Zoonotic Nematodes II

Background & Epidemiology: Angiostrongylus cantonensis (rat lungworm) is the most common cause of eosinophilic meningitis worldwide. It’s primarily seen across Southeast Asia (inc. Philippines, Indonesia,) and the Pacific Basin (inc. Hawaii, Papua New Guinea), and less commonly other parts of the world (e.g., Caribbean nations). Humans become infected when they inadvertently consume L3 larvae present in raw snails, raw slugs, slug slime or contaminated shrimps, fish and vegetables.

Life Cycle: Adult worms live and release eggs in the pulmonary arteries of its definitive host (e.g., rats). Eggs hatch and L1 larvae move up the respiratory tree, are swallowed, and pass out in rat feces. Snails, slugs, and other mollusks and crustaceans ingest the larvae. Inside the intermediate host, the larva molts into an infective L3 form. Humans and rats become infected by consuming L3 larvae present in snails, slugs, or other intermediate hosts. For a diagram please click here.

Humans are accidental hosts. The L3 larva migrates to the brain or (rarely) to the lungs. The death of the larvae induces an eosinophilic inflammation, responsible for most of the symptoms. The natural course of the infection is resolution of symptoms in 1-2 weeks (but can be longer!).

Clinical Presentation: the incubation period is usually less than 6 weeks. Symptoms are those typical of meningoencephalitis. Neurologic complications (e.g., cranial nerve palsies, paresthesias, delirium, seizures, cognitive dysfunction) may persist for weeks-months.

Diagnosis: definitive diagnosis is with serology (Western Blot in serum & CSF).

Background & Epidemiology: Gnathostoma spp. is a parasitic nematode of various mammals (e.g., cats, dogs, mongoose). It’s primarily seen in areas of consumption of raw freshwater fish and crustaceans [Southeast Asia (especially Thailand, Japan, Vietnam), Mexico, South America, among others].

Life Cycle: Unembryonated eggs are released in stool by definitive hosts (e.g., dogs, cats, tigers) and mature in water over 1-2 weeks. Eggs hatch and release L1 larvae, which are ingested by freshwater crustaceans. Once ingested, L1 larvae mature into L3 larvae. Infected crustaceans are then ingested by secondary hosts (e.g., fish, frogs, eels), or paretenic hosts (e.g., snakes) where it matures into an infective L3 stage. Humans mainly become infected when they inadvertently consume infective L3 larvae present in undercooked/raw freshwater fish, frogs, snakes, eels. For a diagram please click here.

In humans, L3 larvae penetrate the gastric wall and migrate through the soft tissues for as long as 10-12 years. Less commonly, larvae can migrate to the CNS.

Clinical Presentation: Incubation period is ~1 month. Most cases are asymptomatic, but when present, symptoms are primarily subcutaneous (e.g, panniculitis, migratory subcutaneous swelling) with eosinophilia. Less commonly, neurological involvement can include eosinophilic meningitis, radiculitis or intracranial hemorrhage.

Diagnosis: diagnosis can be supported through serology (Western Blot).

Background & Epidemiology: Baylisascaris procyonis (the racoon ascarid) is another cause of visceral and neural larva migrans in humans. Infection may occur when humans accidentally consume soil contaminated with raccoon feces. Although human infection is rare, the worm is mostly found in the United States and Canada and has been introduced to other parts of the world (e.g., Europe, China). Primary risk factor is exposure to raccoons.

Life Cycle: Adult worms live and lay unembryonated eggs in the intestine of raccoons. Eggs need weeks to mature in soil. Humans become infected by ingesting soil contaminated with raccoon feces. For diagrams, please click here. In humans, eggs hatch and release larvae, which migrate to distant organs (e.g., eye, CNS).

Clinical Presentation: Incubation period is ~1 month. Similar to toxocariasis, infection can be asymptomatic or generate a visceral or ocular larva migrans syndrome, including eosinophilic meningitis, and chorioretinitis.

Background and Epidemiology: Trichinella spp. are a group of nematodes found across various geographical regions (e.g., North America, Europe, Southern Africa, South and SouthEast Asia). They all share a common mode of transmission: ingestion of raw/undercooked meat containing the parasite.

Life Cycle: Trichinella infects several animals (e.g., wild boar, bears, moose, pigs, horses, crocodiles, foxes, birds, walruses). After humans ingest raw meat, larvae are released and mature to adults in the small intestine. Female worms shed newborn larvae that go into the general circulation and invade skeletal muscle cells where they form “nurse-cell formation” complexes (encystation). Humans are dead-end hosts. With time, some calcify and others do not.

Figure 1. Life cycle of Trichinella
spiralis