Profile

This dermatological disease is caused by infestation with Chorioptes equi which primarily affects the distal limb regions but can extend to abdomen, axilla and groin. The parasites are scale-feeders and live on the surface of the skin. Adult mites can survive in bedding and stable floors for some months, especially if there is accumulated skin debris.

Mite populations are much higher over winter and signs are therefore more recognizable in housed over-wintered horses. This may relate to skin wetting, poor hygiene and the cooler environmental temperatures. The close proximity between housed horses in winter and the movement between stables of staff and tack and equipment means that epidemic infestation can occur in stables. The mites do not survive as well on sun-exposed pastures so populations tend to reduce markedly when horses are at pasture in summer. Carrier horses showing no clinical signs of infestation serve to perpetuate the infection from season to season. In pastured horses a few mites might be found, particularly in the heel bulb and coronary band regions – these cases may be a carrier source that perpetuates the infestation.

Heavy (draught) breeds with dense feathering on the legs are the most frequently affected but thin-skinned, non-feathered horses can be severely affected also – no horse in a challenging environment is exempt from the possibility of infestation. Apart from the tenuous relationship to heavy feathered breeds, there is no colour, sex or age predilection.

The condition is part of the verrucose pastern dermatitis (greasy heel syndrome (see p. 47). The disease is transmitted by direct and indirect contact. Chorioptes equi is possibly species-specific and certainly does not seem to infest in-contact humans, goats, sheep or cattle.

The parasites are easily recognized (see p. 46).

Key points: Chorioptic mange

1. Common cause of pruritus, worse in housed horses in winter months and commoner in feathered legs. The mites survive well in the stable environment especially where skin debris and dander are present and straw bedding is used. Carrier horses harbour a few mites around the distal limb over summer.

2. Usually many parasites (at varying stages and eggs) but clinical signs can be severe with only a few parasites. Pruritus not always proportional to severity of infestation!

3. Also affects body and neck/head and can be generalized, causing scaling and severe self-trauma. Carrier animals may show no signs.

4. Diagnosis is simple by examining brushings with magnifying glass or microscope

5. Treatment involves repeated anti-parasitic washes, clipping of affected areas and stable hygiene. Important to treat all in-contact horses at the same time. Stable hygiene should be improved. Turn-out generally results in reduced mite numbers.

Clinical signs

Pruritus, irritation and restlessness with foot stamping and biting at legs are prominent early signs. Stabled horses in winter seem more liable to the infestation, with some remission over grazing periods. It is likely that some of these cases remain carriers, perpetuating the infestation within a stable yard. The affected horse seeks every opportunity to rub against or kick on posts, fences, concrete steps, walls and rails. Affected groups of horses can often be heard stamping their feet all night. Irritation is worse in warmer weather. Rubbing of the leg often causes the horse to bite or chew at the site (Fig. 9.1). Rubbing of the tail-base and face can also be encountered.

Figure 9.1 Chorioptic mange. This Shire horse mare bit her hind pastern in response to heavy infestation with Chorioptes equi. She persistently stamped her feet and rubbed them on the stable walls.

Patchy alopecia with broken hairs (possibly as result of rubbing and biting) and prominent scaling are common signs (Fig. 9.2). Exudation of serum with matting of leg hair, alopecia and scabs may develop over limited or extensive areas. Severe thickening of the skin and secondary bacterial infections arise from the long-standing, persistent self-inflicted trauma and this is one aspect of the verrucose pastern dermatitis syndrome that plagues many heavy draught breeds and the Shire and Clydesdale in particular (Fig. 9.1).

Figure 9.2 Chorioptic mange showing scaling (A) of the pastern region and ulceration (B) which was caused by self-trauma. This horse is the same as that shown in Figure 9.1.

The severity of pruritus is not proportional to the severity of the infestation – some horses will rub a great deal with few mites while others with heavy burdens will show little (or possibly even no) pruritus. The condition is not restricted to horses with feathered limbs (Fig. 9.3).

