AAEP Convention 2001: General Medicine
Hormone Responses to Feeds
Joe Pagan, PhD, owner of Kentucky Equine Research in Versailles, Ky., discussed glycemic response in growing horses as an indicator of developmental orthopedic disease. He said looking at diet and skeletal disease was nothing new--a study done at The Ohio State University (presented at the 1985 AAEP convention) changed the way horse owners feed young, growing stock. That study defined the relationship between copper and zinc to lower the incidence of physitis and wobblers, which are part of developmental orthopedic disease.
However, even after this feeding regimen was accepted widely, the incidence of osteochondritis dissecans (OCD) did not decrease as dramatically. Thus, researchers started looking for other nutrition factors.
Research done in 1987 found that a high carbohydrate intake resulted in endocrine (hormonal) changes in the horse's system. The theory was that some horses had a genetic sensitivity for developing OCD (much like some families have a higher cancer rate than others). The researcher theorized that if the young, genetically sensitive horse was also faced with obesity or excessive exercise, or by mineral deficiencies, he would be more likely to develop OCD. (As a refresher, when bone forms, cartilage grows first, then it becomes mature cartilage, then it ossifies to bone.)
"With higher insulin and hormone levels and altered levels of other hormones, the cartilage does not mature and ossify to bone," said Pagan.
Research in humans and horses showed that high insulin suppresses chondrocyte (cartilage cell) maturity. "That might be a contributing factor to OCD," said Pagan.
Research at Rutgers by Sarah Ralston, VMD, PhD, in the mid-1990s resulted in a patent for a glycemic response test for OCD prediction in young horses. Pagan licensed that patent from Rutgers to look especially at the predisposition of Thoroughbred foals to developing OCD lesions.
With 218 weanlings on six farms, he looked at the incidence of surgically treated OCD. He found that radiographic OCD lesions often resolved themselves, so he determined that only those lesions the farm manager and veterinarian deemed significant enough for surgery would be included in the study. "A lot of the lesions that weren't operated on after they were found in December or January were gone by July," said Pagan.
The test on these weanlings involved analyzing their feed's carbohydrates, then taking blood samples to measure plasma glucose and insulin after the weanlings were fed a standardized meal. Of the 218 weanlings, 25 had surgery for OCD lesions (eight in stifles, seven in hocks, five in multiple joints, four in fetlocks, and one in the shoulder). Half of the lesions were identified with radiography, not lameness, but still underwent surgery. Pagan found that the 25 weanlings which had surgery had insulin and glucose levels significantly higher than the group which didn't have surgery.
Then Pagan noticed something else--the differences correlated with the feeds the weanlings ate. There was a big difference among farms in glucose and insulin levels and OCD occurrence, ranging from 0% OCD on one farm to 32% on another farm.
"Carbohydrates are not all the same," said Pagan. "What we found is that we could measure the glycemic index of the farm's feed and predict which farms were most at risk for foals to develop OCD. Body weight and condition also appeared to be involved in lesion development."
Pagan said that based on this study, it is prudent to feed foals concentrates that produce low glycemic responses. "If you need X amount of calories, then feed lower glycemic carbohydrates and more alternative energy sources, such as supplemental fat or fermentable fiber," he said.
"A lot of the glycemic responses are due to the speed the horse eats its feed, " said Pagan. "A horse that bolts his feed has a higher glycemic response."
Pagan found that the size of the pellets in pelleted feeds also affects the animal's glycemic response--the smaller the pellet, the higher the glycemic response.
Currently, 10 Thoroughbred farms in Kentucky are feeding a specially designed diet that produces low glycogen response rates. A large field trial will be undertaken in 2002 involving 16 farms. Half will receive traditional feed, and the others are already on low-glycemic feeds.
"The horse in nature would never have glucose or insulin spikes after eating because of their feedstuffs," said Pagan. "We want to mimic hormone responses in young horses as if they were raised only on grass. I predict a big change in the way growing horses are fed, with lower starch and lower glycemic formulas."
The definition of probiotics is "living microorganisms, which upon ingestion in certain numbers exert health effects beyond inherent basic nutrition." Probiotics have become more popular due to their perceived ability to reduce the use of antibiotics, their degree of safety, and an increase in the public's desire for more natural products. In "A Review of Probiotics: Are They Really Functional Foods?" Scott Weese, DVM, DVSc, Dipl. ACVIM, of the University of Guelph in Ontario, Canada, presented the history and basic concepts of probiotic use in human and equine medicine.
