Expalain the characters and laboratory diagnosis of Hepatitis B virus.

Expalain the characters and laboratory diagnosis of Hepatitis B virus.

Hepatitis B VIrus is a member of the Hepadnaviridae family. It is the only DNA virus among the agents which commonly cause viral hepatitis. The viral particle (called the Dane particle) is 42 nm in diameter. The lipoprotein (HBsAg) which  encoats the virus is seen not only as a viral envelope but also by electron microscopy as free non-infectious tubular and spherical structures. These forms of HBsAg circulate in considerable excess compared with the virion and may play a permissive role in viral persistence. However, HBsAg may be present in blood when replication cannot be documented.

Therefore, the presence of HBsAG does not necessarily imply contagiousness.

HBV replicates in hepatocytes and possibly in peripheral blood mononuclear cells. Its genome is the smallest of all known animal DNA viruses. The replicative process is unusual in several aspects; it has an efficient genomic design of four overlapping open reading frames, it utilizes successive strand synthesis and reverse transcription similar to retroviruses, and it has both glucocorticoid- and hepatocyte-specific enhancing elements. Viral replication can be documented by measuring HBeAg (a component of the core gene product) or HBV-DNA (by a non-PCR method) in serum.

Laboratory diagnosis of Hepatitis B virus infection

1. Detection of Viral Markers:Diagnosis of Hepatitis B infection rests on serological demonstration of the viral markers and can be carried out by detection of HBsAg, anti-HBs, HBeAg, anti HBe, IgM anti-HBc, IgG-anti-HBc and HBV DNA in the serum. 
1. HBsAg is the most important serological marker for identifying infection.  It is the first marker to appear in the blood after infection. It is usually detectable 2-6 weeks in advance of clinical and biochemical evidence of hepatitis and persists throughout the clinical course of disease. It  disappears with resolution of infection and persists in chronic infection.

IgM anti-HBc is essential for the diagnosis of acute infection, but is also seen occasionally in very active chronic hepatitis. Anti-HBc antibodies develop and persist after all HBV infections. The loss of HBsAg and development of anti-HBc signals resolution of acute infection. Anti-HBs also occurs post vaccination, but anti-HBc will not be present in such cases. Chronic infection is manifested by persistent HBsAg. Markers of viral replication such as HBeAg and HBV-DNA (non-PCR method) are detectable during the early high replication phase, but are not detectable during the later quiescent low replication phase. HBeAg is not a reliable marker of HBV replication when a precore variant is responsible for the infection. Such cases will be HBeAg negative, anti-HBe positive, but HBV-DNA (by a non-PCR method) positive

Laboratory diagnosis (JAWETZ)

DNA polymerase activity, HBV DNA, and HBeAg, which are representative of the viremic stage of hepatitis B, occur early in the incubation period, concurrently or shortly after the first appearance of HBsAg. High concentration of HBV particles may be present in the blood ( up to 1010 particles/ml) during the initial phase of infection; communicability is highest at this time. HBsAg is usually detectable 2-6 weeks in advance of clinical and biochemical evidence of hepatitis and persists throughout the clinical course of the disease but typically disappears by the sixth month after exposure.

High levels of lgM- specific anti- HBc are frequently detected at the onset of clinical illness. Because this antibody is directed against the 27-nm internal core component of HBV, its appearance in the serum is indicative of viral replication. Antibody to HBsAg is first detected at a variable period after the disappearance of HBsAg. It is present in low concentrations. Before HBsAg disappears, HBeAg is replaced by anti-HBe, signaling the start of resolution of the disease. Anti HBe levels often are no longer detectable after 6 months.

By definition, HBV chronic carriers are those in whom HBsAg persists for more than 6 months in the presence of HBeAg or anti-HBe. HBsAg may persist for years after loss of HBeAg. In contrast to the high titers of lgM –specific anti – HBc observed in acute disease, low titers of IgM anti- HBc observed in acute disease, low titers of IgM anti- HBc are found in the sera of most chronic HBsAg carriers. Small amounts of HBV DNA are usually detectable in the serum as long as HBsAg  is present.

The most useful detection methods are ELISA for HBV antigens and antibodies and PCR for viral DNA.