HORSE PURCHASE AND MEDICATION TESTING:

AN OVERVIEW

Thomas Tobin, PhD, MRCVS

The principal problem associated with examining a horse at the time of purchase is to determine whether or not it is sound for its projected use. While horses are put to varied uses, a purchaser rarely wants an animal that is not sound in wind and limb, or one whose temperament makes it not reasonably easy to manage.and not safe to be around. Unfortu- nately, not all horses are sound in wind, limb and temperament, and failure to meet these requirements, may, indeed, be the primary reason for sale of certain horses. In this re- gard, one of a seller's strategies to make such an animal marketable is to use specific medi- cations to mask its specific unsoundnesses.

The Medications The potential for modifying an animal's

appearance, behavior or performance with medications is limited only by the range of medications available to the seller. There are about 4,000 drugs in common use in human and veterinary medicine, and perhaps an equivalent number of drugs approved for use outside North America, any of which may show up in horses. Beyond this, an unknown number of experimental drugs may also show up in racing horses or horses offered for sale. Because of this very large range of drugs the analytical methodology used in any equine drug testing scheme must be broad in scope. Beyond this, however, the basic ways in which these drugs may be used depends on these medications' pharmacological actions. For example, one of the most common de- fects that a seller may want to mask is a minor lameness. Non-steroidal anti-inflammatory drugs (NSAIDs) such an phenylbutazone are the most popular approach to this problem.

Phenylbutazone and Other NSAIDs The treatment of choice for localized

soft tissue inflammation (bruises, muscle soreness, tendon and joint pain) is phenylbu- tazone, or one of the related NSAIDs. These agents are effective against sore feet (pedal osteitis), jacks (cunean tendon bursitis)

widespread availability of phenylbutazone, it is the medication most commonly used to "touch up" a sore horse before an event where the animal must appear sound. Closely related pharmacologically to aspirin, it is the horses' equivalent of aspirin, and is as com- monly used in the horse as aspirin is in humans.

Phenylbutazone and the other NSAIDs are not a particular problem to detect in horses being sold. They are all acidic drugs, and as such are administered in relatively large doses, give rise to fairly high blood levels of drug, and can almost all be detected in blood during the period of their pharma- cological action. For example, a clinically ef- fective dose of phenylbutazone will give rise to blood levels which are easily detectable for about 24 hours, and should, with a little effort, be detectable for 48 hours. Beyond this, its metabolites, especially oxyphenbu- tazone, will likely be detectable in blood for 72 hours or more; that is, even after the drug's pharmacological effect has worn off. If phenylbutazone were used to mask lameness in a sale horse, a blood sample taken at the sale time would show a pharmacologically effective blood level of phenylbutazone (>2 Ixg/ml or so), and this level is relatively easy to detect.

On the other hand, a sample taken 24 hours or so after the sale, when lameness might appear, would show traces (<l~tg/ml or so) in blood, and low levels of phenylbu- tazone metabolites in urine. Beyond this, because the pharmacology of phenylbuta- zone in the horse is well understood, one can draw some fairly solid conclusion about this drug's likely actions and effects on the horse, if one has good information on blood levels of the drug at a particular time.

The situation is not so clear-cut with other members of the NSAID family. At least 50 NSAIDs are available around the world for use in humans and in horses (flunixen, naproxen, equiproxen, ibuprofen, zomax, etc) Their actions in the horse are not well understood. Some, like Banamine (flunixen), are not detectable in the horse's blood after about 8 hours, although they remain detectable in urine for 48 hours or more. Banamine is an exception, however. In general, these drugs are fairly readily detect-

derived from the hormones of the adrenal cortex which have powerful anti-inflamma- tory effects. Some synthetic members of this group are quite potent, in that they can be given to horses in milligram quantities and still produce useful effects. Like the NSAIDs, they can be given systemically for an overall anti-inflammatory effecL Unlike the NSAIDs, however, corticostemids can also be administered by direct injection into an inflamed area or joint for a local effect. When injected into a bursa or joint in this way, they act to reduce swelling and inflam- mation in the joint, and they also eliminate the inflammatory component of pain. In this way, they can temporarily restore normal action to a joint, and if a long-acting prepara- tion is used, the effect can last for several weeks. Injection of a corticosteroid into a joint is sometimes referred to as "joint tap- ping."

Because the corticosteroids are active in much smaller doses than the NSAIDs, be- cause they can be injected directly into joints to localize their actions, and because their actions can last for a week or more, control- ling their use in sale horses is much more difficult. The blood levels associated with the use of these agents are small, and they can be quite difficult to detect. Because the agents are closely related to natural corticos- teroid hormones, tests used to identify them have to be relatively specific and sophisti- cated. Finally, it is not clear whether blood levels of these agents continue to be detect- able during the entire period that these agents are active after injection into joints. For these reasons, urine samples are much superior for detecting these agents and are strongly rec- ommended for controlling use of the corti- costeroids.

Another problem arising with these agents is that the defect in the horse may not be detected until some time after the sale. By that time, however, the return period for the sale may have expired. A further problem is that if blood and urine samples are drawn a week after the sale, cortieosteroids in a urine sample do not necessarily mean that they were administered before the sale. It is al- most always impossible to tell from a blood sample, and even more difficult from a urine sample, when the drug detected was admini-

272 EQUINE VETERINARY SCIENCE

stered to the horse A useful and conservative policy, therefore, would be to draw a blood and urine sample as soon as possible from a horse to he purchased, or one that has been purchased, to establish clearly that any medi- cations which may he detected were present in the horse when the sale occurred.

Tranquilizers A very common manipulation of horses

before sale is tranquilizing. Since using ace- promazine is almost an integral part of horse- manship, treatment with "ace" or closely related tranquilizers is common for loading and shipping horses. Because of this com- mon usage, the appearance of metabolites of acepromazine in a horse's urine after a sale is not uncommon, and does not neCessariJy indicate an attempt to mislead a buyer. On the other hand, these drugs can be used to alter the behavior of horses, and could render a difficult animal more saleable.

For a short term tranquilizing effect, acepromazine or one of the related phe- nothiazine tranquilizers is the agent of choice. When given IV, acepromazine acts relatively rapidly. The initial effect appears within minutes, and in about 30-60 minutes its actions peak, depending on the dose administered.

