Atypical myopathy: a review and description of the outbreak in the Netherlands during autumn 2009 and spring 2010.

This article reviews the literature on equine atypical myopathy (AM), an acute, severe rhabdomyolysis that occurs in horses at pasture. The prevalence, mortality, clinical signs, pathology, potential aetiology, typical aspects, diagnosis, treatment, and prognosis are described. Horse management, characteristic weather conditions, and possible preventive measures are also discussed. In addition, the characteristics of 54 highly probable or confirmed cases of equine AM occurring between autumn 2009 (27 cases) and spring 2010 (27 cases) in the Netherlands are described. Of the 54 affected horses, nineteen were mares, eleven geldings, and eight stallions; the sex of the other sixteen horses was not recorded. The mortality rate (74.5%) was in the same range as that reported in earlier studies. Many cases were reported at about the same time. Thirty-five horses had been pastured near maple trees, and in fifteen cases the maple trees were known to be infected with the fungus Rhytisma acerinum.

Decreased oxidative phosphorylation and PGAM deficiency in horses suffering from atypical myopathy associated with acquired MADD.

Earlier research on ten horses suffering from the frequently fatal disorder atypical myopathy showed that MADD (multiple acyl-CoA dehydrogenase deficiency) is the biochemical derangement behind atypical myopathy. From five horses that died as a result of this disease and seven healthy control horses, urine and plasma were collected ante mortem and muscle biopsies were obtained immediately post-mortem (2 patients and 7 control horses), to analyse creatine, purine and carbohydrate metabolism as well as oxidative phosphorylation. In patients, the mean creatine concentration in urine was increased 17-fold and the concentration of uric acid approximately 4-fold, compared to controls. The highest degree of depletion of glycogen was observed in the patient with the most severe myopathy clinically. In this patient, glycolysis was more active than in the other patients and controls, which may explain this depletion. One patient demonstrated very low phosphoglycerate mutase (PGAM) activity, less than 10% of reference values. Most respiratory chain complex activity in patients was 20-30% lower than in control horses, complex II activity was 42% lower than normal, and one patient had severely decrease ATP-synthase activity, more than 60% lower than in control horses. General markers for myopathic damage are creatine kinase (CK) and lactic acid in plasma, and creatine and uric acid in urine. To obtain more information about the cause of the myopathy analysis of carbohydrate, lipid and protein metabolism as well as oxidative phosphorylation is advised. This study expands the diagnostic possibilities of equine myopathies.

Equine acquired multiple acyl-CoA dehydrogenase deficiency (MADD) in 14 horses associated with ingestion of Maple leaves (Acer pseudoplatanus) covered with European tar spot (Rhytisma acerinum).

This case-series describes fourteen horses suspected of equine acquired multiple acyl-CoA dehydrogenase deficiency (MADD) also known as atypical myopathy of which seven cases were confirmed biochemically with all horses having had access to leaves of the Maple tree (Acer pseudoplatanus) covered with European tar spot (Rhytisma acerinum). Assessment of organic acids, glycine conjugates, and acylcarnitines in urine was regarded as gold standard in the biochemical diagnosis of equine acquired multiple acyl-CoA dehydrogenase deficiency.

Plasma steroid profiles before and after ACTH stimulation test in healthy horses.

This study describes steroid profiles in equine plasma before and after ACTH stimulation. In human medicine, other steroids have been shown to have a more pronounced reaction to an ACTH stimulation test than cortisol. This study aimed to determine if the same was true for the horse. A total of 11 clinically healthy horses were selected for this study. Ethylenediaminetetraacetic acid plasma samples were taken before and 60 min after stimulation with 1 µg/kg BW of synthetic ACTH administered intravenously. The samples were analyzed for cortisol, 11-deoxycortisol, 21-deoxycortisol, cortisone, corticosterone, 11-deoxycorticosterone, androstenedione, 17-OH-progesterone, progesterone, and testosterone with a liquid chromatography-tandem mass spectrometry (LC-MS/MS). Cortisol, 11-deoxycortisol, cortisone, corticosterone, and 11-deoxycorticosterone showed a significant increase after ACTH stimulation. In conclusion, the LC-MS/MS represents a viable method to measure glucocorticoids and related precursors or metabolites in equine plasma samples. In addition, we were able to show a more pronounced increase of 11-deoxycorticosterone, 11-deoxycortisol, and corticosterone compared with cortisol. These 3 metabolites could potentially serve as more sensitive biomarkers for stress in horses than cortisol.

