19 Skin Conditions The diagnosis of skin disease in the horse often presents a real challenge to the practitioner. One problem is that the condition is often of long standing, allowing ample opportunity for self-inflicted injury and secondary infection. In addition, the owner has probably applied all manner of potions. However, every skin disease should be taken seriously; the lesion may look trivial but it is important to the owner and it may be a disaster waiting to happen. A good history is vital: • How old is the horse? • How long has the horse had the condition? • Has the condition changed? • Are other horses affected? • What treatment has already been used? There is rarely a quick fix with skin conditions, particularly if they are of long standing. Always have a treatment plan and do not be afraid to take samples and photographs to obtain further advice. One of the most useful aids is to brush the feathers into a Petri dish and examine the brushings under the low-power microscope. Parasites, particularly Chorioptes equi, are easily observed. Skin scraping needs to be deep to detect some mites, e.g. Sarcoptes scabiei var. equi, Psoroptes ovis var. equi and Demodex equi. Gram staining is useful to visualize bacteria, particularly from an impression smear. The clinician has to be careful to differentiate between primary and secondary bacteria, as well as contaminants; that is why bacterial culture is rarely worthwhile. It might be considered that a biopsy, which is easily taken with either a 4 mm or 6 mm punch, is the ultimate diagnostic tool; however, the actual sample needs taking with care to sample some normal skin as well as diseased tissue. Also, the pathologist needs to have a full history and photographs to really help the practitioner. A needle aspirate is safer for diagnosis of nodules, as punch biopsies may aggravate equine sarcoids. If pruritus is a prominent sign, the practitioner should consider parasites in the first instance. Parasites can act either as irritants in their own right or as allergens, setting up sensitivity reactions. Termed leg mange as the mite is not found above the carpus or tarsus, this is the most common type of mange in the horse. Some parasitologists consider it to be the same species as Chorioptes bovis and Chorioptes caprae, though clinicians should be aware that there may be cross-over with the host species. Pruritus can be so intense that the horse is continuously stamping its feet, although intensity is not controlled by the number of mites. Often a horse, with millions of mites brushed off into a Petri dish, will show little irritation and yet its neighbour having only a few mites will be very itchy. Leg mange will often start off an attack of pastern dermatitis, which will then develop a bacterial infection. Leg mange can be controlled with regular 10-day i/m injections of doramectin at a dose rate of 1 ml of 1% solution/33 kg. It may also be treated topically with benzyl benzoate applied all over all four legs, to include the carpus and the tarsus, every 10 days. A more modern topical treatment would be a 0.25% solution of fipronil applied over the same area twice, at 10-day intervals. A further treatment suggested is daily washing of the legs with a diluted solution of 5% cypermethrin, which should be carried out for a minimum of 10 days. It is very important to remember to treat all the animals in the group. The most severe mange infection in the horse is caused by this mite; however, it is extremely rare. It occurs all over the body and causes very severe irritation. The mites are often hard to see unless a deep skin scrape is taken. There are invariably secondary bacteria involved and, although their treatment with a prolonged course of a suitable antibiotic is important, the condition will never be cured until every mange mite is dead. The affected horse should receive regular doramectin i/m injections every 10 days at the dose rate of 1 ml of 1% solution/33 kg. In longstanding cases where there is chronic thickening of the skin, this treatment should be supplemented by painting on a 1:1 mixture of 0.5% eprinomectin and dimethyl sulfoxide (DMSO) at 10-day intervals to the severely thickened areas of skin. A very rare mite and not as irritant as either Chorioptes or Sarcoptes, it can be found all over the body but it favours sheltered parts, e.g. under the tail or forelock or even behind the fetlock. The mite is a surface dweller, so doramectin injections are not effective. Topical applications to affected parts every 10 days of either benzyl benzoate or fipronil are effective, as is daily cypermethrin. Various old-fashioned – but extremely toxic and environmentally unfriendly – chemicals such as lime sulfur, toxaphene and lindane were formerly effective in the treatment of all the mange mites