When administering "ace" to a horse, remember the fireman's rule: you can putout a small fire starting with less water. Simi- larly, "ace" is much more effective when administered to an animal before it is put into a stressful situation rather than afterward. In a highly excited horse, "ace" may appear to be virtually ineffective, because of the over- powering effects of the animal's central stimulation.

Small doses of ace (<1 mg) have very subtle effects which can only he seen on the the animal's hematocrit, which falls by about 20% from the normal value of about 38%. The next effect to appear is penile protrusion in the male, which becomes apparent after about a 4 mg dose, and last for about 2 hours. Larger doses depress the animal's breathing rate and depress his behavior, at which point the animal is clearly and unmistakably de- pressed. If the drug has been used skillfully, however, few signs of overt tranquilization may be visible to even the most astute ob- server.

As well as being difficult to detect visu- ally, acepromazine and its relatives are also difficult to detect chemically. As indicated above, a dose of a few milligrams can have a marked effect, but any drug given at the milligram level tends to be difficult to detect. Acepromazine, however, also happens to be a very fat soluble drug, which means that after injection, it virtually disappears from

the blood into body fat. For this reason, acepromazine is, for all practical purposes, undetectable in blood within minutes after its injection and can only be found in urine in the form of its metabolites. Detecting use of acepromazine in a horse is therefore almost totally dependent on obtaining a urine sample where metabolites of acepromazine may be detectable for two to three days after even small doses of this drug.

Another tranquilizer that has been used extensively in the horse is reserpine. Reser- pine is a very effective tranquilizer and has long been used by horsemen, especially for show horses. It has a reputation as the "3 week tranquilizer," able to influence a horse's performance for up to 3 weeks. Remarkable as that statement is, it appears much closer to the truth than the conventional wisdom among veterinarians about this drug.

Like acepromazine, reserpine is active in the horse in doses of about a milligram or so. If you give a large dose (say 2-4 rag) of reserpine to a horse, you see a very clear-cut sequence of events. The horse becomes very depressed, "droopy," his eyelids droop, he sweats, he shows diarrhea, and in the male, the penis extends. Anybody who looks at this horse can clearly see that he is very depressed and abnormal, and he stays like this for up to 2 days.

By the end of 3 days, however, the horse is apparently back to normal, to the casual and sometimes even the expert observer. In reality, however, the horse is very subtly tranquilized, in a way only readily apparent to people quite familiar with the horse, such as his owner and trainer.

With the aid of some very sophisticated behavioral apparatus, we have been able to show that this subtle tranquilization does not peak until 5 days after the dose, and in our hands, lasts for up to 10 days. This effect is very subtle, and would not be detected by a person not quite familiar with the horse. As one might expect, a drug with these charac- teristics has found some very specific uses in the horse.

Reserpine is a very lipid soluble drug, but unlike acepromazine, is easily found in blood. In fact, reserpine is generally not found in urine, so to detect reserpine in horses, all you need is a blood sample, and a laboratory that knows how to do the test. Given these circumstances, and a reasonable (1 nag or so) amount of reserpine in the horse, it is relatively easy to detect and control the use of reserpine.

In summary, therefore, to detect tran- quilizers in a horse, both blood and urine samples are required and most especially urine samples if acepromazine or similar tranquilizers are to be detected.

Other Agents A variety of other agents may be ad-

ministered to horses to make the animal more presentable for purchase. At one time in England, the use of anabolic steroids was widespread in preparing yearlings for sale. Administration of an anabolic steroid has very clear-cut effects on horses, especially fillies and prepubescent males. Anabolic steroids increase appetite, build up muscle and bone, make animals more alert, more aggressive, and improve their coats. These actions all lead to the development of a more marketable yearling, and also serve to add to adult horse's appearance, particularly a mare or gelding.

On the other hand, when given to fillies, or to young broodmares, the masculinizing effects of the anabolic steroids can give rise to subsequent problems with breeding. If sufficient anabolic steroids has been admini- stered, estrus and even ovulation can be suppressed. These effects can last for months after administration of the drug ceases, and such fillies can be significantly more difficult to get in foal. All in all, depending on the amounts of drug administered, and the time since its administration, the anabolic ster- oids' effect can be of considerable signifi- cance for the horse's subsequent perform- ance, especially for its breeding perform- ance.

Analysis for anabolic steroids is diffi- cult. To date, their detection has tended to depend on radioimmunoassay screening, which does not "positively" identify specific drugs. However, depending on the amounts of anabolic steroid administered to the ani- mal, positive confirmation by mass spec- trometry may be possible.

Local anesthetics can, in theory, be used to mask lameness but are less likely to he used as such in practice. This is because a good degree of clinical skill is required for their administration, which means that a veterinarian experienced in their use for nerve or joint blocks will have to administer them. Second, their actions are relatively short-lived. Carbocaine, one of the longer acting agents, lasts for 4 hours, a relatively short period for a horse in an auction situ- ation, where it will be available for viewing for days. This period could be suitable for a horse in a private sale situation, however, where one could predict when the horse would be inspected. If a local anesthetic is used, the drug, or more likely it metabolites, can be picked up in urine for 24 to 48 hours after the last dose. On a strictly chemical basis, therefore, since these drugs "clear" fairly rapidly, they might be considered at- tractive agents to use in a sales setting.

Volume 8, Numbe¢ 3, 1988 273

out above. Like the local anesthetics, the narcotic

analgesics are less than ideal for use in horses being inspected for sale. The classical nar- cotic compounds, of the morphine type, all induce a marked locomotor response in horses in doses similar to those at which they control pain. Also, their actions on pain do not last very long. Unless the dose is very large, they rarely last for more than 2 to 4 hours. In addition, unless a highly potent narcotic analgesic is used, these agents are readily detectable in urine and so offer no special advantage over other agents. While particularly suitable for racing horses, the narcotic analgesics appear to have little to offer for sale horses, and are apparently little used in this area.

