<prompt> A patient presents with a headache, high fever and meningismus. Within 3 days he becomes unconscious. Most probable causative agentNaegleria fowleriAcanthamoeba castellaniiEntamoeba histolyticaTrypanosoma cruzi  <response>: Amongst the given options Naegleria and Acanthamoeba cause amoebic encephalitis.The prognosis of Naegleria encephalitis is uniformly poor, most patients die within a week ---Harrison Acanthamoeba encephalitis follows a more indolent course.---Harrison So, amongst the given options Naegleria causes most fatal amoebic encephalitis.Naegleria fowleri It is a free-living amoeba Exists in three forms - Amoeboid trophozoite, Biagellate trophozoite, Cyst Never forms in the human body Modes of transmission - Human primarily contact the infection by swimming in contaminated water.The amoeboid trophozoite enters the human body through the nasal mucosa and migrates to the brain along the olfactory nerve and blood vessels.Some infections result from inhalation of cysts. Clinical features primary amoebic meningoencephalitis Common in young adults and children.Incubation period 2-15 days 5 days A headache, high fever, nausea, vomiting, meningismus. Cranial nerve palsies 3rd, 4th and 6th. Most patients die within a week 95.N. fowleri also causes humidifier fever, a form of allergic alveolitis resulting from inhaling air from air-coolers. Diagnosis - Detection of motile amoeboid trophozoites in wet mounts of C.S.F. Treatment - High doses of amphotericin B and rifampicin.Acanthamoeba - It is a free-living amoeba Acanthamoeba culbertsoni species is most common, Exists in two stages - Trophozoite, Cyst The appellate stage is absent Modes of transmission - Through contamination of traumatized skin or eye Inhalation. 