Serum acylcarnitine profile in endurance horses with and without metabolic dysfunction.

Mitochondrial ß-oxidation is essential in fat metabolism and can be monitored with blood acylcarnitine profiling, as partly degraded fatty acids accumulate as their carnitine esters. To guarantee continuous energy supply during long-distance exercise, endurance horses oxidise considerable amounts of fat in the mitochondrion. In endurance races over 80 km, glycogen depletion is evident in equine slow-twitch high oxidative muscle fibres and as a consequence, horses participating in endurance races over 80 km rely almost entirely on ß-oxidation of fatty acids. This study investigated mitochondrial fatty acid ß-oxidation in endurance horses exposed to long-distance exercise. Electrospray tandem mass spectrometry analysis of serum acylcarnitine profiles from 10 Arab horses was performed before and after a 160 km endurance race. Results were analysed statistically using ANOVA. Mean speed over the entire race in finishing horses was 16.7 ± 1.2 km/h. Endurance exercise increased mitochondrial ß-oxidation approximately eight-fold (pre-race, 5648.62 ± 1508.52 nmol/L; post-race, 44,243.17 ± 11,504.45 nmol/L; P = 0.001). In these horses, there was an approximately 17-fold increased lipolysis, as demonstrated by elevated serum concentrations of non-esterified fatty acids (NEFA; pre-race, 0.08 ± 0.08 mmol/L; post-race, 1.32 ± 0.36 mmol/L; P < 0.001). In comparison, four Arab horses with poor performance showed an approximately five-fold increase in mitochondrial ß-oxidation (pre-race, 5286.17 ± 3355.16 nmol/L; post-race, 26,660.57 ± 10,064.27 nmol/L; P = 0.009); there was a 29-fold increase in NEFA (pre-race, 0.02 ± 0.01 mmol/L; post-race, 0.58 ± 0.07 mmol/L; P = 0.006) in these horses. Similar post-exercise free carnitine:acetylcarnitine ratios in both groups suggest that the availability of carnitine in long-distance endurance horses might limit performance.

Overtrained horses alter their resting pulsatile growth hormone secretion.

The influence of intensified and reduced training on nocturnal growth hormone (GH) secretion and elimination dynamics was studied in young (1.5 yr) Standardbred geldings to detect potential markers indicative for early overtraining. Ten horses trained on a treadmill for 32 wk in age-, breed-, and gender-matched fixed pairs. Training was divided into four phases (4, 18, 6, and 4 wk, respectively): 1) habituation to high-speed treadmill trotting, 2) normal training, in which speed and duration of training sessions were gradually increased, 3) in this phase, the horses were divided into 2 groups: control (C) and intensified trained (IT) group. In IT, training intensity, duration, and frequency were further increased, whereas in control these remained unaltered, and 4) reduced training (RT). At the end of phases 2, 3, and 4, blood was sampled overnight every 5 min for 8 h for assessment of GH secretory dynamics using pulse detection, deconvolution analysis, and approximate entropy (ApEn). Intensified training induced overtraining (performance decreased by 19% compared with C), which was associated with an increase in concentration peaks number (3.6 vs. 2.0, respectively), a smaller peak secretion pattern with a prolonged half-life (15.2 vs. 7.3 min, respectively), and an increased ApEn (0.89 vs. 0.49, respectively). RT did not lead to full recovery for the overtrained horses. The increased irregularity of nocturnal GH pulsatility pattern is indicative of a loss of coordinated control of GH regulation. Longer phases of somatostatin withdrawal are hypothesized to be the underlying mechanism for the observed changes in GH pulsatility pattern.

Normal function of the hypothalamic-pituitary growth axis in three dwarf Friesian foals.

