Antiseptics/disinfectants

Many of the modern antiseptic compounds have good virucidal, bactericidal and fungicidal effects and some have useful sporicidal effects for spore-forming bacteria and fungi. The commonest compounds used in equine dermatology and wound management include chlorhexidine, povidone-iodine, iodophor, quaternary ammonium compounds, halogenated tertiary amines, inorganic halogenated peroxygen compounds and acidic iodines.

Some antiseptics and disinfectants can be used on the skin but others are harmful. There is no justification for applying chemicals to the horse’s skin that are not specifically designed for topical application. Manufacturers’ instructions should always be followed implicitly. The antiseptics are variously used for their antibacterial properties prior to surgical interference in aseptic surgery and also as wound antiseptics. Most compounds will have some harmful effect on the skin when used in abnormally high concentrations and the specific dilution instructions are vitally important. In any case, many of the iodine-based compounds work more effectively at lower concentrations than at the higher ones and it is very important to ensure correct dilution.

Most agents will have less or more activity against various microbes and the correct selection will enable a more rapid resolution or effect to be gained. Thus, chlorhexidine probably has less antifungal activity than povidone-iodine and some of the newer quaternary ammonium compounds and halogenated tertiary amines have a very wide range of activity against viruses, bacteria and fungal organisms but some are irritant if applied to the skin. Care must be taken to read the labels to check for the range of activity and the value and dangers of application to the skin. Some agents are used solely as environmental disinfectants in order to reduce the microbial challenge to animals.

Chlorhexidine diacetate/gluconate

A phenol-related, bi-guanide antiseptic-disinfectant with few problems and no apparent systemic absorption or local effects. It has a wide range of antibacterial activity and probably has better control over Staphylococcus aureus than povidone-iodine. It has little or no fungicidal effect except at high concentrations and is used primarily as a surgical scrub and as an antiseptic solution (in alcohol).

The presence of pus or other organic material does not diminish its activity and it has good tissue properties with some residual effects in tissues, providing prolonged activity.

Routine use of 0.5% is recommended but, as a wound irrigation solution concentrations of 0.05% should be used. It is important to dilute the concentrate with water rather than saline or other electrolyte solutions which may cause significant precipitation of the compound in solution and consequent loss of effect.

Povidone-iodine

A non-irritating, non-staining, iodophor which comprises iodine solubilized by surface-active agents. It has a broad spectrum of activity with virucidal, bactericidal (and sporicidal) and fungicidal effects which are not affected by blood, serum, necrotic tissue or pus. The rapid bactericidal effects are better at lower dilution – higher concentrations result in reduced bactericidal effects and greater tissue toxicity. The mild tissue toxicity effects are minimized by dilution to less than 0.05%. Solutions of 5% or more actively inhibit leukocyte migration into wounds, while very weak solutions appear to encourage neutrophil and monocyte migration. One per cent solutions cause death of fibroblasts but 0.01% has no detectable harmful effect on these cells.

Acetic (and malic) acid

Natural highly acidic chemical solutions which are widely marketed as wound irrigants. They are effective in the control of Pseudomonas spp. infections on skin and their use should be restricted to old wounds infected with this organism and use as a debriding agent. The solutions should not be applied to a fresh wound or wounds without Pseudomonas spp. complications as they cause severe deleterious effects on all cells associated with wound healing.

Hydrogen peroxide

Available in various dilutions, it has been used widely for its disinfectant properties. It has only weak germicidal effects but an impressive foaming and deodorizing property as a result of liberation of nascent oxygen. The oxygen release can be employed usefully in the control of deep anaerobic infections. Concentrations higher than 5 volumes (v/v) are damaging to tissues, with worse cytological effects against tissue cells (including fibroblasts) than against bacteria. As the chemical can cause microvascular thrombosis, it can retard healing. It can, however, be effective at higher concentrations in the treatment of thrush in the cleft of the frog.

