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This is a rare congenital, inherited cutaneous defect of foals. Possibly a single autosomal recessive gene is responsible, at least in American Saddlebred horses (Scott 1988a, Stannard 1994a, Lieto & Cothran 2003).

There is a complete or partial absence of one or more of the dermal components including the epidermis, dermis, subcutis and fat (Scott 1988b). Although defects on the limbs are probably more common, the body trunk and hooves, and even the tongue and proximal oesophagus may also be affected.

Many of the previous reports of this condition can be attributed to epidermolysis bullosa (see p. 232) rather than cutaneous agenesis (Scott & Miller 2003a).

Clinical signs

Clearly demarcated solitary or regional areas with a complete absence of epidermis and skin appendages are present at birth (Fig. 10.1). The extent of the skin defect may be less severe than the subcutaneous adhesion problems. The commonest and most obvious defects occur distal to carpus and tarsus but it can be over much larger areas. Dermal adhesion in the adjacent tissues can be severely affected and in some cases the defect can be extensive. The margins of the defect are noticeably free of inflammation (Fig. CD10 • 1).

Figure 10.1 Epitheliogenesis imperfecta affecting large areas of the skin of a stillborn foal.

The lesions bleed easily with minor trauma and quickly become infected, possibly leading to septicaemia and death. Foals with limited defects have a reasonable chance of growing normally, but closure of the fault is difficult and requires repeated procedures.

The tongue and the proximal oesophagus can be affected also. Severely affected foals may be aborted.

Differential diagnosis

• Epidermolysis bullosa: a very rare vesico-bullous condition associated with a dermo-epidermal fragility so that the skin can be rubbed off the affected areas; largely restricted to the Belgian breed but can occur in other breeds and donkeys.

Key points: Epitheliogenesis imperfecta

1 Rare condition with a genetic but not necessarily heritable basis.

2 Absence of variable areas of the skin is the most common syndrome but sometimes it is complicated by concurrent defects in the subcutis resulting in loss of adhesion between the skin and the subcutaneous tissues. Involvement of the oesophagus or tongue can be present.

3 Diagnosis is based on signs and the unexplained skin deficits.

4 Treatment options are limited. Repeated reconstructive surgical procedures can be attempted but are seldom effective; wound breakdown and severe infections are common.

5 Prognosis is very poor but foals with limited defects can survive provided that the affected region can be managed.

• Hereditary equine regional dermal asthenia (HERDA): cutaneous adhesion disorder that is restricted largely to Quarterhorses and manifest by skin fragility and lack of elasticity. Signs are usually delayed until the skin is subjected to trauma from harness and weight-bearing. Wounds that result from skin necrosis are very slow to heal and may not do so or may result in severe disfiguring scars.

• Obstetric trauma/trauma/savaging or predation: history of dystocia or overt evidence of trauma.

Diagnostic confirmation

Treatment

No treatment is consistently effective – even skin grafting of the affected area usually fails to close it. Surgical debridement of the margin of the defect may permit some closure but the loss of adhesion in the adjacent subcutis is a major obstacle to a resolution. In this case extensive debridement and reconstructive surgery must be performed if there is to be any chance at all. Very small defects can sometimes be closed with repeated surgical reconstructive measures and very small defects can be managed in the longer term. Some very small localized defects are said to heal adequately with scarring but the protracted non-healing wounds are usually refractory to any form of treatment. Euthanasia is the only course of action when extensive lesions and hoof, mouth or oesophageal defects are present. The possibility of septicaemia must be considered when these cases are presented.

Neither the mare nor the stallion should be used for breeding in view of the suggestion that it may be an autosomal recessive gene.

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This is a very rare complex group of heritable junctional mechanobullous diseases occurring principally in the Belgian and Saddlebred breed of both sexes, characterized by fragile ‘blister’ formation on the skin and oral mucosa, following mild trauma (Kohn et al 1985, Scott 1988a, Stannard 1994a). Other breeds and donkeys are sporadically affected. Although the human condition carries the same name and there are some similarities, there is limited information on how this relates to the equine condition (Linder 2000).

The junctional form of the condition is the most common form and probably results from a lamina lucida defect (Berthelsen & Eriksson 1935, Lieto et al 2002). Variations in location of the clefting within the epidermis are theoretically interesting but have no material significance for the management of the condition. It has been suggested that some of the previous reports of cutaneous agenesis in foals may in fact have been epidermolysis bullosa and so the condition may be slightly more common.

Clinical signs

The condition is recognized by severe blistering of the skin on the newborn foal. The primary lesions consisting of intracutanous bullae are seldom seen. Most rupture quickly with minor trauma or even movement.

