How The Virus Spreads?

Viral infection is the process that the virus invades the host through a variety of ways and proliferates in susceptible host cells. First, sufficient numbers of virions must enter the host. Those cells must be susceptible to infection. The host cells express the receptors that viruses can bind. Then viruses can reproduce themselves by taking over the biosynthetic machinery of the host cells and exploiting it for their purposes. Eventually, virus dissemination spreads from the site of infection to other sites within a host. Viral infection leads almost invariably to some adverse reaction within the host, such as fever or malignant disease.

Portals of Virus Entry

A transmission in the virus, host, and environment is necessary to establish infection. The virus needs to enter a host to initiate a successful infection in the end. There are several different portals of entry to host cells as follows.

Common portals of virus entry. Fig1 Common portals of virus entry. (Payne, 2017)
  • Respiratory tract. Respiratory viruses (e.g. severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), influenza virus) can be spread via direct contact, indirect contact, droplet, and aerosol.
  • Gastrointestinal tract. Viruses can be transmitted via the gastrointestinal tract in several different ways, for example, the fecal-oral route (e.g. Norwalk virus).
  • Genital tract. Viruses that are transmitted via the genital tract as a result of sexual activity are sexually transmitted diseases, such as human papillomavirus (HPV), HIV, hepatitis B.
  • Skin. Accessed into the dermis and subcutaneous tissue by bites of insect vectors, animal bites, or needle punctures, viruses (e.g. rabies virus, encephalitis B virus, dengue virus) can easily gain entry into the bloodstream resulting in infection.
  • Eyes. For example, adenoviruses infect the conjunctiva of the eye or eyelid, causing conjunctivitis.
  • Placenta. Several viruses are able to access from mother to fetus through the placenta, such as the human immunodeficiency virus (HIV).
  • Transplants. Transplanted organs and tissues can also harbor viruses, including HIV, dengue virus, hepatitis virus.

Mechanism of Transmission

The virus transmission includes vertical and horizontal transmission.

  • Vertical transmission (also called maternal-neonatal transmission, perinatal infection) means that the virus is spread from one generation to the next generation. Some neonatal viral infections are typically acquired in the perinatal (intrapartum), including herpes simplex virus (HSV), HIV, hepatitis B, hepatitis C, enterovirus, cytomegalovirus, adenovirus, parechovirus, varicella-zoster virus. vertical transmission often leads to the long-term persistence of the virus within the child.
  • Most viral infections exhibit horizontal transmission, meaning that direct host-to-host transmission occurs. Horizontal transmission can occur between the individuals in the crowd in various ways.

Horizontal transmission versus vertical transmission. Fig2 Horizontal transmission versus vertical transmission. (Payne, 2017)

Dissemination Within A Host

Virions spread to other organs in one of two ways. In the hematogenous spread, viruses spread to target organs using the bloodstream. This can occur through direct injection into the blood or entering the interstitial fluid that virions eventually return to the bloodstream. Since blood circulates throughout the entire body, viruses can use the bloodstream to gain access to their cellular targets in organs other than the ones through which they entered the body. In the neurotropic spread, viruses spread through the body using neurons. Viruses replicate in cells at the local site of infection and then infect neurons located nearby. Viral infection causes devastating results if they reach the central nervous system.

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Reference

  1. Payne, S. Virus Transmission and Epidemiology. Viruses. 2017: 53-60.
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