Drug Detection When a drug is administered to a horse,

its detectability depends on its concentra- tions or its metabolites present in the body fluids submitted for examination, and the availability of a sufficiently sensitive method to detect and unequivocally identify it in the sample submitted. As a general rule, urine is virtually always the superior medium for detection of a drug or drug metabolite, because most drugs, especially drug metabolites, are found in higher concentra- tions in urine than in blood. As a rule of thumb, it may be assumed that the concentra- tion of any drug or drug metabolite in urine will be at least 50 times higher than the corresponding blood level. While this rule does not always hold, it is a very rare drug indeed (e.g. Reserpine) that is detectable only in blood and not at all in urine. For this reason, if at all possible, samples for pre- purchase testing should include urine samples.

Urine Samples Urine samples may be obtained rela-

tively easily from mares by bladder catheterization. One simply runs a catheter into the bladder, applies a little negative pres- sure to the catheter (suck gently on it), and one can usually siphon out a good urine sample. One should lay to obtain at least 200 ml of urine.

Obtaining a sample from a male horse, however, is more difficult. While admini- stration of furosemide virtually guarantees that the horse will produce a urine sample within ten minutes, it also dilutes at least some of the concentrating effect that renal excretion has added to the urine sample. So, although a urine sample obtained with the help of furosemide is vastly better than no urine sample at all, it is not nearly as useful as

a "normal" urine sample. While detecting drugs or drug

metabolites in urine is easy, it is very difficult to draw conclusions from a urine sample about when the drug was administered or whether any significant blood level of drug was present in the horse. This difficulty is because, as a general rule, no direct relation- ship exists between blood and urinary levels of drugs or drug metabolites. Therefore, while a urinary concentration can tell you, with virtual certainty, that an animal was recently administered a certain drug, esti- mates as to when that drug was given tend to be extremely speculative.

Blood Samples Blood samples are easy to obtain, and at

least 20 ml of blood should be provided for useful analysis. For best results, the sample should be drawn into tubes containing ox- alate or fluoride, to poison the sterase en- zymes found in blood, which can break down certain drugs. Alternatively, the blood sample may be drawn into serum tubes. Samples are best not drawn into tubes con- taining separating agents for serum, because the separating agents contain poorly de- scribed chemicals that could lead to inadver- tent contamination of the sample, confusing the interpretation of the test and its forensic validity.

For most NSAIDs, such as phenylbuta- zone, blood levels of this drug are readily detectable for up to two days after its admini- stration. Thus, if the animal is under the pharmacological effect of a drug at the time of sale, then the blood will contain easily detectable levels of the drug. On the other hand, a urine sample will probably contain both the parent or unchanged drug, and also drug metabolites, for a much longer period than the drug is detectable in blood. Urine is thus the superior sample in maximizing the probability of drug detection, but better and forensically more useful results can be drawn from blood level data, if these can be ob- tained.

Shipping and Storage Urine samples are best stored frozen to

prevent any changes likely to occur during storage. Plasma samples (and if necessary, blood samples) can also be stored frozen. The next most satisfactory storage method is re- frigeration, which will hold most samples for several days. On the other hand, if samples are allowed to stand at room temperature for much more than one day, their quality as forensic samples tends to be markedly re- duced.

With the increased availability of drug testing, and the increased awareness of medi- cations' actions and effects on sale horses,

good conservative practice is likely to at least draw a blood sample from any horse being examined in association with a sale. The sample can be stored frozen, and if questions later arise about the horse's condition, the sample can be analyzed. As pointed out ear- lier, however, such testing should optimally include blood and urine samples, as urine sampling greatly extends the scope of test- ing.

In storing such samples, one should be aware that not all analytical laboratories can test for all drugs on short notice. Therefore, if a horse can be returned only within a re- stricted time, it is wise to send the sample to your laboratory as soon as possible, to allow them time to optimize their tests to answer any questions that you may have and, if necessary, allow you to draw further samples.

The Testing Process When samples are received at the labo-

ratory, their receipt is recorded and the sample's quality noted. In addition, if the sample is of urine, its pH and specific gravity are noted. Then a portion of the sample is taken for analysis.

The initial screening analysis of a sample is generally by High Performance Thin Layer Chromatography (HPTLC). The sample is taken, extracted under acidic or basic conditions, and then subjected to HPTLC. As soon as the chromatograms are developed, they are treated with a sequence of oversprays, which demonstrates the pres- ence of different drugs. These methods are very sensitive indeed, and can detect concen- trations of drugs as low as 20 nanograms/ml under optimal conditions. In addition, the scope of these tests is very wide, which makes HPTLC the method of choice for screening for a wide range of drugs. On the other hand, if the search is to be limited to a specific drug or class of drugs, tests known to be optimal for detection of that class of drug will be used.

While a drug can be tentatively identi- fied by HPTLC this kind of identification is only tentative. If necessary, the drug' s pres- ence in the sample is confirmed by mass spectrometry. Mass spectrometry yields evi- dence which constitutes virtually unequivo- cal identification of the drug. With good mass spectral evidence, the probability of ef- fective challenge of the analytical findings' validity is greatly reduced. False negatives, when positive samples are declared negative, is a major deficiency of drug testing. In fact one can never, given today's techniques, declare a sample negative, for two reasons. First, if the chemist is expected to check for all drugs, as in blind testing, the equality of this test will vary. No chemist can test for all

274 EQUINE VETERINARY SCIENCE

drugs at the level of sensitivity that an expert in that analysis can do, and if a chemist is able to test for a broad spectrum of drugs at a good level of sensitivity, then he is doing a very good job. For this reason, there is really no good way to define a false negative, since the circumstances under which the sample would be positive are not stated.

If the sample is being tested for a spe- cific drug or small group of drugs, then a good chemist will specify the sensitivity of his test and say that the sample is negative in that the level of substance present must have been less than a certain concentration. The usual circumstance where this becomes criti- cal is when one lab has called a positive and another lab is doing a referee or confirmation test. In this circumstance, it is perfectly pos- sible for one lab to call a positive and for it to be unconfirmed in another lab if the testing in the second lab is not as sensitive as the testing in the first lab. For this reason, while it is impractical for alab doing broad screening to state the sensitivity levels of all of its tests, and indeed the lab may well not be aware of the sensitivity of its testing for many agents, a good lab should always make a point of determining its sensitivity of any drugs that it calls positive.

