European researchers have delved into an important question around parasitic infections in horses – whether they are beneficial or harmful to the diversity richness and function of the all-important gut bacteria.
The study team focused their research around gastrointestinal strongyles, which pose a major threat to horse health and welfare.
“Given that strongyles inhabit the same niche as the gut microbiota, they may interact with each other,” Núria Mach and her colleagues reported in the journal Frontiers in Physiology.
“These beneficial or detrimental interactions are unknown in horses and could partly explain contrasted susceptibility to infection between individuals.”
It is this differing susceptibility to parasitic infection that has driven the move away from traditional calendar-based worming to programs based around the fecal egg counts of individual horses.
The researchers described a five-month grazing experiment involving 20 worm-free female Welsh ponies, half of which had been assessed as susceptible to strongyle infection and the other half resistant. The susceptible ponies had a record of high fecal egg counts, in contrast to those considered resistant.
All were wormed and kept indoors for three months to clear any parasitic infections, during which time they were fed hay and concentrate, before being placed in the experimental pasture.
Fecal egg counts, blood and biochemical data, body weight and gut microbiological composition were studied in each individual at the start of the experiment and after 24, 43, 92 and 132 grazing days.
The two groups displayed divergent immunological profiles and slight differences in microbiological composition under worm-free conditions, they reported.
After exposure to natural infection, the resistant ponies predictably showed lower fecal egg counts after 92 and 132 grazing days, and maintained higher levels of circulating monocytes and eosinophils (types of white blood cells), while higher levels of lymphocytes (another form of white blood cells) persisted in the susceptible ponies.
Overall gut microbiota diversity and structure remained similar during parasite infection between the two groups, although the susceptible ponies showed a reduction of bacteria such as Ruminococcus, Clostridium XIVa and members of the Lachnospiraceae family, which may have promoted a disruption of mucosal balance at day 92.
In line with this hypothesis, an increase in organisims such as Pseudomonas and Campylobacter, which are linked with chronic inflammatory conditions, was noted in the susceptible ponies when compared to the resistant ponies.
The researchers also noted other changes in the susceptible group in several predicted immunological pathways, including pathogen sensing and lipid metabolism, both critical for the regulation of immune system and energy balance.
The susceptible ponies also had higher protozoan concentrations at day 92, suggesting that strongyles and protozoa may contribute to each other’s success in the equine intestines.
“It could also be that susceptible individuals favor the increase of these carbohydrate-degrading microorganisms to enhance the supply of nutrients needed to fight strongyle infection.”
The researchers said that strongyles, in their preferred niche, were surrounded by gut microbiota and reciprocal interactions between them are expected.
“For the first time, we have characterized how gut microbiological communities and host biochemical and hematological parameters varied between resistant and susceptible ponies under natural parasite infection conditions.”
Resistance, they said, was associated with high levels of circulating eosinophils and monocytes and lower levels of lymphocytes. However, the modifications of gut microbiota composition were modest between the groups.
Therefore, it seemed that horses with fecal egg counts within the range of those shown by the horses in the study – 0 to 800 eggs per gram – do not exhibit major disruption to their gut microbiota structure and composition.
“While the relationship between horse welfare and the severity of strongyle infection remains unclear, this finding may provide extra support for the current fecal egg count cut-offs (200 or 500 eggs per gram) used to decide if treatment is needed or not.”
They continued: “Overall, this study provides a foundation to better understand the mechanisms that underpin the relationship between equines and natural strongyle infection. The profiling of horse immune response and gut microbiota should contribute to the development of novel biomarkers for strongyle infection.”
The full study team comprised Allison Clark, Guillaume Sallé, Valentine Ballan, Fabrice Reigner, Annabelle Meynadier, Jacques Cortet, Christine Koch, Mickaël Riou, Alexandra Blanchard and Núria Mach, variously affiliated with French, Spanish and Swiss institutions.
Clark A, Sallé G, Ballan V, Reigner F, Meynadier A, Cortet J, Koch C, Riou M, Blanchard A and Mach N (2018) Strongyle Infection and Gut Microbiota: Profiling of Resistant and Susceptible Horses Over a Grazing Season. Front. Physiol. 9:272. doi: 10.3389/fphys.2018.00272