Granulosa Cell Tumors in Mares

Granulosa Cell Tumors in Mares

Whinny’s Wisdoms

Springhill Equine Veterinary Clinic

Spring is a-peeping around the corner, days grow longer, and the mares in the barn…well, they can get a little…interesting. Now, I, Whinny, am a field mouse of refined tastes (cheese, anyone?), and I appreciate a good mare’s temperament as much as the next critter. But sometimes, their behavior takes a turn for the puzzling. Is it just springtime shenanigans, or something more concerning?

We all know hormones are powerful little messengers in the body, like tiny knights delivering chemical decrees. But when there’s a surplus, well, chaos can ensue! In mares, one culprit behind this hormonal havoc is a sneaky villain called a granulosa cell tumor (GCT). Now, this might sound scary, but listen close – these tumors are usually benign, meaning they don’t spread like wildfire. However, they’re like mischievous imps, messing with the delicate hormonal balance and causing a whole host of problems.

Imagine a mare’s ovary. Normally, it’s a well-oiled machine, producing just the right balance of hormones for a smooth operation. But a GCT is like a rogue factory, churning out excess hormones like estrogen and testosterone. This throws everything out of whack, leading to a symphony of unwanted behaviors.

One telltale sign is a sudden shift in personality. A sweet-natured mare might morph into a grumpy Gus, exhibiting aggression and even acting stallion-like, with mounting and dominance displays (not cool, dude!). This is all thanks to the testosterone overload, turning our gentle friend into a temporary testosterone-fueled whirlwind.

Another red flag is a disruption in the mare’s reproductive cycle. Normally, mares experience regular estrous cycles, which is like their own internal calendar for breeding. But with a GCT, things can go awry. Persistent anestrus can occur, meaning the cycle disappears altogether, putting breeding plans on hold. On the flip side, some mares might experience continuous estrus, essentially being “in season” all the time. This can be frustrating for both the mare and her handler, as she might exhibit classic estrous behaviors like squealing and frequent urination.

Springhill Equine Veterinary Clinic

Now, before you panic if your mare seems a little off-kilter, here’s the good news: GCTs are detectable! My doctors have a toolbox of tricks to diagnose the problem. One method is rectal palpation, or a transrectal ultrasound. Think of it like a high-tech peek inside, allowing my docs to feel for an enlarged ovary, a signature symptom of a GCT. An ultrasound will show a very large ovary with tons of tiny follicles. The appearance is often described as a honeycomb. But that’s not all! They can also use blood tests to measure levels of equine inhibin, anti-Müllerian hormone, and testosterone to see if they’re out of balance.

Of course, sometimes mares can be grumpy for reasons unrelated to GCTs. Maybe she has a sore tooth or an ache in her leg that’s making her cranky (ouch!). So, a general checkup is crucial to rule out other potential causes of behavioral issues.

But let’s say the diagnosis confirms a GCT. Fear not! Treatment is available, and the most common option is surgery. Here’s the cool part: this surgery can often be done with the mare standing! Vets use a laparoscopic approach, which sounds fancy but basically means they make a tiny incision in the flank and use a special camera to remove the tumor. Minimally invasive and efficient – that’s what I call a win-win!

GCTs might be a challenge, but with proper diagnosis and treatment, mares can get back to their normal, well-behaved selves. The key is to be observant and consult your vet if anything seems amiss. After all, a happy and healthy mare makes for a happy barn, and that’s something every field mouse can appreciate!

Until next week,


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Whinny’s Wisdoms is the official blog of Whinny the Clinic Mouse at Springhill Equine Veterinary Clinic in Newberry, Florida. If you liked this blog, please subscribe below, and share it with your friends on social media! For more information, please call us at (352) 472-1620, visit our website at, or follow us on Facebook!

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