Serial blood samples were collected from three dwarf Friesian foals to examine their endogenous growth hormone (GH) profiles, and the integrity of the GH-insulin-like growth factor-1 (IGF-1) axis was tested in one of them by examining its responses to the administration of GH-releasing hormone (GHRH) and to 10 days of treatment with recombinant equine GH. The basal serum concentrations of IGF-1 in the three dwarf foals were compared with those in nine age-matched normal foals. All the dwarf foals secreted endogenous GH. Stimulation with 7.0 microg/kg GHRH led to a 1400 per cent increase in plasma GH concentration in the dwarf foal tested, and 10 daily subcutaneous treatments with 20 microg/kg recombinant equine GH led to a 100 per cent increase in its serum IGF-1 concentration. The basal serum concentrations of IGF-1 in the dwarf foals were not significantly different from those of the normal foals.

Acinetobacter in veterinary medicine, with an emphasis on Acinetobacter baumannii.

Acinetobacter spp. are aerobic, rod-shaped, Gram-negative bacteria belonging to the Moraxellaceae family of the class Gammaproteobacteria and are considered ubiquitous organisms. Among them, Acinetobacter baumannii is the most clinically significant species with an extraordinary ability to accumulate antimicrobial resistance and to survive in the hospital environment. Recent reports indicate that A. baumannii has also evolved into a veterinary nosocomial pathogen. Although Acinetobacter spp. can be identified to species level using matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF/MS) coupled with an updated database, molecular techniques are still necessary for genotyping and determination of clonal lineages. It appears that the majority of infections due to A. baumannii in veterinary medicine are nosocomial. Such isolates have been associated with several types of infection such as canine pyoderma, feline necrotizing fasciitis, urinary tract infection, equine thrombophlebitis and lower respiratory tract infection, foal sepsis, pneumonia in mink, and cutaneous lesions in hybrid falcons. Given the potential multidrug resistance of A. baumannii, treatment of diseased animals is often supportive and should preferably be based on in vitro antimicrobial susceptibility testing results. It should be noted that animal isolates show high genetic diversity and are in general distinct in their sequence types and resistance patterns from those found in humans. However, it cannot be excluded that animals may occasionally play a role as a reservoir of A. baumannii. Thus, it is of importance to implement infection control measures in veterinary hospitals to avoid nosocomial outbreaks with multidrug-resistant A. baumannii.

Acquired multiple Acyl-CoA dehydrogenase deficiency in 10 horses with atypical myopathy.

The aim of the current study was to assess lipid metabolism in horses with atypical myopathy. Urine samples from 10 cases were subjected to analysis of organic acids, glycine conjugates, and acylcarnitines revealing increased mean excretion of lactic acid, ethylmalonic acid, 2-methylsuccinic acid, butyrylglycine, (iso)valerylglycine, hexanoylglycine, free carnitine, C2-, C3-, C4-, C5-, C6-, C8-, C8:1-, C10:1-, and C10:2-carnitine as compared with 15 control horses (12 healthy and three with acute myopathy due to other causes). Analysis of plasma revealed similar results for these predominantly short-chain acylcarnitines. Furthermore, measurement of dehydrogenase activities in lateral vastus muscle from one horse with atypical myopathy indeed showed deficiencies of short-chain acyl-CoA dehydrogenase (0.66 as compared with 2.27 and 2.48 in two controls), medium-chain acyl-CoA dehydrogenase (0.36 as compared with 4.31 and 4.82 in two controls) and isovaleryl-CoA dehydrogenase (0.74 as compared with 1.43 and 1.61 nmol min(-1) mg(-1) in two controls). A deficiency of several mitochondrial dehydrogenases that utilize flavin adenine dinucleotide as cofactor including the acyl-CoA dehydrogenases of fatty acid beta-oxidation, and enzymes that degrade the CoA-esters of glutaric acid, isovaleric acid, 2-methylbutyric acid, isobutyric acid, and sarcosine was suspected in 10 out of 10 cases as the possible etiology for a highly fatal and prevalent toxic equine muscle disease similar to the combined metabolic derangements seen in human multiple acyl-CoA dehydrogenase deficiency also known as glutaric acidemia type II.

Repeated high dose imidocarb dipropionate treatment did not eliminate Babesia caballi from naturally infected horses as determined by PCR-reverse line blot hybridization.

