The Nematodes


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 proceed.

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.

1. Environmental 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 

exsheath1.JPG (23686 bytes)
 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.


exsheath.JPG (77796 bytes)
The accompanying 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.



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