Figure 9.3 Severe scaling of the distal limb regions in a Thoroughbred cross mare with a non-feathered leg. Note the extensive scaling and crusting with some areas of acute inflammation which were probably due to self-inflicted trauma. Profuse infestation was identified on her and on her stable companions even though they showed very little or no pruritus.

Repeated episodes of self-inflicted damage can cause progressive thickening of the skin and exudation over the areas concerned. In some cases there may also be a degree of hypersensitivity to the mites. Secondary bacterial and fungal infections, fly strike (myiasis) can combine to produce a true verrucose pastern dermatitis (greasy heel) syndrome (see p. 471).

The head, neck and tail-base can be affected as part of the condition or on their own in a few cases. Whole-body (generalized) infestation results in a generally moth-eaten appearance with generalized pruritus, weight loss, irritability and poor exercise tolerance.

In-contact horses may be an important aspect of the clinical investigation both from a direct contagion viewpoint and because of the possibility that an individual may be a non-seasonally/persistently affected carrier animal. Straw bedding appears to be a significant exacerbating factor.

Differential diagnosis

Diagnostic confirmation

Treatment

Careful clipping of long hair and removal of scabs followed by scrubbing of all affected areas with appropriate insecticidal shampoo or powder may be successful in reducing the numbers of mites and providing some relief of clinical signs. Organophosphates and chlorinated hydrocarbon insecticides are effective but are less available now.

Oral ivermectin paste (at 0.1 mg/kg daily for 7–10 days or 0.2 mg/kg twice at 2-weekly intervals) or moxidectin paste (at 0.4 mg/kg q 14 days) appears to help appreciably. However, this approach does not kill the mites – some do die but others are simply rendered less active and they lay fewer eggs. Therefore this alone does not cure the problem (Littlewood et al 1995). There is no advantage in injecting ivermectin over oral dosing – the intestinal absorption is rapid and provides the same blood concentrations very rapidly. There are significant dangers in injecting the drug into horses apart from its ‘off label’ use. Injectable doramectin show some promise as a therapeutic agent but again the effects are unreliable.

Topical washes of the affected horse and the in-contact animals are the main approach. Affected animals should be clipped out first (although in some cases this is anathema to the owners!). A preliminary warm wash with a selenium sulphide shampoo is a very helpful measure prior to the application of fipronil spray (FrontLine, Merial, UK) (see p. 93) or an anti-parasitic rinse. Application of a cat or dog flea collar impregnated with cypermethrin or permethrin is a very helpful approach (see p. 95) and provides a good measure of protection for some weeks. The collars should be tested on one limb first in case the animal shows a local reaction, and must be fitted with a locking buckle to avoid over-tightening and an elasticized section to prevent the collar ‘snagging’ (Fig. 9.4).

Figure 9.4 A cat flea collar impregnated with diazinon placed on the fetlock of a horse to control chorioptic mite reinfestation.

Stables where the disease is established show less response to treatments and isolation of infected horses from foals and yearlings should be considered. Mass treatment is useful in controlling large outbreaks or in stables where recurrent infections are common but reservoir hosts (and possible asymptomatic carriers) might make it impossible to eradicate in spite of the short viability of the mite off the host.

Profile

This is a parasitic skin disease (body mange) primarily of young stabled horses caused by the non-burrowing scale- and fluid-eating mite Psoroptes equi. Psoroptes spp. mites do not appear to be particular about their hosts and so there may be fewer species than once was appreciated. However, it does appear that different feeding habits do occur and so this may be the main species differences. Psoroptes hippotis may be a separate species which may cause some cases of ear irritation and head-shaking. Older horses may become infected if infected younger horses are introduced into the stable. Mites can also be spread by head collars and rugs. Survival in a cool damp environment is possible for up to 2 months or more. Carrier status is not yet established but individual yards are recurrently affected. Degrees of hypersensitivity can occur also.

Key points: Psoroptic mange

1. Uncommon cause of pruritus largely restricted to the head and ears and tail regions. There may be relatively few parasites.

