Diagnosing limb paresis and paralysis in sheep

Abstract

Paresis and paralysis are uncommon problems in sheep but are likely to prompt farmers to seek veterinary advice. A thorough and logical approach can aid in determining the cause of the problem and highlighting the benefit of veterinary involvement. While this may not necessarily alter the prognosis for an individual animal, it can help in formulating preventive measures and avoid the costs - both in economic and in welfare terms - of misdirected treatment. Distinguishing between central and peripheral lesions is most important, as the relative prognoses are markedly different, and this can often be achieved with minimal equipment. This article describes an approach to performing a neurological examination of the ovine trunk and limbs, the ancillary tests available and the common and important causes of paresis and paralysis in sheep.

This ewe is recumbent due to listeriosis but the changes to facial symmetry are subtle (minor changes in the shape of the ocular and nasal aperture and set of the lip and ear) and the animal may be misdiagnosed if the cranial nerve function were not assessed

Fig 1:

Simple reflex arc. The reflex arc provides the basis for neurological examination of the limbs and trunk. At its most simple, it involves just two neurons – the afferent (sensory) neuron and the efferent (motor) neuron. For example, increased tension in a tendon is detected by stretch receptors resulting in activation of the afferent neuron, which, through a direct synapse to a motor neuron, increases motor neuron activity and, thus, muscular contraction

Fig 2:

Scottish Blackface lamb with a compressive thoracolumbar lesion. Spinal lesions in the thoracolumbar region often result in animals adopting a dog-sitting position. The hindlimb myotatic reflexes (eg, patellar reflex) are usually exaggerated in such cases

Fig 3:

Schematic diagram showing upper motor neuron (UMN) and lower motor neuron (LMN) lesions relative to the spinal cord. UMN lesions (1) may occur anywhere cranial to the nerve roots that give rise to the nerves involved in the reflex/limb/muscle group in question. They disrupt the descending (inhibitory) axons that originate in the brain and also disrupt ascending pathways conveying sensation and proprioceptive information. LMN lesions may be central (2) or peripheral (3). Lesions in these locations may affect just motor neuron function or may affect both motor neurons and sensory neurons, leading to a loss of sensation as well as paresis/paralysis

Fig 4:

Limb paralysis due to infection with maedi-visna virus (MVV). Early in the disease course animals often show unilateral hindlimb paresis (a). The disease then progresses to ataxia, hindlimb paralysis (b), recumbency and death. Animals affected with visna are usually a small subsection of those infected with MVV in a flock (Picture b: Valentín Pérez and Julio Benavides)

The lumbosacral space can be felt as a depression just caudal to a line drawn between the two iliac crests

Fig 5:

Radiograph showing (a) vertebral body destruction and disruption of the normal spinal architecture due to vertebral body osteomyelitis affecting the lumbar vertebrae. (b) New bone formation and loss of normal architecture (arrows) around the C2-C3 articulation on the right-hand side caused by a perivertebral abscess

Fig 6:

Myelograph showing (a) narrowing and dorsal deviation of both the dorsal and ventral columns of contrast material at the level of T11-T12. This lamb had vertebral body osteomyelitis resulting in erosion of vertebral bodies of T11 and T12. (b) Pooling of contrast material at the injection site (the lumbosacral junction). This myelograph has been taken too soon after administration of the contrast material as the dorsal contrast column is well established only as far as L3. The lesion in this case was at T10

Fig 7:

Atlanto-occipital joint infection. (a) Sepsis of the atlanto-occipital joint appears to be part of the same septic polyarthritis complex as the more frequently seen septic arthritides of limb joints (eg, carpus and tarsus). (b) and (c) Affected lambs show neck pain, low head carriage, ataxia and tetraparesis, which manifests frequently as flexion of the forelimbs