The largest gathering of equine veterinarians can be found each year at the annual American Association of Equine Practitioners (AAEP) Convention. The 2018 Convention was held in San Francisco, California, and it was the 64th edition of the annual gathering.
In this article, we bring you selected reports from presentations at the 2018 AAEP convention, brought to you by Zoetis. You can find four additional articles providing coverage of equine health topics from the AAEP Convention brought to you by Zoetis here:
Infectious Disease Updates
At a table topic session at the 2018 AAEP Convention, Katie Flynn, BVMS, equine staff veterinarian for the California Department of Food and Agriculture, and Angela Pelzel-McCluskey, DVM, MS, equine epidemiologist of surveillance, preparedness and response Services at the USDA-APHIS-Veterinary Services, led a discussion on infectious disease updates.
Monitoring Disease—It was mentioned that careful surveillance of infectious diseases in the horse world is ongoing and regularly updated. The Equine Disease Communication Center (EDDC) is available to all veterinarians to help keep track of outbreaks. It also allows the public to receive current information on disease status and assists veterinarians in providing the best health advice to clients. It is accessible at equinediseasecc.org.
Herpesvirus—Of special interest to equine practitioners are the continued outbreaks of equine herpesvirus (EHV-1) in its neurologic form (equine herpesvirus myeloencephalopathy or EHM). Flynn described some updated guidelines in regard to EHV-1 infection:
- Some horses are not following the textbook expectations. Even though febrile, the individuals test negative until two days later, so it might be necessary to test again.
- In the face of an outbreak, horse owners want all the horses tested, but Flynn emphasized that only those with clinical signs should be tested.
- Some horses with confirmed EHV-1 had temperatures of 100-100.80 F because they were being treated with firocoxib for musculoskeletal issues. Firocoxib lowers temperature longer than flunixin, so it is possible to miss the fever stage. Flynn recommended keeping a temperature log that states that the horse is on NSAIDs. Treated horses might mean that a lower temperature threshold should be considered for EHV-1 testing.
- It is also important to understand the rules and regulations of your state. Samples from private labs must go to the regulatory lab, and this could delay implementation of quarantine. The horse owner must not remove horses from quarantine until the regulatory lab processes the sample and obtains results.
- There is a requirement for two negative tests on a positive horse before it can be released from a barn. Findings suggest that it might take 36 days rather than 10-11 days to confirm negativity for infection.
Bettina Wagner, DVM, has focused on equine immunology and has been instrumental with her team at Cornell University in developing a diagnostic test for Lyme disease. She commented on a new EHV-1 multiplex assay using nasal swabs and serum samples that might be able to categorize the stage of herpesvirus (EHV-1) infection. While this test is not yet available for field application, it could be helpful in future to determine the exact duration necessary for quarantine.
Wendy Vaala, VMD, DACVIM, of Merck Animal Health, noted that EHM appears to affect multiple organ systems—lungs, spinal cord, eyes, uterus and testes (which can create fertility issues).
Lyme Disease—An updated ACVIM Consensus Statement was published in February 2018 and is available on the ACVIM website or at the link above.
Ticks and Lyme Disease—There is now a website devoted exclusively to tick surveillance: National Equine Tick Survey at equineticks.org.
The group is asking for practitioners to send in ticks from anywhere in the country; then the website will post monthly updates. Cornell is also PCR testing ticks for Anaplasma phagocytophilum and Borrelia burgdorferi.
Cystic Lyme Disease—Thomas Divers, DVM, DACVIM, DACVECC, of Cornell University, brought new details to light about Lyme disease. Motile forms of Borrelia are susceptible to treatment with tetracycline, minocycline or ceftiofur; however, a cystic form is recognized, which, while not necessarily the cause of a problem, is difficult to treat with antibiotics.
