04/13/2024
Most, if not all, of our clients are aware of the potential danger rich spring grass presents to our horses. Statistically, fast growing lush grass is responsible for approximately half of the cases of laminitis seen in this country. In our area, this occurs during the spring as the days warm up and the earth retains the moisture from the winter rains. It is the build-up of sugars in the grass that cause the syndrome of Pasture Associated Laminitis (PAL). The sugars involved in PAL are almost exclusively non-structural carbohydrates (NSC) such as fructans and starch. Structural carbohydrates constitute the structure of the rough, stemmy part of the hay and their digestibility is much reduced.
When NSCs are present in large amounts they can pass, not completely digested, from the stomach and small intestine into the hind gut (cecum and large intestine). The resulting fermentation process causes the hindgut to become more acidic. The resulting increase in permeability of the gut allows “laminitis trigger factors” to be released into the blood stream and be carried to the hoof causing laminitis. These trigger factors are not completely understood but they likely include both endotoxins and vasoactive amines. Endotoxins are released in the now acidic gut following the death of pH sensitive gram-negative bacteria which release endotoxins from their cell wall. Vasoactive amines, such as tryptamine and tyramine, have also been shown to increase in the hind gut with the dropping pH. They cross the gut wall and may be carried to the foot resulting in changes in blood flow to the sensitive lamina. Vasoactive amines are involved in a variety of bodily functions including inflammatory responses, allergic reactions and balance of hormonal equilibrium. Histamine, dopamine, and serotonin are the better-known examples of vasoactive amines.
Certain horse types are more prone to have problems with PAL. Overweight, sedentary horses (Body condition score 8 or greater) and horses who have insulin resistant (IR) issues such as Cushing’s disease or Equine Metabolic Syndrome (EMS) are more likely to suffer laminitis following ingestion of strong spring grass. While I have discussed PAL, it is but one of the potential causes of laminitis. In horses with IR, it is the inability to utilize glucose as a source of energy in the feet that leads to laminitis as the laminar bonds begin to breakdown. In the equine species both the brain and the feet can only use glucose as a source of energy, neither is able to burn fat or protein. In IR cases blood glucose tends to rise but the obligate glucose user organs (feet and brain) still can not utilize it. There is overlap between these causes of laminitis as IR horses are more prone to suffer from PAL.
I wish to give some easy to remember tips to help avoid this syndrome in your horses. When you are aware of what factors cause grass to be higher in sugars these tips are easy to remember. It is the sun, through the process of photosynthesis, that allows grass to accumulate the sugars that are necessary for its growth. When we have cold days and cold nights, less sugar is produced overall thus reducing the danger. When we have warm days and warm nights sugar is produced but is also burned at night as the night warmth increases the metabolic rate of the grass. It is when we have the combination of warm days and cold nights that grasses are at their most dangerous. This describes our current climate situation in mid-April, so this is the time to limit pasture access if you have not done so already.
It follows that the safest time to graze horses is early in the morning from before dawn to the time the sun reaches the grass. This would generally be from 4-10am. Grazing on grass that is primarily in the shade is safer than on grass that receives full sun. Horses that are lean, or best of all, lean and muscular are at the lowest risk as muscle is an efficient burner of sugar and thus can help keep blood glucose levels lower.
While we must be careful about carbohydrate intake in horses generally, horses with access to spring grass warrant special consideration. Avoid any additional source of carbohydrate such as grain or treats. If a concentrate is needed, concentrates high in fat are preferable to those that are higher in sugars. Sweet smelling hays, usually oat or grass hays, are higher in carbohydrates than alfalfa or teff hay. Alfalfa is higher in energy than the other hays on a weight or volume basis but tends to be lower in carbohydrates. Teff hay is consistently low in carbohydrates but is also deficient in protein and nutrients, a vitamin and mineral supplement should be provided to horses eating only teff hay.
Over the last 10 years we have found it useful to monitor blood glucose levels in at risk metabolic horses to help determine if current husbandry methods are efficiently dealing with any metabolic issues. It is simply done through a blood draw and checking levels with a hand-held glucometer (see reference A below). It can be done at a clinic or, with some training, can be done at the barn. If we are monitoring horses over a period of time we do not generally fast before taking the blood but carbohydrate heavy feed should be avoided prior to the draw.
We find normal blood glucose to range between 88mg/dL and 108mg/dL. Generally, glucose levels in the teens do not trigger a laminitic episode but serve for us as a warning sign to continue to monitor or change husbandry, medications or both. Values above 130mg/dL in susceptible horses will trigger us to initiate or modify medication and warrants continued monitoring to assess effectiveness of treatment.
If a single blood draw is being used to assess susceptibility in PAL or EMS horses, they should be fasted for 6 hours before pulling the blood. Blood glucose levels can vary so repetitive blood draws are preferred to a single sample to better assess the individual. Blood glucose levels are but one test in assessing the individual’s current metabolic status and it is through their intermittent but continued monitoring that we find blood glucose to be of most value. It is a simple, inexpensive test that provides an immediate result.
We are hopeful that this brief summary of PAL may help our clients to avoid the dread of finding your horse with sudden pain and warmth emanating from the feet. Generally, both front feet are involved in laminitis but exceptions can occur, one foot may be involved or all 4 feet. It would be unusual to see a case where only hind feet affected in a metabolic individual but it does occur. If signs compatible with laminitis occur, veterinary attention should be sought out. Our approach is to take double block front feet x-rays with the coronary band marked by candle stick wire and take a blood sample, at the very least for glucose. If the horse is in pain we do not generally check hormone levels but we do initiate medical treatment. This can include thyroid supplementation to aid metabolism, chromium picolinate to help glucose uptake and utilization, magnesium to increase sensitivity to insulin and Metformin to lower blood glucose levels (if indicated). Chinese herbs may also be used depending upon the horse’s presentation. While herbs and spices such as chasteberry and cinnamon are used, they are not part of our first line of treatment. I do not generally use higher levels of pain-reducing medication [Phenylbutazone (Bute), Flunixin Meglumine (Banamine) or Firocobix (Previcox, Equi-ox, Osticoxib)] in acute laminitic cases as I feel pain is part of natures warning sign to limit use of the painful feet. We do use low dose flunixin meglumine (generally 125 to250mg) for its anti-endotoxin affect in appropriate acute cases. This low dose is not high enough to significantly decrease pain or discomfort, a higher dose can be more safely used in chronic cases where the risk of further downward displacement or rotation of the coffin bone is reduced.
-Noel S. du Celliee Muller, DVM, ISELP Certified
A. Glucometer- Alpha Track 3 by Zoetis, Pet Blood Glucose monitoring Kit
Note: There is no horse setting, we use the dog setting for consistency.
References:
1. A Horse Owners Guide to Pasture- Associated Laminitis, Scott Puggan et al., Oregon State University, June 2022
2. Dietary fructan carbohydrate increases amine production in the equine large intestine: Implications for Pasture Associated Laminitis, J Anim Science, Nov 2007, pp 2948-2958
3. Equine Laminitis, Chris Pollitt, Hoof Care Publishing, Nov 2001
4. Low dose flunixin meglumine effects on eicosanoid production and clinical signs induced by experimental endotoxaemia in horses, S. D Semrad et al, Equine Vet J, May 1987 pp 201-206
