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Transcript
Behavioral Ecology Vol. 14 No. 1: 40–47
Do bacterial infections cause reduced ejaculate
quality? A meta-analysis of antibiotic treatment
of male infertility
Philip A. Skau and Ivar Folstad
Department of Evolution and Ecology, IB, University of Tromsø, 9037 Tromsø, Norway
Ejaculate quality may limit male reproductive success, and consequently, sperm quality is of importance. Spermatozoa are
perceived as ‘‘non-self’’ by the immune system and are exposed to immunological attacks in the male reproductive tract. To
reduce immunological reactions against their own sperm, males are dependent on the testis being an immunoprivileged site.
Immunoprivilege is obtained by the blood-testis barrier and by local immunosuppression by androgens. Despite this testicular
immunosuppression, an influx of leukocytes may occur in testes. The condition in which males have a heightened level of
leukocytes in semen is called leukocytospermia, and it is associated with reduced fertility. As the ability of immunosuppression by
androgens may depend on current intensities of infectious organisms in the extratesticular soma, only males with high parasite
resistance may be able to bear the cost of immunosuppression and consequently produce high quality ejaculates. This issue is
addressed by a meta-analysis on the effects of broad-spectrum antibiotic treatment of male leukocytospermia-associated infertility.
The analysis showed that antibiotic treatment of leukocytospermic men, without diagnosed genital tract infections, resulted in
a significant improvement of ejaculate quality, that is, an increase in ejaculate volume, sperm concentration, number of motile
spermatozoa, and number of spermatozoa with normal morphology. Moreover, the amount of leukocytes in semen was also
reduced. This suggests that broad-spectrum treatment targeted toward bacterial infections reduces the density of leukocytes in
semen and, at the same time, improves the quality of ejaculates produced. Our results emphasize the importance of parasitic
resistance and immunity as factors that cause variations in ejaculate quality. Key words: antibiotic treatment, ejaculate quality,
immunity, leukocytospermia, male infertility, meta-analysis, parasite resistance, sperm. [Behav Ecol 14:40–47 (2003)]
W
hen assessing the relative importance of pre- and
postcopulatory mechanisms for fitness, the former
have previously been considered to be the most important
determinant of male reproductive success (Trivers, 1985;
Williams, 1975). Postcopulatory mechanisms, such as male
fertilization potential and fertilizing capacity, have therefore
acquired less attention (Ginsberg and Huck, 1989). Male
fertilization potential is, however, sometimes limited by
ejaculate quality, and sperm density in ejaculates is an important determinant of fertilization success (Bonde et al.,
1998; Gibson and Jewell, 1982; Marconato and Shapiro, 1996;
Warner et al., 1995). Males show reduced sperm density in
successive ejaculations (e.g., Freund, 1963; Synnot et al.,
1981), and in humans (Homo sapiens), sperm counts begin to
decrease at ejaculation rates greater than 3.5 times per week
(Freund, 1963). This suggests that the rate of sperm
production or sperm maturation may be limited in some
species (Marson et al., 1989; Synnot et al., 1981). In species in
which males experience sperm competition, which is the
competition between the ejaculates of different males for the
fertilization of an ova (Parker, 1970), males allocate more
resources to sperm production. That is, they have larger testes
and release more sperm per ejaculate relative to males in
species with low risk of sperm competition, (Hosken and
Ward, 2001; Møller, 1988, 1989; Møller and Briskie, 1995).
This suggests that the importance of high sperm numbers is
greater under sperm competition (Birkhead and Møller, 1992;
Møller, 1988; Møller and Briskie, 1995; Parker, 1998).
Address correspondence to P.A. Skau. E-mail: [email protected]
uit.no.
Received 4 April 2001; revised 7 February 2002; accepted 24 April
2002.