Minocycline is only 22-30% bioavailable in horse, whereas in humans and small animals it is 100% bioavailable and penetrates the CNS. Divers noted that once tetracycline reaches the lower gastrointestinal tract, especially if the tract is filled with forage, there is a limited effect on the microbiome, and it might be safer than previously thought. IV tetracycline can be used for seven to 10 days and renal function monitored; then the horse can be switched to doxycycline or minocycline for the remainder of treatment.
Lyme Vaccine or Exposure?—It is possible to tell the difference between Lyme disease vaccine and exposure—with natural exposure, the OSP-A antigen stays elevated while vaccine OSP-A declines.
Exposure to Lyme disease might not necessarily imply disease. It is important to rule out other disease types and causes for noted clinical signs. Some cases treated with doxycycline or minocycline might show improvement because these medications have anti-inflammatory properties even if a horse is not infected with Borrelia. Without a confirmed diagnosis, treatment response might lead to a false positive speculative diagnosis of Borrelia.
Optimizing Risk-Based Disease Protection
Immunization is a large part of an equine practitioner’s horse care regimen, so it helps to know whether one’s method of vaccinating is efficacious to minimize risk of disease.
At the Zoetis Sunrise Session at the AAEP Convention, a panel consisting of Thomas Divers, DVM, DACVIM, DACVECC, of Cornell University; Harold McKenzie, DVM, DACVIM, Virginia-Maryland College of Veterinary Medicine; Tom Chambers, PhD, University of Kentucky Gluck Equine Research Center; and Mark Crisman, DVM, DACVIM, part of the Zoetis Professional Services team, discussed updated information about equine influenza and Potomac Horse Fever.
Influenza Immunity—Vaccinations can be administered either as a standalone antigen in one syringe, or as a polyvalent combination of multiple antigens in one syringe. One study compared immune responses for equine influenza (EIV) with or without equine rhinopneumonitis (EHV) given separately in a different muscle group from core vaccines (tetanus, West Nile virus and rabies).
The EIV-EHV combo given alone in its own muscle group consistently provided better cross-reactive immune responses compared to its inclusion in the polyvalent vaccine. This was true regardless of the vaccine manufacturers (Zoetis, Merck, Boehringer-Ingelheim) represented in the products evaluated.
In summary: For better influenza protection, EIV or EIV-EHV is best administered separately in its own syringe apart from core antigens.
Potomac Horse Fever Immunity and Seasonality—A 91-horse study in southwest Virginia compared the use of monovalent versus multivalent Potomac horse fever (PHF) vaccines. Separate, single monovalent PHF vaccine and monovalent rabies in two di_ erent injection sites were administered to 45 horses, while a single multivalent vaccine that combined PHF and rabies in the same injection was given to 46 horses.
Both vaccine types exhibited poor immunogenicity. Only one-third of the horses seroconverted to a PHF titer >1:4 at any time point post vaccination. Compared to the multivalent product, the monovalent PHF exhibited significantly greater immunogenicity in regard to the number of horses that seroconverted for the first two months following vaccination. However, by three to four months post-vaccination, there was not much difference between the two vaccine methods.
PHF used to occur mostly in late spring and summer, but now cases are extending into fall and early winter. Because the immune response—however much can be achieved—to PHF vaccine lasts only about three months, it is now recommended to booster the vaccine at the end of summer (August).
Another interesting finding about the risk of PHF has to do with insect attractants such as lights. The panel experts recommend turning off night-lights near horses—especially those around water troughs—to avoid luring aquatic insects.
In addition, drought conditions are associated with outbreaks of PHF. Even one wet area in a pasture encourages accumulation of aquatic insects in those areas. Horses also like to congregate around water, where pasture grass might grow better. Keeping horses away from the wet areas might help reduce PHF transmission.
It was also noted that anaplasmosis clinical signs appear similar to PHF clinical signs, and the disease occurs in similar geographic areas. Therefore, it is important to accurately diagnose illness in horses in regions where both diseases occur.
Endocrine Disease in the Aged Horse
Endocrine disease is front and center in many equine practices as horses are aging well into their golden years. One report noted a five to tenfold increase in recent numbers of geriatric patients seen in referral hospitals.
