Virology

Virology Cont.…

Pathogenesis

Viral pathogenesis deals with the interaction between a virus and its host. The route of infection varies with the host and can be via mucosal membranes of respiratory tract, gastrointestinal tract, urogenital tract or conjunctiva of eye or also by breaching the skin which may be arthropod injection, animal bite or needle stick. Viruses causes infection of the host first by breaking the natural protective mechanisms of the body, then invading the immune system of the host and finally by killing off the hosts cells and triggering immune and inflammatory responses. But the outcome of viral infection depends upon response of the host to infection by a virus and on the nature of the host-virus interaction.

Stages of viral infection.

1) Entry into the body

Entry of the viruses into the body is the first step of viral infection. Skin is the best barrier as well mucus, ciliated epithelium, gastric acid, bile, tears etc acts as barrier to infection and protect from viral infection. The virus is mainly enters the body through the respiratory tract, skin conjunctiva, alimentary tract, and genital tract. After enter to the body by breaking natural protective barriers they starts to initiate infection.

1.1  Respiratory tract

Many of the viral infections are caused by respiratory viruses, these viruses enters to respiratory tract by droplets or aerosol generated during the course of coughing, sneezing, talking. Some viruses multiply in respiratory tract then initiate infection and some are spread to other parts of host cells after multiplication. Some example of respiratory viruses are influenza virus, rhinovirus, coronavirus, adenovirus, measles virus, mumps virus, rubella virus etc.

1.2  Skin

Entry of virus by the action of breaking of skin. Some viruses and their descriptive feature of invading.

Viruses	Type of action.
Cowpox	Through minor lesions.
Papilloma virus	Through minor injuries on the surface of skin.
Arbovirus	By bite of insects.
Rabies virus	Through the bite of animals (mainly dog).
Hepatitis B & HIV	Through injection.

1.3  Conjunctiva

Viruses like adenovirus, measles virus are enters through conjunctiva and cause local as well as systemic disease.

Adenovirus: Cause local manifestation of disease.

Measles virus: Cause systemic manifestation of disease.

1.4  Genital tract

Viruses enters into the body through the genital tract mainly during the course of sexual contact. Viruses such as HIV, hepatitis B, hepatitis C, papilloma virus, herpes simplex virus are transmitted through sexual contact.

1.5  Alimentary tract

Alimentary tract is another route of infection. Viruses enter into the body through this tract and multiplications starts there and starts to invading host cells then produce local as well as systemic manifestation of disease. E.g. Rotavirus, Adenovirus, Reo virus etc.

 

1.6  Congenital infection

Congenital infection affects the unborn fetus or newborn infants. The infection in fetus mainly from the infected mother which transmit during the pregnancy; up through the time of delivery. E.g. Rubella, Cytomegalovirus, Enteroviruses etc.

 

2)      Initiation of infection at primary site i.e. infection of target tissue.

Entry  →  Replicate and cause infection

 

3)      Replication of virus and spread to secondary site

Spread mainly by the blood stream (viremia) and lymphatic system.

     →        (Primary viremia)(replication in macrophage, production of virus in large number)
Lymph node  → Blood stream          →        Spleen & Liver                        Blood stream 

                                                                                                                                ￬

                  (Secondary viremia)

                                                                        Skin, Brain, Liver etc.      ←          Blood stream

 

Transmission

The mode of transmission basically depends on the site of the viral replication & secretion and presence or absence of envelop (; any wrapper or covering) in the virus.

1.      The site of the viral replication & secretion

The virus that replicate in the intestinal tract and are excreted in the feces and transmitted by fecal-oral route. The main causes of fecal-oral transmission include lack of adequate sanitation and poor hygiene practices. E.g. enteroviruses, poliovirus, rotavirus etc.

The virus that replicate in the respiratory tract and are secreted in aerosol droplets hence transmitted by inhalation as well as contact with droplets. Respiratory droplets can be generated naturally during the course of talking, sneezing or coughing. E.g. influenza virus, respiratory syncytial virus, rhinovirus, coronaviruses etc.

2.      Presence or absence of envelope in the virus

Envelope viruses are fragile virus. The envelope typically derived from phospholipids and proteins. These virus usually transmitted by respiratory droplets, saliva, mucus, blood or semen, organ transplantation and by ingestion. Envelope viruses are sensitive to the presence of acid and detergents. And are destroy in the gastrointestinal tract of human and hence not transmitted by facal-oral route.

Note: Hepatitis B virus & Corona virus are exception which are transmitted by fecal-oral oute.

E.g. Herpesviruses, Poxviruses, Togavirus, Orthomyxovirus, Retroviruses etc.

Non-envelope viruses are naked viruses. They only need their protein based capsid and host detector proteins to infect the host cells. These can transmitted by the respiratory droplets and fecal-oral route and also often by contaminated objects. They can resist drying, extreme PH & temperature and effects of detergents. And also they can stand with the acidity of the stomach and lytic effect of bile in intestine. E.g. Rhinovirus, Norovirus, Poliovirus etc.