Coronavirus is Affecting Horses Too

COVID-19/SARS-CoV-2 does not infect horses but the lives and welfare of horses are being impacted nonetheless.  They are collateral damage in the need to impose social distancing to slow the spread of the virus.

Many barns are on lockdown because of Coronavirus

Across the country and around the world, events from sales to races to shows are cancelled.  Riding programs in schools are closed.  Access to private barns and lessons is limited and staggered in some areas while others have been locked down completely except for staff.

There have been no reports of shortages of basic supplies and horses are having their core care needs met. Exercise and training are another story.  With limited people and only so many hours a day, exercise can be greatly curtailed leading to both immediate and potential future issues.

To avoid weight gain and possible digestive upset, the diets of all horses must be adjusted.  The following are the average caloric decreases that need to be made depending on the horse’s previous level of activity:

  • Light activity: Reduce by 17%
  • Moderate activity:  Reduce by 28%
  • Heavy activity: Reduce by 38%
  • Very heavy activity:  Reduce by 52%

Those are for horses going from the listed levels of exercise to turnout only. For a different change in activity, subtract the reductions – e.g. from moderate exercise to light = 28% – 17% = 11% reduction in calories.

The easiest way to do this is to reduce everything fed by the same %, so a horse starting with 5 pounds of grain and 15 pounds of hay would go to 4.5 pounds of grain and 13.5 pounds of hay with a 10% reduction.  Another option, which keeps the horses happier, is to substitute hay for grain after you calculate the reductions.

Since grain is more calorie dense, instead of a pound of  commercial grain mix you can feed 2.5 pounds of hay.  In the above example, replacing the 4.5 pounds of grain with hay would give the horse an extra 4.5 x 2.5 = 11.25 pounds of hay! That’s a good bit of diversionary chew time for a bored horse.

For mental health of all horses, the goal should be a bare minimum of 20 minutes of turnout once or twice a day. This is the average amount of time a stallbound horse will spend running, bucking and rolling before settling down.

The loss of regular exercise can have more  serious consequences for horses with metabolic syndrome and muscular disorders where exercise is an important part of helping their metabolism function normally.  Restricted exercise is also more serious for seniors who can lose muscle mass and develop joint stiffness quickly.  Every effort should be made to provide more than minimum turnout or formal lunging for these groups.

Finally, remember that when restrictions are lifted and activities/competitions resume, your horse won’t be as physically ready as he should be if exercise was interrupted. Better to miss a season than risk hurting the horse.

Stay safe!

Eleanor Kellon, VMD

Posted in Equine Nutrition | 2 Comments

Is Your Horse Overweight?

Carrying excess poundage is a serious health issue for any species but when your normal weight is already 800+ pounds the problem is magnified.

Even if pregnant, these horses are grossly obese

Remember basic physics?  Force = Mass x Acceleration. Mass is your horse’s weight.  The stress experienced by the skeletal system, the heart and the lungs is directly proportional to the horse’s weight. Even standing still the pull of gravity has a greater toll on the heavier individual.

A 2017 Canadian study (Kosolofski et al) found 28.6% of horses were overweight or obese.  A 2016 Australian study (Potter et al) found a similar picture in horses with 23.1% overweight or obese but pony types other than Shetland higher at 32.1% and Shetlands led the list at 71.5%. Most owners significantly underestimated how much excess body fat their animals were carrying.

In the United States, a 2018 study in Maryland (Jaqueth et al) also found a serious problem. Of ponies, 41% were overweight or obese and the same was found for 40% of the surveyed full size horses.

Consistent among studies is that owners do not know how to evaluate their animals’ weight.  The first step in correcting this is to become familiar with the Henneke body condition score: .

Many might call this horse too thin but in reality he is a perfect body condition score 5.

The horse’s structural conformation, muscle bulk and muscle tone can lead to mistaken evaluations of body condition score but these are less likely if you carefully follow the Henneke system since it is designed to specifically evaluate the amount of body fat the horse is carrying.