2. Signs are mainly related to pruritus and localized skin thickening with hyperkeratosis. Severe excoriations can occur and this may be complicated by hypersensitivity reactions.

3. Diagnosis is by identification of the mite in ear debris and skin brushings and scrapings.

4. Treatment options are limited to topical anti-parasitic creams and washes and oral ivermectin or moxidectin. Treatment commonly has to be repeated frequently. Hygiene and avoidance of transfer to other horses is important.

5. It is worth checking other in-contact horses carefully. Once eliminated, the condition is relatively easy to avoid. A few horses become long-term carriers.

Clinical signs

Some affected horses show few or no signs at all but close inspection of the pinnal margin in particular may reveal thickening and hyperkeratosis. Obvious clinical signs attributed to the condition include head-shaking/rubbing and tail-rubbing. Moderate to severe pruritus mostly around the ears, mane, body and tail-head are non-specific but important signs (Fig. 9.5). Pruritus is more severe in warmer weather. Scaling (seborrhoea) and exudation are common, particularly along the margins of the ears along the mane and over the tail-base.

Figure 9.5 Psoroptic mange. Psoroptes spp. mites which caused extensive tail-rubbing and severe self-inflicted trauma. This was a severe case.

Aural discharge, head-rubbing, head-shaking (with a side-to-side shake) and the ears held flat (‘droopy’ ears) are characteristic of P. hippotis (Fig. 9.6).

Figure 9.6 Psoroptes hippotis. Infestation of the skin of the ears causing a droopy, lop-eared posture. The margins of the ears showed extensive scaling and thickening.

Tail-head infestation causes tail-rubbing with consequent poor hair density and broken and distorted tail hairs.

Irritability, particularly around the head, is often reported and some cases of head-shaking have been attributed to this infestation.

Differential diagnosis

Diagnostic confirmation

Treatment

The general body and tail itch may respond to anti-parasitic shampoo containing organophosphate, chlorinated hydrocarbon or synthetic pyrethroid insecticides. It may be helpful to clip the areas affected before any washing. The effect of oral ivermectin or moxidectin is uncertain but it may be expected to have at least some effect. Injectable doramectin (off label) is untested but it could be effective. Fipronil is certainly worth a try.

Most horses resent any interference with their ears. Tranquillization may be required. All accumulated wax and debris should be cleaned from the ears. Where such products exist, proprietary canine anti-parasitic ear drops containing gamma benzene hexachloride, benzyl benzoate or synthetic pyrethroids can be used with good effect. However, some horses resent the drops more than the condition and can become permanently head-shy after even one or two applications. Direct applications of human pediculosis creams applied along the ear margins can be useful.

Treatment should be repeated in 10 days to ensure that any newly hatched mites are killed.

Profile

This is an uncommon, highly pruritic skin disease due to the burrowing mite Sarcoptes scabiei. Pruritus is a combined result of the burrowing mite and a hypersensitivity to its proteins. Transmission is by direct and indirect contact although the mite has a poor survival time off the host. It has been eradicated from large areas of the world and is now a rare disease, largely as a result of strict control measures. The removal of effective treatments makes it likely that it will reappear. The large majority of horses affected by this are immunocompromised in some way.

Contact with affected pigs and small ruminants may be a significant source of infection. Transmission to humans (and vice versa) is theoretically possible so it is worth asking if the handlers have any pruritic problems!

Clinical signs

This condition is characterized by intense, almost uncontrollable pruritus. Usually it is reported to start in one site but then rapidly spread to involve the whole body. Lesions frequently begin on the head, neck and ears and may then spread over the entire body (Fig. 9.7).

Figure 9.7 (A–D) This pony was presented with generalized severe pruritus, and Sarcoptes scabiei were identified in skin scrapings. Almost all affected horses are immunocompromised.

Figures courtesy of Dr Marianne Sloet.

The earliest lesions are papules and vesicles on skin with intense pruritus. Rubbing and biting leads to progressive and extensive hair loss and excoriation with serum exudation/crusting and even bleeding. The skin may become very crusty and thickened (lichenified).