2003 International Society for Behavioral Ecology
In species in which females may risk not having their eggs
fertilized owing to poor ejaculate quality among males,
females choosing males with good ejaculates should be
favored by selection. In addition, in such species, females
should allow sperm competition as fertility insurance. Sperm
competition exists in a variety of species, and it has generally
been considered more common among birds and insects
than among mammalian species (Birkhead and Møller, 1998).
However, even if female mammals do not possess proper
sperm-storage organs and if mammalian spermatozoa are
relatively short-lived, recent studies have documented that
sperm competition in mammals is more common than
previously assumed (Gomendio and Roldan, 1993; Gomendio
et al. 1998).
The rate of sperm production in humans is significantly
lower than that of other mammals (Amann and Howard,
1980), and human semen is unique among mammals because
of its large number of morphologically abnormal cells. The
number of spermatozoa in a normal ejaculate is also low in
humans (Birkhead, 1999a). These findings suggest that sperm
competition may not be particularly intense in humans. The
human testis size is, on the other hand, close to that expected
from body mass, indicating intermediate sperm competition
(Birkhead, 1999b).
Sperm competition may occur in situations in which
females engage in extrapair copulations. The extent to which
production of extrapair offspring occurs is currently subject to
controversy, but the most commonly cited estimates are
around 10% (Birkhead, 1999b), which is a relatively small
figure compared with that of other mammals (Birkhead and
Appleton, 1998). However, 10% can still be considered
enough to increase the variation in male reproductive success
and consequently contribute to an important component of
sexual selection in humans. The frequency of extrapair
copulation is, however, probably higher, as not all extrapair
Skau and Folstad
•
Meta-analysis of antibiotic treatment of male infertility
copulations lead to extrapair offspring. Even though the
extent of extrapair copulation in humans is at present quite
small, it may have been of larger importance in earlier stages
of human evolution.
Human females have been suggested to prefer testosteronerelated facial characteristics in mates when conception is most
likely (Penton-Voak and Perrett, 2000). Testosterone-related
facial traits are also preferred when women engage in
extrapair copulations or short-term relationships (PentonVoak et al., 1999; Perrett et al., 1998). Many sexual selected
characters and behaviors in male vertebrates are testosterone
dependent, and testosterone-related traits may honestly
advertise immunocompetence. That is, testosterone is immunosuppressive, and the conditional development of
testosterone-dependent characters reduces individual immunocompetence toward infectious organisms. Testosterone-dependent sex-trait development may thus be a reliable
signal of individual parasite resistance (Folstad and Karter,
1992).
In 1997 both Hillgarth et al. and Folstad and Skarstein
suggested that the adaptive significance of immunosuppression by testosterone was related to the antigenicity of sperm.
Because spermatozoa are differentiated at puberty, after the
immune system has acquired a tolerance to all ‘‘self,’’ they
are perceived as ‘‘non-self.’’ Males are consequently exposed
to new antigens when spermatogonia mature, and the
immune activity involved in the male reproductive system
may lead to autoimmunity (Turek and Lipshultz, 1994). This
is a condition in which the immune system produces
antibodies and sensitized lymphocytes against the body’s
own sperm (Stites et al., 1987). Such autoimmunity does,
however, not normally occur (Turek and Lipshultz, 1994), as
the testis is an immunologically privileged site. This is partly
caused by the blood-testes barrier (Weienbauer et al., 1997),
which isolates the antigenic germ cells from the somatic
immune system. Among seasonal breeders, the blood-testes
barrier is redeveloped before each breeding season and
reabsorbed after breeding (Setchell, 1997), which indicates
the importance of this barrier for development of germ
cells. In addition, immune activity within the testis is also
thought to be inhibited by locally produced hormones such
as testosterone (Lehmann, 1986; Lehmann and Emmons,
1989). The blood-testes barrier and its associated immunosuppressive mechanisms, however, do not seem to hinder
all immunological activity within the testes (Turek and
Lipshultz, 1994), and both sperm antibodies and white
blood cells are able to enter the interior of the testes and
the epididymis. The normal cell population of the testis also
includes macrophages, and human testis macrophages represent about 25% of all interstitial cells in adults
(Weienbauer et al., 1997). Macrophages participate in
normal immunological surveillance by presenting the
antigen for cell-mediated immunity and are capable of
phagocytosis (Janeway and Travers, 1994). The macrophages
in testes are also capable of engulfing numerous spermatozoa (Hughes et al., 1981), and such phagocytosis could
represent a process for removal of ageing spermatozoa
(Tomlinson et al., 1992). However, it has also been
suggested that there could be need for immunological
control over ‘‘outlaw’’ haploid spermatozoa, as conflicts may
arise when diploid and haploid self-interest differ, for
example, as in some forms of meiotic drive (Folstad and
Skarstein, 1997).
