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25% of infertile women do not know the cause. Istock

A virus from the human herpesvirus family has been identified in the uterus lining of women with unexplained infertility, scientists have said. HHV-6A infections also appear to be exacerbated by high levels of hormone estradiol and could explain these women's inability to conceive a child.

In the UK, about 3.5 million couples are infertile. About a quarter of those cases remain unexplained, as scientists struggle to pinpoint the exact problem. This study, published in PLOS Pathogens, could change the way doctors address the issue, by providing them with a new diagnosis tool of feminine infertility.

Little is known about HHV-6A – except that the virus was discovered in 1986 and is one of nine human herpesviruses. It is generally not detectable in the blood or saliva, and its true prevalence in the population is therefore unknown.

Previous research has linked the presence of HHV-6 DNA with miscarriages, pregnancy induced hypertension, and premature preterm rupture of membranes, but so far its impact on fertility had not been identified.

43% of women

In this study, the authors analysed uterus lining samples from thirty women with unexplained infertility and from36 fertile controls. 13 of the 30 infertile women – about 43% - tested positive for HHV-6A, which was not the case for any of the fertile women.

It is unusual to find traces of HHV-6A in the uterus of so many women. The virus is indeed rare in the general population compared with the very similar virus HHV-6B – which in this study was found at equal levels in the peripheral blood of patients and controls. This suggests that HHV-6A could really be to blame for the women's infertility.

The scientists also showed there was a strong correlation between the level of estradiol and the presence of a HHV-6A infection. Women with the virus had a higher probability of having high estradiol levels. Abnormal levels of cytokines – small secreted proteins released by cells to regulate the interactions between cells – were also observed.

It may be interesting to investigate these anomalies further, in order to help women with unexplained infertility. "This is a surprising discovery", says Anthony Komaroff, a professor of Harvard Medical School who has studied HHV-6. "If confirmed, the finding has the potential to improve the outcome for a large subset of infertile women".

Future research could focus on finding an effective treatment against the infection. There are currently no approved drugs specific to HHV-6A or HHV-6B, although doctors may prescribe drugs designed to treat herpesvirus-5. Finding out whether such antiviral treatments would help women with a HHV-6A uterine infection may be a step forward in the fight against infertility.