Plasma and tissue histamine in equine grass sickness

Plasma histamine concentrations in horses administered sodium penicillin, guaifenesin-xylazine-ketamine and isoflurane with morphine or butorphanol

OBJECTIVE

Various drugs administered to horses undergoing surgical procedures can release histamine. Histamine concentrations were evaluated in horses prepared for surgery and administered butorphanol or morphine intraoperative infusions.

STUDY DESIGN

Prospective studies with one randomized.

ANIMALS

A total of 44 client-owned horses.

METHODS

In one study, anesthesia was induced with xylazine followed by ketamine-diazepam. Anesthesia was maintained with guaifenesin-xylazine-ketamine (GXK) during surgical preparation. For surgery, isoflurane was administered with intravenous (IV) morphine (group M: 0.15 mg kg^-1 and 0.1 mg kg^-1 hour^-1; 15 horses) or butorphanol (group B: 0.05 mg kg^-1 and 0.01 mg kg^-1 hour^-1; 15 horses). Histamine and morphine concentrations were measured using enzyme-linked immunoassay before opioid injection (time 0), and after 1, 2, 5, 30, 60 and 90 minutes. In a subsequent study, plasma histamine concentrations were measured in 14 horses before drug administration (baseline), 15 minutes after IV sodium penicillin and 15 minutes after starting GXK IV infusion. Statistical comparison was performed using anova for repeated measures. Pearson correlation compared morphine and histamine concentrations. Data are presented as mean ± standard deviation. Significance was assumed when p ≤ 0.05.

RESULTS

With histamine, differences occurred between baseline (3.2 ± 2.4 ng mL^-1) and GXK (5.2 ± 7.1 ng mL^-1) and between baseline and time 0 in group B (11.9 ± 13.4 ng mL^-1) and group M (11.1 ± 12.4 ng mL^-1). No differences occurred between baseline and after penicillin or between groups M and B. Morphine concentrations were higher at 1 minute following injection (8.1 ± 5.1 ng mL^-1) than at 30 minutes (4.9 ± 3.1 ng mL^-1) and 60 minutes (4.0 ± 2.5 ng mL^-1). Histamine correlated with morphine at 2, 30 and 60 minutes.

CONCLUSIONS AND CLINICAL RELEVANCE

GXK increased histamine concentration, but concentrations were similar with morphine and butorphanol.

Systemic concentrations of antioxidants and biomarkers of macromolecular oxidative damage in horses with grass sickness

REASONS FOR PERFORMING STUDY

The aetiopathogenesis of equine grass sickness (EGS) is unknown. The role of free radical-mediated neuronal damage has not previously been investigated in this condition.

OBJECTIVES

To investigate the potential contribution of oxidative damage and antioxidant status to neurodegeneration in EGS.

METHODS

Systemic levels of surrogate biomarkers were determined in 10 horses with acute EGS and in 2 control populations; 10 healthy horses co-grazing with the 10 EGS horses at the onset of clinical disease, and 10 healthy mares grazing where EGS has not been reported.

RESULTS

EGS horses had alterations in levels of several antioxidants, consistent with oxidative stress, the acute phase response and/or the secondary metabolic complications of EGS. EGS horses had elevated plasma dihydroxyphenylalanine (DOPA) levels.

CONCLUSIONS

The elevated DOPA levels probably reflected a generalised disturbance of catecholamine metabolism rather than increased DOPA production via free radical-mediated oxidation of tyrosine. However, there was no evidence of systemic macromolecular oxidative damage.

POTENTIAL CLINICAL RELEVANCE

Further work is required to determine whether macromolecular oxidative damage occurring at the neuronal level contributes to EGS.

