Lizarraga I, Sumano H & Brumbaugh GW
Pharmacological and pharmacokinetic differences between donkeys and horses.

Equine Vet Educ, 16(2): 102-112, 2004
ISSN: 2042-3292 Equine Veterinary Education (PubMed)

Abstract
Donkeys play a key role in subsistence agriculture and farm work in many parts of the world. Greater populations of donkeys live in developing countries where they rarely receive proper medical assistance. Furthermore, if veterinary care is provided, donkeys tend to be treated as if they were horses, either in developing countries or in the industrialised world. Many physiological features of donkeys (Equus asinus) are different from those of horses (Equus caballus). For example, plasma volume is maintained in dehydrated donkeys even when they loose 20% of normal body water, while horses are, by far, less resistant to this challenge (Matthews et al. 1997a). Donkeys also appear to possess an increased metabolic capacity for certain drugs, which may be related to differences in cytochrome P450 isoenzymes (Peck et al. 1997). Hence, it is reasonable to expect differences in the disposition of drugs between these species, which may alter dosing intervals (Kinabo and Bogan 1989; Horspool and McKellar 1990; Horspool et al. 1994; Mealey et al. 1997). Table 1a, b summarises the values available for pharmacokinetic variables for drugs in donkeys and the corresponding values for horses. In the absence of specific information, however, it has generally been assumed that distribution and metabolism of drugs in donkeys are similar to those in horses. Dosage regimens for many drugs are currently being extrapolated, disregarding not only species but breed and individual variation. Few drugs have been authorised for use in donkeys and only a limited number of dosage regimens have been properly calculated for specific conditions in this species (Table 2). In the UK, for example, griseofulvin, mebendazole and permethrin are the only drugs authorised for use in donkeys (Bishop 1998). To the best of our knowledge, information about pharmacological/pharmacokinetic aspects of drugs in donkeys has not been recently compiled; hence, a review of this subject and its comparison to that for horses became the impetus for this review. The clinical implications of such comparison may stimulate clinicians to treat these species correspondingly.