CASE NOTES

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Ruptured cranial mesenteric artery causing sudden death in a late term triplet bearing ewe

Bruce Watt, Central Tablelands Local Land Services, Bathurst, Gordon Refshauge, NSW Department of Primary Industries, Cowra and Anne Jordan, Elizabeth MacArthur Agricultural Institute, Menangle

Posted Flock & Herd May 2020

Introduction

In ruminants, the cranial mesenteric artery, which branches from the abdominal aorta, supplies blood to the small intestines (Mohamed, 2017). In mature Holstein cows both in the US and Europe, the abdominal aorta and its branches, including the cranial mesenteric artery, can rupture secondary to pathological changes in these vessels (Lamm et al. 2007, Crawshaw et al. 2011). Ruptured mesenteric artery aneurysms are also reported rarely in people (Moore et al. 2011).

In this case, a mature, late-term, first-cross ewe bearing full-term triplets, died from blood loss from a ruptured cranial mesenteric artery. On histopathological examination there were no prior abnormalities seen in the vessel walls of the ruptured artery.

History

The ewes were part of a research project studying management options for ewes bearing triplets. In mid-gestation, a total of 138 ewes were divided into two management groups and allocated pastures and grain to achieve differences in body condition score. In late gestation, the two groups were divided into four paddocks for lambing. The ewes commenced the study (Gestation day 93) in forward condition (BCS 4) and were managed to be maintained in either BCS 4 or to lose condition. The target BCS was 3.5 for the group losing condition, but they averaged 3.7 at Day 142. Four ewes died prior to Day 142, mostly from prolapses (vaginal and spontaneous vaginal rupture).

Calibrated pasture estimates indicated that each paddock offered 1030-1350 kg DM/ha. The ewes were also offered 500g/hd/day of barley grain. The pasture base was a mixed sward including lucerne (Medicago sativa), annual rye grass (Lolium rigidum), barley grass (Hordeum leporinum), and broadleaf weeds, including marshmallow (Malva parviflora) and cape weed (Arctotheca calendula). Grasses were dominant (59-79%), with legumes (22-41%) and broadleaf weeds (7-20%) comprising the remainder.

At the conclusion of the study, a total of 18 ewes had died (13%). Not all ewes that died were examined, but of those that were, in addition to this case and the four ewes that died from vaginal prolapse or spontaneous vaginal rupture, one ewe died from pregnancy toxaemia and another died after being cast.

Necropsy findings

A mature first-cross ewe in BCS 4 was found dead in the lambing paddock. The ewe was bearing large full-term triplets (Figure 1). There was approximately 100 ml of blood in the peritoneal cavity and haemorrhage contained within a 20 cm segment of the omentum (Figure 2). The rumen was noticeably small and compressed and contained green ingesta with some barley. The liver appeared grossly normal. The cranial mesenteric artery was found to be completely severed with a 3 cm blood clot at the site of the rupture (Figure 3).

Figure 1. Dead ewe with full-term triplets

Figure 2. Haemorrhage contained within a segment of omental tissues

Figure 3. Mesenteric artery with fat tissue removed showing site of rupture

Laboratory Findings

Biochemistry

Test	Normal	Units	Results
Urea	2.9-7.1	mmol/L	8.0 H
Glucose	<4.4	mmol/L	7.2 H
BHB	0.00-0.80	mmol/L	0.32
Aq. Hu. Ca	1.13-2.03	mmol/L	1.75
Aq. Hu. Mg	0.65-1.55	mmol/L	0.90
Nitrate	<10	mg/L	<10
Nitrite	<1	mg/L	<1
D-lactate	0.0-0.5	mmol/L	0.1
Ammonia	0-200	umol/L	486 H

These results indicate that the ewe was not suffering from concurrent pregnancy toxaemia, hypocalcaemia or hypomagnesaemia. Mildly elevated urea is often associated with dehydration. Mild hyperglycaemia can be seen with stress in ruminants. The mild elevation in ammonia is considered unlikely to be significant as ammonia levels are known to rise after death.

Histopathology

There was histological evidence of severe haemorrhage into connective tissue surrounding a large artery (presumed mesenteric artery). There was no evidence of chronicity (eg erythrophagocytosis, inflammation, fibrosis, organising thrombus); as such the haemorrhage was considered an acute process. Given the acute nature of the changes, post-mortem haemorrhage cannot be excluded, however the large volume and distribution of perivascular blood was suggestive of active haemorrhage. Several areas of suspected rupture, as well as evidence of arterial fragmentation, are in section. There was no evidence of pre-existing predisposing structural disease in the sections examined (eg degeneration, hypertrophy, neoplasia, inflammation, dissection, or aneurysm). Dissection and aneurysm may be difficult to detect once ruptured, although apart from a small focal area of intimal thickening, there was no evidence of chronic changes (degeneration and repair) associated with abdominal arterial rupture reported previously in dairy cattle (Lamm et al. 2007).

Discussion

We are not aware of any previous reports of rupture of the anterior mesenteric artery in ewes. Copper deficiency has been implicated in arterial rupture in some species. While copper levels were not determined in this sheep or on this property, copper deficiency is regarded as unlikely in the district. A hereditary disorder of the fibrillin 1 gene (Bovine Marfan's syndrome) is a recognised cause of aneurysm in cattle (Robinson and Robinson, 2015).

References

1. Crawshaw, T. Idiopathic arterial aneurysm / rupture causing sudden death in dairy cattle. The Veterinary Record Sep. 2011:216
2. Lamm et al. Characterisation of rupture of abdominal artery aneurysm in dairy cattle. Journal of Veterinary Diagnostic Investigations 2007:19:273-278
3. Robinson W and Robinson N. 'Cardiovascular system' p. 62 in Pathology of Domestic Animals, MG Maxie (Ed). 2015
4. Mohamed R. Anatomical studies on the cranial and caudal mesenteric arteries of the Barbados Black Belly sheep. J. Morphol. Sci., 2017, vol. 34, no. 2, p. 93-97
5. Moore HM, Thapar A, Davies KJ, Herbert P and Davies AH. Case Report: Delayed Diagnosis of Mesenteric Artery Rupture. EJVES Extra (2011) 22, e73ee75