adenovirus are the leading causes of the common cold.
  • Infectious disease.jpg
    Influenza viruses infect the upper respiratory system and sometimes spreads to the lungs causing pneumonia. Influenza has been and continues to be one of the great scourges of man. Influenza viruses produce epidemic disease annually. Irregularly, but with all-too-great frequency, widespread epidemics of influenza occur, occasionally producing a pandemic that involves virtually the whole world. Epidemics attributed to the influenza viruses have occurred throughout recorded history. In the past century, major epidemics occurred in 1890, 1900, 1918, 19S7, and 1968. The great pandemic of influenza in 1918-1919 is estimated to have killed 20--40 million people and accounted for 80% of the deaths in the U.S. Army during World War I.[1]
  • Another virus called the respiratory syncytial virus (RSV) causes a respiratory infection called bronchiolitis in infants and toddlers[6].

Central Nervous System Infections

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Several viruses can infect the central nervous system (brain and spinal cord).

Viral CNS infections have an annual incidence ranging from 0.26 to 17 cases per 100,000 depending on the age and vaccination status of the population. 

Enteroviruses (a genus of positive-sense single-stranded RNA viruses, so named by their transmission-route through the intestine) are the most common cause of viral CNS infection (nearly 60%), followed by arbovirus and herpes virus, such as herpes simplex virus (HSV) and varicella zoster virus (VZV)[7].

Viral Meningitis:

  • Enteroviruses are responsible for 80 to 90% and mumps for 10 to 20% of diagnosed cases of viral meningitis, with many other viruses sometimes incriminated with considerable geographical and seasonal variation.

Viral Encephalitis

  • Japanese encephalitis is the commonest cause of encephalitis in Asia: other causes—with considerable geographical and seasonal variation—include dengue viruses, Enteroviruses (EV71) rabies, Nipah virus, herpes simplex, West Nile virus, and mumps.

Viral Myelitis

  • Viral ‘paralytic’ myelitis is classically caused by poliovirus, which has now been virtually eliminated from the Americas: other causes—with considerable geographical and seasonal variation—include Japanese encephalitis and various coxsackieviruses, echoviruses, enteroviruses and flaviruses.[8]

Skin Infections

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Viruses cause a wide array of skin infections. eg.

  • Herpes simplex viruses (HSV) cause some of the most common skin infections. HSV type 1 tends to cause vesicles in the mouth and on the lips, commonly known as cold sores or fever blisters. HSV type-2 tends to cause genital herpes.
  • The varicella virus causes chickenpox, an illness characterized by itchy fluid-filled bumps on the skin that eventually rupture and scab over. The varicella virus also causes shingles, which is a reactivation of the virus years after the initial bout of chickenpox.
  • Human papillomaviruses (HPV), cause warts[6].

Digestive System Infections

Several types of viruses cause viral gastroenteritis, commonly called the stomach flu.

  • This common illness, characterized by diarrhea, nausea and vomiting, is caused by many different viruses, but not the influenza virus
  • According to a June 2012 "American Family Physician" article, viruses cause 75 to 90 percent of acute gastrointestinal disease in children[9]

Diagnosis

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Viral infections are causing serious problems in human population worldwide - take the recent outbreak of coronavirus disease 2019. The first step in combating viral pathogens is to get a timely and accurate diagnosis. Early and accurate detection of the viral presence is crucial for appropriate treatment, control, and prevention of epidemics[10].

Diagnostic virology has now entered the mainstream of medical practice as a result of several independent developments.

  • The dramatic progress in antiviral therapeutics has increased the need for specific viral diagnoses.
  • Technological developments, particularly in the area of nucleic acid chemistry, have provided important new tools for viral diagnosis.
  • The number of patients at risk for opportunistic viral infections has expanded greatly as a result of the HIV/AIDS epidemic.
  • Modern management of HIV infection and hepatitis C is providing a new paradigm for the integration of molecular techniques into management of chronic viral infections.

These developments are not only increasing the use of diagnostic virology but are reshaping the field.

Multiple methods are used for the laboratory diagnosis of viral infections, including viral culture, antigen detection, nucleic acid detection, and serology. The role of culture is diminishing as new immunologic and molecular tests are developed that provide more rapid results and are able to detect a larger number of viruses[2]

Emergence of Viral Diseases

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In order for a new viral disease to emerge, the causative virus must infect and successfully invade its host, bypassing the complex and sophisticated antiviral defenses that have evolved in all animals. It is to be stressed that necessary host, virological, and environmental factors must, typically, coincide for a disease to emerge[11].

Many of the scariest viruses that have caused past or current epidemics originated in other animals and then jumped to people: HIV from other primates, influenza from birds and pigs, and Ebola probably from bats. So, too, for coronaviruses: The ones behind SARS (severe acute respiratory syndrome), MERS (Middle East respiratory syndrome) and Covid-19 all probably originated in bats and arrived in people via another, stepping-stone species, likely palm civets, camels and possibly pangolins, respectively.

