The pharmaceutical industry in this country has not been very responsive to the needs for safe, effective, and less costly drugs for animal restraint or immobilization. Some progress has been made over the past twenty years, but there is still a long way to go before the ideal immobilization drug may become available. In the meantime, the worker should become adept in using the products currently in the market.
A combination of two or more compatible and complementary drugs may often constitute the best dosage for certain species and for that reason, such combinations-or cocktails are widely used by wildlife workers.
The mixtures of drugs for immobilization purposes usually consist of a primary drug such as a narcotic of cyclohexamine with a neuroleptic added. There are several reasons for including a neuroleptic (sedative) with the primary immobilizing drug, in most cases, the neuroleptic may potentiate the primary drug and negate the commonly encountered side effects of the narcotics or cyclohexamines. This provides for a shorter and smoother induction and safer and less troublesome immobilization.
In addition to the immobilizing drug and a complementary neuroleptic, an enzyme which will speed absorption of these drugs may also be included in the total dosage.
In spite of the good results, which may be obtained through the combination of drugs, this is not recommended by the manufacturers of the drugs involved. One reason is that the required testing of such application. Another reason is purely commercial. The drug manufacturers are interested in selling more of their own products, not in recommending or promoting another company’s products.
The responsibility for the correct use and storage of the drugs rests entirely with the user. This includes the worker, the veterinarian responsible for obtaining and dispensing the drug, and, finally, the agency. The manufacturer of these drugs will not be liable or assume any responsibility for the use, misuse or possible abuse of their products.
In order to correctly calculate the drug dosage required to immobilize a particular animal, the worker must know three factors.
- 1. Estimated weight of the animal. If the weight of the animal is estimated in pounds (lbs), it should be converted to kilograms (kg), as dosages are expressed in mg/kg (milligram of drug per kilogram of body weight) required to produce immobilization. One lb equals .454 kg, and the conversion is made by multiplying the estimated lbs by .454. Thus a 100 lb animal weighs 45.4 kg. Since 1.0 lb is almost equal to 1/2 kg, a simpler conversion can be made by dividing the estimated weight in lbs by 2. By this method the 100 lb animal weighs 50 kg. Because of the difficulty in accurately estimating the weight of an animal and the safety margin of the CNS drugs, this simpler conversion is acceptable for fieldwork.
- 2. The dosage recommended for the species. This is the dosage recommended to produce immobilization in a particular species. Dosage recommendations may be provided by a veterinarian, by the drug package insert, or by consulting dosage tables in reference literature. The suggested dosage tables are given in mg/kg (milligram of drug per kilogram of body weight.)
- 3. Concentration of the drug used. The concentration (solution strength) of the drug is listed on the label of the vial, on the package, and in the package insert. It is given as mg/ml (milligram of drug per milliliter of liquid volume). To minimize the drug volume, and consequent size of the dart, the highest available concentration of a given drug should be used.
For all practical purposes, 1.0 ml (milliliter) is equal to 1.0 cc (cubic centimeter) of liquid volume. On the basis of these three factors, the drug dosage is calculated as follows:
Animal weight (kg) x dosage (mg/kg) / Concentration of drug (mg/ml) =
Equals drug volume in ml or cc.
The manner in which a drug may effect an animal and the level of immobilization produced are subject to a number of factors. In addition to the animal’s weight, its age, sex, physical condition, and mental state at the time of injection, individual animals will have varying degrees of sensitivity to the drug. This can produce different and unexpected results from one individual to another, even within the same species. Excitable animals will usually require a higher dosage than animals which are calm. Also females may require a higher dosage than males to produce a satisfactory level of immobilization.