Drug
resistance and the consequences for treatment
HIV is highly
variable, it is an RNA virus and the reverse
transcriptase has no proofreading function (this means
that when the DNA copy is made no checks are made to ensure the
sequence is exactly right-mistakes, or mutations, are consequently
often encountered). Although some mutations may kill the virus it
is inevitable that drug resistant strains will emerge over long
term treatment. For
example, with continued use of zidovudine, the virus develops resistance
to the drug due to mutations resulting in amino
acid substitutions in the viral reverse
transcriptase.
The introduction
of potent combinations of antiretroviral drugs (highly active antiretroviral
therapy, HAART) has helped to reduce the incidence of opportunistic
infections, and improved survival. With HAART, resistance should
take longer because a virus strain resistant to one drug could still
be sensitive to another. This means that the virus has to develop
multiple mutations to overcome the actions of two or more drugs,
this is certainly not as likely to occur.

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