In part one of this series on acidosis laminitis, we not only learned how to recognise the disease and discovered how it differs from endocrinological and mechanical laminitis but also examined the relationship between acidosis, laminitis and lush grass. However, there was one aspect that we did not uncover: the role of diet and the microbiome in the development and resolution of acidosis. This is what will be analysed in this final article.
Feeding habits of wild, feral and freely grazing horses
Large meals of cereal grains and free access to high-sugar grasses and hay is not a natural part of an equine diet. Horses have evolved as trickle feeders with high fibre content, and eating is a social affair with a set pattern of interactions.
Feeding habits and foodstuffs
When grazing naturally, horses eat to maintain a high level of gut fill (Duncan, 2012), and feeding consists of bouts of uninterrupted grazing separated by non-feeding intervals. Feeding does not occur randomly but is divided into “meals”.
When grazing naturally, horses eat to maintain a high level of gut fill, and feeding consists of bouts of uninterrupted grazing separated by non-feeding intervals
Feeding is a herd activity and part of being social; during mealtimes, horses take short (less than 10-minute) breaks to “look around”, stand guard or walk over and see what their neighbour is eating. There are longer periods between mealtimes for sleeping and resting.
Equine feeding habits are similar to deer, with the types of food eaten including grasses, sedges, leaves, tree shoots and even bark. In the late summer/autumn, fruit and berries are included.
Horses’ meal lengths are affected by the time of day, and herds prefer to eat during the daytime rather than the night.
In April, mealtimes are around 3 hours 20 minutes long, dropping to just 1 hour 10 minutes in October. This change in the length of the mealtime is due to the increase in the fibre content of the grass as the season goes on, suggesting that horses eat to fill their gut and to maintain a high gut fill level. In April, the nutritive value (glucose/starch/protein) of green shoots is high, while the biomass is low, meaning horses must consume a greater quantity before the gut is full. Perhaps this somewhat explains why horses appear to gorge in spring and why turning a horse out onto an unsuitable pasture (ie containing high-sugar grasses) causes a glut of carbohydrates to overflow into the hindgut instead of being digested in the stomach and small intestine.
One management approach for horses prone to acidosis/laminitis might be to feed a hay net of low-quality fodder before turning out, but avoid a grazing mask, which may cause a greater degree of stress to a hungry horse trying to fill their gut with fibre.
During times when flies are a nuisance, mealtimes are much shorter, and horses stop feeding to rub themselves on each other and groom one another. Flies become a nuisance in the UK from early to mid-June onwards, and in the fly season, feral horses graze for much longer periods between the hours of 4 and 8am. During this period, intervals between meals are longer, horses no longer feed until they are full and they have longer periods of walking between meals. The average feeding “bout” in between grooming and rubbing lasts around 25 to 45 seconds – literally snatching a bite to eat. This change in routine coincides with mid-season when glucose/nutrition remains high and fibre content is high enough to allow less time to eat before gut fill level is achieved.
It may be better to bring the horse in from the field throughout the day. This helps them avoid the flies, provides them with a portion or two of low-quality hay […] and reduces the calorie intake
It may be better, therefore, to bring the horse in from the field throughout the day. This helps them avoid the flies, provides them with a portion or two of low-quality hay to nibble and reduces the calorie intake through the day as nature intended.
Acidosis laminitis and the equine diet: pasture dos and don’ts
Fertilised or unfertilised pastures?
Fertilised pastures are reported to have a lower sugar content than unfertilised fields – which could be a bonus – but they also have a higher non-digestible fibre content (Valk et al., 1996), meaning the horse will have to eat more to attain gut fill. Some native low-quality grass species contain less sugar and more fibre, and having an area of the field containing standing hay in the autumn prior to the second flush of lush grass is of obvious benefit.
