Parasites and Pestilence
Human Biology/Microbiology & Immunology103
D. Scott Smith, M.D., M. Sc., DTM &H
Entamoeba
Polecki
Created
By Rachel Cook
May
26, 2004
http://www.practicalscience.com/ep2.jpg
Entamoeba
Polecki is an intestinal protozoan which is best known for its infection of
pigs and monkeys. However, E. polecki is not unique to these animals, as many
human infections have been reported. Due to the rarity of this parasite, however,
human clinical manifestation is not known by names other than E. polecki infection
or amebiasis. In this page, I hope to educate you about this commonly overlooked
parasite and shed some light on its nature, morphology, life cycle, and its
clinical manifestations in incidental human hosts.
Classification:
Entamoeba polecki is best characterized as
a lumen-dwelling protozoa. The complete taxononomy is as follows:
Superkingdom: Eukaryotae
Kingdom: Animalia
Subkingdom: Protozoa
Phylum: Sarcomasigophora
Subphylum: Sarcodina
Superclass: Rhizopoda
Class: Lobosa
Subclass: Gymnamoeba
Order: Amoebida
Suborder:
Tubulina
Family:
Entamoebidae
Genus:
Entamoeba
Species:
Polecki
History
of Discovery:
Entamoeba Polecki was first identified in 1912
in Czechoslovakia by Von Prowazek in the stool samples of two students from
Kampuchea. Characteristic, uninucleated cysts of diameters between 14.2-15.7
microns and nuclei diameters of 3.2-4.2 micron were found-repeatedly in stool
samples taken from these students. Following these cases, E. Polecki was repeatedly
found in pig feces, but no other human cases were reported until 1949. Most
researchers believe, however, that many more human cases existed during this
time and that the infections were either asymptomatic and never identified or
were misdiagnosed as E. histolytica.
Epidemiology and Country
Information:
Although Entamoeba Polecki is rarely found in humans, it has
a widespread and relatively unpredictable epidemiology. The disease is much
more common in rural regions than urban areas. Most commonly, Entamoeba Polecki
is associated with Papua New Guinea, where a study estimated that the prevalence
was as high as 19 percent of the population. This is not surprising given the
economy and culture of this country where pigs play a key role and many pigs
are even allowed to live in residences. There are three other countries in which
E. Polecki is endemic, including Cambodia, Venezuela, and Vietnam. Additionally,
E. polecki infections have been reported in Southeast Asian refugees living
in other locations, namely France, Minnesota, and Venezuela.
Morphology:
The trophozoites of Entamoeba Polecki are rounded with diameters
of less than 10 to more than 20 micrometers, with the majority being between
12-18micrometers. When stained, the nucleus with a small central karyosome is
visible and is either seen evenly distributed or massed at one or both poles.
Stained vacuoles in the trophozoites also show ingested bacteria and yeast.
The peripheral chromatin is most often seen as distributed granules on the nuclear
membrane. The fine granules are either touching each other or having small spaces
in between, but are not uniformly distributed as in the trophozoites of many
similar protozoa.
The cysts of E. Polecki range in size from 9.5-17.5 micrometers,
though normally are between 12-15 micrometers and are spherical or subspherical.
They are almost always uninuclear and contain abundant chromatoidal material
with angular or pointed, or threadlike ends. Glycogen vacuoles are also present
in many of the cysts, in addition to spherical or ovoid shaped inclusion masses.
As in the trophozoites, the peripheral chromatin is generally distributed non-uniformly.
Life Cycle:
There are three main stages to the life cycle
of intestinal protozoa:
1. TROPHOZOITE: The trophozoite
stage is a fragile, vegetative stage in which the protozoan must encyst
to survive in the environment.
2. PRE-CYST: The pre-cyst stage
consists of encysting trophozoites disgorging of any undigested food.
3. CYST: The cyst stage is the
infective, transmissible phase in which protozoan are resistant to harsh
environmental conditions.
Animal
Reservoir:
Pigs and Monkeys are the primary reservoirs for
E. Polecki, though infection has also been documented in goats, sheep, cattle,
and other wild ungulates.
www.foxchasefarm.org/
graphics/pigs.jpg
Vector:
Entamoeba Polecki has no vector.
Transmission:
The exact form of E. polecki transmission is unknown,
but transmission to humans via ingestion of cysts in pig or monkey feces through
contaminated food is considered to be the most likely route. However, there
are many reports of infected individuals that have had no contact with host
animals. This indicates a strong likelihood of human-to-human transmission,
especially in regions such as Papua New Guinea with high disease prevalence.
