| Primary amoebic meningoencephalitis | |
|---|---|
| Classification and external resources | |
Histopathology of amebic meningoencephalitis due to Naegleria fowleri. Direct fluorescent antibody stain. (CDC) |
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| ICD-10 | A06.6, B60.2 |
| ICD-9 | 136.2 |
| eMedicine | ped/81 |
Primary amoebic meningoencephalitis (PAM, or PAME) is a disease of the central nervous system caused by infection from Naegleria fowleri.[1][2]
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Naegleria fowleri propagates in warm, stagnant bodies of freshwater (typically during the summer months), and enters the central nervous system after insufflation of infected water by attaching itself to the olfactory nerve.[3] It then migrates through the cribiform plate and into the olfactory bulbs of the forebrain,[4] where it multiplies itself greatly by feeding on nerve tissue. During this stage, occurring approximately 3–7 days post-infection, the typical symptoms are parosmia, rapidly progressing to anosmia (with resultant ageusia) as the nerve cells of the olfactory bulbs are consumed and replaced with necrotic lesions.
After the organisms have multiplied and largely consumed the olfactory bulbs, the infection rapidly spreads through the mitral cell axons to the rest of the cerebrum, resulting in onset of frank encephalitic symptoms, including cephalgia (headache), nausea, and rigidity of the neck muscles, progressing to vomiting, delirium, seizures, and eventually irreversible coma. Death usually occurs within 14 days of exposure as a result of respiratory failure when the infection spreads to the brain stem, destroying the autonomic nerve cells of the medulla oblongata.
The disease is both exceptionally rare and exceptionally lethal: there had been fewer than 200 confirmed cases in recorded medical history as of 2004,[5] 300 cases as of 2008,[6] with an in-hospital case fatality rate of ~97% (3% patient survival rate). [7]
This extreme mortality is largely blamed on the unusually non-suggestive symptomology of the early-stage disease compounded by the necessity of microbial culture of the cerebrospinal fluid to effect a positive diagnosis. The parasite also demonstrates a particularly rapid late-stage propagation through the nerves of the olfactory system to many parts of the brain simultaneously (including the vulnerable medulla).
For those reasons, it has been suggested that physicians should give an array of antimicrobial drugs, including the drugs used to treat amoebic encephalitis, before the disease is actually confirmed in order to increase the number of survivors. However, administering several of those drugs at once (or even some of them known to treat the condition) is often very dangerous and unpleasant for the patient.
Naegleria fowleri is commonly referred to as an amoeba but is actually a unicellular parasite that is ubiquitous in soils and warm waters. Infection typically occurs during the summer months and patients typically have a history of exposure to a natural body of water. The organism specifically prefers temperatures above 32 °C, as might be found in a tropical climate or in water heated by geothermal activity.[8] The organism is extremely sensitive to chlorine (<0.5 ppm). Exposure to the organism is extremely common due to its wide distribution in nature, but thus far lacks the ability to infect the body through any method other than direct contact with the olfactory nerve, which is only exposed at the extreme vertical terminus of the paranasal sinuses; the contaminated water must actually be deeply insufflated into the sinus cavities for transmission to occur.
Michael Beach, a recreational waterborne illness specialist for the Centers for Disease Control and Prevention, stated in remarks to the Associated Press that the wearing of nose-clips to prevent insufflation of contaminated water would be an effective protection against contracting PAM, noting that "You'd have to have water going way up in your nose to begin with".[9]
This form of nervous system infection by amoeba was first documented in Australia in 1965.[10][11] In 1966, four cases were reported in the USA. By 1968 the causative organism, previously thought to be a species of Acanthamoeba or Hartmanella, was identified as Naegleria. This same year, occurrence of 16 cases over period of two years (1963-1965) was reported in Ústí nad Labem.[12] In 1970, the species of amoeba was named N. fowleri.[13]
In 2010, a 7-year-old girl in Stillwater, Minnesota died of the disease.[14]
In August 2011, a 16-year-old girl in Mims, Florida died after swimming in the St. John's River a week earlier. Doctors found N. fowleri in her cerebral spinal fluid. [15]
As of December 2011, two individuals in Louisiana died after inhaling infected tap water while using a neti pot.[16] [17]
The current standard treatment is prompt intravenous administration of heroic doses of Amphotericin B, a systemic antifungal which is one of the few effective treatments for systemic infections of protozoan parasitic diseases (such as leishmaniasis and toxoplasmosis).
The success rate in treating PAM is usually quite poor, since by the time of definitive diagnosis most patients have already manifested signs of terminal cerebral necrosis. Even if definitive diagnosis is effected early enough to allow for a course of medication, Amphotericin B also causes significant and permanent nephrotoxicity in the doses necessary to quickly halt the progress of the amoebae through the brain.
Rifampicin has also been used with amphotericin B in successful treatment.[18][19][20] However, there is some evidence that it does not effectively inhibit Naegleria growth.[21]
Two cases of similar amoebic infections (caused by Balamuthia mandrillaris) were successfully treated for amoebic encephalitis and recovered, including a 5-year-old girl and a 64-year-old man.[22]The successful use of a combination regimen that includes 1 amebicidal drug (miltefosine) along with 2 amebistatic drugs capable of crossing the brain-blood barrier (fluconazole and albendazole) provides hope for attaining clinical cure for an otherwise lethal condition.
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