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Griseofulvin: A Unique Antifungal Antibiotic For Your Horse

Updated: May 8, 2023

Griseofulvin is unique amongst antibiotics in that it accumulates in the stratum corneum and is highly effective against dermatophytes. It, therefore, is a systemic treatment for ringworm and has been used for this purpose in a variety of species. Both Trichophyton and Microsporum species are affected, but the drug has no antibacterial activity. It is derived from Penicillium griseofulvium.

Two formulations are available: tablets for small animal use and feed additive for use in large animals. The feed additive being a relatively crude mycelial product is more economical than tablets.

The mode of action of griseofulvin on the dermatophyte is due to interference with the polymerization of the microtubular protein into microtubules. This is a fungistatic effect, and the cure depends on the shedding of the infected layers; thus dosing must continue for long enough to allow this to occur. In small animals, this is usually 3-4 weeks, but dosing up to 12 weeks may be required, especially in Microsporum infections which are more difficult to treat than Trichophyton infections. It is recommended that treatment should continue over 1 week, with dosing daily. The dose rate is 15-20mg/kg in small animals and 10mg/kg in large animals.

Side effects in men are depression of the leucocyte count, headache, malaise, and rashes. In animals, toxicity does not seem to be a problem, but the drug is not recommended in pregnant animals (especially the Mare). Animals with impaired liver function should also be treated with care, since metabolism (breakdown to 6-dimethylgriseofulvin and its glucuronide) may be reduced, leading to toxicity.

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