Hexachlorophene

Used primarily in medicated shampoo but it is possibly less effective than many of the other agents and is becoming less available owing to its potential long-term toxicity.

The specific properties of the common agents are described in Table 4.1.

Table 4.1 Relative activity of some of the common disinfectant compounds used in veterinary practice (the more stars, the greater the effect)

Antiviral agents

There are few if any specific antiviral agents currently in use in horses. In theory at least some of the new antiviral drugs (such as aciclovir, trifluoridine, ganciclovir and vidarabine) would be expected to have significant benefits on virally induced skin disease and in particular the herpesvirus conditions. Only aciclovir has any equine reputation because the others are both expensive and largely unavailable in the amounts required for horses (both topically or systemically). Aciclovir is currently limited to treatment of putative herpetiform keratitis. Its action is specifically against herpesviruses.

Topical applications of a 5% aciclovir cream can be used on painful coital exanthema (EHV-3) lesions but there is little to be gained by this given that the condition is self-resolving in almost every case. A number of skin lesions including some defined erosive/ulcerative conditions of the lips and muzzle may respond to aciclovir applied topically but the implications of this are not clear. Oral dosing of aciclovir can also be used but no information is available as to its safety or efficacy.

The modern acidic iodine, tertiary amine and inorganic halogenated peroxygen disinfectants have strong antiviral properties but many are highly irritant to the skin and therefore are inappropriate for application. However, some tertiary amine compounds have been formulated into surgical scrub solutions and can therefore be justifiably used on the skin. Sterilizing the skin with powerful antiseptic solutions is not always an advantage but there are some viruses that are significant commensals.

Prophylactic antibiotic therapy

The use of prophylactic antibiotics is necessary for surgical procedures that carry a significant risk of postoperative infection such as with wounds on the lower limb, in the perineum or around the mouth. Clean procedures are associated with a low risk of postoperative infection and therefore require less than 24 hours of antibiotic coverage. Since in all but surgical skin wounds some infection is likely, elective procedures involving skin wounds should be preceded by at least one full therapeutic dose of a suitable antibiotic. If full aseptic technique can be sustained throughout the procedure (such as in a surgical wound resulting from excision of a small tumour), additional therapy should not usually be necessary. If any break in aseptic technique occurs or the procedure involves complicating factors such as a synovial structure, continued antibiotic administration for up to 5 or more days may be prudent. The aim of prophylactic therapy is to achieve high levels of antibiotics in the skin at the time of surgery. Ideally the concentration of the agent should be 4–8 times the minimum inhibitory concentration (MIC) for the potentially infective bacteria. This requires administration of antibiotics about 30 minutes preoperatively for intravenous dosing and 1–2 hours preoperatively for intramuscular dosing. Care should be taken to ensure that antibiotic administration does not interfere with the anaesthetic protocol (e.g. potentiated sulphonamides should not be used concurrently with injectable α₂-adrenoreceptor agonist sedatives because the combination might induce cardiac dysrhythmias). In addition, the potential systemic toxic effects and the cost of the antibiotic should be considered.

Therapeutic antibiotic medication

Antibiotics are required when suspected or confirmed infection is present. Confirmation is obtained by culture of swabs taken from the site (see above) and subsequent culture, Gram stain and antimicrobial sensitivity test. In the absence of confirmation of an infection, clinical signs of inflammation (heat, pain, swelling, redness) or lack of response to supportive therapy are sufficient to warrant the use of antibiotics. As cultures often take several days to perform, it is often wise to pre-empt the findings by administration of a suitable and logical antibiotic and adjust the drug if necessary when results become available. A clinical judgement of the likely infectious organisms based on experience can usually be made.

Many factors influence the function and efficacy of the drugs and the clinician needs to consider carefully the whole case and make a reasoned selection. In many cases, however, an informed guess has to be used initially. The main questions that need to be considered are:

Streptococcus spp. are involved in a high proportion of equine skin conditions and most of these are fully or partially sensitive to penicillin. Therefore, it has become common practice to use full therapeutic doses of penicillin as a primary approach to skin antimicrobial therapy. This is entirely justifiable.

