Noticeable lesions indicative of the disease generally appear at approximately 1.5 years of age (White et al., 2004; White et al., 2007). This means that HERDA-affected horses rarely show signs at birth and may not exhibit severe signs of clinical disease for the first year and a half (White et al., 2007). However, it has been noted that skin trauma can cause the clinical signs to appear as early as six months (Tryon et al., 2005). Once lesions develop, affected horses can deteriorate quickly as there is no proven treatment (Tryon et al., 2007); the majority of affected horses are euthanized (Tryon et al., 2005).
HERDA is most commonly described in Quarter Horses, but cases have been reported in Appaloosas and American Paint Horses with Quarter Horse bloodlines (White et al., 2004) and in a single Arabian crossbred horse. Males and females are equally affected (Tryon et al., 2005), and a common ancestor appears in both the sires’ and dams’ pedigrees of the vast majority of affected horses (White et al., 2004).
The heritability for HERDA has been calculated at 0.38 +/– 0.13, and it is inherited in an autosomal recessive manner. It was been proposed that HERDA most closely resembles Ehlers-Danlos syndrome (EDS) in people. Although the clinical signs are not exactly the same between HERDA and EDS, genes known to be associated with EDS were proposed as potential candidates for HERDA (Tryon et al., 2005).
Identifying the HERDA locus by traditional linkage mapping methods was challenging. Horse families usually consist of half-siblings and horses have long generation times, meaning that each individual has relatively few offspring in a lifetime as compared to other species. Additionally, by the time the HERDA phenotype manifests itself, affected horses are frequently no longer owned by the breeder and full siblings of affected horses are not produced (Tryon et al., 2007). Based on the available pedigree information in which affected horses had a common ancestor on both sides of their pedigrees, a homozygosity mapping approach was successfully undertaken to map the HERDA locus to equine chromosome 1 (ECA1) (Tryon et al., 2007).
The genomic interval localized by homozygosity mapping contained 20 genes. A non-synonymous mutation in exon 1 of the equine PPIB gene was identified that segregates with the HERDA phenotype. The SNP (c.115G>A), which causes a glycine-to-arginine change in the putative N-terminal domain of the protein, maps to a syntenic block that has been strictly maintained throughout vertebrate evolution. This was the first whole-genome scan used to identify a novel disease gene in the horse (Tryon et al., 2007).
Genotyping of the identified mutation led to an estimated carrier frequency of 3.5%. Pedigree analysis suggests that the HERDA mutation is concentrated within particular lines of cutting horses. It is likely that the disproportionate breeding of highly successful stallions has led to a number of obligate carriers of the HERDA mutation, contributing significantly to the breeding population of Quarter Horses within the past two decades (Tryon et al., 2007).
A study that followed on the heels of the identification of the HERDA locus examined the mutation in a variety of Quarter Horse subgroups delineated by discipline. The HERDA allele was unsurprisingly high in cutting horses, with an allele frequency of 0.142. It was also identified in the reining (0.046), working cow horse (0.057), and Western pleasure (0.064) subgroups. The overall allele frequency was found to be 0.021 in Quarter Horses and 0.008 in Paint horses (Tryon et al., 2009).
Junctional epidermolysis bullosa
Junctional epidermolysis bullosa (JEB) is a skin disease that has been reported in two distinct breeds, the Belgian draft horse and American Saddlebred. Although phenotypically similar, it is caused by discrete genetic mutations in each breed.
First identified in both breeds as epitheliogenesis imperfecta (EI), the disease is characterized by blistering of the skin immediately after birth, primarily at pressure points such as the limbs (Spirito et al., 2002). Hoof loss and oral cavity involvement have been reported (Lieto et al. 2002; Spirito et al., 2002) (Figure 10.2). The progression of the disease is fatal and affected foals are usually euthanized shortly after birth (Lieto et al., 2003). The disease was reclassified as JEB based on similarities to ultrastructural features of human JEB (Johnson et al., 1988; Goureau et al., 1989).
