AAEP 2022: Horse Health and Research

From PPID to PAAG and AMA to PRP, there were a lot of discussions of current and new treatments for our equine patients.
There were multiple presentations at AAEP covering PPID diagnosis and treatments. | Shelley Paulson

Editor’s note: Make sure to visit EquiManagement.com for six additional articles covering equine health and research topics presented at the 2022 AAEP Convention. Brought to you by American Regent Animal Health

The wide range of equine research and field medicine topics discussed at the 2022 AAEP Convention offered veterinarians many live and on-demand presentations to choose from. In this article (and in additional articles on EquiManagement.com), we bring you highlights of some of these discussions.

Measure of Insulin Dysregulation

Wellness Ready Insulin Test
Wellness Ready is a stall-side insultin testing device.

Having the means to measure insulin is critical to the diagnosis and management of pars pituitary intermedia dysfunction (PPID, also known as Cushing’s disease) and equine metabolic syndrome (EMS). At the 2022 AAEP Convention, Emily Berryhill, DVM, DACVM, of the University of California, Davis, described a new stall-side insulin test.

This stall-side application, called the Wellness Ready test, eliminates the many steps required for acquiring, preparing and sending samples to outside labs, then waiting for results. The Wellness Ready Test provides a quantitative measure of whole blood insulin after a 15-minute incubation period.

A validation study compared this test to human insulin radioimmunoassay (RIA) testing using 99 samples from 51 horses. Some horses were fasted, some fed and some underwent oral sugar tests. The stall-side test had a good association between itself and RIA values, although there was an average of 10.4% higher readings on the stall-side test than the RIA measures. For example, if the insulin concentration on RIA was 50, then the stall-side test read-out was 53. Berryhill said this is not enough difference to impact a veterinarian’s clinical decision-making. In addition, literature stated that the RIA tends to measure slightly lower than other assays.

In general, sensitivity ranges from 87-91% and specificity from 92-96% with the Wellness Ready test. These are useful values, especially at an insulin concentration cutoff of 65, reported Berryhill. Overall, the Wellness Ready test has good utility as a stall-side assay for equine practitioners evaluating horses for insulin dysregulation.

Ertugliflozin for Managing Hyperinsulinemia and Laminitis

Tania Sundra, BSc, BVMS, MANZCVS (Eq), of Avon Ridge Equine Veterinary Services in Australia, reported on the use of ertugliflozin for the management of hyperinsulinemia. 

The drug, which is used off-label in the United States, is described as an SGLT2 (sodium-glucose linked transporter 2 inhibitor) developed to treat human diabetes. This medication works at the level of the kidneys, where SGLT2 is responsible for 90% of glucose reabsorption. Ertugliflozin increases urinary glucose output to decrease blood levels of glucose, which in turn decreases insulin concentrations.

A retrospective study from January 2021-February 2022 evaluated 51 horses of median age 17 years with post-prandial hyperinsulinemia and laminitis. Neither diet nor management changes for at least six weeks were able to improve the endocrine or hoof abnormalities in these horses. As part of the study, they received 0.05 mg/kg of ertugliflozin once daily for 30 days.

Within 30 days of treatment, reductions in insulin concentrations and lameness were evident. On Day 0, insulin concentrations were 300. By Day 30, they were 43, and by Days 60-240, insulin concentrations plateaued around 32. Triglycerides increased significantly at Day 30 to 1400 mg/dl, but by Day 60 were decreased to 1 mg/dl. 

Many horses were able to discontinue NSAIDs within seven to 14 days of ertugliflozin treatment. Laminitis improvement occurred despite chronic radiographic changes. Horses also lost weight, likely due to lipolysis, with the most obese individuals losing the most weight. Ten horses exhibited polyuria-polydipsia, likely due to chronic osmotic diuresis from excretion of urinary glucose. Due to a potential for volume depletion from excess urination, treated horses need access to water, remarked Sundra. There were no signs of urinary tract infection.

Sundra stressed the importance of monitoring for hyperlipidemia and hepatic lipidosis with this treatment. If the horse is fasting or feed is withheld, then ertugliflozin should be stopped. The diet should contain less than 10% NSC plus a ration balancer. Following the 30-day 0.05 mg/kg, once daily dose, horses can be maintained on a low dose of 0.025 mg/kg once daily.

