When the results came back, the blood tests showed some mild inflammation in the liver (raised liver enzymes) and a bit of obstruction of the biliary tree (raised alkaline phosphatase, ALP) but the ultrasound was more dramatic showing a 7.5cm abscess in the liver. This seemed to confirm the GPs concerns that this might be an amoebic liver abscess. Chatting it through with the Duty Microbiologist the GP was advised to arrange further blood tests and to make a referral to the surgeons.
What are amoebae?
Amoebae are free living unicellular parasites. They can survive for long periods of time in contaminated water and multiply easily as they reproduce by simple cell division. There are two main amoebae that cause infection in humans (amoebiasis); Entamoeba histolytica and the less virulent but identical looking Entamoeba dispar. Humans are the main hosts for both these parasites.
The biggest risk factors for infection with amoebae are living in unsanitary conditions and exposure to unsafe drinking water or food. Even when a traveller has been careful with sanitation and avoided food contamination, risks can remain as some countries still use “night soil” (faeces collected overnight) to fertilize agricultural land which poses a hidden hazard. At the end of the day you have to have eaten someone else’s “poo” to get amoebiasis! Yep sorry but it’s true…Yuck!!
How does amoebiasis present?
Amoebiasis is usually asymptomatic with most people not even being aware they have this little parasite growing in their guts. The incubation period ranges from a few days up to several months or years so a detailed exposure history is essential.
However, sometimes amoebiasis can cause gastrointestinal or even extra-intestinal infections. Gastrointestinal infection may range from mild diarrhoea and abdominal pain to fulminant (severe) dysentery with bloody diarrhoea and fever. Amoebic dysentery can mimic inflammatory bowel disease but it is important to distinguish the two different diagnoses as steroids will make amoebic dysentery much worse!
Amoebiasis can also lead to a localised granulomatous lesion in the large bowel, called an amoeboma, which can potentially be mistaken for bowel cancer unless samples are sent for histopathological diagnosis.
Extra-intestinal infection occurs when amoebae spread from the bowel either via the blood stream or by direct contact with skin (usually on the perineum). The most common extra-intestinal presentation is a liver abscess, although abscesses can also occur in the lung or brain. Less commonly localised skin infections can occur with painful ulcers at the site of inoculation.
How is amoebiasis diagnosed?
The main method of diagnosis is by microscopy of stool or pus samples, or histopathological examination of tissue samples. Pus from abscesses is typically described as “looking like anchovy paste… it is thick and brown orange…” very much like a paste of anchovies… we do like our food related descriptions in medicine!
In addition, the fresher the sample the better as amoebae will start to die over time…. Ideally the sample should be examined within 30 minutes of voiding but that is rarely practical… just do the best you can!
In non-endemic countries such as the UK serology can also be helpful in the diagnosis of amoebiasis. The test is an immunoflourescent antibody test (IFAT) which is reported as a titre with a positive test being 1/80 or smaller. Essentially this is a dilutional test so 1/1 would be neat serum whereas 1/80 has been diluted 80 fold and is still positive. There can be false positive IFAT tests so they are usually confirmed with a second test called a Cellulose Acetate Precipitin (CAP) test. A positive IFAT and CAP confirm recent or active infection. If the IFAT is positive and the CAP negative there are a number of possible explanations: early infection (repeat the tests if clinically suspicious), a past treated case, past infection or occasionally a false positive.
The main drawback to amoebic serology is when it is applied to patients from endemic countries where they might have had a previous infection or be an asymptomatic carrier. The danger here is that clinicians may stop looking for a cause when they receive a positive test when in fact it is not the cause of the patient’s current illness. Note: asymptomatic or past infection does not need treatment.
How is amoebiasis treated?
The first consideration for the treatment of amoebiasis depends on whether surgical drainage of an abscess is required. The general rule of thumb is that if the abscess is >5cm in diameter it needs draining as antimicrobials alone will not work. Amoebomas in the bowel should also be resected if possible.
After drainage, resection or for gastrointestinal amoebiasis the antimicrobial treatment requires two phases. The first is to treat the active infection in tissues; the second is to eliminate any remaining amoebae from the lumen of the gastrointestinal tract. It can be difficult to get hold of a luminal agent so alternatives are given below. The normal adult doses are:
Treatment of active tissue infection
IV or PO Metronidazole 500mg TDS for 7-10 days
Treatment of luminal infection
PO Paromomycin 10mg/kg TDS for 7 days
PO Diiodohydroxyquin 650mg TDS for 20 days
PO Diloxanide furoate 500mg TDS for 10 days
How can amoebiasis be prevented?
Well first off don’t eat someone else’s poo! After that it is about maintaining sanitary disposal of sewerage and ensuring water and food are safe to eat.
When travelling the best way to ensure clean water is to boil it for one minute (above 5,000 feet altitude this needs to be 3 minutes as water boils at a lower temperature at altitude). Amoebae are killed at temperatures above 50oC but other pathogens in contaminated water need higher temperatures for longer. Alternative methods for sterilising water include filtration and disinfection with chemicals such as iodine or chlorine. Whichever method you use, follow the instructions and ensure the product actually removes or kills ALL potential pathogens. Not all water filters or chemicals are equivalent at removing or killing viruses, bacteria and parasites.
The patient was seen by the surgeons who drained the abscess. Thick brown orange pus was removed which confirmed the presence of amoebae on microscopy. The blood test also confirmed the diagnosis being strongly positive for both IFAT and CAP. The patient was treated with Metronidazole and Paromomycin and made a full recovery, and when it came to the hepatobiliary part of his medical degree course he had a great tale to tell…!