JEB has been described in Belgians from the United States (Kohn et al., 1989) and Canada (Shapiro & McEwen, 1995) and exhibits an autosomal recessive mode of inheritance (Spirito et al., 2002). The known crossing of closely related draft breeds means that sporadic cases of JEB have also been reported in other draft breeds such as the Comtois and Breton (Gourreau et al., 1989).
Research into the molecular cause of JEB in Belgians uncovered a change in laminin 5 expression in tissues from affected foals as indicated by immunofluorescence experiments (Spirito et al., 2002). Further analysis discovered the absence of immunoreactivity to laminin γ2 chain antibodies in the basement membrane of the dermal-epidermal junction in affected foals. In humans and mice, absence of laminin 5 compromises the adhesion of keratinocytes and causes significant areas of skin detachment (Aberdam et al., 1994).
These observations indicated that LAMC2, a gene that resides on equine chromosome 5 (ECA5) and encodes the laminin γ2 chain, was a promising candidate gene for equine JEB. Upon sequencing of the equine γ2 cDNA, a homozygous basepair insertion (1368insC) was discovered in affected horses. This mutation results in a premature stop codon (TGA) in the N-terminal portion of domain III. The location and nature of this mutation results in the absence of the C-terminal domain I/II that is important for the assembly of laminin 5.
Further testing of the identified mutation in more than 150 horses confirmed that JEB in Belgians is autosomal recessive and determined that 48% of the horses tested were carriers (Spirito et al., 2002). Affected individuals from the Trait Breton and Trait Comtois draft breeds were also found to be homozygous for the same mutation (Milenkovic et al., 2003).
Prior to the identification of the equine LAMC2 mutation in Belgians, an identical phenotype had been observed in Saddlebred foals (Lieto et al., 2002), which was thought to have an autosomal recessive mode of inheritance. Approximately 4% of the American Saddlebred breeding population was estimated to carry the disease allele, and the construction of a partial Saddlebred pedigree suggested that the spread of the disease was likely due to a single founder (Lieto, 2002).
Genetic mapping experiments localized Saddlebred JEB to equine chromosome 8 (ECA8), suggesting LAMA3 as a candidate gene. Sequencing of the LAMA3 cDNA identified a deletion that eliminates exons 24, 25, 26, and 27, a total deletion of 6,589 bases and a predicted 169 amino acids, in affected horses. This deletion is predicted to result in the absence of functional laminin 5 molecules in affected horses. Further testing of this mutation in 175 random Saddlebreds identified nine heterozygotes, showing the carrier frequency at 0.051. Belgian foals were tested and none had the LAMA3 mutation (Graves et al., 2008).
The identification of these two laminin 5-associated genes and their association with equine JEB is not surprising considering that the Herlitz variant of JEB in humans is usually caused by a premature stop codon in one of the genes coding for the laminin 5 heterotrimer which results in a lack of expression of the protein chain (Aberdam et al., 1994; Vidal et al., 1995). The identification of these genes in horses supports the idea that equine JEB most closely resembles the Herlitz form of JEB in humans (Lieto et al., 2002). Equine JEB is the first reported hereditary disease in domestic animals with more than one identified causative mutation (Spirito et al., 2002).
Skin Diseases with Suspected Heritable Basis
For some skin diseases in horses a heritable basis is likely but currently unproven. These diseases are often specific to a certain breed, or a couple of closely related breeds, and may have been reported in small families. In most cases, additional work is needed to confidently determine the mode of inheritance and no definite causative genetic mutations have as yet been described.
