Vaginal yeast infections can be caused by a number of fungal species – of which the most common is Candida albicans. That is the same organism that is responsible for most cases of oral thrush, and so it is quite natural to wonder whether or not yeast can be transmitted from the mouth to the vagina, or vice versa, during oral sex.
The answer, however, is not at all clear. While there have been numerous studies of sexual transmission of yeast infections, the results have been decidedly mixed. Some studies have found that both members of a couple are occasionally, but not reliably, infected with the same strain of yeast. However, other research suggests that when this occurs the similarities between the yeast infecting the two partners are usually only superficial. In other words, both colonies of yeast may be from the same general species (i.e. Candida albicans) but their respective metabolic or genetic profiles are different enough to suggest that the strains probably originated from different sources.
Although it isn't precisely the same as sexual transmission, there is somewhat more data on the transmission of vaginal yeast infections to infants during delivery, and it largely shows similar results. As one might expect if the yeast were directly transmitted from mother to infant, thrush infections are more common in infants born vaginally than in those delivered by c-section; however, as the strains infecting the infants have rarely been found to be the same as the ones infecting the mother, direct transmission seems unlikely.
Taken as a whole, the bulk of the evidence on the contagiousness of yeast infections seems to suggest that sexual transmission does not play a major role in vaginal or oral yeast infections, but there is some evidence that suggests that caution may be in order for women who have experienced recurrent vaginal yeast infections. A small study found that clearing up reservoirs of yeast in a partner’s mouth, ejaculate, or rectum – combined with restricting sexual activity until the yeast were eliminated – was an effective way to get rid of recurrent yeast infections in women who had previously repeatedly failed to effectively respond to direct treatment.
Therefore, although most women probably do not need to be particularly concerned about sexual transmission of yeast infections, women who suffer from recurrent vaginal yeast infections may want to discuss the benefits of partner testing with their doctors. In addition, regularly practicing safer sex for vaginal and oral sex may help as well, since doing so may reduce the likelihood of coming into contact with yeast in your partner's secretions.
As for avoiding yeast infections generally, they have been linked to several systemic health conditions such as HIV and diabetes. People on steroids are also at greater risk for yeast infections, as are those who have recently been on antibiotics. That last factor may seem counter intuitive, but yeast are almost always present in our systems. They only become a problem (i.e. a yeast infection) when they overgrow the rest of the normal flora after some sort of physical imbalance - something which the anti-candida diets and other lifestyle interventions also try to counteract.
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