Contrary to our hypothesis, fecal silica analyses (ADIA) from this study did not demonstrate any increase in silica excretion with manufacturer-recommended prophylactic doses of psyllium husk crumbles. Other studies providing therapeutic doses of psyllium as well as combinations of additional therapeutics (prebiotics and probiotics) demonstrated increased clearance of silica with supplementation (2, 13, 15, 27). We did see greater levels of fecal silica excreted from the yearlings. However, these contrasts could be associated with differences in forages fed to yearlings and adults. Higher relative fecal ADF levels in the mature horses seem to indicate either better forage fermentation in the yearlings, reduced fermentation in the mature mares, or possibly better GIT health in yearlings contributing to more overall sand clearance and greater fermentation.
Microbial populations were analyzed from the standpoint of diversity, populations present, as well as their possible functional roles within the GIT. During and after psyllium supplementation, fecal archaea and bacteria were present in abundances similar to most adult horses (7, 20, 28). In considering Faith’s Phylogenetic Diversity, the samples with the richest diversity were those collected during mid-supplementation on Day 7. Greater diversity has been positively correlated with a healthy horse GIT; our findings suggest that prophylactic psyllium was beneficial for improving microbial diversity (29, 30). However, there were no dramatic detectable changes in abundances of microbial populations. Even statistically significant changes in microbial abundances by LEfSe were relatively small with slight reductions in Burkholderiaceae. Slight increases were seen in Fibrobacteraceae and Paraprevotellaceae in yearlings, which contain essential cellulose-degrading bacteria and bacteria found in the GIT of pasture forage-fed horses, respectively (31, 32). In adults, there was a slight reduction in methanogenic archaea Methanocorpusculaceae but slight increases in methanogenic bacteria Methanobacteriaceae, which captures hydrogen and improves fermentation efficiency, and bacterial Moraxellaceae commonly found in younger horses and foals (20, 30, 31, 32). Increased mycothiol biosynthesis activity in the GIT was demonstrated during psyllium supplementation. Mycothiol is a protective antioxidant produced by bacteria in the Phylum Actinobacteria (34). Moreover, an upturn in urea cycle activity is indicative of increased urea utilization by gut bacteria as a nitrogen source during psyllium supplementation (35). Finally, catechol degradation pathways were elevated in GIT bacteria at the time of supplementation because psyllium is catechol rich. Furthermore, four days after the last dose of psyllium at Day 14 there was a spike in bacteria from the phyla Proteobacteria and a drop in bacteria from the phyla Firmicutes; these changes could indicate that after psyllium was no longer present in the GIT, bacterial populations readjusted to gut environment changes. These findings demonstrate that microbial populations were adapting to the psyllium supplied within the GIT. Thus, there is utility to further characterization of the effects of psyllium supplementation on sand excretion and promoting GIT health in horses, particularly at higher levels of supplementation either for prophylaxis or treatment of sand colic (13, 15).
This study does have several limitations. A small number of horses were included (n= 2 groups x 6 horses). Moreover, follow-up studies would benefit from wider representation of various ages of horses. Additionally, more psyllium dosages could have been considered beyond the one manufacturer-recommended prophylactic dose. Furthermore, sand intake was not measured or estimated for the horses during the study, and complete GIT sand quantities excreted were not measured but instead concentrations of ADF and silica in the feces. Thus, total quantities of silica excreted with psyllium supplementation cannot be provided from our measurements. Moreover, the yearlings and mares had differing diets contributing to differences between the forage fiber characteristics in the groups – likely impacting fermentation, the microbial community, and overall gut motility comparisons between yearling and adult groups. We also did not account for breed or sex differences in our analyses. Additionally, we only evaluated microbial populations using 16S amplicon sequencing and did not carry out a metagenomic study which would have had better resolution, thus allowing for distinctions to be found only down to the family level.