Clinically and temporally specific diagnostic thresholds for plasma ACTH in the horse

Factors affecting measurement of basal adrenocorticotropic hormone in adult domestic equids: A scoping review

Measurement of basal adrenocorticotropic hormone (ACTH) concentration is the most commonly used diagnostic test for pituitary pars intermedia dysfunction (PPID). Although several pre-analytical and analytical factors have been reported to affect basal ACTH concentrations in equids, the extent to which these have been evaluated in the context of PPID diagnosis is unclear. The objectives of this scoping review were to identify and systematically chart current evidence about pre-analytical and analytical factors affecting basal ACTH concentrations in adult domestic equids. Systematic searches of electronic databases and conference proceedings were undertaken in June 2022, repeated in October 2022 and updated in August 2023. English language publications published prior to these dates were included. Screening and data extraction were undertaken individually by the authors, using predefined criteria and a modified scoping review data extraction template. After removal of duplicates, 903 publications were identified, of which 235 abstracts were screened for eligibility and 134 publications met inclusion criteria. Time of year, exercise, breed/type and transportation were the factors most frequently associated with significant increases in ACTH concentration (n = 26, 16, 13 and 10 publications, respectively). Only 25 publications reported inclusion of PPID cases in the study population, therefore the relationship between many factors affecting basal ACTH concentration and diagnostic accuracy for PPID remains undefined. However, several factors were identified that could impact interpretation of basal ACTH results. Findings also highlight the need for detailed reporting of pre-analytical and analytical conditions in future research to facilitate translation of evidence to practice.

BEVA primary care clinical guidelines: Diagnosis and management of equine pituitary pars intermedia dysfunction

BACKGROUND

Pituitary pars intermedia dysfunction (PPID) is a prevalent, age-related chronic disorder in equids. Diagnosis of PPID can be challenging because of its broad spectrum of clinical presentations and disparate published diagnostic criteria, and there are limited available treatment options.

OBJECTIVES

To develop evidence-based primary care guidelines for the diagnosis and treatment of equine PPID based on the available literature.

STUDY DESIGN

Evidence-based clinical guideline using the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) framework.

METHODS

Research questions were proposed by a panel of veterinarians and developed into PICO or another structured format. VetSRev and Veterinary Evidence were searched for evidence summaries, and systematic searches of the NCBI PubMed and CAB Direct databases were conducted using keyword searches in July 2022 and updated in January 2023. The evidence was evaluated using the GRADE framework.

