Neil A. Forbes, BVetMed, DECZM (Avian), FRCVS, RCVS, EU Recognised Specialist (Avian Medicine)
Pharmacology: Is all about keeping a bird alive for long enough, and providing a safe, therapeutic medication, at a safe and appropriate level, route and frequency, to maintain an MIC for the relevant pathogen, for sufficient time to destroy the pathogen, or to enable the host to mount a successful immune response to it.
Whilst the principles of emergency and support care as learnt on companion animals may be applied, it will need to be adapted to the special requirements of avian patients.
Until recently, all avian dose rates have been based on companion animal or poultry dose rates. Only now, when further evidence-based trials have been conducted, do we know that there is great interspecies variation in elimination rates of drugs and, hence, recommended doses and frequency of administration.
Off-license drug usage: Very few medications are licensed for birds in general, let alone for specific species, so use of drugs 'off label' is typically unavoidable. Clinicians should always bring this to client's notice and seek their permission to use such medications. It must not be forgotten that medications safe and efficacious in one species may be lethal in another.
Physiology of metabolism and body temperature: Birds have a higher body temperature (40–42°C) and a higher metabolic rate. Flight and maintenance of high body temperature mandate this requirement. In general terms, the rates of metabolic processes increases with body temperature. However, equally the maintenance of a higher body temperature (41°C, rather than 37.5–39°C) is extremely expensive. Birds use 20–30 times more energy to maintain body temperature than reptiles do.
Both respiration and circulation have developed amazing adaptations to get energy and oxygen to cells and metabolic waste products away from cells far more efficiently than in mammals.
Basics of pharmacotherapy: Medication of birds is achieved via the same routes as in mammals, only that administration tends to be required at higher dosages and more frequently. As more and more is learned about pharmacodynamics in birds, it is appreciated that there is significant variation in rates of drug disposition and clearance even between similar species.
Optimal medication requires a specific diagnosis, sensitivity to the pathogen (by testing or anticipation), consideration of toxicities, knowledge of pharmacokinetics and pharmacodynamics of the drug, appropriate administration, simultaneous support, good husbandry, consideration of anatomic and physiologic differences between species, and other considerations. It is well recognised: pharmacokinetics alter significantly during disease process, especially when disease effects organs of absorption, biotransformation or excretion.
Allometric scaling: If a required drug dosage has to be estimated, this should be based on 'metabolic extrapolation' - otherwise known as allometric scaling. In essence, it is the bird's metabolic rate, rather than weight, which is used to calculate the expected dose rate. There are known relationships between weight, surface area and metabolic rate.
As an animal's metabolic rate increases (i.e., speed of metabolic processes in the body), the half-life of any drug decreases; i.e., it is eliminated more rapidly and, hence, administration frequency increases.
Basal metabolism: All birds have a high basal metabolic rate (BMR). Passerines in particular have a very high level (50–60% higher than non-passerines of the same size).
BMR = K(W0.75)
K is a theoretical constant for kilocalories required per 24 hour and varies with respect to species.
K is 129 for passerines and 78 for non-passerines, whilst 70 in mammals.
The formula reflects the relationship between weight and surface area. Clinicians will know that BMR relates to basic body functional requirements. Increased physical activity (breeding, growing, rearing, moulting, etc.) all increase BMR. BMR is also referred to as the 'minimum energy cost' (MEC in kilocalories) utilised per day and may be calculated as
MEC = K x W0.75
We can divide the BMR by the bodyweight to give a specific MEC (SMEC) in units of kcal/kg/day.
SMEC = K x W0.75/W
Where K is the constant equal to the kilocalories used in 1 day by a hypothetical 1 kg animal.
W is the animals mass in kg.
Calculation of dosage: The appropriate dose of a drug will depend on its pharmacokinetics, i.e., its route of absorption, metabolism and clearance. The single most important factor is the patient's surface area-to-volume ratio. The smaller the patient, the greater it is.
