Microbiology and Impact of STIs Part 2 Flashcards
1. Identify basic epidemiologic trends (incidence, patterns of spread) for sexually transmitted infections (MKS-1f) 2. Describe how anatomic, biologic, social and behavioral factors affect STI acquisition and prevention (MKS-1b, CES-1) 3. Identify effective prevention efforts for the major STIs (MKS-1b,d,e,f) 4. List the impact of STI's on women's health and reproductive outcomes (MKS-1b) 5. Explain the mechanism of transmission and effective prevention strategies for STIs which are perinata
1
Q
Describe the distribution of STIs in the US.
A
- While rates of some STI’s are declining, others are increasing (and not merely due to increased surveillance or reporting)
- Rates of new STIs vary from 56,000/year for HIV to about 3 million/year for Chlamydia and over 6 million/year for HPV.
2
Q
Describe the factors that influence the prevalence and incidence of STIs.
A
- The prevalence and incidence of STIs vary widely and reflect:
- the biology of the specific pathogens
- host (age, gender, anatomy, genetic influences)
- environmental (social, cultural, political, economic)
- behavioral (mode of transmission) factors
- Whether infected individuals are symptomatic and seek care and the consequent delays in diagnosis and treatment also influence STI incidence and complications
3
Q
Describe the equation that describes the rate of acquisition or spread of an STI.
A
-
Ro = βcD
- Ro=case reproduction ratio
- β=efficiency of transmission (will be different for each pathogen)
- c=rate of change of sexual partners (sexual concurrency & sexual networks are important, not just total number)
- D=duration of infectiousness (Ro increases for STIs w/ a long duration of infection like HIV)
4
Q
What factors put women more at risk of acquiring STIs?
A
- While behavioral and socioeconomic factors are extremely important determinants of this disparity, biologic factors also play a crucial role
- Biological differences do put women, especially young women, at disproportionate risk of STI acquisition through heterosexual sex
- Per episode of unprotected receptive vaginal intercourse, a woman is almost twice as likely to acquire a pathogen from an infected male partner compared to a man becoming infected from his infectious female partner
5
Q
What are the major biological factors that influence HIV infection?
A
- Status of the mucosal surface
- Viral inoculum presented to it
- Presence and state of activation (immune response) of the target cells
- Biological factors identified as important determinants of sexual transmission of HIV across mucosal surfaces include
- for the infected donor – stage of illness (pathogen load)
- treatment status
- immune status
- presence of another STD, viral phenotype and for the uninfected recipient – presence of an STD
- genetic susceptibility
- hormonal status and integrity of epithelium
- presence of target cells
- mucosal immune response
6
Q
Why are men more likely to pass on HIV?
A
- There is a higher concentration of HIV-1 in semen as compared with cervicovaginal secretions
- Therefore, exposure to semen carries a higher risk than exposure to cervicovaginal secretions that may partially explain the higher risk for gay men and heterosexual women as compared to heterosexual men and lesbians.
7
Q
Why is receptive anal intercourse and receptive vaginal intercourse more likely to transmit HIV?
A
- The mucosal surface of the anus and lower rectum is both more easily traumatized and may contain more target cells (monocytes, macrophages, dendritic cells, lymphocytes) than the epithelium of either the male or female genital tracts, perhaps explaining the higher risk of transmission seen in people who engage in receptive anal intercourse.
- The female genital tract contains target cells capable of becoming infected with HIV.
- These dendritic macrophages called Langerhan’s cells, exist in the submucosa throughout the lower female genital tract which has a much greater surface area than the male genital tract.
- This, coupled with the greater concentration of HIV in semen, may account for the higher rate of transmission from men to women as compared with female to male transmission among heterosexual couples.
8
Q
How do other STD’s increase the chance of HIV infection?
A
- STD’s (both ulcerative and non-ulcerative) may increase the number of these target cells present in genital tract epithelium, thereby increasing the susceptibility of an individual.
- This may explain the “epidemiologic synergy” between other STD’s and HIV and emphasizes the importance of STD diagnosis and treatment in the prevention of HIV transmission.
9
Q
What are some HIV prevention strategies?
A
- Traditional yet effective prevention strategies focus on treatment of the infected partner to lower viral load and use of condoms
- Newer proposed interventions focus on the use of ARVs for pre and post exposure prophylaxis of susceptible individuals, microbicides and circumcision
- Vaccine development has been painfully slow and only this year has even minimal efficacy been demonstrated for the first time.
10
Q
What are some immediate consequenecs of STIs?
A
Some STI’s produce acute infections of the genital tract which require medical therapy to alleviate the immediate consequences (cervicitis, bleeding, pain and discomfort).
11
Q
What are the chornic problems that some STIs such as chlamydia and gonorrhea can cause?
A
- Chronic pelvic pain
- Pelvic inflammatory disease
- Increased ectopic pregnancy rates
- Subsequent infertility
12
Q
What are the outcomes of PID?
A
- If not adequately treated, 20-40 percent of women infected with chlamydia and 10-40 percent of women infected with gonorrhea develop PID
- Among women with PID, subsequent tubal scarring will cause infertility in 20 percent, chronic pelvic pain in 18 percent and ectopic pregnancy in 9 percent
- Approximately 70% of chlamydial infections and 50% of gonococcal infections in women are asymptomatic
- These infections are detected primarily through screening
- The vague symptoms associated with PID cause 85% of women to delay seeking medical care, thereby increasing the risk of infertility and ectopic pregnancy
13
Q
How do STIs increase the risk of ectopic pregnancies?
A
- The total number of ectopic pregnancies in the US in 1992 was estimated at 108,800 or 19.7/1000 pregnancies, the highest level in more than two decades
- Ectopic pregnancy is more dangerous (38 deaths / 100,000 events) than either childbirth (9 / 100,000) or legal abortion (<1/ 100,000)
- In 1992, ectopic pregnancies accounted for approximately 2% of reported pregnancies, and ectopic pregnancy- related deaths accounted for 9% of all pregnancy-related deaths
- Because at least 50% of ectopics are now treated with outpatient medical management (methotrexate), hospital-based surveillance data is no longer an accurate reflection of the incidence.
14
Q
What STIs lead to death and how?
A
- Some STI’s ultimately lead to death in ways unrelated to ectopic pregnancy:
- oncogenic forms of human papilloma virus (cervical cancer)
- hepatitis B (liver cancer)
- HIV (AIDS)
- AIDS is still the 4th leading cause of death for African-American reproductive age women in the US
15
Q
How are STIs transmitted from mother to fetus and what are some of the risk factors?
A
- Maternal infection can be transmitted either transplacentally during gestation or transvaginally at the time of birth
- Similar to sexual transmission, maternal pathogen load, immune status and disease stage are all important factors
- The relevant interface in perinatal transmission is the placenta (maternal-fetal interface) and the genital tract mucosa
- The placental contains macrophages (Hofbauer cells) which may act as the target cell for transplacental infection
- Fetal factors such as gestational age (premature fetuses have more immature immune systems and may be more susceptible to acquiring infection) and post- exposure prophylaxis are important factors
- Relevant obstetric factors include duration of rupture of membranes, invasive procedures and route of delivery
- Breast Feeding is another route of transmission
