Viral Entry Into The Host Body

• Viral entry is the first phase of infection in the viral life cycle when the virus is in contact with the cell of its host and injects virus-related material into the cell.
• The way a virus gets into cells is different based on the kind of virus.
• Three conditions must be met for successful infection of the host in question:
  1. Sufficient virus must be available to initiate infection.
  2. The cells that are infected should be accessible, susceptible to the virus.
  3. Local defense systems against viruses that are hosted by the host are either not present or ineffective.
• To be able to infect its victim, the virus must first be able to enter cells on the body’s surface. The most common sites for entry are mucosal linings in the alimentary, respiratory, and urogenital tracts. outside of the eyes (conjunctival membranes, also known as cornea) along with the skin.

Viral Entry Through Respiratory Tract

• The most frequent route for infection is via the respiratory tract.
• The absorptive capacity that the human lung has is around 140 square meters. Humans are able to rest at a rate of 6 milliliters of air per minute, which brings large quantities of foreign particles and aerosolized droplets to the lungs each time they breathe.
• A large portion of these droplets and particles contain viruses. There are many host defense mechanisms that can prevent respiratory tract infections.
• Mechanical barriers play a crucial function in the anti-viral defense. The tract, for instance, is lined by mucociliary tissue that is composed of ciliated cells and mucoussecreting goblet cells, as well as Mucous secreting glands in the sub-epithelial layer.
• Foreign particles that are deposited inside the nose cavity in the upper respiratory tract get trapped in mucus. They are then carried to the throat’s back, and then swallowed.
• The lower tract of the respiratory system, the particles stuck in mucus are pulled through the lungs to the throat via ciliary action.
• The lower parts in the tract called the alveoli, are devoid of mucus or cilia, however macrophages in the alveoli ingest and eliminate particles. Other humoral and cellular immune responses can also be involved.
• The respiratory tract by way aerosolized droplets released by an infected person by coughing or sneezing, or by contact with saliva of an affected person. Droplets containing large amounts of virus are placed within the nose, whereas smaller droplets make their way to the airways or alveoli.
• To be able to successfully infect the respiratory tract the virus must not be removed through mucus, neutralized with antibodies or destroyed by macrophages in the alveoli.

Viral Entry Through Alimentary Tract 

• The digestive tract is a frequent route for dispersal and infection.
• Drinking, eating, and other activities that involve social interaction regularly introduce viruses into the alimentary tract.
• It’s made for mixing, digestion and absorb food. This provides an excellent chance for viruses to come in contact with the cells that are susceptible and communicate with cells of the lymphatic, circulatory as well as immune system.
• It’s a very hostile environment for viruses.
• The stomach can be acidic, and the intestinal tract is alkaline. Digestive enzymes and bile soaps are plentiful in the epithelium, and mucus is a major component and the lumenal surfaces of the intestines are dotted with antibodies and the phagocytic cells.
• Infecting viruses through the intestinal route should at a minimum be immune to extreme pH, proteases and bile detergents. Actually, viruses without these properties are destroyed when exposed to the digestive tract, and then they can spread to spread to other areas.
• The hostile environment in the digestive tract can facilitate the spread of certain viruses. For instance the reovirus particles are transformed by host proteases within the intestinal lumen to subviral infectious particles, the ones that then are able to infect the intestinal tissues.
• It is not surprising that the majority of enveloped viruses fail to cause infection in the digestive tract since enveloped viruses are susceptible to dissociation caused by detergents like the bile salts.
• Coronaviruses that cause enteric infections are exceptional however it isn’t yet understood why these enveloped viruses can survive the extreme conditions that are found in the alimentary tract.
• The entire surface of the intestinal tract is covered by columnar villous epithelial cells that have the apical surface that is densely filled with microvilli. The brush border, along with a coating of glycolipids and glycoproteins and the mucous layer overlying is permeable to nutrients and electrolytes but is also an impressive barrier for microorganisms.
• However, some viruses, such as enteric adenoviruses as well as Norwalk virus, which is a Calicivirus, are found to replicate widely within intestinal epithelial cell lines.
• The mechanisms through which they overcome physical barriers and get into susceptible cells aren’t yet fully identified. In the mucosa of the intestinal tract are lymphoid follicles coated on the surface by a special epithelium associated with follicles comprised of columnar absorptive cell and M cells (membranous epithelial cells).
• M-cell transcytosis may be the reason why certain enteric viruses gain access to the tissues beneath the inside the intestinal lumen.

Viral Entry Through Urogenital Tract 

• Certain viruses infect the urogenital tract as result of sexual activity.
• The urogenital tract can be guarded by physical barrier such as mucus and a lower pH (in the case of vagina).
• Normal sexual activity may cause minor tears or abrasions on the vaginal epithelium, or in the urethra. This can allow bacteria to get in.
• Certain viruses infect the epithelium and cause locally-occurring lesions (e.g. some human papillomaviruses which cause warts on the genital area).
• Other viruses infect cells within the tissues and then infect cells of the immune system (e.g., human immunodeficiency virus type 1) or sensory and neuronal autonomic cells (in Herpes Simplex viruses). 

Viral Entry Through Eyes

• The epithelium which covers the exposed portion of the sclera as well as the conjunctivae serves as the point to enter for a variety of viruses. Every couple of seconds, the eyelid is brushed over the scleraand bathes it in secretions which wash out foreign particles.
• There is typically no chance for a viral infection to develop in the eyes, save for when damaged by an abrasion.
• Inoculation directly to the eye can occur during ophthalmologic procedures because of environmental contaminants (e.g. uncleanly cleaned swimming pools that are not properly cleaned).
• Most of the time, the replication occurs locally and results as inflammation and irritation of the eye (conjunctivitis).
• The transmission of the virus to the eye is uncommon however, it can happen (e.g. paralytic illness following conjunctivitis caused by enterovirus 70).
• Herpesviruses also can cause cornea inflammation on the site of a cut or damage. This can lead to cornea damage that is immune and even blindness.

Viral Entry Through Skin

• The skin of the majority of animals provides a strong barrier to viral infections because the dead outer layer of skin is not able to sustain viral growth.
• In this organ, entry occurs mostly when the integrity of the organ is compromised by punctures or break.
• The replication is typically limited to the point of entry since the epidermis is without lymphatic or blood vessels, which could open ways for spreading.
• Other viruses are able to gain access into the dermis’s vascularization by bites from arthropod vectors, such as mites, mosquitoes and sandflies.
• Inoculation that is deeper, in the muscle and tissue beneath the dermis. It can be caused through hypodermic needle punctures tattooing or body piercings animal bites, as well as sexual contact with body fluids when they are mixed with skin ulcers or abrasions.
• Contrary to the strict localized replication of the virus in the epidermis, virus that cause infection in sub-dermal or dermal tissues may spread to nearby lymphatic tissues, blood vessels as well as nerve cells. In turn, they could expand to other areas of the body.

References

• Sieczkarski SB, Whittaker GR. Viral entry. Curr Top Microbiol Immunol. 2005;285:1-23. doi: 10.1007/3-540-26764-6_1. PMID: 15609499.
• Dimitrov, D. Virus entry: molecular mechanisms and biomedical applications. Nat Rev Microbiol 2, 109–122 (2004). https://doi.org/10.1038/nrmicro817
• https://www.sciencedirect.com/topics/medicine-and-dentistry/virus-entry
• https://en.wikipedia.org/wiki/Viral_entry
• http://www.columbia.edu/itc/hs/medical/pathophys/id/2004/lecture/notes/pathogenesis_Racaniello.pdf