Scientists have probed deeper into the micro-organisms that inhabit the horse’s hindgut, in the hopes of casting light on the causes of laminitis and colic.
They say the laboratory technique developed for their research could well provide a convenient and cost-effective means of understanding the microbial dynamics underlying colic and laminitis, and test hypotheses for ways to prevent or interrupt the progress of the diseases.
The researchers identified bacteria in the gut in the Veillonellaceae family that reduced lactate levels, which they said could ultimately prove useful in gaining insights into the mechanisms of recovery or resistance. The bacteria could even lead to probiotics to prevent starch-induced colic and laminitis, they said.
University of Massachusetts researchers Amy Biddle, Samuel Black, and Jeffrey Blanchard had their findings published in the peer-reviewed open-access journal, PLoS ONE.
Horses are hindgut fermenters, adapted to grazing continually on marginal forages that change seasonally.
The hindgut comprises roughly two-thirds of the volume of the equine digestive tract. It is here that complex plant material is fermented by microbes to short-chain fatty acids such as acetate, propionate, and butyrate, which provide 60-70 percent of the daily energy needs of the horse.
However, rapid dietary change and modern feeding practices of 2-3 meals a day of starch-based concentrates and/or fructans from rich pasture can disrupt normal hindgut fermentation, causing lactic acidosis, and colic, and predisposing animals to crippling laminitis.
Laminitis accounts for 15 percent of all lameness in horses in the United States, with more than 27 percent of affected horses unable to return to normal work. The mortality rate is 4.7 percent.
Laminitis is characterized by weakened adhesion and eventual detachment of the distal phalynx from the lamellae of the inner hoof wall, resulting in permanent rotation of the coffin bone and severe pain.
Agents released into the bloodstream by bacteria in the gut during lactic acidosis are thought to trigger dietary laminitis. However, the molecular mechanisms underlying induction are unknown.
The researchers said lactate levels in the hindgut are normally low due to the activity of lactate-using bacteria.
“It is unclear why this mechanism fails during conditions of starch induction,” they noted.
While studies have identified lactate-producing bacteria in the gut that prosper during the developmental stage of laminitis, little is known about how the abundance of lactate-using bacteria changes over the same time course, which lactate-users survive the drop in pH arising from the hindgut changes, and which lactate-users are active in the later stages to bring lactate concentrations back to normal levels.
The researchers collected fecal samples from three healthy adult Morgan geldings eating an identical pasture-based diet, then monitored the samples in laboratory conditions to track bacterial metabolites and community shifts over time in response to enrichment with starch and/or lactate.
The researchers managed to identify the lactate-using bacteria that proliferated as lactate levels dropped.
They also identified community differences in cultures lacking the ability to clear excess lactate, which may lead to further insight into why some horses are resistant to starch overload. The identified bacteria with the potential to reduce or prevent lactic acidosis.
The researchers found that the cultures from the three horses differed dramatically in the ability to reduce accumulated lactate.
Differences between horse cultures and treatment conditions were evident at the microbial family level, specifically for the most abundant phyla, the Firmicutes.
The Veillonellaceae, a family that includes known lactate-using species, increased in abundance in all cultures in the control and lactate treatment groups.
In the starch and starch/lactate enrichments, there were dramatic differences between the cultures from horse 3 and those from horses 1 and 2.
At the 48-hour mark, the Veillonellaceae dropped to less than 5 percent in horse 3 cultures, while in horse 1 and 2 cultures this group was the highest in abundance, making up more than 70 percent of the total sequences.
The most abundant family for the starch and starch/lactate horse 3 cultures at time 48 were the Lactobacillaceae, making up greater than 40 percent of the total sequences at this time point.
The Streptococcaceae reached a peak in abundance by 24 hours in the starch and starch/ lactate conditions for all horse cultures, after which the abundance dropped.
In horse 1 and 3 cultures, this decrease was accompanied by an increase in the Lactobacillaceae. However, in horse 2, the abundance of Lactobacillaceae remained relatively low even as the Streptococcaceae declined over time.
The researchers said the accumulation of lactate has long been recognized as an early event in the microbial response to carbohydrate overload.
Their laboratory method was able to simulate many key aspects of starch-induced conditions that have been reported elsewhere in experiments directly on horses.
“Despite a sample size of three horses, we were able to observe between-horse differences in the ability to attenuate [reduce] accumulated lactate as has been described in previous studies.
“Certainly, the microbial dynamics reported here may not reflect the actual community compositions of the caecum and large intestine of horses who recover or resist conditions of lactic acidosis,” they added.
However, the information gathered about lactate-using bacteria that thrive under conditions of low pH following starch enrichment may provide insight into microbial mechanisms of resistance and/or recovery of these debilitating conditions, they concluded.
As well as the Veillonellaceae family, the researchers’ findings also pointed to a relationship between the presence of Megasphaera elsdenii and the reduction of lactate.
“It is possible that other community members with lactate-utilizing capabilities are playing active roles as well.”
The researchers said that understanding the factors stimulating or preventing the growth of lactate-using bacteria in the hindgut could provide valuable information about why some horses are more sensitive to starch induction, and the microbial basis behind resistance.
Biddle AS, Black SJ, Blanchard JL (2013) An In Vitro Model of the Horse Gut Microbiome Enables Identification of Lactate-Utilizing Bacteria That Differentially Respond to Starch Induction. PLoS ONE 8(10): e77599.
The full study can be read here.