The Timeline of Laminitis by: Christy West, TheHorse.com Webmaster November , Article # 10856

Transcription

1 The Timeline of Laminitis by: Christy West, TheHorse.com Webmaster November , Article # What happens within the foot of a laminitic horse? We know that the coffin bone can sink or rotate within the foot of a horse with severe laminitis, but that's fairly late in the game. Researchers are very interested in what happens earlier than that--in what microscopic changes take place before there is enough damage to destabilize the coffin bone. Understanding these changes can help researchers investigate how to prevent those changes and hopefully prevent this painful disease. At the 52nd annual American Association of Equine Practitioners convention, held in San Antonio, Texas, Andrew van Eps, BVSc, MACVSc, a resident at the University of Pennsylvania, discussed a study of findings in this area. Presenting a paper authored by Emma L. Croser, BSc, BVM&S, MVSt, of the University of Queensland (UQ), Australia, and Christopher Pollitt, BVSc, PhD, director of the Australian Equine Research Laminitis Unit at UQ (who were unable to attend the convention), he described what changes researchers saw in serial hoof biopsies taken from horses in which laminitis had been induced. Laminar Microanatomy A horse's hoof is held onto the coffin bone, or the last bone on the end of his leg, by around 600 interlocking, vertical, leaf-like projections called laminae, or more specifically primary epidermal laminae (PEL), van Eps described. Each of those PEL has up to 200 smaller laminae of its own, called secondary epidermal laminae or SEL. These laminae interlock with the same arrangement of laminae on the inner or hoof side of the attachment; these laminae are termed the primary and secondary dermal laminae (PDL and SDL). The laminar system provides about a square meter of attachment area for each hoof. The basement membrane is a sheet of connective tissue that lines every nook and cranny of the epidermal laminae of the hoof wall, he went on. It is "a key component" of the hoof-bone attachment, he noted. Timeline of Laminar Changes with Laminitis The current study evaluated laminar structure with hoof biopsies (punch samples) taken every six hours for 48 hours after induction of laminitis with carbohydrate overload, van Eps reported. Five adult Standardbred horses with no evidence of hoof disease were studied. Changes in the size, shape, and integrity of the laminae and basement membrane (BM) were found as follows:

2 Control horses (no laminitis) Biopsies were normal throughout the 48-hour study period. Secondary epidermal laminae were club-shaped with rounded tips that were never tapered or pointed. They showed a uniform length and angle relative to their PEL. The BM was tightly adhered to the SEL, penetrating deeply into the crevices (also called crypts) between adjacent SEL. Laminitic horses--12 hours after laminitis induction Two of the five horses showed mild microscopic changes; SEL were narrower and elongated, with more pointed tips and more rounded cell nuclei (which normally are oval and oriented perpendicular to the length of the SEL). The basement membrane appeared to have less integrity at the bases of the SDL. Changes tended to occur only on one side of PEL. "This seems to be the localized nature of early disease change and is most likely reflects higher weight bearing on the affected side," notes Croser. "At 12 hours, rounding of SEL cell nuclei is the first observable change," van Eps commented. 18 hours By this time, separation of the BM from the tips of SEL began in the two horses already showing changes, he reported; these separations resulted in teat-shaped, empty pockets of BM. The BM had even less integrity at SEL bases, and in some cases disappeared entirely. Changes were still primarily limited to one side of the PEL. bases. The SEL were thinner and more elongated with more pointed tips, due to stretching as they weakened. The cell nuclei of more SEL were more rounded, and one of the horses was clinically lame. The SDL also became thinner, and appeared to be pulling away from SEL

3 One additional horse showed the mild changes seen at 12 hours in the other two horses showing signs. 24 hours The two horses affected earliest showed progressive worsening of tissue architecture, with changes no longer restricted to one side of the PEL. Secondary epidermal laminae were more elongated, their angles relative to their PEL were more acute, and there were many more rounded nuclei. "There was more loss of BM from crypt-type areas; this is probably one of most telling signs," van Eps said. Two horses still showed no changes, while the later-affected horse showed some worsening of laminar architecture comparable to 18-hour samples of the first two horses. 30 hours The changes previously discussed increased in severity for the three earlier-affected horses, and one of the remaining horses began to show changes comparable to 12-hour changes for the other horses. The last horse also began to show mild changes, along with some partial blockage of a blood vessel with fibrin. "This is an important point," notes Croser. "That image demonstrated that in virtually all horses there was no thrombus (blood clot) formation within the blood vessels. In other research theories, blood clots are hypothesized to cause laminitis by reducing or stopping blood flow. In two slides, small fibrin clots were evident but were not attached to the walls of the vessels and did not have the typical characteristics of thrombi. The important thing about them was that the two small fibrin clots occurred after other microscopic changes were already underway (so they were results of the disease, not causes of it)."

4 36 hours Changes in laminar architecture continued to worsen in all horses, with SDL pulling further away from SEL in the earlier-affected horses. The BM appeared patchy and disappeared entirely between the bases of SEL. 48 hours (postmortem) In earlier-affected horses, the SEL were extremely elongated, and their cell nuclei were spindle-shaped and oriented along the long axis of the SEL (90 to their usual orientation). Extensive BM detachment from SEL was observed, as was empty spaces where laminae had pulled apart. In some cases, complete vesicles (pods) of detached BM were seen. Additional findings "Concurrent events included rapid, explosive proliferation of hindgut Streptococcus bacteria and the absence of Enterobacteriaceae bacteria," said van Eps. "Oligofructose (the carbohydrate used to induce laminitis) in the cecum promotes rapid, massive S. bovis overgrowth. The hindgut epithelium (lining) and BM damage mirrors hoof lamellar damage. There is sloughing of mucosal epithelium and infiltration of leukocytes (white blood cells)--essentially what's happening in the hindgut mirrors what's happening in feet." He also noted that in the feet, edema (fluid swelling) around cells was only evident in one horse, and only at 48 hours. Only mild signs of inflammation were seen in 48-hour samples with advanced lesions. Clinical signs Mildly increased digital pulses were first detected at 13 to 29 hours and became more pronounced from this time on, he reported. The typical constant weight shifting of laminitis was seen in all horses starting 20 to 43 hours after induction. "Lameness in four of the five horses progressively worsened with signs of severe laminitis evident in all four feet," he noted. "The remaining horse exhibited a mild lameness in all four feet that remained constant once detected. All horses developed moderate to marked transient (6- to 8-hour duration) epiphora (watery eyes), blepharospasm (eyelid twitching), and corneal opacity, which was clinically diagnosed as corneal edema, between 30 and 40 hours." He theorized that the corneal BM was affected similar to the laminar BM, but corneal biopsies could not be taken to confirm this theory. The eye problems had all resolved by 48 hours, he reported.

5 Take-Home Message "Laminar histological lesions are present as early as 12 hours (after the causative factor has been administered), before the onset of clinical signs of laminitis occur at around 24 hours," van Eps stated. "Basement membrane disintegration and detachments are early pathological events. Failure of the BM results initially in the stretching of dermal-epidermal attachments, leading ultimately to a complete failure of this attachment under load. Efficacious therapy should commence before 12 hours; we need to treat these horses very early if we want to prevent damage."