There is one further way in which the medication can be abused in the context of a sale. A horse can be treated after a sale, and then drug tested. The new owner can demand that the seller accept the horse back because it was positive for a drug. While an unlikely sounding scenario, it is one that the technol- ogy of drug testing has introduced into the sales arena and one that vendors should be aware oL If the seller has any reason to ques- tion the good faith of the purchase, it would be prudent for him to draw a blood sample, and better still a urine sample, when the horse is inspected or before the horse leaves the premises, to establish the horse's medication status at the time it was sold.

Utility of Pre-Purchase Testing When used in this way, blood and urine

testing of horses offered for sale can greatly help the veterinarian advise his client as to a horse' s suitability or soundness for purchase. The veterinarian has virtually no other way to determine at the time of examination whether or not a horse is under the influence of a specific medication. Simple drawing of a blood sample, or better still a blood or urine sample, can go a long way towards protecting a veterinarian and his client from the effects of medication on a horse's appearance, demeanor and performance at the time of a sale, or indeed, at any other time. (From Equine Data Line, Sept. 1987)

VITAMINS AND THE EQUINE DIET

By Bob Mowry

Vitamins are organic compounds, re- quired by the horse in trace amounts for normal body function and performance. Al- though some vitamins are produced in the horse's body, they are not considered to be a structural component. Nutritionists gener- ally classify vitamins into 2 broad classifica- tions including fat soluble or water soluble vitamins. The fat soluble vitamins A, D, E, and K are dissolved in fat and capable of being stored in the fatty tissues of the horse's body. The water soluble vitamins which include vitamin C and a large complex of B- vitamins, are soluble in water and readily depleted from the horse's body.

The horse's body is capable of produc- ing several vitamins. Vitamin D is synthe- sized in the body in ample amounts to meet the horse's requirements through the conver- sion of ultraviolet rays from the sun to vita- min D_. Vitamin D_ and vitamin D^ con- ;J =:

sumed m sun used forages are converted to an active form of vitamin D in the liver and kidney. Vitamin C is thought to be produced in ample amounts in the horse's liver.

The B vitamins and vitamin K are pro- duced by bacteria located in the cecum of the large intestine. The production of these vita- mins relies on the number and viability of the

cecal bacteria. Fiber obtained primarily from forages is the main nutrient source for the bacteria. Failure to supply adequate fiber, or a reduction in bacteria numbers, would result in decreased B-vitamin and vitamin K pro- duction. In such situations, a supplement should be fed.

Vitamins A and E must be supplied entirely by the diet. Green forages and prop- erly, sun-cured hays are excellent sources of these vitamins, as well as, Vitamin D. Under normal feeding conditions, deficiencies of vitamins A, D, and E are not likely.

Vitamin Supplernentation The requ~ment for vitamin supple-

mentation is minimal for horses fed adequate levels of high quality forages plus concen- trate mixes with vitamin premixes added. The majority of the commercial feed compa- nies will supply a vitamin premix in their concentrate mixes at a level, high enough to ensure adequate fat and water soluble vita- min intake. Addition of supplemental vita- mins to such rations could result in exces- sively high vitamin levels and potential health problems. Although not typical, vita- min toxicities have been reported from over supplementation. Table 1 lists the current NRC recommended levels for the fat and water soluble vitamins.

In general, there are several production situations where vitamin supplementation is

TABLE 1

Dietary Vitamin Requirements for Horses a

Vitamin Class Required Level Potential Toxic Levels

Fat soluble vitamins Vitamin A, IU/kg mature horses at maintenance or work 650 Foals, yearlings, 2-year-olds 800 Mares, last 90 days gestation 1400 Lactating mare, first 3 mos. 11 50 Lactating mare, 3 mos. to weanling 1000

32,500 40,000 70,000 57,500

50,00

Vitamin D, IU/kg 275 150,000 Vitamin E, mg/kg 15.0 no known level Vitamin K none NA b

Water soluble vitamins Vitamin C none NA B Vitamins

Thiamine (B,), mg/kg 3.0 NA Riboflavin (B), mg/kg 2.2 NA Pantothenic acid, mg/kg 15.0 NA Biotin, mg 1-3 NA Pryridoxine (B~) none NA Cyanocobalamin (B,,) none NA Choline none NA Folic Acid none NA Niacin none NA

aAdapted from the National Research Council, Nutrient Requirements oflhe Horse, 4th ed 1987 bNot available

Volume 8, Number 3, 1988 275

required, such as: 1) When feeding poor quality forages,

not properly cured or stored for longer than one year (vitamin A levels are reduced in bleached hay).

2) If an animal has received prolonged antibiotic treatment_ Antibiotics will destroy certain cecal bacteria which will reduce B- vitamin production.

3) Short yearlings (12 months of age) and foals. Cecal development occurs slowly during the foal's life. Adequate forage intake to maintain B-vitamin production is usually not achieved until six months to one year of age.

4) When feeding low levels of forages. This usually occurs in horses undergoing medium to heavy work which are typically fed high grain and low forage diets. In such situations hay intake must be maintained at 5 pounds long stem hay per 100 pounds of body weight to ensure normal digestive tract function. (From Horse Scribbles)

INBREEDING, LINEBREEDING AND CROSSBREEDING

There is a lot of discussion in the horse industry today about inbreeding and line- breeding, and thereafter the perfect cross. In fact, there is very little inbreeding or line- breeding in the horse industry, and therefore almost no crossbreeding from a genetic sense. Since heritability is relatively high in most traits of economic importance in horses the use of inbreeding, linebreeding or cross- breeding is contraindicated.

By definition, inbreeding and line- breeding is simply mating individuals that are related, which tends to concentrate the genes of the common ancestors. Linebreed- ing is a form of inbreeding, which makes the effort to concentrate the genes of a particular ancestor. Crossbreeding is simply the mating of unrelated individuals from two different lines. From a genetic standpoint, crossing two different breeds of horses would be considered crossbreeding, whereas mating two individuals who have different pedi- grees within the same breed, is not a true cross in the genetic sense of the word, unless those individuals are inbred.

