Amoebiasis
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| ICD-10 | A06 |
|---|---|
| ICD-9 | 006 |
Amoebiasis is an infection by a protozoan, typically Entamoeba histolytica. It is usually contracted by ingesting water or food contaminated by amoebic cysts. It is usually included between the waterborne diseases. Amoebiasis most commonly affects young to middle-aged adults.
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[edit] Transmission
Amoebiasis is transmitted by fecal contamination of drinking water and foods, but also by direct contact with dirty hands or objects as well as by sexual intercourse. Additionally, geophagy is a common route of infection in certain cultures.
[edit] Prevention
To help prevent the spread of amoebiasis around the home:
- Wash hands thoroughly with soap and hot running water for at least 10 seconds after using the toilet or changing a baby's diaper.
- Clean bathrooms and toilets often. Pay particular attention to toilet seats and taps.
- Avoid sharing towels or face washers.
[edit] Nature of the disease
Symptoms are usually gastrointestinal including diarrhea, vomiting, abdominal pain or discomfort and fever. Symptoms take from a few days to a few weeks to develop and manifest themselves, but usually it is about two to four weeks. Most infected people are asymptomatic but this disease has the potential to make the sufferer dangerously ill, especially if there is any suggestion of immunocompromise.
Infections that sometimes last for years may be accompanied by
- no symptoms (in the majority of cases),
- vague gastrointestinal distress,
- dysentery (with blood and mucus).
Most infections occur in the digestive tract but other tissues may be invaded. Complications include ulcerative and abscess pain, usually involving the liver, and, rarely, intestinal blockage.
Onset time is highly variable and the average asymptomatic infection persists for over a year. It is theorized that the absence of symptoms or their intensity may vary with such factors as strain of amoeba, immune response of the host, and perhaps associated bacteria and viruses.
In asymptomatic infections the amoeba lives by eating and digesting bacteria and food particles in the gut. It does not usually come in contact with the intestine itself due to the protective layer of mucus that lines the gut. Disease occurs when amoeba comes in contact with the cells lining the intestine. It then secretes toxic substances, including enzymes that destroy cell membranes and allow it to penetrate and digest human tissues, resulting in flask-shaped ulcers in the intestine. These are the same enzymes that are normally used to digest the bacteria it usually feeds on. E. histolytica also feeds on the destroyed cells by phagocytosis and is often seen with red blood cells inside. Especially in Latin America, a granulomatous mass (known as an amoeboma) may form in the wall of the colon due to long-lasting cellular response, and is sometimes confused with cancer.
Theoretically, the ingestion of one viable cyst can cause an infection.
[edit] Diagnosis of human illness
Asymptomatic human infections are usually diagnosed by finding cysts shed with the stool. Various flotation or sedimentation procedures have been developed to recover the cysts from fecal matter and stains help to visualize the isolated cysts for microscopic examination. Since cysts are not shed constantly, a minimum of 3 stools should be examined. In symptomatic infections, the motile form (the trophozoite) can often be seen in fresh feces. Serological tests exist and most individuals (whether with symptoms or not) will test positive for the presence of antibodies. The levels of antibody are much higher in individuals with liver abscesses. Serology only becomes positive about two weeks after infection. More recent developments include a kit that detects the presence of ameba proteins in the feces and another that detects ameba DNA in feces. These tests are not in widespread use due to their expense.
Microscopy is still by far the most widespread method of diagnosis around the world. However it is not as sensitive or accurate in diagnosis as the other tests available. It is important to distinguish the E. histolytica cyst from the cysts of nonpathogenic intestinal protozoa such as Entamoeba coli by its appearance. E. histolytica cysts have a maximum of four nuclei, while the commensal Entamoeba coli has up to 8 nuclei. Additionally, in E. histolytica, the endosome is centrally located in the nucleus, while it is off-center in Entamoeba coli. Finally, chromatoidal bodies in E. histolytica are rounded, while they are jagged in Entamoeba coli. However, another species, Entamoeba dispar, is also a commensal and cannot be distinguished from E. histolytica under the microscope. As E. dispar is much more common than E. histolytica in most parts of the world this means that there is a lot of incorrect diagnosis of E. histolytica infection taking place. The WHO recommends that infections diagnosed by microscopy alone should not be treated if they are asymptomatic and there is no other reason to suspect that the infection is actually E. histolytica.