RESULTS AND RECOMMENDATIONS

The research questions were categorised into four areas: (A) Case selection for diagnostic testing, pre-test probability and diagnostic test accuracy, (B) interpretation of test results, (C) pharmacological treatments and other treatment/management options and (D) monitoring treated cases. Relevant veterinary publications were identified and assessed using the GRADE criteria. The results were developed into recommendations: (A) Case selection for diagnostic testing and diagnostic test accuracy: (i) The prevalence of PPID in equids aged ≥15 years is between 21% and 27%; (ii) hypertrichosis or delayed/incomplete hair coat shedding provides a high index of clinical suspicion for PPID; (iii) the combination of clinical signs and age informs the index of clinical suspicion prior to diagnostic testing; (iv) estimated pre-test probability of PPID should be considered in interpretation of diagnostic test results; (v) pre-test probability of PPID is low in equids aged <10 years; (vi) both pre-test probability of disease and season of testing have strong influence on the ability to diagnose PPID using basal adrenocorticotropic hormone (ACTH) or ACTH after thyrotropin-releasing hormone (TRH) stimulation. The overall diagnostic accuracy of basal ACTH concentrations for diagnosing PPID ranged between 88% and 92% in the autumn and 70% and 86% in the non-autumn, depending on the pre-test probability. Based on a single study, the overall diagnostic accuracy of ACTH concentrations in response to TRH after 30 minutes for diagnosing PPID ranged between 92% and 98% in the autumn and 90% and 94% in the non-autumn, depending on the pre-test probability. Thus, it should be remembered that the risk of a false positive result increases in situations where there is a low pre-test probability, which could mean that treatment is initiated for PPID without checking for a more likely alternative diagnosis. This could compromise horse welfare due to the commencement of lifelong therapy and/or failing to identify and treat an alternative potentially life-threatening condition. (B) Interpretation of diagnostic tests: (i) There is a significant effect of breed on plasma ACTH concentration, particularly in the autumn with markedly higher ACTH concentrations in some but not all 'thrifty' breeds; (ii) basal and/or post-TRH ACTH concentrations may also be affected by latitude/location, diet/feeding, coat colour, critical illness and trailer transport; (iii) mild pain is unlikely to have a large effect on basal ACTH, but caution may be required for more severe pain; (iv) determining diagnostic thresholds that allow for all possible contributory factors is not practical; therefore, the use of equivocal ranges is supported; (v) dynamic insulin testing and TRH stimulation testing may be combined, but TRH stimulation testing should not immediately follow an oral sugar test; (vi) equids with PPID and hyperinsulinaemia appear to be at higher risk of laminitis, but ACTH is not an independent predictor of laminitis risk. (C) Pharmacologic treatments and other treatment/management options: (i) Pergolide improves most clinical signs associated with PPID in the majority of affected animals; (ii) Pergolide treatment lowers basal ACTH concentrations and improves the ACTH response to TRH in many animals, but measures of insulin dysregulation (ID) are not altered in most cases; (iii) chasteberry has no effect on ACTH concentrations and there is no benefit to adding chasteberry to pergolide therapy; (iv) combination of cyproheptadine with pergolide is not superior to pergolide alone; (v) there is no evidence that pergolide has adverse cardiac effects in horses; (vi) Pergolide does not affect insulin sensitivity. (D) Monitoring pergolide-treated cases: (i) Hormone assays provide a crude indication of pituitary control in response to pergolide therapy, however it is unknown whether monitoring of ACTH concentrations and titrating of pergolide doses accordingly is associated with improved endocrinological or clinical outcome; (ii) it is unknown whether monitoring the ACTH response to TRH or clinical signs is associated with an improved outcome; (iii) there is very weak evidence to suggest that increasing pergolide dose in autumn months may be beneficial; (iv) there is little advantage in waiting for more than a month to perform follow-up endocrine testing following initiation of pergolide therapy; there may be merit in performing repeat tests sooner; (v) timing of sampling in relation to pergolide dosing does not confound measurement of ACTH concentration; (vi) there is no evidence that making changes after interpretation of ACTH concentrations measured at certain times of the year is associated with improved outcomes; (vii) evidence is very limited, however, compliance with PPID treatment appears to be poor and it is unclear whether this influences clinical outcome; (viii) evidence is very limited, but horses with clinical signs of PPID are likely to shed more nematode eggs than horses without clinical signs of PPID; it is unclear whether this results in an increased risk of parasitic disease or whether there is a need for more frequent assessment of faecal worm egg counts.

MAIN LIMITATIONS

Limited relevant publications in the veterinary scientific literature.

CONCLUSIONS

These findings should be used to inform decision-making in equine primary care practice.

Clinical implications of imprecise sampling time for 10- and 30-min thyrotropin-releasing hormone stimulation tests in horses

BACKGROUND

The thyrotropin-releasing hormone (TRH) stimulation test is used to diagnose pituitary pars intermedia dysfunction (PPID) using 10- or 30-min protocols. Imprecise sampling time for the 10-min protocol can lead to misdiagnoses.

OBJECTIVES

To determine the effect of imprecise sampling time for the 30-min protocol of the TRH stimulation test.

STUDY DESIGN

In vivo experiment.

METHODS

Plasma immunoreactive adrenocorticotropin (ACTH) concentrations were measured 9, 10, 11, 29, 30 and 31 min after intravenous administration of 1 mg of TRH in 15 control and 12 PPID horses. Differences in ACTH concentrations between sampling times, variability in ACTH concentrations between protocols, and diagnostic classification of PPID were assessed using Friedman's test, Bland-Altman plots, and Fisher's exact test, respectively, with 95% confidence intervals reported and significance set at p < 0.05.

RESULTS

Imprecise sampling time resulted in variable ACTH concentrations, but significant differences in absolute ACTH concentrations were not detected for imprecise sampling within each protocol or between protocols. Imprecise sampling changed PPID diagnostic classification for 3/27 (11 [4-28] %) horses for both protocols. Using the 30-min protocol as a reference, 1/12 (8 [1-35] %) horses returned a negative test result and 5/12 (42 [19-68] %) horses returned equivocal test results that would be considered positive in practice due to the presence of supportive clinical signs.

MAIN LIMITATIONS

Limited sample size and inter-horse variability reduced the ability to detect small but potentially relevant differences.

CONCLUSIONS

Overall, the impact of imprecise sampling was not significantly different between the 10- and 30-min TRH stimulation test protocols. However, diagnostic classification for PPID would have varied between the 10- and 30-min protocols in this population, if clinical signs had been ignored. Precise timing during TRH stimulation tests and contextual interpretation of ACTH concentrations remain fundamental for the diagnosis of PPID.

