What Is A Quasispecies?

If you read enough articles about hepatitis C, you’ll eventually run into this word: quasispecies. Quasispecies are no great mystery, and they are usually unimportant when defining a patient’s genetic version of hepatitis C. The prefix, quasi- means partly or partially. In hep C-speak, quasispecies refers to small, identifiable viral divergences within a patient. Genotype and subtype never change, but quasispecies are always evolving. These small variances are caused by a combination of the patient’s own immune system, and hepatitis C’s error-prone replication system. As soon as a person is infected with HCV, his immune system attacks the virus. This puts pressure on the virus to mutate. As more immune proteins attack, more quasispecies are created. There is some evidence that the creation of quasispecies is a key to viral persistence. In other words, those who do not spontaneously clear the acute phase of HCV may be more likely to have quickly developed a more genetically divergent version of the virus. The phenomenon of viral persistence, or chronic infection is not completely understood.

Since everyone gets hep C from someone else, HCV always exists as a quasispecies. In the human body, it is never not a quasispecies, and each patient will eventually harbor some quasispecies that are uniquely his own. It was once believed that the nucleoside analog, ribavirin, worked by accelerating HCV replication to a speed that spun off so many mutation errors, the quasispecies were unable to pool together fast enough to ward off interferon. So far, quasispecies have never been used as a treatment target.

So why are hepatitis C quasispecies so important? As stated above, everyone gets the virus from someone else. That means their quasispecies is passed along to the next person. It also means that the best, most valuable use of quasispecies is in HCV forensics. Imagine a woman giving birth in a hospital. It’s discovered that both mother and baby are HCV positive. The immediate presumption would be vertical transmission – the baby got infected by the mother – but what if the mother was not the source of infection? What if the hospital used unsafe sterilization practices? Perhaps the baby’s infection came from the hospital, and not the mother. In that case, quasispecies would be used to determine the exact source of infection. The baby’s quasispecies would be checked against the mother’s. These things do happen, and quasispecies are always used to determine the exact source of the virus. Let’s say a doctor found hep C infection in a dental patient. Did the infection come from the dentist, or from the boyfriend’s razor? Suddenly, quasispecies become extremely important. What could cost the dentist a million dollars, might only cost the boyfriend a black eye and a home on the street.

The best way to think about quasispecies is as a viral fingerprint. There are innumerable infections in which the source is initially unknown. This happens more often than most people think, but quasispecies always tell the tale.

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