Pathogenic Significance of Strongyles

Larvae

·         Lesions are restricted in the arterial system of the intestine caused by this species.

·         Lesions are most com­mon in the cranial mesenteric artery and its main branches, and consist of thrombus formation pro­voked by larval damage to the endothelium together with a marked inflammation and thickening of the arterial wall.

·          True aneurysms with dilata­tion and thinning of the arterial wall, although uncom­mon, may be found especially in animals which have experienced repeated infection.

·         Experimental infection of foals with S. vulgaris showed several hundred L₃ as clinical syndrome of fever, inappetence and dullness occurs, sometimes accompanied by colic.

·          At necropsy, these signs are associated with arteritis and thrombo­sis of intestinal blood vessels with subsequent infarction and necrosis of areas of bowel.

·         However, a syndrome of this severity is not commonly reported in foals under natural conditions, probably because lar­val intake is continuous during grazing; it has been shown experimentally that foals may tolerate large numbers of larvae administered in small doses over a long period.

·         In S. edentatus infection there are gross changes in the liver associated with early larval migration, but these rarely result in clinical signs. Similarly, the haemorrhages and fluid-filled nodules which accom­pany later larval development in subperitoneal tissues rarely result in clinical signs.

·         There has been little work on the pathogenesis of migrating larvae of S. equinus.

Adults

• The pathogenesis of infection with adult Strongylus spp. is associated with damage to the large intestinal mucosa due to the feeding habits of the worms / to the disruption caused by emergence of young adults into the intestine following completion of their parasitic larval development.
• These worms have large buccal capsules and feed by ingestion of plugs of mucosa as they move over the surface of the intestine.
• Although the worms appear to feed entirely on mucosal material the incidental dam­age to blood vessels can cause considerable hemorrhage. Ulcers which result from these bites eventually heal, leaving small circular scars.
• the gross damage and subsequent loss of blood and tissue fluids is certainly partly responsible for the unthriftiness and anaemia associated with intestinal helminthosis in the horse.