The Natural Horse In Our Modern World

A look down the rabbit hole of medical science

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Free roaming horses in our manmade world  .  Stress hormones and happy hormones (and neurotransmitters)  .  Ulcers  .  Colic  .  Laminitis  .  Equine digestion . Evolution of the modern horse .
If you are seeking medical advice for your horse talk to a professional. This is for interest only, I cannot guarantee this is wholly accurate.
All this information is widely available on the internet, I have drawn from many sources, referenced after each section. I have tried to bring it together here in a clear digestible fashion.

Free roaming horses

In their natural habitat wild horses would spend many hours per day with their head down, grazing on wild grasses and other vegetation, and occasionally snoozing too. They would also browse shrubs and trees and move seasonally to find healthy vegetation. They would eat small amounts on and off, they don’t need to eat all the time. Over a 24 hour period they may spend 15-18 hours eating and only 3-5 hours sleeping and not all at once, they mostly sleep standing and must lie down for REM sleep.

In our manmade world

Unfortunately, this isn’t possible for many domesticated horses, particularly competition horses where, in some cases, what they are given to eat and when they can eat it may be more aimed at winning than the health of the horse. So, we ought to find a best compromise between what they need for good health and what we are able or want to provide.

Hays and haylage vary in nutrient content, and we supplement with mineral licks and grains, particularly for competition horses requiring a high calorie intake. Many fields lack a natural variety of vegetation, with grasses that are too nutritious and fast growing for horses, and in winter months they may hardly be growing at all.  A diet based mainly on good nutritional forage and turn out on well established organic grazing could be best for digestive health.

Stress hormones & happy hormones (and neurotransmitters)

They are the same for most mammals, for us and horses too!

The hypothalamus, the pea size central control unit, is a gland in the brain that responds to external information from our senses. It is possibly the most important central control unit for our brain and body. It controls the Autonomic Nervous System (ANS) providing rapid neurotransmitter actions mostly in the brain, and it controls the Endocrine system which provides the hormone actions around the body via the bloodstream. The ANS is thought to have two parts, the Sympathetic Nervous System (SNS) more active during stress or excitement, and the Parasympathetic Nervous System (PNS) more active during calm, not to be confused with the Peripheral Nervous System (PNS).
The balance, or homeostasis, of these systems is of course not so simple as one on, one off, and they are effected by more than just scared/calm.
The hypothalamus, and it's teams of hormones and neurotransmitters, also respond to being excited, interested, amused, to enjoyment of activity or food, companionship, feeling cared for, etc.

In a natural stress response to perceived or actual imminent danger:
  • First up it's the Catecholamines group (Dopamine, Norepinephrine, and Epinephrine (Adrenalin)), as neurotransmitters, jolt you into instant action with astounding speed, strength, alertness and precision beyond what you could do if you thought about it for a moment.
  • Joined almost instantly by more actions from Adrenaline, as a hormone, as it raises your heart rate, directs blood where it's needed, etc
  • Next it's Cortisol hormone, which keeps you going further by releasing extra glucose into the bloodstream, releasing extra insulin to tell muscle cells to take up that glucose, and suppressing bodily functions not needed when legging it from danger, such as digestion, salivation (dry mouth), and urination.
  • Keep going and Endorphin hormones kick in, the body's own painkillers. As you leg it further and might be hurting with the effort or hurting from charging through things.
  • Finally as you reach a place of safety, still flowing Endorphins help you relax, and new research suggests its endocannabinoids that give you a feeling of euphoria (runners High) and Dopamine rewards you with a good feeling.
  • As you settle, Catecholamines and Cortisol subside over time to normal levels.
  • Finally finally Serotonin flows again, triggered by the effort of escaping, followed by sensing a calm environment, stabilising your mood and making you feel happy again.
Are these chemicals hormones or neurotransmitters? That simply depends on where they are being used, many act as both, usually more as one than the other, the title indicates which system they are operating in. They all have many functions around the brain and body, and therefore in long term excess (or deficiency) can have many negative effects too.

The following paragraphs describe the good and bad effects 
of raised levels of these chemicals on the brain and body.