Lesions at mucocutaneous junctions and oral mucosa and elsewhere are visible at birth (Fig. 10.2). Further lesions develop shortly afterwards. Collapsed bullae may be found in the mouth, and separation of hooves at the coronary band is sometimes present and foals have sloughed the hoof capsule within hours or days of birth (Frame et al 1988).

Figure 10.2 Epidermolysis bullosa affecting the medial forelimb region in a Belgian foal.

Concurrent dystrophic teeth commonly occur.

Secondary local and systemic infections are common and often result in progressive depression, illness and death.

Differential diagnosis

• Epitheliogenesis imperfecta: overt deficits of skin cover present at birth over various localized or more general regions in the complete absence of any traumatic cause.

Key points: Epitheliogenesis bullosa

1 A rare inherited condition primarily (but not exclusively) restricted to the Belgian breed, affecting the dermo-epidermal junctional regions of limited but sometimes extensive areas of the skin.

2 Minor trauma results in bulla (blister) formation and when these rupture they leave an open ulcerated area that bleeds and is easily traumatized and infected. The limbs are more often affected than the body trunk and head. The mouth and coronary band may be severely affected.

3 Diagnosis is made on the clinical signs and history. The Belgian breed is over-represented.

4 Treatment is not possible and euthanasia is the only effective approach for most cases.

5 Breeding from both parents should probably be stopped.

• Hereditary equine regional dermal asthenia: usually manifest much later in life.

• Bullous pemphigoid.

• Pemphigus foliaceus.

• Systemic lupus erythematosus-like syndrome.

• Scalding and burning injuries: known/suspected history.

Diagnosis

Treatment

No treatment is possible. The mechanisms of treatment for the human condition, in which genetically modified stem cells are used, should not be attempted in horses.

Breeding from both mare and stallion should probably be stopped although currently the evidence for heredity is tenuous.

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A very rare condition most noticeable in younger horses (<2 years of age).

Clinical signs

At birth the foal is usually normal but in the first 6–12 months of life, the coat becomes greasy and staring. The normal patina of horse skin is lost (Fig. 10.3).

Figure 10.3 This foal was diagnosed with primary seborrhoea from birth.

Figure courtesy of Dr Marianne Sloet.

Key points: Generalized primary seborrhoea

1. A rare congenital disorder of epidermal cell hyper-proliferation with inappropriate sebum production.

2. Generalized greasy scale and flake occur over the whole body from a young age. Secondary bacterial and Malassezia spp. infection is common.

3. Diagnosis is only made when the condition has been present from birth or shortly afterwards and when it affects the whole body and causes of secondary seborrhoea have been eliminated. Biopsy is largely unrewarding with non-specific epidermal hyperplasia and hypertrophy and excessive scale.

4. Treatment is palliative/symptomatic with degreasing and descaling shampoos.

5. The prognosis for the animal is good but the skin requires long-term management.

No significant systemic clinical symptoms are associated and there is no pruritus or pain unless secondary bacterial and fungal infections develop.

Differential diagnosis

Diagnostic confirmation

Treatment

Treatment is restricted to degreasing and anti-scale shampoos and if necessary intermittent antibacterial and antifungal shampoos. Advice on breeding cannot be given as the condition is very rare.

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A congenital ‘curly coat’ syndrome has been recently described occurring mainly in Paint horses. A similar syndrome has been recognized in Quarterhorse, Percheron, Arabian, Appaloosa, Missouri Fox Trotter, Tennessee Walking Horses, Morgan and Paso Fino breeds.

Defined genetic lines are possibly more susceptible and the trait appears to be a dominant gene. Currently there is a move towards recognition of the curly-coated horse as a specific breed (Scott 2004).

Clinical signs

Foals are born with an abnormally long, curly hair coat that remains until the adult hair grows through and in some cases the condition remains (Fig. 10.4). The main and tail are also affected. The extent of curliness is very variable. The mane and tail can be affected alone (‘wavy-mane syndrome’) but it can also affect the whole body surface. The winter coat is often a more tightly curled ‘ringlet-like’ pattern while the summer coat is smoother but with prominent ‘waves’.

Figure 10.4 A young Paint horse with a congenital curly coat syndrome.

There may be variable degrees of alopecia with coat and mane and tail thinning (hypotrichosis) especially in the summer coat. The tail is often badly affected and may lose almost all its hair, giving the horse a rat-tailed appearance (termed ‘string tail’).