Key points: Sarcoptic mange

1. A very rare skin disease causing severe or extreme pruritus. Most affected horses are immunocompromised. Contact with affected pigs, small ruminants and humans can be a factor. Pruritus is a result of dermal burrowing and hypersensitivity responses.

2. Pruritus is usually severe, especially in compromised cases where generalized excoriations and severe and distressing itching occur. Severe damage and secondary skin infections are likely. Pruritus is worse if the skin is warmed by sunlight or by rugging.

3. Parasites can be identified in skin scrapings (seldom in brushings). Multiple skin scrapings will maximize the chance of finding the parasite, which is easily recognized. Biopsy may be lucky – a parasite or its track may be recognizable in the generally inflamed skin; it is often complicated by inflammation from self-trauma.

4. It is essential to address any underlying immunocompromising disorder at the outset. Treatment involves repeated anti-parasitic washes (lindane or malathion), clipping of affected areas and stable hygiene. Oral ivermectin or moxidectin may help and ‘off label’ doramectin injections can be justified. The source of the infestation should be sought.

5. The prognosis is poor and complicated by any underlying disease.

Secondary bacterial infections and myiasis can occur. Affected horses often have altered behaviour and temperament and may lose weight significantly, but some of these problems may be an underlying disorder.

Differential diagnosis

Diagnostic confirmation

Treatment

Underlying imunocompromising disorders should be addressed – without this the condition will probably not respond usefully to any treatment.

Topical treatment with chlorinated hydrocarbons or organophosphate insecticides approved for use in horses has been the main approach but most such compounds are no longer commonly available. Malathion, lindane and organophosphate chemical washes may, however, be helpful where they are available. Oral ivermectin or moxidectin at normal dose rates repeated weekly for three doses may be effective. Injectable doramectin (‘off label’) should help. There is no information available on the therapeutic value and the risks associated with topical selamectin as used in dogs.

Washes containing synthetic pyrethroids are very variable in efficacy and usually fail to solve the problem. Similarly 2% lime sulphur has been advocated but it is unlikely to be effective on its own.

Repeated washes and dosing may be needed and it is important to treat in-contact horses at the same time even though these may not have any overt evidence of infestation.

Relief from the pruritus may require tranquillization but this is a short-term expedient.

Corticosteroids will not help in the long term and may exacerbate the problem both in the short and long term.

The prognosis for affected horses is poor.

Control

The disease is reportable in some countries.

Mites can live up to 3 weeks in infested stables and fomites.

Early detection is critical if the condition is to be managed. Isolation of all infected horses and fumigation of all fomites, grooming materials, rugs and harness must be performed. Immediate prophylactic treatment of any potential in-contact horses and elimination of contacts with other species that may be involved may be wise.

Profile

Demodex spp. mites are probably a commensal of the skin of all large animal species. The full life cycle is completed on the host and the parasites can only survive for a few hours at most away from the host. This means that spread between animals is difficult and slow (if at all) – it is best viewed as non-contagious.

The parasites only become clinically significant when the host animal is immunocompromised. Clinically this is a very uncommon disease even though mites can be demonstrated in apparently normal horses. Demodex caballi can be found in and around the eyelids and muzzle and Demodex equi from the body.

Clinical signs

The infestation is invariably asymptomatic in normal horses. Lowering of general body immunity by other conditions (such as debility, concurrent disease, stress or pituitary adenoma) may influence the likelihood of clinical signs developing.

Key points: Demodectic mange

1. A rare skin infestation causing few symptoms. Demodex spp. are probably best regarded as a normal commensal (and opportunistic pathogen) of the skin of large animals. They may become pathogenic in immunocompromised horses.

2. Most cases are identified incidentally.

3. Parasites can be identified in skin scrapings (not in brushings) taken from the eyelids in particular. Multiple skin scrapings will maximize the chance of finding the parasite, which is very easily recognized. The parasite may be recognizable incidentally in biopsies taken for other reasons.