The Catecholamines group:

Dopamine (neurotransmitter), Norepinephrine (neurotransmitter) and Epinephrine 'Adrenaline' (hormone mainly, also a neurotransmitter). A group of chemicals produced (one from the other in that order) in a complex chain of chemical actions, from tyrosine an amino acid obtained by eating protein.
These first three have roles in the instant energetic response. One of dopamine’s jobs is motor control, it plays a part in pulling your hand away from a hot surface. Too much of it and your jittery hyperactive, severe deficiency causes Parkinson's disease.

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Dopamine (neurotransmitter mainly):

It drives motor control, motivation, learning, memory, and gives reward. Dopamine release is increased during fight or flight and afterwards.

Good effects
• Focuses attention.
• Motor control in response to sensory input such as pulling away from danger.

Dopamine release is mostly increased in anticipation of a reward, the greater the anticipation the greater the release. To keep getting that 'high' more and more rewarding things are sought. 

• Thought processing, memory. Remembering you like chocolate bars, or remembering something that was dangerous.
• Increases motivation. The drive, the need, to want something like a chocolate bar, or to get away from danger.
• Reward. Happiness and euphoria. One of the feel-good hormones (neurotransmitter mainly). Reward for achieving a goal such as finding a hidden chocolate bar, achieving a personal best, or reaching a place of safety.

Bad effects
• Panic attacks
• Raised blood pressure
• Hyperactivity
• Addiction. Read the last three good effects again, and again, and…

Norepinephrine (neurotransmitter):

Good effects
• Increases alertness
• Vigilance
• Focuses attention
Bad effects
• Can cause restlessness
• Elevated blood pressure and heart rate
• Irregular heartbeat
• Sweating
• Anxiety etc.

Adrenaline (epinephrine) (hormone mainly):

Neurotransmitter in small amounts involved in the instant reaction to danger.
Hormone in larger amounts from the adrenal glands.
Good effects
• Increases the heart rate.
• Elevates the blood pressure.
• Directs blood to important muscles and organs
• Dilates the airways. Think EpiPens for anaphylactic shock.
• Triggers blood sugar release, the same as cortisol.
Bad effects
• High blood pressure
• Nervousness
• Anxiety
• Sweating and shaking.

Cortisol (hormone):

Released from the adrenal glands, to prepare the body for further physical effort after the immediate reaction.
Good effects
• Tells the liver to convert its stored protein and fat into glucose and release it into the blood.
• Tells the pancreas to release insulin. Insulin enables the body’s cells to take up glucose from the bloodstream, so the muscles get extra energy for running away.
• Suppress functions not needed when running, including digestion.
• Away from stress it acts on your circadian rhythm, rising in the morning to get you going.

Bad effects
• Can lead to excess insulin, which can lead to insulin dysregulation, which is a well known cause of Metabolic Laminitis (3.1), amongst other bad things.
• Suppresses or disrupts digestion in various ways.
• It can also have a negative effect on the stomach lining mucus, (1.1 Gastric Ulcers).

Endorphins (neurotransmitters and hormones):

Good effects
• Interrupts pain signals to the brain. The body’s own opiate pain killers. Entomology “endogenous,” meaning from the body, and “morphine”.
• A feel good neurotransmitter. Boosts mood and self-esteem.
• Reduces stress, depression, and anxiety
Bad effects
• There aren’t any from the amount the body can produce.

Serotonin (neurotransmitter and hormone):

Good effects
Mood stabiliser. Wellbeing and happiness.
Important for brain function
Important role in regulating the digestive system.
Brings on sleep in the circadian rhythm.
Bad effects
Similar to those of too much of a catecholamine.

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Three common digestion related illnesses

1 - Ulcers:

1.1  Stomach (gastric) Ulcers. Typically caused by physical or emotional/psychological stress, also by higher than recommended NSAID doses, particularly on an empty stomach, same as us.

