A 2.5-month-old female Tyrolean Grey calf weighing 99 kg from a cow-calf herd in eastern Switzerland was referred to the Clinic for Ruminants at the Vetsuisse Faculty, University of Berne with a history of pelvic limb weakness. A complete clinical examination indicated that the calf was normally developed for its age and breed. It was alert and responsive, but had a nervous demeanor. Examination of the cardiovascular and respiratory systems disclosed no abnormalities, except for a high heart rate in the first days of the calf's stay at the clinic (136–160 bpm; normal, 60–100 bpm). Heart rate returned to normal within a few days, and tachycardia was attributed to stress upon arrival in a new environment. Respiratory rate and body temperature were normal. Appetite was present, and food prehension, mastication, and swallowing were normal. Examination of the digestive system was within normal limits, as was that of the urinary tract. The musculoskeletal system was normally developed except for slight muscular atrophy of the thighs. Palpable articulations were neither enlarged nor painful. Foot trim was adequate, and the claws had no abnormalities.
Upon neurological examination, the calf's behavior was normal except for its markedly nervous reactions to manipulations. The calf often stood with a wide-based stance in the thoracic and pelvic limbs (Fig 1, Video S1). It showed mild ambulatory paraparesis with moderate-to-severe ataxia of the pelvic limbs and a tendency to fall to both sides (Fig 2, Video S2). Postural reactions were normal in the thoracic limbs and decreased to absent in the pelvic limbs. Segmental spinal reflexes of the thoracic limbs were normal, but decreased in both pelvic limbs. The menace response was absent bilaterally, pupillary light reflexes and vision were normal. Other cranial nerves were normal. The cutaneus trunci response was absent. Tail tone was decreased, but anal tone, defecation, and urination were normal. Palpation of the spine was not painful.
The main lesion was localized in the spinal cord from T3 to S3. At this point, differential diagnoses included degenerative, infectious, and inflammatory lesions, as well as a malformation of the spinal cord.
A CBCa and a blood chemistry profileb were performed. No abnormalities were detected in the CBC, and the chemistry profile disclosed a slight increase in aspartate aminotransferase activity (113 IU/L; reference range, 40–80 IU/L), creatine kinase (CK, 3,152 IU/L; reference range, 170–569 IU/L), and lactate dehydrogenase (LDH, 3,694 IU/L; reference range, 1,052–1,890 IU/L). Urinalysis was within normal limits.
Analysis of lumbosacral cerebrospinal fluid (CSF) revealed 2 leukocytes/μL (reference range, 0–5/μL), a negative Pandy test, and 0.3 g/L albumin (normal, ≤ 0.75 g/L).1 A differential cell count was not performed.
No changes in the animal's neurological status were observed during its stay at the clinic. On day 16 of hospitalization, general anesthesia was induced with xylazine, butorphanol, and ketamine, and maintained by inhalation of isoflurane vaporized in oxygen. Electromyography (EMG)c was performed on the right side of the body, including thoracic and pelvic limb muscles, as well as paraspinal and tail muscles. No spontaneous activity was detected. The calf was euthanized at the end of the EMG procedure with an overdose of pentobarbital. All procedures complied with the Swiss legislation on animal welfare and protection.
A complete necropsy was performed. Macroscopic examination did not identify abnormal findings in any organ system. Brain, cervical, and thoracolumbar spinal cord, and tissue samples from peripheral nerves and skeletal muscles were fixed by immersion in 10% neutral buffered formalin, embedded in paraffin, sectioned at 5 μm, and stained with hematoxylin and eosin (HE). Selected sections of the brain and spinal cord were stained with combined Luxol fast blue HE and modified Bielschowsky stain.
