Original Article

Primary Amebic Meningoencephalitis

Cecil G. Butt, M.D.^†

N Engl J Med 1966; 274:1473-1476June 30, 1966DOI: 10.1056/NEJM196606302742605

Article

THE following 3 case studies, related experimental data and review of the recent literature introduce a new clinical concept, that of primary meningoencephalitis due to the the free-living, soil ameba, commonly referred to as acanthameba (species). It is suggested that in these 3 cases there may be a common etiologic pattern of intranasal inoculation resulting from prolonged swimming in tepid lake water.

Pathological findings in the first 2 cases were presented to the 1964 Scientific Session of the American Society of Clinical Pathologists,1 and Patras and Andujar2 reported similar findings in the brain of an elderly drug addict to the 1965 session. In September, 1965, Fowler and Carter3 submitted preliminary reports of 4 fatal cases of acute amebic meningoencephalitis in children with pathological findings similar to those in the cases referred above,1 and they also considered the possibility of infection through the olfactory route. In our first 2 cases the diagnosis was suggested by the finding of motile amebas in the spinal fluid during life, but this finding has not been reported by others. In none of these 8 cases was there evidence of generalized, or of intestinal, amebiasis. The organisms have not resembled Endamoeba histolytica but have had large karyosomes typical of the soil ameba. The present 3 cases occurred within a 2-mile radius in central Florida whereas those reported by Carter and Fowler3 were all from the same small town in the northern region of the Gulf of St. Vincent in South Australia.

Case Reports

Case 1. On August 12, 1962, a 10-year-old boy began to complain of increasingly severe, persistent frontal headache. For several days he had been swimming and diving in a small, land-locked lake. On the next day there was low-grade fever with occasional vomiting. Mild nuchal rigidity was noted on admission to the outpatient ward at the 136th United States Air Force Hospital, but there was no pharyngeal injection and no signs of infection of the upper respiratory tract. There was no injection or bulging of the eardrums, and the eyegrounds were normal. The spinal-tap fluid was clear, with less than 5 cells per cubic millimeter and a sugar of 70 mg. per 100 ml. He was given acetylsalicylic acid but returned on the following day with nuchal rigidity and a positive Babinski reflex. The white-cell count was 27,000, with 79 per cent neutrophils, 10 per cent band forms, 10 per cent lymphocytes and 1 per cent monocytes. The spinal fluid was cloudy, with 781 cells per cubic millimeter, 73 per cent neutrophils and 27 per cent lymphocytes. Motile amebas were seen in the fluid.

He was transferred to the Orange Memorial Hospital, where an emergency craniotomy did not reveal a brain abscess, although thick, turbid spinal fluid was obtained in which the amebas were again seen. These were approximately 15 to 25 microns in diameter, with a clear and distinct ectoplasm and well defined granular endoplasm. There was directional movement. Indistinct vacuoles were noted, but a definite nucleus could not be seen nor could a contractile vacuole be recognized. These amebas surrounded small debris but did not engulf red blood cells. They survived for a few days in regular E. histolytica culture mediums and in physiologic saline solution but did not thrive. Respiratory paralysis developed, and after operation the patient was placed in a respirator, but despite massive antibiotic therapy, including penicillin, sulfisoxazole (Gantrisin), chloramphenicol, erythromycin (Erythrocin) and tetracycline, he died within 6 days.

