PARASITES AND PARASITIC DISEASES OF
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.
"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
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.
(--->) 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).