The various veterinary medicines will be discussed in the relevant chapters. However, I will generalize in this chapter on anti-infective drugs and anti-inflammatory drugs, both steroidal and non-steroidal. I will end the chapter with a list of medicines I consider should be carried by the ambulatory equine veterinarian.

Lawsonia intracellularis does not seem to respond to any antibiotics other than the macrolides. It is possible that macrolides are bactericidal at higher doses. What is important to remember is that macrolides should never be given intramuscularly in the horse as this causes a very severe local reaction. Erythromycin should be given per os at the rate of 25 mg/kg three times daily, in combination with rifampicin per os at the rate of 10 mg/kg twice daily. This should be carried on for 4 weeks.

Before deciding on an antibiotic it is relevant to consider the factors affecting the route of administration. These are:

• hospitalization;

• temperament of the horse;

• ability of the owner;

• cost.

Even quiet horses will not tolerate long-term courses of i/m injections; i/v injections are the preferred route in this situation. In a hospital situation i/v cathetherization is ideal, but in the field this is hazardous. Jugular abscessation is a nightmare; equally, sarcoid formation has been reported.

Below are presented the antibiotics available for injection in the horse.

Benzathine penicillin G

Products containing this type of penicillin are the so-called ‘long-acting penicillins’. These products achieve such low plasma concentrations in the horse that they cannot be recommended. They must not be injected i/v and they cause severe reactions following i/m injection. They are best avoided.

Ceftiofur

This cephalosporin antibiotic can be given either i/v or i/m. It has excellent tissue penetration and so has many uses.

Gentamicin sulfate

Gentamicin can be given either i/v or i/m. As stated above, it can be nephrotoxic (particularly in foals), so it is best given only once daily.

Oxytetracycline

Penicillin-Na G crystapen

This can be given either i/v or i/m. As stated above, it needs to be repeated ideally at 6 h intervals. Certainly, it cannot be expected to last 24 h.

Procaine penicillin G

This can only be given i/m. It is often given in combination with streptomycin (an aminoglycoside). However, streptomycin has actually been shown not to be active in the horse. So, although these products are licensed in the horse, their use is questionable. There are very few experienced equine practitioners who have not seen a severe reaction to Procaine penicillin G. Therefore, although it may be used in the horse, the fewer times you use it the less likely you are to provoke a reaction.

Trimethoprim-sulfadoxine (TMS)

The i/v and oral routes are both suitable for TMS. Officially it lasts for only 12 h, but common usage for the injectable formulation is once daily. It is very irritant, so should not be given by the intramuscular route. There is a well-documented danger with alpha-2 drugs, so it should not be administered with any of that type of sedative. It is also reportedly ineffective in the presence of pus.

Eye infections need special consideration in the horse. Chloramphenicol is widely used but its use in the horse, which is classed as a food-producing animal, is controversial. Gentamicin is the drug of choice. Amikacin, which is difficult to obtain, is useful for subconjunctival injections. It is also used in joints concurrently with steroids, although this use is controversial as it may be irritant. It is thought by some authors to be unnecessary if full aseptic procedures in joint surgery are adhered to. Treatment for the very serious condition of melting corneal ulceration is difficult, but current thinking is to use tobramycin (an aminoglycoside) and EDTA (ethylenediaminetetraacetic acid) plasma.

Skin infections in the horse, as in other species, require prolonged treatment, e.g. 21 days. Oral treatment is restricted to TMS twice daily. If there is doubt as to the bactericidal effect the normal dose rate may be doubled. If another antibiotic is required enrofloxacin (a fluoroquinolone) can be used, although there is no licensed preparation yet available for the horse. Nevertheless, there is available a 10% oral solution of enrofloxacin licensed for chickens; the daily dose for the horse is 1 ml/kg. There have been recent pharmokinetic studies performed indicating that uptake may not be sufficient. However, it is still widely used and is relatively expensive.