4. Treatment is not usually necessary – the role of regular avermectin anthelmintic doses is not established but this may reduce the overall prevalence still further.

5. The disease is incidental and seldom if ever significant except as part of an immunocompromise condition (stress, debility, steroid or lymphosarcoma).

Patchy alopecia sometimes with scaling, papules and pustules particularly over the face, neck, shoulders and forelimbs may be associated with the condition and/or the underlying immunocompromise. Some cases of leukotrichia and (possibly) leukoderma around the eyes may be caused by this parasite (Fig. 9.8).

Figure 9.8 This horse was severely immunocompromised and had evidence of liver failure. It exhibited persistent pruritus and pigmentary changes around the eyes. A deep scraping revealed copious live Demodex spp. mites. It was not certain that this was the cause of the problem, however.

Figure courtesy of Richard Frost.

Differential diagnosis

Diagnostic confirmation

Treatment

As the condition is usually asymptomatic, it may regress spontaneously. Usually it is not directly treated, though consideration should be given to reasons for impaired immunity and to the treatment of other concurrent disease.

Treatment with repeated avermectin and washes of an organophosphate may be curative if a genuine case is presented.

Regular oral doses of ivermectin or moxidectin probably will (gradually) remove the parasites and as this is a common practice now there may be far fewer horses with any mites at all.

Profile

Infestation with (Neo)Trombicula spp. (‘harvest’, ‘chigger’, forage, ‘scrub itch’ or ‘heel bug’ mites) is relatively common in defined geographical regions (usually associated with chalky and grassland soils) in late summer and autumn. Individual badly infested pastures are usually recognized and then avoidance of autumn grazing may be a useful (if often impractical) suggestion. The adult mites are free-living and feed on plants or invertebrates but the red-orange-coloured larvae require tissue fluids, normally derived from rodents and incidentally from horses and humans. The mites are only on the horse for short periods to feed and so even in severe infestations they may be difficult to find. Clinical disease is associated only with infestation by the larval mite.

A second group of mites behaving in a similar way are the so-called forage mites. These are usually larger and heavier and often have a more hairy appearance. They inhabit straw bedding and so clinical signs are restricted to housed horses. The individual behaviour of these opportunistic parasites varies and so the patterns of disease associated with them are unpredictable. Some are surface feeders and others are simply irritants as they move around in the hair coat (usually of the distal limbs).

Key points: Trombiculidiasis

1. A common, usually transient, seasonal pruritic skin infestation of pastured horses. Regional differences occur in the species involved and the circumstances but most are restricted to chalky soils. Forage mites tend to affect stabled horses − especially those on straw bedding.

2. Mild to severe regionally restricted pruritus under defined management conditions. Usually affects the head and legs more severely but can involve the whole body. Changes in management usually improve matters.

3. LarvalTrombicula spp. parasites can be identified directly as yellow-red or orange dots on the skin. Forage mite species are much larger and usually have a hairy appearance.

4. Treatment is helpful – permethrin washes or fipronil sprays are helpful. Cat flea collars around the legs can help also. The value of avermectin anthelmintic doses is not established but they may reduce the overall prevalence.

5. The disease is incidental and only rarely significant except as part of an immunocompromise condition (stress, debility, steroid or lymphosarcoma).

Tyroglyphus spp. mites occur only in grain and can incidentally affect horses.

Clinical signs

(Neo)Trombicula autumnalis infestation is usually restricted to horses at grass (especially those grazing on chalky soils).

Individual bites are papular, inflamed and exudative and may coalesce to give much wider areas of exudative dermatitis. The condition usually affects the head (nose and face) and the lower legs in late summer and autumn (Fig. 9.9). Repeated and severe biting at the lower legs results in small or larger hairless areas which may become obviously inflamed and exudative after 2–3 days (Fig. 9.10). Irritability, leg-stamping, nose-rubbing and head-shaking reflect the intensely pruritic nature of the infestation – some of the response may reflect a hypersensitivity. Small papules and wheals develop on distal limbs, nose, neck (Fig. CD9 • 1A–C) and the ventral abdomen (where it can cause a form of ventral midline dermatitis). Exudation and superficial inflammation reflect both the local reactions to bites and the self-inflicted trauma. In some circumstances the whole body can be affected.

