Congenital Anomalies and
Inherited Disorders of the Horse
Table of Contents
- Congenital Problems of the Autoimmune
- Congenital Defects of the Eyes and Ears
- Congenital Heart Defects
- Congenital Defects of the Gastrointestinal
- Congenital Disorders of the Musculoskeletal
- Respiratory Abnormalities
- Congenital Sex Determination Disorders
- Congenital Skin or Integument Disorders
- Congenital Diseases of the Urogenital
Congenital anomalies and inherited disorders of the horse include
all of the physical abnormalities which are present upon birth of
the foal and those that are diagnosed later in life. Some anomalies
may be acquired during fetal development while others may be inherited.
There have been a few recent break-throughs in discovering the etiology
of some diseases because of their similarity to a human condition,
e.g., Hyperkalemic periodic paralysis (HYPP) in Quarter horses or
It is often difficult to determine what event(s) may have resulted
in an anomaly. The prevalence or rate at which these anomalies occur
in a given population is unknown since very few cases are reported.
This information sheet will highlight some of the recognized congenital
anomalies of horses (indicated in italics). They are categorized
under the various body conditions, with brief descriptions where
Online Mendelian Inheritance in Animals (OMIA) is an excellent
database and served as a primary source for this information sheet.
It categorizes and compiles genetic conditions in horses and other
species. It is maintained by the Australian National Genomic Information
Service and can be viewed on internet at http://www.angis.su.oz.au/Databases/BIRX/omia/
Congenital Problems of the Autoimmune
- Severe Combined Immunodeficiency (SCID) is a fatal disease of
Arabian and part-Arabian foals. It is caused by a genetic defect
transmitted as an autosomal recessive trait. Affected foals that
attain colostral antibody transfer are clinically normal until
the colostral antibodies decrease. No functional B and T lymphocytes
are produced which leads to a complete lack of antibody production
and defective cell-mediated immunity. Affected foals are lymphopenic
(less than 1,000 lymphocytes per mm3), develop infections and
die by 4.5 months of age (2,3,4).
- Neonatal Isoerythrolysis is not truly an inherited disease.
However, absorption of colostral (maternal) antibodies result
in the destruction of the red blood cells in the newborn foal.
For it to occur, the dam's blood group must be negative for certain
blood types, e.g., Aa- or Qa-. Also, the mare must become sensitized
to the offending antigen (Aa or Qa) by exposure through previous
pregnancies, blood transfusion or transplacental contamination
and the foal must inherit from the sire the antigens (Aa or Qa)
to which the mare has been sensitized. Foals become sick after
nursing. Jaundice (icterus) is preceded by lethargy (drowsiness)
and weakness, and the presence of haemoglobin in the plasma and
urine (haemoglobinemia, haemoglobinuria) rapidly follows. A severe
anaemia (low red blood cell count) ensues. Prompt recognition
and therapy contribute to a good outcome; however, the prognosis
in advanced cases is poor. Mares can be screened before parturition
(birth of the foal) for alloantibodies against a panel of known
erythrocyte alloantigens (5).
Congenital Defects of the Eyes and Ears
- Colobomas are defects and, especially, a fissure of any part
of the eye.
- Absence of the nasal punctum. Tears flow across the eye and
drain from the puncti, which are located at the medial canthus
(near the nose) of the eye, into the nostrils. Blockages or absence
of any part of this drainage system results in overflowing of
tears from the lower eyelid(s).
- Entropion is the inversion (turning inward) of the margin of
the eyelid. It results in chronic irritation of the cornea from
the eye lashes turning inward.
- Congenital cataracts
Congenital Heart Defects
- Various holes or defects between the atria or ventricles occur
as a result of improper fetal development. Other defects are due
to failure of various parts of the fetal circulation to close
after birth. These include patent ductus arteriosus, patent foramen
ovale, or persistent fetal circulation. Persistent right aortic
atrial arch is a failure of regression in the embryo, resulting
in a ring structure that can occlude the esophagus and creating
megaesophagus (dilation of the esophagus).
Congenital Defects of the Gastrointestinal
(GI) System include:
- atresia or blockages of various areas along the intestinal tract.
This condition is uncommon in foals and it is not related to the
Lethal White Disease. The segment of intestines most often involved
is the colon (atresia coli), although it has also been described
in the rectum (atresia recti), anus (atresia ani), ileal and jejunal
segments. The affected animals develop signs of colic during the
first 24 hours of life. Depending on the location of the atresia,
scant mucous (lighter than meconium) may be present in the rectum.
- Lethal white disease is seen in horse breeds that have white
spotting and has been recognized in the following breeds: the
Paint horse, Pinto horse, Quarter horse, Miniature horse and Thoroughbred.
