Technical Bulletin 6
Medicine Use and Equine Formulary
Medicine use and Equine Formulary
INDEX
Page
Introduction3
Good pharmacy practice3
Medicine Use and the Law3
Medicine purchase3
Medicine storage4
Reading the label5
Administration of medicines5
Disposal6
Withdrawal periods6
Medicines used in equine practice7
Antimicrobials7
Anti inflammatories11
Endoparasiticides16
Ectoparasiticides19
Blood Parasite medicines20
Antiseptics/Disinfectants21
Sedatives23
Diuretics26
Cardio-respiratory medicines27
Others29
Drug dosage calculations30
Worksheet problems30
Answers to problems31
Glossary of terms34
References34
Introduction
A large part of SPANA's work involves diagnosing disease and illness in equines and then providing treatment and advice. Much of this treatment is carried out by using medicines. When SPANA first started over 80 years ago, the medicines available to treat animals (and humans) were very limited in number and type, in great contrast to the modern medicines available today. As animal health professionals we have a duty to see that medicines are used correctly and within accepted guidelines. Medicines can potentially do harm if used incorrectly, both to the animal being treated, and sometimes to the vet or technician handling them.
It must also be remembered that medicines alone are unlikely to cure an illness, correct care and nursing (feed, water, rest, and a good environment) are equally as important. As far as advice to owners is concerned, the old adage “Prevention is better than Cure” is most important.
Good Pharmacy Practice
Good practice includes correct storage, utilisation and disposal of medicines. These are described below in more detail.
A decision to use a particular medicine should be taken by a vet after an evaluation of the animal and its condition has been made. A full case history followed by a thorough clinical examination is necessary, before the choice of medicine is agreed. It is particularly important to establish if any other medicine has been given already by the owner or another animal health professional, since some medicines can react badly with others. The use of polypharmacy (using many different drugs) is discouraged as it is usually due to a lack of a proper diagnosis. A drug should be used only if a valid clinical reason for its use exists.
Medicine Use and the Law
In many countries, according to the law, only qualified vets can legally buy many of the drugs listed in this bulletin, and, by law, people who are not vets must only use them on animals that are under a vets care. You should be aware that, if you buy and use these medicines and you are not a registered vet, you may be breaking the law.
If you use a medicine in a way that is not included in the manufacturer’s instructions, even if that way is described in a text book, it is your own responsibility. Such a use might be giving a medicine to a horse or donkey that has not been licensed for use with horses and donkeys. If the animalhealth worker or owner decides to use that medicine on a horse or donkey, he or she must accept that risk.
Medicine Purchase
Medicines should always be purchased from reputable dealers, preferably licensed. The premises should be equipped to store medicines correctly. Beware some traders who sell medicines damaged by being kept in a hot store.
Only genuine medicines with proper printed labels and, preferably, containing a data sheet in the box should be used.
The expiry date must be checked when purchasing medicines.
Some makes/brands of medicine are cheaper than others. Another medicine with a different trade name but the same chemical name may be just as effective and cheaper, although it is sometimes preferable to go with a trusted name.
The strength/concentration of the medicine must be checked to work out the cost of the actual amount of medicine in a container. Some medicines with the same chemical name have more actual medicine in each ml in a bottle
Medicine is often cheaper in larger containers, but it is not necessarily economic to buy such large containers that it will not be used quickly. Medicine left in a container after opening is quickly damaged by air and may become contaminated.
Medicine should only be bought in quantities expected to be used well before the expiry date.
Medicine Storage
Drugs should be kept securely, preferably in a locked cupboard and records must be kept of their administration to each animal. This is particularly important with potentially dangerous drugs such as sedatives and anaesthetics.
If medicines are not stored correctly they will become damaged. They may not work and can become dangerous.
Most medicines require keeping in a cool, dry and dark place. Some must be kept in a fridge; a few require freezing (certain vaccines). Powdered medicines in particularly do not want to be anywhere damp. Some medicines are damaged by sunlight; these ones are often supplied in dark coloured bottles.
It is important to maintain the cold chain once medicines have left the clinic (and between purchasing the medicines and getting them to the clinic). This probably requires the use of insulated cold boxes, especially in hot climates.
