strain responsible for the flu changes every year and we must develop a new vaccine. The Human papillomavirus which causes genital warts, cervical and anal cancers, and head and neck cancers has over 100 strains referred to as genotypes. Most of these strains are harmless to people, but other specific genotypes are able to cause the diseases I listed.
The Hepatitis C virus (HCV) has six major strains, also referred to as genotypes. One of the most striking differences between these genotypes is their response to treatment.
For example, when we used a drug called interferon to treat hepatitis C, patients with genotype 4 could expect to get cured by treatment whereas patients with genotype 1a or 1b were far less likely to experience such a benefit. Fortunately, we now have easy to take medicines that can cure all genotypes of HCV in most cases with just a couple months of treatment (more on this topic in the future).
So what about HIV? There is a lot of variation in HIV strains. First of all, you should understand that there are two major types of HIV: HIV-1 and HIV-2. For the most part, whenever you hear HIV, it is referring to HIV-1. It is by far the major virus that infects people all over the world. HIV-2 is only found in parts of West Africa.
HIV-2 is a deadly virus just as HIV-1 but it causes disease slower than HIV-1. It is also spread sexually, through birth and coming into contact with infected blood. It is possible for a person to be infected by both HIV-1 and HIV-2 but fortunately that isn’t very common. The biggest concern with HIV-2 is that there are some classes of HIV medicines that do not work against the virus. None of the drugs in the Non-nucleoside class (efavirenz, rilpivirine, etravirine, doravirne) can suppress HIV-2. Also, some protease inhibitors don’t work (e.g. atazanavir) while others do (e.g. darunavir).
For HIV-1, the largest category of viruses are in group M and the different strains are referred to as subtypes. Subtypes are based on their genetic similarity and diversity. They are found in different geographical areas. What is amazing to observe is that most developed countries have the same HIV subtype. Throughout North America and Europe but also Latin America and the Caribbean, the predominant strain is HIV-1 subtype B.
This is quite significant because the most research has been done in subtype B virus. Diagnostic tests were developed to detect subtype B and all the drugs we used were developed for their effectiveness against subtype B virus. However, other subtypes predominate in developing countries. The most widespread strain of HIV is subtype C. It is found throughout Southern Africa, parts of East Africa, India and other parts of Asia. Approximately half of all HIV infections in the world are from subtype C virus. As I just mentioned, diagnostic tests early on were developed to detect subtype B virus and oftentimes were not able to recognize infections from other subtypes because of minor differences in their genetic makeup.
This created a problem where HIV infections were sometimes missed here in the US if the person had been infected with a different subtype and may have come from another country. I would say that in the last couple of decades, the tests have been improved such that this is no longer a problem. There are differences between different subtypes in how easily they are transmitted sexually and through birth and also how aggressively they cause disease. For example, in the country of Uganda, most people have either subtype A or subtype D virus. But these two are NOT the same! Subtype A viruses have a significantly higher rate of heterosexual transmission than subtype D viruses.
However, in the absence of treatment, subtype D virus causes more damage to the immune system and progresses nearly twice as fast to AIDS as subtype A. The good