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.
Palomino horse lying down in pasture
Be aware of horses with low-grade colic and fever, since the horses often have been mistaken for having an airway infection and been treated for that for several days.
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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.
The larvae of the 'blood worm' Strongylus vulgaris spend a portion of their life cycle burrowing into the bowel wall, penetrating the arteries of the bowel, migrating into the larger arteries, causing damage to the artery wall, and finally migrating back into the bowel to develop into mature egg laying worms.
It is the passage through the arteries and bowel walls, and later within the large arteries, that severe damage can occur to the blood supply to portions of the intestine.
If there is complete blockage, the bowel will be severely damaged and will become necrotic and rupture, usually with severe signs of colic.
If the damage only partly blocks the blood supply, this can cause small areas of bowel damage, causing low-grade chronic pain, or it can interfere with the actual motility of the bowel wall, leading to faulty digestion, indigestion, the possibility of gut pain, and so leads to colic, rolling end may result in a twisted bowel.
So the usage of the newer, improved pastes and drenches have helped reduce this type of colic.
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.
According to Holberg Pihl, DVM, horse owners should be aware of horses with low-grade colic and fever, since the horses often have been mistaken for having an airway infection and 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.
Press Release by DVM Services