Long-Term Response to Pergolide Treatment in PPID Horses

An old horse with PPID, which is treated with pergolide, which was discussed at the AAEP Convention in 2022.
Research on long-term use of pergolide showed that increased doses might not be necessary and treatment resulted in client satisfaction. | Getty Images

FDA-approved pergolide medication has been on the market for at least a decade. Yet, there is limited data on long-term responses with pergolide treatment of PPID horses. Harold Schott, DVM, PhD, DACVIM, of Michigan State University, discussed a study that reviewed several pertinent questions:

  • Does treatment improve quality of life?
  • Will the dose need to be increased?
  • Does the treatment prolong life?
  • Are clients satisfied with the treatment outcome?
  • Are there potential adverse effects with long-term treatment?

In 2009, a study with 28 horses and two ponies averaging 23 years of age provided initial safety information about treatment with pergolide in the near term. Then, an FDA open field trial tested the efficacy of pergolide administered to 113 horses. The trial found that 76% achieved treatment success. A similar Michigan State University cohort study identified 60% success in 30 client-owned horses during that time period.

This led to an extended-use study of the 30 horses enrolled in the initial Michigan State study to evaluate efficacy and adverse events over a 12.5-year period. Reduced appetite occurred in 15-20% of horses. A few had intermittent diarrhea and laminitis flare-ups, although pergolide is not specified as treatment for laminitis. Seasonal effects of elevated ACTH persist even when horses are on pergolide. Schott mentioned that it might be appropriate to bump up the dose by adding ½ mg pergolide from the end of August through December. 

In general, most horses treated with 1 mg of pergolide per day did well for four to five years with normal endocrine test results. For seven individuals that demonstrated clinical signs of PPID or failed dexamethasone suppression testing after four to five years, the dose was increased to 2-4 mg/day. A horse’s initial status had no effect on survival, with median survival time of 3.3 years (ranging from 0.6-12.5 years).

By five and a half years into the study, 13 horses were still alive, and owners noted continued clinical improvement, with normal endocrine test results in 75%. By 9.5 years, only six horses in the study were still alive, yet only two of the six had normal endocrine test results.

Ten years into the extended-use study, 86% of clients were satisfied. Seventy-one percent strongly agree and 25% agree that treatment improved their horse’s quality of life. Asked if they would do this treatment again, 44% strongly agree, and 44% agree. Regarding the cost of pergolide treatment, 26% said they would pay $500/year; 56% would pay $1,000/year; 10% would pay $1,500/year; and 4% would pay $2,000 or $2,500/year.

Improvements were seen in energy level in 77% of horses in the initial weeks of treatment. Improvements in haircoat were seen in 71% of horses, and improvement to abnormal sweating in 41%.

Schott summarized the extended study findings:

  • There might not be a need for progressive increases in pergolide dose.
  • Endocrine test results can improve over prolonged periods.
  • Treatment improves quality of life but does not prolong life.
  • Client satisfaction is high with extended use.

Use of Polyacrylamide Gel (PAAG) in the Field

Joint management is one area in which most equine practitioners are involved, not just for equine athletes but also for retired and/or elderly horses suffering from osteoarthritis. At the 2022 AAEP Convention, a number of sports medicine topics were discussed at length.

Polyacrylamide gel (PAAG) is a newer product for intra-articular injection in horses to help ameliorate pain and discomfort from osteoarthritis. There are two forms with different cross-linking materials: 4% (Noltrex) and 2.5% (Arthramid Vet). Each product has different properties, explained Scott McClure, DVM, PhD, DACVS, DACVMR, of Iowa-based Midwest Equine Surgery and Sports Medicine. 

The 2.5% PAAG product targets and integrates with the synovium to increase joint capsule elasticity. It is absorbed within 48 hours and remains in the synovial membrane for full integration within four weeks. In contrast, the 4% PAAG product targets and adheres to damaged cartilage to help lubricate the joint surface through its low coefficient of friction. McClure noted that 4% PAAG persists for a longer time than hyaluronic acid (HA) because it isn’t susceptible to degradation by metalloproteinases.