Even though it has been suggested that testicular immune
activity may be adaptive, current evidence indicates that it can
also be pathological (Bronson, 1999; Turek and Lipshultz,
1994). The presence of white blood cells in semen has been
shown to play an important role in male infertility, and there
41
are numerous reports on the negative effects of white blood
cells on sperm function (Branigan and Muller, 1994; Wolff,
1995; Wolff et al., 1990). The negative effects described are,
for example, decreased sperm number and decreased sperm
motility (Wolff et al., 1990). Individuals with high densities of
white blood cells in semen are referred to as leukocytospermic
(World Health Organization, 1993), and the frequency of
leukocytospermia is documented to be from 10–20% among
male infertility patients (Wolff, 1995).
Infectious organisms may interfere with immunosuppression during reproduction and may consequently influence
ejaculate production. Folstad and Skarstein (1997) suggested
that the relationship between immunosuppression and
development of secondary sex traits might be a derived
characteristic of the testicular immunosuppression conducted
by sex hormones. As a consequence of such association,
females could be able to evaluate both the ejaculate qualities
of a male and his resistance toward infectious organisms
from his sex-trait development. Males with high intensities of
parasites should display an increased level of systemic immune
activity and consequently have a heightened level of testicular
immunity. Thus, high parasite intensities could result in
a reduction of ejaculate quality and fertility (Folstad and
Skarstein, 1997).
Infections in the reproductive tract of infertile men have
been acknowledged for decades (Nikkanen et al., 1979). Until
recently, the condition of leukocytospermia has been used as
an indicator of genital tract infection, and it has been
presumed that high densities of leukocytes and antisperm
antibodies have their origin in infections in the genital tract
(Behre et al., 1997). However, a relatively large number of
men who attend fertility clinics exhibit leukocytospermia
without symptoms of genital infections, indicating that there
is not a necessary relationship between infections in the
genital tract and the amount of leukocytes or antisperm
antibodies in semen (Eggert-Kruse et al., 1998; Micic et al.,
1990; Trum et al., 1998; Wolff, 1995). Leukocytospermic men
are consequently of interest as they may show symptoms of
heightened systemic immune activity that is not caused by
genital tract infections (Anderson, 1995; Purvis and Christiansen, 1993). Infections outside the genital tract may be
asymptomatic but could still contribute to an increase in
somatic immune activity and increased influx of leukocytes to
the genital tract.
The most commonly used group of antibiotics in the
treatment of leukocytospermia is broad-spectrum antibiotics,
which are not targeted toward specific bacterial infections and
are used for treatment for a number of nongonadal
infections, for example, pneumonia and gastrointestinal
infections (Dollery, 1999). Yet, even if it has not been
diagnosed as an inflammation in the genital tract or isolated
as a pathological organism in leukocytospermic patients,
leukocytospermia has routinely been treated with antibiotics
(Anderson, 1995; Purvis and Christiansen, 1993). Today there
exists, however, no clear consensus on the effect of such
treatment (Anderson, 1995) or on the importance of
leukocytospermia in general (Kaleli et al., 2000), and
several studies have been conducted with differing results.
