Just to be clear I’m talking about a dog, a female dog, the one that is the subject of this article and the occurrence of probably the rarest and most unusual form of endometriosis I’ve ever seen.
We know that endometriosis can occur in some strange places, like in the brain or in the heart, or in some animals you wouldn’t expect, like monkeys, or in some people you wouldn’t expect, like men or very young children, but this is a stand-out case.
There are several reasons this case stands out so much to me. Firstly is that, looking at all the other odd occurrences of endo, whilst they are indeed odd, they are not entirely unforeseen. For example, endo occurs in humans, so to find it other parts of the human body apart from near the reproductive organs and to find it in young girls or men is weird, but not totally irrational. By the same logic, because we are so closely related to primates genetically, it is not unexpected that some species of ape or monkey will have endometriosis too. What is unexpected though is endo in an animal very dissimilar to humans and one that, by all accounts, has never been shown to have endo and shouldn’t be able to have it.
To summarise this case, an 11 year old female German Shepherd was brought to a vet in Brazil, but died unexpectedly during an examination. Upon dissection the vets found a large growth behind the uterus and ovaries which they later identified as an endometrioma, a large cystic growth filled with solid tissue and blood. Endometriomas are cysts where the outer layer is made up of tissue resembling the normal endometrium and is filled with blood that can become broken down to a brown fluid – giving rise to the name ‘chocolate cyst’. Endometriomas commonly arise in the ovary in women, but can occur in other places like behind the uterus and on the ligaments that hold the uterus in place.
Why would we not expect to see endometriosis in dogs? Mainly because the reproductive system of a dog is very different to that of a human. Dogs do not have a menstrual cycle like humans, they have an estrous cycle and while the hormonal changes across the estrous cycle can be seen as similar to the menstrual cycle, the physical changes are quite different and the time scales between the stages of different cycles vary significantly. One of the most important examples is that if the dog doesn’t become pregnant, the endometrium will be reabsorbed by the uterus, whereas in human the endometrium is shed during menses. This raises the question of how the endometrioma appeared.
For a long time now it has been suggested that endometriosis arises from endometrial cells shed during menses passing backwards through the fallopian tubes and out into the pelvic cavity, where they implant and grow into endometriotic lesions. This is a process termed ‘retrograde menstruation’ and is favoured by many (but not me) as the best explanation of the origin of endometriosis. How then could we have an endometrioma in an animal that doesn’t menstruate? A theory put forward many years ago, called coelomic metaplasia, suggested that certain forms of endometriosis arise from hormonal changes, which induce transformation of the thin layer of tissue that surrounds the reproductive organs (the coelomic mesothelium). This seems a logical explanation for endometriosis of the ovary as the ovaries produce high levels of estrogen. It also provides a decent explanation for the extremely rare instances of endometriosis in men, all of whom had been undergoing hormone therapy for prostate cancer. The authors of this article also point out that tumour and other types of cysts derived from the coelomic mesothelium are fairly common in female dogs, meaning this could be another, overlooked type. Could this explain an endometrioma in a dog? Possibly, although the vets noted no hormonal imbalances in the dog and no pathology of the ovaries that would result in altered hormone production. One other discovery that may give a hint as to how this endometrioma arose was the presence of bundles of smooth muscle found in the cyst. Normally smooth muscle wouldn’t be found in an endometrioma, so its presence suggests not a simple endometrioma, but a uterus like mass. How this might come about is speculative, it could be a birth defect that was only diagnosed when it became serious.
A big question then is – how come this is the first case of an endometrioma being reported in dogs? Dogs are brought to the vet all the time, there are probably millions of dogs visits to vets every year, why is this the first case? The short answer to that is, I don’t really know. It may be that endometriomas are simply not recognised as endometriomas in dogs. In this case study the authors state “based solely on the macroscopic mass, especially the red color and the presence of cavities filled with coagulated blood, an initial diagnosis of hemangiosarcoma was made in this dog”. So it could be endometriomas are being mistaken for cancer in dogs, but I don’t know if microscopic analysis is routinely performed on dog tumours so it’s hard to tell if this is the case.
The incidence of endometriosis in non-human species is an area that has received very little attention save for a few species of monkey used in lab experiments. It may be that endometriosis is extremely rare in non-human species, or it is underdiagnosed either due to lack of awareness or microscopic analysis of animal tumours isn’t a routine procedure. Either way it would appear endometriosis is not a uniquely human concern.