Equine Medicine

Question 1

A foal is born with a predominantly white coat, blue eyes, and within 24 hours develops signs of colic, including abdominal distension and failure to pass meconium. Considering the breeding history includes two frame overo parents, what is the most likely diagnosis?

A. Neonatal Isoerythrolysis
B. Meconium Impaction
C. Overo Lethal White Foal Syndrome
D. Congenital Atresia Ani

Answer 1

Answer: C. Overo Lethal White Foal Syndrome

Explanation:
Overo Lethal White Foal Syndrome (OLWS) is a genetic disorder seen in foals homozygous for the endothelin receptor B (EDNRB) gene mutation. Affected foals are typically born with a predominantly white coat and blue eyes.

Due to intestinal aganglionosis, they exhibit severe colic symptoms shortly after birth, including abdominal distension and failure to pass meconium. The condition is invariably fatal, often leading to humane euthanasia within a few days.

Lethal White Overo Syndrome

Question 2

What genetic mutation is responsible for Overo Lethal White Foal Syndrome, and how does it affect the foal’s development?

A. Mutation in the MYH1 gene leading to muscle atrophy
B. Mutation in the GBE1 gene causing glycogen storage deficiency
C. Mutation in the EDNRB gene resulting in intestinal aganglionosis
D. Mutation in the SCN4A gene leading to hyperkalemic periodic paralysis

Answer 2

Answer: C. Mutation in the EDNRB gene resulting in intestinal aganglionosis

Explanation:
OLWS is caused by a missense mutation in the endothelin receptor B (EDNRB) gene, specifically an Ile118Lys substitution. This mutation impairs the migration and development of neural crest cells during embryogenesis, leading to a lack of enteric ganglia in the distal intestine (aganglionosis).

Consequently, affected foals cannot effectively move ingesta through the intestinal tract, resulting in severe colic and early mortality.

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Question 3

Which breeding strategy is most effective in preventing the occurrence of Overo Lethal White Foal Syndrome in offspring?

A. Breeding two frame overo horses together
B. Breeding a frame overo horse to a non-carrier
C. Breeding two non-carrier horses
D. Breeding two horses without genetic testing

Answer 3

Answer: B. Breeding a frame overo horse to a non-carrier

Explanation:
To prevent OLWS, it is crucial to avoid breeding two carriers of the EDNRB mutation, as this pairing has a 25% chance of producing an affected foal. Genetic testing identifies carriers, allowing breeders to pair a carrier with a non-carrier, thereby eliminating the risk of OLWS in the offspring. Breeding two non-carriers also prevents OLWS but may limit desirable coat patterns associated with the frame overo phenotype.

Lethal Overo White Syndrome