Validation of a low-dose ACTH stimulation test in healthy adult horses

Effects of administration of ascorbic acid and low-dose hydrocortisone after infusion of sublethal doses of lipopolysaccharide to horses

Background

Sepsis is associated with ascorbic acid (AA) depletion and critical illness‐related corticosteroid insufficiency (CIRCI) in humans.

Hypotheses

Intravenous infusion of lipopolysaccharide (LPS) would (a) decrease endogneous AA concentrations, (b) induce CIRCI and (c) administration of a combination of AA and hydrocortisone (HC) would have decreased indices of inflammation compared to either drug alone.

Animals

Thirty‐two healthy horses.

Methods

Randomized placebo‐controlled experimental trial. Horses were assigned to 1 of 4 groups (saline, AA and HC, AA only, or HC only). Treatments were administered 1 hour after completion of LPS infusion. Clinical signs, clinicopathological variables, pro‐inflammatory cytokine gene expression and production, and plasma AA concentrations were assessed at various time points. Serum cortisol concentrations and ACTH stimulation tests were used to detect CIRCI.

Results

There was no effect of drug on clinical signs or pro‐inflammatory cytokine gene expression or production compared to controls at any time point. Administration of AA was associated with higher blood neutrophil counts 6 hours after LPS infusion (11.01 ± 1.02 K/μl) compared to other groups (8.99 ± 0.94 K/μL; P < .009). Adminstration of HC was associated with higher blood neutrophil counts 12 hours after LPS infusion (10.40 ± 0.75 K/μl) compared to other groups (6.88 ± 0.68 K/μl; P < .001). Serum cortisol increased from 5.11 ± 1.48 μg/dL before LPS administration to 9.59 ± 1.83 μg/dL 1 h after completion of LPS infusion (T1) without an effect of treatment (P = 0.59).

Conclusions and Clinical Importance

Ascorbic acid and HC appeared to protect against LPS‐induced neutrophil depletion and could be considered as adjunctive therapy in horses with endotoxemia.

Cortisol and adrenocorticotropic hormone concentrations in horses with systemic inflammatory response syndrome

BACKGROUND

Plasma adrenocorticotropic hormone (ACTH) and serum cortisol concentrations increase with illness-associated stress. Dynamics of plasma ACTH and serum cortisol concentrations in adult horses with systemic illness are undocumented.

HYPOTHESIS/OBJECTIVE

To determine whether ACTH and cortisol concentrations and the ACTH/cortisol ratio vary with survival, the presence of systemic inflammatory response syndrome (SIRS), or ischemic gastrointestinal lesions at admission, or throughout hospitalization.

ANIMALS

One hundred fifty-one adult horses.

METHODS

Prospective study measuring serum cortisol and plasma ACTH at admission and on days 2, 4, and 6 of hospitalization. Horses were grouped by outcome (survival, SIRS status, number of SIRS criteria [SIRS score], SIRS severity group, and the presence of an ischemic lesion). Differences between groups and over time for ACTH, cortisol, and ACTH/cortisol ratio were investigated with a mixed effect model. Receiving operator characteristic curves and odds ratios were calculated for survival and ischemia.

RESULTS

In all groups, ACTH, cortisol, and ACTH/cortisol ratio significantly decreased over time (P < .0001). ACTH, cortisol, and ACTH/cortisol ratio were higher at admission in nonsurvivors, and ACTH and cortisol were higher in horses with ischemic lesions (P < .01). Horses with ACTH above reference interval at admission were 6.10 (2.73-13.68 [95% confidence interval]) times less likely to survive (P < .0001). No significant difference in ACTH, cortisol, and ACTH/cortisol ratio between horses with different SIRS status, scores, or groups were detected, although nonsurvivors had a higher SIRS score (P < .0001).

CONCLUSIONS AND CLINICAL IMPORTANCE

Pituitary and adrenal responses are altered in nonsurviving horses and those with an ischemic gastrointestinal lesion.

Diagnostic accuracy of post-ACTH challenge salivary cortisol concentrations for identifying horses with equine glandular gastric disease

The aims of this study were to better characterize the adrenal response to i.v. adrenocorticotropic hormone (ACTH) in horses with and without gastric disease and to validate and simplify the ACTH stimulation test by determining the diagnostic accuracy of six consecutive sampling time points after ACTH administration for equine glandular gastric disease (EGGD) and equine squamous gastric disease (ESGD). Twenty-six endurance and eventing horses without clinical disease [Sport Horse Population (SHP)] and an independent population of 62 horses [General Population (GP)] were grouped by gastroscopic findings (no/mild vs. moderate/severe EGGD, grade 0-1 vs. 2-4 ESGD, respectively) and underwent an ACTH stimulation test. Salivary cortisol (ng/mL) was analyzed before and 30, 60, 90, 120, and 150 min after i.v. injection of 1 µg/kg BW synthetic ACTH1-24. The association between having moderate or severe EGGD or ESGD and the amount of salivary cortisol was analyzed by means of receiver-operating characteristic (ROC) analysis. The following explanatory variables were considered: cortisol values for every time point, the area under the curve (AUC)-including all time points and corrected for the baseline-and the partial areas under the curve AUC0-90 and AUC90-150. Sampling after 60 min had highest association with moderate/severe EGGD. The diagnostic potential of the ACTH test was higher for the SHP [sensitivity 100% (95% CI 54% to 100%), specificity 75% (95% CI 51% to 91%), ROC-AUC 91% (95% CI 69% to 98%), 1-sided P-value < 0.001] than for the GP [sensitivity 75% (95% CI 48% to 93%), specificity 52% (95% CI 37% to 67%), ROC-AUC 68% (95% CI 51% to 79%), 1-sided P-value = 0.0064]. There were, however, no significant associations with ESGD. The superiority of sampling after 60 min suggests that the initial release of cortisol rather than its peak or the AUC are relevant regarding EGGD. Even though the wide confidence intervals and thus the lack of diagnostic accuracy do not presently support clinical use, characterization of the adrenal response to an ACTH stimulus improves the understanding of EGGD pathophysiology and its relation to stress.