Middle-aged and older horses are prone to developing pituitary pars intermedia dysfunction (PPID). Any age horse can develop equine metabolic syndrome (EMS), especially if obese. A lot of attention is focused on managing these endocrinopathies because of their high association with laminitis. It is reported that more than 90% of laminitis cases seen in the field (outside of sick horses in veterinary hospitals) are a result of endocrinopathies.
At the AAEP Convention, an in-depth session revolved around endocrine disease, particularly as it relates to the geriatric horse. Presenters included Elizabeth Tadros, DVM, PhD, DACVIM, of Michigan State University; Dianne McFarlane, DVM, PhD, DACVIM, of Oklahoma State University; and Catherine McGowan, BVSC, MACVSc, PhD, DEIM, DECEIM, FHEA, MRCVS, of the University of Liverpool.
Aging in horses is associated with decreased insulin sensitivity, an increase in inflamm-aging, and decreasing levels of adiponectin, which is important in regulating glucose levels by increasing insulin sensitivity. In many cases, there can be converging endocrinopathies, according to Tadros.
McFarlane described PPID as an age-related disease, with 20-30% of horses greater than 15 years old testing positive for PPID. Prevalence increases with each year of age. In one study of 450 pituitary glands, no horse under 10 years of age had PPID. Thrifty horses without PPID have a more responsive pars intermedia than lean horses.
It has been proposed, but is still lacking evidence, that a horse with equine metabolic disease might be predisposed to pars pituitary intermedia dysfunction, especially if older than 15 years.
PPID comes on over time due to tissue damage from oxidative stress and accumulation of intraneuronal cellular waste products. Affected horses lack energy, experience hair coat changes such as hirsutism, abnormal shedding and color changes. Muscle mass atrophy, secondary bacterial infections and foot abscesses are common, as are changes in mentation. In addition, PPID is also associated with suspensory ligament degeneration, infertility and changes in sweating patterns—50% sweat extensively while a few experience hypohydrosis.
Obesity occurs in 30% of horses worldwide, according to McFarlane. She said, “If obesity, regional adiposity, easy keeper, laminitis of unknown origin or seasonal laminitis are the clinical signs, one should be concerned with insulin dysfunction (ID).”
The onset of PPID coincides with worsening clinical signs related to obesity, resulting in exacerbation of metabolic dysfunction to lead to hyperinsulinemia. Not all horses experiencing PPID also experience insulin dysfunction (ID), but ID occurs in about 30-60% of PPID horses. PPID concurrent with ID increases the risk for laminitis.
McGowan reported that horses with PPID are 2.7 times more likely to have hyperinsulinemia than age-matched controls. Prolonged hyperinsulinemia is associated with laminitis. In a study of 38 PPID horses and ponies, 29/38 (76%) developed laminitis. Overall, owners recognized only 14/38 (37%) as being laminitic. If the horses were hyperinsulinemic, 63% of owners were more likely to report laminitis. If only mildly hyperinsulinemic, only 18% of owners recognized laminitis.
McFarlane urges client participation to monitor progression of disease via reports of body condition score, shedding patterns, changes in muscle mass and the use of digital photographs for foot health. Observation of hoof rings, widening of the white line, the presence of dropped soles and foot radiographs help to monitor laminitis episodes.
Typical EMS signs include regional adiposity, thriftiness and obesity. These individuals are predisposed to endocrinopathic laminitis due to insulin exposure. Diagnosis of EMS is predicated on testing for ID. It is relevant that obesity is not always present with ID, since an owner might have favorably impacted the horse’s weight with diet and exercise or because a horse suffering from laminitic pain will lose weight. Still, it is important to test for ID, even in the face of managed weight loss, said McFarlane.
Diagnostic testing is important to help define just what you, as the practitioner, might be dealing with in order to implement the best treatment and prevention for laminitis. An initial thorough screening includes a complete blood profile and chemistries, followed by endocrine testing.