Both prophylactic and therapeutic antibiotics in skin disease rely on the delivery of the drug to the skin in therapeutic concentrations. Few antibiotics have been shown, when delivered to the skin of the horse, to achieve an effective minimal inhibitory concentration (MIC) and so careful appraisal of the effects of such therapeutic measures must be taken. There are some antibiotics that are potentially very harmful to horses and so extra care must always be taken to justify the drug selection; there is no point in controlling a skin infection if the drug results in a catastrophic bowel derangement.

The distribution of antimicrobial drugs in the skin depends on its formulation, lipid solubility, state of ionization, protein-binding capacity, route of administration and the delivery efficiency to the site. For instance, a drug that is more soluble in lipid may have better intracellular penetration and therefore prove more effective for use against intracellular bacteria.

The reasons for failure of a detectable therapeutic response to antibiotic therapy include:

Culture and sensitivity results allow the use of antimicrobials to which a particular bacterium is most sensitive in vitro. Enzymatic and pH-related inactivation can usually be avoided if abscess cavities are drained or if septic synovial structures are flushed to remove the acidic debris.

Rapid identification of the infecting organism can sometimes be made from a Gram-stained impression smear from the surface of infected skin, but in dermatological disease the heavy normal commensal bacterial flora of equine skin makes the interpretation difficult. Gram staining may only indicate the type of bacteria, whereas a culture will usually provide a positive identification.

Specific organisms involved in skin disease such as Dermatophilus congolensis may require specific culture methods and the correct method of sampling (see above) should always be observed so as to avoid errors of omission or commission.

Sensitivity testing by disc diffusion or micro-dilution methods can be used to establish whether the bacteria are susceptible, moderately susceptible, or resistant. New rapid methods of determining sensitivity using genetic technology are being developed in an attempt to facilitate correct selection at the outset of treatment.

The minimum inhibitory concentration is the lowest concentration that inhibits bacterial growth and is calculated from serial dilution of the bacteria grown. The antibiotic dose necessary to achieve four times the minimum inhibitory concentration in the blood can be calculated and administered, but delivery to the site may still be a problem. Even the circulatory and blood protein status can have an influence on the delivery of drugs including antibiotics.

Ideally, choice of antimicrobial agent will be based on culture and sensitivity results. However, the decision regarding immediate antibiotic administration must invariably be based on clinical judgement (Fig. 4.2). This requires an assessment of the location and nature of the condition, and more importantly knowledge of the organism(s) most likely to be present. For instance, a kick wound may have a significant infection with Escherichia coli resulting from direct contamination but could also have pathological skin contaminant bacteria such as Staphylococcus aureus, S. intermedius or S. epidermidis. Once the potentially useful antibiotics are selected, the final choice may be based on the route or frequency of administration, the location of the infection, the pharmacokinetics of the drug, the cost of the drug, and the clinician’s preference.

Figure 4.2 This algorithm illustrates the key decisions that should be taken when selecting an antibiotic for use in horses.

The pharmacokinetics should be considered before any antibiotic is administered to a horse. Ideally, a drug administered to treat a dermatological problem needs to be delivered to the skin in an effective concentration without any untoward effect on other organ systems. The correct dose is vital if the drug is to be effective – overdoses are wasteful and under-dosing with antibiotic is a potential factor in resistance development. To calculate the correct dose the weight of the horse should be established accurately if possible. The extent of local irritation, the rate of uptake from the site of injection or application and the excretion pathways carry significant risks if an inappropriate drug is administered. Thus, lincomycin and possibly oxytetracycline may have profound harmful effects on the bacterial flora and fauna of the large colon and intractable diarrhoea may ensue. Similarly, amitraz is an effective ectoparasiticide for dogs and cattle but is extremely toxic to horses, causing severe colic as a result of complete and often irreversible intestinal stasis.

