Equine Malignant Hyperthermia

We've all heard of people and animals that are "sensitive" to anesthesia. Monica Aleman, MVZ, PhD, Dipl. ACVIM, head of the neuromuscular disease lab at the University of California, Davis, reported on a genetic problem called equine malignant hyperthermia (EMH) that can make simple anesthesia deadly for some horses. Malignant hyperthermia-like episodes in the horse have been associated with drugs such as succinylcholine, halothane, isoflurane, and caffeine.

It has been shown that a mutation in the ryanodine receptor 1 (RyR1) gene causes a dysfunction in the sarcoplasmic receptors of skeletal muscles, and that results in the excessive release of calcium into the myoplasm. This triggers a series of events that result in a hypermetabolic state and/or death. Symptoms include hyperthermia (fever), hyperhidrosis (excessive sweating), tachycardia (high heart rate), tachypnea (shallow breathing), dysrhythmias (abnormal heart rhythm), muscle rigidity, and death with acute rigor mortis.

Her study was the first to identify a mutation in the RyR1 gene in the horse; it is unknown if this mutation is responsible for all cases of EMH. A benefit of this discovery is the creation of a faster and more direct screening test to identify susceptible horses, said Aleman.

She currently is accepting samples from horses that practitioners think might suffer from EMH. She can accept hair and whole blood, but prefers muscle. Practitioners should also send signalments and the pedigrees of affected horses. Aleman can be contacted at mraleman@ucdavis.edu.

More information: www.TheHorse.com/ViewArticle.aspx?id=5426.


In 2004, the hereditary disease hyperelastosis cutis (HC), also known as hereditary equine regional dermal asthenia (HERDA), received much attention. One of the leading researchers into this disease is Ann Rashmir-Raven, DVM, MS, Dipl. ACVS, associate professor in the department of clinical sciences at the Mississippi State University (MSU) College of Veterinary Medicine.

In her update, she outlined the specifics of the disease, which is an autosomal recessive connective tissue disorder of economic importance in Quarter Horses and horses of Quarter Horse lineage. It occurs most frequently in popular cutting horse bloodlines. (She noted that in the cutting horse industry alone, 12 of the top 100 cutting sires are confirmed carriers. These 12 stallions have sired about 7,579 total registered foals; approximately 3,790 will be carriers.)

Affected horses have extremely fragile skin that tears easily and exhibits impaired healing. In horses with HC, the skin separates between the deep and superficial dermis. There is no cure. A DNA test for carrier identification is not yet available, so managed breeding strategy is currently the only option for reducing the incidence of HC.

Rashmir-Raven has traced the defects to Poco Bueno and his immediate ancestors. She said Poco Bueno's full brothers Captain Jess and Old Grandad also were likely carriers. Poco Bueno's sire, King, has genetic ties to 4% of known affected horses. In pedigree evaluation, having Poco Bueno or King in both the top and bottom of a pedigree within six generations has a significant correlation with the occurrence of HC.

Rashmir-Raven discussed a study of 44 affected horses. She has up to 24 affected horses in her breeding herd at any given time, 15 additional horses were evaluated and euthanatized, and some client animals continue to be maintained by their owners. In addition, she has evaluated seven confirmed carriers and more than 150 additional pedigrees of affected horses from the United States, Canada, Brazil, and Europe.

She noted that in comparison to other horse populations, HC horses had higher incidences of cryptorchidism, more colic, more respiratory disease, more ophthalmic diseases (including uveitis), and the foals were not as thrifty.

Rashmir-Raven said while there is no known cure for HC, palliative therapy can improve the quality of the affected horse's life. Horses are less apt to develop lesions during the winter; thus suggesting, said Rashmir-Raven, that a solar- or heat-related stress to the skin might contribute to the development or progression of lesions. She said the skin of affected horses improved significantly when they were kept indoors and away from other horses.

Recognizing more subtly affected horses will be important until a DNA test can be developed to positively identify affected and carrier horses.