Tadros described different options to detect ID and PPID. Static testing measures a single blood sample at a single time, checking for resting insulin and endogenous ACTH levels. However, it is possible to miss mild cases of hyperinsulinemia this way.
A noteworthy point made by McFarlane: Specificity of the endogenous ACTH test is only 80%; i.e., 20% that test positive don’t actually have PPID, especially in the fall months, when ACTH normally elevates. She also pointed out that gray horses have higher ACTH concentrations than non-gray horses in the fall, as do horses under stress, such as occurs with illness.
By contrast, dynamic testing relies on hormonal challenge and evaluation of the endocrine response. One benefit of dynamic testing is that it reveals a horse that might be able to compensate homeostatically until the system is perturbed, as it is with a diagnostic endocrine challenge. The TRH (thyroid-releasing hormone) stimulation test is one such example of a dynamic test for PPID, as it amplifies ACTH response to TRH.
At this time, there are no established fall seasonal reference ranges; therefore, this test is best performed from mid-July to mid-November.
An overnight dexamethasone suppression test is another dynamic test for PPID—failure to suppress ACTH with dexamethasone indicates the presence of PPID. The oral sugar test—Karo syrup given after an overnight fast—looks at the response of glucose levels. The presence of hyperglycemia in the baseline and challenge sample is suggestive of diabetes mellitus, which can occur in horses with PPID.
An in-feed oral glucose test uses dextrose powder administered after an overnight fast; insulin and glucose are measured before and after feeding the dextrose.
An insulin tolerance test is performed without fasting. Insulin is given IV, blood is collected 30 minutes later, and glucose is measured. Glucose should decrease to 50% or less of baseline in response to the insulin challenge. If it does not, then it is likely that the horse has peripheral insulin resistance, as well as decreased tissue responsiveness to insulin; i.e., a whole body effect of ID.
While abnormal shedding and a hirsute hair coat are indicative of PPID, Tadros discussed the practicalities of testing a hirsute horse. This is appropriate to achieve baseline values and compare those to treatment response after one to two months on pergolide, she said. Reports advised that 75% of treated horses respond to pergolide therapy. It is important to counsel clients that a horse cannot compete in some venues while taking pergolide and that there might be a specified withdrawal period from the medication.
Overall, McFarlane stressed an integrative approach to diagnosis of an endocrinopathy. Information should come from multiple avenues: signalment, history, clinical signs and diagnostic testing.
Chronic Diarrhea
It is not uncommon for equine practitioners to be called by a client who is upset that his or her horse is passing loose manure. At the 2018 AAEP Convention, a table topic led by Luis Arroyo, LVM, DVsc, PhD, DACVIM, of the University of Guelph, and Ashley Whitehead, DVM, BSc, DVSc, DACVIM, of the University of Calgary, focused on management of cases of chronic diarrhea. They noted that a true diagnosis is obtained in less than 40% of diarrhea cases and that often it is necessary to work through a process of exclusions.
For diarrhea to occur, Whitehead suggested that there must be a problem in the large intestine in addition to the small intestine. It might have to do with aberrations in motility and transit time such that fecal water isn’t absorbed and instead goes out through the rectum.
Often diarrhea cases are associated with some form of dysbiosis not necessarily related to primary intestinal pathology, but rather to stress (training, transport, herd dynamics) or to medications. NSAIDs given within the past 30 days are known culprits in modifying the microbiome. Trimethoprim-sulfa antibiotics alter intestinal microflora for a month.
Whitehead explained that horse owners have a couple of options to help track a potential cause and to alleviate loose stools.
A skin test can be performed for dietary allergens, including grass hay. There are times when changing forage (such as occurs when moving a horse from pasture to hay) can upset the intestinal flora. Dietary exclusion testing might find the source of the problem or help remedy the intestinal changes.
Transfaunation is a treatment that has been successful in managing colitis or intestinal dysbiosis that occurs due to changes in microflora from changes in feed.