Some specific tips and precautions include:

  • Breeds which have a naturally thick or high set neck should be evaluated first without looking at the neck. Wide necks are not necessarily fat.
  • Very heavily muscled breeds may have a crease down their back at normal body condition scores
  • The body bulk should be proportional to the size of the muscle mass at the upper leg/forearm/gaskin.  The body should not look like it’s perched on toothpicks.
  • Except for heavily pregnant mares or horses with abnormally enlarged abdomens, the ribs should not be visible but should be easily felt

While horses do vary somewhat in how easily they gain weight, the bottom line is still always that if calories in is higher than calories burned the horse will gain weight.  The rule of thumb for weight loss is to feed the horse grass hay at a rate of 1.5% of his current weight or 2% of the ideal weight, whichever is larger.

This applies to average grass hays and some may be more calorie dense, requiring a reduction in amount fed. Alfalfa is more easily digestible so will increase the calorie yield per weight of hay fed.  Horses needing to lose weight should never be fed  grain.  Balancers are also higher calorie and should be calculated in the daily ration assuming 1 lb of balancer = 2 lbs of grass hay.

If you are starting an exercise program with your overweight horse, go slowly to avoid joint or tendon/ligament injury. Do not increase calorie intake until the horse has reached and started to go below the target weight.

Horses on a diet must have pasture intake severely limited or prevented with a muzzle. If not losing on the calculated amount of hay, he’s getting too much grass. Ideally, hay should be analyzed to guarantee adequate protein and identify mineral deficiencies or imbalances. Top off the hay diet with a mineral supplement compatible with the hay’s profile, 4 oz of flax seed, 1 to 2 oz of salt and 2000 IU of vitamin E.

Remember, while many horses will overeat if given the opportunity, you are in control. He relies on you, not will power, to protect him from excess weight.

Eleanor Kellon, VMD




Posted in Equine Nutrition | 1 Comment

Nutritional Support for Shedding

Mother Nature provides our horses with a dense coat to protect them in Winter but getting rid of it in the Spring is a hassle.  Shedding is triggered by changes in day length. There are many factors that may either  delay or enhance the process.

Low thyroid function, parasitism, PPID (Cushing’s Disease) and poor  health in general  are among the serious medical conditions that can delay shedding.  You need to contact your veterinarian for advice if any of these issues is suspected. Otherwise, there are several things that may influence shedding.

As the coat becomes older and disconnected from its nutrient supply it will get progressively drier, coarser and lighter in color.  Darker horses can turn a stark mustard-like color if shedding is not progressing normally.  If you see lightening/”bleaching” of your horse’s winter coat it can be a sign that shedding is delayed.

Exercise increases blood flow to the skin and the production of sweat and sebum. Horses being exercised will shed out quicker. A brisk 15 to 20 minute lunge session followed by turning the horse out to roll before grooming will cut the time it takes to get rid of all that hair. Rolling is the horse’s way of auto grooming.

Sebum is the oily secretion of sebaceous glands which is secreted directly into the hair shaft. Sebum coats and protects the hair, helping to prevent dryness and imparting shine. Sebum also makes shedding easier. Dried forages have only half the fat content of fresh grass. That’s a deficit of over 200 grams of fat (7+ ounces) for a horse eating 10 kg (22 lbs) of hay a day.

Vitamin A is low in preserved forages and rapidly progresses to deficient. It is a critical nutrient for normal function of the skin and associated structures, including the hair follicles and sebaceous glands.  Supplementation of 20,000 to 40,000 IU/day supports those tissues in late winter and spring.

Production of the Summer coat is critically dependent on adequate protein and essential amino acid intake. Hair is 95% protein. One of the most striking differences between hay and Spring growths of grass is the high grass protein level. Supplementation of the three most important essential amino acids – lysine, methionine and threonine – may be helpful during shedding. A ratio of 10-5-2 for lysine-methionine-threonine works well.

We are most familiar with the B vitamin biotin in connection with hoof quality but skin and hair follicles, just like hoof horn, are also epithelial tissues.  While the intestinal bacteria produce abundant amounts of most B vitamins, biotin has been identified as one where supplies may be borderline. Supplementation with 20 to 25 mg/day during the high demand shedding period may be wise.

