Gnathostomiasis is caused by invading larvae of roundworm parasites of Gnathostoma species (see http://www.cdc.gov/parasites/gnathostoma/). These parasites have a complicated life cycle involving a variety of mammal and other hosts, including snakes, birds, frogs, eels, crustaceans and freshwater fish (http://www.dpd.cdc.gov/dpdx/HTML/gnathostomiasis.htm) Humans are typically infected when they eat raw or undercooked fish, crabs, or crayfish. The southern African cases have usually acquired the infection by eating raw bream (Tilapia species) that has been marinated in lemon juice, apparently a popular delicacy among tourists to the Delta. The larvae migrate through skin and subcutaneous tissues (the most common presentation), but also sometimes the internal organs, including the central nervous system in the most serious form of the disease. Effective treatment for cutaneous infection is available in South Africa, but treating central nervous system invasion is more difficult.
We strongly advise that freshwater fish caught in southern Africa should not be eaten raw, and that lemon juice does not render raw fish safe to eat.
Information provided by the National Institute for Communicable Diseases, Johannesburg (http://www.nicd.ac.za).