(see Graphic)
Cruising the streets of a red light district to estimate the number of
prostitutes working sounds like a bizarre activity. But the threat of HIV
means health officials need to do such things. Between 1 and 3 per cent of
female prostitutes in Britain may be HIV positive (in Spain almost half of
the 20 per cent or so of prostitutes who also inject drugs are HIV
positive). Without some knowledge about the size of the prostitute
population these figures are meaningless. Their implications for those who
monitor the HIV epidemic and plan health services are vastly different
depending on whether they refer to a percentage of 2000 prostitutes or of
100. But how do you estimate the number of people involved in a covert and
stigmatised activity?
Three years ago, at the University of Glasgow鈥檚 Public Health Research Unit,
Marina Barnard, Alastair Leyland and I began to apply a statistical
technique commonly used by field ecologists to estimate population sizes.
They capture and tag a sample of wild birds or animals and then release
them. The size of the total population can be estimated by taking a second
sample and doing a calculation based on the numbers of tagged and untagged
birds in that sample 鈥� the fewer tagged birds recaptured, the larger the
population size. Our research is based on a refined version of this
technique. We contact individual prostitutes on the streets, ask them for
their initials and date of birth, and then use these details to monitor how
often these 鈥榯agged鈥� prostitutes are contacted by us in the following
months. In 1990, with a grant from the Medical Research Council, we began a
large field study based on this approach.
The project is part of a broader, six-year-old study of prostitution, drug
use and HIV infection in Glasgow鈥檚 city centre. One of the initial goals was
to assess how many prostitutes are also intravenous drug users 鈥� knowledge
that is crucial to gauging the implications of prostitution for HIV
transmission.
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It is now clear from this study that there are some 1150 street prostitutes
in Glasgow, a city with a population of about 750 000. This figure does not
include prostitutes who work from saunas, hotels and brothels. But having
scanned Scottish contact magazines for Glasgow addresses, counted Glasgow鈥檚
saunas and interviewed lobby staff in city-centre hotels, we are confident
that far fewer prostitutes work off the streets in Glasgow than on them.
This is a sensitive area of research and needs to be tackled with delicacy.
While our studies have focused mainly on the women, we have also interviewed
more than 150 of their clients, asking them how often they buy sex, what
the appeal is of commercial sex and how often they use condoms. When asked
about the last time they bought sex, 72 per cent of the clients said they
had vaginal intercourse, and almost 12 per cent of these men said they did
not use a condom. We have a lot of sympathy with the view expressed by the
prostitute who said, 鈥業t鈥檚 not us that鈥檚 needing testing or educating, it鈥檚
the men. They鈥檙e the ones asking for it without a condom . . . they say 鈥榶ou
look clean to me鈥�, and I say 鈥榙o you think AIDS comes stamped on your
蹿辞谤别丑别补诲?鈥�
In our research on street prostitutes, the gathering of statistical
information is combined with an outreach service, in which women are
contacted and offered free condoms, information on HIV risk reduction and
sterile injecting equipment if they need it. In return they are asked to
give us their initials and the day and month of their birth. We also ask
that they provide a sample of saliva for anonymous HIV testing 鈥� so far over
80 per cent have agreed to do so. Each night we set out to contact all the
women working the streets, keeping a record of the total number we contacted
and the number of repeat and first-time contacts.
To estimate the number of women working on the streets we need to know what
size population of prostitutes is most likely to have produced the
particular combination of new and repeat contacts recorded over the course
of field work. The calculation we used can most easily be explained by
considering a simplified situation 鈥� observations taken over just two
nights in the red light area. The total population estimate we call N. On
the first night we contact n1 women, all of whom are new to us. On the
second night we contact n2 women, some of whom we saw on the previous
night. To estimate the total population we multiply the first sample by the
second sample and dividing the total by the number of repeated contacts in
the second sample (m2). The equation is
N = n1 x n2/m2.
While this calculation is fairly straightforward, the behaviour of real
people is a lot more complex. For a start, women are continually entering
the profession, and leaving it 鈥� sometimes permanently, sometimes only
temporarily because they have been imprisoned or because they are pregnant.
