Introduction
Heterophyds are small intestinal trematode (0.5-2 mm in length). Heterophyd trematodes are capable of surviving and reproducing in a wide range of hosts. Many fish eating animals normally become diffinitive hosts of the parasites, in which heterophyidae species are able to reproduce and release eggs. As early as the 1920s researchers had consolidated what seemed to be many species of heterophyds, which had previously seemed to be multiple parasites based on their infecting multiple species (Witenberg 1928).

History
Ohdner named the family Heterophyidae in 1914, combining a set of previous genera which have since been regarded as morphologically similar. He divided the family into five distinct sub-families which have been greatly adjusted over time. Many of the original genera have been subsequently excluded because of morphological differences unknown at the time and new ones included in the family.

Taxonomy
Heterophyids fall in the following Taxonomic path (Stewart 1998):


Classification and Morphology
The main genera known to infect humans are currently known as "Metagonimus, Heterophyes, Centrocestus, Pygidiopsis, Stellantchasmus, Haplorchis and Procerovum [Seo, 1979] (Chai, 1984)." The genera are defined within the family by their morphological features as layed out in Witenberg 1928. His definition is still largely accepted, as his paper is relied upon and widely referenced in the modern literature (Chai et al 1984, Seo et al 1984, literature published in the 1990s and thereafter no longer refers to references before the 1960s but relies upon more recent papers which do). The flukes are in the 0.5-2mm long and 0.3-0.4mm wide. Their tegument is spined. They posess a small oral sucker, pharynx, and simple intestinal system and caeca. The ventral sucker is strong and has approximately 70 spines (CUSRG 1998). Heterophyds do not have a cirrus pouch. Their reproductive organs are grouped except for a separate genital sucker with many chitinous rodlets, characteristic seminal vesicle, and uterus. Some of the details of thes features define the family and distinguish the genera and species therein. Number and location of testes are major determiners of genera.
Transmission
Humans (and other fish eating mammals and birds) are the definitive host, in which the adult parasites lay eggs which are released to continue their life cycle in snails and fish. Humans are infected by eating fish which has not been heated to a high enough temperature to kill the parasites. Pickled and smoked fish can transmit the parasites. The high prevalence of heterophyd infections in eastern asia is largely due to the popularity of raw fish in local cuisines such as sushi.

Clinical Manifestations
The worms attach to the mucosa of the intestine of the final host and release eggs from their residence. They may cause inflamation throughout their invasion. Noticable symptoms of diarrhea, abdominal pain, loss of apetite, and nausea are sometimes present. Eggs released in the intestine can be absorbed into the blood stream and embolize in various regions of the body. Such tissue invasions have been reported to cause cardiac failure and eggs have been found in the central nervous system. Eosiniphilia is often present in Heterophyiasis infections (Parija et al 2003).

Life Cycle

This image
of the standard heterophyd life cycle has been generously provided by the CDC Division of Parasitic Disease.
The original image can be found at http://www.dpd.cdc.gov/DPDx/HTML/Heterophyiasis.htm

Snails and fish living in brackish water are the parasites' intermediate hosts. Miracidia turn into cercariae in snails (going through three stages: Sporocysts, Rediae, Cercariae) and are then released into the water. The cercariae then penetrate the fish, in which they encyst and develop into metacercariae.

Humans and other fish-eating animals are the final (definitive) host. Trematodes encyst in the small intestine, where they develop into adults. Adults reproduce in the intestine, releasing eggs in the stool. The eggs contain a complete and developed miracidium.

The incubation period in humans is between 1 and 15 days (MacLean et al. 1996)


Types of diagnostics used
Stool samples are taken and trematodes are identified through a microscope. Eosinophilia in blood tests is an indication to take stool samples for examination if other risk factors (such as travel to an endemic region and eating raw fish) are present.

Treatment
Disease can be treated with Praziquantel. The dosage is 75mg per kg body weight taken in three doses (of 25mg/kg) over the period of one day (Parija et al 2003, Jong 2002).

Epidemiology
Children may be more susceptible to severe symptoms (Parija et al 2003). Belizario et al 2001 noted that infection rates for men and those aged 15-45 were higher than the rest of the population in San Isidro, Philippines.