Last but not least is the trace mineral zinc. Zinc is the most commonly deficient mineral in forages worldwide. Zinc (along with copper) is required for production of adequate levels of the pigment melanin in dark colors, which protects those gorgeous Spring coats from rapid fading. Zinc is also necessary for the rapid cell division needed to produce a new coat. Zinc is in high concentration in the skin where it supports normal cell multiplication and immune system protections. A dose of 250 mg/day is reasonable.

That’s a lot of bases to cover in enhancing shedding and healthy Summer coats but the good news is if shedding needs some assistance there’s a good chance nutrition can help.

Eleanor Kellon, VMD

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One Medicine

One of the introductory lectures when I started veterinary school talked about how the University of Pennsylvania believed strongly in the principle of One Medicine. The German physician and scientist Rudolph Virchow first proposed this concept and said:

“Between animal and human medicine there is no dividing line – nor should there be. The object is different but the experience obtained constitutes the basis of all medicine”.


This belief led to the creation of the (sadly) now defunct United States Armed Forces Institute of Pathology in Washington, D.C. Even as a lowly intern I was welcome to bring in a tissue biopsy of a tough case, and could discuss questions at any time with some of the best minds available.

There are unique features to every species but under the skin and hide we are all mammals. There are far more things about our physiology that are similar, if not identical, than different.  Research in one species can provide valuable insights for other species as well.

Robin Coombs was a veterinarian who developed an antibody test for autoimmune anemia which is used in both humans and animals.  His work also led to the development of tests for other autoimmune conditions such as rheumatoid arthritis and tests to detect blood and tissue incompatibilities.  Coombs developed his interest in immunology and diagnostics early in his career when working on a test for the disease Glanders which attacks horses and donkeys.

Animals have a long history of use in studying disease processes and treatments intended for human use. Mice, rats, rodents, dogs and monkeys are among the most commonly used. Animals can also benefit from research that was begun in humans. Oral joint nutraceuticals and bisphosphonate drugs (e.g. Os-Phos) were first pioneered in humans, as was hyaluronic acid for joint injection and use of platelet-rich plasma (PLP).

Research like this often follows a parallel course, with equine specific studies appearing alongside human. The animal research has the potential to advance more quickly because it doesn’t have the same level of ethical constraints as when working with human subjects. It is possible to better control important variables like level of exercise. On the flip side, interested human researchers  can provide valuable critical review of the equine research, as in this paper: .

One Medicine is especially applicable to nutrition and supplements. While requirements and digestion may vary, the basic activities of nutrients and supplements at the cellular level are largely the same in all mammals. Using allometric equations, dosages in humans and small animals can be converted to equine on the basis of their metabolism rather than pound per pound.

The concept of One Medicine has always served me well. It opens a world of possibilities that would not exist if we had to rely on equine specific research alone.

Eleanor Kellon, VMD

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Digestive Issues in Horses on Dry Diets

When horses can’t have fresh pasture, either because of stall confinement or the season, the logical next best thing is hay. After all, hay is just dried grass. All the major nutrients – fiber, protein, carbohydrate – are basically the same with one important exception. Water.

Hay averages around 10% water while grass is at least 70%.  Pasture is not only a high moisture food, the water is delivered at the same time as the nutrients requiring digestion. Water is arguably the most important but overlooked component of digestive tract function. Adequate intake is required for:

  • production of saliva, pancreatic juices, bile and other secretions into the GI tract
  • function of digestive enzymes
  • absorption of digested food
  • fermentation of fiber in the hind gut
  • smooth passage of food through the digestive tract

When horses are on pasture they get so much water from grass that they may drink very little water, or none at all.  The move to a hay based diet is a huge change. Water intake is partially driven by how dry the diet is but there is also an important hormonal component.

Thirst is regulated in the brain by cells which “read” the sodium concentration of the blood. If water is low, sodium concentration rises. The first adaptation is to decrease urine production and make it more concentrated. Water also moves from the tissues and intestinal tract into the blood. This is the beginning of disrupted intestinal function. If sodium level continues to rise, the horse will be triggered to drink and restore normal hydration. If it doesn’t, he will stay in this state of blood levels OK but tissues and gut dehydrated.