The extent of protein binding can also influence the efficacy of a compound. A highly protein bound drug is unlikely to reach effective concentrations in the skin without very high systemic doses. A horse suffering from low blood proteins might sustain higher plasma concentrations for longer than a normal healthy horse, but effective uptake from the site of injection may be significantly delayed. The overall clinical status of the horse is therefore important when considering the drug and route of administration.

Spectrum of activity

Some drugs have particular affinity for certain tissues, but unfortunately most seem to have a low affinity for skin. This may relate to the relatively low blood supply or other factors as yet undefined. Delivery of adequate therapeutic concentrations of antibiotics in the skin is sometimes difficult and frequently requires very high doses at frequent intervals.

Most equine skin diseases and wounds carry mixed infections but almost all will have a significant Streptococcus spp. component. Most if not all equine streptococci are penicillin sensitive or partially sensitive and so penicillin is widely used as first line of treatment in skin injuries and disease. However, kick injuries and wounds contaminated by faecal material will most likely have significant Gram-negative infections. Specific infections involving defined organisms may have characteristic sensitivity to antibiotics, but the widespread use of broad-spectrum antibiotics (often in unjustified circumstances) makes resistance a serious potential problem. In general, penicillin is ineffective against Gram-negative organisms but synthetic penicillins have improved activity. β-Lactamase-producing bacteria are resistant to penicillin and so infections with these organisms do not respond to it. Gram-negative organisms usually require aminoglycosides (preferably gentamicin); however, care must be taken to ensure delivery to the skin, which may be less than ideal.

The spectrum of activity also includes the activity of the drug against anaerobic or aerobic organisms. Metronidazole is a very useful drug for treatment of suspected or confirmed anaerobic skin infections (and is particularly useful topically in the foot region) but its tissue residue problems may reduce its availability.

Route of administration

The route of administration of an antibiotic may be dictated by the location of the condition, practitioner preference, environment of the animal, patient or owner compliance and economics. The duration of therapy needed may also influence the decision, since long-term intravenous administration requires a long-stay catheter, hospital environment and normal patent jugular vein(s).

The standard options available for administration of antibiotics to horses are:

Intravenous administration provides a rapid (almost instantaneous), high plasma concentration. This route often requires frequent dosing and may need special training to administer; it is usually therefore expensive. The need for special training can be avoided by the placement of an intravenous catheter; however, this may not be feasible outside the hospital and in any case catheter management can be difficult. It has been established that because its efficacy is concentration dependent rather than time dependent, gentamicin can be administered effectively using 6.6 mg/kg once daily (Magdesian et al 1994) rather than 2.2 mg/kg three times daily and this clearly makes its administration more economical (Geor 1997).

All drugs administered to horses by the intravenous route should be suitably formulated and given slowly to avoid damage to the vein and possible delivery of a bolus of potentially irritant drug to any internal organ. The intravenous route may be chosen to avoid complications associated with tissue damage and local irritant/pain effects of intramuscular injection; all drugs administered by the intravenous route must be specifically formulated for that route.

Possible side-effects of intravenous drug administration include:

Intramuscular administration may achieve good plasma concentrations but these may not be attained until 1–2 hours following administration depending on the formulation and the exact site of injection. Slower absorption, distribution and elimination require less frequent dosing than the intravenous route but levels may fluctuate. Although some special training is necessary for safe administration, clients can usually be trained to administer intramuscular injections. The expense is usually less than for intravenous medication. Large volumes of drug are, however, needed and these can be painful or warrant division of the dose into several sites. Patient resistance can make it difficult to complete longer courses.

Unwanted side-effects are rare but include:

Oral administration is perhaps the easiest for the client and the least traumatic for the animal. However, plasma levels of the drug are achieved slowly and it may be prudent to administer a loading dose of the drug by injection (if such a formulation exists). Furthermore, there are few antibiotics which can be given to horses by mouth. The logistics of administration of the doses which would be required are also problematical for many of the drugs used by this route in other species.