Influence of feeding and other factors on adrenocorticotropin concentration and thyrotropin-releasing hormone stimulation test in horses and ponies

BACKGROUND

The basal (bACTH) and post-thyrotropin-releasing hormone stimulation concentration of adrenocorticotropin (pACTH) are recommended for diagnosis of pituitary pars intermedia dysfunction (PPID). Many factors influence bACTH (e.g., disease, age, month) and some affect the results only in autumn (e.g., breed, colour, sex). There are discrepancies about the impact of feeding on b/pACTH.

OBJECTIVES

To determine whether feeding, month, age, breed, colour, sex and body condition score affect b/pACTH.

STUDY DESIGN

Prospective crossover.

METHODS

Sixty-one animals were divided into groups: healthy, PPID, treated-PPID. The b/pACTH was measured three times (1 mg protirelin; blood collection after 10 min; mid-November to mid-July) after different feedings: fasting, hay, hay + grain. Friedman's test was applied to evaluate the influence of feeding on b/pACTH and linear mixed model to evaluate impact of further factors.

RESULTS

The b/pACTH was not significantly affected by feeding (p = 0.7/0.5). The bACTH was lowest in healthy (29.3 pg/mL, CI 9-49.5 pg/mL) and highest in PPID-group (58.9 pg/mL, CI 39.7-78.1 pg/mL). The pACTH was significantly lower in healthy (396.7 pg/mL, CI 283.2-510.1 pg/mL) compared to PPID (588.4 pg/mL, CI 480.7-696.2 pg/mL) and treated-PPID group (683.1 pg/mL, CI 585.9-780.4 pg/mL), highest in July (881.2 pg/mL, CI 626.3-1136.3 pg/mL) and higher in grey (723.5 pg/mL, CI 577.5-869.4 pg/mL) than other colours (338.7 pg/mL, CI 324.8-452.5 pg/mL). The size of effect for those variables was >0.5.

MAIN LIMITATIONS

Small number of animals, subsequent bACTH measurements were significantly lower in each horse.

CONCLUSIONS

There was no evidence that feeding influences the b/pACTH. There was evidence that pergolide affects the bACTH but it had little effect on pACTH. Further investigation of the impact of month and coat colour on b/pACTH is warranted to better interpret the results.

Pituitary pars intermedia dysfunction with pituitary gland melanotroph adenoma in a Grevy's zebra (Equus grevyi)

A 22-year and 9-month-old female Grevy's zebra (Equus grevyi) showed signs of polyuria, polydipsia, glucosuria, and muscle atrophy. Blood tests revealed hyperglycemia, hypertriglyceridemia, electrolyte imbalance, high levels of adrenocorticotropic hormone (ACTH) and cortisol, and low levels of hormones secreted by the pituitary pars distalis. Pathological examinations revealed a pituitary gland tumor and bilateral adrenal cortical hyperplasia. Pituitary tumor cells showed immunoreactivity for α-melanocyte-stimulating hormone and ACTH. The deposition of amyloid β was observed in the parenchyma and vascular walls of the cerebrum. The zebra showed clinical signs of pituitary pars intermedia dysfunction and was histopathologically diagnosed with pituitary gland melanotroph adenoma. This case report provides insight into neoplastic and endocrine diseases associated with the aging of a zebra.

Evaluation of seasonal influences on adrenocorticotropic hormone response to the thyrotropin-releasing hormone stimulation test and its accuracy for diagnosis of pituitary pars intermedia dysfunction

Pituitary pars intermedia dysfunction (PPID) is an age-related neurodegenerative disorder, affecting >20 % of older horses. There is a need for improved endocrine tests for early disease detection, and the thyrotropin-releasing hormone (TRH) stimulation test has been recommended for diagnosis of early or mild cases. However, it is currently not recommended for year-round use due to marked seasonal variability. The aims of this cohort study were to evaluate effects of month and season on adrenocorticotropic hormone (ACTH) responses to TRH stimulation and to derive monthly cut-offs for PPID diagnosis. Sixty-three horses were assigned to control (n = 17), subclinical PPID (n = 21) and clinical PPID (n = 25) groups, based on a composite reference standard that combined clinical history and examination findings with endocrine test results. TRH stimulation tests were performed monthly for a 12-month period. Circannual changes were evaluated with one- and two-way repeated-measures analysis of variance and receiver operating characteristic curve analysis was used to derive cut-off values for basal and TRH-stimulated ACTH. TRH-stimulated ACTH concentrations were lowest in February-May and highest in August-October. Specificity of both basal and 30 min post-TRH ACTH was generally higher than sensitivity, and TRH stimulation had improved diagnostic accuracy compared to basal ACTH, although its sensitivity was not significantly greater year-round. TRH stimulation tests yielded considerably more positive results than basal ACTH in the subclinical group, but few additional positive results in clinical PPID cases. There were large differences between cut-offs that maximised sensitivity or specificity for TRH-stimulated ACTH, highlighting the importance of considering clinical presentation alongside test results in diagnostic decision-making.