Imidocarb treatment of horses infected with Babesia caballi is supposed to eliminate the infection, but data on the efficacy of this treatment is scarce. The study presented here concerns four Paso Fino horses, which were imported into the island of Curacao on the basis of a piroplasmosis negative complement fixation test (CFT). Upon re-testing with an indirect fluorescent antibody test immediately after arrival in Curacao, two horses appeared to have antibodies to B. caballi and all horses had antibodies to Theileria equi. Subsequent testing with polymerase chain reaction combined with a reverse line blot yielded positive results for both agents in all four horses. Treatment with five consecutive doses of imidocarb dipropionate (4.7 mg/kg BW im q 72 h), temporarily resulted in negative results, but B. caballi and T. equi were detected again in the samples taken at 6 and 18 weeks after completion of the treatment. These results confirm that the CFT is not a suitable test for pre-import testing and that even high dose treatment with imidocarb may not be capable of eliminating B. caballi and T. equi infections from healthy carriers.

Phenotypic diagnosis of dwarfism in six Friesian horses.

An extreme form of abnormal development, dwarfism, is common in man and some animals, but has not been officially reported in horses. Within the Friesian horse breed, congenital dwarfism has been recognised for many years, but no detailed report exists on its phenotype. The most salient feature of the dwarf syndrome is the physeal growth retardation in both limbs and ribs. Affected animals have approximately 25% shorter fore- and hindlimbs and approximately 50% reduced bodyweight. Postnatal growth is still possible in these animals, albeit at a slower rate: the head and back grow faster than the limbs and ribs leading to the characteristic disproportional growth disturbance. Thus, adult dwarfs exhibit a normal, but a relatively larger head conformation, a broader chest with narrowing at the costochondral junction, a disproportionally long back, abnormally short limbs, hyperextension of the fetlocks and narrow long-toed hooves. Furthermore, a dysplastic metaphysis of the distal metacarpus and metatarsus is radiographically evident. Microscopic analysis of the growth plates at the costochondral junction shows an irregular transition from cartilage to bone, and thickening and disturbed formation of chondrocyte columns, which is similar to findings in osteochondrodysplasia.

Effects of exogenous insulin on luteolysis and reproductive cyclicity in the mare.

Insulin is a pancreatic hormone that classically regulates carbohydrate and fat metabolism, but also appears to play a role in various reproductive processes. A preliminary study suggested insulin production by day 10 to 18 equine conceptuses. The aim of the present study was to examine the hypothesis that insulin is the conceptus signal responsible for maternal recognition of pregnancy (MRP) in the mare, or otherwise influences reproductive cyclicity during the MRP period. Six Warmblood mares were treated daily during days 7 to 17 after ovulation of two successive oestrous cycles with either (short and intermediate acting) insulin or control saline. Mares were assigned randomly to treatment, and crossed over during the subsequent cycle. Time of ovulation and corpus luteum surface area were determined by serial transrectal ultrasonographic examination of the mares' ovaries, and daily jugular vein blood samples were analysed for progesterone and luteinizing hormone (LH) concentrations. On day 14 of dioestrus, the luteolytic drive was examined by measuring systemic 15-ketodihydroprostaglandin F(2 alpha) (PG-metabolite) release in response to oxytocin challenge. In addition, yolk sac fluid recovered from 32 day 10 to 14 equine conceptuses was analysed for insulin concentrations. Insulin administration did not affect luteal size, dioestrus length, the interovulatory interval, or circulating LH concentrations. Insulin administration also failed to suppress oxytocin-induced PGF(2 alpha) release, and tended to depress systemic progesterone concentrations. Finally, insulin could not be detected in the yolk sac fluid of day 10 to 14 equine conceptuses by radio-immunoassay. It is concluded that insulin administered daily during days 7 to 17 of dioestrus has little or no effect on reproductive cyclicity in the mare, and is unlikely to be the MRP signal.

Subclinical hypocalcaemia in captive Asian elephants (Elephas maximus).

