Myths, truths about drugs prescribed in veterinary dermatology

Myths, truths about drugs prescribed in veterinary dermatology

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Jul 01, 2007

Corticosteroids and antibiotics are widely prescribed in veterinary dermatology, but there are controversies and even fears about both – over effects, dosages and other factors. Here are some common myths and what is actually known:

Corticosteroids are the most used and abused medications in veterinary medicine. To date, there has been no other type of drug with so many strong effects on the body.

Corticosteroids are considered indispensable in treating many diseases, life-threatening autoimmune disorders, anaphylaxis, transplantation rejection and allergies.

In my practice, they are an integral component in my treatment of severe allergies.

Yet they are feared by many clients (as well as some doctors), who may have unfounded suspicions about potentially life-threatening side effects.

What are these effects, and how likely are they to occur?

First, it should be noted that there is little or no detailed, scientific, peer-reviewed information about corticosteroids in veterinary medicine. What we have are basically guidelines or recommendations derived from clinicians' experiences and extrapolation from human literature. Lacking evidence-based articles, there is a plethora of misinformation.

Corticosteroids have never been proven to "shorten the lives of dogs." I believe that misguided statement probably originated from concerned veterinarians trying to convince owners to find corticosteroid alternatives. That is what I recommend if I believe their continued use would be unhealthy or the patient has experienced unwanted side effects.

I believe the quality of a dog's life may be poor if treated with long-term high dosages of corticosteroids. If not monitored, the animal may experience side effects – but an early, unexpected death is inconceivable.

Side effects are more likely in dogs than cats, more likely in older animals than younger ones and more likely with high dosages and long-term use.

But what constitutes long-term therapy? More than two months? More than six months? A year? No one really knows, but I would estimate more than three months.

The most common side effects of corticosteroids stem from the mineralocorticoid effects, not from glucocorticoid effects.

Although not as potent as the well-known mineralocorticoid aldosterone, glucocorticoids have specific effects on water and electrolyte balance in the kidney. Polyuria and polydipsia associated with glucocorticoids result from inhibition of antidiuretic hormone (ADH) secretion and decreased renal sensitivity to ADH.

Glucocorticoids also enhance potassium excretion and sodium retention by the kidney. They can increase renal excretion and decrease intestinal absorption of calcium, causing depletion of calcium stores.

Prednisone and prednisolone exert a moderate mineralocorticoid effect while methylprednisolone and triamcinolone exert a lesser effect. Since mineralocorticoids promote sodium and chloride retention (thus water retention), an increase in thirst (thus an increase in urine output) occurs.

Some literature states triamcinolone exerts no mineralocorticoid effects, but this cannot be completely true, as I have observed polydypsia and polyuria in dogs treated with oral and injectable triamcinolone.

It is possible, however, that some "pure" glucocorticoids directly exert mineralocorticoid effects. This has been documented in human patients. Polyphagia also is a relatively common side effect.

Finally, excessive panting also has been occasionally reported, but the mechanism of panting from glucocorticoid treatment is not fully determined.

Effects of long-term use

When dogs are treated with corticosteroids longer than several months, one may observe less common side effects, such as muscle wasting (mostly masticatory and dorsal trunk via the catabolic effects), pendulous abdomen (due to weak abdominal muscles and fat redistribution), truncal fat redistribution and, although controversial, proteinuria.

Less common and side effects of long-term corticosteroid therapy include signs of immunosuppression, often manifested by bacterial infections such as cystitis and bullous impetigo (superficial pyoderma). Very rarely, opportunistic microbial infections (fungal, protozoal) are documented, usually when patients are treated concurrently with other immunosuppressant agents such as cyclosporine, azathioprine or luflunomide.

Lastly, glucocorticoids have the ability to cause insulin resistance and thus (in rare instances) cause diabetes mellitus (DM). Cats may be more likely to develop DM from glucocorticoid administration.