Where the oral route can be used the expense will vary with the medication used, but economies can be made simply by home administration. The cost of hospitalization and intravenous catheter placement can be avoided. Patient compliance is usually good and side-effects are usually minimal. Some drug interactions are important, such as the cardiac effects which are reported to occur when α₂-adrenoreceptor agonist sedatives (such as romifidine and detomidine) are administered with potentiated sulphonamides.

Topical antimicrobials are usually in the form of antiseptic solutions, antibiotic creams (often combined with various other pharmaceuticals such as corticosteroids or local anaesthetic agents) and so-called triple antibiotic ointment (a combination of bacitracin–neomycin–polymixin in an ointment base). Topical antibiotics may be useful as a method of antibiotic prophylaxis in surgery. However, it is preferable to apply them to the tissues at the time of incision. This will provide for maximum effect.

Duration of therapy

The duration of antibacterial therapy depends on the situation. In elective procedures, prophylactic antibiotics are useful when administered prior to surgery so that the skin concentrations are high at the time of the surgery. A refined clean procedure may require as little as one preoperative dose. By contrast, severely contaminated wounds involving synovial structures can require in excess of 21–28 days of intensive treatment. Although intravenous or intramuscular administration may be used initially, there are clear advantages in changing to the oral route. However, there are severe limitations in choice, and resistance or non-response to the oral formulations available for horses make long courses difficult. The change to the oral route should probably be delayed until patent sepsis has cleared. For traumatic wounds that do not involve bone or synovial structures, 3–5 days of treatment is usually sufficient. If a drain has been placed, therapy should continue for at least one day beyond the time of drain removal. If signs of infection are present, antibiotic therapy should continue for 2–3 days following resolution of the signs. Usually once drainage is established at the site of an incisional abscess, signs of infection will resolve rapidly.

Table 4.2 shows the commonly used antimicrobial agents and gives a summary of the regimens for their use. It is important to remember that none of these can be definitive and the clinician must make a careful assessment of the case as above before embarking on any therapeutic course. Furthermore, the clinician should be satisfied as to the safety and relevance of the regimen to be used.

Table 4.2 Commonly used systemic antibacterial agents used in equine dermatological disease^a

Chlorinated hydrocarbons

In most countries production and use of these compounds is prohibited due to toxicity and residue problems. Gamma benzene hexachloride and bromocyclen are probably the last of these compounds which are available in some countries. There is little doubt that the compounds were effective and that their loss has a significant implication in the treatment of parasitic infections. Some human anti-parasitic washes and shampoos, however, still have the chemicals and some of these may be safely used on horses where such an approach is legally and ethically acceptable.

Organophosphates

These compounds are now banned in many parts of the world due to the toxic effects on humans of prolonged or repeated exposure. Many of them are, however, still used in the control of insect pests on domestic pets and on agricultural crops and garden plants.

Commonly used insecticidal compounds which have been used for horses include dichlorvos, diazinon, malathion, phosmet and chlorpyrifos. Phoxim (Sebacil, Bayer, Germany) is still available in some parts of Europe and can be an effective compound on horses. These compounds are potentially toxic if used improperly and they may have damaging environmental effects also. All have been used topically in horses (usually as washes or dips but occasionally as pour-on compounds) and have good anti-ectoparasitic effects. Most of the compounds have a potent anticholinesterase activity but vary in their toxicity. Application must be strictly according to the manufacturer’s instructions. Clinical signs of toxicity can develop rapidly following their use, particularly in overtired/exhausted horses or in hot humid weather or as a result of over-frequent dipping or from over-strength applications. Signs of toxicity include salivation, trembling or convulsions, constricted pupils, diarrhoea and collapse. Simultaneous use of other organophosphates (such as dichlorvos-based anthelmintics) makes toxicity even more likely. Treatment of the toxic signs includes administration of atropine sulphate by injection and control of convulsions.