Key points: Congenital curly coat syndrome

1. A congenital disorder mainly affecting Paint horses. This is sometimes viewed as a desirable trait and so active steps are taken to maintain the condition in some breeds and lines within breeds.

2. A prominent curly, dense coat is present from birth and develops into a dense, wavy coat quality. The condition may be restricted to the mane and tail (wavy mane syndrome) but can affect the whole body. Some hair loss is also often present including the tail and mane.

3. Diagnosis is based on the characteristic features and the breeding history.

4. Treatment is not usually requested and in any case there is none!

5. The condition has no apparent deleterious effect on the horse and neither is it associated with any other congenital defect and so the prognosis is irrelevant.

No significant clinical symptoms associated with the condition but some owners are unaware of the condition until a foal is born with it.

Differential diagnosis

Diagnostic confirmation

Treatment

Treatment is neither warranted nor possible. Breeding from ‘curly horses’ has become desirable in some circumstances and a special set of Breed Societies have been set up to study and promote the trait.

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Albinism is a condition in which there is a congenital lack of pigment (melanin) in the skin, hair and iris. Normal melanocytes that have a biochemical defect preventing them from synthesizing melanin are present. Cream-coloured horses lack pigment but are otherwise normal. True albinos have white hair, pink skins and non-pigmented (pink or light blue) irides (and usually have no pigment in the fundus of the eyes). General albinism is an autosomal dominant trait and is only viable in the heterozygous state (Ww). While being white-haired, they may have some coloration of the iris (blue or blue/white streaks). When these horses are mated, non-viable embryos (WW) occur in 25% of conceptions and are resorbed or aborted early in gestation. This is best regarded as one form of lethal white foal syndrome.

Key points: Albinism/lethal white foal syndrome

1. Albinism is an incidental condition that affects ponies more than horses. Affected animals have a white or pink skin (cf. grey horses which have a black or dark skin). The lethal white syndrome is an important and highly fatal disorder mainly affecting foals with Overo-Paint breeding.

2. Albinism does not, in itself, have any disease implication for the horse although the lack of pigment may make the skin more sensitive to sunlight and so actinic dermatitis and squamous cell carcinoma may be more prevalent.

3. The lethal white syndrome is characterized by a severe non-responsive neonatal colic associated with either an atretic (incomplete) gastrointestinal tract or ileus resulting from aganglionosis or both.

4. Diagnosis of albinism is simple. Diagnosis of lethal white syndrome is based on the very typical clinical signs and breeding history.

5. There is no treatment for either condition. Albinotic horses probably need to be given extra sun-protection (mask for face and eyes and light rugs) but most seem to cope well anyway. Lethal white foals must be destroyed immediately; any attempts to restore normal gut function will fail.

Lethal white foal syndrome occurs with the mating of Overo and Paint horses of the pinto breed and is an autosomal recessive trait. This is an inherited condition. Cases have been encountered in other breeds also (Lightbody 2002). Affected foals are white (albinos) but frequently have concurrent, highly fatal defects of the gastrointestinal tracts such as atresia ani, atresia coli and in particular ileocolonic aganglionosis (Knottenbelt & Pascoe 1994, Stannard 1994a). The pathogenesis of the condition appears to be linked to the gene that controls endothelin metabolism.

Clinical signs

Partial albino horses are relatively common, showing pink skin and a white hair coat (Fig. 10.5A). The retina is invariably partially or completely albinotic and the iris may be variously pale blue or white in colour (either completely or partially). Possibly the greatest implication for these animals is their liability to actinic dermatitis (sunburn) and squamous cell carcinoma. Many also have degrees of photophobia and light-related retinal damage can occur.

Figure 10.5 (A) Albino foal and albino mare. Both had albinotic retinas and non-pigmented (blue/yellow) irides. Both lived normal lives. (B) The lethal white syndrome in Overo-Paint-bred horses.

Lethal white foals are a startlingly white colour at birth (Fig. 10.5B); a few have small faint brownish tinges particularly on their ears. Although they may be apparently normal for the first 1–2 hours after birth, they rapidly develop severe intractable and fatal colic as a result of an incomplete intestinal tract or ileus as a result of intestinal aganglionosis (or both). Concurrent abnormalities of the mouth, pharynx, heart or other structures are also sometimes present.

Differential diagnosis (lethal white syndrome only)

Diagnostic confirmation (lethal white syndrome only)

Treatment

Albino foals are often normal and survive normally (cream horses are a form of this), although many are mildly photophobic. Limiting exposure to bright sunlight is often sensible and appreciated by the animal. Early recognition and treatment of actinic dermatitis and squamous cell carcinoma are important.