Medicines must be stored where children and animals cannot gain access to them, and safe from rodent and insect damage.
Once a bottle of medicine has been opened, it should be used up as quickly as possible, since it can become contaminated. Some vaccines must be used immediately.
Part used medicines must not be kept in food or drink containers.
Medicines should be used by their expiry dates.
Medicines should be kept labelled and the directions for use kept with them.
A thermometer should be used to keep a check on the storage conditions for medicines. Most medicines have a maximum and minimum temperature written on the label, and these should be adhered to.
Transport of medicines in mobile clinic vehicles can pose a particular problem. Cool boxes MUST be used, if there is not an efficient refrigerator. Cool packs must be used in the cool box and the lid should be kept on except when removing a medicine to use.
Reading the Label
Medicines usually have two names; the chemical or generic name e.g. oxytetracycline and the trade name given by the manufacturing company e.g. alamycin.
A medicine label should provide the following information either on the bottle, or on an enclosed label:
- The trade name of the medicine
- The chemical name of the medicine
- Dose; this is usually given in mg/kg bodyweight
- The concentration/strength of the medicine (i.e. how much actual drug is in each ml of medicine in the bottle)
- Which animals the medicine can be used in
- Instructions about special care required when using the medicine and contraindications for its use
- How to give the medicine, how often to give it and for how many days
- Withdrawal period for meat and milk, this is to prevent the medicine affecting people if animal products are fed to people
- The total quantity of medicine in the bottle
- The date the medicine was manufactured
- The expiry date –medicines should not be used after this date; it may not work properly and can help to allow resistant microbes to develop in the case of antimicrobials or parasiticides
- The name of the manufacturer
Administration of medicines
Manufacturer’s instructions should be followed when deciding on the route of administration of a medicine, but also note the individual drug entries in this bulletin. Owners are not given injections to administer themselves, but if any medicine is given to an owner to administer in food or on the skin for example, they must be given very clear instructions as to how to do this.
Medicines which have lost their label must not be used.
Strict hygiene should be employed when medicines are given, particularly with injections. This involves using a new sterile needle for each animal, swabbing the top of injection bottles with surgical spirit/alcohol and cleaning the injection site on the skin.
Large quantities of injectable medicines must not be given all at the same site on an animal with intramuscular injections. Usually the maximum quantity of any medicine that can be given at one site is 20mls for an adult horse. Again it is important to follow the manufacturer’s advice on this.
The correct dose must always be given; in order to do this the weight of the animal must be estimated using a weigh tape or the formula in the SPANA Guide to Animal Care. Overdosing can be dangerous for the animal and under dosing (particularly with antibiotics or anthelmintics) and may lead to the development of resistant organisms.
Disposal
Medicines which are past their expiry date or medicines that have changed colour or look damaged should be disposed of.
Drug bottles should be disposed of carefully to avoid injury. It is preferable to remove the label and/or break the bottle to prevent misuse of the contents at a later date.
Incineration is the best method of disposal, but is not always practicable. An alternative is to bury used containers in a deep hole. This should not be within 50m of a water course to prevent leakage and contamination.
Withdrawal periods
This is the period after administration of a medicine during which products from the animal (milk, meat, eggs etc) may not be consumed by people. These mostly relate to medicines in food producing animals, but must be taken into consideration if there is any chance that the equine may end up in the food chain. This is probably not relevant to most of SPANA’s patients.
Medicines used in equine practice
A. Antimicrobials/antibiotics
Criteria to be considered when selecting an antibiotic regime in horses include:
- The presence or likelihood of an infectious process.
- Type of infectious process.
- Bacteria most likely to be isolated.
- Age and immune status of the animal.
- Antibiotic characteristics- the bacteria must be susceptible to the drugs mode of action, an appropriate concentration of drug must be achieved at the site of infection and maintained for a sufficient time to kill the organism, the microenvironment at the infection site must be favourable for the antibiotic action.
- Cost of antibiotics.
- Ease of administration.
- Potential toxic effects.
1. Penicillin G
Penicillin is widely used in many conditions, and is active against gram positive bacteria.
