Modelling the prevalence of HCV amongst people who inject drugs: an investigation into the risks associated with injecting paraphernalia sharing.

In order to prevent the spread of the hepatitis C virus (HCV) amongst people who inject drugs (PWID), it is imperative that any injecting risk behaviour which may contribute to the transmission of disease has its role quantified. To inform public health organisations, mathematical modelling techniques were used to explore the risk of HCV infection through the sharing of injecting paraphernalia (including filters, cookers and water).A mathematical model was developed for the spread of HCV based on the injecting behaviour of PWID in Scotland, with transmission occurring through the sharing of needles/syringes and other injecting paraphernalia. Numerical simulations were used to estimate the transmission probability for HCV through the sharing of injecting paraphernalia such that the modelled endemic HCV prevalence fitted with that observed amongst PWID in Scotland.The transmission probability of HCV through injecting paraphernalia was modelled to be over 8 times lower than that through needles/syringes (approximately 0.19-0.30% and 2.5%, respectively), assuming transmission occurs through a combination of at least filters and cookers. In the context of reported needle/syringe and paraphernalia sharing rates in Scotland, it is estimated that 38% and 62% of HCV infections are contributed by these practices, respectively. If needle/syringe sharing rates were to be twice those reported, the contributions would be 70% and 30%, respectively.Given that the sharing of injecting paraphernalia among PWID is common, HCV transmission through this route could be contributing to the growing healthcare burden associated with this chronic disease. Every effort should therefore be made to establish (a) the contribution that paraphernalia sharing is making to the spread of HCV, and (b) the effectiveness of services providing sterile paraphernalia in preventing infection.