There is no treatment for lethal white foals; attempts to control colic and seizures are fruitless. General anaesthesia gives temporary relief. Attempts to anastomose atretic intestine or enterectomy of affected intestine are a waste of time and effort. This condition should presently be regarded as 100% fatal within 24–72 hours.

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Lavender foal syndrome is a rare, poorly characterized, (supposedly) hereditary disorder of Arabian foals derived from a particular Egyptian line. The name derives from the unusual hair coat colour, which can be tan to brown but which has a slightly purplish, ‘lavender’ appearance. Most cases have been reported in the United States but case series have also been reported in South Africa.

The syndrome is invariably fatal and has a highly complex pathogenesis that involves critical neurological signs.

It may be coupled with other congenital/genetic defects such as cardiac deformities, wry nose or cleft palate and possibly intestinal aganglionosis.

Key points: Lavender foal syndrome

1. A recently described, genetically based coat colour dilution syndrome mainly seen in Arabian horses and their crosses.

2. Prominent neurological symptoms are present from birth and the affected animal usually fails to stand. Convulsions, blindness and opisthotonus are present.

3. Diagnosis is based on clinical signs alone – laboratory features are non-specific and rather disappointing.

4. There is no treatment – the condition is fatal.

5. Parents of the affected foal should probably not be used for breeding but little is known about the syndrome as yet.

Clinical signs

‘Lavender foals’ have a characteristic lavender (bluish/purple) coat colour at birth (Fig. 10.6). The skin is actually normal and there is no dermatological implication. The colour is simply a visible indicator of a critical (lethal) neurological disorder. Affected foals are neurologically abnormal from birth, showing neonatal seizures and convulsions (often even occurring during or immediately after birth). Opisthotonus, paddling, aimless galloping while recumbent and apparent blindness with a light-hungry and startled expression may be present. No periods of remission or sleep occur. Affected foals fail to rise and have no (or reduced/abnormal) suckle reflexes. Death is inevitable.

Figure 10.6 A typical lavender foal. The foal showed all the characteristic neurological signs and was destroyed immediately upon diagnosis.

Figure courtesy of Dr Johann Neser.

Differential diagnosis

Diagnostic confirmation

Treatment

There is no treatment. Attempts to control seizures are fruitless. General anaesthesia gives temporary relief. The condition should presently be regarded as 100% fatal within 24–72 hours (Page et al 2006).

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This condition is a recently described disorder affecting pure bred Fell pony foals under 16 weeks of age (Scholes et al 1988, Richards et al 2000, Butler et al 2006). Severe immunocompromise as a result of a T-cell malfunction with a simple recessive gene is involved. The condition is not similar to any of the other major immunocompromising disorders of foals.

Cutaneous signs are relatively minor but provide a good (if non-specific) early warning system. The panel of signs suggests that other genes may be involved also. There is a very high prevalence of the carrier status in the Fell pony population. Specific breeding lines do carry a major risk of the disease.

Clinical signs

The foal is usually born relatively normal but frequently the coat is rough, russet in colour and there is an abnormal length of many hairs, particularly on the head and neck, giving it a halo-like appearance.

Diarrhoea and failure to thrive as well as increasing lethargy are common. A progressive microcytic, normochromic anaemia is a universal feature (Fig. 10.7).

Figure 10.7 This 5-week-old Fell pony foal had a russet-coloured coat and was presented with all the classic signs of the Fell pony syndrome including anaemia and respiratory and alimentary tract infections. Dermatophilus congolensis skin infection and Candida spp. infections are common in these foals and it is easy to be misled into thinking they are primary conditions.

Immunocompromise results in opportunistic infections including those on the skin and mucosal surfaces. Secondary Candida spp. infection of the oral cavity and intestinal tract, and Dermatophilus congolensis and Staphylococcus spp. infections of the skin are common.

The condition is invariably fatal before 12 weeks of age.

Key points: Fell pony immunocompromise disorder

1. A specific inherited autosomal recessive immunocompromising disorder affecting Fell pony foals under 12–14 weeks of age.

2. Cardinal early signs include a russet halo of long coarse hairs and progressive non-regenerative anaemia. Opportunistic infections including those of the skin are frequently encountered. Other signs include diarrhoea, poor growth and weight gain and oral, respiratory and intestinal infections.

3. Diagnosis is based on breeding history and on the typical signs – usually anaemia is taken as an early indicator.

4. Treatment attempts are fruitless – the condition is 100% fatal.

5. Neither parent of an affected foal should be used for breeding.

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