Determinants of STI transmission

The determinants of STI transmission can be thought of as a continuum1,2 as shown in Figure 1. Opportunities exist at each of these steps to prevent and control sexually transmitted infection. For the primary care physician, many interventions are directly applicable to daily practice.

Figure 1. The continuum of STI ^1,2

Exposure of susceptibles to infected individuals can be influenced through counseling of patients to reduce early sexual debut and concurrent sexual partners, and to promote the performance of safer sexual activities, including:

1. nonpenetrative sexual acts, 
2. consistant and correct use of condom, and 
3. STI and HIV testing followed by commitment to monogamy.¹

Efficiency of transmission during exposure between susceptible and infectious partners can be influenced by a number of primary care physician interventions. Advising patients to use “safer sex” practices such as condom and avoidance of unprotected vaginal or anal intercourse can substantially reduce the transmission of some STI pathogens.

It is important to point out, however, that some viral pathogens such as HSV and HPV are not as effectively controlled as chlamydia or HIV with barrier methods such as condoms. Immunizations for HBV can very effectively prevent transmission of infection in both individuals and the general population if universal immunization is reasonably applied. The application of epidemiologic treatment to patients who have had a known exposure to a STI can reduce transmission presumably by treating patients either before symptoms develop or during the incubation period of the infection. Other forms of post- exposur e prophylaxis have been also been used to prevent HIV infection following needle - stick, or sexual assault exposure.

Finally, physicians can reduce the infectivity of some infected individuals by the use of suppressive antimicrobial therapy. An example of the latter is the use of acyclovir in genital herpes and AZT in HIV-infected pregnant women¹.

Duration of infection is the area where medical care by physicians has the largest impact. This involves monitoring the patient population for the prevalence of varying STIs and developing a patient encounter- based strategy for screening individuals for asymptomatic and new infections. This allows for timely interventions to treat and manage infected individuals to reduce or eliminate transmission of STIs. Incorporating these strategies into daily practice is essential if a physician hopes to have a lasting effect on controlling STI in his or her patients and community. Active case finding through routine screening of sexually active patients and especially those identified with high-risk core groups is an essential first step. Physicians should be aware that patients in the 16-24 year old age group have the highest incidence of STIs.

Table of Trends in the Epidemiology of STI in Canada