Verminous arteritis

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Verminous arteritis lesion in the cranial mesenteric artery of a weanling Quarterhorse colt

Verminous arteritis (thrombotic arteritis, arteriitis thromboticans) is a common cause of colic in horses.

In naturally infected animals, arterial lesions are most commonly seen in the cranial mesenteric artery and its branches. However, lesions have also been found less commonly in other arteries including the abdominal aorta, the renal arteries and the celiac axis.

Cause

Parasites are recognized as a major cause of colic, and Strongylus vulgaris blamed as the chief cause of simple and thromboembolic colic in horses[1][2]. Recently, small strongyles, rather than Strongylus vulgaris, were determined to be the main cause of increased colic incidence on farms[3]. Verminous colic was reproduced by infecting foals with Strongylus vulgaris larva; however, vascular lesions were observed in 90% of horse necropsies, and few of these horses suffered from colic. Therefore, the actual risk of colic associated with this parasite was questioned. Not until the widespread use of ivermectin was it clear that the decrease in thromboembolic colic paralleled the decrease in thrombotic lesions found in the cranial mesenteric artery.

Diagnosis

Diagnosis is primarily made post-mortem, although arteriogram can elucidate the disease process.

The walls of the cranial mesenteric and the ileo-ceco-colic arteries are invariably thickened and contain large amounts of thrombus material in which are found S. vulgaris larvae. This lesion is properly called verminous arteritis. The lumen of the cranial mesenteric artery is usually constricted in its diameter due to the thickening of the wall and the presence of thrombi. The lumen of smaller arteries may be entirely occluded.

Some reports in the literature describe aneurysms of the cranial mesenteric artery and its branches. True aneurysms with dilation and thinning of the arterial wall due to a loss of elastic fibres are unusual and may result from penetration of the elastic layer of the arterial wall by larvae.

In horses that have died of an acute clinical syndrome, infarction and necrosis of areas of the intestine are usually found at necropsy. These areas of infarction invariably coincide with occlusions (due to thrombi and emboli) in arteries supplying blood to the affected region(s) of the intestine.

Pathology

This disease process is an inflammation of the cranial mesenteric artery in horses caused with Strongylus vulgaris larvae. It is characterized by endothelial lesions, creation of thromb masses in the lumen of the artery, as well as narrowing of its lumen. The intima and media of the blood vessel are infiltrated with inflammatory cells, but the arterial wall can also be thinned with a widened lumen of the blood vessel – aneurism formation. This disease is present in all parts of the world in domesticated and wild equids of all breeds and age, but it is more frequent in younger horses. Older horses can also be infected, especially if their immune system is weakened and if there are good conditions for infection[4].

The macroscopic appearance of the cranial mesenteric artery and its branches with verminous arteritis is characterized by enlargement and thickening with hard-elastic, fibrous, consistency, with thrombotic changes[5].

Verminous arteritis development goes through three phases: acute phase of inflammation, chronic phase of inflammation and chronic active phase of inflammation[6].

References

  1. Wright AI (1972_Verminous arteritis as a cause of colic in the horse. Equine Vet J 4:169–174
  2. Gay CC, Speirs VC. (1978) Parasitic arteritis and its consequences in horses. Aust Vet J 54:600–601
  3. Uhlinger C. (1990) Effects of three anthelmintic schedules on the incidence of colic in horses. Equine Vet J 22:251–254
  4. Lyons ET, et al (1987) Common internal parasites found in the stomach, large intestine, and cranial mesenteric artery of Thoroughbreds in Kentucky at necropsy (1985 to 1986). Am J Vet Res 48:268-73
  5. Van Vleet JF, Ferrans VJ, (2007) Cardiovascular System, In: McGavin MD, Zachary JF: Pathologic basis of veterinary disease, fourth edition, Mosby Elsevier, St. Louis, pp:602-606
  6. Maxie MG, Physick-Sheard PW, (1985) Aortic-Iliac Thrombosis in Horses. Vet Pathol 22:238-49