| Introduction |
The hepatitis C virus (HCV) and the human immunodeficiency virus
(HIV) are both bloodborne viruses, are both spread through
needle-sharing and other practices of injecting drug users (IDUs),
and both represent serious health risks for Australian IDUs. Despite
these similarities, in Australia the epidemiology of these viruses
differs greatly. The prevalence of HCV among IDUs is about 65%,
whereas that of HIV is less than 3%; this difference is mirrored by
their respective incidences, about 15% per year for HCV and less than
1% per year for HIV.1,2 While new HIV diagnoses in
heterosexual IDUs are rare, estimates of new HCV infections in the
same group range from 6000 to 10 000 per year.1
Should we conclude from these differences that our current
approaches to prevention of bloodborne viruses among IDUs (which are
based on the harm-reduction philosophy and include needle and
syringe distribution, methadone maintenance therapy, peer
education and advocacy) are ineffective? This is unlikely for two
reasons. Firstly, HIV has a very low incidence in Australian IDUs, but
continues to spread rapidly among IDUs in many parts of the United
States and in other populations lacking equally comprehensive
prevention strategies. Secondly, there have been dramatic declines
in transmission of other viruses (hepatitis B and D) among Australian
IDUs since the mid 1980s.1 The effectiveness of just
one element of our harm reduction programs -- needle and syringe
exchange -- has been well demonstrated,3 and is supported by an
American study which found that IDUs in needle exchange programs were
seven times less likely to be exposed to HCV than those who were
not.4
Can the difference be explained by HCV spreading in a different manner
to HIV? Sexual transmission of HCV is rare5 and is unimportant
epidemiologically, while the reverse applies for HIV. Both viruses
are transmitted between IDUs by blood contact. The determinants of
the epidemics are background virus prevalence, virus
infectiousness and the existence of behaviour that permits spread.
Sharing needles and syringes is generally acknowledged as the most
important means of spread between IDUs, and has consistently been
found to be the major association with HCV transmission among IDUs
worldwide. Since the early 1980s, in response to the threat of HIV
infection, there have been substantial declines in needle-sharing
among Australian IDUs. About 13% of IDUs in Melbourne reported having
shared a needle and syringe in the preceding month in 1994, compared
with 38% in 1989.6 Nevertheless, it remains a
problem, and higher proportions of IDUs who share needles and
syringes are found among substantial minority groups, such as
Vietnamese migrants and prisoners.6,7
There is emerging evidence of the importance of blood contact between
IDUs in ways other than sharing needles and syringes. Videotapes of
groups of IDUs reveal many opportunities for exposure to viruses
through blood contaminating equipment other than needles and
syringes. This includes swabs, spoons, water vials and tourniquets,
as well as fingers, other body parts and surfaces in the immediate
environment.1 For example, a user applying
a tourniquet to a partner's arm might deposit a tiny smear of blood on
skin which is subsequently punctured by a needle, or wipe blood from an
injection site and let the swab fall onto a communal tabletop. The risk
associated with such behaviour is greater for HCV than for HIV,
because the higher infectiousness of HCV8 means smaller amounts of
blood can efficiently transmit the infection. Two studies have
attempted to quantify IDUs' risk of HCV transmission in the absence of
needle-sharing: one found an incidence of about 4% per year among
those who reported never sharing needles and syringes, compared with
about 17% per year among those who did;9 the second, 11.9% compared
with 30.2%.10 Both studies potentially
suffered from misclassification bias, with some of those reporting
never sharing actually having shared. Such bias would exacerbate the
difference between the two incidences, further supporting the
conclusion that the majority of HCV infections among Australian IDUs
are associated with needle-sharing.
Risky behaviour permits the blood contact necessary for both HCV and
HIV transmission, and the higher infectiousness of HCV undoubtedly
explains some of the difference in incidence. Background viral
prevalence is the other critical factor. HCV prevalence is now so high
that even very occasional sharing of needles and syringes carries an
extreme risk of HCV infection, to which must be added the unknown but
non-negligible risk due to "environmental" contamination. With a
low-prevalence virus such as HIV, high-risk behaviour must be far
more frequent and prevalent in a population before the risks of
transmission become large enough to sustain continued spread. Given
the much higher infectiousness of HCV per contact episode and its much
higher prevalence in Australian IDUs relative to HIV, the difference
in incidence comes down to the force of numbers. The Table combines
background prevalences of HCV and HIV, estimates of infectiousness,
and carrier rates to illustrate the difference in infection risk
encountered by IDUs in Australia. On the basis of these crude
estimates, an IDU sharing a needle used by another IDU of unknown
infection status is at somewhere between 150 and 800 times higher risk
of infection with HCV than HIV.
Despite this somewhat sobering picture, it should be noted that there
are tentative indications that HCV incidence in Australian IDUs has
been declining in recent years. Modelling has suggested that
incidence may have fallen from around 18% per year before 1987 to
around 12% per year thereafter;1 a Victorian cohort study
found a decline from 16.6 cases per 100 person-years in 1990-91 to 8.1
per 100 person-years in 1994-95, matched by declines in behavioural
risk;9 and HCV prevalence among
IDUs tested for the first time at a methadone maintenance program in
Melbourne decreased from 75% in 1992 to 50% in 1995.12
The conclusion from all these considerations is that control of the
HCV epidemic requires more intense concentration on reducing
needle-sharing and other risky behaviour, and will require a greater
effort to decrease incidence than HIV has. This has been seen with HIV
infection among IDUs in other countries -- epidemics which have
reached high prevalences have proven much harder to control than
epidemics which have not taken off before interventions began.
Further decreases in needle-sharing will require increased support
for accepted programs (increased funding and reach of needle
exchange programs, pharmacy sales, peer education) as well as
consideration of new strategies. These should urgently include
needle exchange programs in prisons (where needle-sharing is the
norm)13 and youth training
centres, safe injecting spaces, special programs for Aboriginal
IDUs and people of non-English-speaking background, and
deregulation of supply of needles and syringes so they can be
purchased from outlets such as service stations and convenience
stores.
Other strategies, such as promoting transition to the smoking rather
than injecting of illicit drugs, are worth further research and
consideration. Finally, the importance of hygiene with regard to
injecting environments and conditions must be strongly and
routinely stressed. Without serious commitment to new and expanded
strategies -- especially needle exchange -- the HCV epidemic will
continue, with its high and growing toll of illness and cost.
|
|
Centre for Harm Reduction, Macfarlane Burnet Centre for Medical
Research, Fairfield, VIC.
Nick Crofts, MB BS, Director;
Campbell K Aitken, PhD,
Senior Research Officer.
National Centre in HIV Epidemiology and Clinical Research,
Darlinghurst, NSW.
John M Kaldor, PhD, Deputy Director.
Reprints will not be available from the authors.
Correspondence: Dr N
Crofts, Centre for Harm Reduction, Macfarlane Burnet Centre for
Medical Research, PO Box 254, Fairfield, VIC 3078.
Email: crofts@burnet.edu.au
©MJA 1998
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