E = (W/100)0.75 x D
W = weight of bird in grams
D = dose of drug recommended for cat/dog (mg/kg)
E = dose of drug for the bird of a specific weight (W) in mg
The fate of a drug in the body (protein binding, volume of distribution, biotransformation and excretion) will differ between species or groups of birds. There is little interspecies variation in the excretion of 'polar compounds' (e.g., gentamycin), but wide variation and unpredictability in the biotransformation of extensively metabolised drugs, such as chloramphenicol and sulphonamides.
Allometric scaling can be usefully applied to predict dosages required with respect to patient size, but it does not take into account variations in metabolic pathways.
Routes of Administration
Drinking water - least reliable, rarely recommended. Only to be used when birds will voluntarily (i.e., no bad taste) drink a predictable volume.
Flock/individual therapy - very rarely is flock treatment recommended. The downside of flock therapy is counter balanced by stress reduction of avoidance of repeated handling.
In food - must be mixed well, and the patient must be prepared to eat a soft diet. This is often a problem with pet psittacines, as they are only prepared to eat their own normal seed-based food. All new parrot owners should maintain the bird's willingness to eat soft food off a spoon (as during rearing). If a soft food (treat) is given each week off a spoon, then medication (or even just supplementary vitamins) can be given voluntarily at any stage. This is easier in raptors, as medication can be secreted within meat. For groups of waterfowl or poultry, drugs may be mixed with soaked bread, layers mashed or added to oil and mixed with corn.
Gavage tube, effective, invaluable in hospital but potentially stressful and challenging for owner.
Ingluviotomy tube: great technique for medication and food, which avoids stress of restraint.
I/m (pectoral) most reliable, renal portal system renders leg injections less reliable. In smaller birds the available area, (especially for 3–4 times daily administration) can be challenging. Tissue necrosis of pectoral muscles and future flight impairment or pain may be relevant. I/m injections will effect nonspecific soft tissue enzyme levels.
S/c poorly absorbed, no advantage in using this route, may be improved by addition of hyaluronidase. The technique is used commonly in small chicks.
I/v, only relevant whilst intravenous catheter present, so good whilst in hospital.
Local infusion: e.g., sinus flushing - can be very useful, and often easily achieved by the owner in a home environment.
Intratracheally via oropharynx or nebulisation.
Creams and ointments: not often suitable as liable to damage feather structure. Oil-based preparations should not be used.
Drugs mixed with DMSO: for per-cutaneous absorption
Eye drops: preferable to ointments. Beware topical steroid drops as systemic effects will arise.
Powders: e.g., ecto-parasitic preparations.
Recommended Drug Dosages
Whilst previously, clinicians were encouraged to extrapolate using allometric scaling (from companion or poultry doses), there are now a number of excellent (referenced) formularies available. As increased scientific research is undertaken, more referenced species-specific dosages are published. Evidence backed, published dose rates should be used wherever they are available.
Therapeutics for Specific Situations
Clinicians should be familiar with drug types and routes of administration for therapy of the following conditions:
URT - nebulisation, sinus flushing with or without trephination, parenteral therapy
LRT - nebulisation, tracheal topical therapy, air sac intubation, direct intra-air sac applications, parenteral, oral; e.g., itraconazole, voriconazole, terbinafine
Anti-bacterial, anti-fungal, prokinetics, anti-ulcer (e.g., ranitidine), Cox 2-specific NSAID
Antidotes, binding/chelation/purging agents
Routes or administration, types or fluids, benefits and relevance of each
Analgesics, anti-inflammatories, prevention of devitalization of exposed soft tissues
Beware oily substances
Beware parenteral and even topical administration
e.g., procaine penicillin, benzocaine, lidocaine, lignocaine - even in wound powders - low critical toxicity levels
Fungal Therapies for the Following Situations
Air sac aspergillosis
Dermal fungal infections
When treating any condition, make a specific diagnosis, confirm details of the most appropriate medication, confirm safe referenced dose rates in the appropriate species. Ensure route and frequency of medication is appropriate. Ensure nursing, fluid and nutritional support is appropriate and adequate, in order to maintain the bird whilst medical therapy has time to act. Consider the stress caused by therapy against the benefits achieved.