J. Med. Microbiol. Ð Vol. 50 12001), 205±207 # 2001 The Pathological Society of Great Britain and Ireland ISSN 0022-2615 EDITORIAL Balamuthia mandrillaris infection The concept that certain small free-living amoebae identi®ed as due to Acanthamoeba solely on the basis such as Acanthamoeba have the potential to cause of cysts in the tissue sections. Isolation of B. disease in animals, including man, was suggested by mandrillaris ± initially from the brain tissue of a Culbertson in 1961 [1]. This observation was based on mandrill that died of encephalitis in the San Diego remarkable experiments conducted in the late 1950s Wild Animal Park, and subsequently from human and during the manufacture of polio vaccine at the Eli Lilly other animal brain tissue ± and the production of laboratories in Indianapolis, IN, USA [1, 2]. A few speci®c anti-B. mandrillaris serum, were instrumental years later Fowler and Carter in Australia [3] and Butt in differentiating Balamuthia from Acanthamoeba [4] in the USA described the ®rst human cases of encephalitis [10, 11]. amoebic meningo-encephalitis caused by Naegleria fowleri. Since then several hundred cases of central nervous system 1CNS) disease caused by Acanthamoe- Protozoology, cultural characteristics, and ba spp., N. fowleri and Balamuthia mandrillaris have epidemiology been reported worldwide [5±7]. Several cases in animals other than man have also been reported B. mandrillaris, like Acanthamoeba, has a vegetative [8, 9]. Infection is almost invariably fatal. The CNS trophic stage and a dormant cyst stage in its life cycle. disease produced by N. fowleri is called primary The trophozoites measure 15±60 ìm in diameter and amoebic meningo-encephalitis and occurs principally in are characterised by a round nucleus with a large, immunocompetent children and young adults with a spherical, densely staining nucleolus. Binucleate forms history of contact with fresh water; infection is are occasionally seen. In some cases the nucleus may fulminating and nearly always fatal. The infection have more than one nucleolus and this feature distin- caused by Acanthamoeba spp. or B. mandrillaris is guishes B. mandrillaris amoebae from Acanthamoeba, called granulomatous amoebic encephalitis. Acantha- especially in tissue sections [7, 10]. During the early moeba encephalitis occurs principally in immunosup- stages of mitosis the nucleolus and nuclear membrane pressed individuals, chronic alcoholics and debilitated both remain intact, but disappear as mitosis progresses. persons; B. mandrillaris causes encephalitis in the very The nucleus is surrounded by abundant cytoplasm young or very old and in immunosuppressed indivi- containing empty vacuoles, numerous mitochondria, duals. Acanthamoeba spp. also cause a non-fatal, but ribosomes and endoplasmic reticulum. The trophozoites vision-threatening keratitis of the human eye [5±7]. form broad pseudopodia and move slowly. Occasionally, The true incidence of amoebic encephalitis is not the amoebae produce ®nger-like pseudopodia and exactly known. As of 1 Aug. 2000, .190 cases of exhibit spider-like walking movement across the ¯oor meningo-encephalitis due to N. fowleri have been of the tissue culture cells on which they are feeding [11]. reported worldwide, 95 in the USA alone. More than The cysts of B. mandrillaris are usually spherical and 120 cases of granulomatous encephalitis due to measure 6±30 ìm in diameter, with a mean of 15 ìm. Acanthamoeba spp. 184 in the USA, including at least They are usually uninucleate and possess a layer of 50 patients with HIV) and .3000 cases of Acantha- refractile granules beneath the inner cyst wall. Under the moeba keratitis have been reported. There have been optical microscope they resemble Acanthamoeba cysts .85 additional reports of encephalitis due to B. with an outer wrinkled wall 1the ectocyst) and an inner mandrillaris; 40 of these occurred in the USA, and thin wall 1the endocyst), but ultrastructurally they are include .10 patients with HIV. The actual number is tripartite with an additional thick, amorphous, ®brillar probably higher, as the diagnosis is usually made at middle layer, the mesocyst [11]. autopsy, which is seldom done in many countries [5]. N. fowleri and Acanthamoeba spp. are widely dis- Until 1989, the identi®cation of the causative agent of tributed in fresh water, soil and dust throughout the granulomatous amoebic encephalitis, in the absence of world, thus providing a potential source of infection for culture isolation or immunohistological assay, was man and other animals [5±7, 12]. The environmental based on the presence of amoebic cysts in tissue niche of B. mandrillaris is not known, but it is believed sections, as N. fowleri does not produce cysts in tissue to occupy the same