Spider Lamb Syndrome in Sheep
The most important ovine chondrodysplasia is spider lamb syndrome, an inherited semilethal musculoskeletal disease affecting lambs primarily of Suffolk or Hampshire breeds.
Because the syndrome is a recessive genetic disorder, a lamb is afflicted only if both parents pass on the mutation. Thus, identifying carriers of the mutation is critical. Carriers (animals with only one copy of the mutation) are structurally normal with no evidence of disease.
The genetic locus causing spider lamb syndrome is located along the distal end of ovine chromosome 6.
This mutation causes an inactivation of normal fibroblast growth factor receptor 3 (FGFR3), which produces skeletal overgrowth in animals homozygous for the gene. Lambs have pronounced medial deviation of the carpus and hock and are unable to stand without distress. Afflicted animals may be visibly deformed at birth and unable to stand, or seemingly normal for the first 4–6 weeks.
Pathological changes in the skull reveal a rounding of the dorsal silhouette, producing a nasal bridge with a convex curvature (ie, “Roman nose”) and a narrowed elongation of the occipital condyles. Thoracic and lumbar vertebrae are moderately kyphotic, which causes dorsal rounding of the backline (top of the animal's spine). The sternebrae are dorsally deviated, leading to flattening of the sternum. The forelimbs have medially deviated carpal joints, bowed radii and ulnae, and irregular thickening of the growth plate cartilage. The hindlimbs have medially deviated hocks and bowed tibiae, which also have thickened, irregular growth plates. Muscle atrophy is also predominant.
The regulation of liver insulin-like growth factor (IGF) and IGF-binding proteins may be involved in physical manifestations of this disorder.
The condition may be inherited in a simple autosomal recessive pattern.
Dwarfism and Varus Deformity of Texel Sheep
Dwarfism is an autosomal recessive trait. Affected lambs are normal at birth; however, by 2–4 weeks, they show slow growth rate, short neck and legs, forelimb varus deformities, and a wide-based stance. Severely affected individuals usually die within the first few months of life due to reluctance to move.
Postmortem examination identifies denuded cartilage from the major weight-bearing surfaces of the hip and shoulder joints. The trachea is flaccid and flattened.
Muscular Hyperplasia (Double Muscling) in Sheep
Muscular hyperplasia as typically observed in cattle sometimes occurs in sheep with the callipyge (CLPG) gene mutation. The disease is present only in heterozygote males and results in postnatal hyperplasia of the hindquarters and hindlimb musculature.
Muscular Dystrophy of Merino Sheep
Muscular dystrophy of Merino sheep is an autosomal recessive disorder that causes progressive atrophy of muscle fibers and replacement with adipose cells.
Clinical signs include lack of normal growth, decreased flexion of hindlimbs, progressive stiffness that leads to reluctance to move, and eventual death from starvation and weakness. Affected muscles are gray, hard, and atonic due to their high adipose content. Animals usually die by age 2–3 years.
Myotonia of the Goat (“Fainting Goats”)
In myotonia of the goat, an autosomal dominant disorder, a defect in voltage-dependent chloride channels causes decreased skeletal muscle chloride conductance, which presents clinically as muscle spasms at the initiation of muscle contraction. Spasms begin within 2 weeks after birth and are usually initiated by startling the affected animal. Goats assume a "sawhorse" stance with splayed, stiff legs and then fall but are normal between episodes.
Minimal gross anatomical or histopathological changes occur.
