whereYis the pharmacokinetic parameters of interest,Wis body weight, anda
andbare the coefficient and exponents of the allometry. Although not yet critically
evaluated in zoo medicine, this approach might provide a more accurate method
for drug–dose estimation than other approaches. This has been illustrated by
Mahmood ( 2007 ) for lysergic acid diethylamide (LSD) administration to an ele-
phant. Linear extrapolation produced a total dose of 297 mg LSD for the animal.
This was administered and it resulted in death within 2 h (West et al. 1962 ). When
using allometric scaling, the total dose recommended would be 5.3 mg. Finally,
when metabolic scaling was applied, the resulting total dose was between 19 and
56 mg, depending on whether human or cat data were used for the extrapolation
(Mahmood 2007 ). It is unknown if either of these doses would have been safe and
efficacious, but they are 5–15 fold less than the lethal dose derived from linear
extrapolation.
Extrapolation of doses between mammalian and avian species, both qualitatively
and quantitatively, is complicated by several physiological and anatomical differ-
ences (Tables 1 and 2 ). Anatomically, the avian renal cortex is more similar to the
reptile cortex than the mammalian cortex. The physiological consequence is that
glomerular filtration is not held relatively constant in birds, as it is in mammals, in
the face of varying mean arterial pressure and this will impact on the pharmacoki-
netics of drugs in avian species (Frazier et al. 1995 ). Reptiles and birds have a renal
portal system, so that a first-pass effect of renally cleared drugs can occur if the
animal is injected in the posterior half of the body. This system is not fully
understood, and it may not be as restrictive as once believed. However, until
definitive data are available, reptiles and birds should be injected in the anterior
portion of the body when agents dependent upon renal clearance are administered.
Table 2Observed vs. predicted clearance from avians to aviansa
Drug Body
weight (kg)Observed
CL (mL/min)Predicted
Cl (mL/min)% errorEnrofloxacin
Turkey 5.1 37 128 246
Chicken 0.66 18 53 194
Salicylic acid
Turkey 8 61.3 25 60
Duck 3 8 7.4 8
Meloxicam
Turkey 8 7.3 25 241
Duck 3 3 4 33
Flunixin
Turkey 8 24 18 27
Duck 3 7 3.5 50
Gentamicin
Red-tailed hawk 1.3 3.1 1.7 45
Golden eagle 3.6 5 4.1 18
aData from Hunter et al. ( 2008 )148 R.P. Hunter