Figure 3

Life cycle of S. stercoralis. All adult parasites are partenogenetic females (1) that reside in the crypts of small intestine. Their embryonated eggs (2) hatch in the crypts or the intestinal lumen. The L1 rhabditiform larvae are voided in faeces (3) and give a rise either to L3 infective filariform larvae (L3i) or develop to free living adult nematodes depending on environmental conditions. The L3i develops no further if it does not gain access to new host. The L3i enters the dog percutaneously (4) perorally(5). The larvae migrate to the small intestine and molt first to L4 and then tothe parthenogenic adult female(6). The puppies can be infected via milk if the bitch has migrating L3 larvae (7). Transplacental infection does not occur. This parasitic life cycle of S. stercoralis is referred as homogonic life cycle. If environmental conditions are optimal, an alternative route of life cycle (heterogonic life cycle) can take place. Non-infective rhabditiform larvae develop to free-living adult male and female worms (8) that produce eggs. Non-infectious rhabditiform larvae (9) hatching from the eggs will develop to L3i (10). During passage through the host intestinal tract, rhabditiform larvae may rapidly undergo molts into L3i. These larvae can penetrate through the wall of large intestine or perianal skin of the host resulting in migration ending in the small intestine (11). The process is called autoinfection and it is favoured especially in neonatal or immunocompromized hosts. The canine strains of S. stercoralis have been known to infect humans (12). The life cycle was drawn based on the information obtained from following references: [2,4,16–18].