According to the above analysis, we found that the prevalences of HCV/HIV co-infection and HCV mono-infection were high among IDUs enrolled in the Taipei City Hospital MMTP between 2006 and 2010. After controlling for other characteristics, syringe sharing in the 6 months before MMTP enrollment was significantly positively associated with HCV/HIV co-infection but not with HCV mono-infection. Additionally, greater frequency of incarceration was significantly positively associated with HCV/HIV co-infection and HCV mono-infection, whereas smoking amphetamine in the 6 months before MMTP enrollment was inversely associated with HCV/HIV co-infection and HCV mono-infection.
Although many studies have investigated the seroepidemiology of HCV infection among IDUs in Taiwan
[9–11, 22–24], none distinguished between HCV mono-infection and HCV infection, and only one evaluated risk factors of HCV infection
. Chang et al. found that younger age and longer duration of injecting drug use were independently associated with HCV infection among Taiwanese IDUs; however, because that study used a combination of HIV mono-infected and seronegative cases as the reference group, it may have underestimated the effects of HCV infection
A literature review showed that the prevalence of HCV/HIV co-infection among TCH IDUs (13.1%) was not much different from that among whole IDUs population in China in 2007 (12.7%)
 but higher than among incarcerated IDUs in the United States between 2003 and 2004 (4.4%)
 and lower than among incarcerated IDUs in Iran in 2006 (24%)
. Although few studies evaluated the prevalence of HCV mono-infection among IDUs, overall HCV prevalence (91.1%) was higher in our study than in prior reports in Taiwan. For example, HCV prevalence was 59.5–89.6% among incarcerated IDUs between 1997 and 2005
[9–11] and 89.2% among IDUs at the MMTPs in four counties and cities in 2008
. The present overall HCV prevalence was also higher than that reported in most other Asian countries, e.g., 47.8% in India between 2004 and 2006
, 60% in China in 2007
, and 46% in Vietnam between 2005 and 2006
. This extremely high prevalence of HCV among IDUs in Taipei must be addressed by effective HCV prevention and better access to HCV therapy.
Syringe sharing in the 6 months before MMTP enrollment was strongly associated with HCV/HIV co-infection in this study. This finding may reflect that injection practices were the major transmission route of HCV and HIV infection among IDUs
[3, 5]. Because co-infection with HCV and HIV in IDUs has become a rapidly emerging global public health problem
[7, 8], interventions including syringe exchange services and drugs substitution treatments are imperative to prevent IDUs from HCV and HIV infections
We found that greater frequency of incarceration was significantly associated with HCV/HIV co-infection and HCV mono-infection, which suggests that IDUs with a history of incarceration were more likely to have risk behaviors related to blood-borne infection
. Future studies should attempt to identify the risk behaviors that result in such infections.
Smoking amphetamine in the 6 months before MMTP enrollment was significantly inversely associated with HCV/HIV co-infection and HCV mono-infection. Amphetamine and opiates (e.g., heroin and morphine) have contrasting behavioral effects
. Thus, IDUs who smoked amphetamine would be less likely to inject opiates and consequently would have fewer injection risk behaviors and a lower risk of HCV/HIV co-infection and HCV mono-infection. This finding supports the harm reduction policy of providing clean syringes and methadone substitution treatment for IDUs.
HCV incidence among re-enrolled IDUs who were HCV-negative at the preceding MMTP enrollment was 45.25/100 PYAR in this study. HIV incidence among re-enrolled IDUs who were HIV-negative at the preceding MMTP enrollment was 0.53/100 PYAR, which is 74 times higher than that among the general population
. High incidences of HCV and HIV among IDUs who discontinued methadone treatment suggest that HCV and HIV transmission among IDUs in Taipei remains a serious concern. Prior studies showed that interruptions to methadone treatment were associated with increased prevalence of injecting drugs and sharing syringes
, which could raise risks of HCV and HIV infection. Such findings support recommendations to provide HCV and HIV risk reduction counseling, including messages on safe injecting, to IDUs at MMTP enrollment.
Caution is required in interpreting the results of this study. First, we analyzed secondary data; information on the sexual behaviors and occupation of the study participants was not available. However, the sexual behavior of the participants is not likely to have confounded the main findings on risk factors associated with HCV mono-infection and HCV/HIV co-infection because HCV transmission via risky sexual behavior is not common
. Second, the use of cross-sectional data, without information on the temporal sequence between risk behaviors and infection of individual IDUs, precludes causal inference. Third, it is difficult to obtain a truly representative sample of a population of community-dwelling IDUs. It has been estimated that MMTP participants accounted for fewer than 20% of reported heroin-addicted drug users in Taipei in 2010
. Thus, the generalizability of the present findings might be limited to IDUs at the Taipei MMTP.