There are other differences between the two PAAG products discussed:

  • It is OK to administer the 4% PAAG with corticosteroids and biologics. It is recommended to wait 30 days before injecting other treatment medications into the joint following 2.5% PAAG administration.
  • Rest is recommended for 14 days following treatment with 4% PAAG and 10-30 days following 2.5% PAAG. The duration of rest depends on the severity of joint disease.

Complication rates for both are similar at 0.06%, which is better than the rate of complications seen with HA injection.

McClure stressed that it is important to recognize that there won’t be an instantaneous response to PAAG treatment, unlike what is seen with more immediate inflammatory decreases from corticosteroids within 48-72 hours. He explained, “Lameness following PAAG injection creeps down over 45-90 days. However, the horse obtains long-term benefits from PAAG.” 

He noted that prior to injecting with PAAG, it is important to control joint inflammation first with ice therapy, NSAIDs and/or topical diclofenac.

Intra-Articular Stanozolol

As an anabolic steroid, stanozolol can provide beneficial effects when administered into the joint, according to McClure. Studies have demonstrated that stanozolol IA:

  • decreases inflammation and promotes anabolic processes on synoviocytes and chondrocytes;
  • acts as local autocrine response to stimulate production of anabolic growth factors;
  • decreases apoptosis in equine chondrocytes in vitro via stimulation of IGF-1 production and decreased nitric oxide production;
  • decreases in vitro gene expression of MMP-13, MMP-1, IL-6, COX-2 in normal and IL-1-beta-exposed equine chondrocytes;
  • increases TGF-B1 (transforming growth factor), which is associated with decreasing pain from osteoarthritis in humans;
  • has no effect on cell viability; and
  • is a strong anti-inflammatory medication.

From studies comparing dosages, McClure said that fast positive results are seen with 5 mg/joint weekly injections, with improvement achieved after two treatments. A micronized formulation was evaluated through a six-month follow-up of 50 Thoroughbred racehorses treated weekly for four weeks. Half the horses had osteoarthritis, one-third had subchondral bone disease and 13% had synovitis. None experienced adverse reactions, and 77% had beneficial or uncertain response to treatment. No benefit was seen in 22%.

In another double-blind study, 60 horses with osteoarthritis (OA) were split into weekly treatment groups. Forty horses received 5 mg/joint of stanozolol, while 20 horses were given a placebo. Lameness improved in 82.5% (33/40) with stanozolol compared to 10% (2/20) improvement of control horses. Six of the 40 horses treated with stanozolol developed mild distention of the joint that regressed spontaneously and rapidly. Lameness resolved completely in 56% (26/40). There was a better response of 71% in horses with acute OA compared to 58% with chronic OA.

Co-administration of stanozolol with a corticosteroid is contraindicated because that would inhibit the anabolic effect. 

When 5 mg of micronized stanozolol was administered into the tarsocrural joint, stanozolol was found in the systemic circulation but was no longer detectable after 36 hours. That said, this medication is banned for FEI and for racing, and it requires a 47-day withdrawal for USEF. 

McClure recommended its use in young horses with OCD lesions that have been scoped and treated with triamcinolone. Then, two to four weeks later, they can be started on IA micronized (in suspension) stanozolol series. The objective with stanozolol is to allow all joint structures to heal. With this in mind, it is not intended as a “quick fix” but rather should be accompanied by rest and rehabilitation.

Alpha-2 Macroglobulin for Joint Treatment

Alpha-2 macroglobulin (A2M) is an acute-phase protein that can inhibit four classes of proteases and influence modulation of cytokines. It inhibits the inflammatory cascade through a “bait and trap” mechanism to “capture” proteases that degrade cartilage. This arrests osteoarthritis at the molecular level. Once found and inactivated, the molecule is translocated out of the cell and excreted. 

Christina Russillo, DVM, from Virginia Equine Imaging, explained that A2M can be isolated from circulating plasma to produce an injectate through a 60-minute procedure, making it a rapid option for treatment compared to ACS. One huge benefit is that the collection kit can be transported into the field. The injectate is stable for four hours at room temperature and for 12 hours if refrigerated. At minus 18° Celsius (-0.4°F), the product lasts for up to a year. A thawed sample must be used within 12 hours and can’t be frozen a second time. She recommended storing the injectate in 4 ml aliquots in a capped 6-cc syringe with air to allow for expansion during freezing.

