Amoebiasis

Amoebiasis, or Amebiasis is caused by the amoeba "Entamoeba histolytica".cite book | author = Ryan KJ, Ray CG (editors) | title = Sherris Medical Microbiology | edition = 4th ed. | publisher = McGraw Hill | year = 2004 | pages=733-8 | isbn = 0838585299] A gastrointestinal infection that may or may not be symptomatic and can remain latent in an infected person for several years, amoebiasis is estimated to cause 70,000 deaths per year world wide.Fact|date=September 2008 Symptoms can range from mild diarrhea to dysentery with blood and mucus in the stool.

"E. histolytica" is usually a commensal organism. Severe amoebiasis infections (known as invasive or "fulminant" amoebiasis) occur in two major forms. Invasion of the intestinal lining causes "amoebic dysentery" or "amoebic colitis". If the parasite reaches the bloodstream it can spread through the body, most frequently ending up in the liver where it causes "amoebic liver abscesses". When no symptoms are present, the infected individual is still a carrier, able to spread the parasite to others through poor hygienic practices. While symptoms at onset can be similar to bacillary dysentery, amoebiasis is not bacteriological in origin and treatments differ, although both infections can be prevented by good sanitary practices.

Transmission

Amoebiasis is usually transmitted by the fecal-oral route, but it can also be transmitted indirectly through contact with dirty hands or objects as well as by anal-oral contact. Infection is spread through ingestion of the cyst form of the parasite, a semi-dormant and hardy structure found in feces. Free-living amoebae, or "trophozoites", that do not form cysts but die quickly after leaving the body may also be present: these are rarely the source of new infections. Since amoebiasis is transmitted through contaminated food and water, it is often endemic in regions of the world with limited modern sanitation systems, including Mexico, western South America, South Asia, and western and southern Africa.

Amoebic dysentery is often confused with "traveler's diarrhea", or "Montezuma's Revenge" in Mexico, because of the prevalence of both in developing nations. In fact, most traveler's diarrhea is bacterial or viral in origin.

Liver abscesses can occur without previous development of amoebic dysentery.

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, and before handling food.
* Clean bathrooms and toilets often; pay particular attention to toilet seats and taps.
* Avoid sharing towels or face washers.

To help prevent infection:

* Avoid raw vegetables when in endemic areas, as they may have been fertilized using human feces.
* Boil water or treat with iodine tablets.

Nature of the disease

Most infected people, perhaps 90%, are asymptomatic, but this disease has the potential to make the sufferer dangerously ill. It is estimated by the World Health Organisation that about 70,000 people die annually worldwide.

Infections can sometimes last for years. Symptoms take from a few days to a few weeks to develop and manifest themselves, but usually it is about two to four weeks. Symptoms can range from mild diarrhoea to dysentery with blood and mucus. The blood comes from amoebae invading the lining of the intestine. In about 10% of invasive cases the amoebae enter the bloodstream and may travel to other organs in the body. Most commonly this means the liver, as this is where blood from the intestine reaches first, but they can end up almost anywhere.

Onset time is highly variable and the average asymptomatic infection persists for over a year. It is theorised 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, a part of the gastrointestinal tract.Fact|date=July 2008 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 the same substances it uses to digest bacteria, which include enzymes that destroy cell membranes and proteins. This process can lead to penetration and digestion of human tissues, resulting first in flask-shaped ulcers in the intestine. "Entamoeba histolytica" ingests the destroyed cells by phagocytosis and is often seen with red blood cells inside when viewed in stool samples. Especially in Latin America,Fact|date=July 2008 a granulomatous mass (known as an "amoeboma") may form in the wall of the ascending colon or rectum due to long-lasting immunological cellular response, and is sometimes confused with cancer. [cite book|last=Day|first=David W.|coauthors=Basil C. Morson, Jeremy R. Jass, Geraint Williams, Ashley B. Price|title=Morson and Dawson's Gastrointestinal Pathology|year=2003| publisher=John Wiley & Sons, Inc.| isbn=9780632042043]