Repeatability of the ACTH stimulation test as reflected by salivary cortisol response in healthy horses

The aim of this study was to further characterize the ACTH stimulation test as reflected by salivary cortisol response and to measure the short- and long-term repeatability of it in healthy horses as a tool to assess the capacity of the adrenal cortex to secrete cortisol. Nineteen healthy horses were subjected to 3 ACTH stimulation tests. Intervals were 2 wk and 5 mo between the first and second and the second and third tests, respectively. A dose of 1-μg/kg BW synthetic ACTH was injected intravenously. Saliva samples were collected at baseline and at 30, 60, 90, 120, 150, and 180 min after administration for cortisol measurements using a competitive enzyme immunoassay. A repeated measures ANOVA was used to compare values within and among horses. Mean ± SD total increase in cortisol concentrations integrated over the entire sampling period was 34.5 ± 11.0 ng/mL. The highest measured concentration at a single time point was 9.7 ± 2.7 ng/mL and was reached after 122 ± 22 min. For the short- and long-term repeatability, intraclass correlation coefficient was 0.90 and 0.33, respectively. The 3 ACTH stimulation tests results differed significantly among (P < 0.00001) but not within (P = 0.538) individual horses. The Freiberger stallions had a higher salivary cortisol baseline concentration and a lower response to ACTH stimulation as compared with Warmblood mares and geldings. The present study confirmed that the administration of ACTH in healthy horses reliably stimulates the salivary secretion of cortisol and shows that the test is repeatable in the short- and long-term.

A comparison between the equine and bovine hypothalamus-pituitary-adrenocortical axis

In this review, we address the function of the hypothalamus-pituitary-adrenocortical (HPA) axis with special emphasis on the comparison between the bovine and equine species. The pars intermedia of the pituitary gland is particularly well developed in horses and cattle. However, its function is not well appreciated in cattle yet. The Wulzen's cone of the adenohypophysis is a special feature of ruminants. Total basal cortisol concentration is much higher in horses than that in cows with similar free cortisol fractions. Corticotropin-releasing factor (CRF) concentrations in equine pituitary venous blood are lower compared with other species, whereas plasma ACTH concentrations in cows are higher than those in horses. A CRF challenge test induced a more pronounced cortisol response in horses compared with cattle, whereas regarding ACTH challenge testing, the opposite seems true. Based on data from literature, the bovine species is characterized by relatively high basal blood CRF and ACTH and low cortisol and glucose concentrations. Obviously, further lowering of blood cortisol in cattle is easily prevented by the high sensitivity to ACTH, and as a consequence, subsequent increased gluconeogenesis prevents imminent hypoglycemia. Hypoglycemia is less likely in horses given their high muscle glycogen content and their relatively high cortisol concentration. When assessing HPA axis reactivity, response patterns to exogenous ACTH or CRH might be used as a reliable indicator of animal welfare status in cows and horses, respectively, although it is emphasized that considerable caution should be exercised in using measures of HPA activity solely to assess animal welfare.

Serum cortisol concentrations in horses with colic

Few studies have evaluated cortisol concentrations in horses with colic. In humans with septic shock, high cortisol levels are associated with an increased risk of death. The objectives of this study were to compare the serum total cortisol concentrations (STCCs) in horses with colic to those without colic, and to assess whether the STCC relates to the pathological nature or outcome of the disease. STCCs were determined at presentation in horses with colic and in systemically healthy 'control' horses. Horses with colic were grouped based on clinical and clinico-pathological parameters at admission, treatment, lesion type and location, and outcome. Univariable and multivariable logistic regression were performed using two different outcome measures: (a) whether the horse had colic or not (yes vs. no), and (b) horse STCC (≥200 nmol/L vs. <200 nmol/L). Horses were more likely to have colic if they presented with high STCCs (≥200 nmol/L compared with <200 nmol/L). Horses with colic and with STCCs ≥200nmol/L were more likely to have moderate or severe colic signs (compared with mild colic) and heart rates >45 beats per min (compared with ≤45 beats per min). It was concluded that colic in horses is associated with elevated STCCs, and increased STCC in horses with colic appears to relate to the severity of the disease. STCCs may provide additional decision-making and prognostic information in horses with colic but further studies are required to avoid misinterpretations associated with the wide variation in STCCs.