Wednesday, December 10, 2014

Entamoeba histolytica: A Dangerous Stowaway

Figure 1:  A histological slide of amoeba infected brain tissue.
The left side of the image shows a trophozite that has disseminated
to the brain tissue.  The smaller darker spots in are amoebic cysts
that have formed in the brain tissue.
            Amebiasis is an infection of the intestines that is caused by the parasite Entamoeba histolytica.  This parasitic amoeba infects the host through the fecal-oral rout.  However the parasite may spread and infect other organs such as the liver and, or the brain.  Infection of the brain can be fatal (figure 1 [1]). Infection by E. histolytica happens when the host 1).  Ingests a mature cyst containing the parasite, found in food, water, or on unwashed hands that are contaminated with fecal matter.  2). Excystation of the mature cyst (where the parasite escapes from the cyst) occurs in the small intestine.  3). Trophozoites (the active and motile stage of the parasite) are then released and migrate to the large intestine. The trophozoites multiply by binary fission and produce cysts.  4). These cysts are then passed in the feces of the host.  Because of the protection conferred by their walls, the cysts can survive days to weeks in the external environment and are responsible for transmission.  Trophozites can also be transmitted through fecal matter as well, but do not last long in the environment and are destroyed quickly. (Figure 2 [2]). 

Figure 2:  A diagram of E. histolytica life cycle in the human body,
including possible sites where the parasite can disseminate to cause disease.
However In many cases the trophozoites remain confined to the intestinal lumen of individuals who are thus asymptomatic carriers, and cyst passers. This is a non-invasive infection.  In some patients the trophozoites invade the intestinal mucosa and cause intestinal disease, like colitis.  The parasites can also disseminate in the body through the bloodstream, invading extra-intestinal sites such as the liver, brain, and lungs causing extra-intestinal disease.  The result of which can lead to sever pain in certain regions of the body, liver abscess, high fever, and even death. Transmission can also occur during sexual contact, in which not only cysts, but also trophozoites can be transmitted.  All sexually active individuals who practice unsafe oral-anal sex, and immunocompromised individuals are at an increased risk for contracting an E. histolytica infection.

In a study by Salit et al. researchers found a cluster of E. histolytica infections in Canada.  They traced this cluster to several individuals.  Patient A who is a female, patient B who is a female, and patient C who is a male, had all recently traveled to England, Germany and Southern Italy together and returned on September 2007.  Five months later patient A developed anorexia, chills, a fever of 39.5º C, and abdominal pain.  After being admitted to the hospital, she was found to have a liver abscess (an accumulation of puss) and colitis (inflammation of the intestines).  After several tests, it was discovered that she had been infected with a highly virulent strain of E. histolytica that had disseminated and caused her liver abscess.  Three months after, patient C was also found to have two liver abscesses in the same part of the liver as patient A.  The tests concluded that he too was infected with the same highly virulent strain of E. histolytica.  Patient B was infected with the same strain as well, and had developed lymphadenopathy (refers to enlarged lymph nodes).

The researchers found that all three of these patients had been sexually active together, and had other sexual partners who were labeled patients D, F, and G.  Patient A had another sexual partner, patient E, who also was diagnosed with a liver abscess and intestinal colitis as well.  Patients D, F, and G were female homosexuals and bisexuals who were sexually linked to patients A, B, and C.  They all reported engaging in oral-anal sexual activity.  Patients D, F, and G were also found to be infected with E. histolytica but they did not show any symptoms.  It was concluded that although patient E did in fact travel to Iraq and Africa, it was most likely that he contracted the infection from engaging in sexual activity with patient A. [5] These cases are an example of how this parasite can be very easily transmitted from one person to another.  The patients in this study all practiced oral-anal sex with each other, which lead to an increased chance of contracting the parasite from the infected partner.

It is known that individuals with compromised immune system; are more often infected with common microorganisms that would usually not be deadly, individuals infected with HIV-1 for example.  However a recent study by Nagata et al. had some interesting finding when comparing risk factors for amebiasis infections.  They conducted a study on 10,930 patients infected with amebic colitis over a span of seven years.  They studied the infection pattern in Japan from 2003 to 2009 they found that there was a significant increase in the number of infections in this time span.  They performed endoscopic tests on all the patients at that time and confirmed amebic colitis.  The researchers found that in 2003 there were about 3 out of every 1,904 patients that had tested positive for amebic colitis, whereas in 2009 there were 12 in every 1,360 patients that had tested positive.  That is a 5.6-fold increase in the number of cases of colitis related to amebic infections in the span of seven years.

The researchers assessed the risk factors associated with this jump in infections, and concluded that were several factors that can be traced back to individuals who were HIV-1 positive, individuals who had a history of syphilis, and individuals who were sexual activity with commercial sex worker (CSWs).  However when they investigated further into the HIV-1 risk factor, they found that there was no association with the infection itself and being HIV-1 positive, rather it was the risky behavior that these individuals engaged in that led to their higher rate of infection. [4]

            Amebiasis is a serious condition that is caused by Entamoeba histolytica.  It is not a major cause for concern in most industrialized countries.  However when contracted it can be dangerous and sometimes fatal if not treated quickly.  It can cause liver abscess, colitis, and if disseminated to the brain can cause death.  Everybody can get this infection, although sexually active individuals, especially those individuals who practice unsafe oral-anal sex, have to be cautious and take enteric precautions (precautions against possible contamination from bodily fluids of the gastrointestinal (GI) tract of a person suspected of having a GI infection) to keep from getting infected with this parasite.  Also immunosuppressed individuals have to be careful of this infection, as well as people who do not have access to adequate hygiene or a clean water source.


1.     "Amebic Brain Abscess: MedlinePlus Medical Encyclopedia Image." U.S National Library of Medicine. U.S. National Library of Medicine. Web. 17 Nov. 2014.

2.     "Amebiasis." Centers for Disease Control and Prevention. Centers for Disease Control and Prevention. Web. 3 Dec. 2014. <>.

3.     "Amebiasis Life Cycle." Amebiasis Life Cycle. Web. 17 Nov. 2014. <>.

4.     Nagata, Naoyoshi, Takuro Shimbo, Junichi Akiyama, Ryo Nakashima, So Nishimura, Tomoyuki Yada, Koji Watanabe, Shinichi Oka, and Naomi Uemura. "Risk Factors for Intestinal Invasive Amebiasis in Japan, 2003–2009." Emerging Infectious Diseases: 717-24. Print.
5.     Salit, Irving E., Krishna Khairnar, Kevin Gough, and Dylan R. Pillai. "A Possible Cluster of Sexually Transmitted Entamoeba Histolytica: Genetic Analysis of a Highly Virulent Strain." Oxford Journals (2009): 346-53. Print.

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