Insect bite dermal hypersensitivity
Insect bite (dermal) hypersensitivity, or IBH, has been well documented in horses throughout the world (Braverman et al., 1983, Andersson et al., 1988, Littlewood, 1998). This seasonal, chronic dermatitis is also known as summer dermatitis/eczema/sores, sweet itch, and culicoides hypersensitivity (Riek, 1953), and is caused by IgE-mediated reactions to the bites of midges of the genus Culicoides (Quinn et al., 1983, Larsen et al., 1988). Affected horses scratch excessively, which can lead to secondary skin lesions. The most common signs are scratching, thickening of the skin, alopecia, excoriation, scaling, and wounds (Bjornsdottir et al., 2006). No consistently effective treatment is currently available (Marti et al., 2008) aside from environmental changes to aid horses in avoiding biting midges (Bjornsdottir et al., 2006) and corticosteroids to ameliorate the pruritus. Hyposensitization is controversial but has been effective in some cases (Anderson et al., 1996). A heritable basis has been proposed for IBH in certain horse breeds (Marti et al., 1992).
A study that examined the occurrence of IBH in half-sibling Swiss Warmblood families strongly suggested a hereditary component to susceptibility to IBH and proposed a recessive mode of inheritance. The same study indicated that a gene or genes of the equine major histocompatibility complex (MHC), along with other genes and environmental factors, play a role in susceptibility to this condition in some families (Marti et al., 1992).
A “slight but significant” association of the equine MHC with the occurrence of hypersensitivity dermatitis had previously been reported in Icelandic horses (Lazary et al., 1985). IBH generally does not occur in horses in Iceland as Culicoides midges are not found in that region. However, it has been reported to have a particularly high prevalence in Icelandic horses that are exported to areas where Culicoides are present. One study reported the diagnosis of IBH in 34.5% of 330 horses exported from Iceland to Denmark, Sweden, or Germany. The number of affected horses subsequently increased to 49.5% at two years post-export and 47–54% in horses living in areas heavily infested with Culicoides (Bjornsdottir et al., 2006). Consequently, IBH has been recognized as a serious problem in Icelandic horses that are exported, but it is also of concern in Icelandic horses that are born abroad.
It has been suggested that the fact that only 50% of exported Icelandic horses become IBH-affected means that genetic factors are involved. An overall prevalence of IBH in Icelandic horses born in Germany was calculated to be 4.6%, but the prevalence increased to 12.2% when both parents were IBH-affected. However, one study did not find evidence of a sire effect and noted that the estimated heritability of IBH in their sample set was not significantly different from zero (Bjornsdottir et al., 2006). Currently, it is assumed that IBH in Icelandic horses is a complex trait with a large environmental component and it is likely that multiple genes with major effects are involved. Whole genome association analysis has been proposed to identify underlying genetic causes of IBH (Marti et al., 2008).
Genetic studies of IBH have already been undertaken in other breeds. A genome scan for IBH using 50 microsatellites was performed in individuals from the Old Kladruber breed. Significant associations were identified to the microsatellite AHT04 and to SNPs in the FcɛR1 alpha and the IFNγ genes. The study also found that total IgE levels were significantly different between individuals heterozygous for an IFNγ intron 1 SNP and individuals homozygous for the same SNP (Marti et al., 2008).
IBH has some features similar to atopic dermatitis in humans, which is recognized to have a heritable basis and for which associated genetic variants have been identified (Barnes et al., 2010). In humans, the SPINK5 gene has been linked to Netherton syndrome, a disease that has atopic clinical signs (Chavanas et al., 2000, Nishio et al., 2003). Atopic dermatitis has also been reported in horses (Lorch et al., 2001a, 2001b, 2001c) and may have a heritable basis (Reese, 2001). However, analysis of SNPs in equine SPINK5 in Icelandic horses revealed no significant association with IBH (Andersson et al., 2009). Additional studies are required to identify molecular changes associated with this complex disorder.
Chronic progressive lymphedema/Chronic pastern dermatitis
Chronic progressive lymphedema (CPL), also known as chronic pastern dermatitis (CPD), is an inflammatory skin disease common in heavily feathered draft horses. It is widely recognized in horses from the Clydesdale, Shire, and Belgian draft horse breeds, as well as in other draft breeds around the world.
