Myths, truths about drugs prescribed in veterinary dermatology - DVM
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Myths, truths about drugs prescribed in veterinary dermatology


DVM360 MAGAZINE


In general, many of these side effects are more likely when higher dosages ("immunosuppressive") of corticosteroids are administered for long-term treatment. This so-called immunosuppressive dosage can range from 2-4 mg/kg/day in the dog. Anti-inflammatory (anti-pruritic) dosages generally are half or less the lower end of the immunosuppressive dosage. Cats require about double the canine dosage to achieve the same effects.

Antibiotics

The most common bacterium isolated from superficial and deep pyoderma in the dog is Staphylococcus intermedius (less commonly Staphylococcus schleiferi, and rarely Staphylococcus aureus). It is extremely rare to isolate other organisms from superficial lesions, but not uncommon to isolate other bacteria from lesions of deep pyoderma. Aerobic cultures from lesions of deep pyoderma can produce gram-positive (Streptococcus) and gram-negative organisms (Klebsiella, Proteus, Pseudomonas, Escherichia coli, Pasteurella). Because the focus of treatment is resolution of a Staphylococcal infection, cephalexin is the drug of choice (for reasons discussed below).

Cephalexin is a penicillin derivative, a first-generation cephalosporin or a beta-lactam antibiotic that is resistant to most penicillinases secreted by staphylococcal species. Amoxicillin and ampicillin are easily cleaved by penicillinases, rendering these drugs inactive and useless.

Cephalexin is reliable, inexpensive and generally well tolerated with little or no side effects. It is a time-dependent antibiotic, and thus plasma concentrations must remain above the minimum inhibitory concentrations (MIC) over the entire course of treatment to achieve a kill.

One of several controversies with cephalexin is the dosage and dose interval. The established dosage for dogs is 22mg/kg twice daily. It has been shown by veterinary pharmacologists that three-times daily dosages are not necessarily more effective, but that significant under-dosing often fails, as lower dosages can result in much lower plasma concentrations below the MIC.

We all realize that, when calculating a dosage based on a published dose, we never deliver the exact dosage (i.e., 22mg/kg twice daily). Instead, we deliver something a tad lower or higher, depending on the weight of the dog and availability of the drug size (i.e., 24.6 mg/kg).

This is the key to prescribing cephalexin properly to dogs: Always give the dosage at or slightly higher than the published dosage.

For example, it is tempting to prescribe 500 mg cephalexin twice daily to a 30-kilogram dog. This may result in failure, however, being well below the published dosage. Because cephalexin is safe, feel free to prescribe 750 mg orally twice daily. It's that simple.

Finally, because cefpodoxime is popular among veterinarians (and veterinary dermatologists), there are some issues to discuss.

Cefpodoxime is a beta-lactam, cephalosporin antibiotic. It is technically a third-generation cephalosporin, but, at least in the dog, kills mostly gram-positive bacteria like a first- generation cephalosporin. Thus it has virtually an identical spectrum as cephalexin and is not superior to it.

The once-daily dosaging, relatively fewer gastro-intestinal side effects and proven effectiveness have been attractive to veterinarians and clients. For dogs weighing more than 50 or 60 pounds, cefpodoxime can be cost-prohibitive to some.

Otoxicity with topical drugs

Ototoxicity is rare in the dog (most veterinarians feel its less than 1 percent of treated cases) and is mostly unpredictable. There has been, however, an ongoing debate over ototoxicity produced by topical medications applied in the ear canal when the tympanum is ruptured.

There have been many published reports of ototoxicity from topical products used on human patients (antibiotics, antifungals, polyethylene glycol, antiseptics such as chlorhexidine and acetic acid).

Moreover, it appears virtually anything (including debris, mechanical and chemical changes, ph and pus) can potentially cause damage and lead to ototoxicity.

In veterinary medicine, there have been few articles on ototoxicity due to use of chlorhexidine and gentamicin on dogs.

One publication demonstrated the lack of ototoxicity when gentamicin was instilled into ear canals with an intact or a previously mechanically ruptured tympanum.


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Source: DVM360 MAGAZINE,
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