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:

http://www15.bni-hamburg.de/bni/bni2/neu2/getfile.acgi?area_engl=history&pid=131

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:

http://www.lonelyplanet.com/mapshells/australasia/papua_new_guinea/papua_new_guinea.htm

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:

http://www.tulane.edu/~wiser/protozoology/notes/intes.html

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.

http://www.petrx.com/index.asp?PageAction=VIEWPROD&ProdID=564

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:

Intestinal Protozoa

CDC Division of Parasitic Diseases

Tropical Disease Research

Entamoeba Species

Entamoeba Classification

Topics in Infectious Disease Newsletter


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