Metabolic Considerations for Treatment of Birds

Gerry M. Dorrestein

Department of Pathology, Veterinary Faculty, Utrecht University, Utrecht, The Netherlands

Abstract: Allometric scaling or metabolic dosing is based on the principle that the amount of drug to be administered to animals is related more closely to daily energy use than to body weight. Using the formula BMR= K.W^0.75 a table was calculated for the most common weight range of birds presented for treatment in a veterinary practice. Recalculating some pharmacokinetic studies from dose in mg/kg to mg/kcal, these dosage tables became species independent. Based on the calculated daily energy use of the animal, the amount of drug for that animal can easily be extrapolated.

Key Words: Allometric scaling, Antibiotics, Antimycotics, Antiparasiticals.

Even with an increasing amount of pharmacokinetic data in birds, the use of extrapolated drug regimens will continue to be common practice for these species. The extrapolation, using "allometric scaling" from human, mammalian, and avian drugs to birds is complicated and has its limits. Most of the birds have a low bodyweight. This, combined with the knowledge that birds have high basal metabolic rates (BMR’s), makes an accurate dosage of therapeutics very important.

Most formularies for birds are based on information derived from studies in poultry, parrots, or pigeons. The advised individual dosages and drinking water concentrations can be considered to be based on an average bodyweight of 4-500 gram and a drinking water intake of 40-60 ml/kg bw for birds of again 500 grams.^1,2 The average basal metabolic rate as expressed by the formula BMR (kcal) = K.W ^0.75) of the passerine bird (K=129) is 50 to 60 percent higher than those of non-passerines (K=78) of the same body size.^3,4 This knowledge should be used to calculate a dosage regimen and, using this system, the dosages will be expressed in mg/kcal.

For a quick reference, the conversion of body weight (gram) to basal metabolism (kcal/day) is presented in Table 1.

The calculation of the concentration for drinking water medication should be based on the daily water intake. While some desert passerines, such as Zebra finches, have been known to survive for months without drinking water, most small passerine birds drink from 250 to 300 ml/kg body weight daily and may eat up to 30% of their body weight daily.⁵ This is much more than the generally advised 40-60 ml/kg bw for larger parrots.¹ This amount is for an average parrot of 500 grams with a basal metabolism of 46 kcal/day about 0.5 ml/kcal/day. It may be wise to use this amount of total water intake as a starting point for the small birds. Using the dose in mg/kcal and a drinking water intake of 0.5 ml/kcal, the drug concentration per liter will be independent from the species and, therefore, the same for all birds (Tables 2 and 3). This is a good starting point when information from experimental work is not available. The daily water consumption, however, is very much influenced by environmental temperature, type of diet (e.g. fruits), and original habitat of the bird (desert). When using this information for calculating the drinking water concentration for the treatment of birds, the owner should always measure the actual intake and adjust the concentration to reach the desired average drug intake per kcal for the birds that are being treated.

Discussion

Even though there are many complicating factors in dosing birds, not at least because of the many different species that are can be offered for treatment, this method based on metabolism seems more adequate than only dosing on body weight.⁶ This method was initially propagated for use in exotic animals in 1990;⁷ however, it has never become common practice in exotic animal medicine. The main reason for not using this technique may be due to the "complicated" calculations and worksheets that were used for transforming mg/kg to mg/kcal. In reptiles, Sedgwick started to use the term "MEC/SMEC-dose."⁸ This is freely translated as the dose in mg/kcal. By using the Hainsworth constants (K) for the 5 "metabolic taxa" ( Passerine birds 129; Non-passerine birds 78; Placental mammals 70; Marsupial mammals 49, and Reptiles 10), a table can be calculated for the most common weight ranges, e.g., Tables 1 and 3. The calculated doses in mg/kcal are species independent. By reading the "closest" energy use (kcal) for a given animal, the dose can easily be calculated (see Table 3). In the future, all pharmacokinetic studies should calculate the dose in mg/kcal (MEC/SMEC-dose). These doses could be used for any given species.