The hepatitis D virus (HDV, also called the delta virus) is a defective pathogen that requires the presence of the hepatitis B virus (HBV) for infection. The HD virion is composed of an outer lipoprotein envelope made of the surface antigen of the HBV (HBsAg) and an inner ribonucleoprotein structure in which the HDV genome resides. The HDV genome consists of a single stranded RNA which is folded as a rod-like structure through internal base-pairing (figure 1). It is complexed with the only HDV-encoded antigen, the HDAg . (See "Pathogenesis, epidemiology, natural history, and clinical manifestations of hepatitis D virus infection".)
HDAg can elicit a specific immune response in the infected host, consisting of antibodies of the IgM and IgG class (anti-HDV). In HDV infected individuals, the timing of appearance and level of HDV RNA, HDAg, and anti-HDV in serum allow the three HDV-related clinical entities to be discriminated:
- Acute HBV/HDV coinfection
- Acute HDV superinfection of a chronic HBV carrier
- Chronic HDV infection
(See "Pathogenesis, epidemiology, natural history, and clinical manifestations of hepatitis D virus infection".)
DIAGNOSIS OF HDV INFECTION
Due to the dependence of HDV on HBV, the presence of HBsAg is necessary for the diagnosis of HDV infection. The additional presence of IgM antibody to hepatitis B core antigen (IgM anti-HBc) is necessary for the diagnosis of acute HBV/HDV coinfection (table 1).