acvim forum part ii

380 Journal of Equine Veterinary Science September 2004

Meeting Report

The 22nd Annual American College of VeterinaryInternal Medicine Forum was held inMinneapolis, MN, on June 9-12, 2004. As usual,an excellent group of veterinary clinicians and re-

searchers presented a variety of topics on equinemedicine. Summaries of some of the presentations werepublished in the August 2004 issue of JEVS. More sum-maries follow.

ENVIRONMENTAL AIRWAY DISEASE

Dr Bruce C. McGorum, from Scotland, gave a re-view of causal agents of what he described as “equine en-vironmental airway disease.” He said, “Conventional sta-bles often have high levels of airborne organic dust whichcontains potentially pro-inflammatory agents includingbacterial endotoxins, over fifty species of molds, foragemites, plant debris, and inorganic dusts. High levels oftoxic gases such as ammonia may also be present inpoorly ventilated stables. Respirable and total airborneconcentrations of organic dust, endotoxin, and -D-glu-can in the breathing zones of horses in conventional sta-bles may exceed the thresholds for induction of airwayinflammation and hyper-responsiveness in healthy hu-mans, and may exceed the air hygiene standards recom-mended for occupational exposure in man.”

While heaves (or recurrent airway obstruction) is themost commonly recognized equine environmental airwaydisease, there is increasing evidence that inhaled organicstable dust may also cause or contribute to the develop-ment of inflammatory airway disease. In 1 report, highdust environments prolonged the duration of inflamma-tory airway disease and increased the frequency ofcoughing. Inhaled organic dust likely also induces theupper and lower respiratory tract inflammation, whichhas been identified in non-heaves–affected horses afterstabling.

He also pointed out that stables may contain high air-borne concentrations of a wide range of molds (fungi and

0737-0806/$ - see front matter© 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jevs.2004.08.007

ACVIM Forum Part II

thermophilic actinomycetes). Molds contain numerouspotentially proinflammatory agents, including allergens,glucans, proteases, and mycotoxins. One study showedthat inhalation of aqueous extracts of Aspergillus fumiga-tus or Faenia rectivirgula induced a neutrophilic pul-monary inflammatory response and pulmonary dysfunc-tion in asymptomatic heaves horses. He argued that whilethis supports a causal role for these molds, as the re-sponse to these agents was less marked than that follow-ing hay/straw exposure, other agents in stable dust, suchas endotoxins, may also contribute to heaves. In contrast,it has been found that F rectivirgula inhalation inducesairway neutrophilia in control and heaves horses but in-duces airway dysfunction only in the heaves group.

Inhaled endotoxin is an important cause of inflam-matory airway disease in humans. The airborne endo-toxin concentration is the most important determinant forthe development and progression of airway disease inagricultural workers. In horses, inhaled endotoxin con-tributes significantly to the induction of airway inflam-mation and dysfunction in heaves. Evidence of the role ofendotoxin in inducing airway neutrophilia in heavesstems from the observation that depletion of endotoxinfrom hay dust suspension attenuates the airway neu-trophilia and abrogates the airway dysfunction induced inheaves horses by inhaled hay dust suspension.

Hay and straw may contain large numbers of foragemites, and airborne stable dust contains high concentra-tions of Lepidoglyphus destructor antigens. While foragemites may contribute to occupational asthma in humans,their role in heaves is unclear.

As horses with airway inflammation have airwayhyper-responsiveness, other nonspecific environmentalfactors such as airborne inorganic dusts, cold air, and dryair may be important determinants of the day-to-dayseverity of pulmonary dysfunction.

GLUCOSE AND INSULIN METABOLISM

Dr Sarah L. Ralston said that rations containinghigh-starch, high-glycemic-index concentrates have beenstrongly implicated as a cause of increased incidence ofdevelopmental orthopedic disease and other growth-re-lated disorders. There is increasing evidence that this in-

Volume 24, Number 9 381

creased risk may be related to high insulin responses tofeeding (hyperinsulinemia) and concomitant alterationsin growth hormone T-3, T-4, and IGF-1, either because ofhigh-glycemic indices of the feeds or defects or alter-ations in insulin sensitivity. It is hoped that a better un-derstanding of factors affecting glucose/insulin metabo-lism in young horses will lead to improved nutritionalrecommendations that will reduce the incidence ofgrowth disorders while supporting optimal growth.

