By OS
Most people enjoy traveling and want to experience different parts of the world.
Some people, like Todd, even have it on their bucket lists to visit every
country. Traveling comes with many risks though, and one major concern for
travelers is disease. Unfortunately Todd discovered this for himself when he
visited India. Shortly after Todd arrived in India, he went out to an
unsanitary restaurant because it was the only one nearby and he did not feel
like going shopping on his first night there. While there, he noticed that the
water didn’t look completely clean. He figured that water in India just looked
like that, so he drank it anyway. In doing so, he exposed himself to the amoeba
Entamoeba histolytica, which is the
pathogen responsible for amebiasis. Amebiasis causes disease when its cysts are
ingested and is responsible for an estimated 100,000 deaths each year (1). This
disease is common in areas with poor sanitation, including Central America,
South America, Africa, and parts of Asia, so Todd should have been more careful
(2). By staying away from unsanitary restaurants and not drinking potentially
unsafe water, Todd could have avoided being infected by this parasite.
Todd
didn’t realize it for a while, but his body was under attack. The water he
drank carried E. histolytica cysts
down his throat, through his stomach, and into his small intestine (figure 1). Once
the cysts reached his small intestine, the parasite escaped its cyst and
replicated asexually three times, forming eight trophozoites (3). Trophozoites
are the motile form of the parasite that feeds on the host using porins and
proteolytic enzymes. The trophozoites traveled to his large intestine and adhered
to its mucosal lining. Here they degraded the layer of protective mucous and attacked
the epithelial cells of the large intestine. This was followed by the release
of porins, which caused some of Todd’s cells to lyse. The proteolytic enzymes
were then released to partially degrade the ruptured cells. Finally, the amoeba
engulfed the dead cells and used them as a food source. The parasite was
literally eating him alive! The death of Todd’s cells and inflammation of his
colon caused him to experience some of the symptoms associated with amebiasis,
including pain and diarrhea. The
symptoms began to appear five days after infection. At this point, Todd knew something
was wrong, but he figured it was just food poisoning and that it would be done
shortly. He thought his immune system would fight off the infection, but he was
wrong.
 |
| Figure 1. Life cycle of E. histolytica. |
Todd’s
complement, which is part of his innate immune system, could not clear the
infection. E. histolytica is
resistant to the lysis that the complement system normally uses to kill
infections (3). Additionally, some of his IgA antibodies were unable to help
fight the disease. Antibodies normally bind pathogens and either block further
infection or help immune cells destroy the pathogens. However, E. histolytica is able to quickly degrade IgA, allowing for its self-preservation. The parasite also prevented Todd’s T cells from functioning properly. T cells are immune cells that
normally help eliminate specific pathogens. However, Todd’s T cells were unable
to clear the infection because the parasites were secreting proteins to
decrease the T cell activity (4). Todd’s other immune responses were insufficient to kill all of the parasites, so his body could not clear the amebiasis on its own.
The trophozoites kept replicating
asexually and continued attacking his cells. Some trophozoites formed into
cysts, which were excreted in his feces and waited until they found another
person to infect (3). After another week, his amebiasis became more severe and the
trophozoites reached his bloodstream. From here, they could have traveled to
any tissue in his body, including his lungs or brain. Wherever the parasites
travel, they kill cells, so travel to other areas of Todd’s body may have
caused serious damage. Todd was lucky that the E. histolytica cells did not infect his brain or lungs. Infection
of these areas is rare, but can be very serious. However, they did travel to
his liver. When the E. histolytica infection
reached Todd’s liver, it killed some of his liver cells and formed abscesses,
which are clusters of dead cells, immune cells, and parasitic cells (5). As his
liver was infected, Todd started experiencing fever, coughing, and other
flu-like symptoms, as well as intense pain. When he noticed his symptoms
weren’t getting better, he finally went to see the doctor.
The doctor asked about his ailment
and determined the cause may have been due to an ingested parasite. He ordered
a blood test in order to precisely determine the cause. The blood was sent to
be analyzed and an enzyme-linked immunosorbent assay (ELISA) was performed.
This ELISA was done in order to see if Todd had antibodies against several
parasites, including E. histolytica
(6). This can show if someone is infected because the human body normally
creates antibodies to fight against infectious pathogens. The presence of
antibodies specific to a pathogen shows that the body is actively fighting
against the pathogen. The test showed that Todd’s blood did contain antibodies
to E. histolytica, so the doctor
concluded that amebiasis was the cause of his symptoms. Because of this, the
doctor was able to prescribe him metronidazole, which is an amebicidal drug
that is very effective at killing E.
histolytica (7). This helped Todd to finally clear the infection.
Fortunately for Todd, his infection
had not spread too far and it went away after treatment. However, being more
careful could have prevented him from being infected at all. It is always
important to be careful about what you eat and drink, but paying attention can
mean the difference between life and death for immunocompromised or other
vulnerable people. People with AIDS, SCID, or other immune deficiencies need to
pay special attention because amebiasis can be very severe for people with weak
immune systems. Todd plans to continue traveling the world, but now he knows to
be more careful and take precautions to stay healthy. Amebiasis, as well as
many other diseases, can be prevented by taking the necessary precautions when
living in or traveling to areas without proper sanitation. In addition to
avoiding consumption of potentially unsafe food and water, amebiasis can be
prevented by washing your hands often, keeping raw foods away from cooked
foods, and ensuring meat is thoroughly cooked. By spreading this information,
hopefully the prevalence of amebiasis can be reduced.
Bibliography
1. Ali
IKM, Clark CG, Petri Jr WA. 2008. Molecular epidemiology of amebiasis.
Infect Genet Evol. 8:698-707
2. Wertheim HFL, Horby P, Woodall JP. 2012. Atlas of Human Infectious
Diseases. Wiley-Blackwell, Chichester
3. Ravdin JI. 1995. Amebiasis. Clin Infect Dis. 20:1453-1464
4. Salata RA, Martinez-Palomo A, Canales L, Murray HW, Trevino N, Ravdin
JI. 1990. Suppression of T-Lymphocyte Responses to Entamoeba histolytica Antigen by Immune Sera. Infect Immun. 58:3941-3946
5. Espinosa-Cantellano M, Martinez-Palomo A. 2000. Pathogenesis of
Intestinal Amebiasis: From Molecules to Disease. Clin Microbiol Rev. 13:318-331
6. Tanyuksel M, Petri Jr WA. 2003. Laboratory Diagnosis of Amebiasis.
Clin Microbiol Rev. 16:713-729
7. Lasserre R. 1979. Treatment of Amebiasis. Phil J Microbiol Infect
Dis. 8:1-6.
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