Diagnosis of CPL is mainly based on clinical signs including progressive and painful swelling, hyperkeratosis, nodule formation, thick skin folds, and fibrosis of the distal limbs (Figure 10.3), although lymphoscintigraphy has also been used to a limited extent (De Cock et al., 2006). Symptoms progress throughout the horse’s life and there is currently no known effective treatment. The clinical signs in horses can lead to severe disfigurement and interfere with movement, causing significant discomfort (De Cock et al., 2003). Horses with CPL are susceptible to secondary infestations by Chorioptes spp mites and bacterial infections that are very difficult to treat (Rufenascht et al., 2010), both of which can exacerbate lower-leg swelling. The most severe symptoms of the disease are generally observed later in life and severely affected horses are euthanized (De Cock et al., 2003).
Due to the prevalence of this disease in closely related draft breeds, it is assumed that genetic factors play a role (Schaper, 1950; Wallraf et al., 2004). Heritability across different German draft breeds has been calculated at 0.21, with heritability estimates within breeds ranging from 0.14 in South German draft horses to 0.98 in the Rhenish German (Wallraf et al., 2004). Environmental factors are presumed to influence clinical signs and the age at which these signs are observed (Mittmann et al., 2010).
CPL in horses closely resembles chronic lymphedema or elephantiasis nostras verrucosa in humans. Candidate gene approaches to identifying the molecular cause for CPL have been unsuccessful to date (Momke & Distl, 2007a, 2007b; Young et al., 2007), and the mode of inheritance of CPL is currently undetermined.
A study using a whole-genome scan with microsatellites in the aforementioned German draft breeds identified quantitative trait loci (QTL) for CPL (Mittmann et al., 2010). Seven chromosomes were implicated in various breeds and combinations of breeds. Potential candidate genes are present in these regions, but further studies are needed to identify causative mutations.
Determining the underlying genetic cause of CPL in horses is challenging. The prevalence of CPL is known to be high in many breeds and it has a variable age of onset. This makes it difficult to identify appropriate controls for genome-wide association studies. Secondly, a low number of microsatellite polymorphisms have been observed in draft breeds as compared to Thoroughbreds and warm-blooded horses, and it is assumed that the level of polymorphic SNPs will be even lower (Mittmann et al., 2010). Consequently, a large number of SNPs may be required to identify a causal genetic mutation, or mutations, for CPL.
Linear keratosis is characterized by firm, elevated, circumscribed, and linear areas of excessive keratin production (Rook et al., 1979; Muller et al., 1983). One of the authors (SDW) has seen a case that histologically also had a mural folliculitis, as seen in linear alopecia (see below). A rare dermatosis in the horse, it has been primarily reported in closely related horses of the Quarter Horse breed, although cases have occurred in a Morgan, Standardbred, and Percheron (Scott, 1985), and linear epidermal nevi has been reported in a family of Belgian horses (Paradis et al., 1993). This breed predilection suggests a hereditary basis (Scott, 1985).
Single or multiple linear, vertically oriented bands of alopecia and hyperkeratosis over the neck and lateral part of the thorax characterize the disease. It can begin as early as 6 months of age and usually persists throughout the life of the horse (Stannard et al., 1976; Ihrke et al., 1983). Topical therapies are reportedly effective to minimize the surface hyperkeratosis (Scott, 1985).
Equine linear keratosis is believed to closely resemble a linear epidermal nevus in humans, which may have a hereditary basis (Scott et al., 1984; Scott 1985). Currently, no genetic loci have been associated with equine linear keratosis.
Skin/Hair Color Phenotypes
Melanocytes, the cells that contain melanosomes, which in turn produce melanin pigments in the skin and hair, are found in the skin as well as in hair follicles (Thomsett, 1991). Because the skin is often a good indicator of processes taking place in other parts of the body, skin and coat color and/or condition can be characteristic of early or outward signs of diseases that primarily impact other parts or organ systems. In-depth discussions of most of these diseases are presented elsewhere in this book (see Chapter 11 in this volume), so they are only briefly mentioned here.
