An 8.5-month-old Icelandic filly was presented to the primary care veterinarian due to what the owner perceived as a sudden onset of neurological signs. The filly was perceived as being previously healthy, except for an upper respiratory tract infection and fever 3 weeks prior to presentation. The filly was born at the premises and had been with the same owner and at the same farm since birth. The initial clinical signs consisted of head tremors and hypermetria of all four limbs and stiff gait on both thoracic limbs. The filly was treated supportively with dexamethasone (0.1 mg/kg IM for 2 days; Dexadreson® Vet., 2 mg/mL, Intervet International B.V., The Netherlands). However, the condition progressed to marked ataxia of all four legs and difficulty rising within a few days. Based on the history and clinical signs, a progressive cerebellar lesion was suspected. Due to a poor prognosis and the markedly progressive nature of the condition, the owner donated the filly to the University of Copenhagen, Denmark for research. The filly was deemed safe for transportation and brought to the University Large Animal Teaching Hospital for clinical and postmortem examination.
According to the owner, another offspring (male) of the dam, bred to another sire and born 4 years prior had developed neurological signs at the age of 3 months. The clinical signs of this half-sibling also occurred suddenly and were characterized by an apparent loss of orientation and anxious behavior. A detailed neurological examination was not performed at that time and the foal was euthanized within a few days, as the condition did not improve and the foal was disposed without post mortem examination.
The clinical examination was performed the day after arrival to the hospital. The filly was bright and alert, slightly anxious but with normal mentation. It was of normal body condition (score of 4/9) [12] and with rectal temperature, heart rate and respiratory rate all within normal limits. The submandibular lymph nodes were indolent and bilaterally of normal size, and the nares were with minimal serous exudate. The oral mucosa was pink with a capillary refill time of 1–2 s. Auscultation of the heart and lung revealed no abnormalities. Intestinal borborygmi was normal over all four quadrants. No lameness was observed, and the digital pulses were of normal strength on all four legs.
When fed or given treats, as well as when initiating movement and navigating through a narrow space with her head, the filly had marked intention tremors also involving the neck and body, resembling titubation.
During the neurological examination, no head tilt or head turn was observed, the medial and lateral palpebral reflexes were normal, as were the direct and indirect pupillary light reflexes. The menace response was bilaterally absent. The oculocephalic reflex (physiological nystagmus) was normal bilaterally and she had no spontaneous nystagmus or strabismus with her head in a normal position, however, there was a moderate bilateral positional ventral strabismus when the head was moved vertically. There were no signs of muscle atrophy, and activity in the fascial muscles was normal. The filly had normal sensation in the planum nasale, but a mildly exaggerated response to a touch of the face or body was noted.
During walk, tölt and trot, the filly was markedly hypermetric on all four limbs, slightly worse on the left thoracic and left pelvic limbs (see video in Additional files 1, 2 and 3). When rising from a lying position in the stall, the filly would fall towards the left, but she would be able to correct herself and get up.
The filly did not circumduct during circling and was able to back up as well as navigate low obstacles without mistakes, thus suggesting intact proprioception. When the head was elevated, there were no changes in gait. When standing and initiating movement or navigating through a narrow space, the filly had marked truncal sway and intention tremors. Normal tonus was present when the tail was pulled at stand-still, but there was less resistance than expected during walk.
The lesion was neuroanatomically localized to the cerebellum based on the bilaterally absent menace response, the left-lateralizing hypermetria, as well as intention tremors and a bilateral positional ventral strabismus.
The primary differential diagnoses for a progressive cerebellar lesion with no pain, slightly lateralizing to the left, were CA, a congenital anomaly (e.g., cerebellar hypoplasia, sub-cerebellar cyst) or a metabolic condition (e.g., neuronal ceroid lipofuscinosis). Infectious/inflammatory conditions were considered highly unlikely due to the history, but could not be ruled out completely.
Cerebrospinal fluid was obtained immediately after euthanasia via the cerebello-medullary cistern. The aspirate was clear and colorless and cytology findings were within normal range with only a few mononuclear cells and occasional erythrocytes present.
An EDTA blood sample was analyzed for the single base polymorphism in the TOE1 gene associated with CA in Arabian horses [13], but found homozygous for the wildtype-allele (LABOKLIN, Bad Kissingen, Germany).
Hematological and biochemical parameters were within normal ranges, except for a slightly increased level of fibrinogen (4.14 g/L; ref. 1–4 g/L; Additional file 4), which was considered clinically non-significant. A nasal swab sample taken by the primary care veterinarian prior to admission to the university hospital was positive for Streptococcus equi subsp. zooepidemicus and Equid herpesvirus type 5.
The filly was euthanized by an overdose of barbiturate intravenously and necropsied. No gross lesions were found, with the exception of localized subcutaneous hemorrhage and edema above the right sacral tuber, several chronic gastric ulcerations in the non-glandular portion and some Gasterophilus larvae attached to the epithelium. The central nervous system (CNS) was unremarkable and with a total brain weight of 469.5 g and a cerebellar weight of 33.5 g (7.1%). The brain was fixed in toto by immersion in 10% neutral buffered formalin, as was the spinal cord, peripheral nerves, skeletal musculature and specimens of visceral organs. After fixation, the brain was embedded in agar, sectioned in 4 mm parallel transverse slices, visually inspected for gross lesions, and then subdivided into smaller pieces. Gross lesions were not observed. Selected regions of the CNS, spinal cord and other tissues were processed for histopathology, paraffin embedded, sectioned at 4 μm and stained with hematoxylin and eosin. Sections of the cerebellum were also stained with Luxol fast blue and immunohistochemically analyzed for expression of glial fibrillary acidic protein (Z0334 Agilent Technologies Denmark ApS, Glostrup, Denmark) using the UltraVision HRP system (Fisher Scientific, Fremont, CA, USA).
Histopathological examination revealed an extensive and diffuse loss of Purkinje cells. Wide parts of the Purkinje cell layer lacked Purkinje cells and appeared as a spongy zone, while scattered normal-appearing Purkinje cells were present in other parts alongside degenerated Purkinje cells. These appeared either as large cells with pale cytoplasm, large pale nuclei and poorly defined borders, sometimes only as cytoplasmic remnants, or as shrunken hyperchromatic cells with sparse amounts of cytoplasm (Fig. 1a, b). Shrunken Purkinje cells were also scattered in the profound molecular layer. Depletion of granule cells was mild and inconsistent. Immunohistochemistry revealed astrogliosis in the Purkinje cell and profound molecular layers (Fig. 1c). Other parts of the brain, the spinal cord, peripheral nerves, skeletal muscle and other tissues were unremarkable, except for a chronic granulomatous steatitis in the sacral tuber area.
Examination of six-generation pedigree data revealed that the parents of the CA-affected filly were genetically related though a common ancestor. The ancestor (A) was a stallion born in Iceland in 1968. He was related to the filly’s dam through one of his daughters and to the filly’s father through two of his daughters (Fig. 2). Pedigree analysis of the half-sibling that developed neurological signs 4 years previously revealed a genetic relationship to the sire of stallion A.