The risk factors for PID strongly reflect those of any sexually transmitted infection - young age, multiple sexual partners, lack of condom use, lower socio-economic status and Black Caribbean/Black African ethnicity. What is less certain is why some women with lower genital tract infection go on to develop upper genital tract disease - what factors encourage infection to spread from the vagina or cervix to the endometrium and fallopian tubes?
Cervical mucous provides an important barrier to ascending infection. Young women, with anovulatory cycles, have thinner cervical mucous and this, combined with higher rates of cervical ectopy and riskier sexual behaviour, may account for their high rates of PID. The ability of the immune response to control and contain infection will also determine the risk of upper genital tract involvement. Part of that immune response is genetically determined and an increased risk of PID is observed in women of HLA sub-type A31, while women with HLA
DQA 0501 and DQB 0402 have lower rates of infertility following a diagnosis of PID. It is also possible that certain strains of bacteria are more likely to cause PID than others but the evidence for this is limited, for example, serogroup A Neisseria gonorrhoeae, serovar F Chlamydia trachomatis.
Differences in behaviour have been linked to the risk of PID. A clear association can be seen between vaginal douching and PID but more recent longitudinal studies suggest that douching does not cause PID - rather it would appear that the vaginal discharge and menstrual irregularities associated with PID may themselves lead to more douching . Women who smoke are at higher risk of PID but it is unclear whether this is a marker for sexual high-risk behaviour or a direct effect of smoking itself.
Many women with PID also have bacterial vaginosis with an overgrowth of the normal commensual bacteria in the vagina and loss of vaginal lactobacilli. These same vaginal commensual bacteria are often isolated from the upper genital tract raising the possibility that bacterial vaginosis may lead to PID. Longitudinal studies do not support a direct causal association, although women who catch gonorrhoea or chlamydia infection are at higher risk of PID if they also have pre-existing bacterial vagi-nosis suggesting some synergy between the different infections .
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