HIV/AIDS Epi Update - May 2004
Oral Sex and the Risk of HIV Transmission
Introduction
At A Glance
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Unprotected oral sex has been
associated with HIV infection in
some studies.
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Poor oral health and the
presence of other STIs may
increase the risk of HIV transmission
through oral sex.
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The actual risk of HIV
transmission through oral sex is
difficult to assess since research
subjects may underreport
sexual activities that are of
higher risk.
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Oral sex, particularly unprotected
receptive fellatio with
ejaculation, should be considered
as a potential risk
behaviour for HIV transmission.
The risk of HIV transmission through unprotected anal and
vaginal intercourse is well known. Estimates of the probability of
per-sex-act (receptive penile-anal intercourse with ejaculation)
HIV transmission among homosexual men in the USA range from 0.005
to 0.03 during the asymptomatic phase of infection1 to as high as
0.1-0.3 during primary HIV infection.2 Analyses of data
from North American and European studies of long-term heterosexual
couples estimate the per-sex-act probability of HIV transmission
through penile-vaginal intercourse to be approximately 0.001.3
However, the independent risk of HIV transmission through
orogenital contact has been more difficult to study and is not as
well understood.
One study calculated the per-sex-act probability of HIV
transmission in a cohort of men who have sex with men (MSM) and
determined that for unprotected receptive anal intercourse, the
probability was 0.82% per act, for unprotected insertive anal
intercourse 0.06%, and for unprotected receptive oral intercourse
with ejaculation 0.04%.4 This remains the only study
available that provides a probability for oral transmission, and
further study is required to corroborate these estimates.
Another study attempted to calculate the population-attributable
risk percentage (PAR%) for HIV prevalence associated with fellatio.
PAR% refers to the incidence of a disease (in this case, HIV) in a
population that can be attributed to a certain risk behaviour (in
this case, fellatio). The study focused on MSM and found that the
PAR% was 0.18% for MSM who had had one partner in the previous six
months, 0.25% for two partners, and 0.31% for three
partners.5
This report updates current information on oral sex and the risk
of HIV transmission. Current recommendations on the use of condoms
for oral intercourse are also reviewed.
Laboratory and Animal Studies: Evidence Links HIV Infection to
Oral Intercourse in Humans
- An animal study found that the minimal dose of simian
immunodeficiency virus (SIV) (a virus closely related to HIV-1)
required to achieve infection in adult rhesus monkeys through oral
exposure was 6,000 times lower than was the minimal dose needed to
achieve infection after rectal exposure. The researchers concluded
that, as with oral exposure of rhesus monkeys to SIV, oral exposure
of humans to HIV-1 likely carries the risk of
infection.6
- In a laboratory study designed to explore the oral transmission
of HIV by seminal fluid and colostrum, normal donor samples of
human milk, colostrum, seminal fluid and blood were separately
combined with samples of saliva and HIV-infected white blood cells.
