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Micro Chapter 69: Sexually Transmitted Diseases
Page 698 – STD’s, the bug that causes each one, and how common they are
Chlamydia is the most common STI, followed by gonorrhea
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About half of all preventable cases of infertility among women is a result of infections with
chlamydia and gonorrhea
Neisseria gonorrhoeae is increasing in resistance to antibiotics
Certain kinds of HPV can cause cervical cancer
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About ¼ of women in the US are infected with genital HPV, most of which are low risk
o Some though can involve cervical dysplasia and cervical cancer
o Only a small # of women progress to cervical cancer
Movement of on STI through a population depends on how transmissible the agent is, the rate of getting
new partners, the partner’s sexual history, and the duration of being infected
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Transmissibility (infectivity rate) – the risk of acquiring infection during a single contact with an
infected partner
o The infectivity rate varies with the type of sex act, the properties of the agent, and the
immune status of the person
 Ex: HIV has specific surface molecules to bind receptors in anal epithelium, so
anal sex increases the rate
Studies show most people don’t have that many sex partners, and STD spread is due to a
minority of people that have lots of partners, called the core
o Sex among core members sustains the diseases, and sex outside the core spreads them
Duration of infectivity is the length of time a person is able to transmit the infection
Almost half of STDs in the US happen in people under 25 years old
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Not only do behavioral risks cause this, but an adolescent cervix also is anatomically at greater
risk for chlamydia or gonorrhea, and therefore pelivic inflammatory disease (PID)
STDs are also more common in minority groups in the US, and those in low economic settings
Page 699 – STD complications experienced only by women
Most agents that cause STDs enter the body through mucosal or squamous epithelial layers of the
cervix, urethra, rectum, oral pharynx, and vagina
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Exception: HIV is mainly transmitted through sex, but can also be spread by blood
Nearly all STD-causing agents are sensitive to chemical and physical factors, and are practically
never found free in the environment
The most common reservoir of an STD is an asymptomatic person
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All agents that cause STDs are able to resist host defense and are infectious
o They can attach to and enter tissue easily
The acute manifestations of the most common STDs cause either mucopurulent cervicitis and urethritis,
or genital ulcer disease
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STD causing agents tend to cause primary lesions at or near the site of entry
o It’s not uncommon for this primary lesion to be somewhere hard to see, and go
unnoticed
o The most serious consequences of STDs are due to progression to chronic infections:
 Pelvic inflammatory disease (PID)
 Anogenital cancer – includes cervical cancer
 Secondary and tertiary syphilis
 Recurrent herpes infection
o These chronic infections can lead to htings like:
 Fallopian tube scarring and adhesions – can cause infertility, chronic pelvic pain,
and ectopic pregnancy
 Congenital diseases – ex: syphilis
 Increased risk of acquiring HIV – due to genital ulcers or changed genital mucosa
 Adverse outcomes of pregnancy – includes abortion, low birth weight
Pelvic inflammatory disease (PID, aka female upper reproductive tract infection)
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PID is an ascending infection of the uterus, fallopian tubes, ovaries, and adjacent peritoneum
It’s most often caused by N. gonorrhoeae (gonorrhea) or C. trachomatis (chlamydia)
PID often causes irreversible infertility, ectopic pregnancy, and chronic pelvic pain
Over half of cases are subclinical, meaning they’re asymptomatic or only shower minor
symptoms until late in the disease
Endogenous organisms from the lower GU can sometimes get involved
o Includes mycoplasmas and E. coli
Usually, a primary episode of gonococcal and chlamydial PID is followed by episodes of PID
caused by the endogenous organisms
Gonococcal or chlamydial cervical infection may damage the endocervical canal, break down the
mucus plug in the endocervix, and allow these pathogens, as well as endogenous vaginal
organisms, to ascend to the upper genital tract
Adolescent females have a larger zone of ectopy than older women
o This area isn’t protected by cervical mucus, and causes increased susceptibility to
gonococcal and chlamydial infections, because its columnar cells are preferred sites for
the microbes to attach to
Cilia moving mucus towards the uterus, and contractions during menses (expel endometrium)
are some host defenses to prevent spread
Oral contraceptives may decrease the risk of chlamydial PID
Intrauterine devices may increase the risk of PID within the first few months of placement
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Vaginal douching increases the risk for PID
Most cases of gonococcal PID happen during or near the end of menses
Hormone changes during the menstrual cycle may lead to changes in the cervical mucus plug,
allowing passage of organisms, especially when estrogen is ↑ and progesterone is ↓
Gonococcal PID – N. gonorrhoeae infection causes slowed to stopped movement of cilia on the
epithelial cells, which then get selectively removed from the cell due to toxic LPS or murein
o Gonococci then uses pili to attach to the nonciliated epithelial cell, then get internalized,
move to the basal part of the cell, & exit into the subepithelium to cause inflammation
o This causes progressive mucosal cell damage and submucosal invasion, leading to WBCs
and an inflammatory response, as well as a purulent exudate
o Tissue damage is caused by lipooligosaccharide (LOS) and peptidoglycan on the bacteria
Chlamydial PID – the bacteria attaches to the cell and then gets endocytosed
o The phagosome then doesn’t fuse with lysosomes, due to stuff on the surface of the
elementary bodies (term for infectious part of the organism)
o These chlamydia-containing phagosomes are called inclusions
o In the phagosome, the elementary bodies differentiate into reticulate bodies (metabolic
form of the agent)
o Eventually, the reticulate bodies reorganize into elementary bodies, and get released to
infect adjacent cells
o Chlamydia cause more of a macrophage response than gonorrhea, but neutrophils do
still respond, mainly early in the inflammation
Coinfections by gonococci and chlamydiae – when gonorrhea stimulates endocytosis in
nonciliated epithelial cells, it changes the surface of the cell, allowing easy uptake of chlamydia
Bacterial vaginosis - Asymptomatic or symptomatic disruption of normal vaginal flora
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Characterized by a smelly vagina discharge
The most common vaginal infection in the world
The normally dominant colonizing lactobacilli get ↓, and other bacteria ↑ in #
Bacterial vaginosis has also been linked with causing PID
HIV and its role in other STDs:
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HIV infection risk can be increased by other STDs, and treatment for STDs may not respond
normally due ot HIV, making the STD more severe
o So HIV and STDs amplify each other
The risk for HIV increases a lot when there are STDs
o Exceptions are HPV and anogenital warts
The compromised immune system of people with AIDS may cause severe cases of other STDs
It’s likely that the inflammatory changes caused by STDs facilitate HIV entry by changing the
barrier function of the genital mucosa epithelium
o Ulcer causing STDs (ex: syphilis, herpes) create an easy portal of entry for HIV
o All STDs recruit inflammatory cells, which can then be targets for HIV invasion
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Local inflammation also produces cytokines, that can enhance local HIV replication
Hepatitis B and HPV can be prevented by vaccines