There are two types of gene action, qualitative and quantitative. Quantitative or additive gene action simply means that the more good genes a horse has, the better the horse will be. When heritability is high, we simply mate the best to the best and get as many good genes in the offspring as possible to produce a high-quality horse. Research

has indicated that most of the traits of eco- nomic importance to the horse (size, speed,length of stride, conformation, and intelligence) are highly heritable (40% to 60%). Therefore, simply mating the best animals available is the best way of getting maximum number of "good" genes into the offspring and therefore produce the best horse.

Qualitative gene action means that each gene pair has a specific combination which gives a maximum result (heterosis).-When heritability is low, such as in reproduction, then breeding programs which will maxi- mize the largest number of ideal combina- tions of genes, tend to give the most heterosis to the offspring. In order to take advantage of qualitative gene action, two inbred lines of horses are needed. Unfortunately, inbreeding tends to concentrate the bad genes as well as the good genes, and inbreeding in normally detrimental to size, vigor and quality. How- ever, if two inbred lines are developed when mated, they should produce the maximum number of ideal gene combinations and therefore produce offspring that are superior to either of the parents-- genetically termed heterosis.

In developing a breeding program for horses, qualitative gene action has limited importance. First, it is very important in the color breeds where, in many cases, you have one or two gene pairs that dictate the color markings of the horse. In that case, mating certain color (i.e. lines) horses produce a maximum number of the desired color.

Second, although it is normally undesir- able to inbreed because of the detrimental ef- fects, horses that are inbred have a more concentrated genetic makeup and, therefore, are more prepotent. The inbred individual must be a superior individual itself because it can stamp its bad characteristics on its off- spring as easily as the good ones.

In general, inbreeding, linebreeding and consequently, crossbreeding, have very lim- ited value in the horse because heritability is high and, therefore, quantitative or additive gene action is the most important_ As a result, simply mating the best individuals available should prod)ace the maximum result. For example, in the race industry, the philosophy of mating a horse from a line of distance runners with a line of sprinters could poten- tially yield a genetic combination which re- suits in a horse that cannot go the distance or be fast_ Additive gene action would indicate that the horse would have some genes which would make him a distance runner and some which would make him a sprinter and, there- fore, probably not be either one. The best combination genetically would be to mate two horses which have the demonstrated

ability to run the distance and the speed desired. This type of mating program would concentrate the genes for these traits and product the best results. In fact, the statistics indicate that winners bred to winners consis- tently produce the largest percentage of winners. Therefore, horse producers should simply identify and mate the best stallion and mares available to obtain the best results. (From Horsin" Around)

HORSE FLIES LINKED TO EIA TRANSMISSION

By Bob Mowry

Mosquitoes have traditionally been identified as the main vector in the transmis- sion of equine infectious anemia (EIA). However, data presented by Dr. Lane Foil during the 5th International Conference on Equine Infectious Diseases indicates that horse flies and mosquitoes are the main transmitting vector.

Different vectors were introduced to a group of EIA negative sentinel and EIA positive horses. When an all-horse fly popu- lation was introduced into the herd, the nega- tive sentinels showed a high level of serocon- version. No EIA positive transmission cases were reported when a high mosquito popula- tion was introduced into the herd. The re- search findings also indicated that horse flies prefer to feed on mature horses and had little contact with foals. On the average, only 2% of the fly population was noted on the foals at any one time and there was not a single case of a foal being infected with EIA.

Producers should use sound manage- merit practices to limit fly and mosquito populations during the appropriate seasons. Proper waste management, including routine manure removal and the use of insecticide products are encouraged. (From Horse Scribbles)

LEG STRAIN

Is it possible for a horse to regain its original tendon or ligament strength once the structure has been injured? Researchers at the University of Illinois College of Veteri- nary Medicine at Urbana are not sure what the answer to that question is, but they are looking into it.

Detailed study of suspensory ligament anatomy, coupled with another project de- signed to measure strain placed on a horse's tendons under various situations, is permit- ring veterinary researchers to make inroads

276 EQUINE VETERINARY SCIENCE

EQUINE SCIENCE UPDATE

to understanding equine lamenesses and their treatment.

Tendonitis and related tendon problems are probably the most common racing injury in horses. As Dr. Kevin Keegan, a veterinar- ian at the UI College of Veterinary Medicine, explains, "It is generally thought that tendons are injured by a mechanism of 'overstreteh- ing.' Yet no one knows how much stretch or strain a normal tendon can sustain without damage, or how much stretch or strain a tendon undergoes when a horse is simply standing o r walking."

To address this issue, Dr. Keegan is surgically inserting flexible strain gauges into flexor tendons and suspensory liga- ments. Using custom built electronic cir- cuitry, these gauges can measure the strain a tendon is subjected to during mild exercise. Once a value for normal strain has been measured, these gauges can be used to evalu- ate tendon support techniques used during convalescence of a tendon injury.

"A tendon that heals without support and gradually increasing loads heals haphaz- ardly, lbroducing excessive scar tissue, and scar tissue is never as strong as the original tissue," Dr. Keegan points out. "If we can justify support techniques for optimal heal- ing, we can more accurately give recommen- dations for the treatment of tendon injuries."

Dr. David Wilson, also a veterinarian at the college, is studying the horse's suspen- sory ligaments. These ligaments provide support to the horse's legs and are often a factor in lamenesses. To learn more about treatment of suspensory ligament injuries, Dr. Wilson is charting the structural changes this ligament undergoes with age. He will then determine whether similar structural changes occur with training as well. The goal is to learn more about suspensory ligament structure so that new techniques can be ap- plied in treatment of injuries.

For example, ultrasound is used to de- tect areas of ligament tearing and hemor- rhage. Yet because the suspensory ligament contains up to 40 percent muscle, sometimes the muscle is mistaken for ligament tearing. Familiarity with the details of suspensory ligament anatomy could enhance diagnosis.

The new knowledge gained from these research projects, coupled with an aggressive diagnosis and treatment program at the col- lege, permit veterinarians at the college to evaluate each individual case thoroughly and accurately. A complete spectrum of diagnos- tic tools provides the horse owner with a broader sense of the problem and an accurate idea of how to proceed.

Ultrasound equipment for assessment of soft tissue injuries, arthroscopic evalu- ation of joint cartilage, and radiologic evalu- ation of bone fractures all provide specific

information which can be applied to an over- all picture of the horse's injuries. For ex- ample, an X-ray might show that a horse has a splint problem, but upon examining the same injury with ultrasound, the injury is narrowed down to an inflamed suspensory ligament.

