Soil-Transmitted Helminths in Tropical Australia and Asia

Abstract

Soil-transmitted helminths (STH) infect 2 billion people worldwide including significant numbers in South-East Asia (SEA). In Australia, STH are of less concern; however, indigenous communities are endemic for STH, including Strongyloides stercoralis, as well as for serious clinical infections due to other helminths such as Toxocara spp. The zoonotic hookworm Ancylostoma ceylanicum is also present in Australia and SEA, and may contribute to human infections particularly among pet owners. High human immigration rates to Australia from SEA, which is highly endemic for STH Strongyloides and Toxocara, has resulted in a high prevalence of these helminthic infections in immigrant communities, particularly since such individuals are not screened for worm infections upon entry. In this review, we consider the current state of STH infections in Australia and SEA.

Keywords: Ancylostoma ceylanicum; Ascaris lumbricoides; Australia; South East Asia; Strongyloides stercoralis; Trichuris trichiura; hookworm; soil-transmitted helminths.

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.

Figure 2

Distribution of STH in South-East Asia and Australia, modified from Brooker et al., [64].

Figure 3

Prevalence of S. stercoralis in Asia and Australia based on community-based studies, hospital-based studies, and prevalence in immigrants and refugees. Modified from Schäret al., [84].