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.