She pointed out that mixtures of corn, oats, and bar-ley with molasses (“sweet feeds”) cause rapid (within 15minutes) increases in blood glucose and insulin and lowblood pH that are sustained for up 3 to 4 hours after feed-ing. Oat starch is more readily digested in the stomachand small intestine than is corn or barley starch, makingthe glycemic index of oats potentially higher than that ofcorn, though the higher fiber content of hulled oats prob-ably mitigates glycemic response. The reported glycemicindices of hulled oats and corn are roughly equivalent,

but “naked” oats cause more rapid and higher glucose re-sponses following feeding. Pelleting and extrusion ap-pear to affect the availability of carbohydrates. Pelletedand extruded concentrates tested to date cause lower glu-cose and insulin changes than textured feeds with thesame basic formulation. Concentrate feeds containing re-stricted amounts of molasses and starch and higher thanusual levels of fiber (12%-20%) and fat (5%-12%) havesignificantly reduced glycemic indices with concomitantreduction in postprandial glucose and insulin responses.Concentrates that contain >10% fat, however, adverselyimpact calcium absorption and bone mineralization inyoung horses.

Traditionally, the glycemic indices of hays and mostforages were assumed to be relatively low. It has beencommonly stated that “lush” spring pastures containhigher-soluble carbohydrates, contributing to the risk oflaminitis in adults and spring growth spurts in yearlings,though late fall pastures may contain even higher con-


centrations of soluble sugars than in spring. Forage con-tent of water-soluble nonstructural carbohydrates (NSC)can vary from <6% to as high as 33%, especially if theforages are stressed by drought or cold weather.

In a study conducted in Colorado, orchard grass,perennial rye, and timothy grasses had 27% NSC themorning following a freeze, whereas alfalfa had only 8%to 10% growing under similar conditions. By afternoon,the perennial ryegrass NSC had increased to 33%. It ispresumed that the predominant component of the NSC isfructan, which is not amenable to enzymatic digestionand is thought to contribute more to the colonic fermen-tative changes leading to laminitis than alterations in glu-cose/insulin responses to feeding. The glycemic indicesof high- versus low-NSC hays in young horses are cur-rently being investigated by Dr Ralston.

Horses adapted to forage have lower glucose toler-ance (decreased insulin sensitivity) to standardized dex-trose challenges compared with those fed rations con-taining 30% to 50% high-starch concentrates. In 1 study,weanling fillies adapted to a high-starch (45%) concen-trate fed at 50% of caloric intake had lower glucose andinsulin responses to a standardized low-dose oral dex-trose challenge (0.25 g dextrose/kg BW) compared withthose fed a restricted-starch (30%) high-fat (8%), fiber(20%) ration in equicaloric amounts. Serum glucagonslike peptide 1 (GLP-1), a gastrointestinal hormone thatstimulates pancreatic beta cell activity, was also in-creased in the high-starch-fed fillies.

As reported in other studies of high-fat/fiber rationsin horses, the high-starch ration had a significantly higherglycemic index and insulin response than the high-fat/fiber ration. In a study using 12 Quarter Horse wean-lings, there were no consistent differences in growth hor-mone or IGF-1 post-feeding in weanlings fed concentraterations (70% of total) containing 2.2% versus 10% fat,though Dr Ralston has found a large degree of variabilityin the responses that may have precluded detection of dif-ferences in the relatively small sample sizes. A study hasshown that glucose responses to the test meals are higherin 2.2% fat-fed weanlings after 30 days of feeding thanthose on the 10% fat ration,and insulin response washigher at 60 days.