Case 2. An 18-year-old high-school student living in Orlando, Florida, noticed a gradual onset of increasing and persistent headache on or about September 2, 1962. He had been water skiing and swimming strenuously over the Labor Day weekend and had been diving to the bottom of a small, land-locked lake for a lost wristwatch. The headache increased, but he continued to swim and to attend school. He went to school on September 5 but had a temperature of 102.4°F. that afternoon. On the following day he was taken to a physician's office at 1 p.m., complaining bitterly of headache, which was so severe that he refused to answer questions although he seemed coherent. At the Orange Memorial Hospital Emergency Room there was nuchal rigidity but no evidence of upper-respiratory-tract infection or of otitis media. The eyegrounds were normal. The white-cell count was 20,000, with 88 per cent neutrophils and 12 per cent lymphocytes. The spinal-fluid count was 1069 cells per cubic millimeter, virtually all of which were neutrophils or large monocytic cells, and the spinal-fluid sugar was 56 mg. per 100 ml. Large doses of chloramphenicol and penicillin were given intravenously, and within 12 hours, sulfadiazine, 2.5 gm. every 12 hours, was added. The patient's condition became worse, and on the evening after admission a repeat spinal tap revealed a white-cell count of 15,200, all neutrophils or large monocytic cells. The spinal-fluid sugar was 20 mg., and the protein 450 mg. per 100 ml. At that time T/Sgt. Mike Phelan, who had seen Case 1 and who was a part-time member of the laboratory staff, noted suspicious activity in some of the larger cells. He warmed the slides with a hot penny, and active, directional amebas were seen. The parasites were similar to those previously noted, but appeared slightly smaller. Once again, cellular activity was preserved in E. histolytica culture medium for a few days, but propagation could not be obtained. It had been planned to transmit this specimen to the Communicable Disease Center, but one of the persons to whom this was entrusted decided that there was no growth in the tube and discarded the specimen.

Case 3. An 18-year-old college student had been visiting his family in Winter Park, Florida, and doing extensive swimming and water skiing in the small lake in which Case 1 had been swimming. On August 26, 1965, he noticed the onset of headache, but continued his usual activities, including swimming. The headache became more severe over the next few days, and vomiting developed. He was admitted to the Winter Park Memorial Hospital at 12:05 p.m. on August 30, 4 days after the onset of headache. On physical examination there were no signs other than those related to a fulminating meningoencephalitis. The nares were somewhat congested, but the eardrums gave no evidence of injection or bulging and the eyegrounds were normal. Lumbar puncture revealed many white cells, with 95 per cent neutrophils. A few gram-negative bacilli, both large and small, were noted on the smear, but these did not grow on culture. The course was inexorably downhill despite massive antibiotic therapy, including intravenous administration of sulfadiazine, chloramphenicol and penicillin. The patient died 6 days after the onset of the headache.

Autopsy Findings

The pathological findings in these 3 cases are discussed together since they are identical in all pertinent respects. In each the gastrointestinal tract was smooth throughout, and the viscera showed only mild congestion. Each exhibited a mild interstitial pneumonitis, with some hemorrhage, but neither granulomatous lesions nor organisms were seen. In each case the basal areas of the brain tended to be gray and necrotic, and a thick, yellowish-gray, purulent exudate completely filled the cisternae. These changes were marked in the region of the olfactory bulb but were not thought to be appreciably more severe here than in other areas of the ventral surface. In Cases 1 and 2 a number of pathologists studied the brain sections, but amebas were not identified until the slides were forwarded to Dr. C. G. Culbertson, who demonstrated numerous parasites in the cerebellum and cerebrum of both patients (Fig. 1Figure 1 Solitary Ameba with a Large Endosome and a Distinct Nuclear Rim in the Cerebrum of Case 2.). In Case 3 the prosector, Dr. J. G. Jones, did not identify amebas in the original sections, but after he had allowed me to review additional sections, hundreds of amebas could be found in advance of the severe inflammatory reaction. The amebas were best demonstrated by the iron hematoxylin stain, and in retrospect it was thought that the difficulty in original diagnosis was confusion with gitter cells (Fig. 2Figure 2 Many Amebas Superficially Resembling Gitter Cells in Case 3.) and with neurons because of the absence of differential staining by the amebas. A sharply distinct cytoplasmic outline and clear nuclear detail were considered mandatory for the positive identification of amebas. Uniformly, the nucleus had a large central or slightly eccentric karyosome and a distinct nuclear rim, features quite unlike those of E. histolytica and identical in all respects to those of the soil ameba.

