Pathogenic Neisseriae: Gonorrhea, Neonatal Ophthalmia and Meningococcal Meningitis (page 4)
(This chapter has 7 pages)
© Kenneth Todar, PhD
Host Defenses
Infection stimulates inflammation and a local immune (IgA) response.
Inflammation
focuses the host defenses but also becomes the pathology of the
disease.
It is not known whether the secretory immune response is protective.
Serum
antibodies also appear, and IgG and complement may be components of the
inflammatory exudate. But whether the immune defenses provide much
protection
against reinfection has not been clearly shown. In any case, immunity
is
expected to be strain specific so that reinfection may occur.
Not everyone exposed to N. gonorrhoeae acquires the disease.
This
may be due to variations in the size or virulence of the inoculum, to
natural
resistance, or to specific immunity. A 50% infective dose (ID50)
of about 1,000 bacteria has been determined based on experimental
urethral
inoculation of male volunteers. No data is available for females.
Nonspecific factors
have been implicated in natural resistance to
gonococcal
infection. In women, changes in the genital pH and hormones may
increase
resistance to infection at certain times of the menstrual cycle. Urine
contains bactericidal and bacteriostatic components against N.
gonorrhoeae.
Factors in urine that may be important are pH, osmolarity, and the
concentration
of urea. The variability in the susceptibility of gonococcal strains to
the bactericidal and bacteriostatic properties of urine is thought to
be
one of the reasons some males apparently do not develop a gonorrhea
infection
when exposed.
Most uninfected
individuals have serum antibodies that react with
gonococcal
antigens. These antibodies probably result from colonization or
infection
by various Gram-negative bacteria that possess cross-reactive antigens.
Such "natural antibodies" may be important in individual natural
resistance or susceptibility to infection.
demonstrated.
Infection with N.
gonorrhoeae stimulates both mucosal and
systemic
antibodies to a variety of gonococcal antigens. Mucosal antibodies are
primarily IgA and IgG. In genital secretions, antibodies have been
identified
that react with Por, Opa, Rmp and LOS. Vaccine trials have suggested
that
specific anti-fimbrial antibodies inhibit the fimbrial-mediated
attachment
of the homologous gonococcal strain. In general, the IgA response is
brief
and declines rapidly after treatment; IgG levels decline more
slowly.
Anti-Por antibodies apparently are bactericidal for the gonococcus. IgG
that reacts with Rmp blocks the bactericidal activity of antibodies
directed
against Por and LOS. Genital infection with N. gonorrhoeae
stimulates
a serum antibody response against the LOS of the infecting strain.
Disseminated
gonococcal infection results in much higher levels of anti-LOS antibody
than do genital infections.
Strains that cause
uncomplicated genital infections usually are
killed
by normal human serum and are termed serum sensitive. This
bactericidal
activity is mediated by IgM and IgG antibodies that recognize sites on
the LOS. Strains that cause disseminated infections are not killed by
most
normal human serum and are referred to as serum resistant.
Resistance
is mediated, in part, by IgA that blocks the IgG-mediated
bactericidal
activity of the serum. Serum from convalescent patients with
disseminating
infections contains bactericidal IgG to the LOS of the infecting
strain.
Individuals with
inherited complement deficiencies have a markedly
increased
risk of acquiring systemic neisserial infections and are subject to
recurring
episodes of systemic gonococcal and meningococcal infections,
indicating
that the complement system is important in host defense. Gonococci
activate
complement by both the classic and alternative pathways. Complement
activation
by gonococci leads to the formation of the C5b-9 complex (membrane
attack
complex) on the outer membrane. In normal human serum, similar numbers
of C5b-9 complexes are deposited on serum-sensitive and serum-resistant
organisms, but the membrane attack complex is not functional on
serum-resistant
organisms.
Treatment
The
current
CDC
Guidelines recommend treatment of all gonococcal
infections
with antibiotic regimens effective against resistant strains.
Currently recommended
antimicrobial agents are ceftriaxone, cefixime, ciprofloxacin, or
oflaxacin.
Cephalosporins
remained the foundation of gonorrhea treatment in the 2010 CDC STD
treatment guidelines. These updated guidelines increased the
recommended dosage of ceftriaxone to 250 mg and included broadened
recommendations for combination therapy: a cephalosporin, preferably
ceftriaxone 250 mg as a single intramuscular dose, should be
administered with a second antimicrobial. Combination therapy treats
frequently co-occurring pathogens (e.g., Chlamydia trachomatis) and might
hinder the spread of cephalosporin antimicrobial resistance. Sex
partners should be referred and treated.
Control
There is no effective vaccine to prevent gonorrhea. Candidate vaccines
consisting of PilE protein or Por are of little
benefit.
The development of an effective vaccine has been hampered by the lack
of
a suitable animal model and the fact that an effective immune response
has never been demonstrated. Condoms are effective in preventing the
transmission
of gonorrhea.
The evolution of antimicrobial resistance in N. gonorrhoeae
may
ultimately affect the control of the disease. Soon after their
introduction, gonococcal resistance to sulfonamides occurred rapidly and
was common by the 1940s. Penicillin was then found to be effective for
gonorrhea treatment and became the therapy of choice for several
decades. During this time, however, the gonococcus acquired genetic
mutations that conferred increasing penicillin resistance,
necessitating increasingly higher doses of penicillin to ensure
treatment success. Penicillinase-producing
strains of N. gonorrhoeae
were first described in 1976. Strains that produce
this enzyme are highly resistant to penicillin.
