Large strongyles and a potentially reemerging and fatal equine disease

While equine medicine has been focused on the emergence of small strongyle resistance to deworming products we have forgotten about the elephant in the room, large strongyle disease. Large strongyles are the cause of cranial mesenteric arteritis which causes colic and death in horses. There is evidence that the disease is returning(1). When I first started practicing I would frequently attend a horse dying from colic due to large strongyle damage to the arteries supplying the bowel. Before the appearance of ivermectin, large strongyles were hard to control.

Ivermectin solved the large strongyle problem. Because of their sensitivity to the drug and their long larval development period, they were easily controlled with the new products. Large strongyle caused colic and death disappeared from my practice.

Fast-forward three decades later, the frequent use of ivermectin and other avermectins results in a new problem: small strongyle resistance to these products. Larval cyathostominosis has become a serious problem, even in frequently dewormed horses. Frequent deworming was identified as the cause of growing resistance. In response, parasite control became based on the parasite burden in the horse as measured by fecal egg concentrations. And that is a problem as far as large strongyles are concerned. Large strongyles are not large egg producers so a serious infection is not readily identified. Next is that even a small exposure to large strongyle larvae over time can cause life-threatening disease.

Not well studied are anthelmintic resistance patterns in large strongyles. Where such studies have been done benzimidazole resistance has been found and reduced prepatent periods to the avermectins evident (2).

We are in a bit of Catch-22. Strongyle resistance vs prevention of large strongyle damage to the cranial mesenteric arteries. We do not know if current recommendations of limited dewormer use with application based on increased fecal egg counts will prevent this disease. At home, I am including at least yearly use of an avermectin and increasing my environmental parasite control efforts like keeping pastures cleaned of manure and appropriate pasture rotation.
DrO

References:
(1)A retrospective study of the prevalence in equine postmortems of cranial mesenteric arteritis caused by Strongylus vulgaris in Alberta (2010 to 2022)
Can Vet J. 2024 Jun;65(6):587-593.
Authors
Katrice A Domshy 1 , Ashley E Whitehead 1 , Jocelyn Poissant 1 , Dayna A Goldsmith 1 , Carolyn Legge 1 , Cameron G Knight 1 , Erin K Zachar 1 , Sara S Loch 1 , Jennifer L Davies 1
Affiliation

1 Faculty of Veterinary Medicine, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4.

PMID: 38827589
PMCID: PMC11132149 (available on 2024-09-01)

Abstract
in English, French

Background: Strongylus vulgaris is one of the most pathogenic nematodes affecting equids. Larval migration through the cranial mesenteric artery (CMA) with attendant arteritis and thromboembolism can result in fatal non-strangulating intestinal infarction. Once considered a historical disease, recent studies have described the reemergence of this pathogen in several European countries; however, little is known of the current prevalence of S. vulgaris in the Canadian horse population.

Objective: To determine the prevalence of active S. vulgaris cranial mesenteric arteritis in horses submitted for postmortem examination to the Diagnostic Services Unit (DSU) at the University of Calgary Faculty of Veterinary Medicine.

Animals and procedure: We conducted a retrospective review of all equine postmortem cases submitted to the DSU between July 1, 2010 and June 30, 2022. Over 12 y, 510 horses > 2 mo of age from Alberta were submitted to the DSU for necropsy. Active cases were defined as those with endarteritis and thrombosis in the CMA or its branches. Those cases with only intimal scarring of the CMA were classified as historical.

Results: The prevalence of all CMA lesions (both historical and active) over the study period was 17.3% (88/510). Active S. vulgaris cranial mesenteric arteritis was documented in 6.1% (31/510) of equine postmortems and the sequelae of verminous arteritis were the cause of euthanasia or death in 1.5% (8/510) of the cases submitted.

Conclusion and clinical relevance: Even after historically intense efforts to eradicate this parasite, the continued effects of S. vulgaris are demonstrated by the results of this study. Strongylus vulgaris should not be regarded as a parasite of the past and verminous arteritis remains an important differential diagnosis for horses in western Canada presenting with mild colic or dull demeanor and anorexia of duration > 24 h. Furthermore, S. vulgaris should be taken into careful consideration when implementing antiparasitic control strategies. Practitioners should remain current on prevention, diagnosis, and treatment of this potentially reemerging and fatal equine disease.

(2) A national survey of anthelmintic resistance in ascarid and strongylid nematodes in Australian Thoroughbred horses
Ghazanfar Abbas, et al
.Int J Parasitol Drugs Drug Resist. 2024 Apr; 24: 100517. 
Published online 2023 Nov 29. doi: 10.1016/j.ijpddr.2023.11.006
PMCID: PMC10757041
PMID: 38064906