Prior to collection, alpha-2 agonist sedatives and analgesics should be withdrawn for three days, and NSAIDs should be withdrawn for five days. It is important to wait 24 hours following joint analgesia to inject A2M, and to wait two weeks following IA corticosteroid treatment before using A2M. She noted that A2M can be used in conjunction with shockwave, laser therapy or bisphosphonates.

Time to onset for lameness relief is similar to more traditional therapies, said Russillo. She noted that it is especially useful for treatment of multiple joints to address cartilage injury and to provide OA pain relief for horses with endocrinopathies that won’t do well with corticosteroid treatment. 

Withdrawal time for FEI competition is less than what is typical for other joint therapies.

In studies conducted at Russillo’s clinic, lameness in bilateral coffin joints treated in 19 horses improved in 79% by one grade of the AAEP lameness scale. In distal hock joints, flexion improved in 86%, with 57% improving one grade and 11% improving two grades. Neck pain improved in 71.4%, with 43% improving two grades. 

She reported that the benefits achieved with A2M as an orthobiologic in treating inflammatory synovitis are especially important for competition horses, reducing lameness and joint effusion, improving flexion tests and reducing pain with palpation.

Kester News Hour Reports

Kester News Hour presentation at 2022 AAEP Convention.
Drs. Amy Johnson (above), Kyla Ortved and Sherry Johnson presented on surgical and medicine topics during the Kester News Hour at the 2022 AAEP Convention.

The Kester News Hour at the 2022 American Association of Equine Practitioners Convention featured surgical topics covered by Kyla Ortved, DVM, PhD, DACVS, DACVSMR, of the University of Pennsylvania, and Sherry Johnson, DVM, DACVIM, of Equine Sports Medicine and Rehabilitation. Amy Johnson, DVM, DACVIM, of the University of Pennsylvania, discussed medicine topics. 

Bisphosphonates

There has been recent concern about lingering effects of bisphosphonates on equine bone. Ortved described an extremely long half-life of bisphosphonates of eight to 10 years. However, she reported that research has only detected it in the horse for 40 days. 

There is a current attempt to develop a more sensitive assay with paired blood and urine samples [Riggs, C.M.; Thompson, S.L.; So, Y-M., et al. Tiludronic acid can be detected in blood and urine samples from Thoroughbred racehorses over 3 years after last administration. Equine Vet J 2021;53:1287-1295. DOI: 10.1111/evj.13395].

The study looked at 24 horses who had been administered Tildren over the past several years. Most received a one-time administration. However, three horses received multiple doses, one horse received multiple doses close together in time, and one received a 1/5 dose with regional limb perfusion.

Three years out from the injections, assays identified Tildren in blood and urine in all 24 horses. This has significance for implementation of medication rules, especially in racehorses and for pre-purchase exam evaluations. To date, there is no clear knowledge regarding long-term impact of bisphosphonates on bone health. It is also not clear whether high concentrates in urine and blood are representative of bone concentrations. 

Timing for Gathering TRH-Stimulation Testing Samples

The thyroid-releasing hormone (TRH) stimulation test is helpful in diagnosing PPID (pituitary pars intermedia dysfunction or Cushing’s disease) in suspect horses. It’s also useful in identifying early cases. This test is used commonly in equine practice, especially as more horses are living longer, since older horses are at risk of developing PPID. 

A study looked at the timing of collection of the second blood sample at 10 minutes following administration of intravenous TRH [Thane, K.; Uricchio, C.; Frank, N. Effect of early or late blood sampling on thyrotropin releasing hormone stimulation test results in horses. J Vet Intern Med 2022 Mar;36(2):770-777. DOI: 10.1111/jvim.16362. Epub 2022 Jan 20].

Amy Johnson described the study of 24 horses aged 3-28 years with blood collected at nine, 10 and 11 minutes after the horses received IV TRH. At exactly 10 minutes, six were diagnosed with PPID (>200 pg/ml) and one horse was equivocal at (110-200 pg/ml). Horses with samples taken a minute early or later than 10 minutes resulted in five horses with a change in laboratory interpretation—moving from positive to equivocal, equivocal to positive or normal to equivocal.

