Naegleria fowleri (Brain eating amoeba) : Habitat, Life Cycle, Pathogenicity
- Naegleria fowleri (Brain eating amoeba), the only pathogenic species of naegleria is named after Fowler who, with Carter described it first from Australia in 1965.
- It is found worldwide in warm fresh waters.
N. fowleri has 3 stages in its life cycle—a dormant cyst form, an amoeboid trophozoite form and a flagellate form (hence classified as an amoeboflagellate).
- The amoeboid form is about 10 to 20 μm, showing rounded pseudopodia (lobopodia), a spherical nucleus with a big endosome, and pulsating vacuoles. This is the feeding, growing and replicating form, seen on the surface of vegetation, mud and water.
- In water, some of them get transformed into a ‘pear-shaped form’ with 2 flagella.
- This rapidly motile flagellate form is the main infective stage, more so than the trophozoite. The flagellate can revert to the amoeboid form.
- Cysts develop from the trophozoites and are seen in the same locations as trophozoites. The cysts are
spherical. They are the resting dormant form and can resist unfavourable conditions such as drying and chlorine up to 50 ppm.
- The trophozoites can withstand moderate heat (45°C), but die at chlorine levels of 2 ppm and salinity of 0.7 per cent.
Mode of infection
- Human infection comes from water containing the amoebae and usually follows swimming or diving in ponds.
Naegleria fowleri causes Primary amoebic meningoencephalitis (PAM).
- Patients are mostly previously healthy young adults or children.
- The amoebae invade the nasal mucosa, pass through the olfactory nerve branches in the cribriform plate into the meninges and brain to initiate an acute purulent meningitis and encephalitis (primary amoebic meningoencephalitis).
- The incubation period is 2 days to 2 weeks.
- The disease almost always ends fatally within a week.
- Over 200 cases of PAM have been reported from many countries, including India. Most cases have been from the developed countries.
Diagnosis can be made by CSF examination.
- The fluid is cloudy to purulent, with prominent neutrophil leucocytosis, elevated proteins and low glucose, resembling pyogenic meningitis. Failure to find bacteria in such specimens should raise the
possibility of PAM.
- Wet film examination of CSF may show the trophozoites.
- Cysts are never seen CSF or brain. At autopsy, trophozoites can be demonstrated in brain histologically.
- Culture can be obtained in agar seeded with Escherichia coli or in the usual cell cultures used for virus isolation. Both trophozoites and cysts occur in culture.
- Amphotericin B has been used in treatment with limited success.