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New hypothesis proposed for how chlamydia might increase cancer and ectopic pregnancy risk

16 August 2021 

A review of evidence by researchers at the University of Bristol and University of Edinburgh has suggested a possible new means by which chlamydia could lead to an increased risk of cancer and ectopic pregnancy. The hypothesis also provides a possible explanation for how pelvic inflammatory disease may be triggered in some women.

Black & white image of shredded paper with text Chlamydia

The review, published in the Journal of Infectious Diseases, looked at evidence from lab-based studies, animal models and clinical studies on the role of chlamydia in diseases of the reproductive tract. 

The researchers’ analysis of the studies’ findings suggests that chlamydia induces a particular type of change in reproductive tract cells known as ‘epithelial to mesenchymal transition’ (EMT), which can lead to inflammation and cell growth. Their hypothesis is that this chlamydia-triggered cell change contributes to the development of further disease.

“Chlamydia is a bacterial infection that stimulates EMT, which may persist after the chlamydia infection has cleared,” explains Dr Paddy Horner from the NIHR Health Protection Research Unit in Behavioural Science and Evaluation at University of Bristol, who led the review. 

“We think that the association of chlamydia with ovarian and cervical cancer could be explained by the persistence of EMT changes in combination with DNA damage caused by chlamydia following chlamydia infection”, he said.

“Also, we know that EMT cells impair the integrity of the lining of the infected reproductive tract cell, making it more susceptible to invasion by other bacteria. This increases the risk of pelvic inflammatory disease from those invading bacteria. 

“Furthermore, epithelial (barrier) cells in the fallopian tube that have previously been infected with chlamydia have more receptors on their surface, which are associated with an increased risk of ectopic pregnancy. There is evidence that these cell surface receptor changes could be caused by EMT.

“If our hypothesis about the role of EMT following chlamydia infection in women is correct, it could help explain some of the recent epidemiological observations about chlamydia and reproductive disease which are difficult to account for using current concepts about the immune response to chlamydia.

“It would also support the English National Chlamydia Screening Programme’s shift to earlier testing of women, as the shorter the duration of infection, the lower the risk of developing EMT changes. Further down the line, this could lead to the development of new tests for identifying women at increased risk of ovarian cancer and ectopic pregnancy and interventions that could reduce these risks.

“Obviously a lot more research is needed before we can be sure that our hypothesis is correct, but the evidence from this review suggests that further research in this area would be fruitful and could have important benefits both for patients and in the prevention of chlamydia-induced disease in the long-term.”

Munira Oza, Director of the Ectopic Pregnancy Trust, said: “This analysis helps to further our understanding of one of the possible risk factors for ectopic pregnancy and we would welcome more research in this area. 

“It also highlights the importance of the change of focus of the National Chlamydia Screening Programme to opportunistically making proactive offers of a chlamydia test to young people without symptoms to reduce the risk of reproductive harm. 

“With early detection through the screening programme and much-needed education to reduce the stigma of chlamydia, we hope that many women and families might be spared the health risks and heartache of ectopic pregnancy. We encourage young women to screen when given the opportunity.”

Chlamydia is the most common bacterial sexually transmitted infection worldwide. If left untreated, it can cause pelvic inflammatory disease, ectopic pregnancy, tubal factor infertility, and chronic pelvic pain due to tubal scarring.

Paper: Is There a Hidden Burden of Disease as a Result of Epigenetic Epithelial-to-Mesenchymal Transition Following Chlamydia trachomatis Genital Tract Infection? by Paddy Horner, Heather Flanagan and Andrew Horne in Journal of Infectious Diseases.


Further information

About the NIHR Health Protection Research Unit [HPRU] in Behavioural Science and Evaluation at the University of Bristol 

The NIHR HPRU in Behavioural Science and Evaluation at University of Bristol is one of 14 HPRUs across England, part of a £58.7 million investment by the NIHR to protect the health of the nation. 

The NIHR HPRU in Behavioural Science and Evaluation is a partnership between Public Health England and University of Bristol, in collaboration with MRC Biostatistics Research Unit at the University of Cambridge and University of the West of England. 

Each NIHR HPRU undertakes high quality research that is used by PHE to keep the public safe from current and emerging public health threats.

About the NIHR 

The mission of the National Institute for Health Research (NIHR) is to improve the health and wealth of the nation through research. We do this by:

  • Funding high quality, timely research that benefits the NHS, public health and social care;
  • Investing in world-class expertise, facilities and a skilled delivery workforce to translate discoveries into improved treatments and services;
  • Partnering with patients, service users, carers and communities, improving the relevance, quality and impact of our research;
  • Attracting, training and supporting the best researchers to tackle complex health and social care challenges;
  • Collaborating with other public funders, charities and industry to help shape a cohesive and globally competitive research system;
  • Funding applied global health research and training to meet the needs of the poorest people in low and middle income countries.

NIHR is funded by the Department of Health and Social Care. Its work in low and middle income countries is principally funded through UK Aid from the UK government.

About The Ectopic Pregnancy Trust 

The Ectopic Pregnancy Trust provides information, education, and support to those affected by ectopic pregnancy and to the health professionals who care for them. The charity seeks to raise awareness about the condition so that the public are aware of symptoms to enable women to get medical care quickly that can be life-saving. It ensures women and families leave hospital with relevant information and have someone to talk to about their experience. The NHS signposts to The EPT's resources and its Ectopic Pregnancy Patient Information literature was Highly Commended in the 2019 BMA Patient Information Awards.