Histologically, bilaterally symmetrical (Wallerian-type) axonal degeneration and axonal loss were the major lesions and were most conspicuous on modified Bielschowsky-stained sections (Fig 3). Lesions involved most ascending and descending tracts, but primarily targeted the dorsal spinocerebellar tract and the fasciculus gracilis where the lost axonal fibers were replaced by astrogliosis (Fig 3). On combined Luxol fast blue and HE-stained sections, the deep blue staining of the normal white matter was replaced by an eosinophilic staining in these areas (Fig 3). In most of the remaining white matter tracts, including the ventral and lateral corticospinal tracts, the rubrospinal tract, and the spinothalamic tract, scattered degenerated axons were observed. The Fasciculus cuneatus was spared. Lesions of the dorsal spinocerebellar tract appeared to be more severe in the cervical spinal cord, and lesions of the descending tracts appeared to increase in severity in the thoracolumbar cord. Degeneration was characterized by distension of myelin sheaths in the white matter, which was devoid of normal axonal profiles and contained fragmented myelin, foamy macrophages, and cellular debris (digestion chambers). Occasionally, axonal spheroids were observed. Inflammatory infiltrates were absent. Dorsal and ventral nerve roots were unremarkable. In the brain, mild-to-moderate axonal degeneration and loss were present in the longitudinal medial fascicle, reticular formation of the medulla, pyramids, cerebellar peduncles, cerebellar white matter, cerebral peduncles, and internal capsule. In addition to axonal damage, cell bodies of red nucleus neurons were swollen and chromatolytic (Fig 3). Occasionally, swelling and chromatolysis of large neurons also were observed in the reticular formation of the medulla, medial vestibular nucleus, and ventral horns of the spinal cord. Wallerian-type axonal degeneration also was observed in the femoral nerve (Fig 3). Changes compatible with neurogenic muscle atrophy were present in skeletal muscle.
A 2nd calf with similar clinical signs had been observed on the 1st calf's farm of origin shortly before its hospitalization (Paganini, personal communication). Although it was not possible to perform a complete clinical evaluation of the 2nd calf, information about the genealogy of both calves was obtained from the Tyrolean Grey cattle breeding organization and analyzed. This allowed a pedigree to be derived that was suggestive of autosomal recessive inheritance (Fig 4). According to the Tyrolean Grey cattle breeding organization, approximately 40 cases have been recorded in Austria and Switzerland in the last few years. Affected calves showed progressive ataxia in the pelvic limbs, starting at the age of 1–1.5 months, which progressed to a point where the animals became recumbent. Most of them were slaughtered between 8 and 10 months of age.
Several central nervous system (CNS) diseases in cattle have typical clinical features and are confined to specific breeds because of their genetic nature. Hereditary degenerative disorders of the CNS in calves include among others degenerative axonopathy,2 spinal dysmyelination,3,4 bovine progressive degenerative myeloencephalopathy (BPME or weaver syndrome),5–10 and spinal muscular atrophy (SMA).11–16 The predominant clinical feature in the calf described here was moderate-to-severe spinal ataxia, leading to a tentative diagnosis of diffuse spinal cord disease with the most pronounced lesions in the segments between T3 and S3. The absent menace response could not be explained by this localization, and additional cerebellar signs were lacking. The absent menace response was attributed to excitement in this calf.1 Degenerative, infectious, and inflammatory lesions, as well as malformation of the spinal cord were considered as possible differential diagnoses.
Blood and CSF analyses did not indicate signs of infectious or inflammatory processes. The moderately increased activities of the muscle enzymes CK, AST, and LDH were considered to be secondary to increased muscular activity. Moderately increased muscle enzyme activity has been reported in animals with weaver syndrome.7,8,10 A malformation was considered unlikely because the calf had been normal at birth and developed clinical signs after more than 1 month of life.
These considerations and the fact that clinical signs were progressive led to the suspicion of a degenerative spinal cord disease. Furthermore, the occurrence of similar cases in the same breed suggested a genetic defect as the cause of disease.