Geographical Distribution
Heterophyds are prevalent in Brazil, Mediterranean countries, Japan, Egypt (Chai et al 1990), Ukraine, Hawaii, Southeast Asia, Korea (Seo et al 1984, Chai et al 1984), the Middle East (Belizario et al 2001). In the past 20 years, cases have been reported in Alaska, Canada (Dixon 1997), the United States (Butt 2004). However, though Heterophyiasis was considered an "emerging" infection in the Philippines (as studied in San Isidro) by Belizario et al 2001, Kim et al 2003 reported finding no cases of trematodes in Roxas City, no more than 250 kilometers of water away. It is dificult to understand how a disease transmitted by fish and surviving up to 10 years in humans could either only be in certain areas of the Phillipines or have dissappeared. It is most likely that Heterophyidae is still present in all areas, and the second search was either not thorough enough or not specific enough to find heterophyidae, which has eggs very similar to some other species (Parija et al. 2003).

Public health measures and prevention strategies
Belizario et al, among others, noted the great need for an educational campaign to teach people about the dangers of eating raw fish and the proper way to prepare fish. Fish should be skinned and its gills removed before cooking fully. Raw, pickled, and smoked fish from endemic areas should be avoided. Such a program has not yet been enacted beacause disease is greatly underdiagnosed due to its similarity to common ailments such as acid peptic disease and peptic ulcer disease (Belizario et al. 2001).

References
  1. Witenberg, G. "Studies on the trematode Family Heterophyidae." 1928 Ann. Trop Med. Parasit. 23:131-268.
  2. Parija, Subhash C., Marrie, Thomas J., Koirala, Shekhar. "Trematode Infection." eMedicine Journal 4/4 (2003). May 3, 2004 <http://author.emedicine.com/MED/topic2301.htm>.
  3. "DPDx - Heterophyiasis." Centers for Disease Control & Prevention, National Center for Infectious Diseases, Division of Parasitic Disease. 2004. May 19, 2004 <http://www.dpd.cdc.gov/DPDx/HTML/Heterophyiasis.htm>.
  4. Jong, Elaine. "Intestinal parasites." Primary Care; Clinics in Office Practice 29.4 (December 2002) [pages unknown]
  5. Belizario Jr., VY, Bersabe, MJ, de Leon, WU, et al. "Intestinal Heterophyidiasis: an emerging food-borne parasitic zoonosis in southern Philippines." Southeast Asian Journal of Tropical Medicine and Public Health 32 (Suppl 2) (2001): 36-42.
  6. Chai, Jong-Yil, Seo, Byong-Seol, Lee, Soon-Hyung. "Studies on Intestinal Trematodes in Korea: XI. Two Cases of Human Infection by Hererophyes heterophyes nocens." Korean Journal of Parasitology 22.1 (1984): 37-42.
  7. Seo, Byong-Seol, Lee, Soon-Hyung, Chai, Jong-Yil, Hong, Sung-Jong. "Studies on Intestinal Trematodes in Korea: XII. Two Cases of Human Infection by Stellantchasmus falcatus." Korean Journal of Parasitology 22.1 (1984): 43-50.
  8. Kim, Bong-Jin, Ock, Mee-Sun, Chung, Dong-Il, Yong, Tai-Soon, Lee, Kyu-Jae. "The intestinal parasite infection status of inhabitants in the Roxas city, the Philippines." The Korean Journal of Parasitology 41.2 (June 2003): 113-115.
  9. Chai, Jong-Yil, Lee, Soon-Hyung. "Intestinal Trematodes of Humans in Korea: Metagonimus Heterophyids and Echinostomes." The Korean Journal of Parasitology 28 (December 1990): Suppl 103-122
  10. Dixon, BR. "Fish—and shellfish borne—trematode infections in Canada." Southeast Asian Journal of Tropical Medicine and Public Health 28 Suppl 1: 58-64.
  11. Stewart, T. "Detailed Taxonomy of the Parasitic Helminths." Cambridge University Schistosome Research Group. 1998. May 23, 2004 <http://www.path.cam.ac.uk/~schisto/Taxonomy/Old.Taxonomy.html>.
  12. [No author listed] "Digenean Flukes Causing Human Disease" Cambridge University Schistosome Research Group. 1998. May 23, 2004 <http://www.path.cam.ac.uk/~schisto/OtherFlukes/Human.flukes.html>.
  13. MacLean, J Dick, Arthur, J Richard, Ward, Brian J, Gyorkos, Theresa W, Curtis, Mark A, Kokoskin, Evelyne. "Common-source outbreak of acute infection due to the North American liver fluke Metorchis conjunctus." The Lancet 347(1996): 154-158.




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