PARASITES AND PARASITIC DISEASES OF DOMESTIC ANIMALS


Cyathostomes

(Small strongyles)

Life cycles

Cyathostome species have similar life cycles. The cecum and colon of horses are the predilection sites for all species and all life cycles are direct.

Pre-parasitic phase

"Strongyle-type" eggs are laid by female worms and are passed in the feces of infected horses (---> A). A first stage larvae (L1) develops inside each egg, then hatches. This L1 develops and molts into a second stage larva (L2) which, in turn, develops and molts into a third stage larva (L3) but retains the L2 cuticle as a protective sheath. Ensheathed L3's are the infective stages for the definitive hosts (equines) of these nematodes and they are non-feeding stages surviving on food granules accumulated by the feeding first and second stage larvae.

Temperature and moisture control both development and survival of these free living stages, with optimal development taking place at approximately 25'C and 80% humidity. Under optimal conditions development of eggs through to infective third stage larvae may take as little time as 2 or 3 days.

L3s actively migrate from their host feces (in which they have developed) onto the surrounding pasture thereby increasing the chances of being ingested by grazing horses.

Parasitic phase

(--->) Horses are infected by ingesting sheathed L3's while grazing(B). These pass through the stomach (C) and exsheath in the small intestine (D). Parasitic third stage larvae pass on to the cecum and colon(E) where they pass into the crypts of Lieberkuhn and penetrate the mucosa and, in some species, the   submucosa. Here, larvae become encysted by host fibroblasts and molt to L4's. The majority (98%) of encysted larvae are found in the walls of the cecum and ventral colon. L4's emerge from their tissue cysts and resume development in the lumen of the large intestine with the majority(about 95%) of mature adult cyathostomes being found in the lumen of the ventral and dorsal colon. The prepatent period (depending on the species) is from 6-14 weeks but may be greatly prolonged when emergence of larvae from the mucosa is delayed due to arrested development(hypobiosis) at the early third stage(EL3).

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Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000