Diagnosis of equine pituitary pars intermedia dysfunction

Equine pituitary pars intermedia dysfunction (PPID) is common in aged horses. The majority of horses respond well to treatment, but treatment is lifelong, meaning accurate diagnosis of PPID is important. Similar to any condition, there is no perfect laboratory test to diagnose PPID and accuracy is affected by the characteristics of the population in which the test is being evaluated. This review details the importance of consideration of clinical factors and diagnostic test accuracy. Basal adrenocorticotrophic hormone (ACTH) concentration is used most frequently in practice and has very good diagnostic accuracy when used in combination with clinical judgement and the correct application of diagnostic thresholds. The thyrotropin-releasing hormone stimulation test can be used in horses with equivocal test results following basal ACTH testing, or to evaluate subtle cases due to its improved accuracy.

Deciphering reference intervals and clinical decision limits in equine endocrine diagnostic testing

Reference intervals (RIs) and clinical decision limits (CDLs) are frequently established to facilitate interpretation of values of endocrine biomarkers in the diagnosis of disease. Despite their commonplace use in clinical decision-making, these concepts can be confused. Comparing a test result with a RI provides an estimation as to whether or not the individual is healthy, whereas comparison with a CDL facilitates identification of individuals with a particular disease state or at greater risk of adverse clinical outcomes. In practice, there will also be a range of results for which the discriminative ability of the test is insufficient to inform a specific diagnostic decision. Including a range of uncertain test results, or 'grey zone', between positive and negative avoids the constraint of a binary decision in classifying an individual with a test value above (or below) a single cut-off value as diseased. This review will detail the application of both RIs and CDLs, including defining the range of uncertain test results, in the context of equine endocrinology.

Investigation of breed differences in plasma adrenocorticotropic hormone concentrations among healthy horses and ponies

Plasma adrenocorticotropic hormone (ACTH) concentration is commonly measured to diagnose pituitary pars intermedia dysfunction (PPID). Several intrinsic and extrinsic factors affect ACTH concentrations, including breed. The objective of this study was to prospectively compare plasma ACTH concentrations among different breeds of mature horses and ponies. Three breed groups comprised Thoroughbred horses (n = 127), Shetland ponies (n = 131) and ponies of non-Shetland breeds (n = 141). Enrolled animals did not show any signs of illness, lameness or clinical signs consistent with PPID. Blood samples were collected 6 months apart, around the autumn equinox and spring equinox, and plasma concentrations of ACTH were measured by chemiluminescent immunoassay. Pairwise breed comparisons within each season were performed on log transformed data using the Tukey test. Estimated mean differences in ACTH concentrations were expressed as fold difference with 95 % confidence intervals (CI). Reference intervals for each breed group per season were calculated using non-parametric methods. In autumn, higher ACTH concentrations were found among non-Shetland pony breeds compared with Thoroughbreds (1.55 fold higher; 95 % CI, 1.35-1.77; P < 0.001), and in Shetland ponies compared with Thoroughbreds (2.67 fold higher; 95 % CI, 2.33-3.08; P < 0.001) and non-Shetland pony breeds (1.73 fold higher; 95 % CI, 1.51-1.98; P < 0.001). In spring, no differences were identified among breed groups (all P > 0.05). Reference intervals were similar among breed groups in spring, but upper limits for ACTH concentrations were markedly different between Thoroughbred horses and pony breeds in autumn. These findings emphasise that breed should be accounted for when determining and interpreting reference intervals for ACTH concentrations among healthy horses and ponies in autumn.

Equine Endocrine Disease: Challenges With Case Definition for Research

Equine endocrine disease is an important area for equine research, requiring an appropriate case definition for inclusion and criteria for exclusion from disease. Defining a case for research may be different from criteria for clinical diagnosis. Further, clinical diagnosis recommendations have been changing regularly, making this area challenging for equine scientists. This review discusses the diagnosis of major equine endocrine diseases, pituitary pars intermedia dysfunction, equine metabolic syndrome and insulin dysregulation, focusing on the most appropriate diagnostic methods for research case definitions. Different diagnostic methods, including use of reference intervals and clinical decision limits, will be discussed with their relative merits for use in case definition for research.

