There are many free-living amoeba, but only four have been causally associated with disease in humans. These are Acanthamoeba spp., Balamuthia mandrillaris (only known species of Balamuthia), Naegleria fowleri and Sappinia pedata. The family Acanthamoebidae consists of the two genera called Acanthamoeba and Balamuthia.¹ These organisms live as parasites when they invade humans and animals.

The amoeba belonging to Acanthamoeba and B. mandrillaris can lead to various illnesses in humans. Examples are as follows (there is a predilection in immunocompromised or debilitated hosts):^1,2

• Granulomatous amoebic encephalitis (GAE) - caused both by Acanthamoeba and Balamuthia
• Amoebic keratitis (AK) - caused by Acanthamoeba spp.
• Other species of the Acanthamoeba can cause illness in immunocompromised hosts, e.g. skin lesions or sinusitis

Distribution

These organisms are ubiquitous and found worldwide. They are found in soil, dust, air and water (e.g. swimming pools, domestic and sewage). They have also been isolated in hospitals, contact lenses and cell cultures. In cell cultures they are commonly contaminants. In fact, this is how they were discovered in the 1950s - they grew on cell cultures grown for the polio vaccine. Acanthamoeba can also be found in fish and has been isolated from the nasal and throat mucosa of healthy humans.

Both Acanthamoeba and Balamuthia can act as hosts for other bacterial infections, e.g. legionellosis. Further research into this area is ongoing.^2,3

N. fowleri is also ubiquitous and found in soil and warm water. Sappinia spp. is a more recently recognised amoeba that also causes CNS disease.

Lifecycle

There are 2 stages:^1,2

1. Active feeding stage:
   • During this stage the trophozoites are actively dividing by feeding on bacteria, yeast and algae or axenically (i.e. not associated with any other organisms).
2. Dormant cyst stage:
   • Cysts form once there is a change in the environment of the trophozoites, e.g. nutrient deprivation or changes in temperature. The cysts are resistant to chlorination and antibiotics.

Granulomatous amoebic encephalitis¹

Granulomatous amoebic encephalitis (GAE) is caused by both Acanthamoeba spp. and Balamuthia.

• Description - chronic, slowly progressive infection of the CNS. May also involve the lungs.
• Causative organism - several Acanthamoeba spp. and Balamuthia spp. may cause GAE.
• Incubation period - unknown but estimated at weeks to months. The route of infection is aerosol or direct inoculation with haematogenous spread to the CNS.
• Epidemiology - GAE is a very rare cause of disease and most publications are case reports.¹ Most cases are not identified until postmortem due to the lack of good and reliable diagnostic tests and secondary infections being more common.
  Commonly seen in immunocompromised patients, including those with neoplasia, systemic lupus erythematosus, human immunodeficiency virus and tuberculosis. However, cases have been seen in the immunocompetent - for example, Balamuthia infections in children.⁴
• Risk factors - alcoholism, drug abuse, chemotherapy, corticosteroids and organ transplantation.
• Presentation:
  • Symptoms - headache, confusion, fever, lethargy, nausea and vomiting, seizures, photophobia and neck stiffness. Patients may become frankly psychotic.
  • Signs - neck stiffness and focal neurological deficits. The latter include the following: hemiparesis, cranial nerve deficits (e.g. facial nerve palsy), diplopia, ataxia and positive Babinski's sign and positive Kernig's sign. Patients may also develop raised intracranial pressure.
• Diagnosis - cerebrospinal fluid (CSF) smear (usually lymphocyte predominance and low glucose), culture, immunofluorescence or polymerase chain reaction (PCR).⁵ In AIDS patients the CSF may be lacking in cells, making diagnosis difficult. Brain biopsy may be required. CNS imaging, e.g. CT and MRI scanning, may reveal enhancing or non-enhancing lesions and is thus non-diagnostic. Balamuthia does not grow on agar plates, unlike Acanthamoeba. However, similar to Acanthamoeba, Balamuthia is difficult to isolate from CSF specimens.²
• AIDS patients - may have disseminated infection. They may also have chronic sinusitis, otitis and skin lesions. Cases of vasculitis and osteomyelitis have also been reported.
• Differential diagnosis - bacterial or viral meningitis, toxoplasmosis or CNS vasculitis (last two in AIDS patients).
• Prognosis - mortality is high with GAE, reaching almost 100% when both skin lesions and CNS disease occur together.
• Treatment - GAE is treated with pentamidine, usually in combination with one or more of the following: ketoconazole, hydroxystilbamidine, paromomycin, 5-fluorocytosine polymyxin, sulfadiazine, trimethoprim-sulfamethoxazole and azithromycin. Similar medications are used in the treatment of Balamuthia.²