It is excreted by the kidneys and concentrated in urine.
It is inactive in respiratory secretions.
Poor systemic absorption after oral administration.
Allergy to penicillin is seen occasionally as acute anaphylaxis. Immune mediated anaemias are also seen rarely. Neurological signs are occasionally seen with the procaine form (due to the procaine, not the penicillin)
i. Sodium penicillin
This crystalline salt is used when a rapid effect and high concentration are desired.
Dose
20 000 iu/kg every 6 hours.
Route of Administration
iv or im
ii. Procaine penicillin
Trade names available locally
Combi-kel 40 (penicillin 200 000 iu/ml + streptomycin 200 mg/ml)
Pen & Strep (penicillin 200 mg/ml + streptomycin 250 mg/ml
This is possibly the most used antibiotic in equine practice, often used in combination with an aminoglycoside antibiotic to broaden the spectrum of activity.
Dose
20 000 iu/kg every 12 hours.
Route of Administration
im only
iii. Benzathine penicillin
Trade names available locally
Shotapen (benzathine penicillin 100 mg/procaine penicillin 100mg/streptomycin 0.164 iu/100ml)
Slow absorption and low peak serum concentrations, therefore generally not useful in equines, but can be effective against very susceptible organisms with low MICs (minimum inhibitory concentrations) involving the urinary tract.
Dose
20 000 iu/kg every 24-48 hours
Route of Administration
im only
2. Aminoglycosides
Used for their gram negative aerobic antibacterial action.
Generally used in combination with penicillins.
Must be given parenterally (not orally) to achieve systemic therapeutic concentrations.
Indicated in the initial treatment of life threatening infections caused by gram negative aerobic bacteria.
The major toxic effects are nephrotoxicity and ototoxicity. Nephrotoxicity can be potentiated by the use of other nephrotoxic drugs, loop diuretics (furosemide), hypotension and age. Care must therefore be taken in animals with already compromised renal function.
i. Streptomycin
Trade names available locally
Combi-kel 40 (penicillin 200 000iu/ml + dihydrostreptomycin 200mg/ml)
Shotapen (benzathine penicillin 100mg/procaine penicillin 100mg/streptomycin 0.164 iu/100ml)
Pen & Strep (procaine penicillin 200 mg/ml, streptomycin 250mg/ml)
More nephrotoxic than gentamicin.
Dose
10 mg/kg once daily (CM)
2 mg/kg bid (Rose)
Route of Administration
im
ii. Gentamicin
Trade names available locally
Totamicine (3g/100ml = 30mg/ml) Not indicated for equines
Gentamycin 50 (50mg/ml)
Commonly used in equine medicine, often along with procaine penicillin. Once daily dosing is recommended.
Dose
6.6 mg/kg once daily.
Route of Administration
iv or im. iv is generally recommended as im use can cause local myositis. If the medicine is given im it must be given at several sites due to the generally large volume required.
3. Potentiated sulphonamides.
Trade names available locally
Trisulmix poudre
Bactrim forte tablets (human) (800mg/160mg per tablet)
Avemix 150
Kombitrim 240 (sulfamethoxazole 200mg/ml + trimethoprim 40mg/ml)
Borgal (200mg/ml sulphadoxime and 40mg/ml trimethoprim)
Norodine
A broad spectrum antibiotic mixture consisting of Trimethoprim with one of the sulphonamide drugs. Not used for difficult gram negative or serious anaerobic infections, but a good choice for minor skin wounds, respiratory infections and foal diarrhoea.
Rarely immune mediated crystallurea has been found as a side effect.
There have also been reports of sudden death occurring when intravenous potentiated sulphonamides have been used alongside the alpha 2 adrenergic sedatives detomidine and romifidine.
Dose
Recommended dose rates in text books vary but as a general guide:
15 mg/kg every 12 hours if used intravenously.
30 mg/kg every 12 hours if given orally.
These are the total doses of trimethoprim plus the sulphonamide component.
Route of Administration
Intravenous and oral forms are available. The oral form, usually a powder.
4. Tetracyclines
A broad spectrum bacteriostatic drug, which can also be used against mycoplasma infections.
Some resistance has been reported.