The National Research Council (1989) recommendsthat weanlings receive rations that contain 70% concen-trates to meet their energy, protein, and mineral needsand that even yearlings receive 45% to 60% of their totalration in the form of concentrates. In a recent survey offeeding practices on 58 Thoroughbred and Quarter Horsebreeding farms, it was found that 62% of the farms fed50% or less of the rations provided to weanlings in the

form of concentrates. As little as 40% of the total caloricintake in the form of concentrates has supported normalto high growth rates in Belgian/Quarter Horse cross-weanlings fed hays of varying quality and nutrient con-tent. This translates to approximately 0.75% to 1.0% ofthe foal/weanling body weight per day, divided into 2 or3 feedings. Similarly, growth rates and bone densities ofThoroughbred and Quarter Horse weanlings fed a high-fat concentrate at only 50% of the total ration with freeaccess to grass hay did not differ from those offered 65%of their recommended intake in the form of a high-starchration. Researchers at Virginia Polytech Institute havebeen promoting the use of high-fat (8%-10%) and fiber(>12%) rations for young horses as a means to reduceglycemic/insulinemic responses to concentrates formu-lated for growth. For yearlings, good-quality roughagealone has supported moderate growth rates with no inci-dence of developmental orthopedic disease.

Dr Ralston recommended that nursing foals shouldbe introduced to concentrates when they are 1 to 2months of age. The concentrate should contain 14% to18% protein and have added calcium, phosphorus, cop-per, and zinc in a formulation designed specifically forgrowing horses. Fat and fiber contents of 5% to 10% and>12%, respectively, will reduce the glycemic index of thefeed and potentially reduce the amounts needed to be fed.Pelleted and extruded feeds are preferable to texturedsweet feed mixes, especially in foals from bloodlines po-tentially predisposed to developmental orthopedic dis-ease. The higher percentages of protein and calciumshould be used if only grass hay is available. The lowerpercentages can be used with legume or legume/grass-mix hays. Concentrates should be fed at the rate of 0.50%to 1.0% of body weight, with the emphasis on maintain-ing lean to moderate body condition. If lower amountsare fed (less than 0.75% body weight), the mineral con-centrations in the feed will need to be increased. She rec-ommended that after weaning, maintain the horses on thesame type of feed until at least 18 months of age. Longyearlings’ rations can be reduced to 12% to 14% proteinwith lesser concentrations of minerals but still above thatusually found in mixes formulated for adult horses.Maintaining yearlings on the weanling concentrates willnot hurt and may help, especially if horses are still grow-ing fairly rapidly.

FAT SUPPLEMENTS FOR HORSES

Dr Robert J. Van Saun pointed out that consumptionof fatty foods is often perceived as inappropriate nutri-tion, especially from a human health perspective. Morerecently, however, consumption of higher-protein and -fat


diets in lieu of high-carbohydrate diets has been advo-cated relative to weight reduction. Carbohydrate bashinghas become a new trend in human nutrition faddism.

He asked, “Is a fat-supplemented diet for horses ahealthier diet?”

To understand the role of dietary carbohydrates(CHO) relative to their impact on health and the potentialof fat to minimize these health problems, one must un-derstand differences in dietary CHO sources. DietaryCHO can be generally characterized into 2 groups, eitherstructural or nonfiber. Structural CHO, often termed di-etary fiber, are composed of complex cell wall compo-nents quantified by neutral detergent fiber and are slowlyfermented by intestinal microbial populations. NonfiberCHO contain 2 subsets of compounds, those readily hy-drolyzed (CHO-H) by intestinal enzymes (ie, sugars,starches) and a group of easily fermentable complexCHO (CHO-RF) associated with the inner plant cell wall

(ie, pectins, glucans, fructosans). Excess consumption ofCHO-H can result in a variety of health problems relatedto abnormal glucose metabolism (eg, obesity, develop-mental orthopedic disease, exertional myopathy) and ex-cessive starch fermentation (eg, colic, founder).

He calculated that a traditional equine diet consistingof primarily good-quality forage with free choice min-eral/vitamin supplements would provide only 3% to 4%of total calories from fat and 16% to 20% from protein,with the remainder coming from carbohydrates.Hydrolyzable CHO accounted for less than 33% of totalCHO in hay- or pasture-based diets in 1 study, althoughthere was some seasonal variation with pasture. Compo-sitional analysis of legume, grass, and mixed haysshowed average CHO-H content was 17% of total CHOacross all hay types. As a percent of total CHO, averageneutral detergent fiber content increased from 58%(legume) to 80% (grass) hays, while average CHO-RF


content declined from 24% (legume) to 3.5% (grasses).In contrast with hay, concentrate supplements contain upto 97% CHO-H.

