Amoebiasis

Notification requirement for amoebiasis

Notification is not required.

Primary school and children’s services centre exclusion for amoebiasis

Exclude until there has not been a loose bowel motion for 24 hours.

Infectious agent of amoebiasis

Amoebiasis is caused by the parasite Entamoeba histolytica. The parasite exists in two forms: an infective cyst and a potentially pathogenic trophozoite. It should not be confused with the morphologically identical non-pathogenic E. dispar and E. moshkovskii.

Identification of amoebiasis

Clinical features

Most infections are asymptomatic, but occasionally clinically important intestinal or extra-intestinal disease may result.

Intestinal disease varies from acute with diarrhoea with fever, chills and bloody or mucoid diarrhoea (amoebic dysentery) to mild abdominal discomfort with diarrhoea containing blood or mucus, alternating with periods of constipation or remission.

Intestinal amoebiasis may rarely be complicated by:

• granuloma of the large intestine
• colitis, colonic perforation and haemorrhage
• perianal ulceration.

Dissemination via the bloodstream may lead to extra-intestinal amoebiasis. This is most commonly manifested as abscess formation in the liver. It can occur less commonly in the brain or lungs. Rarely, ulceration in the perianal region can occur as a direct extension from intestinal lesions, and penile lesions may occur in men who have sex with men.

Diagnosis

The presence of trophozoites containing red blood cells is strongly suggestive of invasive (symptomatic) amoebiasis.

Diagnosis is confirmed by microscopic examination for trophozoites or cysts in:

• fresh or suitably preserved faecal specimens
• smears of aspirates or scrapings obtained by proctoscopy
• aspirates of abscesses or other tissue specimens.

Repeated stool specimens may be needed to establish a diagnosis and are best obtained in the morning. Examination of at least three specimens will increase the yield of organisms from 50 per cent in a single specimen to 85–90 per cent.

Microscopy does not distinguish between amoebas; therefore, additional tests are required.

Serology (indirect haemagglutination [IHA] and enzyme immunoassays [EIA] for Entamoeba) and polymerase chain reaction (PCR) are useful in the diagnosis of extra-intestinal disease, such as liver abscesses, when stool examination is often negative. Serology and PCR specific to E. histolytica are also important in the differentiation of strains of the pathogenic E. histolytica from strains of the other non-pathogenic Entamoeba.

Ultrasound and CT scans are also useful in the identification of amoebic liver abscesses and can be considered diagnostic in the presence of a specific antibody response to E. histolytica.

Incubation period of Entamoeba histolytica

The average incubation period is 2–4 weeks. Patients may, however, present months to years after the initial infection.

Public health significance and occurrence of amoebiasis

Occurrence is worldwide. Prevalence rates tend to be higher:

• in areas with poor sanitation
• in institutions for the intellectually disabled
• among men who have sex with men
• among travellers returning from developing countries.

Amoebiasis most commonly affects young adults and is rare below the age of 5 years. Amoebic dysentery is very rare under the age of 2 years, when dysentery is more commonly due to Shigella.

Outbreaks have occurred following water supply contamination, as the cysts are resistant to chlorine.

Reservoir of Entamoeba histolytica

Humans, who are often asymptomatic and pass the cysts in faeces, are the reservoir.

Mode of transmission of Entamoeba histolytica

Amoebiasis can be transmitted by:

• ingestion of faecally contaminated food or water containing amoebic cysts
• indirect hand contamination from contaminated surfaces
• oral–anal sexual contact with a chronically ill or asymptomatic carrier.

Period of communicability of amoebiasis

Cases are infectious as long as cysts are present in the faeces. In some instances, cyst excretion may persist for years.

Susceptibility and resistance to amoebiasis

All non-immune people are susceptible to infection. People with E. dispar do not develop symptoms. Re-infection is possible but rare.

Control measures for amoebiasis

Preventive measures

General public health measures to prevent disease transmission focus on:

• public education on the importance of personal hygiene
• public education about the importance of hand hygiene after defecation and before preparing or eating food
• providing information to intending travellers about the risks involved in eating uncooked vegetables and fruits and drinking potentially contaminated water
• public education about the possibility of transmitting the disease via sexual contact.
• protecting public water supplies from faecal contamination
• investigating the food preparation practices of any implicated local food premises
• boiling and purifying water before consumption in endemic areas.

Control of case

Treating clinicians should consult the current version of Therapeutic guidelines: gastrointestinal and seek expert advice.

Treatment of asymptomatic carriage is recommended to minimise the spread of disease and to lessen the risk of developing invasive disease.

Metronidazole or tinidazole are the usual treatments. Paramomycin is used to eradicate carriage within the colon; it is not registered for use in Australia but is available via the Special Access Scheme.

For amoebic liver abscess, tinidazole should be given for 5 days or metronidazole should be given for 14 days. Eradication therapy for carriage should also be given and specialist advice should be sought.

Control of contacts

Consider faecal screening for household members and institutional contacts. Faecal screening is advised for fellow travellers of a confirmed case. Confirmed carriers should also be treated.

Control of environment

Environmental measures to control disease transmission focus on:

Outbreak measures for amoebiasis

In the event of a cluster of cases, public health measures involve:

• confirming laboratory results
• undertaking an epidemiological investigation to determine the source of infection and common mode of transmission
• taking appropriate measures to eliminate any common vehicles of transmission, such as contaminated food or water, to prevent further cases.

Special settings

People who are suspected of having acquired their infection in an institutional setting should be investigated as appropriate by the department.