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Balamuthia mandrillaris pathology

Authors: A/Prof Patrick Emanuel, Dermatopathologist, Clinica Ricardo Palma, Lima, Peru; Dr. Alex Ventura León, Pathologist Universidad Peruana Cayetano Heredia, Lima, Peru; Dr. Cesar Ramos, Dermatologist, Lima, Peru. DermNet NZ Editor in Chief: Adjunct A/Prof Amanda Oakley. Copy edited by Gus Mitchell. October 2019.

Table of contents


Balamuthia mandrillaris (B. mandrillaris) is a free-living amoeba well known in endemic areas for causing potentially fatal neurological infection. It often presents primarily in the skin as an indurated plaque on the central face or — less commonly — on other parts of the body (figure 1).

Balamuthia mandrillaris skin lesion

Histology of Balamuthia mandrillaris

A skin biopsy of a plaque due to B. mandrillaris, shows a dense dermal infiltrate with loose granulomas accompanying an infiltrate rich in plasma cells and lymphocytes (figures 2,3). Characteristically, there are multinucleated giant cells free in the dermis, outside of the loose granulomas (figure 3).

The causative organisms can be extremely difficult to find. The clinical presentation and unusual infiltrate (figures 2,3) should prompt a search for B. mandrillaris with serial sectioning of the tissue block.

The organisms are quite characteristic. B. mandrillaris have a granular to vacuolated cytoplasm with irregular contours (pseudopods) and contain a nucleus with a large central karyosome (figure 4, arrow).

Balamuthia mandrillaris pathology

Special studies for Balamuthia mandrillaris

The B. mandrillaris organisms do not stain with Periodic acid-Schiff (PAS), Gomori Methenamine-Silver Nitrate Stain (GMS) or acid-fast bacilli (AFB). The amoeba cannot be cultured on regular agar plates, as the organism does not feed on bacteria. It can be cultured on primate liver or human brain cells. Polymerase chain reaction (PCR) studies may help in the identification of the organism.

Differential diagnosis of Balamuthia mandrillaris

Acanthamoeba species — these tend to cause ulcerated lesions with epithelial hyperplasia rather than indurated plaques. Culture or PCR studies can be helpful

Naegleria fowleri — this infection usually does not present primarily in the skin. Rather it usually presents as an erosive upper airway infection. N. fowleri is more common in North America while B. mandrillaris is more common in South America. Culture or PCR studies can be helpful.



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  • Norgan AP, Sloan LM, Pritt BS. Detection of Naegleria fowleri, Acanthamoeba spp, and Balamuthia mandrillaris in Formalin-Fixed, Paraffin-Embedded Tissues by Real-Time Multiplex Polymerase Chain Reaction. Am J Clin Pathol. 2019 Nov 4;152(6):799-807. doi: 10.1093/ajcp/aqz103. PMID: 31415080. PubMed

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