Cushings and immune system - technical question!?

Yes. The reason we don't use corticosteroid ointments on wounds is because they suppress the cytokines required to launch inflammatory responses needed to clot and heal the wounds and ward off infections. The beneficial antiinflammatory effects of corticosteroids being injected or inhaled is always weighed against their immunosuppressive effect along with other considerations. Inhalers used by asthmatics employ corticosteroids to alleviate allergic and.hypersensitivity symptoms and restore breathing.

Equine Cushing's (PPID) involves pituitary gland oversecretion of ACTH (adrenocorticotropic hormone). ACTH from the pituitary normally targets the cells of the adrenal cortex to trigger secretion of the cortical steroid hormones as needed. Oversecretion of ACTH means the adrenals are in turn triggered to oversecrete their hormones (glucocorticoids, mineralocorticoids and androgens) even when they are not needed, which among other things has an immunosuppressive effect throughout the body.

Excess hormones such as cortisol ( ex: hydrocortisone) function like the cortisone derivatives routinely administered as drugs to suppress inflammatory responses triggered by allergies. The excessive circulating corticosteroids in equine Cushing's suppress immune responses triggered by allergies in the same way.

Pergolide mesylate (FDA approved brand Prascend) is a dopamine agonist. This means it functions in the same way that endogenous (manufactured within the body) dopamine does to inhibit the excessive secretion of ACTH.

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Add per cyproheptadine.......... I know this isn't relative to the question, but since it came up, cyproheptadine is a serotonin antagonist as opposed to a dopamine agonist, and the role of serotonin in regulation of pars Intermedia dysfunction is uncertain. It was originally prescribed for horses because it had been used effectively in treating human Cushing’s disease, however human Cushing's differs significantly from equine Cushing's in the mechanisms of its development. Serotonin concentration in the equine pars intermedia is basically the same in horses definitively identified with PPID as it is in normal horses, so the rationale behind treatment with cyproheptadine is still questionable. Comparison studies of groups of horses treated with either pergolide or cyproheptadine have shown that cyproheptadine is not as effective in improving clinical signs of PPID, but in horses showing minimal response to pergolide alone, the combination of the two drugs may improve treatment efficacy.

Since an allergic response is a reaction of the immune system, it would make sense for a horse with a limited immune system to no longer suffer from allergies. The only horse I have known with Cushings did not have allergies or react to bug bites for the time I knew her, while my healthy gelding would get visible bug bites.

Sometimes as horses get older, they do get relief from allergies without developing Cushings. I had an OTTB with allergy related COPD. From the time he was 10ish, it got progressively worse until he was 18ish - at which point it basically disappeared overnight. Vets just said he "outgrew" it, that it happened sometimes. We evented him until he was almost 25. We retired him when my daughter went to college. About 2 years later, at pasture and in glowing health, he coliced and could not be saved. Necropsy showed no signs of a suppressed immune system; it was a mechanical failure.

I've also known a horse with severe allergies that worsened with age and Cushings. That horse did not have a suppressed immune system, as far as I know; he succumbed to chronic laminitis (cushings related).

I currently have 2 ponies with Cushings (22 and 24) and I've had 5 now deceased horses with Cushing's syndrome, three of them with suppressed immune systems, two with other issues. If Cushing's has suppressed your horse's immune system, it's imperative that you treat the cushings in order to help that immune system. If you don't... the horse will die. Of my 5 cushings patients, one died of a cerebral event from the pituitary expansion impinging on the brain (at 35), one died of unrelated causes (at 18), one died of a coffin bone infection of unknown origin (at 32), one died of chronic lung inflammations (30), and one of liver failure due to infection of unknown origin (32). I had all but the 18 yr old for more than 20 years each, and all were given the best treatment available for their issues, including Pergolide for the Cushings. All but the 18 year old lived for at least 5 years after the diagnosis. The two ponies are still going strong.

This may seem like a lot, but I've had multiple horses for over 50 years, that I've kept at my home, under my care, throughout their lives. Right now I have six horses; if all goes well I'll keep these until they also die of issues of old age, like Cushings, as well.

Cushings is such a strange syndrome, with so many different symptoms possible, that I doubt you can prove or disprove your theory. It all depends how the syndrome affects your horse, and as I personally know of cases where allergies improved without cushings and where allergies worsened with cushings, I suspect it is, at best, apocryphal.

It is an interesting theory, though! Good Luck with your horse - I hope you have many years left to enjoy together.

Edit: Found an interesting connection between cushings and itchy skin - cyproheptadine is used to treat both conditons:

http://www.drsfostersmith.com/product/prod_display...

It acts as an antihistamine and serotonin blocker, and treats both allergies and equine cushing's sucessfully.

I don't know what your vet has prescribed for your horse's cushings, but in your situation I definitely would ask about cyproheptadine. I have a friend who has used this successfully for cushing's; it would be interesting to hear your results if you use it.