Feces are collected from a healthy horse and are placed in a stocking net tied at the top. This is added to 10 liters of pre-warmed salt water (salt added to make a physiologic saline solution) in a metal bucket. The manure is “massaged” in the bucket to the color of black tea. The stocking-net of manure is discarded, and about eight liters (if possible) of the supernatant is given via nasogastric tube to the diarrheic horse. This treatment might be administered daily for three days, although results might be achieved with a single dose.
Dietary Supplement Effects on Insulin and Adipokine
High circulating levels of insulin are known to trigger laminitis in horses. One protein, adiponectin, is important to enhancing insulin sensitivity through increased fatty acid oxidation and inhibition of liver glucose production. Obese individuals tend to have lower adiponectin levels and are more at risk for insulin dysfunction (ID).
Human studies have found that a plant polyphenol (resveratrol, which is found in foods such as red grapes) improves insulin sensitivity and mimics the effects of calorie restriction. In rats, combining resveratrol with leucine facilitated improvements in insulin sensitivity with smaller amounts of resveratrol.
Jane Manfredi, DVM, MS, PhD, DACVS-LA, DACVSMR, of Michigan State University’s College of Veterinary Medicine, presented at the 2018 AAEP Convention on a supplement for horses with equine metabolic syndrome (EMS) and insulin dysregulation (ID). The supplement contains a synergistic combination of resveratrol and an amino acid complex containing leucine (SPAAB+L).
Fifteen adult horses with EMS and ID were in the study. Body condition scores were 7 out of 9, and the average age was about 10. The SPAAB+L supplement was administered for six weeks at either a high dose or low dose. Testing was done July through August, with analysis pre- and post-supplementation for insulin with an oral sugar test (OST), BCS, weight, adiponectin, triglycerides, non-esterified fatty acids and tumor necrosis factor.
No difference in results was seen between horses fed high or low doses of SPAAB+L supplement. While the horses lost an average of 20 kilograms in weight, there were no changes in body condition score. However, Manfredi suggested that there might have been loss of visceral fat, which is known to contribute to diabetes in humans. Weight loss might be explained by the “browning” of fat, which is fat packed with mitochondria. This is comparable to fat browning triggered by cold temperatures— there is increased metabolic activity and calorie burn.
Manfredi explained that the microbiome of EMS/ID horses is different from normal horses, and resveratrol alters the gut microbiota with beneficial effects. Supplementation with resveratrol and leucine resulted in higher levels of adiponectin, as well as lower insulin concentrations at 60- and 75-minute time points with the OST. Use of this supplement is meant as a complementary treatment for EMS/ID.
Oral Sugar Test
Obesity in horses is a problem worldwide, ranging from 20-51%. Also noteworthy: It is thought that 89% of laminitis cases might be due to underlying endocrinopathy. An experimental infusion of insulin in healthy horses elicited laminitis. Pasture carbohydrates correlate with high basal insulin levels in the spring and fall. Therefore, it is suspected that grazing horses are likely experiencing hyperglycemia and hyperinsulinemia.
At the 2018 AAEP Convention, Erica Macon, BS, MS, PAS, of the University of Kentucky Gluck Equine Research Center, reported on a study on the seasonal effects on body weight, body condition score (BCS), crest neck scores (CNS) and insulin responses to the oral sugar test (OST). Physical features and OST responses of 11 insulin-dysregulated horses (ID) were compared to 11 healthy non-insulin-dysregulated controls (NID). All horses in the study were tested for ACTH levels, and PPID was ruled out.
The results were informative:
- The body weight of both groups (ID and NID) did not alter with season.
- The BCS and CNS scores were higher in the winter for both the NID and ID horses compared to spring and fall.
- Season did not affect responses to the OST in the NID horses but did in the ID horses. Responses to the OST among ID horses were higher in the winter and spring.
- Pasture protein and NSC (nonstructural carbohydrates) varied between seasons; hay did not.