The hypothesis tested here is that a general reduction in
the level of bacterial infections after broad-spectrum antibiotic treatment of leukocytospermic men will reduce the
somatic immune activity and lead to a reduction in the level of
leukocytes in testes and epididymis. This will subsequently
lead to an improved ejaculate quality. This hypothesis is
addressed with a meta-analysis of the results from studies
conducted to examine the effects of antibiotic treatment of
leukocytospermic infertile men without diagnosed infections
in the reproductive organs.
Behavioral Ecology Vol. 14 No. 1
42
METHODS
Identification and classification of outcome variables
To identify clinical trials, a computer search of the databases
Medline, EmBase, and Cochrane Collaboration was performed. This was conducted to identify all published articles
in any languages published in the period from 1966–2000.
These sources of information should provide a near complete
coverage of published data on the subject. The following
keywords were used: leukocytospermia, male infertility,
antibiotic treatment of male infertility, immunological infertility, male genital tract infection, antisperm antibodies,
asthenozoospermia (i.e., loss or reduction of sperm motility),
and oligozoospermia (i.e., low numbers of spermatozoa in
ejaculates). To reduce the risk of missing studies, a search of
the Science Citation Index was also performed by using the
name of the first author of papers obtained from the
databases. A search on the references (and references of
references) cited in the primary sources was also performed.
Only published papers were included in the analysis.
The definition of normal semen values is given by the
World Health Organization (1993). Semen volume is the total
volume of an ejaculate and is classified as normal above 2.0
ml. Sperm concentration is the number of spermatozoa per
milliliter semen and is classified as normal above 20 million
sperm per milliliter. A semen sample has normal motility if
more than 50% of the spermatozoa show forward progression.
The evaluation of sperm motility is often subjective, and
various methods are in use. The morphology of a sperm cell is
defined by its shape and form, for example, whether the head
is round or tapered, or whether a different tail or a neck or
mid-piece defect is present. A semen sample with more than
15% cells with normal morphology is classified as normal.
According to the World Health Organization, leukocytospermia is defined as the presence of more than 106 white blood
cells per milliliter semen.
Identified trials
A total of 23 clinical studies of antibiotic treatment of male
infertility were identified. To be included in the analysis, each
study had to report original data, a quantitative measure of
effect size, or p values. Moreover, we excluded studies in which
leukocytospermia was assumed to be caused by local
infections causing pathology (e.g., in cases of prostatitis).
Studies on leukocytospermic patients showing bacteriospermia were, however, included in the analysis because bacteriospermia was most likely not caused by a local infection
in testes or epididymis but rather by infections situated
elsewhere or by contamination in the urethra (Bieniek and
Riedel 1993; Busolo et al., 1984; Purvis and Christiansen
1995). Following these selection criteria, we were left with
12 studies. In 11 of these, the effect size was converted from
p values. Often researchers only stated whether their results were significant or not, and consequently we could only
extract p values of .05, even if the original results may have
been significant at higher levels. This procedure will result in
conservative effect-size estimates. The most commonly applied
antibiotics were doxycycline, erythromycin, and trimethoprim
in combination with sulfamethoxazole, which are all considered broad-spectrum antibiotics. The trials were classified
according to their different outcome measurements: sperm
counts, motility, morphology, ejaculate volume, and the
release of leukocytospermia. Pregnancy rates are the best
measure of fertility, but the lack of sufficient data in the
identified studies did not allow fertility as an outcome variable
to be included in the meta-analysis.
Some authors have reported that poor studies tend to
exaggerate the overall estimate of treatment effect and thus
lead to incorrect inference (Khan et al., 1996), whereas others
have failed to find that the inclusiveness of poor studies affect
the overall estimate in meta-analysis (e.g., Greenwald and
Russell, 1991). It might consequently be difficult to find exact
rationales for inclusiveness of studies in meta-analysis (Greenwald and Russell, 1991). Because the total number of available
studies is rather low in the present analysis, no attempt was
made to exclude studies because of poor methodological
quality. Studies included in the analysis are given in Table 1.