Making the jump from one species to another isn’t easy

  • Successful viruses have to be tightly adapted to their hosts.
  • To get into a host cell, a molecule on the virus’s surface has to match a receptor on the outside of the cell, like a key fitting into a lock.
  • Once inside the cell, the virus has to evade the cell’s immune defenses and then commandeer the appropriate parts of the host’s biochemistry to churn out new viruses.
  • Any or all of these factors are likely to differ from one host species to another, so viruses will need to change genetically — that is, evolve — in order to set up shop in a new animal.

Host switching actually involves two steps

  • The virus has to be able to invade the new host’s cells: That’s a minimum requirement for making the host sick.
  • To become capable of causing epidemics, the virus also has to become infectious — that is, transmissible between individuals — in its new host. That’s what elevates a virus from an occasional nuisance to one capable of causing widespread harm[12].

Resources

"Why the world needs viruses to function" -you may wish to read this interesting article

Some exerts below

  • Viruses seem to exist solely to wreak havoc on society and bring suffering to humanity. They have cost untold lives over the millennia, often knocking out significant chunks of the global population – from the 1918 influenza epidemic which killed 50 to 100 million people to the estimated 200 million who died from smallpox in the 20th Century alone. The current Covid-19 pandemic is just one in a series of ongoing and never-ending deadly viral assaults.
  • Most people are not aware of the role viruses play in supporting much of life on Earth, because we tend to focus only on the ones that cause humanity trouble. Nearly all virologists solely study pathogens; only recently have a few intrepid researchers begun investigating the viruses that keep us and the planet alive, rather than kill us[13].

References

  1. 1.0 1.1 Winn W.C., Walker D.H. (1994) Viral Infections. In: Dail D.H., Hammar S.P. (eds) Pulmonary Pathology. Springer, New York, NY. https://doi.org/10.1007/978-1-4757-3935-0_12 Available from:https://link.springer.com/chapter/10.1007/978-1-4757-3935-0_12#citeas (last accessed 8.11.2020)
  2. 2.0 2.1 Storch GA. Diagnostic virology. Clinical infectious diseases. 2000 Sep 1;31(3):739-51.Available from;https://academic.oup.com/cid/article/31/3/739/297506 (last accessed 8.11.2020)
  3. Healthline plus Viral Infections Available from:https://medlineplus.gov/viralinfections.html#cat_42 (last accessed 8.11.2020)
  4. Very well health Infections Available from:https://www.medicalnewstoday.com/articles/196271 (last accessed 8.11.2020)
  5. Life Persona Common Viruses Available from:https://www.lifepersona.com/10-diseases-caused-by-the-most-common-viruses (last accessed 8.11.2020)
  6. 6.0 6.1 Healthfully Viral infections Available from:https://healthfully.com/list-of-diseases-caused-by-viruses-4722379.html (last accessed 8.11.2020)
  7. Amy C. Walker, ... Nicholas J. Johnson, in Evidence-Based Practice of Critical Care (Third Edition), 2020 Available from:https://www.sciencedirect.com/topics/medicine-and-dentistry/central-nervous-system-viral-infections (last accessed 8.11.2020)
  8. Jeffery KJ, Read SJ, Peto TE, Mayon-White RT, Bangham CR. Diagnosis of viral infections of the central nervous system: clinical interpretation of PCR results. The Lancet. 1997 Feb 1;349(9048):313-7. Available from:http://www.enetmd.com/content/viral-infections-central-nervous-system
  9. Churgay CA, Aftab Z. Gastroenteritis in children: part I. diagnosis. American family physician. 2012 Jun 1;85(11):1059-62.Available from:https://www.aafp.org/afp/2012/0601/p1059.html (last accessed 8.11.2020)
  10. Reta DH, Tessema TS, Ashenef AS, Desta AF, Labisso WL, Gizaw ST, Abay SM, Melka DS, Reta FA. Molecular and Immunological Diagnostic Techniques of Medical Viruses. International Journal of Microbiology. 2020 Sep 4;2020.Available from:https://www.hindawi.com/journals/ijmicro/2020/8832728/ (last accessed 8.11.2020)
  11. James N. FENNER'S VETERINARY VIROLOGY. Elsevier Academic Press; 2017. Available from:http://scitechconnect.elsevier.com/wp-content/uploads/2017/01/Epidemiology-and-Control-of-Viral-Diseases.pdf (last accessed 8.11.2020)
  12. Smithsonian Magazine COVID 19 Available from:https://www.smithsonianmag.com/science-nature/how-viruses-evolve-180975343/ (last accessed 8.11.2020)
  13. BBC future Why the world needs viruses to function Available from:https://www.bbc.com/future/article/20200617-what-if-all-viruses-disappeared (last accessed 8.11.2020)