Squamous cervix cells covered with rod-shaped
bacteria, Gardnerella vaginalis
(source)
Bacterial vaginosis is a common disease among
reproductive-aged women:
[It] is characterized by the loss
of normal vaginal flora, predominantly hydrogen peroxide-producing Lactobacillus spp., and the increase in
the number and species of other bacteria in vaginal fluid. The decrease in
lactobacilli and increase in numerous facultative and anaerobic bacteria, some
of which have only been recently characterized, may lead to changes in the
characteristics of vaginal fluid, such as thin discharge and odor. (Koumans etal., 2007)
Its incidence correlates with the number of lifetime
sex partners, and this correlation holds true even when one controls for a
series of socioeconomic variables: poverty, smoking, body mass index, douching
frequency, education, and oral contraceptive use (Koumans et al., 2007). It
thus seems to be sexually transmissible, with suspicions falling particularly
on the bacterium Gardnerella vaginalis:
Sexual activity is a risk factor
for BV, and most experts believe that BV does not occur in women who have never
had vaginal intercourse [12,13].
Epidemiologic studies are strongly supportive of sexual transmission of BV
pathogens. There is a high occurrence of BV and concordance of flora in women
who have sex with women, further suggesting sexual transmission is important in
this setting [14-16]. It
is not clear, however, whether one type of sexual activity may be more
important in the pathogenesis of infection than another. As an example,
oral-genital sex may be a more important risk factor than penile intromission
into the vagina. (Sobel, 2012)
The incidence of bacterial vaginosis also correlates
with ethnicity, being 51.4% of non-Hispanic blacks, 31.9% of Mexican Americans,
and 23.2% of non-Hispanic whites. This correlation likewise holds true when the
above socioeconomic variables are held constant. High incidences have also been
found in sub-Saharan Africa (Pepin et al., 2011).
After studying these ethnic differences in vaginal
flora, Ravel et al. (2010) concluded that they were normal and not
pathological:
From these data we conclude that
vaginal bacterial communities not dominated by species of Lactobacillus are
common and appear normal in black and Hispanic women. The data from this study
are in accordance with the results of Zhou et al. (17, 18), who studied the
vaginal bacterial communities of white, black, and Japanese women. The reasons
for these differences among ethnic groups are unknown, but it is tempting to
speculate that the species composition of vaginal communities could be governed
by genetically determined differences between hosts. These might include
differences in innate and adaptive immune systems, the composition and quantity
of vaginal secretions, and ligands on epithelial cell surfaces, among others.
The ultimate cause may be vaginal pH, which is
higher in blacks and Hispanics than in Asians and non-Hispanic whites (Ravel etal., 2010). Or it may be differences in cytokine concentrations, with
differences in vaginal pH being due to the differences in vaginal flora
(Nomelini et al., 2010).
There has thus been a co-evolution between the vaginal environment and certain strains of vaginal bacteria. This co-evolution would have followed different trajectories in different human populations. In a monogamous population, possibilities for sexual transmission would have been sporadic and difficult to sustain. The picture is different in a population with a high incidence of polygyny, especially if the males often inherit or steal some of their co-wives from other males. Such a context would have favored bacteria that can spread from one co-wife to another and then to other sets of co-wives when circumstance permit.
But why wait for the right circumstances? Why not
make them by manipulating the host’s behavior? Such behavioral manipulation
sounds like science-fiction, yet it has been demonstrated in a wide range of
animal species, often in surprisingly precise ways. So how could our bacterium manipulate
its host? It wants to hop from one set of co-wives to another, but the regular
male partner is standing in its way. What should it do?
First, it should facilitate female-to-female
transmission among the co-wives. Second, it should disable the male’s
propensity for mate guarding. Better yet, it should reverse the polarity,
causing him to feel not jealousy but pleasure at the idea of being cuckolded.
This kind of manipulation occurs in the isopod Caecidotea intermedius. A parasite, Acanthocephalus dirus, infects it as a
temporary host before infecting one of several freshwater fishes. When the
parasite is still soft and immature, it cannot survive a fish eating its isopod
host. It thus seeks to reduce this risk by suppressing conspicuous host behaviors,
like mate guarding. Later, when the parasite becomes hard and mature, it can
survive consumption of its host and now stimulates mate guarding (Galipaud et
al., 2011; Mormann, 2010).
Cuckoldry is fatal to reproductive success, so any
such tendency would soon flush itself out of the gene pool. For example, the
neural networks for mate guarding might become more insensitive to outside
tampering. This change, however, would in turn favor those parasites that could
maintain such tampering. The eventual outcome would be an evolutionary
compromise where mate guarding is impaired, but not enough to prevent
reproductive success. The situation is different, though, if the parasite
spreads to another population that has never developed such immunity.
Many sexual fetishes have been around for a long
time and are often traceable to the ancient Greco-Roman world. Cuckold envy,
however, seems relatively recent, the oldest references dating back to 17th
century England (Kuchar, 2011, pp. 18-19). We may thus be looking at a sexually
transmitted parasite that entered England with the expansion of world trade in
the 17th century. But from where? Probably from a highly polygynous culture
area, like West Africa.
This parasite might be a vaginal bacterium that
first acquired sexual transmissibility and then an ability to manipulate host
behavior. It might alternately be a strain of vaginal yeast. Indeed, vaginal
strains of Candida albicans show a
similar adaptation to sexual transmission via the partner’s mouth, i.e., they adhere
better to saliva-coated surfaces than do other strains (Schmid et al., 1995).
References
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