Efficacy of viral transmission by parenteral exposure

Injecting a substance contaminated with HCV HBV or HIV into the bloodstream is a particularly efficient means for transmitting infection - even more so than sexual transmission. Thus, drug use plays a key role in how epidemics first develop in regions. In the case of HIV, once it enters the drug-using population, epidemics have accelerated rapidly - starting in injection drug users and eventually spreading to the general population (Crofts et al., 1998). The prevalence of HIV among people who inject drugs was found to have risen from 0 to 30 percent within 12 months in many cities, including New York, Edinburgh, Bangkok, Ho Chi Minh City, Manipur, Riuli, Kaliningrad, Svetigorsk, Odessa, and Nikolayav (Stimson et al., 1998).

The trajectory of HCV and HBV infections in drug-injecting populations is even more dramatic. It is estimated that by the percutaneous route, HCV is 10 times and HBV is over 30 times more efficiently transmitted than HIV (Kiyosawa et al., 1991; Mitsui et al., 1992). This is evidenced by patterns in incidence and prevalence of these infections among new injectors. Data consistently suggest that HBV and HCV transmission increase rapidly within the first year of injection, whereas HIV transmission lags and increases at a slower rate (Garfein et al., 1996; see also Figure 3.5). Rates of HCV and HBV incidence are generally between 10 and 35 per 100 person years in the first few years after injection

Figure 3.5 Estonia HIV epidemic was preceded by... Incidence of reported HCV/HBV infections and absolute number of HIV-positive cases in Estonia, 1988-2002. Source: Uuskula et al. (2002).

initiation (Hagan etal., 1995; Miller etal., 2002). A study in Baltimore showed that the prevalence of HCV for injection drug users who had been injecting for 1 year or less was 65 percent (Garfein et al, 1996). Among young injection drug users between the ages of 15 and 30 across the US, a recent multicenter study reported the incidences of HIV, HBV and HCV to be 1.3, 11.3, and 15.8 per 100 person-years respectively (Garfein et al., 2006). In Pakistan, the HIV infection rate among injection drug users in Lahore has been estimated to be 12 percent, but the prevalence of hepatitis C has been estimated to be as high as 89 percent. There is further evidence that transmission of hepatitis C (and hepatitis B) occurs not only through direct needle sharing, but also through sharing of drug-use paraphernalia - including cottons, cookers, and even drug tourniquets and environmental contamination (Hagan et al, 2001; Thorpe et al, 2002). Evidence consistently suggests that unless safe injecting practices are adopted throughout an injector's lifespan, he or she will most likely acquire HCV. Moreover, even low levels of needle sharing are probably sufficient to sustain an HCV epidemic. The same is not true for HIV (des Jarlais et al., 1995b).

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