Excreted in kidneys and in the bile.
Avoid tetracycline use in animals with renal failure.
Use with care in young animals as it can cause bone and teeth abnormalities.
These drugs were rarely used in the past with equines as they were said to cause diarrhoea. Recent evidence suggests they are no more likely to cause diarrhoea in the general population of equines than other antibiotics, although it is not advised to use the long acting or high concentration preparations commonly used in ruminants.
i. Oxytetracycline.
Trade names available locally
Terramycine 50 (5g/100ml = 50mg/ml)
Alamycin 10 (100mg/ml)
Dose
5 mg/kg every 12 hours.
Or 10 mg/kg once daily.
Route of Administration
Usually iv. Injection site reactions have been reported.
5. Metronidazole
Trade names available locally
Metronex (oral paste 10g syringe)
Flagyl (human preparation in tablet form)
Used for anaerobic infections. Metronidazole is well absorbed orally and also from the rectum. It is excreted by the liver and kidneys. Toxicity is uncommon although anorexia has been reported as a side effect.
Dose
15 mg/kg every 6 hours
Route of Administration
Orally, usually in paste or tablet form.
Can also be given rectally.
The above information relates to antibiotic preparations for oral or parenteral use. There are many other topical and ophthalmic antibiotic preparations available.
B. Anti-inflammatories.
1. NSAIDS (Non Steroidal Anti-Inflammatory Drugs.
NSAIDS are drugs that inhibit part of the cyclo oxygenase (COX)enzyme systems that convert arachidonic acid into prostaglandins, thromboxanes and leukotrienes. There are two COX enzyme systems:
- COX I enzymes are thoseresponsible for producing prostaglandins which are necessary for some normal physiological processes and have protective effects on the gastro-intestinal tract and kidneys.We do not want to inhibit these COX1 enzymes.
- COX II enzymes play a major part in inflammation, and these are the ones we want to inhibit.
- The ideal NSAID would inhibit COX II, but have no effect on COX I. At the present time, there is no ideal NSAID that does this.
- However there are NSAIDS that have only small effects on COX I while inhibiting COX II, these include meclofenamic acid.
- NSAIDS that have a greater effect on COX I are more likely to lead to toxicity, these include aspirin.
Side effects of all NSAIDS can include the following:
Local irritation it is advised not to administer by the im route.
Gastric irritation gastric ulcers
acute enterocolitis
protein losing enteritis
Renal effectsalthough rare.
Bone marrow suppressionpancytopaenia, occasionally seen in horses treated with phenylbutazone, or aspirin.
Do NOT use NSAIDS together as the side effects are cumulative.
i. Phenylbutazone
Trade names available locally
Equipalazone
Phenylarthrite (200mg/ml)
Butosyl (Fort Dodge186mg/ml)
Equizone powder (1g)
Arthricidine (200mg/ml)
*Dexatomanol (70mg/ml phenylbutazone + dexamethasone, cinchocaine, ramifen)
*Dexaphenylarthrite (180mg/ml phenylbutazone, + dexamethasone)
*The above two products are examples of polypharmacy and should be avoided. It is not generally recommended to use steroids and NSAIDS at the same time.
Phenylbutazone (Bute as it is often known) is the most commonly used NSAID in equines. It has the advantage of being relatively cheap. It is used particularly in musculoskeletal problems, in particular low grade lameness. It also has an antipyretic effect.
It must not be used in animals that may be used for human consumption.
Dose
4.4 mg/kg every 12 hours on the first day.
2.2 mg/kg every 12 hours after that for up to 4 days.
2.2 mg/kg once daily can be used after the 5th day.
NOTE: many commercially prepared formulations have an incorrect dose given, the correct dose rate for phenylbutazone is 4.4 mg/kg maximum and the quantity of drug to be given in mls should be calculated from this only.
Route of Administration
Injections should be given by the iv route only. im injections will cause tissue irritation, and possible necrosis.
Oral formulations as powders, pastes or tablets are commonly available. This route avoids the possibility of phlebitis. Peak absorption from the oral route is 6 hours, or delayed if fed.
ii. Flunixin meglumine.
Trade names available locally