Strongylus vulgaris (commonly known as the blood worm) used to be a common cause of colic before the advent of ivermectin, which proved to be very effective against the parasite. The larvae migrate up the arteries supplying the intestines, causing damage as they go. This can lead to thrombus formation and ischaemic damage, typically involving the large intestine.
Anthelmintic resistance, particularly among the cyathostomins (small red worms), has led to reduced use of anthelmintics. In some countries, including Sweden, anthelmintics have been restricted to use on prescription only. There is concern that this may be allowing Strongylus vulgaris to make a comeback.
What is the current relationship between gastrointestinal parasites and colic? Research has been carried out at the Swedish University of Agricultural Sciences, in Uppsala, Sweden. The work, by Ylva Hedberg-Alm and colleagues, is published in the journal Animals (Basel).
Blood and faecal samples were collected from horses presented to the Horse Clinic, at the University Animal Hospital. Each horse with a gastro-intestinal condition (colic, colitis, peritonitis, weight loss) was matched with a control horse of similar age seen during the same week for a condition unrelated to the gastrointestinal tract. The owners were questioned about management and history
The findings were compared between those presenting with colic and those with problems unrelated to colic. A total of 137 cases and 137 controls were included in the study. Age and gender distribution was similar between the groups.
Faecal samples were examined for strongyle egg counts, and for Anoplocephala perfoliata (tapeworm) eggs. The presence of S. vulgaris was confirmed using a polymerase chain reaction (PCR) test. (S. vulgaris eggs look like those of other strongyles, including the more common cyathostomins.)
Blood samples were evaluated for antibodies to S. vulgaris using an ELISA test.
The researchers found that exposure of Swedish horses to S. vulgaris was common. Most horses, both cases and controls, were positive in the S. vulgaris ELISA test. In addition, horses with peritonitis had significantly higher ELISA values compared to controls and to other gastrointestinal diagnoses, suggesting S. vulgaris was involved in this case group.
They also found that despite new legislation, 29% of owners did not use faecal analyses to monitor the parasite burden. The use of additional methods to diagnose specific parasites, such as S. vulgaris, was low. Owners rarely used pasture management methods to reduce the parasite burden on the grazing.
The authors suggest that, considering exposure to S. vulgaris appears high, there is a need for education in specific faecal diagnostics and pasture management.
For more details, see:
Ylva Hedberg-Alm, Johanna Penell, Miia Riihimäki, Eva Osterman-Lind, Martin K. Nielsen, and Eva Tydén
Animals (Basel). 2020 Apr; 10(4): 638.