[small aside, my fingers wanted to type “COVID” instead of Cushings which…wow]
It’s been a while since I’ve done a Cushings update for Tristan, and this spring we did actually have some developments, so it’s time.
Longtime readers will remember that Tristan was diagnosed with Cushings in the fall of 2015, and I post periodically on how we are maintaining him with this strange, tricky, but nearly universal disease.
In February, the barn manager and I decided that injecting Tristan’s hocks again would be a good idea. Putting steroids into a Cushings horse’s body is always a dicey proposition, and I try to be as prepared as possible before doing it, so that meant a new ACTH test.
Surprise for me, the vet is now doing a new type of ACTH test, called a TRH suppression test. Basically, they pull blood, inject TRH (thyrotropin-releasing hormone), wait a period of time, and then pull blood again. The TRH more or less pokes at the pituitary gland to try to make it angry and stressed. A normal horse’s pituitary gland will react a little bit to that – a Cushings horse’s pituitary gland will react a lot.
For reference, we did a regular ACTH test in May 2021 ahead of his June 2021 hock injections:
Great news! Right in the middle of the range, on one mg of pergolide (as a Prascend pill) per day.
Here’s what the TRH test looked like in March 2022:
Super interesting change, right? His baseline ACTH levels are right where we want them to be, even a little bit better than last year. After the TRH injection, he shot way up – as you can see, nearly double the ideal reference interval. What does that mean, exactly? Reference intervals usually have an ideal range and then deviation from them tells you a lot more, so I set out to find out what a positive result of 210 meant in practical terms.
The most useful information I found came from this article, from Boehringer Ingleheim from 2020, based on actual scientific evaluation of the test:
There we go! According to the chart, Tristan is still positive for Cushings, which, of course he is. The good news, though, is as I suspected, right in the middle there: under “Equivocal.” He’s only what could be described as a mild positive, with 210 pg/mL hitting only a smidge over the true positive measurement. As the chart says, it’s subject to ongoing research, but this is the more nuanced look I was hoping to see. I’m glad to have access to the better testing, even though the cost was 2x a regular ACTH test.
Armed with that information, I made two decisions: the first was to bump him up to 1.5mg of pergolide a day, or a pill and a half of Prascend. It’s not an unexpected decision; Cushings is a progressive disease, and this bump up was always going to happen at some point. That mild positive, after all, is while medicated, which is an indication that the pergolide isn’t controlling his disease quite as well as it might.
The second decision was to go ahead with the hock injections. I did have a brief conversation with our lameness vet first, and shared the testing results with him, and he felt comfortable going ahead. I was also able to talk to him in-person while he did the injections, and stayed with him for a while after the injections. That was a few weeks ago, and all has gone well since.
In other Cushings-related things: he adjusted to the higher dose of pergolide just fine, with no real sign of difficulty there. He’s in really good shape overall, and his main Cushings symptoms are what they have always been: difficulty in building & keeping muscle, and a weakened immune system overall. He’s shedding out well, his energy is good, he’s still a nice easy keeper relative to the amount of food he gets, and honestly, he’s in pretty darn good shape for a 27 year old horse.