Small alterations in timing significantly altered test results, Johnson concluded. Early or late draws occurred in 75% of samples—these results differed by ≥10% compared to the exact 10-minute draw. Of these, 21% yielded a different diagnostic interpretation.

This study is relevant considering that with accurate TRH-stimulation testing, ACTH concentrations are reported to decrease in 88% of those treated accurately, including those monitored for treatment improvements.

Synovial Sepsis Following Intra-Articular Injection in Ambulatory Settings

It is common for field practitioners to inject joints in settings when a horse is not able to be brought into a hospital. A study evaluated 1,122 horses with 3,866 intra-articular (IA) injections in 1,623 medication sessions [Krause, D.M.; Pezzanite, L.M.; Griffenhagen, G.M.; et al. Comparison of equine synovial sepsis rate following intrasynovial injection in ambulatory versus hospital settings. Equine Vet J 2022;54:523-530. DOI: 10.1111/evj.13485].

Ortved said of 643 field injections, 278 (42%) received antimicrobial drugs (AMD) with the IA medicationwhile 365 (57%) did not. In contrast, with 980 injections done in a hospital setting, 406 (41%) received AMDs while 574 (59%) did not. This comparison demonstrates a similarity between intra-articular injection strategies done in both field and hospital settings.

Only four of 1,122 horses developed joint sepsis, equivalent to 0.1% frequency, which is very low. One infection developed after IRAP, two after triamcinolone and one after PRP. 

In summary, neither the injection setting nor the use of concurrent IA AMD impacted the infection rate. 

Toxic Effects of Amikacin in Equine Joints

Several research studies were discussed at AAEP that could help equine vets choose options for treating horses with joint issues. | Shelley Paulson

Ortved described the effects of amikacin on synoviocytes and chondrocytes even when given in low concentrations. Toxicity to these cells is not just isolated to amikacin but exists across many anti-microbial drugs used in joints [Pezzanite, L.; Chow, L.; Piquini, G.; et al. Use of in vitro assays to identify antibiotics that are cytotoxic to normal equine chondrocytes and synovial cells. Equine Vet J 2021 May;53(3):579-589. DOI: 10.1111/evj.13314].

Because of toxic effects of amikacin coupled with the study that demonstrated very low sepsis rates from IA injections, there is no indication to routinely add antimicrobial drugs when injecting joint or synovial structures. There is one exception to this: It is necessary to add amikacin to an intra-articular injection of PSGAG due to the risk for potentiation of Staphylococcus sp. 

Use of Polymyxin B 

Polymyxin B is commonly used in equine practice to bind endotoxin. Gentamycin is used as an antibacterial against gram-negative infections. A study examined the different side effects experienced with polymyxin B alone compared to polymyxin B combined with gentamycin [van Spijk, J.N.; Beckmann, K.; Wehrli Eser, M., et al. Adverse effects of polymyxin B administration to healthy horses. J Vet Intern Med 2022 Jul;36(4):1525-1534. DOI: 10.1111/jvim.16470. Epub 2022 Jul 7].

Six healthy horses were used in a prospective, blinded, randomized cross-over trial with two protocols: a) polymyxin B (6000 IU/kg IV every 24 hours for 7 doses); or b) polymyxin B at that dose for seven doses plus gentamycin (10 mg/kg IV every 24 hours for 4 doses). The horses were given a washout period of three weeks between trials.

All horses developed transient ataxia that worsened by two grades with cumulative doses of polymyxin B combined with gentamycin, reported Amy Johnson. Renal effects also developed with co-administration. The study concluded that polymyxin B should be avoided in ataxic horses or those with underlying renal compromise.

Gentamycin Toxicity

Another side effect can occur with gentamycin use: induced auditory loss. It is reported that up to 47% of humans with a history of aminoglycoside treatment have hearing loss.