He pointed out that equine diets are often supple-mented with starch-based carbohydrate sources to in-crease dietary energy supply to meet daily nutritionalneeds. Physiologic states of growth, lactation, and activ-ity often have daily energy requirements that exceedavailable energy from forage diets alone. Poor foragequality and a need to increase body weight are other sit-uations where supplemental carbohydrate sources areused. As concentrate is added to the diet, risk of adversehealth problems increase. Replacing CHO-H sourceswith fat can help minimize or eliminate CHO-associatedhealth problems.

Suggested health benefits to added dietary fat in-clude improved coat quality, especially with polyunsatu-rated fats, reduced potential for colic or laminitis fromhigh-digestible carbohydrate diets, and improved athleticperformance. Positive health effects of fat in preventinglaminitis and colic are related to minimizing excessiveCHO-H intake.

Fat supplementation also seems to be protective for avariety of syndromes associated with abnormal muscleenergy metabolism. Exertional rhabdomyolysis (ER) is acollection of exercise-related myopathies with similarclinical signs that differ in underlying etiology. My-opathy syndromes include sporadic ER, chronic ER, re-current ER, and polysaccharide storage myopathy.Regardless of the underlying mechanisms of ER amongbreeds or relatedness of ER to other clinical syndromes,clinical signs of muscle dysfunction in most horses canbe controlled by dietary modification to reduce the num-ber of calories from CHO-H (ie, sugar and starch) and toincrease total number of calories from fat. For horseswith any of the ER syndromes, diets should maintainmore than 1% body weight intake from forage and have

20% of total daily calories from fat, with <15% of totalcalories from CHO-H. To achieve this level of caloric dis-tribution from fat, at least 480 mL, which is approxi-mately 1 lb of vegetable oil, is provided per 1000 lb ofbody weight per day. Commercial fat supplements andrice bran may also be used; however, many of these prod-ucts contain large amounts of CHO-H. Other dietary nu-trients need to be balanced appropriately to meet dailyneeds.

STRATEGIC USE OF EQUINE VACCINES

Dr Hugh G. G. Townsend said that without reason-able knowledge of specific disease immunology, epi-demiology, vaccine safety, vaccine efficacy, and duration

of immunity, effective vaccination programs for the indi-vidual or group cannot be devised. He used the term“strategic vaccination” to describe vaccination programsbased upon knowledge of disease immunology, patho-genesis, and epidemiology in combination with currentdata on the response of horses to vaccination and studiesof vaccine efficacy. Strategic vaccination suggests the useof vaccines of known efficacy, timed to achieve greatestprotection in periods of greatest risk of disease and em-ployed in circumstances where significant reduction inthe risk of disease is likely.

However one calculates cost, the use of efficaciousvaccines is most likely to be cost-effective in situations ofmoderate risk. In principle, it is possible to overwhelmthe immunity achieved through vaccination if the chal-lenge is sufficiently severe. For this reason, disease pre-vention programs must employ all reasonable means ofrisk reduction. If this is not done, then the recommenda-tion to vaccinate will result in less than optimal benefitand perhaps no benefit at all.

Regardless of efforts to increase the safety of all vac-cines, the process of vaccination and the outcome of theprocess itself are always accompanied by some cost orrisk to the animal, its owner, and the individuals whocarry out the procedure. Throughout their lives, all ani-mals are at varying risk of infectious disease. In circum-stances where there is little or no risk of disease withinthe duration of immunity of the vaccine, vaccination can-not provide measurable benefit and the cost or risk of per-forming the procedure will outweigh the benefits.

The prospect of designing strategic vaccination pro-grams is greatest when applied to infectious diseases witha relatively high incidence. These diseases cluster in cer-tain locations, ages, and classes of animals and are usu-ally associated with several well-known risk factors.Infectious diseases that occur commonly provide logicaltargets for vaccine development and for ongoing researchto assess and improve their efficacy. Diseases of particu-lar interest in this regard include influenza, strangles, andWest Nile virus.