Figure 9.9 Trombiculidiasis. Diffuse dermatitis of the face caused by Trombicula autumnalis infestation.

Figure 9.10 Trombiculidiasis. Extensive dermatitis which arose as a result of self-inflicted trauma due to severe and persistent trombiculid mite infestation. Note that the dermatitis was not restricted to the white areas.

Forage mite infestation will affect different parts of the body exposed to hay/straw and other forage sources. Some can affect the head and legs and others affect the body when the horse lies down. Overhead hay/straw storage may be a source of mites on the back. The mites tend to remain fairly localized so collection is easy but numbers may be low.

Differential diagnosis

Diagnostic confirmation

Treatment

Horses usually lose the infestation quickly and so should only be treated if absolutely necessary. It is better to remove the source of infestation if possible or to move the horses to a non-infested pasture.

Topical applications of sulphur washes, and permethrin powders and washes are helpful. Regular spraying with fipronil may be useful. Attention must be paid to the possible source of infection.

Forage mites may live in preserved forage or bedding so infestation can sometimes be ‘out-of-season’ and affect different regions of the body.

Profile

This pruritic dermatological disorder is due to cutaneous infestation with the poultry (red) mite Dermanyssus gallinae. Direct or indirect poultry contact is always present. The mites are blood-feeders and localized hypersensitivity responses may be encountered.

Key points: Poultry (red) mite

1. A pruritic condition due to incidental/opportunistic bites of a variety of poultry blood-feeding ectoparasites including (most often)Dermanyssus gallinae Poultry or pigeon exposure is always involved.

2. Pruritus restricted to specific regions such as the back, face and legs is the usual presentation. Some individuals will be more/less affected than others.

3. The parasites are easily identified but they do not stay on the horse for long. Therefore extensive and repeated brushings are often required. Circumstantial evidence is often strong! The mites may be found in and on wooden rails used for perching.

4. Treatment is not usually necessary – but washes and dusting with a permethrin compound will help limit the effects. Washing any wooden roosting/nesting sites and eliminating or treating the chickens is also helpful.

5. Removal of the contact between the horses and the poultry is obligatory and curative but removal of the poultry can in fact make matters worse (at least in the short term) unless the environment is cleaned as well.

Clinical signs

There is severe, often progressive, pruritus over the face and distal limbs initially but often extending to the whole body. Horses become irritable and stamp and bite at the body and legs (Fig. 9.11). Some cases cause severe excoriation – the extent of pruritus may be disproportionate to the number of mites present. Small pruritic skin papules and crusts may be present in areas readily accessible to the mite (legs, face and ventrum).

Figure 9.11 This mare developed severe facial and body trunk pruritus as a result of exposure to poultry. Poultry mites do not prefer to live on the horse and so they may not be found at all. Only a few mites were identified after extensive brushings were taken.

Differential diagnosis

Diagnostic confirmation

Treatment

Washing or spraying of the horse with an approved (licensed) pyrethroid or fipronil insecticide can be very effective but it is usually necessary to repeat all treatments every 7 days in summer and 10–14 days in winter.

Remove horses from proximity to poultry or remove poultry from the stable environment. The effect of such separation may not be obvious for up to 6 months or more as the mites are very robust and can survive for some time in crevices in walls and floors and in wooden rafters on which the chickens once roosted. Treatment of buildings and all equipment with an appropriate insecticide may be more difficult than it sounds. Treatment of the poultry is sometimes helpful but even a few mites will survive to perpetuate the problem. Complete elimination of poultry, pigeons and other roosting birds is essential.

Profile

Flea infestations can occur either as result of contact with flea-ridden cats, dogs, rodents and wild animals in the stable yard or through contact with poultry infested with the poultry flea Echidnophaga gallinacea. The poultry flea prefers poultry but can feed off livestock whilst the cat/dog fleas species are much less host selective. In spite of the sometimes overwhelming population, horses are seldom affected clinically. This may reflect a tolerance of the bites rather than the fact that fleas would not feed on horses. All fleas are blood-feeders and so there can be some clinical effects from high challenge.