discussed above; however, these have all been banned in most countries. The most effective replacement wash available contains selenium sulfide. Although these mites live in the hair follicles and sebaceous glands, they can be found in horses not showing any sign of skin disease. However, they very occasionally appear in very large numbers with an accompanying staphylococcal infection, the treatment of which has to be addressed with both parenteral and topical antibiotics. The mite can be eliminated with formamidine washes, the only one readily available being amitraz; however, great care must be taken if using this medication (see Chapter 18). It should be stressed that the horse is not affected by ear mites. There are two types of lice seen on horses, both capable of causing intense irritation. The constant rubbing will show as hair loss and eventually infection of the skin with secondary bacteria (see Fig. 19.1). Certain individuals seem to be more susceptible, particularly if affected with another disease or malnutrition. Both species, Damalinia equi (the biting louse) and Haematopinus asini (the sucking louse), are readily controlled by topical insecticides, e.g. 5% cypermethrin. This needs to be repeated at 10-day intervals, as the eggs are not killed by the chemical. Injectable doramectin is effective only against the sucking louse. Pour-on ivermectin preparations, available for cattle, should not be used in the horse as they will damage the skin and cause permanent hair loss; oral ivermectin preparations are not effective against lice. Fig. 19.1. Self-trauma resulting from louse infestation. Throughout the world there are many species of tick capable of infesting the horse. They can be divided into two types: the Argasidae (soft ticks) and Ixodidae (hard ticks). All have four developmental stages, egg, larva, nymph and adult. Not only are the ticks species specific, but so too are the different developmental stages; on the other hand, some tick species may prefer certain hosts but are very adaptable and colonize many mammalian hosts. Soft ticks very rarely infest horses, unless they are kept in close proximity to pigs in the tropics; however, hard ticks are a very serious problem in the horse, causing debilitation from blood loss, irritation with accompanying skin disease and spreading of numerous diseases. There are over 650 species of hard tick living off many different hosts that are not always mammalian. Most species have a three-host life cycle, some only a two-host and a very few have a single host. Obviously, the last of these are easier to control, except that resistance to an acaricide can build up more rapidly. Different species prefer different sites on the body, and these may vary between the immature and adult forms. The following tick genera may be found on the horse: • Amblyomma maculatum is a particular problem in horses in tropical and semitropical areas of North and South America; Amblyomma hebraeum, Amblyomma variegatum and Amblyomma lepidum are all parasitic in sub-Saharan Africa. • Boophilus kohlsi is parasitic for horses in the Middle East. • Dermacentor nitens is found in horses’ ears in tropical America; Dermacentor marginatus, Dermacentor reticulatus and Dermacentor silvarum are found on horses in both Europe and Asia. • Hyalomma anatolicum causes severe problems for horses in Asia; Hyalomma scupense is seen in Eastern Europe and Russia. • Ixodes ricinus is found on horses in the UK, Continental Europe, North Africa, North and South America. • Margaropus winthemi lives on zebra and horses in southern Africa. • Rhipicephalus sanguineus and Rhipicephalus appendiculatus are both rarely found on the horse, the former worldwide and the latter in Africa. Ticks can be controlled on equines with organophosphates and chlorinated hydrocarbons; however, in many countries their use has been banned (see Chapter 18). Natural pyrethrins and the synthetic pyrethroids are safer and fairly effective if applied twice weekly, or more often in very rainy weather. Although free-living, these will infest the horse’s legs and cause severe irritation. As the name, suggests Trombicula autumnalis is seen in the autumn (fall) in the UK; it can, in fact, cause problems from June onwards, but it is killed by the first frosts. It is the six-legged larva that causes the irritation, biting firmly into the skin, so a very deep skin scraping is required for diagnosis. Trombicula alfreddugesi is seen in America and Africa, though normally associated with poultry. The poultry mite will also cause intense irritation on the horse. It lives in the wooden parts of the stable or hen house and feeds mainly at night, but some authorities have found it feeding during daylight hours. All the insects listed below may cause varying degrees of pruritis (see Fig. 