  • Physical stress (often accompanied by psychological stress) causes ulcers to occur on the band between the lower and upper stomach and in the upper
    stomach. Being exercised or transported with an empty stomach basically causes the acid, continuously produced in the lower part, to be splashed up there.
  • Emotional or psychological stress can cause ulcers in the lower stomach, here’s how. Just like for us, being upset causes a release of hormones which then cause all sorts of problems. For a horse being stuck in a stall for long periods, or unable to see other horses in close company (friends), or a sudden change of environment, can all cause stress. Cortisol is released, the ‘fight or flight’ hormone, this somehow causes a slowdown in the production of prostaglandins in the stomach lining mucus, they help protect the lower stomach lining from stomach acid.
  • NSAIDS. Too high doses or prolonged high doses can interfere with the protection of the lower stomach lining from acid, partly by disrupting the production of prostaglandins.

1.2  Colonic Ulcers. Are much less common. Researchers say they can occur in any part of the colon, though usually these are the causes and locations…

  • NSAIDS. Can cause ulcers, usually in the Right Dorsal Colon, the last big bit. Again it’s too high doses being given to competition horses, some horses are more sensitive to NSAID side effects at lower doses.
  • Volatile Fatty Acids. If undigested grain feed reaches the  caecum, first of the hindgut fermentation tanks, excessive production of VFAs can occur here.
Sources: Vetmed, Science Direct, Today's Veterinary Nurse.

2 - Colic:

Colic for horses is regarded as pain in the gastro-intestinal system. There are many types and causes. Here are the most common causes.

2.1  Gas colic. Can occur anywhere in the system. Can be caused by eating more grain than forage, or a quick change to eating too much food too rich in sugars, when the system has not had time to adapt. The two classics being fresh grass in the spring and sugar rich grain, especially if some of this undigested food gets past the enzymes of the small intestine, into the hindgut where the fermentation microorganisms will have a party and produce far too much gas. And the only way out is the back way!
2.2  Spasmodic colic. Where the peristalsis movement is disrupted, and the muscles go into painful spasms. The main cause is thought to be over excitement, a sudden stressful event. The other typical cause is again a sudden change of diet causing gas, the peristalsis movement can’t shift the gas, becomes irregular and goes into painful spasms.
2.3  Impaction colic. The hindgut is long and complex with many changes of direction. There are a few ways impaction can occur, where food gets stuck, build up, and causes painful stretching. Such as, again eating too much too quickly and maybe while standing still too much, eating too much straw bedding can also contribute, a build-up of sand, large volumes of poorly chewed forage or grains ingested with little saliva or additional water to drink.
Regular exercise does help food move through the gut.
2.4  Stomach distention colic. Eating to much too quickly causing the small stomach to be painfully over stretched.
2.5  Displacement colic, twisting or entrapment. Movement resulting in twisting or squeezing against the outside of the body. Can be caused by swellings in the small or large intestines due to gas or other means.
Sources: My Horse University, Horse Vet, Horse and Hound, Horse Journals.

3 - Laminitis:

A symptom, not an illness. Is pain from the lamellae tissues bonding the outer hoof to the bone. A painful condition something akin to a toenail coming off but far more serious. Laminitis is not a disease in itself but a symptom of other health issues, some short term, some long term, requiring greater management. So, what has it got to do with digestive health? Have you ever noticed white patches, ridges, bumps, and thin areas on your fingernails after lengthy illness? Your nails are affected by your health too.

What are lamellae? Without getting too medically technical here, the outer hoof as it grows down is bonded to the last bone in the foot, the coffin bone (pedal bone in US), by interlocking scaley tissues, the lamellae (laminae US) on each part, with a gluey layer in between.

There are numerous often interacting causes. Poor digestive health is a major factor in inflammatory laminitis, and a contributing factor in metabolic laminitis. Another form or cause is overload laminitis. Inflammatory laminitis Is pain from this firm tissue, lamellae, becoming swollen, enflamed, “itis” means inflammation. Metabolic laminitis is pain from the lamellae due to the bone beginning to separate from the inner hoof wall. The lamellae are weakening, lengthening, and beginning to break down due to lack of nutrient from blood sugars, they can no longer transfer the weight to the hoof. As it gets worse the coffin bone begins to peel away from the hoof inner wall and point down and sink down in the foot. Overload laminitis is pain from the lamellae from standing too much weight on that foot for long periods, usually because of pain or injury in another leg.