All samples, in normal physiologic volumes, prevented saliva from
inactivating the HIV-infected blood cells, leading the researchers
to conclude that successful oral transmission of HIV by seminal
fluid, milk and colostrum may occur.7
- Another study took oral tissue samples from non-infected
subjects and exposed them to three types of HIV. The researchers
found that normal human oral keratinocytes (NHOKs), which are
produced in the mouth, can become infected with HIV and transmit
the virus to adjacent leukocytes. Though certain orally produced
glycolipids can inhibit HIV replication and the infectivity noted
was lower than in blood plasma, the results still demonstrate the
risk of potential HIV oral transmission.8
Oral Sex between Homosexual Males: Not as Safe as once
Perceived
Several epidemiological studies have examined the risk of HIV
infection through unprotected receptive oral intercourse (receptive
fellatio):
- In a 1996-1999 study of MSM with a recent diagnosis of HIV
infection, it was found that 7.8% of subjects (eight of 102) were
probably infected through receptive oral sex.9
- In a 1986-1988 prospective study of HIV infection and AIDS
among MSM in the Netherlands, four of 102 cases of seroconversion
(3.9%) likely occurred as a result of receptive oral
intercourse.10
- In a 1990-1992 study of gay men with newly diagnosed HIV
infection, six of 37 patients (16.2%) who had been infected within
a year before testing claimed receptive oral sex as the only
possible route of their infection.11
Several studies have also explored the possibility of HIV
transmission through unprotected insertive orogenital intercourse
(insertive fellatio) or insertive oral-anal sex (insertive
anilingus):
- In a prospective study of HIV infection among MSM in the
Netherlands, five of 102 seroconverters (4.9%) may have been
infected through insertive orogenital or oral-anal
intercourse.10
- In an early cohort study of MSM, two of five cases of HIV
seroconversion were attributed to insertive orogenital
sex.12
Additional reports or studies, while not distinguishing the type
of oral sex between MSM, further suggest the possibility of HIV
transmission through oral-penile/oral-anal contact:
- In the UK, 13 cases of HIV transmission through orogenital
contact had been reported to the public health authorities up to
December 1998. In two of these cases, the reporting physician was
not convinced that orogenital contact was the only
risk.13
- In a US study describing the clinical and epidemiological
features of primary HIV infection, four of 46 patients reported
having had only unprotected orogenital contact during the suspected
sexual 94% with a seronegative regular partner, 91% with a regular
partner of unknown serostatus, and 88% with a seropositive regular
partner. For casual partners the rates were 92% with unknown or
seronegative partners and 73% with seropositive
partners.19
- In the Polaris study in Ontario, researchers examining the
difference between recent seroconverters and HIV-negative MSM
reported that 97% of the sample of seroconverters (n = 62)
practised unprotected oral sex, as did 73% of HIV-negative MSM (n =
121). Further, 55% of recent seroconverters and 27% of HIV-negative
MSM reported exposure to ejaculate while engaging in unprotected
oral sex.20
Oral Risk Behaviours among Heterosexuals
- In a study of female street youth involved in prostitution in
Montreal, researchers found that condom use was extremely low
during oral sex. Only 5% of girls involved in prostitution and 4%
of girls not involved in prostitution used condoms while performing
fellatio.21
Female-to-Female Transmission of HIV through Oral Intercourse:
Truth or Bias?
To date, there have been several reports of HIV transmission
through orogenital contact between lesbians
(cunnilingus).22 A number of researchers have suggested,
however, that bisexual activity may be underreported by gay women,
and therefore that not all the cases of female-to-female
transmission of HIV infection are authentic.23
Possible Transmission of HIV Between Heterosexual Partners as a
Result of Oral Intercourse
- There are several reported cases in the literature of women who
acquired HIV infection after performing oral sex on their
seropositive male partner (receptive fellatio).24
- Cases of infection in men following oral sex with their female
partners have been reported, including one in which a man was
apparently infected through fellatio involving a
prostitute.25,26
- In a study of 741 MSM in the Netherlands, orogenital contact
was identified as an independent risk for HIV acquisition, although
this result was not statistically significant.15
- In a US study, homosexual males who were participating in a
hepatitis B study were found to have a higher risk of HIV infection
from both orogenital and oro-anal contact.16
- In the Omega cohort in Quebec, 10 out of 629 MSM participants
(1.6%) seroconverted and listed only unprotected receptive oral
intercourse as the possible route of their
infection.17
- In Australia, researchers looking at MSM and risk behaviours
found that five out of 75 recently seroconverted subjects (6.7%) in
the study were likely infected by oral intercourse. The authors
note that it is difficult to be certain of the actual mode of
transmission. The subjects had varying risk profiles: for example,
