Inherited and Congenital Diseases

Myotonia Congenita

Myotonia congenita is an inherited disease of goats that manifests as transient tetanic spasms of the skeletal mus­culature initiated by visual, tactile, or auditory stimuli.

Etiology and Pathogenesis

Caprine myotonia congenita appears to be inherited as an autosomal dominant gene with incomplete penetrance. Two classes of affected goats exist regarding clinical signs: mild and severe. The mildly affected goats are assumed to be heterozygous and severely affected goats homozygous for the responsible gene. Breeding studies argue against the condition being sex linked.

Only voluntary striated skeletal muscles are involved in this disease. The defect appears to be at the level of the muscle fiber membrane and may be caused by a reduced number of chloride channels in myotonic fibers (Bryant and Owenburg 1980). This leads to a reduction in chloride conductance with decreased accommodation, increased membrane excitability, and abnormal repetitive muscle fir­ing (Bryant et al. 1968; Adrian and Bryant 1974). The myo­tonic discharge may be triggered by any sustained depolarization, such as a burst of nerve impulses in response to surprise, noise, or handling. The disease is characterized by a generalized, sustained tetanic contrac­tion of skeletal muscles lasting for several seconds to approximately one minute. For reasons that are unclear, withholding water from myotonic goats for up to three days results in disappearance of the myotonic response. Reintroduction of water is accompanied by return of myo­tonia (Hegyeli and Szent-Gyorgyi 1961).

Epidemiology

Caprine congenital myotonia was first recognized in goats in Tennessee in the 1880s and was first described in the veterinary literature in 1904 (White and Plaskett 1904). Such goats were favored because they did not wander or easily escape confinement. They also became popular as an amusement. Recognition of these goats as a model for human myotonia congenita received growing attention in the 1930s (Kolb 1938).

Veterinarians may encounter myotonic goats in research herds maintained for studies of human myotonia congen­ita. The goats are also maintained as pets or curiosities, and their popularity as such seems to have grown in recent years, at least when gauged by media publicity. These ani­mals are also being used increasingly as meat goats because of the heavy muscling that occurs in their hindquarters. Both males and females can be affected. Crossbreeding of a myotonic goat and a non-myotonic breed of goat can pro­duce a myotonic offspring. The breed is included in the American Livestock Breeds Conservancy Conservation list and an International Fainting Goat Association has been established to promote, protect, and preserve the breed.

Clinical Findings

The myotonic condition is not usually recognized in affected goats younger than 2 weeks of age and is clearly recognizable by 6 weeks. Younger goats may show some slight stiffening during handling. Buck kids tend to show more pronounced signs than females. Episodes of myoto­nia congenita can be triggered by visual, auditory, or con­tact stimuli, such as the sudden appearance of strangers, clapping, or handling for vaccinations. Mildly affected goats develop marked rigidity in the hindlimbs. They visi­bly stiffen and when running away exhibit a “bunny­hopping gait,” caused by impaired mobility of the hindlimbs. Severely affected goats show general muscular rigidity, stiffen, and fall over. Hyperextension of muscles can be so severe that recumbent animals involuntarily roll over on their backs with legs extended in the air.

Clinical Pathology and Necropsy

There are no detectable abnormalities in hematology or clinical chemistry. Routine necropsy and histologic exami­nation reveal no lesions. Electromyographic (EMG) studies of the live animal may be helpful in diagnosis. Affected goats produce a distinctive response characterized by an audible “divebomber” sound, typical of the persistent, high-frequency discharge of muscle associated with a hyperexcitable muscle cell membrane (Steinberg and Botelho 1962). Myotonic goats have some characteristic ultrastructural abnormalities. Mitochondria from myo­tonic fibers have an altered lipid composition (Harris 1983). Myotonic muscle fibers from myotonic goats have an increased proportion of fast isomyosins relative to slow iso­myosins compared to non-myotonic goats, suggesting that the abnormal action potential pattern of myotonia leads to a redistribution of the myosin isoforms (Martin et al. 1984).

Diagnosis

The presumptive diagnosis is based on the characteristic pattern of intermittent, sustained muscle stiffening in response to external stimuli in goats with a history of myo­tonic lineage. When the background of goats is unknown, EMG tests can support the diagnosis. Tetanus is a progres­sive condition in which muscular rigidity becomes persis­tent rather than intermittent. The author (DMS) has seen a pygmy goat with intermittent rigid collapse that, on nec­ropsy, had extensive lesions of CAE virus infection in the cerebrum.

Treatment and Control

Goats with myotonia congenita are not usually treated. In humans, the condition is managed by daily administration of quinine sulfate or procainamide. Because fainting goats are often bred with the goal of maintaining the defect for research purposes, control is not an appropriate topic of discussion. The use of these goats solely for amusement purposes should be discouraged on humane grounds.

Various Congenital Skeletal Abnormalities

Achondroplastic dwarfism is a breed characteristic of pygmy and dwarf goats. Akabane virus infection is dis - cussed earlier in the chapter as an infectious cause of congenital skeletal deformity. Lupinosis is also discussed above as a possible phytotoxic cause. Ingestion of Veratrum californicum by does during gestation also has the potential of producing hypoplasia of metatarsal and metacarpal bones in kids, in addition to the more com­monly known sign of cyclopia (Binns et al. 1972). There are numerous reports of spontaneous, congenital, skele­tal deformity in goats, of unknown etiology. Hemivertebra of the fourth thoracic vertebra, causing spinal injury and hindlimb weakness, was seen in a 2-month-old Saanen cross kid and was presumed to be hereditary (Rowe 1979). Increased limb length with knuckling of unknown cause was recognized in an Osmanabadi kid as a cause of dys­tocia (Kulkarni and Deshpande 1987). Tibial agenesis has been reported in a newborn Toggenburg kid (Giddings 1976). Radial agenesis and ulnar hypoplasia occurred in two Saanen kids from the same kidding (Baum et al. 1985). Complete absence of a forelimb at birth has also been observed in a West Alficau DwanT goat (Onawunmi et al. 1979). Lateral luxation of the patella secondary to a hypoplastic lateral trochlear ridge may be congenital in goats, though the condition is usu­ally not observed until goats are older than 1 year of age (Baron 1987). Various other sporadic congenital devia­tions of the forelimbs and hindlimbs have also been described (Koch et al. 1957).

An inherited condition of short tendons occurs in up to 4.5% of Australian Angora goats. Severely affected kids may die, but milder cases are correctable by splinting or plaster casting. The trait is transmitted as a recessive auto­somal allele (Baxendell 1988). An inherited metacarpal gait has been reported in Saanen goats in Iran (Bazargani and Khavary 1976). It is believed to be a recessive, sex- linked trait, as only males of twins of different sexes were affected at birth. These animals walked on their metacarpi and died within a few weeks of age. Peromelia, the agene­sis of the distal parts of the limbs, has been recorded in Angora goats in Denmark and identified as being inherited as an autosomal recessive defect with varying degrees of expressivity (Nielsen and Arnbjerg 1992; Agerholm et al. 1997).