Gnathostomiasis

Gnathostomiasis is a parasitic infection caused by larval Gnathostoma species, predominantly Gnathostoma spinigerum. Infection follows consumption of undercooked or raw freshwater fish, eels, frogs or other paratenic hosts harbouring infective third stage larvae. It is endemic particularly in Southeast Asia and parts of Latin America.

It is characterised by migratory cutaneous swellings, reflecting wandering larval migration through host tissue.

In a small but serious proportion of cases the central nervous system is invaded, producing a severe, potentially fatal eosinophilic meningoencephalitis. Take a specific dietary and geographical exposure history in any returning traveller with a compatible migratory swelling or unexplained eosinophilic central nervous system disease.

Pathophysiology

• Humans are an accidental, dead end host. The larvae cannot mature into adult worms in human tissue, unlike in the natural definitive host, typically cats or dogs, where the adult worm resides within the gastric wall.
• The larvae instead migrate erratically through human subcutaneous and, in a proportion of cases, deeper tissue for a prolonged period, sometimes years. This produces the characteristic intermittent, migratory pattern of clinical manifestations.
• Central nervous system involvement is uncommon. It reflects direct larval migration into neural tissue, most classically after larval entry via a peripheral nerve root or through direct cranial penetration.
• The migrating larva causes substantial local tissue destruction, producing a severe haemorrhagic, eosinophilic meningoencephalitis or myelitis.

• The characteristic and commonest manifestation is intermittent, migratory subcutaneous swellings, often painful, pruritic and erythematous. They appear at variable sites over the body and typically resolve over days to weeks. They recur at a different location days to months later, reflecting ongoing larval migration through subcutaneous tissue.
• Cutaneous larva migrans like tracks can occur where the larva migrates more superficially within the dermis.
• Visceral involvement is less common. It can affect essentially any organ the larva encounters and produces organ specific symptoms according to the site.
• Central nervous system gnathostomiasis is the most severe manifestation. It presents with severe radicular pain, often the earliest symptom, reflecting nerve root larval entry.
• Progressive myelitis or meningoencephalitis follows, with headache, altered consciousness and focal neurological deficit. The cerebrospinal fluid characteristically has a haemorrhagic component, because larval migration through neural tissue destroys local tissue.
• Mortality is substantial, and survivors have a significant risk of permanent neurological deficit.
• Seek a dietary history of consumption of raw or undercooked freshwater fish, eel, frog, snake or other relevant paratenic host tissue. Certain raw fish preparations common in parts of Southeast Asia and Latin America are recognised risk exposures.
• Also ask about travel to or residence in an endemic region. Do this in any patient with a compatible migratory swelling or unexplained eosinophilic neurological presentation.

• Diagnosis is frequently made on the characteristic history (migratory, intermittent subcutaneous swellings), the relevant dietary and geographical exposure history, and supportive peripheral eosinophilia. The larva itself is often not amenable to tissue sampling because it constantly migrates.
• Serological testing supports the diagnosis where available, predominantly through specialist tropical medicine reference laboratories because routine availability is limited. Cross reactivity with other helminthic infections can complicate interpretation.
• Biopsy of a subcutaneous swelling, or direct extraction of the larva, provides definitive confirmation by histological or morphological identification. Extraction is occasionally possible if a swelling is examined promptly while the larva remains at that site.
• Central nervous system disease requires magnetic resonance imaging, which shows characteristic migratory tract signal change, sometimes with visible haemorrhage.
• Cerebrospinal fluid examination is also needed. It characteristically shows an eosinophilic pleocytosis with frequent xanthochromia or overt blood staining, reflecting the haemorrhagic nature of larval migration within the central nervous system.

• Albendazole 400 mg twice daily for twenty one days is standard first line treatment for cutaneous and subcutaneous disease. Ivermectin 200 micrograms per kg as a single dose (sometimes repeated) is an effective alternative.
• Surgical extraction offers immediate, definitive cure of a specific lesion where the swelling is identified and the larva can be localised and removed. It is often impractical because the larva migrates and relocates frequently, so pharmacological treatment is more generally applicable.
• Central nervous system gnathostomiasis requires urgent specialist neurological and infectious disease input.
• Management combines antiparasitic treatment (albendazole, generally at the standard dose and duration) with corticosteroid therapy for the significant inflammatory and haemorrhagic tissue response, and full supportive neurological care.
• Prognosis is guarded, because substantial tissue destruction has often already occurred by the time of diagnosis and treatment.
• Treat the cutaneous manifestations symptomatically alongside definitive antiparasitic treatment, with antihistamines for pruritus and analgesia for pain from active migratory lesions.
• Prevention is the primary and entirely effective strategy: cook freshwater fish, eels, frogs and other relevant paratenic host species thoroughly before eating. Avoid raw or undercooked preparations of these foods when travelling in or residing within endemic regions.
• No vaccine is available, and central nervous system involvement can be severe should infection occur.