VIRUS : INFECTIOUS AGENT THAT REPLICATES INSIDE THE CELL OF LIVING ORGANISM.

Viral diseases are the most common cause of human illness .However many of the viral infections remain asymptomatic while other produce viral disease.

Virus is a small parasite that cannot reproduce by itself .Once it infects a susceptible cell, a virus can direct the cell machinery to produce more viruses. Most viruses have either RNA or DNA as their genetic material.

Some of the common and important viral diseases are-

Viral haemorrhagic fevers

Viral haemorrhagic fevers are a group of acute viral infections which have common features of causing haemorrhages, shock and sometimes death.

"Viruses causing haemorrhagic fevers were earlier called arthropod porne ( or arbo) viruses since their transmission were considered to be from arthropods to humans .According to the routes of transmission and epidemiologic features ,they are classified into four groups –"

• Mosquito borne such as yellow fever ,dengue fever ,Rift valley fever.

• Tick borne such as Crimean haemorrhagic fever, Kyasanur forest fever.

• Zoonotic such as Korean haemorrhagic fever, Lassa fever.

• Marburg virus disease and Ebola virus disease by unknown route.

Aedes aegypti mosquitoes are vectors of mosquito porne viral haemorrhagic fevers .Viral haemorrhagic fevers are the most common problem the world over ,especially in developing countries .

Examples-

• Yellow fever

• Dengue fever

Yellow fever

"Monkeys carry the virus without suffering from illness and the virus is transmitted from them to humans by Aedes aegypti as vector."

Yellow fever is characterized by the following clinical features -

Sudden onset of high fever ,chills ,myalgia ,headache ,jaundice ,hepatic failure ,renal failure ,bleeding disorders and hypotension .

Dengue Haemorrhagic fever (DHF)

"Dengue is caused by virus transmitted by the bites of mosquito Aedes aegypti ."

Dengue fever or break bone fever – Dengue fever in an uncomplicated way is a self limited febrile illness affecting muscles , any joints with severe back pain due to myalgia.

Dengue haemorrhagic fever – It is a severe and potentially fatal form of acute febrile illness characterised by cutaneous and intestinal haemorrhages due to thrombocytopenia, haemoconcentration ,hypovolaemic shock and neurologic disturbances .It is most in children under 15 years of age .

Diagnosis of DHF is confirmed by-

• Serologic testing for detection of antibodies

• Detection of virus by immunofluorescence method and monoclonal antibodies

• Rapid methods such as reverse transcriptase-PCR and fluorogenic –ELISA.

Chikungunya virus infection

"It is primarily a disease in non human primates but the infection is transmitted to humans by Aedes aegypti mosquito ."

Clinically ,the disease is characterized by abrupt onset of fever ,severe arthralgia ,migratory polyarthritis affecting small joints ,chills,headache,anorexia,nausea,abdominal pain,rash and ocular symptoms such as photophobia.

Influenza virus infections

"It is an important and common form of communicable disease .It’s general clinical features range from a mild afebrile illness similar to common cold by appearance of sudden fever ,headache ,myalgia ,malaise, chills and respiratory tract manifestations such as cough ,soar throat to a more severe form of acute respiratory illness and lymphadenopathy."

Etiologic agent –Influenza virus is a single stranded RNA virus belonging to coronaviruses depending upon it’s antigenic characteristics of the nucleoprotein and matrix .

Influenza virus infections are three types.They are – Type A – It is responsible for most serious and severe forms of outbreaks in human beings while type B and type C cause a milder form of illness.

There are two subtypes of influenza virus are found . They are-

• Avian influenza virus A /H5N1 virus commonly called Bird flu.

• Swine influenza virus A /H1N1 virus commonly called Swine flu.

Bird flu

H5N1 virus infection causes severe acute respiratory syndrome (SARS) which is the human form of bird flu with having similar symptomatology.

Swine flu

H1N1 virus is primarily an infection in pigs .Human beings acquire infection by direct contact with infected pigs. Then further transmission of H1N1 flu occurs by person to person contact.

Varicella Zoster virus infections

"It is a member of herpes virus family and causes chicken pox in non immune individuals and herpes Zoster (shingles) in those who had chicken pox in the past."

Lymphogranuloma venereum

"LGV is a sexually transmitted disease caused by Chlamydia trachomation and it is characterized by mucocutaneous lesions and regional lymphadenopathy."

The condition begins as a painless, herpes like lesion on the cervix, vagina or penis .The organisms are carried via lymphatics to regional lymph nodes .The involved lymph nodes are tender fluctuant and may ulcerate and drain pus.

Cat –scratch disease

"It is caused by Bartonella henselae ,an organism linked to rickettsiae but unlike rickettsiae .This organism can be grown in culture .There is regional nodal enlargement which appears about two weeks after cat scratch ,sometimes after thorn injury."

Rabies

"It is a fatal form of encephalitis in human caused by rabies virus .The virus is transmitted into the human body by a bite by infected carnivores such as dog, wolf, fox and bats."

Symptoms are difficulty in swallowing, painful spasm of the throat termed hydrophobia. Other clinical features are irritability, seizure and delirium.