19.2). Little bigger than Musca domestica, the housefly, the most notable of these is Stomoxys calcitrans. It is difficult to control, as it only feeds once or twice daily. The numbers can be radically reduced by removing the dung pile from the proximity of the stable. They will cause an urticarial reaction in certain individuals (see below). Fig. 19.2. Acute pruritis resulting from flying insect worry. Although bigger and are more aggressive than the stable fly, both Chrysops spp. (the deer flies) and Tabanus spp. cause large weals on the horse, potentially leading to more permanent lumps, particularly in the saddle area, called eosinophilic granulomata (see below). These flies will cause intense irritation to the horse but do not cause any other reactions; they are found worldwide. The most notable is Simulium colombaschense, which is found in the Danube valley in Europe. Survival of larvae requires fast-flowing water. Mercifully, these flies are restricted to tropical Africa. As they spread trypanosomiasis (see Section 5.4), which is a fatal disease of equines, any localized damage is the least of the horse owner’s worries. The bite is very painful, sometimes leading horses to stampede. Although not actually causing any skin lesions, Gasterophilus intestinalis will lay a large number of eggs on the horse’s skin. The horse then ingests these, which hatch into larvae in the stomach; after feeding in the stomach, larvae pass through the intestine and pupate on the soil, after which the adult then emerges and the life cycle is complete. There are two other species, the nose bot Gasterophilus haemorrhoidalis and the throat bot Gasterophilus nasalis, the larvae of which bore into the tongue and buccal mucosa and will be seen when carrying out dental procedures. The best method of control is to kill the larvae in the stomach with oral ivermectin. It should be noted that, although moxidectin is claimed to kill bot fly larvae, the kill rate is not high and many survive. Regular grooming of horses helps remove bot fly eggs and thus prevent infection. In temperate climates frost will kill the adult flies, so the species has to survive as larvae in the stomach; therefore, if oral ivermectin is given after the first frost in winter the larvae will be killed and there will be no flies the following summer. Obviously, this does not occur in the tropics or subtropics. As the name implies, these flies need damp sand in which to pupate. They cause constant worry to the horse, particularly around wounds; their saliva appears to delay wound healing, and so it is vital they are controlled. They are rarely recorded in temperate climates but are common in specific areas in the tropics and subtropics. Wounds should be covered or smeared with an oily cream containing acriflavin and benzyl hexachloride (BHC). Attacking mainly horned ruminants, these flies are found throughout the world; however, they will also attack horses when kept in close proximity to cattle. Haematobia irritans seems to be the most damaging. Normal pyrethrin fly repellents are very effective for the horse; it should be noted that ‘spot-on’ treatments used in cattle are not suitable for horses, as the skin will be damaged. Basically bovine parasites, these flies have been eliminated from the UK. They lay their eggs on the legs of cattle, the eggs hatch and the larvae then bore through the skin and travel for 9 months through the tissues to reach the deep layers of the skin on the back. The larvae emerge and pupate on the ground, the adults emerge and the life cycle is complete. Elimination in the UK has been achieved by strict treatment of all cattle with an appropriate insecticide in the spring before larval emergence. If the eggs are laid on a horse, the larvae may travel through the tissues to the back but, as the horse is not the natural host, they will form a calcified nodule and the life cycle is broken. There are two species parasitic for domestic animals, Hypoderma bovis and Hypoderma lineatum, both occurring in North America, Europe, North Africa and Asia. There is considerable confusion in the terminology of this worm in various parts of the world. They are in known in North and South America as the larvae of a particular blowfly, Cochliomyia hominivorax, which is an obligate myiasis-causing parasite. This has been eliminated in the USA by rigid control methods using now-banned insecticides. There is a similar parasite, Chrysomya bezziana, found in Africa and Asia. Treatment is described under cutaneous myiasis, below. Occurring mainly in wool sheep, animals are attacked by blowfly larvae. In the sheep, Lucilia sericata is the most important blowfly in Europe and Australasia, the larva being termed the primary striker; there are also secondary strikers, e.g. Phormia terra-novae and