Gray horses are born colored and gradually lose hair pigment as they age but maintain dark skin pigment. Gray coat color in horses is dominant. It is well known that gray horses have a high incidence of dermal melanomas that generally take the form of firm, black nodules under the tail base and in anal, perianal, and genital regions, as well as the perineum, lips, and eyelids (Swinburne et al., 2002). Although some melanomas are benign, others, depending on the age of the horse and the histopathology of the tumor (Valentine, 1998), can metastasize to internal organs. Melanomas are reported in 70–80% of gray horses older than 15 years of age (Sutton et al., 1997, Fleury et al., 2000).
Other characteristics that often accompany the gray phenotype include skin depigmentation that appears as patches of red pigmentation (Fleury et al., 2000), speckling of the coat, and the speed with which the coat turns gray. These features are widely variable among gray horses. Reduced longevity has also been reported in gray horses (Comfort et al., 1958).
The gray coat color locus in horses is associated with a 4.6 kilobase (kb) insertion in the STX17 gene (Rosengren Pielberg et al., 2008). The duplication was detected in all gray horses tested but was not observed in non-gray horses. A high level of expression of STX17 was seen in melanomas from gray horses. Horses that are homozygous for the STX17 duplication are more homogenously white as compared to heterozygotes and have a higher incidence of melanoma, patches of depigmentation, and exhibit almost no speckling.
In addition to STX17, there is a highly significant association between a horse’s genotype for the ASIP gene, an MC1R antagonist, and the incidence of melanoma (Rosengren Pielberg et al., 2008). However, ASIP genotype has no significant effects on graying, patchy depigmentation, or speckling. These results show that the ASIP gene influences dermal melanocytes and implies that melanoma in gray horses is promoted by increased MC1R signaling.
Researchers also noted in melanomas from gray horses a markedly higher expression of NR4A3, a gene located in the same genomic interval as STX17. Expression of NR4A3 in horses heterozygous for gray occurs only from the gray haplotype, suggesting that a cis-acting regulatory mutation, possibly the STX17 duplication, underlies the upregulation of expression.
The lack of pigment in the skin, hair, and iris is known as albinism. Albinos are recognized by white hair, pink skin, and pink or light blue eyes. Albinism is an autosomal dominant trait: all albinos are heterozygous, while homozygotes are not viable. Aside from their appearance, albinos are normal but have a tendency to sunburn, are prone to squamous cell carcinoma, and might suffer from photophobia and light-related retinal damage (Knottenbelt, 2009).
In humans, oculocutaneous albinism (OCA) has a genetic basis; more than 36 mutations have been identified in the tyrosinase gene for type I OCA (Giebel et al., 1990, Oetting & King, 1993). As in horses, the affected individuals in these studies were heterozygotes. Tyrosinase mutations have also been implicated in murine albinism (Kwon et al., 1988, Jackson & Bennett, 1990).
The genetics of equine depigmentation phenotypes, including albinism, is heterogeneous, and is discussed in detail in Chapter 11 in the “White spotting and depigmentation patterns” section.
Lethal white foal syndrome (LWFS)
Foals affected with this disease are all white or nearly white and may be deaf and/or have blue eyes. The white coat is caused by the absence of melanocytes in the skin rather than a biochemical absence of pigment, as is the case in albinism. In addition to effects on melanoctyes, other cells derived from the neural crest, such as the enteric ganglia, are also absent, causing severe intestinal blockage/colic and resulting in the death of affected foals within 12 hours of birth. There is no treatment for LWFS.