Evaluation of stress markers in horses during hippotherapy sessions in comparison to being ridden by beginners

Hippotherapy has been used for decades and its benefits to human patients have largely been proven, whether being applied to those with physical or mental disabilities. There have been a plethora of animal welfare studies recently, pertaining especially to ridden horses. This study aimed to investigate stress markers in horses during hippotherapy sessions to address the ethical considerations raised by using horses for therapy. A ridden stress ethogram was established and validated specifically for this study via subjective observation and video recording of a ridden session involving intermediate-level riders. The experiment entailed eight healthy horses undergoing two ridden sessions on separate days, one with disabled riders and one with beginners. Several parameters associated with physiological responses to stress were evaluated at rest, such as heart rate, plasma adrenocorticotropic hormone [ACTH], serum and salivary cortisol. These parameters as well as the behavioural stress score from the ethogram scale were measured during both sessions. No significant differences were found between heart rate, plasma ACTH, and stress scores. Serum and salivary cortisol were significantly lower during the hippotherapy session than during the session with beginners. The current study found no evidence of compromised welfare when horses were used as a therapeutic aid during hippotherapy sessions compared to their usual ridden activity. Although these results indicate that hippotherapy may be ethically justified as it benefits humans without causing harm to the horses, the present study was small, and the results should be interpreted with caution.

A one-health lens offers new perspectives on the importance of endocrine disorders in the equine athlete

Endocrine disorders are associated with joint pain and tendon injury in humans, but the effects in the horse are only starting to be understood. Similar patterns of clinical signs and injury appear to affect horses and humans for both orthopedic and endocrine disorders, supporting the use of a one-health approach to tackle these issues. In this Currents in One Health, we will discuss common equine endocrinopathies, current testing recommendations, dietary management, genetic predispositions, and endocrine disorders' effects on performance. Our aim is to use a one-health lens to describe current comparative research so that veterinarians can employ cutting-edge preventative, diagnostic, and therapeutic recommendations. Identified key gaps in knowledge include whether equine metabolic osteoarthritis exists, if steroid joint injections are safe in horses with endocrine disorders, and if the return to performance percentage improves with concurrent treatment of endocrine and musculoskeletal disorders. Key takeaways include that the relationship between endocrine disorders and musculoskeletal disease in the horse goes beyond laminitis to include lameness, muscle atrophy, suspensory ligament degeneration, osteochondritis dissecans, and potentially metabolic osteoarthritis. Approaches learned from human and equine comparative studies can offer insight into injury recognition and management, thus mitigating the impact of endocrine disorders on performance in both species. Readers interested in an in-depth description of current and future research involving pathophysiology, novel interventions, and multiomic approaches to identify individuals with athletic limitations induced by endocrine disorders are invited to read the companion Currents in One Health by Manfredi et al, AJVR, February 2023.

Markers for internal neoplasia in the horse

The diagnosis of internal neoplasia in horses is challenging. Increased production of hormones physiologic for adult animals (e.g., adrenocorticotropin, norepinephrine, and erythropoietin) or typical for the foetal phase (alpha-fetoprotein, anti-Müllerian hormone, and parathyroid-hormone-related protein) might aid in tumour diagnostics. Thymidine kinase-1 and alkaline phosphatase are examples of intracellular enzymes, whose activity in the blood may increase in some neoplasia cases. Furthermore, inappropriate production of abnormal monoclonal or autologous antibodies can accompany lymphoma and multiple myeloma. Many of those tumour markers lead to clinical or laboratory changes, called paraneoplastic syndromes, such as hypercalcaemia and erythrocytosis. The interpretation of the results of the tumour marker measurements in horses is complicated due to many factors affecting the markers' concentration or activity (e.g., young age, pregnancy, and inflammation) and other diseases triggering the same changes. Moreover, the presence of paraneoplastic syndromes is inconsistent, which leads to low sensitivity of those substances as tumour markers. In conclusion, screening for neoplasia in horses is not recommended. The measurement of tumour markers should be performed only in risk groups with suspicious clinical or laboratory findings, and the results should be interpreted with caution. It is advisable to add inflammatory markers to the tumour profile or repeat the measurements.