If we take into consideration all that we currentlyknow about the epidemiology of equine influenza, DrTownsend believes it is clear that vaccination againstequine influenza, as generally taught and practiced inNorth America, could be much more strategic than it cur-rently is. All horses entering high-risk venues shouldhave received a vaccine of published efficacy within theprevious 6 months. Horses that exceed this 6-month limitwhile resident in the environment should be revaccinated.The risk of adverse responses to these vaccines is smalland the potential benefit to the population is great.


In contrast with influenza, strangles is not an impor-tant disease of the equine athlete. Most recently reportedepidemics have involved brood mare and rescue farms,groups of experimental animals, horse feedlots, and rid-ing stables. Risk factors include age, crowding, and en-vironmental contamination. Outbreaks may last formonths or years, and the risk of repeated outbreaks incertain environments is recognized. The key to preven-tion and control of strangles rests with the early identifi-cation of both clinically diseased and nonclinical carrieranimals, along with appropriate control procedures in-cluding effective quarantine, sanitation, and hygiene.This, in combination with concerns about the safety ofvaccination, suggests that a logical strategy may be tolimit the use of these vaccines to horses placed at highrisk of exposure. A potentially risky but intriguing strat-egy, begging critical assessment, would be to combinevaccination of clinically normal animals with other ef-forts to control the spread of disease in outbreaks and sit-uations of endemic disease.

Dr Townsend said that until more information on theepidemiology of equine herpesvirus (EHV) respiratorydisease and the efficacy of EHV vaccines becomes avail-able, it will not be possible to develop and test strategicvaccination programs against this disease. Many equinerespiratory vaccines contain antigens against EHV, butbased on current data, there appears to be no justificationfor designing vaccination programs around these vac-cines or for the selection of particular multivalent vac-cines simply because they contain EHV antigens.

Currently, West Nile virus continues to spread acrossNorth America, causing clinical signs of disease in largenumbers of horses. Two manufactures have licensed vac-cines on the market. Both claim 12-month duration ofimmunity, backed by experimental challenge studies.Neither company has published the results of their stud-ies in peer-reviewed literature. However, there is mount-ing independent field evidence showing that vaccinationdoes aid in the prevention of clinical signs of disease.

He pointed out that epidemiologic data on West Nilevirus infection and disease of horses are being collectedby a substantial number of research groups as well as fed-eral, state, and provincial government agencies in boththe United States and Canada. Data are also being col-lected on the infection of mosquito, bird, animal, andhuman populations. As field data continue to becomeavailable (on the efficacy and duration of immunity of thevaccines, the distribution, intensity, and seasonal occur-rence of infected populations of mosquito vectors, andthe epidemiology of the disease in horses), it should bepossible to develop excellent strategic vaccination pro-grams against this disease.

Although few are likely to question the need foryearly vaccination of horses against tetanus, it seemsprobable that given current practices of hygiene, surgery,and the use of antibiotics, the routine yearly vaccinationof horses is having little or no impact on the occurrenceof this disease. Theoretically, it should be possible to de-velop a more strategic vaccination program againstequine tetanus. However, owing to the rarity of this dis-ease and strongly held convictions regarding the need foryearly vaccination, the likelihood of changing our cur-rent approach to tetanus vaccination is small.

Dr Townsend summarized by saying, “It is becomingincreasingly evident that simple “1 size fits all” vaccina-tion programs for the horse are not consistent with diseaseepidemiology, vaccine efficacy, duration of immunity, ormanagement of different classes of animals. The use of thesame routine yearly administration of multivalent vaccinesover a wide range of circumstances cannot be viewed as ei-ther strategic or appropriate. The development and produc-tion of safe, highly efficacious vaccines for the horse withprolonged duration of immunity provide extremely com-plex and difficult challenges for all those who attempt it.As a result, all equine vaccines have their limitations.Careful planning and ‘tailor made’ vaccination programsfor individual horses and groups of horses are required totake full advantage of current equine vaccines.”

acvim forum part ii

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