The condition is seen primarily in American Paint horses, especially those with the frame overo white spotting pattern (Vonderfecht et al., 1983; McCabe et al., 1990), and is similar to Hirschsprung disease in humans. A mutation in the EDNRB gene is known to cause Hirschsprung disease and is also associated with lethal white spotting in mice and rats. Likewise, a missense mutation in the equine EDNRB gene is responsible for LWFS (Metallinos et al., 1998; Santschi et al., 1998; Yang et al., 1998).
Lavender foal syndrome (LFS)
Like LWFS, Lavender foal syndrome (LFS) has no effective treatment and the disease is lethal. Outwardly, affected individuals exhibit a characteristic dilute coat color that is described as lavender, pale gray, pewter, or light chestnut. Foals with LFS have multiple neurologic abnormalities that prevent them from standing and nursing normally. The condition is most commonly reported in Egyptian Arabians where it has an autosomal recessive mode of inheritance. A single base pair deletion in the MYO5A gene, which is associated with Griscelli syndrome in humans, causes a frameshift and a premature stop codon. The identification of the LFS mutation represents the first successful use of whole genome SNP association study in the horse (Brooks et al., 2010).
Arabian fading syndrome (AFS)/pinky Arab syndrome
Arabian fading syndrome (AFS) is most often reported in horses from the Arabian breed, but it has also been observed in Welsh mountain ponies and Clydesdales. It is characterized by a loss of pigmentation and hair. Skin biopsies from affected horses reveal a loss of melanin from the epidermal basal cells. It can occur spontaneously at any age and is more frequently observed in gray horses (McMullan, 1982; Mullowny, 1985; Yager & Scott, 1985). Depigmentation of the muzzle, lips, periorbital regions, perineum, sheath, and hooves has been noted, and patches of depigmentation on the body are usually permanent. These regions of depigmentation generally do not show inflammatory or traumatic changes and do not appear to be excessively pruritic. The condition is not harmful to the horse, although the pink skin is more susceptible to sunburn and affected horses have an increased susceptibility to squamous cell carcinoma.
As the disease is most often encountered in a certain breed, a heritable basis has been proposed. Similar cases of hypopigmentation have been reported in certain family lines in Belgian Tervuren dogs (Mahaffey, Yardrough, & Munnell, 1978), and an autosomal recessive form has been reported in black Angus cattle (Foreman et al., 1994). Susceptibility to patchy depigmentation of the skin and hair in humans is genetically complex and has been associated with more than one locus (Jin et al., 2010). The molecular cause of AFS is currently unknown.
Foal immunodeficiency syndrome (FIS)
Foal immunodeficiency syndrome (FIS), also known as Fell pony syndrome or Fell pony immunocompromise disorder, affects young purebred Fell (Scholes et al., 1988; Butler et al., 2006) and Dales ponies (Anonymous, 2009). Primary cutaneous signs, including a rough, russet-colored coat, are characteristic of the disease and can provide an early warning of a serious condition. Coat hairs can be abnormally long, especially around the head and neck, which gives the foal a “halo-like appearance” (Richards et al., 2000). Opportunistic infections typically develop on the skin and mucosal surfaces but can occur in different organ systems. There is no treatment for FIS and it is fatal at an early age (Knottenbelt, 2009).
The disease has an autosomal recessive mode of inheritance (Thomas et al., 2003) and the carrier rate in the United Kingdom and the Netherlands has been estimated at 40–60% (Butler et al., 2006). Recent genome-wide SNP analysis associated FIS with a 2.6 Mb region on ECA26, and proposed a mutation in SLC5A3 responsible for the compromise of the immune system (Fox-Clipsham et al., 2011).
Hairs are produced in hair follicles, emerge from ostia in the surface of the skin, are important for thermal insulation and sensory perception, and protect the skin against injury. The length, thickness, and density of hair correlate with the ability of the hair coat to regulate body temperature (Scott & Miller, 2003).
