Modelling the hepatitis C virus epidemic in Australia
Introduction
The prevalence of hepatitis C virus (HCV) infection worldwide has been estimated to be about 3% with 170 million people affected (Sy and Jamal, 2006). The prevalence was estimated to be 3.9 million people in the United States (US), and about 1% in Europe, although it varies among different countries. For example, the prevalence was estimated to be 1.3% in France, compared to 0.5% in the United Kingdom (Sy and Jamal, 2006). In most countries, injecting drug use remains the main driver of the epidemic. Estimates of infection incidence and prevalence have helped public health strategy development and increased awareness of the magnitude of the burden of disease associated with HCV infection.
In Australia, over 225,000 diagnoses of HCV infection had been reported to State and Territory surveillance systems by the end of 2005 (Australian Government Department of Health and Ageing (AGDHA), 2006). Studies of HCV risk factors indicate that around 80% of prevalent HCV infections in Australia are attributable to injecting drug use (Kaldor et al., 1992, Strasser et al., 1995). Other important populations of people infected with HCV are migrants to Australia from countries of high HCV prevalence, and recipients of blood or blood products in the period prior to routine screening of blood for HCV in 1990 (Archer et al., 1992, Ismay et al., 1995, Mison et al., 1997, Li et al., 1998, Wong et al., 1999).
It has been widely acknowledged that there was reduction in the heroin supply in Australia from late 2000 (Day et al., 2003, Rouen et al., 2001, Topp et al., 2003, Maher et al., 2005). A number of indicators, including opioid overdose deaths, drug-related arrests, needle and syringe distribution figures, new HCV diagnoses among young adults and surveys of injecting drug users suggest there has been a reduction in both the number of IDUs and the frequency of injecting. While some data suggest that the reduction in heroin injecting has to a limited extent been replaced by injecting other drugs, principally amphetamines, it is thought that the overall amount of injecting has decreased (Topp et al., 2003).
Despite the expectation that a decrease in injecting drug use should lead to a decrease in the incidence of HCV there is only limited evidence that this is the case (Maher et al., 2006). In fact some studies have found no evidence of a decrease in HCV incidence amongst IDUs (van Beek et al., 1998, NCHECR, 2005).
This paper uses mathematical modelling to explore the impact of the change in injecting drug use in Australia since 2000 on HCV incidence and prevalence. It updates previous modelled estimates of HCV incidence and prevalence in Australia in light of new data surrounding injecting drug use. Incidence of HCV through other routes of transmission is also incorporated, in particular the introduction of infections into Australia by HCV-infected migrants from countries of high HCV prevalence.
Section snippets
Methods
HCV incidence was modelled based on three main populations of interest: (i) injecting drug users; (ii) immigrants from countries with high HCV prevalence; and (iii) people exposed through other routes of transmission. A natural history model of HCV disease progression and mortality rates was then applied to estimate the number of people were HCV antibody positive in Australia between 1960 and 2005.
Results
The estimated trend in numbers of regular IDUs between 1970 and 2005 are shown in Fig. 3. Each of the five indicators used showed some decline in injecting drug use between 2000 and 2005, but this ranged from fairly modest declines for total needle and syringe units distributed, to much more rapid and substantial declines based on opioid overdose deaths. The estimated total number of IDUs (regular and occasional) peaked in 1999 at 320,800 (lower and upper limits of 233,300 and 384,900) and
Discussion
The models estimate a decline in annual HCV incidence from over 14,000 in 2000 to less than 10,000 in 2005, predominately driven by a decline in the number of injecting drug users in Australia since 2000. Despite this decline, injecting drug use remains the major route of exposure to HCV in Australia with the models estimating that 82% of the 264,000 people who were HCV antibody positive by the end of 2005 were exposed to HCV through injecting drug use.
The models used here are based on
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