Equine endotoxemia: pathomorphological aspects of endotoxin-induced damage in equine mesenteric arteries

Coagulation Activation by Lipopolysaccharides

Lipopolysaccharides at approximate plasma reactivities >3 ng/mL or β-glucans at >0.5-1 μg/mL are toxic for human blood; lipopolysaccharide interacts with membrane components of susceptible cells (eg, monocytes) activating phospholipase A2that destroys the cell membrane. Cell fragments (microparticles or DNA) possess polynegative niches that activate intrinsic hemostasis. Pathologic disseminated intravascular coagulation arises. Blood vessels are obstructed by disseminated thrombi, and vital organ areas become ischemic. Multiorgan failure threatens life of the patient. Diagnosis and therapy of pathologic disseminated intravascular coagulation is of extreme clinical importance. For early diagnosis of pathologic disseminated intravascular coagulation, specific activation markers of coagulation (eg, plasmatic amidolytic thrombin activity) or the plasmatic lipopolysaccharide or glucan reactivity can be measured. A new treatment target might be kallikrein or factor XIIa; 10 to 20 mM arginine is the approximate 50% inhibitory concentration against the contact phase of coagulation. The complex interaction between cell fragments and hemostasis causes pathologic disseminated intravascular coagulation in sepsis.

Endotoxin-induced Injury of the Central, Autonomic and Enteric Nervous Systems and Intestinal Muscularis in Thoroughbred Horses

To evaluate the effects of endotoxin on the morphology of the equine central, autonomic and enteric nervous system and intestinal muscularis, six Thoroughbred horses with experimentally induced endotoxaemia were examined. The lesions in the central nervous system consisted of perivascular oedema around arterioles, suggesting brain oedema, and ring haemorrhages around veins, similar to those in human patients with septic shock. In the cranial mesenteric ganglia, neuronal cell bodies became pink or red, with shrinkage of cytoplasm indicative of ischaemic changes; intramural and perivascular infiltration by erythrocytes and neutrophils occurred around arterioles in the epineurium (acute focal interstitial inflammation). In addition, transmission electron microscopy revealed oedema of the endoneurium and mesoaxon in the nerve fascicles running inside or outside the ganglia. Myenteric neurons showed shrinkage of the cytoplasm with multiple cytoplasmic vacuoles, suggesting ischaemic changes. Oedematous degeneration and coagulation necrosis of smooth muscle cells, with dissociation of the cells, were prominent in the tunica muscularis. It is suggested that arterionecrosis elicited by endotoxin and frequently observed in the autonomic and enteric nervous system and intestinal muscularis, was the result of vasoconstriction or vasospasm.

Mesenteric Arterionecrosis in Natural and Experimental Equine Endotoxaemia

To test the hypothesis that mesenteric arterionecrosis (MA) occurs in horses with naturally occurring endotoxaemia (ET) and in those with experimentally induced ET, the mesentery and gastrointestinal tract of 21 Thoroughbred racehorses (15 with spontaneous colic suspected to be due to ET, and six with experimentally induced ET) were examined. MA, which occurred in 13 of the 15 horses with spontaneous colic and in all six of the cases of experimental ET, was morphologically similar in the two groups of animals. This suggested that the pathogenesis of the MA was fundamentally similar in the two groups, and that MA is a pathognomonic feature of equine ET. In addition to histolysis of the arterial walls associated with infiltration of blood components, changes were noted in the medial smooth muscle including formation of many intracellular vacuoles within single smooth muscle cells, cytoplasmolysis, necrosis with granules and vacuoles, and coagulation necrosis; similar changes have been observed in cases of prolonged angiospasm or vasoconstriction. It is suggested that the effects of sustained arterial contraction leading to intimal and medial damage influence the pathomorphogenesis of MA.

Effect of Acute Sublethal Endotoxaemia on In Vitro Digital Vascular Reactivity in Horses

Endotoxaemia is a syndrome linked to the development of equine laminitis; however, the relationship between them is uncertain. The aim of this experiment was to evaluate the effect of an experimental acute sublethal endotoxaemia model on in vitro equine palmar digital vascular reactivity. Rings of arteries and veins of each forelimb were obtained from 11 clinically healthy horses submitted to two surgical procedures, 3 weeks apart. Before the second surgery, 0.25 microg/kg of lipopolysaccharide from Escherichia coli O55:B5 in saline, was administered i.v. in 30 min. After 3 h, the vessels were harvested and submitted to in vitro vascular reactivity experiments and histopathology. The response to depolarizing Krebs solution (DKS, 40 mm), phenylephrine (PHE), acetylcholine (ACh) and sodium nitroprusside (SNP) were evaluated. All horses showed colic pain and watery diarrhoea, tachycardia, tachypnea, hyperthermia and leucopenia. Concentration-response curve (CRC) to PHE was shifted to the left in arteries rings from endotoxemic horses without any effect on vein rings. The CRC to ACh was shifted to the right with a reduction in the maximal response. The response to SNP and DKS was similar between groups. There was no evidence of histopathological effects. The increased response to PHE in digital arteries together with a reduction of the endothelium-dependent response to ACh in arteries and veins, confirm the existing reports where endotoxaemia was found to modify the digital vascular reactivity during the acute phase. As the digital endothelial function is impaired, there may be an increased potential to develop a digital prothrombotic state with a reduced vasodilatory capacity.

Arterionecrosis of the Equine Mesentery in Naturally Occurring Endotoxaemia

This report describes the mesenteric arteriolar lesions in a Thoroughbred racehorse with endotoxaemia due to colic. The vascular lesions consisted of a striking loss of medial smooth muscle cells, associated with granular cell debris derived from necrosed muscle cells, plasma insudation, erythrocyte infiltration and the deposition of a fibrinoid substance (fibrinoid degeneration) in the entire arterial wall, possibly produced by the infiltration of blood components through endothelial cell junctions into the arterial wall. The morphology of the mesenteric arteriolar necrosis closely resembled that seen in experimental equine endotoxaemia and in horses that died from colic; it also resembled that of Shiga toxin-induced arteriolar lesions in oedema disease of swine and of the arterionecrosis in human cerebral arteries that may lead to hypertensive intracerebral haemorrhage.