Calliphora erythrocephala. There are no primary strikers in the horse. Larvae of all these species and the species of screw-worm described above require a wound before they will attack a horse or donkey, particularly if wounds remain untreated; except in the very debilitated animal, clean wounds are not attacked. However, larvae will be seen in hooves that have not been picked out, in wet conditions; they will not penetrate intact hoof. Treatment should be carried out urgently on all wounds to prevent cutaneous myiasis or, as it is commonly called ‘fly strike’. In the animal with myiasis the surrounding hair is clipped away – this is particularly important in the donkey – and the wound washed vigorously with clean water; the use of a hose is particularly recommended. Any maggots remaining are removed, using forceps if necessary. The wound should be treated as described in Chapter 12. If organophosphorus wound powder is available, this is to be dusted on; however, in countries where such powder has been banned the wound and surrounding area should be dressed with ointment containing benzene hexachloride (BHC) and acriflavin; this is marketed as ‘summer fly cream’. There are more than 300 species of mosquito found worldwide, the best known being Anopheles sp., which spreads malaria in humans. Mosquitoes in large numbers will disturb horses. They transmit equine encephalomyelitis and West Nile fever. Diethyltoluidine is the only effective repellent, and that is effective for only 6 h under dry conditions. There are over 80 species of these so-called gnats or midges worldwide. Their saliva causes a very bad hypersensitivity reaction in certain individuals, there being a genetic predisposition. The disease syndrome is called ‘sweet itch’ in the UK, ‘Queensland itch’ in Australia and many other names elsewhere. In the UK midges bite only during the warmer months of the year, but in hotter climates they bite all year round. The expected sites for the midges to bite are the mane and tail, but if these parts are denied to the insects some species will bite elsewhere, so that the horse may itch on its belly or head. Often, the animal will have been itching for some time, so the entire mane and tail hair will have been rubbed off; the skin may also be broken and a secondary infection established. It is vital to stop insects from actually biting the animal. A sensitivity reaction to Culicoides is particularly strong in certain individuals, and appears to be inherited. It certainly is a welfare problem if it is not controlled. Treatment options are many and, on the whole, ineffective, and can be divided into treatments that prevent the insects biting the horse, treatments that desensitize the horse and treatments that lessen the reaction of the horse. The ultimate treatment is complete separation of insects from the horse, which is, in the main, impossible; the best option is the ‘Boett’ rug, an extremely fine-meshed rug that covers the horse totally excepting holes for its legs, anus and mouth. In most cases this is effective; other ‘fly rugs’ are useless. The midges tend to bite at dawn and dusk, so housing the horse at these times is largely ineffectual as the midges will fly inside unless there is very small-gauge netting over all the windows and doors, which makes the building unbearably hot. Culicoides are not good fliers, so wind is a deterrent. High-speed fans are useful, while fly repellents are generally ineffective, the only effective example being N,N-diethyl-meta-toluamide (DEET). Unfortunately, this is really only effective for 6 h under dry conditions, even less in rainy conditions, and so is therefore impracticable. Thick, oily preparations like benzyl benzoate can be helpful if smeared copiously over the bitten areas, but these are extremely messy and their application has to be frequent. Desensitization of the horse requires expensive laboratory preparation after a blood sample has been submitted. A desensitizing injectable preparation is prepared, which then has to be injected into the horse, initially very frequently but eventually at longer intervals. This laboratory preparation is expensive and results are very variable. There are various products available containing certain fatty acids and amino acids which, it is claimed, help lessen the reaction if given before and throughout the likely period of exposure. Lastly, I will discuss medications that actually treat the reaction seen in the horse. As the condition is so pruritic, there is a large amount of self-mutilation, which in itself will cause more histamine release and secondary infection. Infection can be controlled by antibiotics but the irritation is much harder to control; the drug of choice is dexamethasone, but long-term use will be extremely hazardous on account of the danger of