Standing hay and the nutritive value of seed heads
Standing hay is useful for horses to eat from the autumn through into the winter and has several benefits. Figure 1A shows one such field in February 2023. In July 2022 (Figure 1B), the field contained numerous plantain seed heads, containing many secondary plant compounds, vitamins and antioxidants that support good gastric health, which were all eaten by February 2023.
Seeds stay longer in the gut of horses (up to two weeks) and go through many digestive actions, some of which provide remarkable benefits to horse health. When ingested, seeds are abraded (the surface is worn away or damaged) and crushed by the grinding action of the teeth. They are then warmed up and moistened before being soaked in acid in the stomach and small intestine. Finally, they are colonised by the gut bacteria in the caecum and large intestine.
Proteolytic bacteria adhere to the surface of the seeds and excrete enzymes as they digest the hard outer surface. These enzymes have significant health benefits to the horse, including the modulation of inflammation (gastric ulcers), reduction in swelling of the mucous membranes (helpful for gut wall integrity) and improving circulation.
Some seeds grow better after being eaten and are a great way to increase the biodiversity of equine pastures:
- 10,000 varieties of grass seed
- 19,000 species of Fabaceae (legumes)
- 4,400 species of sedges and rushes
- 1,265 species of flowering plants, such as Cistaceae
- 2,500 species of small shrubs and herbs, including the common nettle
The equine gut microbiome and acidosis laminitis
Changes in the microbiome profile of horses with acidosis
Prior to the onset of laminitis, there is an increase in the species of lactic acid-producing bacteria (lactobacilli) and an increase in the bacteria that thrive in an acidic environment, plus a reduction in the bacteria that digest fibre (fibrobacteres) (Al Jassim et al., 2005) (Figure 2).
In one study, lactobacilli were isolated from different gut sections in horses with laminitis caused by acidosis (Al Jassim et al., 2005), indicating the ability of lactobacilli to adapt to changes in pH. Lactobacillus salivarius adheres to the gut wall and has been isolated from the colon and rectum. It is known to break down amino acids, possibly producing vasoactive amines such as histamine. Lactobacillus delbrueckii has also been found in the gastrointestinal tract of horses and humans, where it is identified as the main causative agent for the human form of gastrointestinal D-lactic acidosis.
“Vasoactive” is a term given to substances that are a vasoconstrictor (causing constriction or narrowing of the blood vessels) or produce a dilating effect (causing dilation of the blood vessels). In the case of horses with laminitis, the effect of the histamine is hypothesised to contribute to the onset of laminitis by constricting the vessels to the hoof, causing ischaemia.
Why is there an increase in bacteria that thrive in an acidic gastric environment?
As the lactic acid bacteria increase and the pH of the gut drops, the number of bacteria that thrive in an acidic environment also increases.
A low hindgut pH favours a particular group of bacteria called spirochaetes (Figure 3). Spirochaetes are long, slender and tightly coiled like a miniature spring; this shape together with an inbuilt motor and hook system enables them to cross connective tissues and the gut wall barrier. The speed of travel allows them to escape from the host’s immune system. This family contains some of the most pathogenic disease-related bacteria, such as Treponema pallidum (syphilis), Leptospira (leptospirosis), Borrelia (Lyme disease) and Treponema pertenue (yaws).
Spirochaetes are commonly found in the equine microbiome and are considered part of the core equid gut microbiome common to all horses (Dougal et al., 2012). Even though most spirochaete species found in the equine gut microbiome are not considered true pathogens, high levels are thought to be detrimental to health because all spirochaetes can translocate across the gut wall. Reduction is, therefore, recommended. Spirochaetes multiply in the blood and can survive in extravascular sites even when the bacteria in the blood have been eliminated.
The species found in the equine microbiome include:
- Treponema bryantii
- Treponema porcinum
- Treponema brennaborense
- Treponema saccharophilum
- Treponema amylovorum
- Treponema socranskii
- Treponema parvum
- Treponema berlinense
Treponema spp need vitamins and minerals to thrive, including calcium, vitamins B1, B3, B5, B6 and B9, and biotin. They also need inulin and fructo-oligosaccharides, arabinoxylan and guar gum, which are found in many equine probiotics.