Sadly, antibiotic therapy is not always successful, for many reasons. The owner should be helped to avoid poor compliance. The clinician can prevent problems by making the correct diagnosis and choosing the correct antibiotic. The dosage, route and frequency should be optimal. Obviously, known drug interactions should be avoided. Unfortunately, antibiotics do not dissolve foreign bodies! Naturally, the clinician cannot be blamed for the depressed immune system of the patient or the antibiotic resistance of the bacteria.

These can be divided into systemic agents given either by i/v injection or orally. There are also topical agents.

Amphotericin

Griseofulvin

The commonly used griseofulvin is a very effective treatment for Trichophyton spp. These infections are normally caught from cattle. Equine animals should be given 5 g/50 kg of 7.5% powder in their feed daily for 7 days. However, griseofulvin is not effective against Microsporum equi (equine ringworm). Care should be taken by pregnant women when handling griseofulvin, as it is teratogenic. It should be remembered that ring-worm, particularly Trichophyton spp., is a zoonosis. Vigorous scrubbing and strong disinfectants that damage the skin should be avoided. Normal washing with soap is preferable. Women and children are particularly susceptible.

Miconazole

Miconazole is a good topical treatment for M. equi. It should be used as a shampoo twice weekly, but it is expensive.

Natamycin

Natamycin is also a good topical treatment for M. equi, and it too is expensive. It is supplied in powder form for re-suspension. The suspension is then sponged or sprayed on to the affected area every 4–5 days.

Virkon S

The last on this list is a broad-spectrum virucidal disinfectant. It is a very effective disinfectant and also can be used to treat M. equi. It should be made up as a 1% solution, i.e. one 50 g sachet/5 l water. This solution can be used either as a spray or sponged on to affected areas every 48 h. This use is not licensed in the UK but is widely used off-licence. It is relatively inexpensive.

I will divide these into two groups. The first group is used to treat blood-borne parasites, mainly in the tropics. The second group is used to treat other protozoal conditions.

Group 1

Diminazene aceturate

The most hazardous of this group of drugs, it is extremely effective against Babesia equi and Babesia caballi. However, as less dangerous drugs are available to treat this protozoan these should be considered. On the other hand it is the most effective drug to treat trypanosomiasis. This includes Trypanosoma brucei (surra), Trypanosoma vivax (acute ‘fly’), Trypanosoma congolense (chronic ‘fly’), Trypanosoma evansi (camel ‘fly’) and Trypanosoma equiperdum (dourine). The first three of these trypanosomes are spread only by tsetse fly. Therefore, you will only see them in a tsetse area. In areas with a high density of tsetse fly only game animals can survive. Cattle can be kept for short periods under prophylactic treatment with trypanocidal drugs used prophylactically. Donkeys and mules can be kept in the same manner for short periods. None the less, it is too dangerous to treat horses prophylactically and therefore such areas should be avoided by horses. If trypanosomiasis is suspected in the equine a diagnosis should be made rapidly from a blood smear. Treatment should only be carried out if the diagnosis is certain. T. evansi is spread by biting flies outside of tsetse areas. Donkeys, and to some extent mules, appear to develop a premunity and do not readily succumb to the infection. However, the organism is very serious in horses. Once again a rapid diagnosis should be made on blood smears, and treatment should then be carried out. T. equiperdum is a venereal disease of horses, mules and donkeys. The trypanosomes can be found with care in the pus coming from the vulva and the urethral opening in males.

Diminazene aceturate can be used to cure all these infections. It is supplied in 1.05 g sachets for reconstitution in 12.5 ml water to make a 12.5% solution. This is the normal dose for an adult cow. Often this is made up in a non-sterile manner, which can cause abscesses in cattle. It will cause massive problems if given in such a manner to horses. A protocol for treatment of a 500 kg horse is to dissolve the 1.05 g sachet in 50 ml warm, sterile water in the most aseptic manner possible. This is then injected i/v at blood temperature, as slowly as possible, preferably though a catheter unless the horse is so ill that it is unlikely to move.

