The molting process in nematode growth and development involves two steps: synthesis of
a new cuticle and exsheathment or shedding of the old. The process of exsheathment is
particularly important in the order Strongylida since it plays a vital role in
transmission to the definitive host. The infective stage for most stronglyid species is a
third stage larva still enclosed in the loosely-fitting cuticle (sheath) from the
preceding second stage. This outer sheath plays a protective role for these larvae but,
because it completely encloses the larva, it also prevents feeding. In species infecting
grazing ruminants transmission occurs by ingestion of ensheathed third stage larvae during
grazing. The first process to occur is exsheathment since without it, infection can not
The exsheathment process in Haemonchus contortus in ruminants has been
reasonably well delineated. The actual exsheathment of Haemonchus third stage larvae takes
place in the rumen and appears to be a three-stage process.
conditions in the rumen - primarily levels of dissolved carbon dioxide (influenced by pH,
temperature and reducing agents) activate neurosecretory cells in the nerve ring, at the
base of the esophagus.
2. These activated
neurosecretory cells release hormones (including nor adrenaline)
which act directly on the excretory cell to stimulate the uptake of water which, in turn,
activates enzymes present in crystalline form.
3. The accompanying
diagram shows the third, and final, stage
| in the exsheathment
process. Activated enzymes,
including leucine aminopeptidase, are released into the excretory duct and pass into the
space between the two cuticles. Enzyme action weakens the cuticle (A).
The cuticle breaks at this point and the anterior end detaches as a cap (B).
The exsheathed L3 (C) wriggles out leaving the sheath (D)
(second stage cuticle) behind.
The importance of exsheathment is because it
initiates infection. Without exsheathment, in nematodes like Haemonchus, infection would
not occur. The process of exsheathment during infection is not only host specific but is
also site specific within the host. This ensures that nematodes will not exsheath when
ingested by abnormal hosts. For example, Haemonchus contortus will not infect horses
because they lack a rumen and therefore infective Haemonchus larvae ingested by horses
will not receive the necessary stimulus to begin the infective process.
image shows Haemonchus larvae exsheathing in a petri
dish after stimulation with carbon dioxide. Larva A still retains its sheath while
larva B is
wriggling out of its sheath (C).
A cast, empty sheath, its larva having escaped, is labeled D.