What are the benefits of the Treponema species in the equine gut microbiome?
Treponema species have some benefits when present in the equine gut microbiome, but only within a recommended average of 7 to 9 percent.
Treponema spp help and assist other members of the gut microbiome, such as Bacteroides succinogenes and Ruminococcus alba, in breaking down the woody, stemmy parts of plant material. They consume glucose and convert it into succinate, butyrate, acetate and formate. Of these, succinate is an important antioxidant; however, when levels are too high, inflammation and imbalance can occur in the gut.
Plant antimicrobials to reduce Treponema and the rationale for using them
With an average occurrence of 9 percent, Treponema spp are a core member of the microbiome of horses. However, when an imbalance or dysbiosis of the microbiome (such as acidosis) occurs, there is an increase and a rise in percentages of Treponema to 17 percent or above. Horses with gastric ulcers have a different microbiome profile than healthy horses, demonstrating levels of Treponema higher than 9 percent and a type of dysbiosis linked to ulceration.
There is, however, scientific evidence to support the use of antibiotics to reduce Treponema spp, with infections responding to penicillin, erythromycin and azithromycin with procaine penicillin G given intramuscularly to horses (Zeng et al., 2021; Dwivedi et al., 2015). Unfortunately, evidence of Treponema antibiotic resistance has recently been found (Dwivedi et al., 2015). To combat antibiotic resistance there is a growing interest in the use of plant-based antimicrobial compounds such as terpenoids, either as an extract or by using the whole plant.
To combat antibiotic resistance there is a growing interest in the use of plant-based antimicrobial compounds such as terpenoids, either as an extract or by using the whole plant
There is a long history of ethnomedicinal use of plants to reduce Treponema infections (Vermani and Garg, 2002; Soviati et al., 2020). Using evidence of before and after from Equibiome’s database, sarsaparilla (Smilax) and acacia catechu are both effective in reducing overgrowths of Treponema at a daily dose of 5g for sarsaparilla root and 2g of acacia catechu for one month.
Other plants with antimicrobial properties include any member of the salvia family, including sage (Salvia officinalis), the name of which originates from the Latin “salvere”, meaning “be well”! This is an important medicinal plant, and a daily dose of six to seven fresh leaves can be used to help maintain a healthy gastrointestinal tract. Sage will promote good bacteria and be effective against the over-proliferation of Gram-negative bacteria (Treponema). It can also be used as a grooming tool for horses with long-term or chronic acidosis that have a build-up of lactate in the muscles.
Other useful plant-based medicines
Other plants (family name in parentheses) that contain terpenoids like salvicine include:
- Amaranthus spinosus (Amaranthaceae)
- Piper betle (Piperaceae)
- Pongamia pinnata (Leguminaceae)
- Sida cordifolia (Malvaceae)
- Ocimum tenuiflorum (Labiateae)
- Curcuma longa (Zingiberaceae)
- Swertia chirata (Gentianaceae)
- Phyllanthus niruri (Euphorbiaceae)
- Abrus precatorius (Leguminaceae)
- Aloe vera (Asphodelaceae)
- Senna alata (Leguminaceae)
- Pistia stratiotes (Araceae)
Plants used to treat syphilis (family name in parentheses) include:
- Cassia fistula (Leguminaceae)
- Mucuna pruriens (Leguminaceae)
- Solanum surattense (Solanaceae)
- Azadirachta indica (Meliaceae)
- Terminalia chebula (Combretaceae)
- Phyllanthus niruri (Euphorbiaceae)
- Gloriosa superba (Colchicaceae)
- Areca catechu (Arecaceae)
- Gmelina arborea (Labiateae)
Phyllanthus niruri (Euphorbiaceae) has also been used as a remedy for gonorrhoea.