Guaranteeing adequate salt (sodium chloride) intake protects against dehydration. Your horse needs salt, even in winter, because sodium is lost in the manure and urine every day.  If salt intake is too low, the horse will stay in a constant state where tissues and intestinal contents are dehydrated to keep blood salt and water levels normal (above).  When salt intake is normalized, blood concentration rises quicker and the horse is triggered to drink.

You can also support digestive health by feeding a probiotic supplement which also has high digestive enzyme activity. This helps ensure that protein, fat and simple carbohydrates get digested where they should, in the small intestine, while fiber passes back to the hind gut.

Feeding an additional soluble fiber source, like psyllium husk, also greatly supports the hind gut. This should be fed after fully hydrating it with water. The fiber will carry water to the hind gut contents and also serves as a prebiotic, enhancing growth of fiber fermenting organisms.

Finally, make sure the horse has access to water that will not freeze over. Better yet, slightly warmed water encourages intake. You cannot rely on the horse eating enough snow to support good gut function.

Cold weather and dry diets pose some challenges to good digestive health but understanding the issues, and the relatively simple fixes, will help you keep your horse’s GI tract functioning well.

Eleanor Kellon, VMD


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Fasting Insulin in Equine Metabolic Syndrome

Abnormally high insulin is a key feature of Equine Metabolic Syndrome and responsible for the high laminitis risk but there has been considerable uncertainty regarding the best way to test for abnormal insulin.

The earliest suggestion was to fast the horse or pony before testing insulin level. This seemed to make sense since humans are fasted before diabetes testing. However, horses are not humans. Equine Metabolic Syndrome is not the same as Human Metabolic Syndrome or human type II diabetes.

It became evident that many horses and ponies with metabolic syndrome were testing negative after fasting.  Part of this was because horses do not typically maintain high glucose and insulin levels when fasted in the way many metabolic syndrome humans do. An even bigger problem was what was being called “normal”.

To be a positive result, a threshold of 20 uIU/mL for a fasting insulin (aka basal insulin) was set.  If below 20 uIU/mL, the test was considered negative.  However, a review of reported equine insulin levels in published studies and PhD dissertations shows a level of 20 uIU/mL is more typical of a normal response to a grain meal than fasting. This high threshold resulted in a high percentage of the animals classified as positive on the test being correctly diagnosed (high specificity) but a very large number of cases were miscategorized as normal (low sensitivity).

Various challenge tests were then devised to pick up cases missed by fasting insulin. These included grain, Karo syrup feeding or glucose powder in a high fiber meal.  They are more time consuming, more expensive and not without risk. Another approach would be simply to have a more accurate threshold.

Olley et al (2019) did exactly that. They compared the fasting insulin levels of 62 horses and ponies with results of a CGIT – combined glucose and insulin test. The CGIT is an intravenous test for insulin resistance.  They determined the upper limit of normal for a fasting insulin should be 5.2 uIU/mL – not 20! The authors pointed out that the problem wasn’t that insulin is a poor test; it was the cutoff that was to blame.

The findings aren’t a surprise to anyone familiar with the many other prior studies looking at equine insulin. The 5.2 uIU/mL ceiling is in line with previous reports. However, one problem with using fasting insulin as a screening test is that many assays may not be reliable at these low levels.

The Equine Cushing’s and Insulin Resistance Group,,  has never recommended using fasting insulin because of the large number of incorrect results with the 20 uIU/mL guideline and because high insulin occurs as a response to what is eaten.  It stands to reason that insulin after a bucket of sweet feed won’t be the same as after munching hay or not eating at all for an extended period. Olley also pointed out insulin “normal” ranges have to be defined by the feeding situation.

ECIR recommends testing horses with access to known low sugar/starch hay or pasture from the night before, overnight and on the day of testing. Insulin and glucose are tested.  The glucose:insulin ratio and insulin resistance proxies of RISQI and MIRG, as defined by the work of Dr. Treiber’s group (2005, 2006), are used to classify the horses and ponies. ECIR has a calculator on their information site which can compute the proxies for you: . Cornell’s diagnostic laboratory is recommended as they use the same methodology for testing insulin as the original work.