Horses often nurse vigorously at birth and are fine until their
GI tract proximal to the lesion fills up, then they get colicky
and look "bloated". At that point, they don't nurse
any more. The cause of the intestinal obstruction is a lack of
nerve cells in the distal portion of the large intestine (aganglionic
megacolon). This is thought to be due to a fault in the proliferation
and/or migration of nerve stem cells from the neural crest of
the developing embryo. The disease is similar to the human Hirschsprung
Disease. It is caused by a mutant allele. Homozygosity for the
Overo allele (both alleles are OO) results in white or nearly
white foals. Heterozygous animals (Oo) can be tested for the presence
of this allele and therefore prevent heterozygous carriers being
mated. Lethal white foal syndrome is recessive (i.e. the only
horses to show this disorder are homozygotes for the Overo allele).
- Brachygnathia refers to a malocclusion between the mandible
and the maxilla. Mandibular brachygnathia is most common and refers
to an abnormal shortening of the mandible which results in an
overshot upper jaw or parrot mouth.
- Cleft palate is a longitudinal opening in the hard palate which
separates the mouth from the nasal pharynx. Signs are usually
evident at birth or shortly after nursing when milk drains from
the nostrils. An incident level of 0.1 to 0.2 % is reported. A
physical exam must be done in a neonate with this sign. Aspiration
pneumonia is a common sequel. Surgical therapy is the only option
but it may not be recommended.
Congenital Disorders of the Musculoskeletal
- Flexural, Contractural and Angular limb deformities include
flexor laxity (flexor ligaments are too loose), flexural deformities
of the fetlock and coffin (club foot where the superficial digital
flexor tendon is too tight), varus (e.g., bowlegged) and valgus
(e.g., knock-kneed), affecting mostly the carpus (knee), fetlock
and tibio-tarsal (hock) regions. These problems are often self-limiting
and often resolve with adequate treatment and support.
- Patellar luxation. There is a congenital form of this condition
which has been described, most commonly in miniature horses and
Shetland ponies. The luxation occurs laterally (the patellar can
be displaced or moves to the outside of the knee) because of hypoplasia
of the femoral trochlea and shallowing of the intertrochlear groove.
In newborns, it becomes apparent when the foal tries to stand
but fails to extend the stifles, so the foal acquires a crouching
position. Femoral nerve paralysis may also produce this sign.
- Skeletal malformations include wry nose (twisted nose), torticollis
(twisted neck), scoliosis (lateral deviation of the back), lordosis
(ventral dorsal deviation of the back).
- Incomplete closures of the bony spinal canal (cervical meningomyelocele,
- Hydrocephalus (abnormal accumulation of fluid in the cranial
vault with resultant enlargement of the head)
- Digital malformations - supernumerary (polydactyly) digits,
hypoplasia of phalanges, e.g., navicular
- Malignant hyperthermia syndrome (hyperthermia of anaesthesia)
is a progressive increase in body temperature, muscle rigidity
and metabolic acidosis, leading to rapid death, seen when an anaesthetic
- Hernias (are defects in the muscle wall which permit intestines/organs
to move into an abnormal location)
- Diaphragmatic hernia is a congenital opening in the thoracic
diaphragm, permitting the displacement of abdominal organs
into the thorax.
- Umbilical hernia is the incomplete closure of the abdominal
wall at the umbilicus.
- Inguinal hernias are commonly seen in certain breeds, particularly
Standardbred and draft horses. Usually they happen in male
foals and a large scrotal sac will be seen. They will rarely
present colic signs. It is recommended to reduce the hernia
on a daily basis and wait for 4-6 months as most of them will
resolve. In cases presented with colic or when edema appears
in the inguinal/ventral area (ruptured hernia), it is advisable
to surgically repair it.
- Dwarfism refers to the failure of appropriate growth resulting
in a smaller horse. A dwarf horse can be proportionate or disproportionate.
Proportionate dwarfs are a result of a deficiency in growth hormone
while disproportionate dwarfs result from abnormal thyroid hormone
levels. The latter results in a foal with musculoskeletal immaturity,
characterized by delayed cuboidal bone development, a large head,
silky hair coat, floppy ears and mandibular brachygnathia. Determination
and interpretation of either growth hormone assays or thyroid
hormone function is not entirely developed or understood in the
equine, hence the importance of clinical diagnosis. Efforts towards
characterizing thyroid function and growth hormone levels should
be undertaken to prevent overdiagnosing this condition. A nitrate
toxicity theory has been confirmed in certain foals born with
"congenital hypothyroid syndrome".
- Tying-up Syndrome (Equine rhabdomyolysis syndrome, Exertional
rhabdomyolysis, Myoglobinuria). Some forms of this disorder are
thought to be inherited as an autosomal recessive trait. However,
the published data is inconclusive on this point (6).
- Hyperkalemic periodic paralysis or HYPP is marked by sudden
attacks of paralysis which, in severe cases, may lead to collapse
and sudden death. It is an inherited mis-sense mutation in the
gene encoding the alpha chain of the adult skeletal muscle sodium
channel, resulting in increased sodium permeability across the
skeletal muscle cell membrane. Quarter horse, Paint horse and
Appaloosa progeny tracing back to the Quarter horse sire, "Impressive",
can be affected with this disease and must be eliminated from
any reproductive program.