3.1  Metabolic laminitis

  • Is now generally accepted to be the most common form, digestion does play an important part, so it is relevant here. We’ll start with the troubled lamellae and work our way up to the causes.
  • All tissue in the body gets nutrients from the bloodstream, the greatest being sugars and oxygen for energy. The lamellae tissues begin to weaken when they are not getting enough blood sugar. Body tissues get sugars from the blood via receptors on the vessel walls that take it and pass it through into the tissue. Generally, around the body the hormone insulin, released from the pancreas, bonds to these receptors and makes them take up sugar from the blood. If these is too much insulin in the blood the receptors can begin to ignore it or not respond to its signal to open up, this is known as Insulin Resistance, a part of a newer term ‘Insulin Dysregulation’, the sugar then goes to be stored in the liver and fat.
  • Insulin at work: Eat food > glucose in the blood > insulin released > glucose distributed > glucose level in the blood goes down > insulin level goes down again.
  • Insulin resistance: Eat food > glucose in the blood > insulin released > body tissue receptors ignore insulin and don’t take up glucose > glucose still in the blood so more insulin released > glucose slowly stored in the liver and fat > feel hungry and lack energy > eat more food > until blood glucose levels begin to return down to normal, insulin continues to be released.
  • It has been known for a long time that high levels of insulin cause laminitis, but how exactly is still not totally clear, as it’s now known that there are few if any receptors in the hoof lamellae area that respond to signals from insulin, instead there are receptors that respond to a similar hormone called Insulin-like Growth Factor-1 (IGF-1).
  • Many studies are ongoing, and some of the possible causes found may be: High levels of insulin somehow disrupt pickup of the IGF-1 hormone signal for those receptors to take in blood sugars. High levels of insulin disrupt blood perfusion (cause flow resistance in the capillaries and tissue), as high levels of anything in the blood do. High levels of insulin cause an over production of endothelin-1, one of the hormones that controls blood vessel size on demand, this one causes them to narrow.

So what causes insulin dysregulation? Three common causes for horses are EMS, PPID (Cushing’s Disease), and poor digestive health which is also important when considering the first two.

  • Equine Metabolic Syndrome (EMS) has multiple facets, a major one is insulin dysregulation of which resistance is a major part. One cause of insulin dysregulation seems to be when fat cells release stored hormones, the significant one here is excess cortisol (the fight or flight hormone again), which causes an increase in the release of insulin. Therefore, overweight horses may be susceptible to EMS. Genetics and lack of exercise also play a part. So it’s obvious a carefully controlled diet will help here, keep blood sugar levels in balance with the demand from exercise.
  • Pituitary Pars Intermedia Dysfunction (PPID or formerly Equine Cushing’s Disease) is most common in older horses. It is caused by a degradation of neurons between the hypothalamus and the pituitary gland, along with the subsequent loss of dopamine, which affects communication between these two. The middle lobe of the pituitary gland, the pars intermedia goes into over production of various hormones that control many systems in the body such as metabolism, reproduction, growth, blood pressure, mental health, and others. Among the effects on metabolism is insulin dysregulation. So here again a healthy balanced diet is important.
  • Poor digestive health as a cause of insulin dysregulation is pretty much already explained, it’s a result of too much glucose sugar in the blood, and the cycle of too much insulin being released.

3.2  Inflammation laminitis

The acid produced from digestive problems such as colic, diarrhoea, or a sudden large intake of sugars can cause lesions, weakening of the intestinal walls. This can result in bacterial toxins getting into the blood, and the immune system releasing inflammatory cells (disease fighting cells) into the blood. These bacterial toxins can cause infection around the body including the lamellae, and the increase of blood flow to these infected cells, caused by the immune system cells going to fight the infection, cause inflammation. The resulting inflammation in the lamellae causes pain due to the immense pressure from the weight of the horse. The pressure will restrict blood flow. Add on flow restriction due to the volume of toxins and immune system cells in the blood, and you have the beginnings of metabolic laminitis type symptoms.

3.3 Overload laminitis

Overload laminitis can lead to inflammation laminitis symptoms, and then to loss of blood flow resulting in metabolic laminitis like symptoms.