Natural insecticidal compounds

A number of natural compounds with variable insecticidal properties have been used over many years in treatment of ectoparasitism in horses. These include rotenone/derris, lime sulphur and natural pyrethrins. These compounds are usually derived from plants or naturally occurring chemicals. They are almost all very safe and non-toxic but their effects are often marginal. All these substances have a limited duration of effect which limits their therapeutic value. Lime sulphur has a particularly unpleasant smell and can cause burning and irritation of the skin when used in strengths over 5% but it has a significant effect on surface-feeding mites and on some dermatophytes.

As the more effective drugs are discontinued the value of these compounds may need to be reviewed.

Synthetic pyrethroids

Commonly used compounds include deltamethrin, permethrin, cypermethrin and fenvalerate. These compounds have almost no known toxicity at any dose and so are extremely safe. They are frequently used topically against skin parasites. Piperonyl butoxide is commonly added to preparations to enhance the activity. They have a higher efficacy and longer duration than the natural pyrethrins which they have largely replaced. The synthetic pyrethroids are particularly useful as repellents for Culicoides spp. midges and a few species of fly but must be applied at least weekly and immediately after rain wetting in order to achieve continuous protection. Cypermethrin and permethrin have been used as repellents in fly fringes and tags for attachment to head collars and the mane and tail hair (Fig. 4.4).

Figure 4.4 This horse has a cypermethrin-impregnated tag fastened to its forelock. Others were attached to its mane and tail to act as an effective fly repellent.

Other agents

General considerations for the use of corticosteroids

Corticosteroids are anti-inflammatory, immunosuppressive drugs which are frequently used in immune-mediated and pruritic skin diseases and in diseases where non-infective inflammation plays a major role in the aetiopathogenesis. They are powerful drugs and must be used carefully and correctly. Oral medication is usually preferable to parenteral injection as the former provides a better sparing effect on the pituitary–adrenal axis and thereby minimizes their secondary, undesirable effects.

The drugs are commonly employed in the treatment of insect bites, insect hypersensitivity, habronemiasis, allergies and in some immune-mediated diseases including vasculitis, urticaria, pemphigus (in all its forms), systemic lupus erythematosus-like syndrome and sarcoidosis.

Specific diseases do not always respond to standard treatment regimens and may require either a higher or lower than normal dose rate. Once remission is established, the dose should be gradually reduced over 2–3 weeks to the lowest dose which maintains the desired therapeutic effect. Once this dose is established, an attempt should be made to use alternate-day treatments as this will have a negligible effect on the pituitary–adrenal axis and will minimize side-effects.

Some of these drugs do not always have the expected effect in every case and alternatives should then be tried. Thus, it is usual to institute treatment with prednisolone as it has the fewest problems and is conveniently administered by the oral route. Dexamethasone should then be used if no effect is detectable within 24–48 hours. Again it is important to reduce the drug dose to a minimum effective dose as soon as possible.

It is important when selecting a corticosteroid to consider its specific pharmacokinetics. For example, prednisone is widely used in other species but it cannot be converted into an effective metabolite in horses. There is little evidence for its efficacy and it probably has no effect beyond placebo in most cases – it therefore has no material value in dermatological therapy. The drug distribution of methylprednisolone acetate is such that its value in dermatology is also very limited. Its supposed depot formulation results in poor volume distribution and unpredictable blood concentrations. Triamcinolone is possibly the compound that is associated with most of the largely irrational fear of corticosteroids in horses. Evidence-based medicine suggests that it is not as liable to induce laminitis as it was previously thought (Cornelisse & Robinson 2004, McCluskey & Kavenagh 2004). As might be expected, there are limits to the efficacy of any drug and potent glucocorticoids are no exception. Maximal doses should be accepted and not exceeded. Furthermore, there may be circumstances when the pharmacokinetics are impaired or altered and then the dangers may be far greater. No drug should ever be administered without due consideration for the possibility of side-effects and the possibility that standard doses may be excessive in some circumstances. As a good principle, drugs should only be used when specifically indicated and should be withdrawn as soon as they no longer have a genuine benefit. This principle applies as much to corticosteroids as to any other type of drug.