Hypotrichosis/mane and tail dystrophy/follicular dysplasia
Hypotrichosis, also known as follicular dysplasia or mane and tail dystrophy, is a condition in which the amount and density of hair is significantly reduced, most commonly accompanied by thinning or lack of hair around the eyes and muzzle. The skin of affected horses is reported to be scaly and thin. Although severe forms are rare, it is presumed to be a hereditary condition in some lines of Arabians and has also been seen in other breeds to a lesser extent. It is generally not uncomfortable for the horse and there is no treatment.
Some forms of this condition primarily affect the hairs of the mane and tail. Affected tail hairs are characteristically sparse and brittle. Biopsies can be used to confirm the presence of a reduced number of hair follicles. The condition is well recognized, and even accepted as normal, in some lines of Appaloosas, suggesting a heritable component (Knottenbelt, 2009).
Comparatively, autosomal recessive forms of hypotrichosis in humans are associated with the LIPH (Ali et al., 2007) and DSG4 (Schaffer et al., 2006; Shimomura et al., 2006) genes. These genes may represent potential candidates for the analogous condition in horses.
Curly coat syndrome
Foals with curly coat syndrome are born with an abnormally long, curly hair coat and a curly mane and tail. The affected horses often have hypotrichosis that causes “string tail” or “scanty tail” and alopecia caused by follicular dysplasia that is presumed to be genetically determined. The curliness of the coat varies from tight ringlets to waves in the winter to a smoother, slightly wavy summer coat. It has mainly been described in Paint horses in which a dominant mode of inheritance has been suggested (Thomas, 1989). Curly coat syndrome has also been described in other breeds including Quarter Horse, Percheron, Arabian, Appaloosa, Missouri Fox Trotter, Tennessee Walking Horse, Morgan, and Paso Fino. It is thought to be recessive in these breeds (Knottenbelt, 2009).
There is currently no available treatment for curly coat syndrome and the phenotype is primarily considered cosmetic, with no direct negative effects on the affected horse. To date, registries that represent the curly horse include the American Bashkir Curly Registry, the American Curly Horse Association, and the International Curly Horse Organization (Scott, 2004).
Molecular causes for the curly coat syndrome are not well understood. A genomic scan for the regions known to carry candidate genes for hair texture was undertaken to identify genetic markers associated with dominant curly coat (Cothran et al., 2009). Evidence of linkage or association was not observed. It has been proposed that Curly coat syndrome in horses is similar to Wilson’s disease and non-photosensitive trichothiodystrophy in humans, which are associated with mutations in the TTDN1 gene (Nakabayashi et al., 2005). Additional research is needed to uncover loci associated with this trait in either species.
Linear alopecia is characterized by circular areas of baldness that are oriented vertically, most commonly in areas of the neck, shoulder, and lateral thorax. Histologically it is typified by a mural folliculitis. The condition is rare in horses but has been seen in different breeds, most predominantly the Quarter Horse (Fadok, 1995). The lesions do not appear to be painful or pruritic (Scott & Miller, 2003).
Alopecia areata is one of the most common human autoimmune diseases (Gilhar & Kalish, 2006). It is known to be genetically determined and several loci have been implicated in susceptibility. Similarly, multiple loci are involved in the disease in the mouse model (Sundberg et al., 2004). In the horse, circular areas of alopecia are seen. Histologically, a lymphocytic infiltrate surrounding the base of the hair follicle is seen; however, in long-standing cases this infiltrate may not be present. Presence of antibodies targeting the hair follicles have been documented (Tobin, 1998). Lesions may be focal or progress to whole body alopecia (often sparing the mane and tail). Most horses regrow the hair but this may take up to two to three years. Corticosteroids may hasten the hair regrowth.
As described here, evidence exists for a hereditary basis for a number of equine skin conditions. Advances in genetic tools have facilitated the discovery of the causative mutations for some phenotypes. It is anticipated that further improvement of genomics tools will enable researchers to gain a better understanding of the more complex conditions and develop genetic tests to help breeders produce foals that are free from serious dermatological issues.
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