What causes thrush?
Thrush is an infection caused by types of fungal yeast called Candida spp. Small numbers of Candida spp. commonly live on the skin and around the vaginal area. These are usually harmless. The immune system and the harmless germs (bacteria) that also normally live on the skin and in the vagina usually stop Candida spp. from thriving. However, when conditions are good for Candida spp., numbers multiply and may invade the vagina and cause symptoms.
The conditions most liked by Candida spp. are warm, moist, airless parts of the body. This is why the vagina is the most common site for candidal infection. Other areas of the body that are prone to candidal infection include the groin, the mouth and the nappy area in babies.
Most causes of thrush are a result of Candida albicans but sometimes other types of Candida spp. such as Candida glabrata or Candida tropicalis are the cause.
Who gets thrush?
Up to three quarters of all women will have at least one bout of thrush in their lives. In most cases it develops for no apparent reason. However, certain factors can make thrush more likely to develop. The vagina contains mucus and some harmless bacteria which help to defend the vagina from candidal infection (and other germs). These natural defences may be altered or upset by certain situations:
- When you are pregnant.
- If you have diabetes.
- If you take antibiotic medication.
- If your immune system is not working normally. For example, if you are on chemotherapy for certain cancers, if you are taking high-dose steroids, etc.
So, in these situations, you may be more likely to develop thrush.
There is some relationship between thrush and the female hormone oestrogen. Thrush is much more common in the "reproductive years", ie the years between starting to have periods and stopping for menopause. Some women are prone to thrush in certain times of their menstrual cycle - for example, before a period. The hormone changes of pregnancy make thrush more likely. It may be that hormone pills such as hormone replacement therapy (HRT) and the combined oral contraceptive (COC) pill make you more prone to thrush. This is not yet certain, however.
What causes recurring thrush?
Some women develop repeated (recurrent) thrush. Recurrent thrush is defined as a bout of thrush four or more times in a year. Of women who develop a first bout of vaginal thrush, about 5 in 100 of them will get problems with recurrent vaginal thrush. In most cases, the reason why this occurs is not known. Some women just seem more prone than usual to develop thrush. However, women with uncontrolled diabetes and women with a poor immune system may be more likely to develop recurrent thrush. There is some debate as to whether women taking HRT or the COC pill are more likely to develop recurrent thrush - the evidence is not yet clear.
Further reading and references
Sexually Transmitted Infections in Primary Care; Royal College of General Practitioners and British Association for Sexual Health and HIV (Apr 2013)
Candida - female genital; NICE CKS, November 2016 (UK access only)
Martin Lopez JE; Candidiasis (vulvovaginal). BMJ Clin Evid. 2015 Mar 162015. pii: 0815.
British National Formulary; NICE Evidence Services (UK access only)
Abad CL, Safdar N; The role of lactobacillus probiotics in the treatment or prevention of urogenital infections - a systematic review. J Chemother. 2009 Jun21(3):243-52.
Fischer G, Bradford J; Vulvovaginal candidiasis in postmenopausal women: the role of hormone replacement therapy. J Low Genit Tract Dis. 2011 Oct15(4):263-7. doi: 10.1097/LGT.0b013e3182241f1a.
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