Management of pituitary pars intermedia dysfunction in practice: A clinical audit

In horses/ponies with pituitary pars intermedia dysfunction (PPID), pergolide mesylate treatment, with monitoring of therapeutic response, is recommended by contemporaneous literature and equine endocrinologists. However, it is unknown whether these recommendations are adhered to in private practice. This clinical audit aimed to compare treatment and monitoring of PPID cases in veterinary practice against available recommendations. Case data and basal plasma adrenocorticotropic hormone (ACTH) concentrations from all equids tested for PPID between 2012 and 2016 from a single veterinary practice in the UK were obtained. Records were reviewed and information on treatment and monitoring over the subsequent 2-6 years was extracted and compared with published recommendations. After exclusions, the audit population was 480 animals (median age, 20 years). The most common presenting signs were laminitis and/or historical laminitis (51.2%) and hypertrichosis and/or delayed coat shedding (24.5%). Based on seasonally adjusted reference intervals for basal ACTH concentration, 51.7% (n = 248) of animals were classified as positive, 37.1% (n = 178) as negative and 11.3% (n = 54) as equivocal for PPID. Records were available for 459 animals; of which pergolide treatment was initiated in 78.7% (n = 185/235) of positive cases, 19.2% (n = 10/52) of equivocal cases and 6.4% (n = 11/172) of cases classified as negative. Overall, 87.2% (n = 129/148) of cases commenced treatment as per recommendations. Only 77.7% (n = 160/206) of pergolide-treated animals had documented PPID monitoring and of these, only 48.1% (n = 77/160) had follow-up basal ACTH testing in the first 1-3 months following diagnosis. The findings confirm that management of PPID in veterinary practice fell below contemporaneous recommendations, especially for monitoring.

Prospective Case Series of Clinical Signs and Adrenocorticotrophin (ACTH) Concentrations in Seven Horses Transitioning to Pituitary Pars Intermedia Dysfunction (PPID)

Simple Summary

Pituitary pars intermedia dysfunction (PPID) is a common disease of the geriatric horse population. The most common clinical sign of PPID is hypertrichosis, or a long hair-coat with delayed shedding. Hypertrichosis is the most easily recognized clinical sign of PPID. However, the presence of hypertrichosis is often associated with severe end-stage disease. There is little research investigating sub-clinical or early PPID and the clinical signs associated with these stages of disease. The benefit of being able to recognize early PPID, is that we are able to begin treatment earlier on in disease process, potentially reducing the deleterious consequences of PPID and improving survival. Laboratory tests are available to more accurately diagnose PPID, and these tests include the basal ACTH and TRH-stimulated ACTH tests. Basal ACTH is easy to perform and is recommended in cases where clinical disease is suspected. The TRH-stimulation test improves diagnostic accuracy in early PPID cases. This study documents both test results and clinical signs associated with the transition from subclinical to clinical PPID, so that we are better able to recognize potential early PPID, as well as interpret results in these horses.

Abstract

Poor recognition of subtle clinical abnormalities and equivocal ACTH concentrations make early diagnosis of PPID difficult. Progressive clinical findings and corresponding ACTH concentrations in horses transitioning to PPID over time have not been documented. Seven horses with ACTH concentrations equivocal for PPID (utilizing locally derived, seasonally adjusted diagnostic-cut off values (DCOV)) and no clinical signs of PPID were selected. Sequential measurement of basal and thyrotropin-releasing hormone (TRH)-stimulated ACTH concentrations and recording of clinical findings occurred from October 2017 to November 2021 in a prospective case series. In two horses, marked hypertrichosis developed. Although 1/11 basal ACTH concentrations were below DCOV in 2018, subsequently all basal ACTH concentrations in these two horses without treatment were greater than DCOV. One horse was treated with pergolide which normalized basal ACTH concentrations. Four horses developed intermittent, mild hypertrichosis, and one horse never developed hypertrichosis. Basal ACTH concentrations in these five horses were greater than DCOV in 63/133 (47.4%) of testing points. TRH-stimulated ACTH concentrations in these five horses were greater than DCOV in 77/133 (57.9%) of testing points, sometimes markedly increased and greater than the assay upper limit of detection (LoD) of 1250pg/mL. TRH-stimulated ACTH concentrations were most frequently positive in late summer and early autumn, with 24/37 (64.9%) of TRH-stimulated ACTH concentrations greater than the DCOV in February and March. Horses transitioning to PPID can have subtle clinical signs and equivocal ACTH concentrations. However, TRH-stimulated ACTH concentrations can be markedly greater than DCOV, especially in late summer and early autumn (February and March) allowing for identification of subclinical and transitional cases.

Pituitary Pars Intermedia Dysfunction (PPID) in Horses

Substantial morbidity results from pituitary pars intermedia dysfunction (PPID) which is often underestimated by owners and veterinarians. Clinical signs, pathophysiology, diagnostic tests, and treatment protocols of this condition are reviewed. The importance of improved recognition of early clinical signs and diagnosis are highlighted, as initiation of treatment will result in improved quality of life. Future research should be targeted at improving the accuracy of the diagnosis of PPID, as basal adrenocorticotropic hormone (ACTH) concentration can lack sensitivity and thyrotropin releasing hormone (TRH) used to assess ACTH response to TRH stimulation is not commercially available as a sterile registered product in many countries. The relationship between PPID and insulin dysregulation and its association with laminitis, as well as additional management practices and long-term responses to treatment with pergolide also require further investigation.