Cutaneous acanthamoebiasis¹

Caused by both Acanthamoeba spp. and Balamuthia spp.

• Skin lesions - hard nodules or nonhealing, indurated skin ulcers can occur.
• Treatment - skin lesions are difficult to treat and even harder when the CNS is also involved. Regimens including itraconazole, pentamidine, and 5-flucytosine have been used. Topical chlorhexidine and ketoconazole are also used in addition to systemic therapies.
• Prognosis - 76% mortality is associated with skin disease alone (higher when granulomatous amoebic encephalitis (GAE) is also present - see above).

Amoebic keratitis¹

• Description - amoebic keratitis (AK) is a progressive disease of the cornea, which is sight-threatening
• Causative organism - several Acanthamoeba spp. may cause AK.
• Commonly seen in - immunocompetent patients. However, infection does not confer immunity and reinfection is common.
• Risk factors - poor contact lens hygiene, corneal abrasion or exposure of the eye to contaminated water.
• Epidemiology - the incidence of AK is 3 per 100,000 and around 85% of cases occur in people who wear contact lenses. In a Scottish study the incidence amongst wearers of soft contact lenses was 14.9 per 100,000.⁶An epidemic of AK occurred in the USA in the 1980s which was related to contaminated contact lenses and solutions.
• Presentation - secondary bacterial infection occurs commonly, making it difficult to diagnose.
  • Symptoms - watering of eyes, eye pain with photophobia, blurred vision and irritation are common.
  • Signs - include ptosis, conjunctival hyperemia, episcleritis, scleritis and loosening of the corneal epithelium. Stromal infiltrates can be seen with a bright light. Rarely, trophozoites can infiltrate the corneal nerve and retina, leading to chorioretinitis.
• Diagnosis - corneal scrape or biopsy.
• Differential diagnosis - herpes keratitis or fungal keratitis.
• Treatment - wide epithelial debridement if infection is detected early - but try to achieve medical resolution first.⁷ Therapy should include the cationic antiseptic agents, of which chlorhexidine or polyhexamethylene biguanide (PHMB) is the most effective. This is in combination with propamidine isethionate and neomycin as part of triple therapy. These may have to be used for prolonged periods, e.g. more than a year. Imidazoles have also been used but success rates are not great. In severe cases, enucleation may be necessary.
• Prevention - killing Acanthamoeba spp. from the contact lens. Tap water should not be used to rinse contact lenses. The British Contact Lens Association gives advice to those who wear contact lenses (see under Internet and further reading below).

Primary amoebic meningoencephalitis⁸

Primary amoebic meningoencephalitis (PAM) is caused by N. fowleri.

• Description - acute, rapidly progressive CNS infection, which is usually fatal.
• Causative organism - although there are over 30 Naegleria spp. the condition is only caused by the N. fowleri variety.
• Incubation period - unknown.
• Risk factors - swimming in contaminated warm water. The amoebae pass through the olfactory mucosa to the CNS. No human-to-human spread has been described.
• Epidemiology - 1 case per 2.6 million exposed. Most reports are from the USA and India.⁸
• Presentation - similar to bacterial/viral meningitis.
  • Symptoms - headache, photophobia, nausea and vomiting.
  • Signs - pyrexia, neck stiffness and localising signs, e.g. cranial nerve (CN) palsies when encephalitis develops. Patients can present in a comatosed state.
• Diagnosis/investigations - these should include tests for any suspected meningoencephalitis, i.e. full blood count, and CT scan of the brain. Definitive diagnosis rests on identifying the trophozoites in the CSF or biopsy specimens. PCR is being used in research centres with good results. Serology testing is unlikely to be helpful as the short duration means there is virtually no time for an antibody response to be initiated.
• Differential diagnosis - bacterial or viral meningoencephalitis.
• Treatment - amphotericin B is the drug of choice. Most evidence is based on case reports and amphotericin is usually combined with rifampicin and other broad-spectrum antibiotics. Drugs are usually administered intravenously but intrathecal use has also been described.
• Prognosis - mortality is over 95% and usually within a week of presentation. Better cases relate to early detection and treatment but often there will be residual neurological problems.
• Prevention - chlorination of swimming pools.