Sources: Royal Veterinary Collage London, University of Liverpool, The Laminitis Site, American Association of Equine Practitioners.

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Equine digestion

All domesticated animals have been selectively bred over hundreds, in some cases thousands of years, for the physical and behavioural attributes we desire. Compared to the pace of evolution these have been rapid changes. Many things about all breeds are still basically the same as their wild ancestors, including their digestive systems.

The equine digestive system is defined in two parts, the foregut which includes the stomach and small intestine, and the hindgut or large intestine which includes the caecum, large and small colon. For an adult 500kg horse the whole system is about 30m long with a capacity of about 180L.

Teeth. Horses chew in a complex sweeping action of both side to side and forward backward movement as well as up and down, and the upper jaw is wider than the lower jaw, creating more area for this action to grind fibrous foods. A diet high in grains and low in forage requires less lateral chewing action that does not pass beyond the outer edges of the teeth, resulting in sharp points developing on the outside edges. If these are not filed down or ‘floated’, they can cause significant effect on efficient chewing, rate of eating, mood, and poorly chewed forage may get stuck causing choking. Less forage and more grain also means the continuously growing teeth will not be worn down by the silica content of the grasses and need regular filing/floating.

1 - Foregut

1.1  Oesophagus. They have a long neck and oesophagus. For food to pass easily along this it must be well chewed and contain plenty saliva. Preventing horses bolting their food down is important to avoid choking.

1.2  Stomach. Horses have only one chamber to their stomach, it does have an upper and lower part, and it is the smallest stomach in proportion to body size of all domesticated animals, about 7.5-15 Litres, and only 10% of the whole system. The small size is because they eat little and often so don’t need to store food, which normally passes quite quickly through, the stomach is also not very stretchy as it doesn’t need to be. Gas colic can be a problem as horses cannot burp or be sick, because the valve at the stomach entrance remains tightly shut. The only way out for gasses is the back way!

Not much of the fibrous food they eat can be digested by the stomach, mainly digestion of proteins and fats starts here. The main function is adding acid and gastric juices to break down the fibrous food into smaller parts and make it easier to pass on into the intestines where most of the digestion occurs. The acid is secreted continuously in the lower half of the stomach which has a special lining which releases a thick layer of mucus containing bicarbonate that resist the acid. The upper half of the stomach, which is basically an expansion of the oesophagus separated by a band of tissue, does not have this acid resistance and is where, without a steady flow of food taking the acid away, ulcers will occur if acid gets up there.

An unnatural diet high in grains and low in forage can cause problems throughout the system. Because of their lack of bulk grains can pass quickly through the stomach leaving it empty again and vulnerable to acid attack, and get stuck low down in other parts. Grains and pellets are mostly rapidly digested in the small intestine and can give a horse a sugar rush, some horses have learned to get this by refusing to eat until they get their grains. If some passes quickly on into the hindgut it can create a lot of gas causing painful gas colic.

.1.3  Small intestine. About 20-25m long with a volume about 5 times that of the stomach and is about 30% of the whole system. Food again normally passes relatively quickly through here. This is where enzymes digest soluble carbohydrates, the sugars from starches, amino acids from proteins, fatty acids, and vitamins A, D, and E.

2 - Hindgut

This is where equine digestion has really evolved. How to digest cellulose that ruminants need a four chambered stomach, lots of chewing the cud, and lots of time sitting around and sleeping, to do. The hindgut is about 62% of the whole system, where the bulk of the food spends most of its time. Its where fibres are broken down by fermentation by microorganisms (bacteria, protozoa, and fungi) more or less the same as in ruminant stomachs.

The caecum and large colon are where most fibre is digested, in a similar way to ruminant stomach chambers, into Volatile Fatty Acids (a fermentation biproduct of the bacteria’s digestion of the cellulose), from which the horse gets a lot of its energy.

2.1  Caecum. About 1m long and 30L, a kind of large muscular sac with one way entry and exit valves both at the top and is often where impaction colic can occur if a lot of forage is eaten too quickly, as it will pass through the foregut too fast and not be broken down enough and be too dry.