The effect of pergolide mesylate on adrenocorticotrophic hormone responses to exogenous thyrotropin releasing hormone in horses

Thyrotropin releasing hormone (TRH) stimulation testing is often used to support a diagnosis of pituitary pars intermedia dysfunction (PPID) in horses although it is unclear whether or not repeat TRH stimulation testing post-treatment is a valid means of assessing response to medical therapy. Laboratory submissions from 64 suspected equine PPID cases were examined including the initial pre-treatment TRH stimulation test and a follow up test within 100 days of starting medical therapy with pergolide. In a subset of cases, further follow-up tests were examined beyond 100 days of starting treatment. Results from tests conducted between 1 July and 30 November 30 were excluded. Significant improvements were seen in both the baseline and TRH-stimulated adrenocorticotrophic hormone (ACTH) concentrations within 100 days with no further improvements seen in the subset of cases examined thereafter. Although 88% (n=56/64) of all cases showed a decreased response to TRH post-treatment, only 24% (n=9/38) of horses with positive pre-treatment TRH stimulation tests normalised following treatment, with a further 34% (n=13/38) improving into an equivocal test outcome category. Most commonly (42%; n=16/38), horses with positive pre-treatment TRH stimulation tests remained positive following treatment, although 75% (n=12/16) of these showed a numerically lower post-treatment response to TRH. These results will help inform practitioners of expected changes in TRH stimulation test results when assessing response of horses with PPID to medical therapy with pergolide.

Impact of blue light therapy on plasma adrenocorticotropic hormone (ACTH) and hypertrichosis in horses with pituitary pars intermedia dysfunction

Blue light therapy can be used in horses to alter the natural photoperiod and inhibit winter hair coat growth. Seasonal increases in ACTH occur in the fall season but are exaggerated in horses with pituitary pars intermedia dysfunction (PPID). Additionally, PPID horses frequently present with hypertrichosis. Thus, blue light therapy was proposed as a potential management tool for hypertrichosis and for investigating the impact of photoperiod manipulation on ACTH. Eighteen PPID horses, aged 18 to 31 yr, from a university-owned research herd were selected and assigned to either the control group (n = 10) or the treatment (blue light therapy) group (n = 8) based on age and clinical history, which included the results of multiple endocrine tests. Consistent daylength of approximately 14.5 h was maintained for the treated horses from July 15 through approximately late October via the extension of natural daylength using wearable masks that provided short wavelength blue light (465 nm) to 1 eye. The control group was exposed to only the natural photoperiod during this time. All horses were housed on the same farm and remained on pasture for the duration of the study. On Day 0, thyrotropin-releasing hormone (TRH) stimulation tests were performed to confirm PPID status; there were no differences between the 2 groups in resting plasma ACTH or plasma ACTH at 10 min after TRH administration. To determine an effect of treatment on ACTH, blood was collected via jugular venipuncture for measurement of ACTH at sequential timepoints over a 16-h period in mid-October. Hair weights were also assessed throughout the study. No differences in resting plasma ACTH were observed between the 2 groups across the seasonal analysis (July and October) or during the 16-h testing. The PPID horses receiving blue light therapy had lighter hair weights compared to the PPID control horses. These results suggest that blue light therapy does not alter ACTH concentrations but could potentially be used as an additional management tool for hypertrichosis in PPID horses. Manipulation of the photoperiod using blue light therapy did not affect seasonal changes in ACTH in this study.

Repeatability of a thyrotropin-releasing hormone stimulation test for diagnosis of pituitary pars intermedia dysfunction in mature horses

BACKGROUND

Pituitary pars intermedia dysfunction (PPID) is a common endocrinopathy of horses diagnosed with a thyrotropin-releasing hormone (TRH) stimulation test.

HYPOTHESIS/OBJECTIVES

Describe the repeatability of TRH stimulation in horses with and without PPID in winter and autumn.

ANIMALS

Twenty adult horses; 6 controls and 6 with PPID tested in autumn, 8 controls and 6 with PPID tested in winter with 3 controls and 3 with PPID tested in both seasons.

