Cyclosporiasis pathophysiology: Difference between revisions
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==Pathogenesis== | ==Pathogenesis== | ||
===Life Cycle=== | |||
Unsporulated oocysts of Cyclospora cayetanensis are excreted from infected persons. Under adequate temperatures (23-27ºC), these take about 7-15 days to sporulate, in order to become infectious. | |||
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When food or water contaminated with infectious oocysts is ingested by a susceptible host, the oocysts excyst and sporozoites are released to infect epithelial cells of the duodenum and jeju- num. Asexual multiplication results in type I and II meronts. The latter differentiate into sexual stages or gametocytes. The macrogametocyte is fertilized by the microgametocyte and pro- duces a zygote. Oocysts are then formed and excreted into the environment as unsporulated oocysts. The extended period for oocysts to sporulate and become infectious raises questions as to where and how sporulation occurs. Unsuccessful attempts to infect animals or cells with sporulated oocysts suggest the need for a specific, unknown trigger to initiate infection (63). | |||
Thus far, C. cayetanensis infections have been identified only in humans. The role of other animal species as reservoirs or inter- mediary hosts has been examined, with conflicting results. Cyclo- spora oocysts have been found in feces of chickens (37, 62, 70), ducks (62, 197), and dogs (27, 37, 62, 194) collected in countries where Cyclospora is endemic (27, 62, 63). However, attempts to experimentally infect a variety of animals (63) and humans (5) have been unsuccessful. The presence of Cyclospora in stools from animals could be explained by eating or coprophagic habits of the surveyed animals. Nonetheless, it cannot yet be ruled out that animals may play a role in the dissemination of Cyclospora by contaminating water and food products. Two reports indicate that Cyclospora can be propagated in guinea pigs (191) and albino mice (162). These studies could not be reproduced, and further analyses are needed to determine the susceptibility of these ani- mals to infection by Cyclospora and their suitability as hosts or reservoirs of C. cayetanensis. | |||
Insects, rotifers, and free-living nematodes could also play a role in the dissemination of oocysts, a phenomenon that has been described for Cryptosporidium (67, 74, 75, 90, 179) and other food-borne pathogens. The nematode Caenorhabditis elegans can ingest and excrete infectious bacteria (23, 98, 99) as well as Cryptosporidium parvum oocysts. Although the oocysts of Cyclospora are larger and may not be ingested by C. elegans, other, larger species of free-living nematodes could have a significant role in Cyclospora oocyst dissemination. Therefore, the role of free-living nematodes and other mechanical vectors in the contamination of produce and other food crops needs to be examined further (90). | |||
Cyclospora oocysts have been detected in nongastrointesti- nal samples. There are two reports of oocysts in the sputa of HIV patients with a history of pulmonary tuberculosis (55, 93), suggesting that Cyclospora could be considered an opportunis- tic pathogen. Travel to rural areas and ingestion of contami- nated foods could be modes of infection. Accidental inhalation of oocysts has also been suggested (93) | |||
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* When freshly passed in stools, the oocyst is not infective (thus, direct fecal-oral transmission cannot occur; this differentiates Cyclospora from another important coccidian parasite, Cryptosporidium). | * When freshly passed in stools, the oocyst is not infective (thus, direct fecal-oral transmission cannot occur; this differentiates Cyclospora from another important coccidian parasite, Cryptosporidium). | ||
Revision as of 17:47, 18 September 2014
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]
Overview
Fresh produce and water can serve as vehicles for transmission and the sporulated oocysts are ingested (in contaminated food or water). The oocysts excyst in the gastrointestinal tract, freeing the sporozoites which invade the epithelial cells of the small intestine.
Pathogenesis
Life Cycle
Unsporulated oocysts of Cyclospora cayetanensis are excreted from infected persons. Under adequate temperatures (23-27ºC), these take about 7-15 days to sporulate, in order to become infectious.
- When freshly passed in stools, the oocyst is not infective (thus, direct fecal-oral transmission cannot occur; this differentiates Cyclospora from another important coccidian parasite, Cryptosporidium).
- In the environment, sporulation occurs after days or weeks at temperatures between 22°C to 32°C, resulting in division of the sporont into two sporocysts, each containing two elongate sporozoites.
- The oocysts excyst in the gastrointestinal tract, freeing the sporozoites which invade the epithelial cells of the small intestine.
- Inside the cells they undergo asexual multiplication and sexual development to mature into oocysts, which will be shed in stools.[1]
Various chemicals in the host's gastrointestinal tract cause the oocysts to excyst and release sporozoites; generally, two are observed per oocyst. After these sporozoites invade the epithelial cells, they undergo merogony, a form of asexual reproduction that results in many daughter merozoites. These daughter cells may either infect new host cells and initiate yet another round of merogony, or they can take on a sexual track via gametogony: daughter merozoites become male macrogamonts — which form many microgametes — and female macrogamonts. After fertilization has occurred via male microgamete fusion with female macrogamont, the zygote matures into an oocyst and ruptures the host cell, from which point it is passed with the stool. The oocysts that are passed are not, however, immediately infectious. Sporulation can take anywhere from one to several weeks, meaning that person-to-person transmission is not a likely problem. This differentiates C. cayentanensis from Cryptosporidium parvum — a closely related organism that causes a similar disease — since C. parvum oocysts are immediately infectious upon release from the host.
Life Cycle
The protozoan lives out its lifecycle intracellularly within the host’s epithelial cells and gastrointestinal tract. Infection is transmitted through the oral-fecal route, and begins when a person ingests oocysts in fecally contaminated food or water. Various chemicals in the hosts gastrointestinal tract cause the oocysts to excyst and release sporozoites; generally, two are observed per oocyst. After these sporozoites invade the epithelial cells, they undergo merogony, a form of asexual reproduction that results in many daughter merozoites. These daughter cells may either infect new host cells and initiate yet another round of merogony, or they can take on a sexual track via gametogony: daughter merozoites become male macrogamonts — which form many microgametes — and female macrogamonts. After fertilization has occurred via male microgamete fusion with female macrogamont, the zygote matures into an oocyst and ruptures the host cell, from which point it is passed with the stool. The oocysts that are passed are not, however, immediately infectious. Sporulation can take anywhere from one to several weeks, meaning that person-to-person transmission is not a likely problem. This differentiates C. cayetanensis from Cryptosporidium parvum — a closely related organism that causes a similar disease — since C. parvum oocysts are immediately infectious upon release from the host.
Transmission
The protozoan lives out its lifecycle intracellularly within the host’s epithelial cells and gastrointestinal tract. Infection is transmitted through the oral-fecal route, and begins when a person ingests oocysts in fecally contaminated food or water.
- Fresh produce and water can serve as vehicles for transmission and the sporulated oocysts are ingested (in contaminated food or water).