The bloodworm, Strongylus vulgaris, is small enough to fit into horses’ arteries, but it can cause big—even life-threatening—problems for its equid hosts. Researchers have determined that the survival rates for horses with bloodworm-associated nonstrangulating intestinal infarctions (NSII) are poor whether they’re treated medically or surgically.

As bloodworms migrate through the horse’s intestinal arteries, they cause damage that leads to clotting. When the clots detach or large amounts of larvae build up in smaller arteries, blockages can occur. The result is intestinal wall infarction and inflammation, a condition called NSII or thromboembolic colic.

“If left untreated the intestine will rupture and the horse will die from shock,” said Tina Holberg Pihl, DVM, an associate professor at the University of Copenhagen, in Denmark.

In a recent study looking at treatments and outcomes for 30 horses with bloodworm-associated NSII, Pihl said most patients did not exhibit severe colic signs. As such, she said, the condition is difficult to diagnose because most horses aren’t visibly uncomfortable.

“Be aware of horses with low-grade colic and fever,” she said. “Often the horses had been mistaken for having an airway infection and had been treated for that for several days.”

She also noted that 70% of horses were admitted to the hospital for NSII treatment during the winter.

Nine horses received medical treatment, none of which survived. Surgeons performed exploratory laparotomy (abdominal surgery) in 21 horses, 11 of which were euthanized due to their presumed poor prognosis, the researchers said. Of the nine horses that underwent intestinal resection to repair bloodworm-caused damage, just three survived to discharge. In a bit of good news, the researchers said the surviving horses returned to athletic function for at least two years following discharge.

“If horses are taken to surgery faster than in this study, survival can be increased,” Pihl said.

Previous research identified the greatest risk among young horses, she said. However, this study contradicts those findings.

“This can be explained by the fact that the young horses less than 4 have the highest egg shedding and are most often treated with anthelmintics (dewormers) twice yearly,” she said.

Timely diagnosis is the key to increasing survival rates. A new research project collaboration between University of Copenhagen, University of Oslo, the Swedish Agricultural University, and the University of Kentucky Gluck Equine Research Center is currently underway to identify better diagnostic aids, she said.

The study, “Nonstrangulating intestinal infarctions associated with Strongylus vulgaris: Clinical presentation and treatment outcomes of 30 horses (2008-2016),” was published in the Equine Veterinary Journal.