The possibility of obtaining parasite from other domestic animals such as goats,
sheep, cattle, and wild ungulates also exists.
Clinical Presentation
In Humans:
Infection with Entamoeba Polecki is almost always
asymptomatic in humans, but debate remains about the possibility of nonspecific
symptoms such as diarrhea, bloody stools, fever, nausea, vomiting, abdominal
cramps, inspiratory restriction, and weight loss. There is conflicting evidence
in the published research on this infection, though the CDC reports that any
gastrointestinal symptoms must be attributed to other, non-amoebic causes.
Incubation Period:
The incubation period of Entamoeba Polecki is currently unknown.
However, Entamoeba polecki greatly resembles E. histolytica which has an incubation
period that varies between a few days and a few weeks, depending on the infective
dose.
Diagnosis:
The main method for diagnosing Entamoeba Polecki is by the
identification of trophozoites in feces, utilizing preserved, stained, and microscopically
examined stool specimens. This method of diagnosis can be difficult, however,
given the morphologic similarities between E. Polecki and other intestinal amoebas
such as E. histolytica and E. hartmanni. For a definitive diagnosis, electroimmunotransfer
blots are frequently used to identify the antigenic structure of the parasite.
Serologic tests have been shown to be insufficient in distinguishing between
the three aforementioned Entamoeba species.
Drug Therapy:
Entamoeba Polecki has been successfully treated with the use
of three antiparasitic drugs. Metronidazole, Ornidazole, and Furamide have been
proven effective, though Metronidazole is the most common and debatably most
effective. This drug is effective at a dosage of 750mg three times a day for
5, 7, or 10 days. Ornidazole and Furamide have been shown to treat the parasite
in combination with Metronidazole, though it is still not known if these drugs
are effective on their own. Interestingly, all the the commonly employed antiamebic
drugs have been ineffective in the treatment and management of this parasitic
infection.
Public Health and
Prevention Strategies:
Because both zoonotic
and fecal-oral transmission have been suggested for Entamoeba Polecki, many
public health and prevention strategies are possible and should be considered.
Limited contact with pigs and monkeys is the most obvious. Additionally, health
education strategies such as improving personal hygiene, sanitary disposal of
feces, and hand washing are necessary for the prevention of transmission. Most
importantly, proper cleaning, handling, and cooking of food will be essential
public health interventions.
Useful Links: References:
References:
Burrows, RB. "Morphological Differentiation of Entamoeba
hartmanni and E. polecki from E. histolytica," American Journal of Tropical
Medicine and Hygiene 1959, Sep 8 (8): 583-589.
Chancin-Bonilla, L. "Successful Treatment of Human
Entamoeba polecki infection with Metronidazole," American Journal of Tropical
Medicine and Hygiene 1980, Jul 29(4): 521-523.
Gay, JD, et al. "Entamoeba polecki Infection in
Southeast Asian Refugees: Multiple Cases of a Rarely Reported Parasite,"
Mayo Clinical Proceedings 1985, Aug 60(8): 523-530.
Levin, RL & Armonstrong, DE. "Human Infection
with Entamoeba Polecki," American Journal of Clinical Pathology 1970, Oct
54(4): 611-614.
Lubinsky, D. "The occurrence in Pakistan of a human
Entamoeba of the Polecki type," Parasitology 1952, Mar 42(1-2): 48-51.
Markell, John, & Krotoski. "Medical Parasitology,
Eighth Edition," Philadelphia, PA, 1999.
McMillan, B & Kelly, A. "Attempts to Cultivate
Entamoeba polecki von Prowazek, 1912," Trans Royal Society of Tropical
Medicine and Hygiene 1972, 66(2): 366-367.
Salaki, JS, Shirey, JL, & Strickland, GT. "Successful
Treatment of Symptomatic Entamoeba polecki Infection," American Journal
of Tropical Medicine and Hygiene 1979, Mar 28(2): 190-193.
Verweij, Jaco, et. al. "Detection and Identification
of Entamoeba Species in Stool Samples by a Reverse Line Hybridization Assay"
Journal of Clinical Microbiology 2003, Nov 41(11): 5041-5045.
Verweij, Jaco, et. al. "Genetic Variation Among
Human Isolates of Uninucleated Cyst-Producing Entamoeba Species," Journal
of Clinical Microbiology 2001, Apr 39(4): 1644-1646.
Questions or Comments? Contact Rachel Cook