Theoretically, the ingestion of one viable cyst can cause an infection. [cite web |url=http://vm.cfsan.fda.gov/~mow/chap23.html |title=US FDA/CFSAN - - Entamoeba histolytica |work=Bad Bug Book |publisher=Food and Drug Administration: Center for Food Safety & Applied Nutrition |date=2007-12-28 |accessdate=2008-09-26]

Diagnosis of human illness

Asymptomatic human infections are usually diagnosed by finding cysts shed in 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 three 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 amoeba 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" cyst has up to 8 nuclei. Additionally, in "E. histolytica," the endosome is centrally located in the nucleus, while it is usually off-center in "Entamoeba coli." Finally, chromatoidal bodies in "E. histolytica" cysts are rounded, while they are jagged in "Entamoeba coli". However, other species, "Entamoeba dispar" and "E. moshkovskii", are also commensals 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".

Typically, the organism can no longer be found in the feces once the disease goes extra-intestinal.Fact|date=September 2008 Serological tests are useful in detecting infection by "E. histolytica" if the organism goes extra-intestinal and in excluding the organism from the diagnosis of other disorders. An Ova & Parasite (O&P) test or an "E. histolytica" fecal antigen assay is the proper assay for intestinal infections. Since antibodies may persist for years after clinical cure, a positive serological result may not necessarily indicate an active infection. A negative serological result however can be equally important in excluding suspected tissue invasion by "E. histolytica".Fact|date=September 2008

Relative frequency of the disease

In older textbooks it is often stated that 10% of the world's population is infected with "Entamoeba histolytica".Fact|date=September 2008 It is now known that at least 90% of these infections are due to "E. dispar". Nevertheless, this means that there are up to 50 million true "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.Fact|date=September 2008 Infection is more common in warmer areas, but this is both because of poorer hygiene and the parasitic cysts surviving longer in warm moist conditions.

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 classes of drugs are needed to treat the infection, one for each location. Metronidazole, or a related drug such as Tinidazole, Secnidazole or Ornidazole, is used to destroy amoebae that have invaded tissue. These are rapidly absorbed into the bloodstream and transported to the site of infection. Because they are rapidly absorbed there is almost none remaining in the intestine. Since most of the amoebae 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 (also known as Furamide) is used in the US and Iodoquinol (also known as Yodoxin) is used in certain other countries. Both tissue and lumenal drugs 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 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 (or one of the other lumenal amebicides above) must always be given afterwards

Doses for children are calculated by body weight and a pharmacist should be consulted for help.

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. [cite journal | author=Mondal D, Petri Jr WA, Sack RB, "et al." | title="Entamoeba histolytica"-associated diarreal illness is negatively associated with the growth of preschool shildren: evidence from a prospective study | journal=Trans R Soc Trop Med H | year=2006 | volume=100 | issue=11 | pages=1032–38 | doi=10.1016/j.trstmh.2005.12.012 ]

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. [cite web | title = FDA Bacteriological Analytical Manual | url = http://www.foodinfonet.com/publication/fdaBAM.htm | accessdate = 2008-03-26]

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. [cite journal |author=Markell EK |title=The 1933 Chicago outbreak of amebiasis |journal=West. J. Med. |volume=144 |issue=6 |pages=750 |year=1986 |month=June |pmid=3524005 |pmc=1306777 |doi= |url=] There were 1,000 cases (with 58 deaths). In 1998 there was an outbreak of amoebiasis in the Republic of Georgia. [cite journal |author=Kreidl P, Imnadze P, Baidoshvili L, Greco D |title=Investigation of an outbreak of amoebiasis in Georgia |journal=Euro Surveill. |volume=4 |issue=10 |pages=103–104 |year=1999 |month=October |pmid=12631887 |doi= |url=http://www.eurosurveillance.org/em/v04n10/0410-222.asp] One hundred and seventy-seven cases were reported between 26 May and 3 September 1998, including 71 cases of intestinal amoebiasis and 106 probable cases of liver abscess.

References