Entamoeba histolytica

The entamoeba histolytica parasite is the cause of a gastrointestinal illness known as ‘amoebiasis’ which is similar to several other types of parasitical food poisoning.

This parasite mainly affects humans although it has been found in other animals such as cats and dogs. However, they are not usually responsible for the spread of this infection due to the fact that they don’t excrete this parasite within their faeces.

It is the main cause of amoebic dysentery. Plus it has infected around 50 million people around the world and is the cause of a great deal of illness within developing countries. It tends to affect young and middle aged people although anyone can develop this disease.

Most infected people do not present with any symptoms but some people become seriously ill as a result of this illness. In these cases this infection can persist for up to several years.

This illness has the ability to infect other parts of the body such as the liver, heart or brain where it can cause long term damage. This illness is more severe in people with compromised immune systems.

The entamoeba histolytica parasite

This is a single cell organism (a protazoa) which forms part of the Entamoeba family. This genus contains 6 types of parasites which are found within the human digestive system but only one – the entamoeba histolytica parasite causes this infection.

This parasite exists in the form of cysts which are able to survive only for a short period of time outside their normal environment. They particularly thrive in warm or moist conditions, especially in connection with undercooked or improperly stored foods.

Causes of amoebiasis

This infection occurs as a result of ingestion or contact with parasitical cysts. These cysts are a form of the entamoeba histolytica parasite which enter the human body through the mouth and cause amoebiasis.

Methods of transmission include:

• Consuming infected food or water
• Contact with infected soil or animal faeces
• Anal sex
• Eating salads, fruits or vegetables which have been washed in infected water.
• Handling objects which have already been infected or touching an infected surface.

Once these cysts are ingested they release smaller version of themselves in a process called ‘excystation’. In this process, small organisms called ‘trophozoites’ are released which invade cells within the lining of the large intestine.

These parasites have the ability to burrow into the walls of the intestines causing large, crater like ulcers to form. This results in an infection which spreads into the bloodstream, enabling it to reach many parts of the body.

The liver is usually affected but other areas include the heart, brain, lungs and spleen.

Asymptomatic amoebiasis

Many people become infected but find that they do not develop symptoms of this illness. This is known as ‘asymptomatic amoebiasis’and accounts for the majority of cases.

In these cases, the infection remains dormant for a long period of time, often several years without any symptoms: or if they are any symptoms they tend to be very mild. Others may find that they carry this infection for several years – without any symptoms, but then surrender to these.

Someone with this form of amoebiasis is still able to transmit this infection to others even if they do not show any symptoms.

What is the difference between ‘asymptomatic amoebiasis’ and the normal amoebiasis infection?

The difference is that with the asymptomatic version, the parasite lives within the digestive system but does not penetrate the cells within the intestine.

Why? The answer is a protective layer of mucus called the ‘epithelium’.

This layer contains cells and acts as a lining within the small and large intestines. With asymptomatic amoebiasis these parasites do not usually come into contact with this layer and exist by consuming bacteria and food debris within the digestive system.

But, in amoebiasis cases, these parasites manage to invade cells within this protective layer which causes this infection. These parasites then release a substance which attack cell membranes and can even break down tissues.

This breakdown of tissues within the intestines leads to the formation of large ulcers.

Symptoms of amoebiasis

These symptoms appear very similar to bacterial dysentery although this illness is not caused by a bacteria but rather a parasite instead.

But, the treatment is very similar for both bacterial and parasitical dysentery. Another common factor is good hygiene: both of these illnesses can be prevented by adopting a sensible hygiene plan.

Most people are asymptomatic in respect of this illness but there are others who, unfortunately, develop symptoms of this illness which include:

• Tiredness
• Weight loss
• Nausea
• Vomiting
• Bloody diarrhoea
• Abdominal pain/cramps
• Lesions (within the intestinal wall)

These symptoms appear between 2 to 4 weeks after first exposure to this parasite. The severity of these symptoms ranges from mild diarrhoea through to a severe, dysentery like illness with mucus and blood in the diarrhoea and a distended abdomen.

Complications of amoebiasis

In the majority of cases, the infection is localised within the gastrointestinal tract. But this infection may spread to other parts of the body, for example the liver and cause the following diseases:

• Liver abscess
• Brain abscess
• Lung abscess

Abscesses can also develop in other areas of the body. But whatever part of the body they do affect these are all serious medical conditions which require medical treatment.

Diagnosing amoebiasis

The symptoms of this are similar to bacterial dysentery or various forms of food poisoning so it easy to misdiagnose this illness.

The most efficient way of diagnosing this is to obtain a stool sample. This is, basically, a small sample of faeces which is examined under a microscope for signs of these parasitical cysts.

Several samples will be required to obtain an accurate diagnosis as these cysts are not excreted from the body on a regular basis.

A physical examination will be performed followed by a series of tests. These include a blood test and an abnormal liver function test. Ultrasound or MRI/CT scans if the illness has spread to the liver.

Treatment for amoebiasis

This involves fluid replacement and medication (antibiotics).

It is important to replace fluids lost as a result of the diarrhoea, and to replace electrolytes as well. The word ‘electrolyte’is used to describe essential salts which the body needs to function on a daily basis.

Examples of these electrolytes include sodium, potassium and calcium.

These can be replaced by means of ‘oral re-hydration therapy’. This takes the form of a powder which contains electrolytes and is dissolved in water.

Babies and young children will require their fluids to be administered intravenously.

Medication such as metronidazole or tinidazole can be given which will ease any symptoms as well as treat liver abscesses.

Preventing amoebiasis

This parasite can infect food and water so food safety is of paramount importance. There are a few measures you can put in place to reduce the risk of amoebiasis which include:

• Boiling untreated water especially when travelling or camping
• Washing fruit and vegetables before eating them
• Washing your hands, utensils and kitchen worktops before and after food preparation.
• Ensuring that all food is cooked at the correct temperature
• Follow all cooking instructions carefully; this also includes microwave cooking times.
• Washing your hands every time you visit the toilet or after you have been in contact with an infected animal.
• Ensuring that any cooked food is covered when left out to cool down.

Find out more in our preventing food poisoning section.