Measures of effect size were transformed into Pearson’s
product-moment correlation coefficients by using the formulas given by Rosenthal (1991: 19). The sampling distribution of r is usually skewed, and in order to meet the assumption
of normality, we transformed r values into zr values. The
relationship between r and zr is given by Sokal and Rohlf
(1995: 575).
The results from studies that include small samples are
more subject to chance than those of larger studies and
should therefore be given less weight. In this study we used
a weighted average of the results, in which the larger trials
are given more influence than that of the smaller ones. The
overall effect size was consequently calculated from the
mean effect size adjusted for sample size (after zr transformation) by using the equation provided by Rosenthal (1991:
73–74). If the result of the studies differ greatly, it may not
be appropriate to combine the results into one overall measure of effect. It is, however, unclear how to assert whether or
not this is appropriate. One approach is to statistically
examine the degree of similarity between the outcomes of
the studies, that is, to test for heterogeneity across studies.
Such tests examine whether the results of a study reflect
a single underlying effect rather than a distribution of effects.
Statistically significant heterogeneity may be caused by
methodological variation between trials; for example, study
designs, method of randomization, and different methods of
measuring sperm quality or quantity. If the test shows
homogenous results, then the differences between studies
are assumed to be a result of sampling variation alone.
Heterogeneity was calculated by using the formula provided
by Rosenthal (1991: 73–74).
The ‘‘file-drawer problem’’ is one aspect of the more general
problem of publication bias (Rosenthal, 1979). There are
reports suggesting that the set of available studies does not
reflect the total set of studies ever conducted, because some
reports are not published owing to the lack of statistically
significant results (Begg and Berlin, 1988; Rosenthal, 1991).
Rosenthal (1991) suggested calculating the fail-safe number,
which is an estimate of the number of studies with zero effect
needed to nullify an overall effect. This procedure was
conducted by using the equation provided by Rosenthal
(1991: 104). Moreover, the lack of consensus regarding the
effect of antibiotic treatment of leukocytospermic men may
have reduced publication bias in this particular field of science.
RESULTS
Effect of treatment
The meta-analysis revealed that there was a significant positive effect of antibiotic treatment for the following sperm
parameters: sperm volume, sperm concentration, sperm
motility, and sperm morphology (Table 2). Antibiotic treatment also significantly reduced the number of leukocytes in
ejaculates of male infertility patients. Thus, in general, males
treated with antibiotics were relieved from leukocytospermia
and produced ejaculates of higher quality.
Skau and Folstad
•
Meta-analysis of antibiotic treatment of male infertility
43
Table 1
Treatment period (in days of treatment) and the total number of patients enrolled (n) in each of the 12 studies included in the meta-analysis
Treatment and study design
Treatment of leukocytospermia with doxycycline and trimethoprim. The
treatment group was compared with a control group of untreated patients.
Treatment of bacteriospermia with trimethoprim/sulfamethoxazole. The
treatment group was compared with a control group of patients given
placebo drugs.
Treatment of leukocytospermia with doxycycline. The treatment group was
compared with a control group of patients given placebo drugs.
Treatment of leukocytospermia with ciprofloxacin; evaluation of treatment
group before and after medical treatment, and treatment with trimethoprim and
sulfamethoxazole; evaluation of treatment group before and after medical treatment.
Treatment of leukocytospermia with doxycycline. The treatment group was
compared with a control group of untreated patients.
Treatment of seminal infection with doxycycline. The treatment group was
compared with a control group of untreated patients.
Treatment of bacteriospermia with doxycycline. Evaluation of treatment
group before and after medical treatment.
Treatment of oligoasthenozoospermia (small amounts of spermatozoa) with
doxycycline and erythromycin. Evaluation of treatment group before and after
medical treatment.
Treatment of asthenospermia (loss or reduction of motility) with
erythromycin. The treatment group was compared with a control group
of patients given placebo drugs.