Key points: Fleas

1. Fleas from farm stock and domestic pets as well as wild animals can affect horses because they are not host-specific. The poultry flea can also infest horses but prefers its own host. Sporadic outbreaks can occur but once established the fleas are extremely difficult to eliminate. Fleas remain attached to the skin for some hours and so they can sometimes be seen.

2. The signs are seldom obvious but localized foci of mild to intense pruritus can be identified usually after the flea has left the site. Examination of the animals and birds in contact is helpful. Sometimes the humans will also be bitten and this is strong epidemiological evidence.

3. Diagnosis can be made by identifying and capturing the fleas. A small bleb of petroleum jelly will prevent them from escaping! Flea dirt can be found by combing the hair in the affected regions. The dander is placed on damp blotting paper and observed for red discolouring of the blotting paper as the ‘flea dirt’ dissolves.

4. Anti-parasitic washes may be effective but their effects are short and so they may have to be repeated often.

5. Control is the solution – either a concerted effort to treat the animals and poultry and the environment together using controlling sprays such as imidocloprid or separation of the source animals/birds from the horses are the main options.

Clinical signs

Prominent pruritus and secondary alopecia and excoriation occur where fleas attach. The resulting localized dermatitis is often roughly circular.

Most often the eyelids, face, ears, chest, shoulders are affected and limbs. Fleas are visible to the naked eye and are firmly attached to the skin.

Differential diagnosis

Diagnostic confirmation

Visible adult fleas, with mouth parts buried in the skin, may be seen. Parasites remain attached for some time and so they can be seen with an illuminated magnifying glass on the skin surface. ‘Harvesting’ the fleas for diagnosis can be difficult; a small bleb of petroleum jelly can be applied and then a pair of tweezers can be used to grab the parasite and place it in formalin for microscopic identification.

Treatment

Topical washes with insecticidal shampoo, flea powders or aerosols (licensed synthetic pyrethroids, organophosphates and chlorinated hydrocarbons are effective). There is no information on the value of ivermectin but as the fleas are blood-feeders it is reasonable to suppose that this or moxidectin would be a useful addition.

The fleas are very hardy and so elimination can be a real problem. Sustained treatment of source animals/birds and their own environments or significant and strict separation of horses from source animals including poultry can be effective. Often, however, removal of the others species simply results in a bigger challenge to the remaining animals. Ongoing control measures including regular stable spraying with anti-flea aerosols such as imidocloprid may be helpful but there is no reported information on the likely benefits. Poultry farmers and advice organizations may be a good source of information on the latest effective control strategies.

Profile

The Culicoides family (also known as ‘no-see-ums’, midges or gnats) includes many hundreds of species each with its own geographical location and breeding/feeding habits and some have a host preference. Many geographically restricted species of mosquito and Culicoides attack horses. Individual species of Culicoides tend to attack different areas of the horse (Pascoe 1973). It is important epidemiologically and clinically to know what Culicoides spp. midges there are in any particular geographical region.

Apart from their nuisance effects both Culicoides spp. and mosquitoes (Culex spp., Aedes spp. and Anopheles spp.) have an important second role in the transmission of some significant viral diseases (including the viral encephalitides such as West Nile virus and African horse sickness) and some other infectious diseases (including those caused by Onchocerca cervicalis and Leishmania spp.). These transmissions often require specific species of Culicoides; for example African horse sickness is probably transmitted only by Culicoides imicola. Both Culicoides and mosquitoes are also involved in the hypersensitivity disorders known as insect bite hypersensitivity (sweet itch/Queensland itch) (see p. 284) (Fadok & Greiner 1990).