The neurology and enterology of equine grass sickness: a review of basic mechanisms

Autonomic dysfunction constitutes a prominent clinical feature of equine grass sickness (EGS). Significant injury to the nervous control of the alimentary system is life threatening, partly because of dysphagia but also because of the failure of the unique regulatory mechanisms in equine digestion involving water and electrolyte balance. The neuropathology also indicates the presence of a somatic polyneuropathy. The morphological features of EGS are similar to those of excitotoxic neuronal degeneration, which resembles neuronal apoptosis. It is difficult to ascertain from published accounts the degree of damage to central neurones: the distribution is well documented and selective but the proportion of damage is poorly quantified. If lesions involve a significant number of regulatory neurones they should produce functional deficits. Any clinical assessment of horses, especially those with chronic EGS, should include a thorough neurological examination. Although this will not necessarily improve the outcome of the case, it may enable the rational selection of animals with a reasonable prognosis for recovery which is partly determined by the extent of CNS lesions. The evidence supports the following pathogenesis. There is an initial lesion in the enteric nervous system of susceptible horses. In the acute form of EGS, massive enteric neuronal damage occurs first functionally, then structurally leading to generalized alimentary smooth muscle atony, enhanced secretions and altered fluid fluxes. Severe distension of the stomach and small intestines rapidly develops, which augments the intestinal ileus by intersegmental inhibitory reflexes and causes colic and dehydration. In subacute cases, failure of intestinal bicarbonate buffer together with alimentary stasis rapidly reduces caecal-colonic fermentation. Thus the osmolality of large intestinal digesta reduces and water travels out of the bowel along osmotic gradients. Water returns to the circulation, but is eventually lost in the gastric and small intestinal secretions. The observation that pathological lesions may not be seen in the prevertebral ganglia within the first few days of acute cases supports the view that a functional deficit precedes structural lesions which may be secondary to a retrograde degeneration. It is therefore possible to resolve the observations that less damage may be seen in prevertebral ganglia and elsewhere in peracute and acute cases with the more common finding that greater neuronal damage is present in acute than in chronic cases. These different observations are probably time dependent. Chronic EGS occurs when there is less initial enteric nerve damage which may lead to less secondary prevertebral ganglionic pathology, and more time for functional and structural compensatory mechanisms to develop. Denervation hypersensitivity develops at target sites both in the gut and in peripheral somatic nerves which may account, in part, for the clinical signs of patchy sweating and muscle tremors. Raised circulating adrenaline levels may also account for generalized sweating, may contribute to gastrointestinal atony and may affect pacemakers at the pelvic flexure. Many of the features of EGS make worthwhile the re-investigation of Clostridium botulinum Group III toxins, which are known to prevent vesicular exocytosis, stimulate neurosecretion, produce neuronal chromatolysis and inhibit neutrophil migration. Also, evidence from other species suggests that increased nitrergic neuronal activity can account for many of the clinical signs of EGS, namely dysphagia, generalized ileus, gastric dilatation, sweating, peripheral vasodilatation, tachycardia, salivary hypersecretion, muscle wastage and cachexia.

Quantification of histamine in plasma and pulmonary fluids from horses with chronic obstructive pulmonary disease, before and after 'natural (hay and straw) challenges'

A commercial radioimmunoassay kit was used to quantify histamine concentrations of plasma, bronchoalveolar lavage fluid (BALF) and pulmonary epithelial lining fluid (PELF) of normal horses and horses with chronic obstructive pulmonary disease (COPD), before and after 'natural (hay and straw) challenge' (NC). There were no significant changes in the concentrations of histamine in plasma or BALF at 0.5 or 5 h after NC, but the PELF histamine concentration of COPD affected horses was significantly increased at 5 h, but not at 0.5 h, following NC. As the histamine concentrations of whole BALF lysates were significantly correlated with the numbers of metachromatically staining cells, presumed to be mast cells and/or basophils, these findings support involvement of a late phase, IgE mediated, hypersensitivity reaction in the pathogenesis of equine COPD.

Plasma histamine levels in laminitic horses and in horses treated with a corticosteroid

In seven sound horses histamine plasma levels were evaluated by means of a radioenzymatic method using the enzyme histamine-N-methyltransferase (mean = 4.46 nmol/l, SD = 1.55 nmol/l). Histamine levels of laminitic horses were significantly higher (n = 11; mean = 12.46 nmol/l, SD = 3.24 nmol/l). In three sound horses plasma histamine levels were determined over a two week period following a double administration of 10 mg dexamethasone-21-isonicotinate/100 kg body weight. Histamine levels remained within the range of sound control horses.