Furthermore, some women work very frequently, but most work only
occasionally. To get a greater sense of these comings and goings, we have
made our observations over periods of several months. This gives us a more
accurate estimate of the population size but makes statistical analysis so
complicated that a computer is needed.
Our most recent results, taken over a seven-month period in 1991 are shown
in the graph. This work involved 53 nights of field work and more than 150
hours of observation, ranging from early evening to past midnight. On the
basis of this we estimate that around 1150 women a year work on the streets
in Glasgow. Saliva samples taken during the study suggest that approximately
29 are HIV positive (2.5 per cent). The research also shows that as many as
70 per cent of the total population are injecting drugs. Interestingly, we
found that drug-injecting prostitutes work just as frequently as those who
do not inject drugs, a fact that has important implications for how we
assess the risk that HIV will spread.
Our approach has its limitations. An early assumption of field ecologists
was that all animals or birds in the population being studied were equally
likely to be captured. This was quickly disproved 鈥� within a population,
some individuals are 鈥榯rap shy鈥� and others are 鈥榯rap friendly鈥�. We have
noticed a similar tendency in our work. Those women with whom we have a good
relationship and who value the service we provide will often seek us out.
Others deliberately avoid contact or are in such great demand that we
seldom see them. During our research there was one woman whom we repeatedly
glimpsed as she stepped into and out of clients鈥� cars. It took some five
months鈥� work before we could speak to her directly.
Another source of error in our calculations may stem from prostitutes
attempting to conceal their identity by using different names on different
occasions. Many working women use aliases as a way of separating their
working role from their private lives. Others have different reasons for
wishing to keep their identity secret. For example, one young woman we
contacted in the red-light area, whom we had not seen for almost two years,
explained: 鈥業 was only 15 at the time and things were getting a bit heavy
for me, so I knocked it on the head and went back to the children鈥檚 home.
Now that I鈥檓 17 I thought it was OK to get back into it.鈥�
She now felt able to reveal her real initials and date of birth rather than
the false details she had provided in the past. Unfortunately, we have no
way of knowing to what extent false identification has affected our
results. All we can do practically is assure the women we contact that any
information they provide will be regarded as confidential.
Research like this involves the surveillance of a hidden population involved
in a stigmatised activity. Such surveillance has been deemed acceptable
because of AIDS. Nevertheless we must be sensitive about balancing the needs
of individuals and those of society. Our research identified few
HIV-positive prostitutes, but there are already plans to use the technique
in countries such as Zimbabwe where HIV is much more prevalent among
prostitutes because the rate of infection in the population as a whole is
high. This may well lead to calls for their enforced registration, together
with an increase in the stigma of commercial sex work. For some this is an
acceptable price to pay to mitigate a public health threat. For others it
conveniently glosses over the fact that female prostitutes are only one half
of the commercial sex equation.
In developed countries, what matters most in assessing the likely
contribution of prostitution to the spread of HIV is the link with
injecting drugs. Previous studies in the US and northern Europe detected
relatively few HIV-positive prostitutes. In an article in the British
Medical Journal, we recently reported HIV data on 159 street prostitutes in
Glasgow. Although all the non-injecting women were HIV-negative, 3.5 per
cent of those who injected drugs were HIV-positive. By comparison, studies
of heterosexual men attending clinics for sexually transmitted diseases in
developed countries show that about 1 per cent are HIV positive.
There are good grounds for extending the kind of research that I have
described here to prostitutes鈥� male clients. In Glasgow, we are already
investigating patterns in client activity. Is there a small number of men
who contact prostitutes regularly, or a much larger number who use them
only occasionally? However, extending quantitative research to clients, who
have even more invested in their invisibility, is likely to be a daunting
and perhaps impossible task.
Neil McKeganey is a senior lecturer in the Public Health Research Unit at
Glasgow University. He is the author (with Marina Barnard) of AIDS, drugs
and sexual risk: lives in the balance (Open University Press, 1992)