one had a penile piercing that could have caused transmission,
another had gingivitis and dental treatment, and another had had
protected anal intercourse.18
Prevalence of Unprotected Oral Sex among Homosexual Males
- The Omega cohort study in Quebec examined the prevalence of
unprotected oral sex among 400 MSM by the HIV serostatus of their
partners. Researchers found rates for unprotected oral sex of
seropositive male partner (receptive fellatio).24
- Cases of infection in men following oral sex with their female
partners have been reported, including one in which a man was
apparently infected through fellatio involving a
prostitute.25,26
- On the other hand, a study in Madrid of 135 serodiscordant
couples reported over 19,000 instances of unprotected oral sex
between spouses without one seroconversion, showing that this
behaviour requires further investigation.27
Potential Co-Factors for HIV Transmission During Oral Sex
Saliva that does not contain blood presents no potential for
transmission, as research has shown that an enzyme in saliva
inhibits HIV. In general, the mouth and throat are well defended
against HIV: the oral mucosal lining contains few of the cells that
are the most susceptible to HIV.28 Other research notes that saliva
contains several HIV inhibitors, such as peroxidases and
thrombospondin-1, and that the hypotonicity of saliva disrupts the
transmission of infected leukocytes (white blood
cells).29
Case reports identify factors potentially associated with
increased risk of HIV transmission through oral sex: oral trauma,
sores, inflammation, concomitant sexually transmitted infections,
ejaculation in the mouth, and systemic immune suppression.13 For
receptive fellatio, poor oral health and taking ejaculate in the
mouth is a hazardous combination that increases the risk of HIV
transmission.30
- In a 1996 cross-sectional study of crack cocaine smokers, oral
lesions were associated with HIV infection among persons who
reported receptive oral sex.31
- A 1993 study of female sex trade workers found that crack users
who inconsistently used condoms when performing oral sex on their
clients were more likely to be infected with HIV than were those
who consistently used condoms when performing
fellatio.32
- Of the eight MSM in the Options Project in San Francisco in
2000 who may have acquired their HIV infection through receptive
oral intercourse, three reported oral problems, including
occasional bleeding gums.9
Oral Sex and "Safer Sex Counselling": Existing Views
and Recommendations
- The Canadian AIDS Society (CAS) currently classifies insertive
fellatio between men, or between women and men, as carrying a
negligible risk of HIV transmission regardless of condom use.
Receptive fellatio between men, or between men and women, is
classified as carrying negligible risk if a condom is used and as
low risk if a condom is not used (whether or not semen is taken in
the mouth). The CAS currently cautions that the risk of
transmission from receptive fellatio is increased if lesions or
sores are present in the mouth.33
- With respect to insertive cunnilingus between men and women or
between two women, the CAS regards this practice as carrying a
negligible risk of HIV transmission if a barrier is used and as low
risk if no barrier is used (regardless of menstrual status).
Receptive cunnilingus between men and women or between two women is
regarded as carrying a negligible HIV risk.33
- Both insertive and receptive anilingus, with or without a
barrier, between partners of the same sex or opposite sex are
viewed by the CAS as carrying a negligible risk of HIV
transmission.33
- The CAS emphasizes that the risk of transmission of HIV (or
other STIs) from any of these types of oral intercourse can be
effectively reduced by the proper use of a latex barrier (condom or
dental dam), and thus advocates the avoidance of unprotected
orogenital or oro-anal contact.33
Conclusions
The risk of HIV transmission through oral sex is difficult to
assess because HIV seroconverters may underreport other higher risk
sexual practices. A literature review identified exposure to HIV
through unprotected oral intercourse as an independent risk factor
for HIV acquisition in only three (12.5%) of 24 epidemiological
studies designed to examine risk of HIV from different sexual
exposures.34 It indicates that the importance of oral
sex to HIV transmission is a complex result of the relative
frequency of oral sex (among other activities), the infectivity of
oral secretions and its modification by oral pathology, and the
resistance to infection of inhibitory substances in
saliva.34 Also, the HIV incidence and prevalence in the
community, the role of antiretroviral therapy and the extent to
which personal prophylaxis is adopted will influence the
contribution of oral sex to HIV transmission.34
While oral sex is a lower risk activity than unprotected anal or
vaginal intercourse, repeated exposures may increase the risk.
Although the risk of acquiring HIV through oral sex is low, the
higher rates of practising oral sex indicate that it may contribute
to significant numbers of HIV cases among MSM. Safer sex practices
should consider oral sex, particularly unprotected receptive
fellatio with ejaculation, as a potential risk behaviour for HIV
transmission.
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