"IUinois has one of the nation's top programs in diagnostic nuclear medicine scans," says Dr. Gordon Baker, head of the UI College of Veterinary Medicine's equine section. "We are able to pinpoint bone mi- crofractures and chips early, permitting horses to be taken out of training and rested before the injury causes permanent damage or necessitates longer layoffs."

Once an injury is diagnosed, itis impor- tant for the horse's owner or trainer to follow clinical recommendations. Rest is especially important in healing lamenesses. Without strict adherence to recommendations, laser therapy, ultrasonography, thermography and other high technology treatment methods will not be beneficial. (From Horse Scribbles)

UPDATE ON P O T O M A C HORSE FEVER

Although the agent causing Potomac Horse Fever (PHF) has been identified as the rickettsia Ehrlichia risticii, its route of trans- mission remains unknown. Various arthro- pods have been suggested as vectors, and while much attention has been focused on the American dog tick (Dermacentor vari- abilis), several other blood-sucking insects can be temporally and geographically associ- ated with the infection as well.

Several groups of scientists, supported by grants from Mort'is Animal Foundation, are attempting to uncover the method of transmission of E. risticff.

Scientists at North Carolina State Uni- versity are currently evaluating the compe-

tence of not only D. variabilis, but of black flies (Simulium sp.) and horseflies (Tabanus sp.) as well, to serve as vectors of the PHF organism- According to these scientists, for an arthropod to serve as a biologic vector, the following mustoccur: uptake ofthcpathogen (ingestion), development within the vector, and output (transmission).

To confirm that the requirements of vector competence are being met, the North Carolina scientists will allow flies to feed, through a latex membrane, on blood contain- ing a known quantity of infective cells and then allow the flies to feed on susceptible mice. These mice will be examined for E. risticii antibody weekly for six weeks. (Ear- lier studies indicated antibody tilers in mice peak at four weeks after infection.)

Other flies will be fed on infected mice at the acute stage of the infection (i.e. Day 12 post-inoculation). These flies will then be fed on susceptible mice which will be followed serologically, at weekly intervals, for six weeks. Finally, all three stages of D. vari- abilis (larva, nymph and adult) will be fed similarly on infected, then non-infected, lab animals to ascertain the tick's competence to transmit the disease.

Follow-up experiments will involve isolating the specific location of the Ehriichia organisms within the arthropod vector's body and attempting to isolate the organism from the vectors at varying intervals to deter- mine how long the vector retains viable Ehrlichia.

Veterinary scientists from the Virginia- Maryland Regional College of Veterinary Medicine at Virginia Tech also have been looking attbe role of arthropods as vectors in the transmission of PHF. To date, they have been unable to conclusively demonstrate transmission of PHF by D. variabilisin either pony or mouse studies. Other insects being investigated by the Virginia Tech scientists include the black legged tick (Ixodes scapu- /w/s), "punkies" or "no-see-urns" (Culicoi-

T A B L E 1

Prevalence of Antibodies to E. Risticli in Non-Equine Mammals

No. of No. of No. of Proportion Premises Animals Animals Positive

Species Sampled Tested Positive* (%) Dog 10 27 0 0 Cat 10 48 8 16.6 Cattle 6 75 0 0 Sheep 2 15 0 0 Pig 1 14 3 21.4 goat 3 3 1 33.3 Barn mice 7 24 0 0 Rats 7 131 0 0 White-footed mouse 3 40 0 0 Meadow vole 3 65 0 0

*Positive for antibodies against PHF by IFAT (1:40)

Volume 8, Number 3, 1988 277

des sp) and the stable fly (Stomoxys sp). The scientists at Virginia Teeh have

added an additional piece to the puzzle. They have examined, serologically, wild and domestic (non-equine) mammals to deter- mine their role as potential reservoirs and amplifier hosts of PHF. Their results are shown in Table 1.

A third group of scientists investigat- ing the transmission of PHF is atthe Univer- sity of Illinois. Previously, based on evi- dence of serologically positive dogs in the field, this group determined that dogs are susceptible to experimental infection with E. risticii and a paper discussion their findings is currently in press. These investigators are now looking in to the role of cats as potential carriers ofE. risticii. To date, all cats inocu- lated with E. risticii have developed signifi- cant antibody titers and, from some of the experimentally infected cats, E. risticii has been recovered from the blood. This, coupled with the serological findings of naturally infected cats by the University of Illinois as well as Virginia Tech, sl~ongly suggests that cats may, indeed, be a natural reservoir host ofE. risticii. To confirm this, the Illinois scientists will now challenge ponies with the cat-derived E. risticii. (From Morris Animal Foundation Practitioner's Update)

HORSEPASTURES: THE MYSTERY FEED

Pastures for horses are considered an "unknown" and are often characterized by lack of information, uncertainties, myths and "guessing" management. However, the use of pasture to supply a mature horse with 10- 15 pounds of forage dry matter per day is likely the most economical feed source. Since economics and efficiency of produc- tion are not normally a high priority with many horse owner, pasture management receives low priority. A few factors which may influence interest in the use of pasture are outlined below.

Pasture as an Exercise Area Most of the top horses spend too much

of their time in stalls and corrals. They are exercised, but in most cases, this may not be comparable to the exercise obtained in large pastures where horses can walk and run at will. Characteristics of desirable horse pas- ture which are related to exercise include:

-Dense, smooth sod which tolerates the hoof action of running or pawing animals;

-Absence of rocks, stumps, holes and junk which could cause leg or hoof injury;

• Fences should be made of smooth wire, boards, pipe, e t e - - no barbed wire or

TABLE 1

Dally Nutrient Requirements of Horses at Varying Production Stages ~,=

Mature Production Body Wgt. DE3 C.P.' Ca' P Stage (Ib) Mcal) (Ib) (g) (g) Main tenance ' 880 13.9 1.2 18 11

