It is much easier to talk of the Hepatitis C virus as if it is a single organism but in fact it is a range of viruses, similar enough to be called Hepatitis C virus, yet different enough to be classified into subgroups.
Viruses are microscopic and no person could ever see them with the naked eye. Indeed, HCV is so small that there’s been no confirmed actual sighting of it using any type of microscope yet developed.
Consequently, a better way to understand the terms HCV ‘genotypes’ and ‘subtypes’ is to compare them to things that we can more readily relate to.
The group of birds we call ‘raptors’ (birds of prey) have evolved into different main types. Imagining raptors as being Hepatitis C viruses, you could take one major raptor type, such as eagles, and imagine these as being one of HCV’s main types (genotypes).
But eagles as a group are made up of different sub types such as the American Bald Eagle and Australia’s Wedge Tailed Eagle and Sea Eagle. You could imagine each of these as being one of the HCV subtypes that make up an HCV genotype.
Within each of above particular types of eagles, there are further differences. All Wedge Tailed Eagles, for example, differ from each other in regard to wing span, weight, color, beak size, etc. Similarly, within a Hepatitis C sub-type, individual viruses differ from each other ever so slightly. Such viral differences are not significant enough to form another sub-type but instead form what’s known as quasi-species. It is believed that within an HCV sub-type, several million quasispecies may exist. Scientists predict that people who have Hepatitis C, have billions of actual viruses circulating within their body. Although there may be one or two predominant sub-types, the infection as a whole is not a single entity and is composed of many different quasispecies.
Biologists are generally not known for creativity when it comes to naming things – hence Hepatitis C virus. The most commonly used classification of Hepatitis C virus has HCV divided into the following genotypes (main types): 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11. As we’ve highlighted, HCV genotypes can be broken down into sub-types, some of which include:
1a, 1b, 1c
2a, 2b, 2c
4a, 4b, 4c, 4d, 4e
It is believed that theHhepatitis C virus has evolved over a period of several thousand years. This would explain the current general global patterns of genotypes and subtypes:
1a – mostly found in North & South America; also common in Australia
1b – mostly found in Europe and Asia.
2a – is the most common genotype 2 in Japan and China.
2b – is the most common genotype 2 in the U.S. and Northern Europe.
2c – the most common genotype 2 in Western and Southern Europe.
3a – highly prevalent here in Australia (40% of cases) and South Asia.
4a – highly prevalent in Egypt
4c – highly prevalent in Central Africa
5a – highly prevalent only in South Africa
6a – restricted to Hong Kong, Macau and Vietnam
7a and 7b – common in Thailand
8a, 8b & 9a – prevalent in Vietnam
10a & 11a – found in Indonesia
It’s believed that of the estimated 160,000 Australians with HCV, approx. 35% have subtype ‘1a’, 15% have ‘1b’, 7% have ‘2’, 35% have ‘3’ (mostly being 3a). The remaining people would have other genotypes.
Genotype and treatment
Current scientific belief is that factors such as duration of a person’s HCV infection, their HCV viral load, age, grade of liver inflammation or stage of fibrosis may play an important role in determining response to interferon treatment. Recent studies have suggested that a person’s HCV subtype (or subtypes) may influence their possible response to interferon, or interferon-ribavirin combination treatment. Worldwide trials are being conducted which will soon shed more light on this belief. We’ll publish any reports as they come to hand.
Genotypes and Genetic Variation of Hepatitis C Virus by G. Maerterns & L. Stuyver, reviewed by Dr Greg Dore of the National Centre in HIV Epidemiology & Clinical Research.
From The Hep C Review; Ed 23, December 1998; Paul Harvey