Hepatitis A virus (HAV) is a small, unenveloped, symmetrical RNA virus (picornavirus). The HAV was first isolated by Purcell in 1973. Since the 1980s specific antibody tests have helped reveal the epidemiology, clinical manifestations, and natural history of HAV infection.
Infection with the virus ranges from mild symptoms of nausea to, in very rare cases, liver failure.
Symptoms are usually worse and the illness more often severe in older patients.

Spread is normally by the faecal-oral route although there are occasional outbreaks through food sources.
Hand washing and good hygiene around food and drink prevent spread of infection.
Active and passive immunisation are used in those at risk of infection. Travellers to certain countries, injecting drug users and those in contact with infected individuals are at risk of infection.

The most important determinant of illness severity is age and there is a direct correlation between increasing age and morbidity and mortality.
Most deaths from acute HAV infection occur in those over the age of 50, even though infection is uncommon in this age group.

Pathophysiology
Humans appear to be the only reservoir for the HAV.
The incubation period usually lasts 2-6 weeks. The time to onset of symptoms may be dose-related.

Viral replication depends on hepatocyte uptake.
After uptake, the viral RNA is uncoated, and host ribosomes bind to form polysomes.
Viral proteins can then be synthesised with the viral genome being copied by a viral RNA polymerase.
Assembled virus particles are then shed through the biliary tree into the faeces.

Shedding of the HAV is greatest during the anicteric prodrome of infection (between 14 and 21 days after infection). This corresponds to the time when transmission is highest.

Epidemiology
The anti-HAV seroprevalence rate is presently decreasing in many parts of the world, but in less developed regions and in several developing countries, HAV infection is still very common in the first years of life and seroprevalence rates approach 100%.

Hepatitis A is the most common form of acute viral hepatitis worldwide.

The highest risk areas of the world for hepatitis A infection include: the Indian subcontinent (in particular India, Pakistan, Bangladesh, and Nepal), Africa, parts of the Far East (except Japan), South and Central America, and the Middle East.

The incidence rate is strongly related to socio-economic indicators and access to safe drinking water.

In developed countries, reduced encounters with HAV in the young have resulted in a decline in herd immunity.

RISK FACTORS,CLINICAL FEATURES,DIFF:DIAGNOSIS,INVESTIGATIONS,MANAGEMENT- IN PART 2- FOLLOWS.

G Mohan.