Treatment of accessory gland infection with doxycycline. The treatment
group was compared with a control group of patients given placebo drugs.
Treatment of leukocytospermia with doxycycline. The treatment group was
compared with a control group of untreated patients.
Treatment of leukocytospermia with doxycycline. The treatment group
was compared with a control group of untreated patients.
Treatment
period
n
Reference
14
41
Yanushpolsky et al., 1995
30
20
Merino and Carranza-Lira, 1995
10
25
Erel et al., 1997
14
7
10
76
Keck et al., 1998
30
24
Omu et al., 1998
30
24
Cardoso et al., 1998
90
56
Micic, 1988
30
65
Baker et al., 1984
30
33
Comhaire et al., 1986
30
50
Branigan and Muller, 1994
28
53
Branigan et al., 1995
Carransa-Lira et al., 1998
The drugs doxycycline, erythromycin, trimethoprim/sulfamethoxazole, cephalexin, and ciprofloxacin are all considered broad-spectrum
antibiotics. Studies with patients showing bacteriospermia are included, as bacteriospermia in these cases most likely was not caused by infections
situated in reproductive tissue.
Fail-safe number and heterogeneity
The fail-safe number for the different parameters ranged
from six to 68 (Table 2). No firm guidelines can be given as to
what constitutes an unlikely number of unretrieved and
unpublished studies, but a robust result has been suggested to
be five times the number of studies included in an analysis
plus 10 (Rosenthal, 1991: 106). Consequently, one of the
present results, those on morphology, had a fail-safe number
higher than this rule of thumb.
Sperm morphology showed the highest heterogeneity of
the different sperm parameters, but none of the included
parameters showed significant heterogeneity among studies.
This is interesting, as several of the included studies show low
sample size.
DISCUSSION
Treatment of male infertility with broad-spectrum antibiotics
significantly reduces the density of leukocytes in ejaculates. In
addition, treatment also significantly increases ejaculate
quality. That is, treatment increases ejaculate volume, sperm
concentration, the number of motile spermatozoa, and the
number of spermatozoa with normal morphology.
Leukocytospermia has a heterogeneous etiology, including
infections, inflammations, and autoimmunity (Barratt et al.,
1990), making the immediate cause for this condition quite
complex and unclear. Most cases of leukocytes in semen are
presumed to originate from some form of infection (Behre
et al., 1997). However, the majority of men with leukocytospermia have semen cultures that fail to show genital tract
infections (Anderson, 1995; Barratt, 1988; Hillier et al., 1990).
A heightened level of seminal leukocytes in individuals in
whom no infection in the genital tract is identified may have
its cause in infections situated outside the genital tract
(Bieniek and Riedel 1993; Busolo et al., 1984; Purvis and
Christiansen 1995). Thus, a wide variety of infections may
cause leukocytospermia. We have only included studies of
antibiotic treatment of male infertility, and this may have
contributed to our low effect sizes, as only bacteriological
infections should be affected by treatment. Other infectious
organisms of potential importance for a male’s ability to
down-regulate the immune response could be viral, protozoan, or metazoan infections. HIV-infected men are, for
example, shown to have low seminal quality (Anderson et al.,
1990), and ejaculate quality decreases with the progression of
the infection. Moreover, treatment of HIV-infected men
with antiviral drugs reduces seminal levels of leukocytes and
also improves ejaculate quality (Politch et al., 1994). It can,
however, not be excluded that these effects could be caused by
the presence of HIV in testes tissue (Fan et al., 2000). Yet,
negative effects on host sperm quality, sperm quantity, and
fertility have also been observed from nongonadal infections
(Baker, 1991; El-Gamal et al., 1991; Liljedal et al., 1999;
MacLeod, 1951; Sheriff, 1987; Wang et al., 1987; see also
Måsvær M, Liljedal S, Folstad I, in preparation).