Once the presence of amebas in Cases 1 and 2 had been established, it was thought important independently to confirm Culbertson's4 ^, 5 experimental data on the pathogenicity of the soil ameba and to prove the pathogenicity of the species living in lakes in which the boys had been swimming. At the time this experiment was begun the animal pathogenicity of strains from lake water had not been established.

Materials and Methods

Water samples were taken from representative lakes including the 2 in which the boys had been swimming. Amebas were cultured and isolated from this material in accordance with the methods previously described.6 This consists in the addition of a gently centrifuged sediment to an agar plate that has been impregnated with neomycin to hold back extraneous growth and with an organism either alive or dead that is capable of stimulating amebic excystment. In these experiments a variety of E. coli, the Singh organism, which has a proved capacity for producing excystment of the acanthameba was used.

Subcultures to agar plates containing only dead Singh organisms were made, and when satisfactory numbers of trophozoites were obtained, they were harvested by washing with distilled water. This solution was injected into the brains of mature Swiss mice. There were a number of unsuccessful experiments in which no neurologic defects were obtained and in which the amebas could not be retrieved at autopsy. In a successful experiment with 5 white mice there was no immediate response to the intracerebral inoculation of 0.05 ml. of suspended amebas. In approximately five days, however, focal paralysis began to develop in individual mice, with circling movements and other forms of neurologic malfunctioning. Death usually occurred within twenty-four to seventy-two hours of the onset of neurologic disease, and all 5 mice died within six to fifteen days. All had evidence of active amebic infiltration within the brain some distance from the site of inoculation. The histologic findings varied somewhat. In those that died early there was a diffuse purulent meningoencephalitis similar to that in the human cases (Fig. 3Figure 3 Amebas and Encephalitis in a Mouse Brain (Death Ten Days after Inoculation).). In those that lived longer there seemed to be more of a granulomatous response, with glial cells and macrophages surrounding isolated amebas. Intranasal inoculations were attempted but were not successful, presumably because it was difficult to obtain large numbers of active trophozoites and many of these inoculations were made with encysted forms.

Discussion

Culbertson has now produced over 2000 cases of experimental amebic meningoencephalitis via the intranasal route in animals from mice to monkeys.7 Pathological findings have varied from those associated with an overwhelming purulent and necrotic meningoencephalitis to a smoldering chronic encephalitis. Similar purulent meningoencephalitides with negative bacterial cultures are not uncommon, particularly in children.

The soil amebas are stimulated to excyst to the infectious or trophozoite form by gram-negative bacilli,8 which are present in heavy concentrations in the fecal stream, in decaying vegetation and in some small lakes during the latter part of summer. However, lake swimming and a tropical climate would not necessarily be common factors in the etiopathogenesis, for many liquid mediums, such as those in drainage ditches, swamps and sewage, support large numbers of gram-negative bacilli and would be suitable for stimulating amebic excystment. Accidental intranasal inoculation of such material whether from lake or sewage would duplicate virtually all salient features of the animal experiments.

The difficulty in diagnosis in these cases suggests that other cases may be missed at the bedside and at the autopsy table. Furthermore, as previously suggested,9 the human disease may not necessarily be fatal, for recovery has been reported in at least 1 case of encephalitis thought to be due to E. histolytica 10 and some of Culbertson's experimental animals are giving signs of recovery from infection. It is probable that the pathogenicity of the amebas for human being will depend on the effectiveness of the inoculum, including the virulence of the strain and the numbers and activity of the trophozoites. Many strains of acanthameba, including some with proved animal pathogenicity, have been recovered from the noses and throats of human volunteers,11 but clinical disease has not been associated with these isolates.

*From the Department of Pathology, Orange Memorial Hospital.

I am indebted to J. D. Jones, M.D., for inclusion of Case 3.

Source Information

ORLANDO, FLORIDA

†Chief, Department of Pathology, and director of laboratories, Orange Memorial Hospital.

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