Primary Amoebic Meningoencephalitis: Report of a Case
Khurshid Ahmad, Athar Saeed Dil*, Ijaz Ahmad, Shagufta Hussain,
and Noor Khan Lakhnana
Department of Pathology, Pakistan Institute of Pathology, Islamabad.
* NIH, Islamabad.
During the course of a routine review of old departmental records recently, we came across a
rare case of Primary Amoebic Meningoencephalitis (PAM), proven to have been caused by a free-living fresh water amoeba, which had been presumptively identified as Naegleria fowleri. Fortunately, the clinical and laboratory features of the case had been
neatly recorded and meticulously documented. For unknown reasons, the case was relegated to distant memory in the departmental archives and was never reported in medical literature at that particular point in time.
Primary Amoebic Meningoencephalitis (PAM) is unique in three respects. It is a singularly rare condition. Only about 140 cases have been reported
in the entire medical literature1; the condition is
almost always fatal and, for all practical purposes,
it is clinically indistinguishable from acute pyogenic (bacterial) meningitis.
To the best of our knowledge and belief,
the present report is the first authentic, laboratory confirmed case of PAM due to a free living amoeba being reported from this country.
A 20 year old young medical student traveled from Faisalabad to Islamabad with a two day history of headache that had progressively worsened. On arrival in Islamabad, he was seen in Neurology outpatient department. Since there was no pyrexia or neurological signs of meningeal irritation, the patient was prescribed analgesics and sedatives and sent home as a case of tension headache.
On the morning of the third day, patient’s headache was persisting despite analgesics, he developed pyrexia and vomited twice. Again no specific signs of meningeal irritation were detectable and he was continued on conservative treatment.
In the afternoon however, the patient had high fever (103O F), became drowsy, disoriented and combative but responded to painful stimuli. He was brought to the casualty department where the CMO, detecting an element of neck stiffness, performed
a lumber puncture. Spinal fluid was frankly turbid and under pressure. It was collected into three sterile containers and sent to the laboratory for urgent routine examination and culture. Blood samples were also drawn for complete blood count and routine chemistry.
A clinical diagnosis of acute bacterial meningitis was made. Appropriate antibiotics were started immediately and the patient was shifted to the neurology ward.
Laboratory examination of CSF revealed a turbid fluid with a small coagulum, a total WBC count of 13200 / ul with 95% neutrophils, glucose 23 mg / dl and protein 450 mg / dl. Microscopy of a direct Gram stain smear showed no microorganisms. CBC showed a total WBC count of 14500/ul, neutrophils 85%, lymphocytes 14% and monocytes 01%. Routine blood chemistry including glucose, urea, creatinine and arterial blood gases were all within normal limits.
Second tube of sterile CSF was inoculated on blood agar, chocolate agar (incubated in candle jar), and maconkey agar. As had been the standard practice in the pathology department, remaining tube of CSF sample was left in the incubator overnight. In spite of a negative Gram smear on repeated attempts, initial laboratory findings supported the clinical diagnosis of acute pyogenic meningitis and the patient was continued to be treated on antibiotics.
By midnight, patient’s condition markedly deteriorated, he became unconscious, was shifted to intensive care unit and put on a ventilator.
Next morning, examination of inoculated culture plates after overnight incubation revealed no bacterial growth on any of the three media. At this stage it was decided not only to repeat both a Gram smear as well as culture but also to examine a wet preparation from the CSF sample that had been stored in the incubator overnight.
Figure 1: Neglaria Fowleri (Amebic Form) in Spinal Fluid seen under Phase Contrast (X 400)
Examination of the wet mount by phase contrast microscopy revealed numerous motile amoebae, roughly of the size of a WBC, showing directional movements by means of blunt, bulbous pseudopodia, and granular cytoplasm, some of them showing vacuoles (Fig. 1). The amoebae were identified as Naegleria species on morphological grounds alone. Smears were sent to NIH for
staining by Wheatley’s Trichrome stain2. Considering that in all instances of human PAM where
the responsible organism had been cultured and isolated was found to be N. fowleri1, the patient
was presumptively diagnosed as a case of Primary Amoebic Meningoencephalitis due to Naegleria fowleri.
While the earlier therapy was continued,
the patient was immediately started on Amphotericin B and Metronidazole both intravenous as well as intrathecal. However, in spite of intensive resuscitation measures, he continued to deteriorate and finally expired on the morning of 5th day of illness.
The first question that we asked ourselves was as to how and why the detection of amoebae was missed in the first instance when the spinal fluid was examined microscopically for leucocyte count. It was standard practice in the department that all turbid fluids were suitably diluted using a diluting fluid which contained acetic acid and gentian violet. This not only facilitated counting in the chamber and dissolved any RBC present, but also enabled the examiner to report “chamber differential” without resorting to differential Leishman staining3. The diluting fluid, however, effectively inactivates any viable organisms, if present, in the spinal fluid, including the amoebae which will lose their amoeboid movements. This is what precisely happened in this case. All the amoebae were inactivated by the diluting fluid and were in fact counted as leucocytes.
Neaglaria fowleri, a 10 – 20 um small protozoan, is one of the three species of pathogenic free living amoebae, which is responsible for the majority of cases of primary amoebic meningoencephalitis in humans. Other two are Acanthamoeba and Hartmanella. It is found in soil and water, including soil contaminated pools and brackish and fresh water lakes. Infection usually occurs in young adults in summer months who give a history of swimming in a pond or lake about a week before the onset of illness. Amoebae gain entry through the nasal passages and reach the brain directly through the cribriform plate. The illness is rapid in onset, brief in duration and almost invariably fatal. The drug of choice is Amphotericin
B to which Naegleria is susceptible. However in the vast majority of cases, by the time the condition is diagnosed it is already too late. To date only six cases are reported to have survived4. All were diagnosed and treated in the early phase of illness; four
were treated with Amphotericin B alone1, 5, one
with Amphotericin B, Meconazole and Rifampicin1 and one with Amphotericin B, Rifampicin and Chloramphanicol along with drainage of the brain abcess4. In this particular case, the diagnosis was too late for the therapy to be of any benefit.
An effort was however made to find out the source of infection. The young man was residing in the college hostel. There was no history ofthe patient having gone out for swimming during the last few days or weeks. However, he was a religious boy and was known to belong to a Tablighi group in the college. We were able to get information from his class mates that he had gone out ona short tablighi trip
a few days before his illness. It was presumed that during this trip, the patient performed ablution with water which was possibly contaminated with Naegleria fowleri. While rinsing the nostrils, he might have sniffed water up his nose permitting entry of the amoebae to the nasal passages and, through the cribriform plate, into the brain. There is at least one confirmed report of this mode of transmission when a Muslim farmer in Nigeria had become infected during the ritual washing before prayers6.
Although widely believed to be rare, possibility of primary amoebic meningoencephalitis should always be considered in the differentialdiagnosis of smear negative and culture negative cases of acute pyogenic meningitis. Early diagnosis is crucial to the outcome. Direct examination of a wet mount of all cloudy spinal fluids without centrifugation and after a short period of incubation, should constitute an essential part of routine examination of all CSF samples. Saving any residual CSF samples in the incubator or at room temperature, but never in the refrigerator, is a time tested and useful practice.