1100 16.4 1.4 23 14 1230 18.8 1.6 27 17

(!~: ~ i ~ ; ::~?:::iiiiiiiiiii~i~ ~iiiiiiiiii:.iiii:: ~iiii::iiiiiii~:,~:~ iiiiiiiiiiiiiiiiiiiiiiiiiiiiii~ii~i~:i~iiiii~iii~iii~ii~i~i~i~3~iiiii~ ii!iiiill iiiiiiiiiiiiii~iiiiiiiiiiiii

Lactat ing Mare 880 23.4 2.5 40 27 (first 3 months) 1100 28.3 3.0 50 34

. . . . . . . . . . . . . . . . . . . . . . . . . 1230. ...................... 33 , t .................... 3 ,5 ..................... 60. . . . . . . . . . .40 ...... ~ ~ ~ g i~iM ~ i iiiiiiiiiiii::i::i~i::i::i~i::i ::i:: i ~iiiiiiiiiiiiiiiiiiiiiiiiiiiii iiiiiiiiiiiiiiiiiil ~ ~i~: iil iiiiiilililili::iiiiiiiiiiiiiiiiii::ii~;~iiiiiii::iiiiiiiiiiiiiiiiiiiiii iiiiiiiiiiiiz ~i~i~iii~i~iii~iiiii:.iiiiiiii ii~iiiiiiiili ~ ~iiiiii:~::

i ~ i ~ : .m~:th~ iii :.iiiiiiiiii ] iit~Oiii::iiiiiiiiiiii ili::iiiiii2~: 3 i iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii i i i i.:2i~iiiiiiiiiiiii:: i il i i i i i iiiiiiiiiiiii~ ~l !iii iil iill ilii:i :ii :ii iiiiiii~iiiiiiiiiiiii ~:: i:: i::::::ii ii ii i:.::::::i:: i:: :::::::::::::::::::::::: ::::::::::::::::::::: i~iiiii i i:: i iii i::ii i i i ii::iii!:: i:,i: '[ ::230i::i::::::::::i i::::::::::::::::i:: ::::::::i:: :::: :::: :,:: il ii ii :::: :::: :::: :::: ::i ii:2~i3!:: ::: :,:: i:: i:: ::iiii:: i:: :::: :::: ii iiii ii il il :,iii ii ::i ::::::::::::::::::::::::::::::: :::: :::::::::::::::: :::: :::: :::: :: :::: :::: :::: :::: :::: ::~ 9iii::::::i:::: :::::::::::::::::::::::: :: :::: :: :: i i :::: ii ::ii::~i:: ::iii :::: :::: i:: ii Weanling 880 13.0 1.4 27 20 (6 months of age) 1100 15.6 1.7 34 25

1230 16.9 1.9 37 27

i~iiii~i:, i::ii!:-:::::::::::::::::::::: i:-i~ i:-iii::i::i :::::::::::::::::::::::::::::::::::: ::i ::i:: i::ill :'~ ~3~i :::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::: i::i::ii i~ s~s ~i:iii~iii~iii::i::i::i::i::i::i::i::i:i :.::! ~ :Oi:.:-i::i::ii :.i i::i:: i:: i~i:.i:.i::i::i:: ::!::!:-:.i ~ ::::::::::::::::::::::::::::: i!i:. i ~!:,!:, i::i:: i~ S:iiiiii::iiii: Long Yead ing 880 14.4 1.3 22 15 (18 months of age) 1100 17.0 1.6 28 19

1230 19.1 1.6 32 22 T ~ ~e~ ~i o~ :d. !~!iiiiiiiiiiii::iii::::~i~i:.ii i ii ~ ~oiiii!! i!!!!! i! i!i!iiii::::!i!:.i::i::!::;::iiiiiii!iiiiiii~ a;gi::!~i::i::i::i::iiiiiiiiiiii i iiiiiiiii~ !~ ::: ~i i i~ i iii::iiil i i::!~i ! :. i iiii~i::i ::i::::~ii::~oiiiiii!iiiiiiiiii::i::i iiiiiiiiiii ~ ~iii::ii~i::iii (Z~ ~ S ~:ag~)i::i::i::ii:: i::~ :! ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: i::i:.::il ii~i~iiii~S :: :. ::::::::::::::::::::::::::::::::::::::::::::::::: ~::~:~

Nutrien! Flequirements of the Horse, NAS, 1978. ~Dry matter basis ~D.E.--digestible energy, C.P.= crude protein, Ca=calcium, P=phosphorus 4Nonworking horses and broodmares in first 8 months of pregnancy fed to maintain body weight.

woven wire; -Absence

marshes of poisonous plants and

Pasture as a Source of Feed Too many pastures are almost bare of

forage because they are too small and too many horses are stocked in small areas. This deprives horses of many benefits derived from lush, green pasture which supplies many nutrients including possible unidenti- fied factors. Horses may eat .75-1.25% of body weight as forage, depending on work load and physiological stage.

Calculating Nutrients Supplied from Pasture

If properly managed, pastures can sup- ply an excellent source of nutrients to horses in a variety of production stages (Table 1), while minimizing the need to purchase feeds. Pasture quality is directly related to several factors, such as fertilization, forage specie selection, pasture renovation, stocking rate and the stage of maturity of the forage. Fortu- nately, these factors can be controlled for optimum, high quality production through sound management practices. Is the horse consuming enough nutrients from pasture alone? When is the pasture overgrazed?

When should long stem hay be supplemented to compensate for overgrazed pastures? These are just a few of the questions con- fronting the horseman.

In order to answer these important ques- tions horsemen must have a basic under- standing of the nulrient content of different forages and how that nutrient content is af- fected by stage of maturity. Forages that arc consumed at an immature growth stage have a higher leaf area and less fibrous stems resulting in a higher nutrient content. Mature forages are more fibrous and lower in nutri- ent content due to the higher proportion of stems to leaves. In general, immature pas- tures will appear emerald green in color and have a minimum number of seed heads (grasses) or blooms (legumes). Mature pas- tures will have a higher percentage of seed heads and blooms, may appear dark green, yellow or brown in color, and will be lower in nutritive value.

Visual appraisal only allows producers to estimate forage quality. Chemical analyz- ers provide more exact measurements of nutrient content. Fresh forage or hay can be analyzed by the Department of Agriculture feed testing service for a nominal fee. The county extension agent should be consulted for additional information on this service.

