Endometriosis is a painful condition where the tissue that normally lines the endometrium grows outside the uterus. It affects around 1 in 10 women and the NHS has named it as of the top twenty most painful health conditions. Despite this, doctors still have no idea what causes endometriosis. This means diagnosis and treatment can be a lengthy and painful process for patients.

The lack of information around endometriosis is another example of the sheer size of the gender health gap, but there is some good news: an increasing number of researchers are studying endometriosis and beginning to understand more about this condition. 

This year it was announced that researchers at Sydney’s Royal Hospital have made a major breakthrough, having grown tissue from every known type of endometriosis, they’re now able to observe changes and compare how different types of endometriosis respond to treatments. 

More recently, researchers at Nagoya University in Japan published a study which examined the link between the development of endometriosis and Fusobacterium nucleatum, a type of bacteria. Cue the headlines, breaking news and clickbait: Endometriosis may be caused by bacteria!

So, is this the case? Have researchers finally discovered the cause of endometriosis? Let’s get into it.

What does the study actually show?

First things first, what does the study actually show, beyond the headlines? Well, to begin with, researchers examined endometrial and endometriosis tissue samples collected from women with and without endometriosis.

In 64% of the samples from women with endometriosis, fusobacterium nucleatum was present. By comparison, this bacteria was only present in 7% of the samples from women who didn’t have endometriosis. Alongside this, fusobacterium nucleatum levels were higher on vaginal swab samples from endometriosis patients than it was among people who did not have the condition. 

That basically means more than half of the people with endometriosis also presented with the fusobacterium nucleatum bacteria. 

But the study doesn’t end there. With that data, the team at Nagoya then used a mouse model of endometriosis to run further investigations. 

When the bacteria was introduced to the mouse model, the number and size of endometrial “lesions” increased compared to the control group of mice who weren’t infected with the bacteria. The researchers then ran tests with antibiotic treatment of metronidazole and chloramphenicol, which was shown to reduce these lesions. 

What does that mean when it comes to the cause of endometriosis?

The study does reveal that fusobacterium could play a role in facilitating endometriosis development. But it’s not as simple as that. 

There are some key elements missing from the study and gaps in knowledge when it comes to endometriosis in humans. 

The missing links

If you aren’t familiar with the concept of mouse models, don’t worry – I’ve got you. I spoke to Frances England, who has a Masters of Research in Stem Cell Biology and Medicine from the University of Cambridge and is currently a Wellcome Trust Scholar in the final stages of completing her PhD in Stem Cell Biology. She works with mouse models every day and filled me in on how endometriosis mouse models work. 

Here’s what she said. 

"A mouse model is supposed to be a model or replication of a disease in a mouse in a lab. But it’s really tricky when it comes to endometriosis. This is because, fun fact: most mice don’t menstruate or have periods like humans have periods. They have an estrous cycle which is completely different, which means they can’t get endometriosis. 

One school of thought behind endometriosis is that it arises out of retrograde menstruation. In simple terms, this is where your menstrual blood – rather than flowing out of your body – washes back up and ends up in other areas of your body, but obviously this doesn’t happen with mice. Because of this human endometriosis is very tricky to mimic in mouse models. This is part of why research into endometriosis is so limited."

The Nagoya University study on fusobacterium and endometriosis transplanted endometrial tissue into mice, as opposed to endometriotic tissue, disregarding the distinct differences between the two. As a result of this, it can’t actually be generalised and be applied to endometriosis in humans. Although the presence of bacteria in the mouse model of endometriosis seemed to increase the number and size of the “lesions”, it isn’t necessarily the cause of the lesions, and could just be a result of chance occurrence in this case. 

Beyond this, it's important to consider that other bacteria present in the vagina might also play a role in the development of endometriosis, however, this is currently unknown. Speaking to New Scientist, Christine Metz at the Feinstein Institutes for Medical Research in New York said “I would find it unusual that it’s a single pathogen that’s causing endometriosis, rather than a cluster of pathogens or many pathogens independently contributing to the disease.”

The bottom line

The study on the role of fusobacterium in facilitating endometriosis is a great step forward. It presents a new area for exploration when it comes to the cause of endometriosis, which has so far been focusing on retrograde menstruation, which some medical professionals view to be outdated. But it isn't the answer to the question 'What causes endometriosis?'. 

Further research is absolutely needed for us to say for certain that fusobacterium could be the cause of endometriosis but it’s incredible to see new research breaking through in the field.