Imidocarb

Isometamidium

Formerly a useful drug in cattle to treat T. brucei, T. congolense and T. vivax, widespread resistance has since been encountered in cattle and so I would not advise its use in the horse.

Quinapyramine

Also formerly used in cattle as either the sulfate or chloride preparation. There is now widespread resistance in infections of T. brucei, T. congolense and T. vivax. I do not advise its use for these infections in the horse, but it has been used to treat T. equiperdum. I would not advise its use because it favours the formation of a carrier state. It also causes severe abscessation when given i/m. It is a suspension and on no account should it be given i/v.

Suramin

Can be used to treat T. evansi and T. equiperdum. However, resistance is widespread and therefore I cannot advise its use.

Group 2

Metronidazole

Metronidazole can be used to treat protozoal diseases, but its principal use in horses is to treat anaerobic bacteria. These may be in the alimentary system, wounds or pedal infections. The best method of administration is by mouth, at the rate of 15 mg/kg every 8 h. The drug is supplied in 500 mg tablets. It may cause inappettance, which is a difficult side effect to deal with as often the owners are unable to get sufficient medicine into the horse. Metronidazole is also supplied as a 20 mg/kg solution in 50 ml sachets. This solution is best used topically on wounds or hoof infections. Nevertheless, some workers have used it i/v at 20 mg/kg every 24 h for 3–5 days to treat clostridial enteritis and peritonitis. The drug per se can cause diarrhoea, so this side effect is very worrying in horses with existing enteric problems. I would advise against i/v use.

Nitazoxanide

Nitazoxanide can also be used to treat EPM, and is almost as effective as ponazuril. However, it may cause side effects and so should be given at half the normal dose rate, i.e. 25 mg/kg daily by mouth for 5 days. If all is well a dose of 50 mg/kg should be given for a further 23 days.

Ponazuril is the drug of choice for EPM, having a cure rate of 70%. It should be given by mouth at 5 mg/kg every day for 28 days. It is now available in paste form.

Pyrimethamine

Pyrimethamine can be used to treat toxoplasmosis and equine protozoal myeloencephalitis (EPM). There is a synergism between pyrimethamine and potentiated sulfonamides, and so the two drugs are often given together. Pyrimethamine should be given by mouth at 0.1–0.2 mg/kg daily. Sulfonamides should be given at the normal dose rate of 15 mg/kg. This regime may be given for several months until a cure is effected.

Rather than give a long list of the anthelmintics available I, will describe the helminths that cause problems and then suggest methods of control. This will include the strategic use of anthelmintics, bearing in mind the widespread resistance in other herbivores.

Lungworm (Dictyocaulus arnfieldi)

Dictyocaulus lives in donkeys without causing symptoms. Nevertheless, the donkey can act as a carrier and infect mules and horses. The presence of this worm will cause coughing in these species. It is readily controlled by ivermectin paste given by mouth at 0.2 mg/kg. It is not controlled by moxidectin paste or any other family of anthelmintics. Some practitioners give ivermectin by i/v injection or in the cattle pour-on formulation. Both these methods of administration are not advised. No resistance to ivermectin has been recorded in lungworm.

Large bowel worms (Strongylus vulgaris, Strongylus edentatus and Strongylus equinus)

Foal worm (Strongyloides westeri)

Strongyloides is now, in my experience, rarely seen, but it can cause diarrhoea in foals that will be persistent unless the worm burden is eliminated. It is sensitive to all classes of modern anthelmintics and no resistance has been recorded.

Horse ascarid (Parascaris equorum)

The horse ascarid worm can occur in both foals and adults. There are reports of it becoming resistant to certain wormers, but there are no reports of resistance to pyrantel wormer at normal doses so this must be the drug of choice. Piperazine is also effective and no resistance has been reported. A dose rate of 250 mg/kg should be given by naso-gastric tube. It is very unlikely that a group of these worms will obstruct the small intestine (SI) of the young foal, although it is possible. The adults live in the SI. The larvae migrate through the lungs and are coughed up and so return to the SI, taking approximately 17 days.

Horse pin worm (Oxyuris equi)