[edit] Relative frequency of the disease
Nevertheless, this means that there are about 50 million E. histolytica infections and approximately seventy thousand die each year, mostly from liver abscesses or other complications. Although usually considered a tropical parasite, the first case reported (in 1875) was actually in St Petersburg in Russia, near the Arctic Circle. Infection is more common in warmer areas, but this is both because of poorer hygiene and because the parasite cysts survive longer in warm moist conditions.
[edit] Treatment
E. histolytica infections occur in both the intestine and (in people with symptoms) in tissue of the intestine and/or liver. As a result two different sorts of drugs are needed to rid the body of the infection, one for each location. Metronidazole, or a related drug such a tinidazole, is used to destroy amebae that have invaded tissue. It is rapidly absorbed into the bloodstream and transported to the site of infection. Because it is rapidly absorbed there is almost none remaining in the intestine. Since most of the amebae remain in the intestine when tissue invasion occurs, it is important to get rid of those also or the patient will be at risk of developing another case of invasive disease. Several drugs are available for treating intestinal infections, the most effective of which has been shown to be Paromomycin (also known as Humatin); diloxanide furoate is used in the UK. Both types of drug must be used to treat infections, with metronidazole usually being given first, followed by paromomycin or diloxanide. E. dispar does not require treatment, but many laboratories (even in the developed world) do not have the facilities to distinguish this from E. histolytica.
For amebic dysentery a multi-prong approach must be used, starting with one of:
- Metronidazole 500-750mg three times a day for 5-10 days
- Tinidazole 2g once a day for 3 days is an alternative to metronidazole
In addition to the above, one of the following luminal amebicides should be prescribed as an adjunctive treatment, either concurrently or sequentially, to destroy E. Histolytica in its cyst form, especially in the colon:
- Paromomycin 500mg three times a day for 10 days
- Diloxanide Furoate 500mg three times a day for 10 days
- Iodoquinol 650mg three times a day for 20 days
For amebic liver abscess:
- Metronidazole 400mg three times a day for 10 days
- Tinidazole 2g once a day for 6 days is an alternative to metronidazole
- Diloxanide furoate 500mg three times a day for 10 days must always be given afterwards
Doses for children are calculated by body weight and a pharmacist should be consulted for help.
[edit] Herbal treatments
In Mexico, it is common to use herbal tinctures of chaparro amargo. 30 drops are taken in a small glass of water first thing in the morning, and 30 drops before the last meal of the day, for seven days straight. After taking a seven day break from the treatment, it is resumed for seven days. Some mild cramping may be felt; it is claimed this means that the amoebas are dying and will be expelled from the body. Many Mexicans use the chaparro amargo treatment regularly, three times a year. The efficacy of such treatments has not been scientifically proven.
A 1998 study in Africa suggests that 2 tablespoons per week of papaya seeds may have some antiamoebic action and aid in prevention of amoebiasis, but this remains unconfirmed.
[edit] Complications
In the majority of cases, amoebas remain in the gastrointestinal tract of the hosts. Severe ulceration of the gastrointestinal mucosal surfaces occurs in less than 16% of cases. In fewer cases, the parasite invades the soft tissues, most commonly the liver. Only rarely are masses formed (amoebomas) that lead to intestinal obstruction.
Entamoeba histolytica infection is associated with malnutrition and stunting of growth.[1]
[edit] Populations at risk
All people are believed to be susceptible to infection, but individuals with a damaged or undeveloped immunity may suffer more severe forms of the disease. AIDS / ARC patients are very vulnerable.
[edit] Food analysis
E. histolytica cysts may be recovered from contaminated food by methods similar to those used for recovering Giardia lamblia cysts from feces. Filtration is probably the most practical method for recovery from drinking water and liquid foods. E. histolytica cysts must be distinguished from cysts of other parasitic (but nonpathogenic) protozoa and from cysts of free-living protozoa as discussed above. Recovery procedures are not very accurate; cysts are easily lost or damaged beyond recognition, which leads to many falsely negative results in recovery tests. (See the FDA Bacteriological Analytical Manual.)
[edit] Outbreaks
The most dramatic incident the USA was the Chicago World's Fair outbreak in 1933 caused by contaminated drinking water; defective plumbing permitted sewage to contaminate water. There were 1,000 cases (with 58 deaths). In recent times, food handlers are suspected of causing many scattered infections, but there has been no single outbreak.
[edit] References
- ^ Mondal D, Petri Jr WA, Sack RB, et al. (2006). "Entamoeba histolytica-associated diarreal illness is negatively associated with the growth of preschool shildren: evidence from a prospective study". Trans R Soc Trop Med H 100 (11): 1032–38. DOI:10.1016/j.trstmh.2005.12.012.