Treatment of leukocytospermia with antibiotics resulted in
a significant reduction in the level of leukocytes in semen.
Behavioral Ecology Vol. 14 No. 1
44
Table 2
Results from the meta-analysis of male infertility with broad-spectrum antibiotics
Parameters
No. of
Studies
n
Weighted
effect size
Unweighted
effect size
Heterogeneity
test
Fail safe
number
Sperm volume
Sperm concentration
Motility
Morphology
Leukocytospermia
4
8
9
9
5
182
327
360
333
258
0.20*
0.16*
0.20**
0.22***
0.23**
0.19
0.19
0.24
0.35
0.25
0.98
0.14
0.18
0.08
0.17
6
25
48
68
26
Weighted mean effect size and unweighted effect size of the outcome variables sperm volume, sperm
concentration, sperm motility, and sperm morphology. The number of studies, sample size (n), and
effect sizes (mean zr values) are given. The heterogeneity test provides information about the difference
between correlation coefficients (p values are reported), and the fail-safe number is the number of
studies with zero effect needed to nullify the mean effect size.
* p , .01, ** p , .001, *** p , .0001.
Leukocyte density was also the parameter that showed the
largest response to treatment. Current research suggests that
some leukocytes may be adapted to remove defect and
immature sperm cells in the testicular environment (Tomlinson et al., 1992), and it is documented that leukocytes are
present in testes as early as the seventh week of gestation
(Weienbauer et al., 1997). This suggests that the presence of
leukocytes in testes is not only of a pathological nature.
However, densities of leukocytes in semen are positively
associated with levels of antisperm antibodies (Wolff, 1995),
and antisperm antibodies are able to bind to the surface of
sperm cells and reduce the acrosome reaction (Bandoh et al.,
1992). This will, in turn, reduce the number of sperm cells in
the ejaculate capable of fertilizing an ovum. A reduced level of
leukocytes in semen should consequently be associated with
improved ejaculate quality and improved fertility.
Antibiotic treatment resulted in an increased ejaculate
volume. The volume of ejaculates show large differences both
between and within individuals, and is strongly related to
ejaculation frequency (Freund, 1963). Ejaculate volume may,
however, also be influenced by infections outside the
urogenital tract that reduce the elasticity in prostate and
seminal vesicles (Behre et al., 1997). In humans, ejaculate
volume is not a commonly used measure of ejaculate quality,
and volumes of ejaculates are not positively related to
pregnancy rates, except at exceptionally low volumes (Bonde
et al., 1998; World Health Organization, 1993). Because
ejaculate volume, for a large part, is determined by the
production of seminal fluid in the prostate and the seminal
vesicles, it should not be affected by immune activity in testes
or epididymis. It could consequently not be expected that
a reduced immunological activity in testes resulting from
antibiotic treatment would have a strong effect on ejaculate
volume, and volume was also one of the outcome parameters
that showed low response to treatment.
Antibiotic treatment lead to an increased sperm concentration. This was the parameter showing the lowest response
to treatment. Sperm numbers appear to be related to the
number of sperm surviving during the passage through the
female tract (Gomendio et al., 1998), and sperm concentration is also strongly associated with the probability of
pregnancy (Bonde et al., 1998). It is, however, not obvious
how antibiotic treatment should affect sperm concentration,
because sperm cells of the treated individuals are produced
before medication. One explanation may be that the number
of macrophages in testes has been reduced. In general,
macrophages in testes have been considered to be of negative
influence on sperm cell production (Rossi and Aitken, 1997;
Wang and Fanning, 1997), and as macrophages are shown to
remove sperm cells through phagocytosis (Kurpisz and
Fernandez, 1995), a reduction in macrophage density in the
testes may lead to an increased number of sperm cells.
