Hepatitis delta virus infection in the world: epidemiological patterns and clinical expression

In the developed world, HDV infection is confined to groups at particular risk, such as the drug addicts. Infection is acquired predominantly by coinfection through the continuous recruitment of new drug abusers and the disease occurs often in clustered outbreaks of severe hepatitis. In areas of intermediate endemicity as in the Mediterranean basin, the epidemiology of HDV is composite. It results from an epidemic pattern similar to that seen in northern Europe, without groups at particular risk, and from an endemic pattern in the HBsAg-positive population. The endemic spread of HDV follows the modes of transmission noted in hyperendemic areas of the 'Third World', i.e. superinfection from carrier to carrier of the HBsAg, with only limited extension from the carrier domain to the normal population. However, because the network of carriers is more dispersed and transmission mechanisms less efficient, dissemination of the infection is much slower than in hyperendemic areas. Thus, the virus circulates constantly in a minority of carriers rather than occurring in epidemic waves. When the transfer of HDV from patient to patient is rapid, as in coinfected drug addicts or superinfected carriers in hyperendemic areas, infection results in severe acute and chronic hepatitis. When circulation of the virus is relatively slow, as in the Mediterranean basin, infection has a bimodal clinical expression, running a subclinical course in some of the patients and a progressive course in the others. Clinically overt hepatitis D is more likely than are other forms of viral hepatitis to advance rapidly to cirrhosis. However, when the cirrhotic stage is reached, this is often a stable condition compatible with a prolonged survival.