In horses, it is thought that probiotics might be most useful in preventing and treating antimicrobial-associated diarrhea, treating Clostridial colitis, salmonellosis, chronic diarrhea, preventing nosocomial diarrhea, and preventing acute infectious (especially rotaviral) diarrhea in foals.
Weese said that certain probiotics might be effective in treating or preventing certain conditions; however, all probiotics will not be effective for all conditions. Unfortunately, there is little information on what is effective in the horse or what dosage should be used. Assumptions cannot be made from human medicine, nor from research done on other species--some probiotics will react differently in the equine system, and some might not have any effect at all.
Several problems associated with probiotics include lack of regulation, lack of efficacy studies, and poor quality control. These problems make it difficult to determine dosages, which are critical for ensuring that colonization of the organism has taken place. Also, dosage will probably vary for compromised versus healthy horses, and sometimes the dose required for colonization might be impractically high, meaning that a horse wouldn't be able to ingest that much. Therefore, the search for equine-origin organisms continues, since using a species-specific organism might produce better results.
Since no studies have shown probiotics to be effective in horses, it's hard to make specific recommendations. If a probiotic is to be used, Weese recommended that organisms that have been shown to be effective in other species would be preferable. These include Lactobacillus acidophilus, Lactobacillus rhamnosus, and Lactobacillus casei. However, because probiotic effects are specific to individual strains in these species, it's impossible to say if they will be effective without equine research. Weese recommended that horses should be fed at least 10-100 billion of the bacteria per day.
"Hopefully, future studies will provide insight allowing for more specific selection of probiotic preparations," said Weese. "In the meantime, an understanding of the general principles of probiotics and of their application in other species can allow the practitioner to make reasonable choices."
Diagnosis and treatment of Cushing's disease have advanced significantly; however, before 1999 no studies had compared the two most used medications--pergolide, a dopaminergic agonist, and cyproheptadine, a serotonin antagonist. Harold Schott II, DVM, PhD, presented "The Michigan Cushing's Project" during the General Medicine session at the convention.
In a study that ran from 1997 to 1999, Michigan State University and Michigan veterinarians followed 77 horses which were confirmed to have Cushing's disease based on the characteristic clinical signs and the results of either a low-dose dexamethasone suppression test (DST) or a thyrotropin-releasing hormone stimulation test (TRH). Horses were divided into three groups--a control (non-treated) group, those treated with pergolide, and those treated with cyproheptadine.
It was found that clinical improvement and a reversal of abnormal endocrine test results was better with pergolide than with cyproheptadine. In fact, treatment with cyproheptadine was found to be of little benefit when responses were compared with untreated horses.
In addition, it was found that measurement of plasma adrenocorticotropin (ACTH) concentration alone is not a reliable endocrine test for diagnosis of Cushing's disease, since both false positive and false negative results were obtained when compared to DST or TRH results for diagnosis.
White Line Disease/Canker
White line disease (hoof wall separation) and canker (chronic inflammatory hoof overgrowth, usually of the frog) are foot/ hoof concerns that are often linked to environmental conditions (i.e., unhygienic, high-moisture, and hot weather). Both were discussed in a roundtable discussion.
"It is interesting to note that 'white line disease' is relatively infrequent in hard-working horses," said Bill Moyer, DVM, head of the Department of Large Animal Medicine and Surgery at Texas A&M University and moderator of the session. "Today's horse is pretty much an apartment dweller, and this problem might be linked to lack of exercise and confinement, as it is unusual in horses that live outside. Horses I treated when I was younger simply did not experience this problem with any frequency."
Farriers and veterinarians discussed the efficacy of various treatments for drying out hoof tissues and killing the opportunistic bacteria present in both conditions. Formaldehyde was mentioned as an option; Moyer commented that it works, but can be dangerous if not properly diluted. He suggested using less dangerous agents in most cases.
Many veterinarians and farriers treat white line disease by resecting (cutting out) the separated hoof wall so that the infection can be aired out and easily treated. These often-extensive resections require antiseptic topical medication (such as iodine) and packing, which is usually changed daily. One attendee mentioned success with alcohol packing following resection.
