Figure 1.

Lifecycles of soil-transmitted helminths (STH), S. sterocoralis, and Toxocara. 1. Adult worms reside in the gastrointestinal tract (GIT). Hookworm, A. lumbricoides, and S. stercoralis adults reside in the small intestine while T. trichiura adults reside in the cecum and ascending colon. Female worms produce eggs which are passed in the stool of an infected person. 2. T. trichiura, Toxocara, and A. lumbricoides eggs mature in soil but do not hatch. Hookworm eggs hatch in soil and mature into L3 hookworm larvae. S. stercoralis eggs hatch into rhabditiform larvae in the gut, which are then excreted via the faeces. Rhabditiform larvae then mature into infective filariform larvae or free-living adults. 3. Infectious L3 filariform larvae of hookworm and S. stercoralis penetrate the skin directly, enter the circulation and migrate to the GIT after passing into the lumen of the lungs. 4. Mature eggs of Toxocara, A. lumbricoides, and T. trichiura are swallowed by the host. The eggs hatch, releasing larvae in the GIT. T. trichiura larvae hatch in the small intestine and mature into adults in the colon while Toxocara and A. lumbricoides larvae penetrate the gut. Toxocara larvae are carried by the circulation to a variety of tissue types while A. lumbricoides larvae are carried to the lungs. 5. Hookworm and A. lumbricoides larvae penetrate the alveolar walls and ascend the bronchial tree to the throat and are swallowed. Once they reach the small intestine the larvae mature into adults. S. stercoralis can also follow bronchial migration, or they can penetrate straight to the GIT. 6. Toxocara larvae can be carried to any tissue type. As humans are dead-end hosts the larvae do not undergo further development once they reach these sites, they can cause local reactions, known as the disease toxocariasis. Ocular toxocariasis, where the larvae penetrate the eye, can result in blindness. 7. S. stercoralis can also undergo autoinfection, where the rhabditiform larvae become infective filarial form larvae in the small intestine and penetrate the gut or perianal region. The filariform larvae can then disseminate to throughout the body. 8. Strongyloides rhabditiform larvae develop into free-living adults that produce eggs from which rhabditidorm larvae hatch. Rhabditiform larvae then develop into infectious filariform larvae and penetrate a human host. The free-living cycle exists for one generation cycle only.