laminitis. Prednisolone is safer but, on the whole, is much less effective; oral administration at a dose of 1 mg/kg has been recommended. NSAIDs are much safer but also less effective. Antihistamines are largely ineffective, even at high doses given three times a day. A very common reaction in the horse, owners often associate it with a high-protein diet or food allergy. Food allergies will occur in horses but are extremely rare, and are described in Section 8.9. Normally, urticarial reaction results from the bite of a fly to which the horse is hypersensitive. Raised urticarial plaques will occur all over the body or just in specific areas, the most common sites being the neck and body. These plaques may well become infected and will leak yellow serum, but are not normally pruritic. The standard treatment in the early stages is by dexamethasone, but the clinician may be reluctant to use this medication if there is any possibility of laminitis developing. The safest course of action is treatment with NSAIDs and antibiotics if the area becomes infected. Rugs should be avoided, but hosing with clean, cold water is useful. Antihistamines, e.g. hydroxyzine (see Section 5.9) can be tried as treatment but, like dexamethasone, this drug should not be used in the pregnant mare. Normally seen in the saddle area, these raised areas (singular, granuloma) approximately 1 cm in diameter are not painful and rarely become infected; they are caused by an overreaction to the bite of either a fly or tick. In many cases after some months they will regress spontaneously. Dexamethasone can be injected into the actual lesion and will speed remission, while steroid creams are of doubtful value. Surgical removal is an option but is not really necessary. Occurring in Europe (but not in the UK), the Middle East, North Africa and Asia, this is an unclassified pox virus; conditions include equine molluscum contagiosum and viral papular dermatitis. Pox virus disease shows initially as vesicles on the mucosa and papules on the skin, these latter developing into pustules normally occurring on the mouth, nose and face, but have also been recorded on the vulva and prepuce. The disease is not venereal. Eventually the lesions will regress, leaving scabby areas that then are seen as areas of alopecia. There is no treatment. Autogenous vaccines have been tried, though unsuccessfully. A venereal disease caused by equine herpes virus 3, described in Section 17.7. A very common disease of young horses caused by a species-specific papovavirus, it presents as multiple warts mainly on the face and occurs extremely suddenly; it may even be seen on the newborn foal. It is sometimes seen on the testicles when a young horse is castrated. There is no treatment, and the warts will suddenly regress. Clinicians should be aware that if the warts suddenly occur in horses over the age of 6 they may well not regress, but such cases are extremely rare. Caused by Dermatophilus congolensis, a Gram-positive organism showing branching mycelia or rows of coccoid bodies, this is a common, superficial infectious disease seen worldwide. It is sometimes called ‘rain scald’ or, if on the legs, ‘mud rash’; as the names imply, moisture and the organisms are required in combination for signs to appear. The moisture may be sweat in hot weather; it can occur in winter when the coat is long, or in summer when there is little hair cover. The hair bunches up to form a very small tuft which, when plucked, leaves a small, triangular lesion that is often purulent. It is a self-limiting disease, although the course of the healing is aided by antibiotics and oily creams, with the affected area kept dry. The condition can be spread by rugs and clippers. Both Staphylococcus aureus and Staphylococcus intermedius cause dermatitis in the horse. The bacteria gain entry from wounds or from micro-abrasions. They can be seen as Gram-positive cocci, their spread being by saddles, blankets, girths, bridles and other tack. The disease also is common on the legs, particularly the white areas, and can be spread by continuous movement through deep mud. The notable feature of the condition is pain; lesions, particularly on the legs, will cause even the quiet horse to react violently. Cleaning the areas is important and is best carried out with running water; the area is then dried with paper towels and antibiotic ointment applied. Parenteral antibiotics and NSAIDs are also helpful. The condition can take on a chronic nature, forming granulomatous lesions, called botryomycosis. These lesions will contain bacteria and really should be termed pyogranulomata. Unfortunately, they will not regress with antibiotic treatment as the bacteria are walled off within the lesion; the only effective treatment is radical excision. Streptococcal organisms