Equine Veterinary Journal

Reason for performing study: Platelet activation occurs in human obstructive airway diseases and in laboratory animal models. However, there is limited evidence that platelets may be involved in equine recurrent airway obstruction (RAO) and other inflammatory diseases. This study investigated whether platelet activation also occurred in RAO.

Hypothesis: Platelet function is altered in ponies with active RAO. This alteration can be detectedex vivoby measuring platelet adhesion.

Methods: Anin vitroplatelet adhesion assay measuring acid phosphatase (AcP) activity colorimetrically was adapted for use with equine platelets and responses to selected agonists were established. Platelet adhesion and aggregation was evaluatedin vitroon platelets isolated from 6 ponies with RAO before, during and after a 7 h natural antigen challenge. Three ponies with no history of airway disease were also studied.

Results: Adhesion of equine platelets to serum coated plastic was detected at concentrations of 10–100 times 10⁹/l. Adhesion increased in response to stimulation with platelet activating factor and thrombin, but not equine interleukin 8. Prior to the antigen challenge, adhesion of nonstimulated platelets was low and increased significantly (P<0.05) 24 h after initiation of the challenge in RAOs, but not in the normal animals. No changes in platelet aggregation were noted in either group.

Conclusions: The described assay offers an alternative method to evaluate platelet function in healthy and diseased horses and can detect changes not observed using a classic aggregation assay. Circulating platelets are activated 24 h after antigen challenge of ponies with RAO and may play a role in pulmonary inflammation and/or the pathophysiology of RAO.

Potential relevance: Investigating platelet function in RAO and airway inflammation may reveal new aspects of the pathogenesis of inflammatory lung disease in the horse.

A role for platelets in allergic airways disease has been postulated and changes in the responsiveness of circulating platelets have been demonstrated following antigen challenge of asthmatic human subjects. In this study agonist‐induced aggregation of equine platelets in vitro has been compared before and after exposure of horses to a controlled hay and straw challenge. Prior to challenge the response of platelets, from horses with chronic obstructive pulmonary disease (COPD) and normal animals, to adenosine diphosphate (ADP) and platelet activating factor (PAF) did not differ. Five hours after initiation of the challenge, there was a modest but significant decrease in the response of platelets from the COPD horses to PAF, but not to ADP. Platelets from normal horses were not less sensitive to either agonist at this time. Twenty four hours after challenge the responses of platelets from COPD horses to both agonists were the same as pre‐challenge values. These results demonstrate that antigen challenge alters the responsiveness of platelets from allergic horses to PAF and this change is suggestive of PAF release accompanying allergen exposure in the horse.

Knowledge of cardiac output is expected to help guide the treatment of hypotension associated with critical illness and/or anaesthesia in neonatal foals. However, a practical and safe method of measuring cardiac output has not been described for the foal. Lithum dilution, a new method of cardiac output determination not requiring cardiac catheterisation, has recently been reported in mature horses. We compared this method to thermodilution in isoflurane‐anaesthetised foals age 30–42 h and found good agreement between the 2 methods in a range of cardiac outputs 5.4–20.41/min. The lithium dilution technique is a practical and reliable method of measuring cardiac output in anaesthetised neonatal foals, and warrants investigation in critically ill conscious foals.

Carpal osteochondral injury is frequently observed in strenuously trained horses. It is clear that the integrity of articular cartilage and subchondral bone are intimately related, although there is controversy about which component is the most important. Calcified cartilage provides the mechanical link between soft hyaline cartilage and stiff subchondral bone so it is essential to understand the tissue's response to exercise. Middle carpal calcified and uncalcified (hyaline) cartilage thickness was investigated in horses undergoing high‐ and low‐intensity exercise. Twelve untrained horses (18–21 months) were paired, and randomly assigned to an exercise group. Group 1 underwent 19 weeks progressive high‐intensity training on a high‐speed treadmill. Group 2 underwent walking exercise only. Osteochondral specimens were obtained from 8 test sites per carpus immediately after euthanasia. Histomorphometric measurements of total cartilage, hyaline layer, and calcified zone thickness were obtained from decalcified and undecalcified samples. Mean ± s.d. (μm) calcified cartilage thickness for dorsal cartilage from Group 1 was 271 ± 73 and from Group 2 was 163 ± 49; for palmar cartilage from Group 1 was 195 ± 42 and Group 2 was 150 ± 52. Group 1 horses had significantly thicker total cartilage (P<.0001) and calcified zone (P<.0001) than Group 2, but there was no difference in hyaline layer. Within each group all dorsal sites had thicker calcified cartilage (P<.003) than palmar sites, but no difference in hyaline or total cartilage. These findings indicate that high‐intensity exercise leads to greater calcified zone depth without alteration in hyaline cartilage thickness, and that this response is maximal at sites that withstand high, intermittent loads. Increasing calcified cartilage thickness with exercise may maintain the articular surface stiffness gradient in the face of alterations in hyaline cartilage and/or subchondral bone stiffness.