2.2  Large colon. About 3.5m and 55L, is where microbial fermentation digestion continues and absorption of B vitamins and trace minerals occurs.

2.3  Small colon. About 3m and 18L, is where water is reclaimed back into the body before the remainder passes out as poo.

Sources: Edinburg University, American Association of Equine Practitioners, My Horse University, Equine Wellness Mag, Equisearch.

The deepest depths of the page:

Evolution of the modern horse

Today’s equids:

Domesticated and wild, horses, donkeys, zebras etc, have a unique digestive system, due to a change in the environment of their very early ancestors 20 million years ago. Unlike most other grazing animals, they only have one stomach chamber and it’s a small one. And no, it’s not because they are a fast animal, antelopes have 4, kangaroos have 2, and rhinos have 1.

56 – 34 million years ago. Eocene (Greek for dawn) period.

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A time of evolution of many animal groups including mammals. The climate was warm and humid, plant life was widespread lush forests and very little grass lands. Digestive systems don’t show in fossil records, so little detail is known of their evolution.

  • Early Eocene: Eohippus or Dawn Horse.

A hoofed browsing mammal about 5 hand high, with 4 padded toes or hooves at the front and the rear 3. Fossils have been found in North America and Europe and they were so different to modern horses that it was sometime before the connection was made via later discoveries. It had a single small stomach system well adapted to the abundance of easily digested high energy fruits and lush leaves at the time.

From Eohippus there evolved many branches, many extinct. The main evolutionary changes then were of the teeth, the animal itself changed little. More and larger molars appearing for specialist adaptations to its browsing diet. All modern equids still have these teeth today, though very adapted to grazing.

  • Mid Eocene: Orohippus.
  • Late Eocene: Epihippus.

34 -23 million years ago. Oligocene period.

  • Early and mid Oligocene: Mesohippus.

From Eohippus fossil records suggest the continued evolution towards the modern horse occurred mostly in North America. Mesohippus was becoming more horse like, still only 6 hands high and still browsing.

  • Late Oligocene: Miohippus.

23 – 5.3 million years ago. Miocene period.

In the early Miocene the climate cooled and became drier, in North America forests were dying off and dry course low nutrition grasslands were spreading.

  • Early Miocene: Parahippus and many branches.

Some branches spread north and across into Eurasia. Remaining in North America the Parahippus evolved into a grazing animal and went on to become the modern horse. Their molars and pre-molars became more adapted to the side-to-side motion required to grind down grasses, developed larger and stronger protrusions, and a greater depth of crown material to longer withstand the abrasive nature of grasses due to their high silicon content.

  • Mid to Late Miocene: Merychippus.

The transition to grazing teeth was complete. Merychippus was about 10 hands high, the lower leg bones had fused for fast running and ligaments attaching the remaining central toe to the ankle and leg bones became strong and spring like. still with three toes.

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5.3 to 2.6 million years ago. Pliocene period.

  • Early Pliocene: Pliohippus, and many other branches.

Pliohippus was single toed.

  • Mid Pliocene, 4 -4.5 million years ago: Equus.

Finally, all forms of Equus evolved with even springier ligaments and longer straighter teeth. This included Equus Caballus, the modern horse.

2.6 million – 11,700 years ago. Pleistocene period.

Equus successfully spread all over North America, some into South America, and north across the Bering land bridge all over Eurasia and into Africa.

10,000 to 8,000 years ago.

They disappeared from North and South America and non could spread back from Eurasia because sea level had risen submerging the Bering land strip. It’s not known why they disappeared and there are various theories, humans had been there since 20,000 years ago, so hunting is one possibility.

In Central Asia and Eastern Europe Equus Cabullus thrived and various breeds developed. Of these it’s thought Przewalski’s Horse of Central Asia (there is some debate as to whether these are wild or ferral), the Tarpan of Eastern Europe and the Ukranian Steppes, and the Forest Horse of Northern Europe formed the origins of the domesticated horse. From the first two came the southern “warm-blood” horses. And from the Forrest Horse the heavier “cold-blooded” horses.

16th Century.

Horses finally returned to their ancestral lands with the Spanish.

Source: Britannica.