Another, not mutually exclusive, explanation of the increased
sperm concentration after treatment may be that the number
of immunological cells with a cytotoxic effect in testes, such as
cytotoxic T-lymphocytes, may have been reduced. Cytotoxic
cells have been shown to inflict oxidative stress on spermatozoa (Aitken et al., 1994), which leads to a reduction in
sperm motility and altered morphology of sperm cells (Rossi
and Aitken, 1997). If treatment results in a lowered oxidative
stress because of the reduced level of cytotoxic cells in testes,
fewer sperm cells are likely to be abnormal and therefore are
less likely to be engulfed by macrophages. Although this may
result in differences in the numbers of sperm cells ejaculated
between individuals, variation in sperm numbers in the
ejaculate appears not to provide a full explanation of
differential fertilizing capacity. Even when an approximately
equivalent number of sperm from two or more males have
been inseminated in females, systematic differences in
reproductive outcome between males have been found
(Martin and Dzuik, 1977). This suggests that differences in
fertilizing capacity between ejaculates may also be related to
the quality of the ejaculated sperm cells.
Antibiotic treatment resulted in a significant improvement
of sperm motility. Motility is considered to be an important
factor in male fertility (Drobnis and Overstreet, 1992; World
Health Organization, 1993; for an alternative view, see Ben
Chitrit et al., 1995; Bonde et al., 1998). In mice, it was shown
that subordinate individuals produce sperm cells with lower
motility compared with that of dominant individuals (Koyama
and Kamimura, 1999), and this lower motility may be caused
by low levels of androgens in subordinate mice (Bronson and
Desjardins, 1971; Gandelman, 1980). Androgens, which may
be immunosuppressive, could influence the level of white
blood cells and antisperm antibodies in semen. Antisperm
antibodies present in semen may bind to the surface of
spermatozoa (Bronson, 1999), and such binding is shown
to considerably reduce sperm motility (Barratt et al., 1990;
Purvis and Christiansen, 1995; Turek and Lipshultz, 1994). A
reduction in the intratesticular immune activity may therefore
reduce the amount of antisperm antibodies and consequently
lead to improved motility.
The level of morphologically normal sperm cells increased
as a result of antibiotic treatment. Large numbers of cytotoxic
cells present in the testes have been shown to negatively affect
sperm morphology (Aitken et al., 1994), and the formation of
Skau and Folstad
•
Meta-analysis of antibiotic treatment of male infertility
normal spermatozoa is dependent on low levels of testicular
immune activity. Therefore, a reduction in immune activity in
the testes should lead to a higher number of spermatozoa with
normal morphology. Moreover, sperm morphology seems to be
an important determinant for spermatozoa motility, as spermatozoa with odd morphology usually show dysfunctional
movement. Thus, morphology and motility are closely associated variables (Katz et al., 1982). It has also been suggested that
the proportion of sperm with normal morphology is strongly
related to the likelihood of pregnancy, independent of both
sperm concentration and ejaculate volume (Bonde et al., 1998;
Liu and Baker, 1994).
It has clearly been illustrated in the case of the stomach
ulcer–producing bacteria Helicobacter pylori that infectious
organisms of relatively low pathogenicity may play an
important role in human illness (Bourke et al., 1996). In
accordance with this view, antibiotic treatment of infertility
patients, without clear symptoms of genital tract infections,
leads to a significant decrease of leukocyte numbers in
semen and an increase in ejaculate quality. This suggests that
a reduction in the level of parasites may lead to a reduction in
the somatic immune activity, a lower influx of immunocompetent cells in the testes, and a higher ejaculate quality. Thus,
increased pathogenicity from infectious organisms associated
with immunosuppression may be an unrecognized cost of
ejaculate production.
We are currently conducting a follow-up study on the
published effects of corticosteroid treatment on sperm quality
in humans.
We thank Monica Martinussen for guidance in meta-analysis, Yngve
Figenschau for providing information about andrology, and Mathilde
Berg, Henning Klausen Ståle Liljedal, Marthe Måsvær, Ingebrigt
Uglem, and two anonymous referees for commenting on earlier
versions of the manuscript.
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