This review focuses on free-living amoebae, widely distributed in soil and water, causing opportunistic and non-
opportunistic infections in humans: Acanthamoeba spp., Balamuthia mandrillaris, Naegleria fowleri, and Sappinia diploidea. Diseases include primary amoebic
meningoencephalitis (N. fowleri), granulomatous amoebic
encephalitis, cutaneous and nasopharyngeal
infections (Acanthamoeba spp., Balamuthia mandrillaris, S. diploidea), and amoebic
keratitis (Acanthamoeba spp). Acanthamoeba, Balamuthia, and Naegleria have been repeatedly isolated; S. diploidea has been reported only once, from a brain
infection. Antimicrobial
therapy for these
infections is generally empirical and patient recovery often problematic. N. fowleri is highly sensitive to the
antifungal agent amphotericin B, but delay in diagnosis and the fulminant nature of the disease result in few survivors.
Encephalitis and other
infections caused by Acanthamoeba and Balamuthia have been treated, more or less successfully, with antimicrobial combinations including
sterol-targeting
azoles (
clotrimazole,
miconazole,
ketoconazole,
fluconazole,
itraconazole),
pentamidine isethionate,
5-fluorocytosine, and
sulfadiazine. The use of
drug combinations addresses resistance patterns that may exist or develop during treatment, ensuring that at least one of the drugs may be effective against the amoebae. Favorable drug interactions (additive or synergistic) are another potential benefit. In vitro
drug testing of clinical isolates points up strain and species differences in sensitivity, so that no single
drug can be assumed effective against all amoebae. Another complication is risk of activation of dormant
cysts that form in situ in Acanthamoeba and
Balamuthia infections, and which can lead to patient relapse following apparently effective treatment. This is particularly true in
Acanthamoeba keratitis, a non-
opportunistic infection of the cornea, which responds well to treatment with
chlorhexidine gluconate and
polyhexamethylene biguanide, in combination with
propamidine isothionate (
Brolene),
hexamidine (Désomodine), or
neomycin. Acanthamoeba spp. may also be carriers of endosymbiotic bacteria (Legionella and Legionella-like pathogens) and have been implicated in outbreaks of
pneumonias in debilitated hosts. As with other
infectious diseases, recovery is dependent not only on antimicrobial
therapy, but also on patient's immune status, infective dose and virulence of the ameba strain, and on how early the disease is diagnosed and
drug therapy initiated.