The objective of this study was to document the development of biochemical heterogeneity from birth to maturity in equine articular cartilage, and to test the hypothesis that the amount of exercise during early life may influence this process. Neonatal foals showed no biochemical heterogeneity whatsoever, in contrast to a clearbiochemical heterogeneity in mature horses. The process of formation of site differences was almost completed in exercised foals age 5 months, but was delayed in those deprived of exercise. For some collagen‐related parameters, this delay was not compensated for after an additional 6 month period of moderate exercise.

It is concluded that the functional adaptation of articular cartilage, as reflected in the formation of biochemical heterogeneity in the horse, occurs forthe most part during the first 5 monthspostpartum. A certain level of exercise seems essential forthis process and withholding exercise in early life, may result in a delay in the adaptation of the cartilage.

Reasons for performing study: Training at a very young age may influence the characteristics of the collagen network of articular cartilage extracellular matrix (ECM) in horses.

Objectives: To investigate whether increasing workload of foals results in significant changes in the biochemical composition of articular cartilage ECM.

Methods: Thoroughbred foals (n = 33) were divided into 2 different exercise groups from age 10 days‐18 months. One group (PASTEX; n = 15) was reared at pasture; the other (CONDEX; n = 18) underwent a specific additional training programme that increased workload by 30%. At mean age 18 months, 6 animals from each group were subjected to euthanasia. The proximal articular surface of the proximal phalanx of the right hindlimb was examined for the presence of damage using the cartilage degeneration index (CDI). Samples were taken from 2 sites with known different loading patterns. Slices were analysed for DNA, glycosaminoglycans (GAG), collagen and post translational modifications of collagen (formation of hydroxylysylpyridinoline [HP] and pentosidine crosslinks, and hydroxylysine [Hyl]), and exercise groups and different sites compared.

Results: There were no differences in CDI betweenPASTEXandCONDEXanimals, indicating the absence of extra joint damage due to the exercise regimen. There were site‐related differences for most biochemical variables, corroborating earlier reports. All biochemical variables showed differences betweenPASTEXandCONDEXgroups at one of the sites, and some at both. GAG and collagen levels were lower in theCONDEXgroup whereas Hyl, HP crosslinks and pentosidine crosslinks were higher.

Conclusions and potential relevance: A measurable effect of the conditioning exercise was demonstrated. The margin between too much and too little work when training foals may be narrower than intuitively presumed.

Reasons for performing study: An objective measure of neck, back and croup pain is needed in horses with musculoskeletal injuries, vague upper limb lameness or poor performance.

Objectives: To establish mechanical nociceptive thresholds (MNTs) within the axial skeleton and evaluate the effects of subject status and ridden exercise.

Methods: Thirty‐six mature horses (10 nonridden; 26 actively ridden) were used to assess MNTs evoked by a pressure algometer (PA) with a 1 cm²rubber plunger tip at 62 midline and bilaterally symmetrical anatomical landmarks along the axial skeleton. Pressure was applied at a uniform rate of application until a local avoidance reaction was noted. The repeatability of 3 consecutive measurements was evaluated. Left‐to‐right comparisons and the effects of subject status and ridden exercise on MNTs were assessed.

Results: MNTs were repeatable and increased in a cranial‐to‐caudal gradient within the axial skeleton. Typically, there were no significant left‐to‐right differences. Within‐horse variability was less than between‐horse variability. Higher MNTs were measured in young, heavy, non‐Thoroughbred, castrated males, and in horses that were ridden and actively exercised.

Conclusions and potential relevance: PA provides an objective, noninvasive, and repeatable tool to measure mechanical nociception in horses. MNTs vary in horses with differing subject status and ridden exercise level.

Reasons for performing study: Pressure algometry (PA) is a potential modality for objectively measuring mechanical nociceptive thresholds (MNTs) in horses. Its ability to differentiate musculoskeletal pain sites from nonpain sites is unknown and must be assessed prior to its clinical application.

Objectives: To assess the ability of PA to detect induced musculoskeletal pain.

Methods: Twenty clinically normal mature horses (2 groups of 10 horses, over 2 years) were used to measure MNTs before and after implantation and removal of 2 fixation half‐pins in the dorsal spinous processes of 2 adjacent vertebrae. To assess hyperalgesia, MNTs near the surgical sites were pooled and compared to surrounding landmarks. MNTs were also compared on 2 consecutive days immediately after surgical implantation. To assess longer‐term adaptation or sensitisation, changes from the beginning to the end of the study were evaluated.

Results: The precise surgical sites could be recognised due to substantial and localised decreases in the MNTs compared with surrounding landmarks. At most sites distant to the surgical sites, the distribution of the median percentage change in MNTs was centred around 0 (suggesting unbiased repeatability). MNTs compared from the beginning to the end of this study did not demonstrate obvious adaptation or sensitisation.

Conclusions and potential relevance: PA provides a quantitative and repeatable method for assessing the presence of musculoskeletal pain. Further studies are needed to evaluate PA clinically for assessing musculoskeletal injuries and pain management strategies in horses.