Sappinia amoebic encephalitis⁹

Caused by S. pedata; (S. diploidea is another species but infections in humans have not been reported).

• Description - meningoencephalitis associated with cerebral tumour-like lesion, described in one case only.
• Incubation period, mode of spread and risk factors - all remain unknown. It is likely to be reach the CNS either through the nasal mucosa or the bloodstream.
• Epidemiology - only one case described in the literature.
• Presentation - sinus infection was followed by headache, vomiting and photophobia.
• Diagnosis - CT brain scan in the single reported case revealed a tumour-like mass. PCR is likely to be a very important tool in diagnosing this particular infection; the infection was thought to be S. diploidea but PCR confirmed it to be S. pedata.
• Treatment - in the reported case, the cerebral lesion was surgically removed and azithromycin, pentamidine, itraconazole and flucytosine were also administered. The patient survived.

Document references

1. Marciano-Cabral F, Cabral G; Acanthamoeba spp. as agents of disease in humans. Clin Microbiol Rev. 2003 Apr;16(2):273-307. [abstract]
2. Visvesvara GS, Moura H, Schuster FL; Pathogenic and opportunistic free-living amoebae: Acanthamoeba spp., Balamuthia mandrillaris, Naegleria fowleri, and Sappinia diploidea. FEMS Immunol Med Microbiol. 2007 Jun;50(1):1-26. Epub 2007 Apr 11. [abstract]
3. Shadrach WS, Rydzewski K, Laube U, et al; Balamuthia mandrillaris, free-living ameba and opportunistic agent of encephalitis, is a potential host for Legionella pneumophila bacteria. Appl Environ Microbiol. 2005 May;71(5):2244-9. [abstract]
4. Intalapaporn P, Suankratay C, Shuangshoti S, et al; Balamuthia mandrillaris meningoencephalitis: the first case in southeast Asia. Am J Trop Med Hyg. 2004 Jun;70(6):666-9. [abstract]
5. Qvarnstrom Y, Visvesvara GS, Sriram R, et al; Multiplex real-time PCR assay for simultaneous detection of Acanthamoeba spp., Balamuthia mandrillaris, and Naegleria fowleri. J Clin Microbiol. 2006 Oct;44(10):3589-95. [abstract]
6. Seal DV, Kirkness CM, Bennett HG, et al; Population-based cohort study of microbial keratitis in Scotland: incidence and features. Cont Lens Anterior Eye. 1999;22(2):49-57. [abstract]
7. Lindquist TD; Treatment of Acanthamoeba keratitis. Cornea. 1998 Jan;17(1):11-6. [abstract]
8. Parija SC et el; Naegleria infection, eMedicine, Aug 2009
9. da Rocha-Azevedo B, Tanowitz HB, Marciano-Cabral F; Diagnosis of infections caused by pathogenic free-living amoebae. Interdiscip Perspect Infect Dis. 2009;2009:251406. Epub 2009 Aug 2. [abstract]

Internet and further reading

• British Contact Lens Association; (BCLA)
• da Rocha-Azevedo B, Tanowitz HB, Marciano-Cabral F; Diagnosis of infections caused by pathogenic free-living amoebae. Interdiscip Perspect Infect Dis. 2009;2009:251406. Epub 2009 Aug 2. [abstract]
• Crum-Cianflone NF; Acanthamoeba, eMedicine, Jun 2008

Acknowledgements