Whenever prolonged corticosteroid treatment courses are unavoidable, alternate-day therapy should begin as soon as the case becomes stable. Prednisolone and dexamethasone can be used in this way and so are the preferred choices in most cases. Dexamethasone and betamethasone have a longer duration of effect than prednisolone and so have less sparing effects but can still be used in alternate-day regimens. Anti-inflammatory (low) doses of prednisolone may not cause side-effects, whereas immunosuppressive (high) doses administered daily carry significant risks including immunosuppression (with a risk of infections), laminitis, polydipsia, polyphagia and behavioural alterations.

Prolonged (or sometimes even short) high-dose (or low-dose) courses may induce acute, often life-threatening laminitis, although there are few recorded confirmed scientific reports to this effect. The risks of laminitis should not be underestimated but are probably not as great as once feared. The risks appear to be much higher in cases which have active laminitis or which have had, or are susceptible to, laminitis. Under these conditions the use of corticosteroids should be considered particularly carefully. Triamcinolone seemingly has a greater potential for causing laminitis than other members of the group but is commonly only used in very small doses for the intralesional treatment of collagen necrosis nodules (and occasionally for intra-articular injections).

Complete failure of corticosteroid therapy should be followed by a careful reassessment of the case and, if the diagnosis is confirmed, treatment with aurothioglucose can be attempted as an alternative approach with the same objective. Azathioprine, a potentially toxic immunosuppressive drug, can also be used when corticosteroids are contraindicated for other reasons or fail to bring a resolution.

Systemic corticosteroid therapy (Stannard 1994)

Route of administration

The preferred route of administration for each of the compounds varies.

Parenteral/injection

The drug is injected intravenously, intramuscularly or subcutaneously. Over the period during which the drug remains in the bloodstream it exerts a continuous effect on the area of inflammation. There will also be continuous adrenosuppressive and mineralocorticoid effects. These will remain for as long as the drug is at concentrations capable of exerting the effect (even very low doses can have significant effects on the pituitary–adrenal axis). There will be a progressive, slow reduction in circulating drug concentrations and so there is no abrupt withdrawal of the drug.

Intralesional cortisone injection

Very high local concentrations of the drug can be achieved locally without large overall doses and can therefore provide prolonged local effect. It is applicable to nodular lesions in particular. The overall dose of the compound used (usually triamcinolone acetonide or methylprednisolone actetate) should always be considered. Multiple small injections soon add up and it is easy to exceed the safe upper limit overall when many very small doses are being given. As a basic principle this has little to commend it – there are local and potentially systemic side-effects that are often more serious than the original condition. Injection locally commonly causes calcification and severe local immunosuppression can result in bacterial proliferation.

Oral corticosteroids

The oral route is generally considered to be safer than injections. Provided that the dose is correct, absorption from the gastrointestinal tract is rapid and effective in delivering a therapeutic concentration in the blood. As a basic principle the safest, most effective drug should be used – this usually means prednisolone, which has a high safety margin in horses. The action of prednisolone stops quickly after therapy ceases. Therefore it is useful for alternate-day therapy and has minimal adrenal–pituitary axis suppression. Oral dexamethasone appears to have a rapid and useful intestinal absorption rate. However, the nature of the drug is such that it has a much longer duration in the bloodstream and so has a lesser sparing effect on the pituitary–adrenal axis than prednisolone. Nevertheless there are circumstances when its increased anti-inflammatory effects are valuable.

Note

All single daily doses of oral glucocorticoids should be administered in the morning.

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