Page content: Victorian statutory requirement | Infectious agent | Identification | Incubation period | Public health significance & occurrence | Reservoir | Mode of transmission | Period of communicability | Susceptibility & resistance | Control measures | Outbreak measures | Additional sources of information

Victorian statutory requirement

Notification is not required.

School exclusion: exclude until diarrhoea has ceased.

Infectious agent

Entamoeba histolytica is a protozoan parasite that exists in two forms: an infective cyst and a potentially pathogenic trophozoite. It should not be confused with the morphologically identical non-pathogenic Entamoeba dispar.

Identification

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

Intestinal disease varies from an acute form with diarrhoea which may be bloody and associated fever and abdominal discomfort (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
• 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. This can occur less commonly in the brain or lungs.

Method of diagnosis
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 as cysts are shed intermittently in asymptomatic and mild infections. The presence of trophozoites containing red blood cells is indicative of invasive amoebiasis.

Serology using indirect haemagglutination (IHA) and enzyme immunoassays (EIA) is useful in the diagnosis of extra-intestinal disease such as liver abscesses, when stool examination is often negative. Serology is also important in the differentiation between strains of the pathogenic E. histolytica and strains of the non-pathogenic E. dispar.

X-ray, ultrasound and CT scans are also useful in the identification of amoebic abscesses and can be considered diagnostic in the presence of a specific antibody response to E. histolytica.

Incubation period

The average incubation period is two to four weeks. Patients may present months to years after the initial infection.

Public health significance & occurrence

Occurrence is worldwide. Prevalence rates tend to be higher in:

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

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

Reservoir

Humans are often asymptomatic carriers.

Mode of transmission

Amoebiasis can be transmitted by:

• ingestion of water contaminated with faeces containing amoebic cysts
• ingestion after faecal contamination of hands
• contaminated raw vegetables
• unprotected oral-anal sexual contact.

Period of communicability

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

Susceptibility & resistance

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

Control measures

Preventive measures
General public health measures to prevent disease transmission focus on:

• public education on the importance of personal hygiene
• providing information to intending travellers about the risk involved in eating uncooked vegetables and fruits and drinking contaminated water
• public education about the possibility of transmitting the disease via sexual contact.

Control of case
Treating clinicians should consult the current version of Therapeutic guidelines: antibiotic (Therapeutic Guidelines Limited) and seek expert advice. Metronidazole and either diloxanide furoate or paramomycin are the usual treatment. Neither diloxanide furoate or paramomycin are registered for use in Australia - contact the NPS Therapeutic Advice and Information Service on 1300 138 677.

For amoebic liver abscess metronidazole should be continued for 14 days and specialist advice should be sought. Passage of Entamoeba cysts or trophozoites in the absence of acute dysenteric illness does not warrant antimicrobial therapy.
Surgical aspiration of abscesses may be necessary. Cyst eradication with diloxanide furoate may be indicated in cyst carriers. Seek expert advice.

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:

• protecting public water supplies from faecal contamination
• investigation of the food preparation practices of any implicated local food premises.

Outbreak measures

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

• confirmation of laboratory results
• undertaking an epidemiological investigation to determine 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
Persons who are suspected of having acquired their infection in an institutional setting should be investigated as appropriate by the Department of Human Services.

Additional sources of information

Centers for Disease Prevention and Control, Atlanta USA