278 EQUINE VETERINARY SCIENCE

TABLE 2

Nutrient Composi t ions of Pasture Forage Dry Matter'

State of DM' D.E. C.P. Ca P Grasses Maturity % Mcal/Ib % % %

Specie Bermuda Early 28 1.42 17.1 .89 .32

Mature 39 1.24 5.8 .40 .18

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::U25 ~ u ~ ................... Ea~iff .................... 2 8 ................. ii3;~ .................. i ~ i .................. ~Si ...................... ~38

Mature 35 1.19 8.9 .42 .30 i ~ i i i i i i i i i i i ! i i i i ! i~iiiii!!iiiiiiiiiiiiii~iiii::iiiiiii~i~i::i::i::iiii~i ~ i i i i i i iiiii::i::!::!::~::i::i::i~i~i::f:~i~::i::i::i::i::i::i::i::iii::i::~ii::i~::~ii~i~ii~ ~:::: :::: :: :: :: :: i ~/:i~

L e g u m e s Specie

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: !ii:j:j ::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

Clover, rod Early 20 1.40 21.1 2.26 .38 .................................................................... M~ture ........................ 28 ................ ! .28 .................. 14,9 ................ 1,0! .................. 27

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . : . : . : . : . : . : . : . : + : + : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . . : . . . . . . . . . . . . . . . . . . . . . . . . . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : . : .

~ U.S.-Canadian Tables of Feed Composition. 2 Rev. ed, NAS, 1969. 2D.M.- Dry matter

The percentage dry matter must be considered when calculating nutrient availa- bility. Grazed or fresh forages are extremely high in moisture, containing only 20 to 35% dry matter (Table 2). The high moisture content of fresh forages dilutes their nutrient content, compared to nutrient levels ex- pressed on an air-dry (90% DM) or dry- matter (100% DM) basis. Thus, an animal may need to consume many more pounds of fresh, grazed forage than the same forage dried, to achieve the same nutrient intake. For example, a 1000 pound horse would need to

eat about 65 pounds of fresh grass (30% DM) or about 22 pounds of hay (90% DM) to achieve the same level of dry matter and nutrient intake.

In addition to providing adequate acre- age of high quality, immature forage, horse- men must also be aware of the horse's ability to consume enough dry matter. Since the horse has a relatively small digestive tract, gut fill is another factor limiting forage in- take. Normally a horse can consume 2.0- 3.5% of its body weight in dry matter before gut fill occurs and intake steps.

TABLE 3

Comparison of Nutrient Requirements and Nutrients Provided by Pasture Forage

1100 lb. mature body weight

DE C.P. Ca P (acal) (tb) (9) (g)

Long yearling 17.0 1.6 28 19 2-year-old 13.9 1.1 20 13 Late pregnancy 18.4 1.6 34 23 1st 3-months' lactation 28.3 3.0 50 34

Nutrients Supplied From Pasture Bermudagrass

Early 15 Ib din/54 Ib as fed* 21.3 Mature 15 Ib din/38 Ib as fed 18.6 Eady 22 Ib din/79 Ib as fed 31.2

Grass/legume mixed 60% early rescue 40% early ladino clover 15 Ib din/62 Ib as fed 20.4 22 Ib dm/90 Ib as fed 29.9

*Example:

2.5 60.6 21.8 0.9 27.2 12.6 3.7 88.9 31.9

2.9 57.2 19.1 4.2 83.9 28.0

15 Ibs. Bermudagrass (D.M. basis) x 1.42 mcal/Ib D.E. (Table 2) = 21.3 mcal DE 15 Ibs. Bermudagrass (D.M. basis) x 17.1% C.P. (Table 2) = 2~5 Ib C.P. 15 Ibs. Bermuda grass (D.M. basis) x .89% CA (Table 2) x 454 grams/Ib = 60.6 gram Ca 15 Ibs. Bermudagrass (D.M. basis) x .32% P (Table 2) x 454 grams/Ib = 21.8 gram P.

I

Volume 8, Number 3, 1988

Several pasture feeding programs for horses, having an estimated mature body weight of 1100 pounds, in difference produc- tion situations are compared in Table 3. Long yearlings, two-year-olds or mares in late pregnancy, grazing immature Bermuda grass or fescue-clover mixed pastures, can obtain the necessary digestible energy (DE), crude protein (CP), calcium (Ca), and phosphorus (P) intake to meet their nutrient require- ments, by consuming 15 pounds of forage (dry matter basis) daily. However, lactating mares consuming the same amount of Ber- muda grass pasture will be deficient in en- ergy, protein and phosphorus. The same lac- tating mare cculd supply all of her nutrient requirements from 60% low endophyte res- cue: 40% ladino clover pasture is she were able to consume 90 pounds per day on an as- fed basis. Unfortunately, her limited diges- tive tract will normally notpermit such intake levels.

A similar nutrient deficiency occurs in pregnant mares forced to graze mature Ber- muda grass pastures. Such nutrient deficien- cies can be corrected by supplementing a balanced concentrate mix in moderate amounts in addition to the pasture. Trace minerals and vitamins should be supple- mented free choice with all pasture. Loose trace mineralized salt (granular form) or trace mineralized salt blocks will provide adequate trace minerals if the soil is not trace mineral deficient. However pastures, known to be trace mineral deficient should be sup- plemented with a 1% trace mineral-vitamin premix added directly to a concentrate mix. Trace mineralized salt alone will not provide high enough trace mineral levels to meet the deficiencies in such soils.

Caution must be observed with over- grazed pastures. In such situations, horses are forced to graze weeds, some of which may be noxious. Horses will avoid consuming such weeds when adequate forage is available. One of the first warning signs of an over- grazed pasture is the consumption of wood. Horses are attempting to replace the fiber deficiency, due to a lack of forage, with the fibrous wood product. Although the con- sumption of wood will not cause digestive problems, it is not economically feasible. Long stem forages should be supplemented in such situations.

The utilization of quality forages, pref- erably as pasture, should be the basis of a sound, economical feeding program. During periods of raising feed costs, horsemen must use the economical grazing benefits to elimi- nate or reduce the need for hay and possibly grains. However, the entire basis of such feeding programs depends on the accuracy of determining pasture quality and nutrient availability. (From Horse Scribbles)

279