Spontaneous rupture of a major blood vessel is an uncommon cause of sudden death in grazing livestock. Spontaneous rupture of the aorta has been observed infrequently, and in single animals, including a calf and stallion (Jubb et al, 1993). This report describes two sheep in a flock which died suddenly on the same day.
Two of 200 mixed age white dorper ewes died suddenly following shearing in January on a southern tablelands property. The sheep were yarded the previous evening, from an improved pasture paddock of predominantly phalaris. They travelled about one and a half kilometres to the yards, with none showing evidence of fatigue. Lambs had been weaned about one month previously. The ewes had been vaccinated with a six-in-one clostridial/CLA vaccine two weeks previously, when a couple of ewes had been found suddenly dead in the paddock. They had also been drenched with a triple combination drench (BZ/Lev/Aba) at that time.
The affected ewes were in body condition score 3.5 and 4. They were each shorn by a different shearer. The shearers reported the ewes resisted restraint. They struggled and kicked, then began to shake. One collapsed and died on the board, while the second ewe slumped at the bottom of the exit chute and died. Each shearer reported other ewes in the mob also resisted restraint, and no undue force or pressure was applied to the affected ewes.
Post mortem examination of the ewes showed pallor of the gums in otherwise externally unremarkable carcases. Sheep one was a full-mouth mature size ewe which had reared a lamb. There were two flattened 100mm x 20mm blood clots free in the left thorax, and extensive haemorrhage in the dorsal mediastinum (See Figure 1).
The heart had stopped in systole. A 2cm linear tear was located in the thoracic aorta (See Figure 2).
Sheep two was a three-year old smaller ewe that had also reared a lamb. Her left chest cavity contained a large bright red blood clot of similar size and shape to the compressed lung beneath (See Figure 3).
The thoracic aorta showed both a 1cm transverse tear and a conjoined 3cm linear tear (See Figure 4).
In both cases, the adjacent aortic wall appeared of similar thickness. There was no gross evidence of thrombosis or vasculitis.
Spontaneous rupture of the thoracic aorta in animals is regarded as a rare event (Jubb et al, 1993). The occurrence of two cases simultaneously in the same flock is unusual, and suggests the presence of other risk factors in the affected flock.
Arterial rupture in humans commonly occurs at the site of an aneurism. Where cases of spontaneous rupture of the aorta in humans are not associated with an aneurism, there is frequently a history of long-term steroid use, hypertension, or trauma (Yokoyama et al, 2000).Spontaneous rupture of the aorta is reported in horses, including during racing and in a stallion during mating. An association with increased arterial pressure is assumed (Jubb et al, 1993). The condition has also been observed in horses following the severe thoracic trauma caused by a fall. The shearers and manager involved with these sheep claimed the affected animals were not subjected to excessive force, but noted they, like many of their cohorts, actively resisted restraint. This would be expected to elevate blood pressure.
Spontaneous rupture of the abdominal aorta occurred experimentally in 50% of turkeys suffering from lathyrism (Terpin and Roach, 1987). Lathyrism is caused by ingestion of an amino acid in the seeds of leguminous plants of the genus Lathryus, known also as chickling pea or grass pea. It causes inhibition of the enzyme lysyl oxidase leading to a neurotoxic paresis of the hind limbs, and weakened elastin and collagen in blood vessels. This condition is also reported in humans and other animal species especially horses. In this case, no plants similar to Lathryus were observed in the paddock, although at least two species of Lathryus plants have been occasionally identified in southern New South Wales (PlantNET, 2013).
Spontaneous aortic rupture occurs experimentally in copper-deficient pigs, associated with a deficiency of the same copper-containing enzyme lysyl oxidase (Jubb et al, 1993). The condition has not been reported in cases of naturally occurring copper deficiency, and symptoms of copper deficiency have not been noticed in stock on the property on which these sheep grazed. Copper deficiency in grazing livestock is rare in this district.
No information was available on the pedigree of the affected sheep. The younger of the two ewes was home-bred, while the second had been purchased a couple of years previously.