METHODS

Thyrotropin-releasing hormone stimulation was performed on 2 consecutive occasions, 1 week before and 1 week after the winter solstice and the autumn equinox. Blood was collected before and 30 minutes after IV injection of 1 mg of TRH. ACTH concentration was determined by a chemiluminescent assay. Repeatability and test-retest reliability were assessed by repeated measures analysis of variance, intraclass correlation coefficient and within-horse coefficients of variation (CV). Bland-Altman plots were generated to visualize agreement between repetitions.

RESULTS

In winter, no week effect was detected on the results of the TRH simulation and the test had an excellent test-retest reliability. In autumn, after-TRH ACTH concentrations were significantly lower on week 2 (P = .02) and the test only had a good test-retest reliability. There were significantly larger within-horse CV during autumn (P = .04) and after TRH stimulation (P = .04). There were 2 misclassifications in winter and 4 in autumn.

CONCLUSIONS AND CLINICAL IMPORTANCE

The TRH stimulation test was repeatable when performed 2 weeks apart in winter; however, in autumn, more variability in after-TRH ACTH concentrations resulted in decreased repeatability.

Influence of endocrine disease on l-lactate concentrations in blood of ponies

Background

Blood l‐lactate concentrations are higher in people with developing or established diabetes mellitus and insulin resistance.

Objectives

To investigate whether blood l‐lactate concentrations are positively correlated with measures of insulin dysregulation (ID) or increased autumnal ACTH concentrations in ponies.

Animals

Systemically healthy client‐owned ponies (n = 101).

Methods

Prospective case‐control study. Blood samples were obtained from 101 clinically healthy ponies. Breed, weight, height, and subjective and objective measures of body condition were recorded. Blood l‐lactate, glucose, triglyceride, total adiponectin, and ACTH concentrations were measured and an oral sugar test (OST) was carried out. Correlations between blood l‐lactate and variables of endocrine health were determined.

Results

Using a seasonal cutoff point of ACTH concentrations ≥47 pg/mL, 55 ponies had increased autumnal ACTH concentrations and 45 did not. Using a basal insulin concentration of >50 μiU/mL, 42 ponies were diagnosed with ID and 58 were not. Using a 60 minutes after OST cutoff point of >45 μiU/mL, 57 ponies had ID and 37 did not. Blood l‐lactate concentrations were significantly lower in obese (average body condition score ≥ 7/9) compared to nonobese ponies (0.6 mmol/L; range, 0.0‐1.9 mmol/L vs 0.8 mmol/L; range, 0.3‐2.7 mmol/L; P = .01). No other significant correlations were detected. No differences were detected between ponies with and without increased autumnal ACTH concentrations (0.7 mmol/L; range, 0.0‐2.7 mmol/L vs 0.7 mmol/L; range, 0.3‐1.8 mmol/L; P = .84) and with and without ID (0.7 mmol/L; range, 0.3‐2.7 mmol/L vs 0.8 mmol/L; range, 0.0‐1.6 mmol/L; P = .63).

Conclusions and Clinical Importance

Results do not support an effect of endocrine status on l‐lactate concentrations in blood of ponies.

Evaluation of the sensitivity and specificity of basal plasma adrenocorticotrophic hormone concentration for diagnosing pituitary pars intermedia dysfunction in horses: A systematic review

Measurement of basal adrenocorticotrophic hormone (ACTH) is currently used to diagnose pituitary pars intermedia dysfunction (PPID) in horses, yet a systematic review of the evidence for its use has not been undertaken. This study aimed to systematically review evidence regarding the sensitivity and specificity of the basal ACTH diagnostic test. Electronic databases were systematically searched in January 2019, September 2020 and January 2021, for English language publications published prior to these dates. Screening, data extraction and quality assessment of publications was undertaken by the authors using predefined criteria. Study design, methodology and information reported in included studies were assessed using Standards for Reporting of Diagnostic Accuracy (STARD) checklists. Risk of bias and applicability were appraised using the Quality Assessment tool for Diagnostic Accuracy Studies (QUADAS-2) quality assessment tool. Due to identified biases and marked between-study variations, meta-analysis was not undertaken. After removal of duplicates, 415 publications were identified, of which 25 were evaluated in full, with 11 of these meeting inclusion criteria. In most studies, basal ACTH was reported to have good sensitivity (overall median 75.5%; interquartile range [IQR], 64.0-86.5%; range, 36.0-100%) and excellent specificity (overall median, 95.2%; IQR, 84.2-98.9%; range, 63.3-100%). However, QUADAS-2 and STARD assessment highlighted that studies did not utilise optimal study design and/or study populations for the evaluation of a diagnostic test and the majority were subject to bias, or provided insufficient information to fully assess possible biases. Based on this review, basal ACTH performed better at ruling out PPID than detecting it.