locations as N. fowleri and whereas Acanthamoeba does. Hence many cases were Acanthamoeba spp. 206 EDITORIAL Unlike Acanthamoeba spp. and N. fowleri, B. man- that may be helpful in the diagnosis of B. mandrillaris drillaris does not grow on non-nutrient agar plates infection has been developed [25]. coated with gram-negative bacteria [11]. It is probably for this reason that B. mandrillaris has not been Cerebrospinal ¯uid 1CSF) obtained by lumbar puncture isolated from nature, as free-living amoebae are usually may show lymphocytic pleocytosis, normal or slightly isolated by inoculating environmental samples on low glucose level, and mild elevation of proteins, bacteria-covered agar plates. B. mandrillaris grows usually without amoebic trophozoites or cysts. The well at 35±378C on monkey kidney cells or human CSF resembles that of aseptic meningitis in most cases. lung ®broblasts, as well as in a complex chemical medium containing fetal bovine serum [13], which is Neuroimaging by computed tomography and magnetic helpful in the screening of isolates for drug suscept- resonance imaging of the head are helpful in the ibility and potential treatments [13, 14]. diagnosis [5, 23, 24]. Intravenous contrast material may reveal ring-enhancing masses suggestive of a brain Experimental animals such as out-bred mice have been abscess or a brain tumour [5, 23]. Chest X-ray may infected by intranasal instillation of culture-grown B. show focal areas of consolidation in the pulmonary mandrillaris, resulting in disease similar to that seen in tissue. man and death of the animals [10, 11]. A congenitally immunode®cient mouse model has also been described, con®rming the `opportunist' characteristics of B. Pathogenesis and pathological features mandrillaris infection [15]. B. mandrillaris produces multifocal, subacute or chronic granulomatous encephalitis with trophozoites and cysts within the central nervous system, as well as Clinical features and diagnosis cutaneous nodules, ulcerations and pneumonitis. Neu- ropathologically, infection is characterised by brain Clinically, encephalitis due to B. mandrillaris resem- oedema and a subacute necrotising haemorrhagic bles that caused by Acanthamoeba, namely the pres- encephalitis. A modest lymphocytic in®ltrate is often ence of a single or multiple space-occupying les- present, depending on the immunological status of the ions. Neurologically, hemiparesis, aphasia and seizures host. This is usually composed of CD4 and CD8 T may appear early. Personality and mental status cells and B lymphocytes, together with a few plasma abnormalities may be present, associated with head- cells, macrophages and multinucleate giant cells. ache, stiff neckand cranial nerve palsies mainly Amoebic trophozoites and cysts are usually present affecting the third and sixth cranial nerves. Cerebellar within perivascular spaces and within the necrotic CNS ataxia, diplopia and low grade fever have been reported parenchyma. Focal chronic leptomeningitis may be in some cases. These symptoms closely mimic ± and seen in the areas near the parenchymal lesions. are often mistaken for ± a bacterial or viral Arteritis with trophozoites and cysts may be seen in encephalitis, leptomeningitis and tuberculous meningi- the same areas. Pneumonitis, and ulcerative dermatitis tis. Raised intracranial pressure can lead to papilloe- with the presence of trophozoites and cysts have also dema, coma, herniation of cerebellar tonsils and death. been described [5]. Invasion and penetration into the The direct cause of death is usually acute broncho- CNS probably arises by haematogenous spread from a pneumonia, hepatic or renal failure, septicaemia, and primary focus of infection in the lower respiratory tract brain oedema leading to uncal and cerebellar tonsillar or skin [5]. The ulcerated skin lesions may serve as a herniation [5, 16±23]. portal of entry of the amoeba or may represent `terminal' haematogenous dissemination of the infec- The diagnosis is made when the amoebic trophozoites tion. and cysts are identi®ed in tissues or the agent is isolated in culture. Brain and skin biopsies are useful B. mandrillaris can infect healthy and immunosup- for culture and for histological examination with pressed hosts of both sexes, and there may be no haematoxylin and eosin or special stains like Gomori's history of swimming or exposure to contaminated methenamine silver and periodic acid-Schiff. However, water. Encephalitis due to B. mandrillaris usually runs immunohistological techniques, e.g., indirect immuno- a long protracted and insidious clinical course with an ¯uorescence assay, or electron microscopy are neces- unknown incubation period, that is de®nitely longer sary to identify B. mandrillaris in tissue sections [5± than 10 days