A study of 10 horses administered 6.6 mg/kg gentamycin daily for a week developed a variety of hearing loss issues. Auditory function was measured by brainstem auditory-evoked-response testing at the end of the treatment week and one month later [Aleman, M.R.; True, A.; Scalco, R.; et al. Gentamicin-induced sensorineural auditory loss in healthy adult horses. J Vet Intern Med 2021 Sep;35(5):2486-2494. DOI: 10.1111/jvim.16221. Epub 2021 Jul 28].

Six of the 10 horses lost hearing, with two developing unilateral complete deafness and four developingunilateral partial deafness. One month later, three horses were still affected. Four had regained their hearing, two were partially deaf, and one new horse in the group of 10 was completely deaf.

With this in mind, practitioners should be circumspect about potential auditory and renal risks when using gentamycin. 

Effect of Exercise on Cytokine Concentration in IRAP Collection

In humans, exercise and surgical stress can adversely affect cytokine concentrations. It is desirable to have high concentrations of IL-1-Ra and low concentrations of TNF-alpha. 

A study looked at the effects of equine exercise prior to collection of blood for autologous conditioned serum (ACS) production. The researchers evaluated the potential of finding decreased concentrations of specific cytokines (IL-1-Ra; IL-10) and increased TNF-alpha and IL-1-beta [Hale, J.N.; Hughes, K.J.; Hall, S.; et al. The effect of exercise on cytokine concentration in equine autologous conditioned serum. Equine Vet J2022, 1-6. DOI: 10.1111/evj. 13586].

Whole blood was collected from eight Standardbred horses one hour and 24 hours after a single bout of exhaustive exercise pacing on a 1000-meter sand track. Sherry Johnson explained that at one-hour post-exercise, IL-1-Ra concentrations had decreased and TNF-alpha concentrations had increased, the opposite of the desired concentration effect for treatment with ACS. There was no difference at any time point for IL-10 or IL-1-beta in relation to exercise. There was also no difference in cytokine levels between fit-to-race horses compared to those not fit to race.

In summary, a single bout of intense exercise appears to transiently decrease the favorable cytokines while increasing the bad when blood is obtained within an hour of exercise. It is recommended to wait to collect blood for ACS for at least 24 hours following a single exercise bout.

Plasma-Rich Protein (PRP) Clears Staph Aureus Biofilms

The bio-active fraction of PRP lysate (Bio-Ply) was used in a study to compare tarsocrural joint infection induced with Staphylococcus aureus and the response to treatment with amikacin alone or amikacin with Bio-Ply [Gilbertie, J.M.; Schaer, T.P.; Engiles, J.B.; et al. A platelet-rich plasma-derived biologic clears Staphylococcus aureus biofilms while mitigating cartilage degeneration and joint inflammation in a clinically relevant large animal infectious arthritis model. Frontier Cell Inf Micro 2022 May;12:1-18. Doi.org/10.3389/fcimb.2022.895022].

Horses were euthanized three weeks after treatment. Ortved said that in all joints, bio-floats (collection of biofilms) were present in synovial fluid one day following induction of infection. Including Bio-Ply in the treatment decreased bacterial loads compared to amikacin alone, with complete bacterial clearance in 5/6 horses by the end of the study. In addition, those with the combination treatment had significantly decreased pain scores starting on Day 4. Inflammatory mediators were decreased in synovial fluid, and there was less cartilage and joint degradation with amikacin plus Bio-Ply compared to amikacin alone. 

The researchers concluded that there are superior antimicrobial properties and preservation of joint healing when Bio-Ply is included in intra-articular joint treatment of sepsis. Commercialization of this product is now in the beginning stages.

Subchondral Bone Cyst Treatment with Absorbable Implant

Cystic subchondral bone lesions within a joint are variable in shape, size and location. Ortved described a study that used an absorbable implant in 38 racehorses under 2 years old [Ravanetti, P.; Lechartier, A.; Hamon, M.; et al. A composite absorbable implant used to treat subchondral bone cysts in 38 horses. Equine Vet J 2022; 54:97-105. DOI: 10.1111/evj.13428].

The cystic lesions were drilled and curetted, and a screw was placed. Post-operatively, the horses received controlled exercise for 12 weeks. At 120 days, radiologic evaluation showed that 77% had filling of the lesions. Ninety-five percent were sound at 120 days, and 71% went on to race. The authors reported that there is no need for screw removal following use of the implant. 

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