The proxies ECIR uses have been developed from what is universally recognized as the “gold standard” in insulin resistance testing, the FSIGTT – frequently sampled intravenous glucose tolerance test. They were then field tested on ponies living at pasture, with no fasting before testing. The pretest feeding protocol was designed to mimic that.

For more background visit

Eleanor Kellon, VMD



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Anemia in Horses

Let’s get one thing out of the way right up front.  Insufficient iron is not the cause of your horse’s anemia, and feeding more iron is not going to fix it. In fact, it might make things worse.  Even if your horse has a source of blood loss, like bleeding ulcers, he has more than enough iron stored in his body to replace that.

Anemia is an abnormally low number of circulating red blood cells

Except for foals raised without access to soil or with heavy parasite burdens, iron deficiency does not exist in horses. Too much iron causes oxidative stress which makes red cells more fragile. It can also cause secondary deficiencies of other minerals critical for red cell production, like copper. If you can’t get iron out of your head, prove it’s not iron deficiency with a serum iron or a full iron panel from KSU’s Comparative Hematology laboratory.

Iron deficiency also causes a characteristic type of anemia called microcytic hypochromic. This means that the red cell size, measured by MCV on the lab report, will be low and hemoglobin concentration,  MCHC, is also low.  If you don’t see this, it isn’t iron deficiency.

Full blown anemia is rare in performance horses but they may develop drops in their red cell counts, which normally run on the high end of normal. The cause here is usually oxidative stress from exercise which makes the red cells more fragile and easily damaged. The solution is to avoid excess pro-oxidants like iron supplements (especially intravenous) and institute supplementation with antioxidant nutrients commonly deficient such as vitamin E, selenium, copper and zinc. Because of the stress they are under and potential compromise of the gut microbiome, B vitamin supplementation may also help.

The most common cause of anemia is horses of all ages is anemia of chronic disease. When the body is under attack from organisms (e.g. Lyme, internal abscess, Strangles), bowel or other organ disease, cancer, severe wounds or burns, iron is made less available because it fuels inflammatory processes and growth of organisms.  This type of anemia will resolve when the underlying condition is corrected.

Older horses with or without PPID (pituitary pars intermedia dysfunction, aka Cushing’s disease) very commonly have a mild anemia with red cells counts and hematocrit/PCV hovering at or just below the lower limits of the laboratory ranges.  The combination of less exercise and an age-related drop in metabolic rate decreases the oxygen requirement of senior horses so some drop in red cell numbers is  physiological. There can also be an element of anemia of chronic disease in many older horses. In any case, the anemia is almost always not the primary problem.

Finally, there are infections that attack the red cells directly, like the Equine Infectious Anemia (EIA) virus, Babesia/Piroplasmosis, Leptospira and Ehrlichia.  These horses are usually obviously ill, have fever and sudden drops in red count.  The horse’s overall condition will alert the veterinarian to look for these disorders.

Sound nutrition is important for creation and maintenance of good red blood cell parameters.  In addition to adequate protein and calories, pay attention to:

  • vitamin E for antioxidant defenses (do not rely on the E in multiingredient mixtures; add it separately)
  • B complex vitamins (especially in older, stressed and compromised horses)
  • generous levels of the usually deficient selenium, zinc and copper, with low to no added iron to compete for absorption

On the herbal front, Echinacea has been documented to support healthy red blood cell production in horses.

Eleanor Kellon, VMD

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Sulfate – Essential to Health

Sulfur in the form of sulfate is essential for life in organisms from bacteria to plants to horses.  Sulfate is an ion with a negative charge, SO4-2

Copper sulfate – preserving animal health since the 18th century

Bacteria in the soil form sulfate from sulfur and oxygen. Soil sulfate may complex with other minerals or be absorbed by plants in its free form.  Most of the sulfate in plants is used to manufacture the sulfur containing amino acids. Plants also need sulfate to make chlorophyll and chemicals that protect them from insects.