The majority of the significant insects are blood-feeders and their life cycle demands warmth, moisture and suitable vegetation conditions for the individual species, so they tend to be very seasonal in most parts of the world. Where this is so, the diseases they transmit and the specific primary effects tend also to be seasonal. Their size and fragility mean that they are most active in calm conditions (with wind speeds less than 6–7 km/h). The various species have preferred feeding times and preferred feeding sites on the host animals. Thus, some feed in mornings, others in evenings. Some are ventral feeding, while others prefer to feed on the dorsum of the host.

Under favourable climatic conditions enormous populations of the midges and mosquitoes are present and challenged horses usually show severe irritation during these wave attacks. The irritation rapidly abates as the population of insects falls, leaving only those horses with hypersensitivity as a serious clinical problem. It is important to remember that horses can develop hypersensitivity responses to any mosquito or midge (or indeed any other biting insect).

Key points: Culicoides /mosquito irritation

1. This is a worldwide problem relating to the bites of Culicoides and other mosquito species. The condition results in a painful, papular and pruritic skin disorder and in some cases worry/behavioural changes. Swarm attacks are common but some individuals seem able to tolerate the multitudes of bites without any problem. The greater importance lies in the transmission of important diseases and to individuals that develop a hypersensitivity to the bites.

2. Swarm attacks cause more problems because of the large number of bites. Bites are restricted to the preferred feeding sites of the prevalent species. Individual bites are papular, pruritic and sometimes oedematous. Large numbers of bites in one location cause coalescent areas of erythema, pain and pruritus.

3. Diagnosis is usually simple. Known circumstances of exposure are almost impossible to avoid.

4. Avoidance by stabling or insect repellents can be effective to some extent. Severe attacks may warrant short-term systemic corticosteroid treatment. Localized bite sites may benefit from topical corticosteroids/antihistamines and local anaesthetic creams.

5. A relatively few cases develop progressive hypersensitivity problems (insect bite hypersensitivity). Eliminating populations of Culicoides is probably impossible but controlling numbers through traps, breeding site disruption and environmental management can help.

Clinical signs

Bites are followed immediately by short-term, localized pain, and pruritus develops within 2–12 hours. Papular erythematous foci with a surround area of oedema (wheals) develop but in many cases the skin changes are not really recognizable until self-trauma causes skin excoriation. The bite site itself is seldom visible with even a magnifying glass (but can be identified histologically) (cf. biting flies which leave an obvious central crust). Bites are usually restricted to defined anatomical sites of the preferred feeding habits of the species involved; the majority of species seem to affect the head, base of mane, the withers and tail. Ventral biting species bite the ventral abdomen or the upper medial limb regions (axillae/groin) and the distribution of the pruritus varies accordingly. Individual horses may be bitten mercilessly (Fig. CD9 • 2A–C).

Heavy attacks particularly by mosquitoes may precipitate an urticarial response over variable areas of the skin.

Secondary dermal bacterial or fungal infections can occur if self-trauma is significant.

In contrast to the allergic disorder which affects individuals, most or all of the horses in a group will usually be affected to some extent simply as a result of common exposure. A few individuals may not be affected in spite of the challenge of many bites. Self-inflicted trauma can cause moderate to severe superficial skin abrasions and excoriation with serum exudate (Fig. 9.12).

Figure 9.12 Culicoides irritation. Acute inflammatory focus due to Culicoides spp. bites on the flank. The horse showed no sign of hypersensitivity. Similar intensely pruritic lesions are caused by mosquitoes and a few fly species.

Individual horses may develop the allergic condition even after a single attack, thereafter remaining allergic and becoming progressively more affected in succeeding seasons. Some breeds are apparently more sensitive and others (such as the Icelandic and Welsh ponies and the Shire horse) are more inclined to develop an allergic reaction.

The relationship between insect bites (of any type) and the common nodular skin disease ‘collagen necrobiosis’ or ‘collagenolytic granuloma’ (see p. 292) is not established. Some cases of this disorder have obvious tracts leading to the skin while others do not. The location of the nodules on the dorsum of the back and the sides of the neck tends to indicate that mosquito and Culicoides bites are at least less likely to be involved than other heavier biting insects.