Five related ß-lactamase plasmids
of
different sizes have been identified. The prevalence
of penicillin-resistant
strains has increased dramatically in the United States since 1984.
During the 1980s tetracycline-resistant strains of gonococci emerged and
became widespread in the United States, complicating gonorrhea therapy.
>Plasmid-mediated
resistance of N. gonorrhoeae to
tetracycline
was first described in 1986 and has now been reported in most parts of
the world. This resistance is due to the presence of the streptococcal
tetM determinant on a gonococcal conjugative plasmid. Strains
with multiple
chromosomal
resistance to penicillin, tetracycline, erythromycin, and cefoxitin
have
been identified in the United States and most other parts of the world.
Fluoroquinolone-resistant
N.
gonorrhoeae (QRNG) were observed sporadically in the United States during the
1990s and 2000s. During this period, fluoroquinolones were widely used
for treatment of gonorrhea because they were safe, effective,
inexpensive, and available in oral forms. Fluoroquinolone resistance is
caused by the acquisition of parC and gyrA mutations that alter binding
sites on enzymes DNA gyrase and topoisomerase IV. QRNG strains emerged
in East Asia during the 1990s. In the early 2000s. QRNG emerged in the United States, initially in Hawaii and California. By 2007, the prevalence
of QRNG was >5% among isolates collected throughout the United
States, prompting CDC to no longer recommend the use of
fluoroquinolones for gonorrhea treatment. Spectinomycin, an
alternative treatment, had not been available in the United States
since 2006, so cephalosporins (such as cefixime and ceftriaxone) were
the only remaining antimicrobials recommended for treatment of
gonococcal infections.
Cephalosporins remained the foundation of gonorrhea treatment in the
2010 CDC STD treatment guidelines. These updated guidelines increased
the recommended dosage of ceftriaxone to 250 mg and included broadened
recommendations for combination therapy: a cephalosporin, preferably
ceftriaxone 250 mg as a single intramuscular dose, should be
administered with a second antimicrobial. Combination therapy treats
frequently co-occurring pathogens (e.g., Chlamydia trachomatis) and might
hinder the spread of cephalosporin antimicrobial resistance.
Unsuccessful
treatment of gonorrhea with oral cephalosporins, such as cefixime, was
identified in East Asia, beginning in the early 2000s, and in Europe
within the past few years. Ceftriaxone-resistant isolates have been
identified in Japan, France and Spain. There is growing evidence that
cephalosporin resistance might be emerging in the United States.
Likewise, there is some evidence that cefixime susceptibility might be
threatened. The acquisition of a penA gene encoding a penicillin
binding protein (PBP2) and overproduction of an efflux pump in N. gonorrhoeae appears responsible,
at least in part, for reduced susceptibility to cephalosporins.
The development and spread of cephalosporin resistance in N. gonorrhoeae, particularly
ceftriaxone resistance, would greatly complicate treatment of
gonorrhea. Previously recommended antimicrobials likely cannot again be
routinely prescribed for empiric gonorrhea treatment. N. gonorrhoeae maintains
previously acquired antimicrobial resistance phenotypes, even if the
antimicrobial is no longer used for treatment. In 2011, 11.8% of
isolates in the United States were penicillin-resistant, 22.7% were
tetracycline-resistant, and 13.3% were fluoroquinolone-resistant.
>Tailpiece
Gonorrhea is the second most commonly reported notifiable infection in
the United States; >300,000 cases were reported in 2011. In the
United States, health inequities persist; the incidence of reported
gonorrhea among blacks is 17 times the rate among whites, likely
because of structural socioeconomic factors.
The only natural host for N. gonorrhoeae is humans. Gonorrhea
has all but disappeared in Scandinavia and several other European
countries. However, the disease is very common in the United States.
CDC estimates that more than 700,000 persons in the U.S. get new
gonorrheal infections each year. Only about half of these infections
are reported to CDC.
Gonorrhea is
transmitted almost exclusively by sexual contact. Any
sexually active person can be infected with gonorrhea. In the United
States, the highest reported rates of infection are among sexually
active teenagers, young adults, and African Americans. Persons who have
multiple sex partners are at highest risk. Rates of gonorrhea are
higher in males and in minority
and inner-city populations.
Gonorrhea is usually
contracted from a sex partner who is either
asymptomatic
or has only minimal symptoms. It is estimated that the efficiency of
transmission
after one exposure is about 35 percent from an infected woman to an
uninfected
man and 50 to 60 percent from an infected man to an uninfected woman.
More
than 90 percent of men with urethral gonorrhea will develop symptoms
within
5 days; fewer than 50 percent of women with genital gonorrhea will do
so.
Women with asymptomatic infections are at higher risk of developing
pelvic
inflammatory disease and disseminated gonococcal infection.
Last year (2012) the
World Health Organization reported that cases of drug-resistant
gonorrhea had spread across the world.
In England, gonorrhea cases soared by 25% as drug-resistant strains
began to take hold worldwide. The UK Health
Protection Agency (HPA) reported 21,000 new
cases diagnosed in 2011, with more than a third of cases in men that
have sex with men and more than a third in people who have had
gonorrhea before. Effective treatment with antibiotics has been
compromised by growing resistance to antibiotics, including
cephalosporins that are normally recommended as treatment.
A strain of gonorrhea that was resistant to all recommended antibiotics
was found in Japan in 2008. Public health officials have warned that it
could transform a once easily treatable infection into a global health
threat.
chapter continued
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