Sulfate  forms of trace minerals like zinc and copper are highly bioavailable. The intestinal tract can also absorb the sulfate and has a special receptor for it called NaS1 – the sodium sulfate cotransporter. The same receptor is found in the kidney and reabsorbs sulfate from filtered blood to retain what the body needs. Sulfate absorption decreases when supply is abundant and vice versa.

While the body can, and does, absorb sulfate ions, sulfate can be generated from sulfur amino acids and this is the major source.  Sulfate is formed in the terminal breakdown of those amino acids.  Studies have shown that a completely sulfur/sulfate free diet will still support life if it supplies the required amounts of sulfur containing amino acids.

Free sulfate ions combine with ATP to form a compound called PAPS which supplies the sulfate for all body functions which require it – and there are a lot of them:

  • sulfation – adding a sulfur group – is one of the main pathways for detoxifying and then excreting things like heavy metals and toxins of many different kinds including xenobiotics
  • production of bile salts
  • glycosaminoglycan synthesis (sulfates of chondroitin, heparin, dermatan)
  • elimination of steroid and thyroid hormones
  • activation of some hormones and cellular receptors
  • mucus production, including the mucus which protects the stomach from ulceration
  • essential for the formation of myelin sheaths which protect the nerves

The level of sulfur amino acids and sulfate in the diet is decreasing in many areas.  Air pollution control legislation has dramatically reduced the amount of sulfur dioxide in the air.  This used to be a rich source of sulfur for the soil, where bacteria converted it to sulfate.  Since sulfur is a key nutrient for plant growth, fertilizer mixtures increasingly need to contain sulfur.

You can check for the adequacy of sulfur amino acids in your horse’s hay by making sure the analysis includes the percentage of sulfur then check the N:S – nitrogen sulfur ratio. To determine nitrogen, divide the percentage of crude protein by 6.25. The N:S should be between 10:1 and 15:1. Also, to support normal amino acid production in the hay the sulfur should be at least 0.2%. These recommendations are from research on cattle but since most sulfur in the diet is in the form of sulfur amino acids it should work as a minimum guideline for the horse as well.

In summary, sulfate is an important nutrient for the horse.  Some comes directly from sulfate in the diet but most is derived from the metabolism of sulfur amino acids. Levels in the diet are dropping as sulfate in soil becomes less plentiful. Guard against this by supplementing 2.5 to 5 grams of methionine/day for the average size horse.

Eleanor Kellon, VMD

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Optimizing the Immune System

The horse’s immune system is always busy. Even when the horse is healthy it is performing its background functions of removing dead or damaged cells, guarding all the mucus membranes that are portals into the body from the outside world (intestinal tract, eyes, respiratory tract, genitourinary tract), and reacting to any organisms or foreign substances that make it past the body’s barriers of skin and mucus membranes.

Herbs can be part of an effective immune support program

The immune system is never inactive, but in health it performs all these functions silently without any outward signs.  Let a serious challenge come along though and it’s like poking the bear. The signs of a fully activated immune response come to the surface such as fever, muscle and joint pain, lethargy, inflammation. When stimulating the immune system, be careful what you wish for!

As with all body systems, there is a network of checks and balances for the immune system to keep reactions in check. This is termed homeostasis. For optimal functioning there must be support of all arms of the immune system, including reactions to regulate the responses.

The first step in supporting a strong immune system is nutrition. The horse needs both adequate quality protein and sufficient calories to maintain a normal body condition. On the vitamin and mineral end, B vitamins, vitamin A, vitamin E, vitamin C, magnesium and the trace minerals selenium, zinc and copper are particularly important to both vigorous reactions and the control countermeasures that keep them in check.

Immune system activity generates large amounts of reactive oxygen species (ROS), also known as free radicals or pro-oxidants.  Damage to surrounding tissues and the immune system cells themselves can result from this friendly fire. The antioxidant vitamins, glutathione, N-acetyl-cysteine and enzyme systems such as copper and zinc superoxide dismutase are the body’s first line of defense against these but the protection can be boosted by other plant based sources of antioxidants such as citrus bioflavonoids, brightly colored fruits plus herbals like Turmeric and Pau d’Arco.

Spirulina is a powerful antioxidant and also demonstrated to support the production of protective IgA antibody along mucus membranes rather than the IgE that is associated with allergic reactions.