In the question and answer session, Rashmir-Raven was asked about the use of Cosequin (a joint health supplement) for HC horses. She said that there was anecdotal evidence from owners of affected horses that giving Cosequin was helpful. Recent studies on fibroblasts, cells that make connective tissue (such as collagen), suggest that there might be some skin benefit with Cosequin usage.

More information: www.TheHorse.com/ViewArticle.aspx?id=5425.

R. Equi Pneumonia in Foals

Rhodococcus equi is the most common cause of severe pneumonia in foals, said Keith Chaffin, DVM, MS, associate professor of medicine at Texas A&M University's College of Veterinary Medicine. However, despite intensive research, the reasons some foals get this pneumonia and some don't are not known. A study from two breeding farms in Texas with endemic problems of R. equi has helped shed light on this problem.

Chaffin said data from this study showed peripheral blood lymphocyte (PBL) populations in two- and four-week-old foals were different for foals that later develop pneumonia than for foals that don't develop it. The differences were most pronounced during the second week of life, which is the period when foals most commonly become infected with the bacterium. Chaffin said it is possible that there is a subset of foals that are relatively immunosuppressed compared with other foals, and thus are more susceptible to infection with R. equi.

A new finding from this study was that total white blood cell counts and segmented neutrophil concentrations in peripheral blood for foals that later develop R. equi pneumonia were significantly lower than those of unaffected foals at two and four weeks of age.

More information: www.TheHorse.com/ViewArticle.aspx?id=5427.

Neonatal Septicemia

Septicemia is loosely defined as bacteria or bacterial components in the blood. In foals, septicemia is the most common cause of death in the first week of life, noted Simon Peek, BVSc, MRCVS, PhD, Dipl. ACVIM, clinical associate professor of large animal internal medicine, theriogenology, and infectious diseases at the University of Wisconsin's School of Veterinary Medicine. In a study of 56 septicemic foals, he said a sepsis score that gave a numeric value to a number of variables was the most reliable indicator of survival rate. However, foals that had bacteria cultured from blood had a lower survival rate (57%) compared to foals that didn't have a positive blood culture (72%). These survival numbers were higher than those found with previous studies.

He said that under conditions where finances allow, administration of broad-spectrum antibiotics and hyperimmune equine plasma, along with referral to a facility that can provide the intensive support care needed, might result in a positive outcome in up to 72% of all cases.

More information: www.TheHorse.com/ViewArticle.aspx?id=5428.

Oral Antibiotic

When a horse has to be on an antibiotic for long periods, the oral route of administration is preferable. However, the choices available to horse owners are limited. Cornelia D. Nieuwoudt, PharmD, director of the pharmacy at Texas A&M University and formerly with Johns Hopkins University, discussed using a gel formulation of the antibiotic enrofloxacin in horses.

Nieuwoudt said enrofloxacin offers a broad spectrum of activity. For her study, she used a cattle enrofloxacin injectable that she reformulated into an oral, flavored gel. She believes this would not be difficult for veterinarians in the field to do.

She found that the formulated enrofloxacin gel maintained its full potency for three months (the length of her trial) if it was protected from light and stored at room temperature, or if it was stored in a refrigerator. Refrigeration is preferred over room temperature. When exposed to heat, the formulation was not very stable and lost potency very quickly. Nieuwoudt also found that the active ingredient was absorbed by the horse via the oral route and produced serum concentrations sufficient to treat infections caused by a variety of bacteria. However, it is always prudent to use an antibiotic with the narrowest spectrum of activity.

While there were few complications, some horses developed mouth lesions, which could be avoided by rinsing the horse's mouth with water after gel administration. Although not observed in this trial, the use of any antimicrobial can, in some instances, result in diarrhea. She said it is advisable to schedule dosing after fasting the horse for 12 hours, which is not difficult if the horse is stalled overnight.

More information: www.TheHorse.com/ViewArticle.aspx?id=5430.