Ric Redden, DVM, owner of the International Equine Podiatry Center in Versailles, Ky., cautioned that good communication with a client is essential to success of a
resection. "It's a big burden for a client," he warned. "Ninety days is about an average turnaround for these horses, with clients changing the bandages every day."
Attendees also asked about the effectiveness of hoof supplements and isoxsuprine in a white line disease treatment regimen. Moyer said that there is no evidence of a beneficial effect of either common treatment.
Racing Performance and Yearling Endoscopic Evaluation
Many have questioned the validity of a one- to two-minute respiratory endoscopic evaluation in predicting the future performance of the adult horse. Scott Pierce, DVM, and Rolf Embertson, DVM, both of the Rood and Riddle Equine Hospital in Lexington, Ky., conducted a study of 816 Thoroughbred yearlings to test the endoscopic exam's predictive value.
Pierce evaluated the yearlings' arytenoid cartilage function and epiglottic character, grading each on a scale of one to four with four being the worst measure. Of the 816 yearlings that went through the Keeneland and Fasig-Tipton sales in 1998, 19% had grade I arytenoid function and 79% were graded at II (both of which were considered normal), and 1.6% graded at III (no grade IV scores were given). There was no difference in racing performance (number of starts, earnings, or earnings per start) between grades I and II horses. Grade III horses had significantly fewer starts and less earnings during the 3-year-old year than grade I and II horses.
For epiglottic structure, 80% were normal, 14% grade I, 3% grade II, 2% grade III, and less than 1% grade IV. Grade IV horses were not compared to the others because there were too few (two horses). There was a significant difference between the earnings of 2-year-old grade II and III horses compared to normal horses, but the difference disappeared when comparing them as 3-year-olds.
Earnings per start were also significantly lower for these horses during the 2-year-old year, but again there was no difference at three years. The researchers theorized that physical maturation, surgery, and/or training might have all played a part in closing the gap between the horses with poor epiglottic structure and normal horses at age three.
One other finding in the study was that 74 horses with noticeably small airways had significantly decreased earnings at three years.
Nasal Strips: Beneficial or Not?
Two studies presented at the convention relayed different messages on the efficacy of an external nasal strip device (Flair nasal strips) in exercising Thoroughbreds. In one experiment presented by Ray J. Geor, BVSc, PhD, Dipl. ACVIM, nasal strips were shown to be effective in reducing exercise-induced pulmonary hemorrhage (EIPH). Gordon J. Baker, BVSc, PhD, MRCVS, of the University of Illinois, presented his study that showed the use of a nasal strip had no effect on equine pulmonary gas exchange, anaerobic metabolisms, or incidence of EIPH.
The adhesive Flair nasal strip is affixed above the horse's nostrils. Adapted from human use, it is designed to reduce nasal passage collapse and decrease transmural (involving the entire wall) lung pressure. This decrease in pressure also decreases the forces that are thought to cause EIPH (see article #938 at www.TheHorse.com).
Geor's study separately examined the effects of nasal strips and furosemide (a diuretic commonly used in racehorses that increases urine production, decreases blood volume and body weight, and lowers blood pressure in the lung). He also looked at the combined effect of the two. After high-speed treadmill exercise, bronchoalveolar lavage (BAL) was used to detect blood cells in the lungs of the horse. He found that the nasal strip reduced the red blood cell count by more than 40%. Combined use of furosemide and nasal strips significantly attenuated EIPH, although there was limited difference between the trials with furosemide and furosemide/nasal strip combination.
"It is important to recognize that neither treatment completely prevented EIPH," said Geor. "The nasal strips may decrease the overall work of breathing."
Baker's research revealed that the nasal strip did not improve exercise-induced hypoxemia (deficient oxygenation of blood) or hypercapnia (the presence of excessive amounts of carbon dioxide in blood), nor did it diminish lactate and ammonia production (indicative of strenuous exercise), or the incidence of EIPH.
"We were unable to detect any significant differences between the control horses and the ones wearing the nasal strips," said Baker. "Our observation showed no difference in the severity and occurrence of pulmonary hemorrhage."
Baker and Geor collected different types of data, used different means to evaluate it, and further research trials should help clarify the efficacy of nasal strips.
POLL: Managing Working Horses