Some herbs are well documented to support the maintenance of both a protective and balanced immune system, including Echinacea, Ginseng, Astragalus and arabinogalactans from Larch. Other naturally occurring products support alert defenses by providing gentle stimulation to the rich immune system of the intestinal tract – the GALT or gut-associated lymphoid tissue.  These include the beneficial strains of probiotic organisms and mannanoligosaccharides from the cell wall of Saccharomyces cerevisae yeast.  Cells originating  from activity in the GALT communicate with the immune system throughout the body.

From directly battling disease to enabling robust responses to vaccines, the time to think about immune system support is at least 4 weeks before the challenges may come along. By properly fueling the immune system in advance it will be prepared to defend your horse in the best way possible.

Eleanor Kellon, VMD

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Lyme Disease in Horses

Tick season will be starting up again soon and with it the risk of Lyme Disease. The first report of horses carrying antibodies to the organism causing Lyme Disease, Borrelia burgdorferi, appeared in the scientific literature 35 years ago. Skepticism abounded regarding Lyme Disease (LD) actually existing in horses (antibodies only confirm exposure, not infection or disease) and continues to some extent today.

In North America, Lyme Disease is most often spread by deer ticks

The disease is spread by the bite of infected Ixodes/hard-shelled ticks. In people, the first symptom is often a rash that expands around the bite site and resembles a bull’s eye target. This has not been observed in horses.  Fever, fatigue and body aches may occur or there may be no other signs. Treatment at this stage can be curative.

Untreated infections can progress to more serious problems as the organism migrates to tissues other than the skin. It has an affinity for the joints and nervous system. It may also attack the eyes or heart.

LD is most frequently suspected in horses which show unexplained lameness/arthritis and stiffness that shifts in locations. This is the most common symptom of late LD in people. However, in horses these cases are often poorly documented as having been definitely caused by B. burgdorferi.  A particular severe form of laminitis has also been suspected to be caused by LD and it may invade tendon sheaths.

There are more confirmed cases of LD involving the eyes (uveitis) and nervous system (neuroborreliosis) in horses.  A skin condition, cutaneous pseudolymphoma, has been found in infected horses.

Diagnostic efforts usually involve blood tests for antibodies. Problems with this include cross-reaction with other infections in some assays, trouble differentiating between active infection versus past exposure, and false positives from vaccination. One of the best tests for sorting through these issues is the Lyme Multiplex .

The organism is a bacterium of the spirochete type and treatment is with antibiotics.  Unfortunately, by the time a horse is diagnosed they are in the advanced disseminated phase when actual cure is extremely difficult and even when remission of signs is achieved the horse may need to be treated again in the future.

Researchers have been unable to experimentally induce LD in horses using deliberate exposure to infected ticks. Although the infection was successful, no clinical signs developed. This makes it very difficult to study the effectiveness of various antibiotics. However, in one trial of experimental infection, in treatment started 3 months after the start of the infection only 4 weeks of  intravenous tetracycline was successful in eliminating the organism. Half of the test ponies treated with  intramuscular ceftiofur and 25% of those treated with oral doxycycline also cleared the infection.

Four weeks of intravenous treatment is both impractical  and prohibitively expensive for most owners. Unless dealing with a serious neurological case, most practitioners rely on treatment with oral doxycycline or minocycline which is usually well tolerated.  Some will start with a few days to a week of the intravenous tetracycline.  Response is typically good although, as above, it may need to be repeated in the future.

Protective measures include keeping your horse’s environment closely mowed and free of brush. Use tick repellant chemicals especially when venturing into areas that may harbor ticks. Permethrin is best and also an excellent fly repellant. There are no equine LD vaccines. Veterinarians may use canine vaccines in horses but the protection has not been carefully evaluated. A typical course for best antibody levels is a 2 injection initial series with boosters at 6 month intervals.  The Multiplex assay can be used to monitor vaccine titers.

Lyme is a complex disease with potentially serious consequences.  Speak with your veterinarian about the best program for protection and monitoring in your area.

Eleanor Kellon, VMD

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