Cushing's Disease

A general session on medicine included Babretta Breuhaus, DVM, PhD, associate professor of equine medicine in the department of clinical sciences at North Carolina State University, discussing a review of thyroid function and dysfunction. Hypothyroidism (underactivity of the thyroid gland) often is a mis-diagnosed malady, she said, and horses with clinical signs of hypothyroidism often have normal thyroid function. Thyroid dysfunction is not well understood, and the "prevalence of true hypothyroidism in horses is unknown," she added.

Often, she said, horses suffering from laminitis, obesity, and poor fertility are administered thyroid hormone supplementation. But in many of these cases proper documentation or accurate diagnosis of hypothyroidism is non-existent. As a result, owners spend a good deal of money on therapy that does no good and can actually do harm to the horse, she told the group.

She then discussed normal thyroid function as well as hyperthyroidism (overactivity of the thyroid gland) and hypothyroidism. Hyperthyroidism in horses, she said, is rare. Hypothyroidism might also be quite rare, but often becomes the diagnosis when a horse is overweight with a cresty neck, has reduced exercise tolerance, or is predisposed to suffer from mild, recurrent bouts of laminitis.

"Horses fitting this description have traditionally been supplemented with thyroid hormone medication, and some clinicians continue to do so at this time with or without evaluating the status of thyroid function first," she said. "However, currently there is no good documentation that thyroid hormone supplementation in these horses results in weight loss, reduction of fat deposits, or decrease in the frequency of bouts with laminitis. More recent information suggests that the clinical signs in these horses are more likely associated with Cushing's disease or a newly described condition that has been termed equine metabolic syndrome."

Frank Andrews, DVM, MS, Dipl. ACVIM, professor of large animal clinical sciences at the University of Tennessee (UT), discussed a new approach to testing horses for Cushing's disease. One of the standard tests has been the dexamethasone suppression test. Andrews said the test has produced both false positives and false negatives.

UT researchers did a study involving a dexamethasone suppression/thyrotropin-releasing hormone stimulation test to diagnose early cases of Cushing's. In this study involving 44 horses, Andrews said the combined test proved more accurate than the dexamethasone suppression test alone.

Urinary Incontinence

Harold Schott II, DVM, PhD, Dipl. ACVIM, associate professor in the department of large animal clinical sciences at Michigan State University (MSU), said there can be a number of causes of urinary incontinence. The bad news is that in a MSU study of 37 horses, only 20% of those afflicted could be treated successfully.

Mucus Accumulation

Joel Lugo, DVM, MS, Dipl. ACVS, of MSU discussed a study that looked at the accumulation of mucus in horses with heaves. Therapy targeted at reducing inflammation during the attacks also reduced the amount of mucus buildup, thus helping alleviate obstruction.

More information: www.TheHorse.com/ViewArticle.aspx?id=5442.

Pigeon Fever

This disease has nothing to do with birds; it got its name from the inflammation seen in the chest region of some affected horses that makes them look like they have a puffed-out pigeon's breast. Also known as dryland distemper, the disease is caused by Corynebacterium pseudotuberculosis. Research at the University of California, Davis, has indicated that insect vectors play a significant role in the transmission of this disease in horses.

C. pseudotuberculosis can also cause ulcerative lymphangitis (elephant leg), subcutaneous (beneath the skin) abscessation, internal abscessation, bacteremia (the presence of bacteria), and abortion in horses.

Betsy Vaughan, DVM, of the University of California, Davis, discussed using ultrasound to diagnose systemic (internal) C. pseudotuberculosis infections of abdominal organs, which can occur concurrently with typical outward signs of the disease or might not be associated with external abscessation. Horses with systemic infections had clinical signs of depression, lethargy, anorexia, weight loss, fever, and colic. She said ultrasound also is useful to monitor treatment and resolution of internal infection involving abdominal organs.

Vaughan noted that C. pseudotuberculosis infection is one of the most common diseases diagnosed in California, and it is also present in other parts of the world, including Texas and Colorado. "The recent spread of the external form of the disease into states not previously affected should make practitioners more aware of the possibility of internal infection in states outside California," she said. "Many cases may go undiagnosed without the use of ultrasound."

