The Viruses

There are a number of viruses that need to be considered. The main ones that will be considered are Hepatitis A-E.

Hepatitis A

The key characteristics of the virus are:

• It is a picornavirus
• It is RNA based
• It does not have a lipid envelope
• Its primary mode of spread is faecal-oral.

Infection from Hepatitis A is acute. The incubation period is approximately 2 weeks and then symptoms typically last up to 10 days. A major problem with Hepatitis A is that children are often asymptomatically affected and therefore there is increased risk of spreading the infection to another person. Additionally, the severity of the infection increases with age.

Serology: Initially levels of IgM rise and provide some short-term protection. However, levels of IgM quickly decline and are replaced by IgG, which provides longer term protection. Vaccine: There is a vaccine with a formalin-killed virus.

Hepatitis B

This virus is critical and has many important features. The key characteristics of the virus are:

• It is a hepadnaviridae
• It is a DNA virus
• It does have a lipid envelope
• Transmission is parenteral (i.e. methods other than the mouth or the alimentary canal) – vertical transmission (mother to child) does commonly occur. Infection in early childhood is often asymptomatic and also had a greater risk of chronicity!

The lipid envelope is usually a product of the previous cell that the virus occupied. The structure of the virus is important and is demonstrated in the image below:

There are two important components of the hepatitis B virus. The first to consider is the surface antigen (HBaAg). The surface antigen behaves as an important decoy when the virus infects the liver. Components of the surface form aggregates (filaments and spheres) which then enter the bloodstream and travel elsewhere in the body. These aggregates are NOT infectious, however, they are still considered as ‘non-self’ and so they become a target for antibodies. This allows the hepatitis virus to replicate more easily within the liver hepatocytes.

The other important antigen is the core antigen (HBcAg). The importance of this is identified when the genetic code of the virus is considered. The gene responsible for the production of the core antigen is referred to as the core protein. The gene has a region associated with it known as the ‘preC’ region.

• If only the core protein is transcribed then the normal c antigen is produced and this is not actively secreted out of the cell.
• However, if the core protein is transcribed ALONG WITH the ‘preC’ region then, after some peptide modification, an e antigen is produced. This is actively secreted out of the cell and therefore behaves as a good marker to identify viral replication rates.

A Hepatitis B infection can either be acute or chronic. With an acute infection, a patient can either mount an effective immune response and clear the infection, or alternatively they may go in to liver failure. The typical serology for an acute hepatitis B infection, with recovery, is shown below:

The surface antigen peaks around 3 months but then slowly declines as antibody levels rise. Usually, after about 10 months the patient clears the infection and antibodies remain (unless they go in to fulminant hepatitis). It must be noted that this infection, compared to Hepatitis A, occurs over MONTHS.

In a more chronic Hepatitis B infection, over years, an individual fails to produce any HBeAb and so they can’t clear the infection. This can lead to cirrhosis and even hepatocellular carcinoma. Vaccine: There is a vaccine available with a genetically engineered HBsAg.

Hepatitis D

Going out of alphabetical order, however Hepatitis D is closely associated with Hepatitis B.

The Hepatitis D virus is a defective virus which requires a Hepatitis B infection and virus in order to function. As demonstrated by the image, the virus utilises the Hepatitis B surface antigen in order to cause infection in the hepatocytes. Infection can either be:

• Co-incident with Hepatitis B – i.e. infection occurs at the same time as the Hepatitis B infection – more of an acute infection. The infection is limited by the rate of replication of the Hepatitis B virus as it ultimately relies on the HBsAg.
• Superinfection on Hepatitis B – this is where the individual is already infected with Hepatitis B. As there is already plenty of the Hepatitis B virus around, the Hepatitis D virus thrives and rapidly increases in number. This is commonly a more chronic infection.

Vaccine: Vaccination against Hepatitis B prevents infection by Hepatitis D due to the reliance.

Hepatitis C

The key characteristics of the virus are:

• It is a hepacivirus
• It is a RNA virus
• It does have a lipid envelope
• Transmission is parenteral (i.e. methods other than the mouth or the alimentary canal) – very common amongst IV drug users, with transmission via needles.

Compared to Hepatitis B and D, Hepatitis C leads to a milder infection and is often asymptomatic.

• 1% have acute infections
• Over 50% become chronic and a very smaller percentage of these will develop hepatocellular carcinoma.

The virus has a relatively low replication and the RNA gets damaged very easily and has no method of repair. As a result of this variability, it has proven extremely difficult to produce a vaccine against Hepatitis C. Treatment: Interferon α and ribavirin

Hepatitis E

The key characteristics of the virus are:

• It is a hepevirus
• It is an RNA virus
• It does not have a lipid envelope
• Transmission is faecal-oral

Younger adults tend to be highest effected with an average incubation period of 6 weeks. Infection can even be asymptomatic and tends to be self-limiting. It leads to a mainly acute infection and so is comparable to Hepatitis A. Vaccine: None

Summary

• There are 5 key Hepatitis Viruses
• Hepatitis A causes an acute infection, is RNA based, spread via faecal-oral contact, has no lipid envelope and there is a vaccine available.
• Hepatitis B can cause both acute and chronic infections, is DNA based, spread parentally, has a lipid envelope and has a vaccine available. It has key surface and core antigens.
• Hepatitis D is a defective virus that relies upon Hepatitis B to cause infection. Infection may be acute or chronic.
• Hepatitis C causes primarily chronic infection, is RNA based, has a lipid envelope, spread parentally and can be treated with medication. A vaccine has proven difficult to develop.
• Hepatitis E causes acute infection, is RNA based, has no lipid envelope and is spread faecal-orally.

Quiz

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References and Recommended Reading

• Essentials of Microbiology for Dental Students
• Infectious Disease Essentials