The hypothesis that hypocalcaemia may play a role in dystocia in captive Asian elephants (Elephas maximus) was investigated. The objectives of the study were to measure the total calcium concentration in elephant plasma; assess the changes in parameters of calcium metabolism during a feeding trial; investigate a possible relationship between calcium metabolism and dystocia; and assess bone mineralisation in captive Asian elephants in vivo. The following parameters were measured: total and ionised calcium, inorganic phosphorous and magnesium, the fractional excretions of these minerals, intact parathyroid hormone, 25-OH-D(3) and 1,25-OH-D(3). Radiographs were taken from tail vertebrae for assessment of bone mineralisation. The mean (sd) heparinised plasma total calcium concentration was 2.7 (0.33) mmol/l (n=43) ranging from 0.84 to 3.08 mmol/l in 11 Asian elephants. There was no significant correlation between plasma total calcium concentration and age. Following feeding of a calcium rich ration to four captive Asian elephant cows, plasma total and ionised calcium peaked at 3.6 (0.24) mmol/l (range 3.4 to 3.9 mmol/l) and 1.25 (0.07) mmol/l (range 1.17 to 1.32 mmol/l), respectively. Plasma ionised calcium concentrations around parturition in four Asian elephant cows ranged from 0.37 to 1.1 mmol/l only. The present study indicates that captive Asian elephants might be hypocalcaemic, and that, in captive Asian elephants, the normal plasma concentration of total calcium should actually be around 3.6 mmol/l and normal plasma concentration of ionised calcium around 1.25 mmol/l. Given the fact that elephants absorb dietary calcium mainly from the intestine, it could be concluded that elephants should be fed calcium-rich diets at all times, and particularly around parturition. In addition, normal values for ionised calcium in captive Asian elephants should be reassessed.

Hormonal responses to acute exercise, training and overtraining. A review with emphasis on the horse.

Overtraining is an imbalance between training and recovery leading to symptoms associated with a neuroendocrine dysbalance called the overtraining syndrome, a disease characterized by behavioral, emotional and physical symptoms similar with depression. Although the prevalence of overtraining is high in human and equine athletes, at present no sensitive and specific test is available to prevent or diagnose overtraining. Nowadays, it is believed that combination of different (hormonal) parameters appear to be the best indicators of overtraining. Therefore, this review provides a summary of previous literature examining the response of the hypothalamic-pituitary-adrenal (HPA) axis and the growth hormone-insulin-like growth factor-I (GH-IGF-I) axis to acute and chronic exercise as well as overtraining in humans and horses. The exercise induced hormonal responses seem to be equal for the equine as well as the human athlete, which makes comparisons possible. Repeated bouts of exercise are suggested to provide a way to detect subtle changes in hormonal responses in the individual athlete, which may make them an important tool in detecting early overtraining. This should be combined with corticotropin releasing hormone (CRH) stimulation tests and basal ACTH and GH pulsatility determination. Further research is needed to establish the correct training intensity and rest period for the exercise test in equines.

Equine metabolic myopathies with emphasis on the diagnostic approach. Comparison with human myopathies. A review.

This review gives an overview of the presently known human and equine metabolic myopathies with emphasis on the diagnostic approach. Metabolic myopathies are muscle disorders caused by a biochemical defect of the skeletal muscle energy system, which results in inefficient muscle performance. Myopathies can arise in different levels of the metabolic system. In this review the metabolic myopathies are categorized in disorders of the carbohydrate metabolism, lipid metabolism, mitochondrial myopathies (other than those described in lipid metabolism), disorders of purine metabolism, primary disorders involving ion channels and electrolyte flux and secondary or acquired metabolic myopathies.

Disturbed bovine mitochondrial lipid metabolism: a review.

In mammals, excess energy is stored primarily as triglycerides, which are mobilized when energy demands arise and cannot be covered by feed intake. This review mainly focuses on the role of long chain fatty acids in disturbed energy metabolism of the bovine species. Long chain fatty acids regulate energy metabolism as ligands of peroxisome proliferator-activated receptors. Carnitine acts as a carrier of fatty acyl groups as long-chain acyl-CoA derivatives do not penetrate the mitochondrial inner membrane. There are two different types of disorders in lipid metabolism which can occur in cattle, namely the hypoglycaemic-hypoinsulinaemic and the hyperglycaemic-hyperinsulinaemic type with the latter not always associated with ketosis. There is general agreement that fatty acid ß-oxidation capability is limited in the liver of (ketotic) cows. In accord, supplemental L-carnitine decreased liver lipid accumulation in periparturient Holstein cows. Of note, around parturition concurrent oxidation of fatty acids in skeletal muscle is highly activated. Also peroxisomal ß-oxidation in liver of dairy cows may be part of the hepatic adaptations to a negative energy balance (NEB) to break down fatty acids. An elevated blood concentration of nonesterified fatty acids is one of the indicators of NEB in cattle among others like increased ß-hydroxy butyrate concentration, and decreased concentrations of glucose, insulin, and insulin-like growth factor-I. Assuming that liver carnitine concentrations might limit hepatic fatty acid oxidation capacity in dairy cows, further study of the role of acyl-CoA dehydrogenases and/or riboflavin in bovine ketosis is warranted.