- Guttural pouch tympany is a distinct soft swelling at the throat
latch area. Palpation of the swelling resembles the palpation
of a balloon. Radiographs revealed a large air-filled cavity.
Horses may also have a respiratory noise, dyspnea (difficult breathing),
a cough, dysphagia (difficult swallowing) and aspiration pneumonia.
- Choanal atresia is a failure to regress of the bucconasal membrane,
producing an airway obstruction at the junction of the nasal cavity
and the pharynx. Bilateral cases usually die unless an emergency
tracheotomy is performed. Unilateral cases may go undiagnosed;
however, asymmetry of airflow can be detected. Surgical treatment
by a transnasal or laryngotomy approach or through an endoscope
has resulted in some success.
Congenital Sex Determination Disorders
- The intersex disorders hermaphrodite and pseudohermaphrodite
occur when an individual has a mixture of male and female characteristics.
It is often due to an abnormality of the sex chromosomes. The
disorders include gonadal hypoplasia (small testes or ovaries),
gonadal dysgenesis. XY female type, Swyer syndrome, Sry -XX hermaphroditism,
and XX male pseudohermaphroditism.
- An example is a horse with stallion-like behaviour with gonads
retained in the abdomen which histologically were testes but,
when karyotyped (looking at the chromosomes), the stallion had
the normal 64 chromosomes, two of which were XX (female) chromosomes.
This stallion was a 64 XX male pseudohermaphrodite.
Congenital Skin or Integument Disorders
- Hereditary junctional mechanobullous is a condition in Belgian
foals where the basement membrane in the hoof separates and results
in sloughing of the hooves by 12 days of age (7).
- Undermined skin or Hyperelastosis cutis (Ehler's-Danlos syndrome
in humans) is seen in Quarter horses and Haflingers. Ehlers-Danlos
syndrome in humans is an autosomal dominant trait with variable
- Dilute Lethal or Lavender Foal Syndrome occurs in Egyptian and
part-Egyptian Arabian horses and is usually fatal within 48 hours
of birth. Foals are born with difficulty (dystocia), fail to stand
or nurse and have neurological problems (intermittent joint rigidity
and rapid eye movements). The foals are called lavender'
because the hair coat has a diluted lavender or pink colour. This
may be due to abnormal clumping of the pigmentation in the hair,
but could also be attributed to cyanosis (lacking oxygen) caused
by the long and difficult birth. The foal is often larger than
normal. On postmortem, vacuolations of the neurons are found (8).
- Roan coat colour occurs at a low frequency (generally less than
5%) in many horse breeds. Hintz and van Vleck (1979) provided
convincing evidence that roan coat colour in Belgian horses is
due to heterozygosity for an autosomal gene that is lethal when
Congenital Diseases of the Urogenital
- Ruptured bladder. The etiology of this condition is believed
to be traumatic and occurs most commonly in male foals during
parturition, although the signs of uroperitoneum (urine in the
abdomen) do not appear until 2-5 days of age. These animals can
be suffering from very severe metabolic disturbances and their
correction previous to surgical repair is of paramount importance.
Therefore, ruptured bladder is not a surgical emergency but a
medical one. Clinical signs involve lethargy, abdominal distention,
decreased appetite and mild colic. Affected foals may urinate
normally although commonly they are seen to strain and make frequent
attempts to urinate. The back is seen ventroflexed (flexed downwards)
as a difference with meconium impaction where the back is dorsoflexed
(flexed upwards). Serum electrolytes and metabolites will be affected
and are characterized by increased potassium (hyperkalemia), decreased
sodium (hyponatremia), decreased chloride (hypochloremia) and
presence of urea in the blood (azotemia). Diagnosis of uroperitoneum
is based on comparison of levels of creatinine in peritoneal fluid
- Patent Urachus is where urine is detected leaking from the umbilicus.
This could be a normal finding in foals up to 5 to 7 days of age.
However, failure of the urachus to close could result in septicemic
(blood infection) foals. It is not a surgical emergency and time
and medical treatment should be considered before engaging in
a surgical procedure.
- Recto-vaginal, urethro-rectal fistula are characterized by a
bypass of urine into the rectum or feces into the vagina or urethra.
These types of abnormalities are usually a part of a larger picture
where other congenital abnormalities related to the urogenital
tract are present. In addition, in rare occasions, some of these
cases may also present an atretic segment of bowel (section of
bowel without an opening).
- Ectopic ureter is unusual in horses. There is no predisposition
of sex or breed. It may go undetected for years or the foal may
present signs of urinary incontinence because the ectopic ureters
usually open distal to the trigone of the bladder. Unilateral
or bilateral cases may occur and some cases may also present with
hydro-ureter (dilated fluid filled ureter) or hydronephrosis (dilated
kidney). Surgery has been successful in some cases.
- Uterus unicornis signifies the presence of a single uterine
horn rather than two.
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AJVR 1996; 57: 286-290.
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