cause systemic disease in the horse, e.g. Streptococcus equi, which causes ‘strangles’ (see Section 13.3). However, Streptococcus zooepidemicus and Streptococcus equisimilis will cause skin disease, which may take the form of an infected wound, particularly a puncture wound or a serious cellulitis. In very serious cases there may be ulcerative lymphangitis. Aggressive treatment is required, with high doses of antibiotics and NSAIDs. If you see many horses affected with a skin condition in one group it is likely that the cause is a fungus and is called ‘ringworm’. This is a real misnomer, as it has nothing to do with a worm and, in the majority of cases, the skin lesion is not ring-shaped, although it can be and in humans it is often seen as a ring-shaped lesion. The main organism affecting horses worldwide is Trichophyton equinum; however, alternatively they may be infected with Microsporum equinum, which is the most common pathogen in the UK. Neither of these infections cause pruritis, except in the foal; on the other hand, if a horse becomes infected with a non-equine fungus, e.g. Trichophyton verrucosum from cattle or Microsporum canis from dogs, it will usually be pruritic and become secondarily infected. Luckily, the equine-specific fungal infections, T. equinum and M. equinum, are not very contagious to man, although precautions should be observed; in addition, those infections appear to be self-limiting, so treatment for the in-contact human is rarely necessary. Although griseofulvin at 10 mg/kg daily by mouth for 14 days is said to treat the horse (provided it is not intended for human consumption), it gives rather disappointing results except for horses infected with T. verrucosum, when it is very effective. For treatment of either T. equinum or M. equinum infections, use topical applications of either miconazole or natamycin. The disinfectant Vircon®, used as a wash, seems to be helpful not only in reducing the level of fungal infection on the horse but also for cleaning tack, head collars, brushes and rugs; it is rather expensive for stable cleaning, but very effective. All wooden surfaces should have creosote applied; this environmental clean-up is well worthwhile. Equine ringworm, which is a very over-diagnosed disease, is in fact difficult to recognize. The Wood’s lamp is of no use for T. equinum, as this fungus does not cause fluorescence, although fluorescence may be seen with M. equinum. Microscopic examination is difficult, but culture on dermatophyte test media is very useful; these plates, which have an added dye, should be examined daily to ensure that growth and any change in dye colour are not missed. EnvironmentaI fungi can also cause lesions, known as cutaneous pythiosis, caused by Pythium insidiosum. This fungus requires rotting vegetation in a warm, marshy environment; it then appears as an opportunist pathogen on the equine skin, usually on the lower limbs, but can also affect the back, probably if the horse rolls. It may be seen on impression smears stained with Giemsa. Infection is restricted to tropical and subtropical areas. The equine sarcoid is by far the most common nodule affecting the horse’s skin; it also affects the skin of ponies, donkeys, mules and zebra – over 80% of the very rare Cape Zebra found in South Africa are affected, which leads to a supposition that there is an inherited susceptibility. Strictly speaking, an equine sarcoid is not a tumour but an aggressive fibroblastic nodule, but it is easier for the lay person to understand the condition if clinicians describe it as ‘skin cancer’. Owners are also heartened if told that sarcoids will never metastasize to internal organs. The cause is still unknown, although there is certainly a link with a bovine papilloma virus; it is extremely likely that they are spread by flies. The incidence varies worldwide; 9% of horses in the UK have sarcoids. They can occur anywhere on the body, the most common sites being the groin, the neck and around the ears; they are rare on the lower limbs but are very difficult to remove from these sites (see Figs 19.3 and 19.4).
19.1 Introduction
19.2 Aids to Diagnosis
19.3 Mange Mites
Chorioptes equi
Sarcoptes scabiei var. equi
Psoroptes ovis var. equi
Demodex equi
Ear mites
19.4 Lice
19.5 Ticks
19.6 Other Skin Parasites
Trombiculid mites
Dermanyssus gallinae
19.7 Flying Insects
Stable fly
Horsefly
Blackfly
Tsetse fly
Bot fly
Sandfly
Horn fly
Warble fly
Screw-worm
Cutaneous myiasis
Mosquito
Culicoides
Culicoides sensitivity
19.8 Urticarial Reaction
19.9 Eosinophilic Granulomata
19.10 Viral Skin Conditions
Horse pox
Coital exanthema
Viral papilloma
19.11 Bacterial Skin Conditions
Dermatophilus
Staphylococcus
Streptococcus
19.12 Fungal Skin Conditions
19.13 Equine Sarcoid
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
