equine grass sickness

The mysteries surrounding equine grass sickness (EGS) are becoming less riddling. New investigations using DNA-analysis technology are giving veterinarians a peek at what’s going on in affected horses’ guts. This research isn’t only helping researchers better understand the disease; it’s also giving hope for the development of a simple, noninvasive diagnostic test.

“There is currently no way to predict when or which horse will develop EGS, and diagnosis is only possible by taking a biopsy of the gut wall, which involves invasive surgery and is costly for the horse owner,” said Joy Leng, PhD, MSc, BSc, a postdoctoral researcher at the University of Surrey School of Veterinary Medicine, in Guildford, U.K.

“Therefore, there is an urgent need for a quick, noninvasive diagnostic test,” she said.

Grass sickness is a deadly disease that’s centralized in the U.K., and particularly Scotland, where it kills about 600 horses a year. Many scientists believe the “mal seco” disease in South America is the same as EGS. While researchers have hypothesized about how EGS develops, they’ve yet to confirm its cause.

In their study, Leng and her fellow researchers investigated the DNA of microscopic bacterial colonies in samples from 19 EGS horses and their healthy pasturemates, as well as from a few hospitalized horses with other forms of colic. They evaluated fecal and urine samples, since the kidneys are indirectly affected by digestion processes and gut composition. The scientists used a relatively new technique—sequencing bacterial DNA—which has only been available for the past decade.

“This technique allows for the identification of bacteria that cannot be grown within the lab using more traditional microbial techniques,” Leng said. “We chose this relatively new technique to give us unique, and previously impossible, insights into how gut bacterial populations might be involved in EGS.”

The results from the fecal samples revealed that EGS horses had significantly reduced bacterial diversity compared to the other horses, she said. Healthy gut usually has many different “healthy” bacteria that keep the digestive system running properly.

They also identified four potential EGS metabolite biomarkers—small chemical molecules found in higher or lower quantities in the urine of the affected horses compared to healthy horses:

  • 4-cresyl-sulfate;
  • Hippurate;
  • Trimethylamine-N-oxide (TMAO); and
  • O-acetyl carnitine.

This opens the possibility of creating a simple diagnostic test that requires just a urine or fecal sample.

Whether a future diagnostic test could ultimately help prevent the disease’s fatal consequences is uncertain, said Leng.

“How early in the course of disease changes can be detected in urine is unknown; further research would be needed to answer this question,” she said. “Similarly, it is currently unknown whether early detection of affected horses will allow treatment which can reverse the progress of disease. There is currently no effective treatment once a horse has developed EGS.”

Interestingly, Leng said, their work did not show a connection between the bacterial species Clostridium botulinum and EGS. That was surprising given that for the last 100 years, people have generally believed this was a bacterium responsible for EGS, she added. It doesn’t prove that it’s not related, though—it only indicates that they can’t prove the relationship.

“Our work offers novel approaches to diagnosing this disease in the live horse and also highlights the profound changes in gut bacterial populations associated with EGS,” Leng said. “Further development of this work may indicate how manipulation of the horse’s gut bacteria, such as with probiotics or prebiotics, could offer simple and affordable disease prevention strategies.”

This research was funded by the Equine Grass Sickness Fund.

The study, “Exploration of the Fecal Microbiota and Biomarker Discovery in Equine Grass Sickness,” was published in the Journal of Proteome Research.