Behaviour of hepatitis C virus

Abi Berger, Science editor

Hepatitis C virus was identified in 1989 in the United States. Parenteral and sexual transmission is responsible for most hepatitis C infection worldwide. There is a difference in carriage rate between hepatitis B and C. For example, over 80% of people infected with hepatitis C virus become chronic carriers compared with up to 20% of those infected with hepatitis B virus.

One possible explanation for this difference between the two viruses is that hepatitis C evades the immune system more easily than hepatitis B because it mutates more rapidly. This theory is supported by the observation that one person may be infected by several subtypes of hepatitis C simultaneously. Vaccine development will prove difficult for the same reason.

Fetomaternal transmission of the two viruses also differs. In mothers infected with hepatitis B virus the vertical transmission rate may be over 90%. The immaturity of the neonatal immune system at least partly accounts for this inability to mount an immune response sufficient to clear the virus. With hepatitis C virus, however, which has recently been shown to be present in the uterine muscle as well as in blood, vertical transmission is only about 6% (but higher if the mother is HIV positive).

The difference may be partly accounted for by the factors that influence infectivity of the two viruses. Hepatitis B is much more likely to be transmitted from mother to infant if there is a high concentration of the virus in the mother's blood. This explains the ethnic differences that are observedfor example, the transmission rate is over 70% in Chinese women but less than 10% in white women. This ethnic difference does not seem to apply to hepatitis C infection.

Alcohol intake and obesity are both thought to be associated with more severe hepatitis C, although the exact interaction is unknown. Advanced liver disease, for example, is far worse in people infected with hepatitis C who also have a high alcohol intake than in those with a low intake. About half of patients with hepatitis B infections respond to interferon compared with 15% with hepatitis C. Ongoing trials of interferon and antivirals together may prove more fruitful. Although infection with hepatitis C virus does not necessarily cause abnormal liver function, precirrhotic damage confirmed by biopsy is one reason for starting treatment with interferon.

Abi Berger Science editor, BMJ  

© British Medical Journal 1998

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Where is the evidence for proportions of carriers?

Mark Rumble, GP, Great Yarmouth , Leonard Ley Surgery, 43 King Street

eBMJ, 15 Jan 1998 [Response]