Increased adrenocortical response to adrenocorticotropic hormone (ACTH) in sport horses with equine glandular gastric disease (EGGD).

This study tested the hypothesis that adrenocortical function would be altered in horses with equine gastric ulcer syndrome (EGUS). Twenty-six sport horses competing at national or international levels in eventing (n=15) or endurance (n=11) were subjected to a gastroscopic examination and an adrenocorticotropic hormone (ACTH) stimulation test. Salivary cortisol concentrations were measured before (baseline) and after (30, 60, 90, 120 and 150min) IV ACTH injection (1µg/kg bodyweight). Within EGUS, two distinct diseases, equine squamous gastric disease (ESGD) and equine glandular gastric disease (EGGD), can be distinguished. ESGD was diagnosed in 8/11 (73%; 95% confidence intervals [95%CI], 43-92%) endurance horses and 5/15 (33%; 95% CI, 14-58%) eventing horses. EGGD was observed in 9/11 (82%; 95% CI, 53-96%) endurance horses and 9/15 (60%; 95% CI, 35-81%) eventing horses. The presence or severity of ESGD was unrelated to the presence or severity of EGGD. ACTH stimulation induced a larger increase in cortisol concentration in horses with moderate EGGD than in horses with mild EGGD. Cortisol concentration during the entire sampling period (total increase in cortisol concentration during the entire sampling period [dAUC], 31.1±6.4ng/mL) and the highest measured concentration at a single time point (maximal increase in cortisol concentration [dMAX], 10.3±2.3ng/mL) were increased (P=0.005 and P=0.038, respectively), indicating that horses with glandular gastric disease exhibited increased adrenocortical responses to ACTH stimulation. These results suggest that EGGD might be associated with an enhanced adrenocortical sensitivity. Further investigations are warranted to confirm the association between adrenocortical sensitivity and EGGD and to elucidate the pathophysiological mechanisms involved.

The role of quantitative electromyography (EMG) in horses suspected of acute and chronic grass sickness.

REASONS FOR PERFORMING THE STUDY: Clinical evidence of motor neuron involvement in equine grass sickness (EGS) has not been reported. HYPOTHESIS: Quantitative electromyography (EMG) analysis can elucidate subtle changes of the lower motor neuron system present in horses with EGS, performed ante mortem. METHODS: Fourteen horses diagnosed clinically with acute, subacute or chronic EGS were examined and quantitative EMG performed. Previously published data on healthy horses and horses with proven lower motor neuron disease (LMND) were used as controls. In 8 horses post mortem examination was performed, and in 7 muscle biopsies of the lateral vastus muscle underwent histopathology and morphometry. RESULTS: Clinical electrophysiological evidence of neuropathy was present in 12 horses. Analysis of data from the first 4 horses resulted in 95% confidence intervals (CI) of nontransformed data for motor unit action potential (MUP) duration in subclavian, triceps and lateral vastus muscle of 11.0-13.7, 14.8-20.3 and 12.2-17.2 msecs, respectively, and for MUP amplitude 291-453, 1026-1892 and 957-1736 microV, respectively. For number of phases the 95% CI was 3.6-4.4, 2.9-3.6 and 2.9-3.4, respectively, and for number of turns 5.0-6.5, 4.3-5.3 and 3.7-4.6, respectively. No changes in duration of insertional activity were measured. Pathological spontaneous activity was observed in all horses. EGS as evidenced by degenerative changes in the autonomic ganglia in combination with minor degenerative changes of the spinal lower motor neurons was observed on post mortem examination in all 8 available autopsies. In muscle biopsies of 4 out of 7 horses changes consistent with slight neurogenic atrophy were found. CONCLUSIONS AND POTENTIAL RELEVANCE: EMG results demonstrated the presence of a neuropathy of skeletal muscles in all horses suspected to have EGS. The combination of clinical and electrophysiological evidence may aid differential diagnosis of neurogenic disease in cases of weight loss and colic.

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.

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.