Infection with the parasitic nematode (roundworm) Angiostrongylus. The two species that infect humans are A. cantonensis and A. costaricensis. A. cantonensis, the rat lungworm, is the most common cause of human eosinophilic meningitis (meningitis with eosinophils in the cerebrospinal fluid). The meningitis is due to the presence of the rat lungworm larvae in the brain and the patient's local reactions to them. A. costaricensis causes abdominal, or intestinal, angiostrongyliasis which mimics appendicitis, with eosinophilia (an increase of eosinophils in blood). There is no effective treatment for A. cantonensis or A. costaricensis.