As of 1990, approximately 200 cases of amoebic meningoencephalitis had been reported world-wide. Of these, approximately 75% have been PAM. In 2000, a confirmed case of PAM was reported in New Zealand. The diseases are similar in that both are caused by free-living amoebae and are almost always fatal, but they differ in their aetiologic agents, epidemiology, clinical presentation, pathogenesis and pathology.
Primary amoebic meningoencephalitis (PAM)
This is a fulminant disease of previously healthy children and young adults that progresses rapidly and is fatal within a few days of symptoms onset.
It is caused by an infection with Naegleria fowleri, a free living amoeba found worldwide in fresh water: steams, lakes, ponds, reservoirs, well water, municipal water supplies, natural hot springs, chlorinated swimming pools, spa pools, and soil. The amoebae are thermophilic, multiplying best at temperatures of 37 deg C to 45 deg C. Most infections occur in the summer.
Clinical presentation
3 - 7 days after exposure, a rhinitis may occur as a result of amoebic ulceration of the nasal mucosa. This is followed by an abrupt onset of severe bifrontal headache, fever, nausea and vomiting. Meningeal signs are present in approximately 85% of patients. Patients lapse into coma within a few days. Death typically occurs within 7 - 10 days of symptoms onset.
Diagnosis
Typically presents as a culture-negative meningitis. The CSF protein is typically increased up to 400 g/L with the CSF glucose normal or decreased. Moderate numbers of erythrocytes are usually present. The CSF WBC count varies from several hundred to many thousand. The differential count shows a predominance (>80%) of neutrophils. Gram stain and cultures of CSF are negative.
Motile trophozoites can be demonstrated in freshly collected CSF which has not been refrigerated or frozen.
Therapy
The Infectious Disease Service should be consulted if this diagnosis is being considered. Because of the paucity of survivors, there is no obvious treatment of choice. Aggressive therapy with intravenous and intrathecal amphotericin B has been used successfully in two patients.
Granulomatous amoebic meningoencephalitis (GAE)
This is a more insidious disease that effects immunocompromised, debilitated, and chronically ill patients. Although both diseases are rare, accurate diagnosis is important because a few patients given early and aggressive therapy have survived. Furthermore, these diseases must be distinguished from other clinically and radiographically similar causes of encephalitis and meningitis.
Granulomatous amoebic meningoencephalitis is caused by amoebae of the genus Acanthamoeba. They have been isolated from the same water sources as N.fowleri , as well as from sea water, bottled mineral water, air conditioning units, dialysis fluids, vegetables, contact lens cases, and the nose and throat of healthy individuals.
Clinical presentation
Of the approximately 200 reported cases of amoebic meningoencephalitis reported world-wide up until 1990, approximately 25% have been granulomatous amoebic meningoencephalitis (GAE). The clinical course is usually more protracted than primary amoebic meningoencephalitis (PAM), most dying weeks to months after the onset of symptoms. Because nearly all patients have an underlying disorder or predisposing condition, the clinical presentation is more variable than PAM.
Diagnosis
There are no clinical signs or symptoms or laboratory tests which can distinguish GAE from other chronic meningitides such as tuberculous, fungal, or viral meningitis. The CSF shows a moderate pleocytosis with a predominance of lymphocytes. Polymorphonuclear cells may predominate early in the disease. The CSF protein increases steadily during the infection and the CSF glucose is low to normal. Acanthamoeba species have been cultured from the CSF.
Therapy
Only six patients have been known to survive GAE. There is no proven treatment of choice.