More information: www.TheHorse.com/ViewArticle.aspx?id=5429.


Next on tap was a discussion of the impact a salmonella outbreak can have at a veterinary hospital and what can be done to prevent it. Magdalena Dunowska, DVM, PhD, postdoctoral fellow in the veterinary teaching hospital at Colorado State University (CSU), said the James L. Voss Veterinary Teaching Hospital at CSU was hit with salmonella outbreaks in 1996 and 2001. The 1996 outbreak, she said, resulted in the hospital being closed for three months at a cost of more than $500,000.

As part of a study to determine how to cut down on harmful bacteria, a footbath for workers was compared to disinfectant misting, she said. The ammonia-based solution in the footbath did little to alleviate the harmful bacteria, but fogging the facility with 180 liters of 4% peroxygen disinfectant with backpack foggers did have a highly positive effect. The peroxygen formula appears to be safe for animals, Dunowska said, but there have been no scientific tests authenticating that belief. Because of the lack of data, it is suggested that all animals be removed from the facility during the fogging procedure.

Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, professor of clinical sciences at CSU, evaluated hygiene protocols and reducing bacterial load on the hands of staff performing routine examinations. This study compared washing hands for 15 seconds with antibacterial soap, application of a 67% alcohol-based hand sanitizer, and using a chlorhexidine-alcohol lotion.

The result: "The alcohol-based sanitizer and the chlorhexidine-alcohol lotion reduced the bacterial load on hands after a physical examination more than washing hands for 15 seconds with antibacterial soap containing 0.3% triclosan," she said.

More information: www.TheHorse.com/ViewArticle.aspx?id=5432.

Wound Management

Ted Stashak, DVM, MS, Dipl. ACVS, emeritus professor of veterinary medicine and surgery at Colorado State University, and Laurie Goodrich, DVM, MS, Dipl. ACVS, of Cornell University, moderated a table topic on wound management.

Stashak said that for heavily contaminated and infected wounds, the wet-to-dry plain gauze or antimicrobial-impregnated gauze (Kerlix AMD antimicrobial dressing) can be effective, but it is often used too long.

For plain gauze, a dilute 0.01% (1 mL/ 100 mL) povidone iodine or 0.05% (1:40 dilution) chlorhexidine or physiologic saline solution alone are most commonly used to wet the material. If Kerlix is used, the gauze is moistened with saline. When applied to the wound surface, the moisture liquefies the wound exudate, allowing movement of fluid and bacteria through the gauze. As the dressing dries, fibrin from the wound bed causes temporary bonding of the dressing to the wound bed. As the dressing is peeled off the wound, fibrin, debris, bacteria, and necrotic (dead) tissue are removed (debrided) from the wound surface. Since the debridement process is non-selective, and healthy newly formed tissue can be damaged, he recommended using them for one or two bandage changes only. Once the wound appears clean, another dressing type is indicated.

He advised against using an occlusive dressing that promotes moist wound healing on an infected wound. It is better to use a dressing that "lifts and stores" the exudate (pus) away from the wound surface.

Stashak says many newer dressings are designed to create a moist wound healing environment, which allows the wound fluids to remain in contact with wound. A moist wound free of infection provides an environment rich in white blood cells, enzymes, and growth factors beneficial to wound healing. The enzymes released primarily from the white blood cells cause "autolytic debridement" of the wound (the wound cleans itself), which appears to be selective for necrotic (dead) tissue. Under these dressings, autolytic debridement usually occurs 72-96 hours after wounding, thus cleaning the wound in preparation for the repair phase. Wound healing is stimulated by growth factors.

The wound healing product Acell was discussed. Stashak concluded that he thinks there is a place for Acell therapy in wound management, but that its use needs to be better defined.

More information: www.TheHorse.com/ViewArticle.aspx?id=5452.


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