The Parasite

Classification
Synonyms

History of Discovery
Morphology
Life Cycle

Classification

  • Protozoa
  • Phylum: Acomplexa
  • Class: Sporozoa
  • Subclass: Coccidia
  • Order: Eucoccidiorida
  • Family: Eimeriidae
  • Genus: Cyclospora
  • Species: cayentanensis

*microscopy of Cyclospora with acid-fast staining. image from http://www.cdfound.to.it/html/cyc3.htm.

Some Dissention:

The name Cyclospora cayentanensis was given to this parasite in 1994. Other members of the Cyclospora genus are known to infect a range of vertebrates including reptiles, insectivores, and rodents. However, some recent phylogenetic studies have suggested that Cyclospora might be better classified in the genus Eimeria. The Eimeria genus is the largest genus of coccidian parasites and currently includes more than 1500 species. If Cyclospora were reclassified it would be the first Eimeria species shown to infect humans.


*www-biol.paisley.ac.uk/biomedia/ text/txt_pictures.htm

A light micrograph of an Eimeria oocyst, containing sporocysts and the encased sporozoites. This species of Eimeria is an important parasite of poultry.

Proponents of reclassification claim that Cyclospora shares more homology with members of the Eimeria genus. They claim that proper classification could potentially lead to better research and understanding of the properties of Cyclospora. For example, similarities with certain species of Eimeria could suggest possible natural animal reservoirs, effective cell culture systems or applicable animal models for Cyclospora. The following phylogeny was constructed by a group of researchers and used to suggest the need for reclassification. It was derived by analyzing the small subunit ribosomal RNA of various coccidian species.


*Pieniazek and Herwald. Emerging Infectious Diseases. Vol 3, No 3. September 1997. www.cdc.gov/ncidod/eid/ vol3no3/pienz.htm

Synonyms

Before the official name of Cyclospora cayentanensis was given to this protozoan, synonyms included "coccidian-like body", "cyanobacterium-like body" (both CLB), "large Cryptosporidium", "blue-green algae," and "crytopsporidium muris –like oocyst."

History of Discovery

1977-78
first 3 documented cases of cyclosporiasis, reported by Ashford in Papua New Guinea (termed "undescribed coccidian")
1983
first documented case in Haiti (termed "Big Crypto")
1985
first documented case in Peru (termed "Cryptosporidium muris-like object")
1986
first documented U.S. cases, in four travelers returning from Haiti and Mexico
1990
first documented US outbreak, Chicago
1993
organism confirmed to be a coccidian parasite by induction of sporulation and was placed in Cyclospora genus
1994
a documented waterborne outbreak in Pokhara, Nepal
1994
name Cyclospora cayentanensis proposed; species name refers to Cyentano Heredia University in Lima, Peru where the major studies of the parasite were conducted
1995
Trimethoprim-sulfamethoxazole shown to be an effective treatment in placebo-controlled trials
1995
foodborne transmission first suggested when illness of an airline pilot was associated with food prepared in a Haitian kitchen
1996
phylogenetic relationship to Eimeria species suggested through genetic studies
1995-99
multiple foodborne outbreaks in North America linked to various types of fresh produce


*The comparative sizes of other protozoan parasites. http://www.journals.uchicago.edu/CID/journal/issues/
v31n4/000497/000497.html, Herwalt. Clinical Infectious Diseases
.

To place these discoveries in a comparative context, the first documented cases of other protozoan parasitic infections were as follows: Giardia lambia in the late 17th century, Enatmoeba histolytica in the late 19th century, Isopora belli in the early 20th century, and Cryptosporidium parvum in 1974.


Morphology

*image from www.nih.go.jp/~tendo/atlas/ japanese/cyclospora.html

The morphology of the Cyclospora cayentanensis oocyst is one of the main features currently used for diagnosis. The oocysts contain two sporocysts, each which contains two crescent-shaped sporozoites. They are spherical and approximately 8 to 10 microns in diameter. Unsporulated oocysts, after they are passed from the human host, have a greenish central morula containing 6 to9 refractile globules. Fine structure studies have revealed that the unsporulated oocyst has an outer fibrillar coat, a defined cell wall and membrane. After sporulation the sporozoites within the sporocysts contain a membrane-bound nucleus and micronemes (a small twisted organelle in the anterior which is a defining characteristic of members of Apicomplexa).


Life Cycle

*diagram from http://www.dpd.cdc.gov/DPDx/HTML/ImageLibrary/Cyclosporiasis_il.htm

The entire life cycle of Cyclospora can be completed within in the human host and with a brief environmental phase. Unsporulated Cyclospora oocysts are released into the environment through human feces. They are resistant to many environmental pressures and can survive freezing, certain toxins, and chlorination. Sporulation occurs in the environment at appropriate temperatures and oxygen concentrations. One laboratory study documented sporulation in 7 to 12 days at 22 to 32 C.

The sporulated oocysts, containing 2 sporocysts each enclosing 2 sporozoites, are ingested and move to the gut. Once the oocysts reach the gut the sporocysts are released and the sporozoites exit from the sporocysts. The sporozoites then penetrate the gut epithelial cells in the small intestine, especially the jejunum. Initially, the parasite reproduces asexually and forms meronts which contain multiple merozoites, which can infect nearby cells. This is the main pathogenic stage of the organism, and is responsible for most of the symptoms of cyclosporiasis. This stage is also self-limiting and acute infection usually subsides.

Sexual reproduction can also occur in the gut. Some of the meroizoites will enlarge and form the female gamete (macrogamete). Others transform into the male flagellated sperm-like gamete (microgamete). Sexual reproduction produces unsporulated oocysts which are then excreted into the environment and are eventually able to pass through another stage of infection.

Below are two pictures of the sporulation of Cyclospora oocysts:


*DIC microscopy of wet mounts of stool specimens; image from http://www.dpd.cdc.gov/DPDx/HTML/ImageLibrary/Cyclosporiasis_il.htm

This first set of microscopy images shows the progression from an unsporulated to a sporulated oocyst, from Day 0 to Day 5 and 10. In Day 5 and 10, one can observe the two encased sporocysts. The inner sporozoites are not visible at this point. The rupture of the oocyst and release of the sporocysts is depicted in the last image.

*DIC microscopy and autofluoresence of wet mounts of stool specimens; image from http://www.journals.uchicago.edu/CID/journal/issues/v31n4/000497/000497.html

This second set of microscopy images shows a similar progression from an unsporulated oocyst to a sporulated oocyst in the top row of images. Image B represents rupture, also termed excystation, which releases the sporocysts from the oocyst. Image C shows a sporocyst and two free sporozoites. The sporozoites are normally not visualized with microscopy but mechanical stimulation causing excystation allowed for their release from the sporocyst. Image D demonstrates how an unsporulated oocyst looks through DIC microscopy, while image E demonstrates how this same oocyst would look through ultraviolet fluorescence microscopy. Both of these latter methods are used for Cyclospora diagnosis and are described further below.