Multiple horses losing condition despite access to feed and dying.
Above: The lungs did not collapse and the ribs had left an impression.
Time and location: Sep-December 2020, inland WA.
Case definition: Horses dying following a period of chronic weight loss, no neurological signs.
Disease mapping: Horses had been rotated between two sites on the property but losses and decline in body condition was seen in those spelled in the southern part of the property for the past months. Two horses in poor condition brought back to homestead in early September and continued to deteriorate. All horses were brought back to the homestead mid-September and two were euthanised by the owner within a couple of days.
Gross findings: Bloods were taken from three stockhorses in light condition, 5-6 years of age, at the end of September to evaluate liver function (to rule in or out Kimberley Walk About disease that can occur in horses grazing Crotolaria species). The horses showed no signs of depression, exercise intolerance, coughing or nasal discharge coming into the yards. One of these horses continued to deteriorate rapidly in the next week, became dyspnoeic and tachypnoeic, and was euthanised. Severe lung consolidation, oedema and emphysema were seen at post mortem. Another two followed the same course of events within a fortnight.
Laboratory findings: The initial blood samples showed evidence of a chronic, active inflammatory response (elevated fibrinogen, hypoalbuminaemia and hyperglobulinaemia) in two of the horses. The third horse had mild elevation in GLDH suggesting mild hepatocellular injury. Histopathology included severe, proliferative bronchoalveolar pneumonia with Clara cell hyperplasia, severe interstitial and septal fibrosis, and degenerative hepatopathy with hepatocellular megalocytosis. This suggests a toxic agent inducing the lung pathology, most likely of plant origin.
Animal / management / environment risk factors: The respiratory changes were consistent with a chronic progressive respiratory syndrome seen in horses associated with the ingestion Crotalaria species plants, known as Jaagsiekte disease. The syndrome is characterised by undulating fever, rapid respiration, laboured breathing, subcutaneous and pulmonary emphysema, interstitial pneumonia and sometimes hepatic fibrosis. Other pyrrolizidine alkaloid containing plants which have been associated with respiratory disease in horses include Chromolaena odorata (trifid weed), Ageratina adenophora (crofton weed) and Trichodesma zeylanicum (camel bush). A plant survey of the southern paddock identified camel bush plants that had been grazed.
Recommendations: To prevent access to the plants by relocating the horses and establishing facilities at the homestead to enable feeding of more hay for longer (until the season breaks).
Above: Bronchial epithelial hyperplasia and mixed inflammatory infiltrates with fibrosis indicating a chronic inflammatory process.