Similar syndromes are known in other breeds, including spinal dysmyelination of crossbred Brown Swiss calves,3,4 degenerative axonopathy in Holstein-Friesian calves,2 weaver syndrome of Brown Swiss cattle,5–10 and SMA in Brown Swiss,11–13 horned Hereford,14 Red Danish,15 and Holstein Friesian calves.16 These disorders are caused by genetic defects that lead to degenerative spinal cord disease. Although the clinical signs themselves resemble those observed in the Tyrolean Grey calf described here, some signs, especially the age at onset, indicated a different entity. Neuropathologically, the lesions were compatible with a degenerative central and peripheral axonopathy affecting multiple systems (multisystem axonal degeneration or long tract degeneration). The pattern of axonal involvement (ie, involvement of the peripheral nerves in the absence of lesions in the ventral nerve roots) suggested a distal axonopathy, in which the most distal portions of the axon are the first to degenerate. It has been proposed that a metabolic or toxic derangement of neurons prevents maintenance of the most distal axonal processes by the neuronal cell body (ie, “dying back axonopathy”). Because metabolic aberrations precede morphological changes in the neurons, distal axonopathy might become evident before neuronal chromatolysis and therefore be the major lesion observed in affected animals.17 However, degenerative changes of the neuronal cell bodies were present in this calf and could be an indication of a primary insult to the neuronal cell body with secondary changes in the distal portions of the axons. Degenerative axonopathies with lesions resembling those of the axonopathy in the Tyrolean Grey calf described here have been described in Brown Swiss cattle with weaver syndrome5 and in Holstein-Friesian calves with congenital axonopathy.2 In both disorders, however, the topography of the lesions is different with axonal degeneration affecting all funiculi, but primarily the lateral and ventral funiculi. In addition, Purkinje cell loss and the presence of “torpedos” in the cerebellar cortex associated with weaver syndrome were lacking in the Tyrolean Grey calf.
The clinical signs and age of onset of these diseases also differ from those observed in the present case. Clinical signs of congenital axonopathy in Holstein-Friesian calves are present at birth. Affected calves are unable to stand and lie mostly in lateral recumbency with extended limbs, opisthotonus, and variable other neurologic abnormalities.2 The clinical signs observed in this Tyrolean Grey calf were strikingly similar to those of weaver syndrome in Brown Swiss cattle.5,7–10 However, the 1st clinical signs of this disease are not seen before 5–8 months of age, whereas the Tyrolean Grey calf was only 2.5 months old when it was presented for examination.
Genealogic information obtained from the Tyrolean Grey cattle breeding association for the calf described in this report and another affected calf in the same herd allowed identification of a common ancestor suspected to have been a carrier of the defect causing the clinical syndrome described here. Pedigree analysis indicated familial relationships among affected cattle, with an equal sex distribution of affected animals, suggestive of an autosomal recessive genetic defect. The history of the Tyrolean Grey cattle breed is well documented by breeding organizations. Given the relatively small gene pool, the incidence of genetic defects and the likelihood of phenotypic expression of recessive traits are increased in comparison to breeds with larger populations. The Tyrolean Grey cattle ancestor cow “Gusti” was the mother of 2 famous bulls used widely for artificial insemination, a fact that likely contributed to rapid dissemination of the defective gene in the population. The mutation involved in the pathogenesis of this axonopathy has been investigated and is described in detail elsewhere.18 Bulls identified as carriers by affected progeny or by genetic testing currently are excluded from breeding programs (Hausegger, personal communication).
In conclusion, we describe the clinical and pathologic features of a newly identified hereditary degenerative spinal cord disease affecting young calves of the Tyrolean Grey breed. Neurologic signs consisting of spinal ataxia and proprioceptive deficits mainly in the pelvic limbs were similar to those observed in older animals with weaver syndrome. The principal pathological lesion was a bilaterally symmetrical central and peripheral axonopathy. Pedigree analysis suggested autosomal recessive inheritance. Because of the progressive course of the disease, affected calves become recumbent and must be culled before the age of 1 year. The Tyrolean Grey cattle breeding organization has implemented measures to identify carrier animals and eliminate the defective allele from the population.