Tuesday, December 11, 2012

Cryptosporidium essay

by YS

As the second leading cause of human gastrointestinal disease in the United States, risk of the waterborne transmission of Cryptosporidium is a serious global issue in drinking water safety (1). Oocysts of Cryptosporidium have thick cell walls making them resistant to most environment challenges and are extremely robust in capable of surviving for extended periods of time. There are several different known species of Cryptosporidium, yet Cryptosporidium hominis (parvum genotype 1) and Cryptosporidium parvum (parvum genotype 2) are the two most common species that cause the disease in humans, called Cryptosporidiosis (2).

Figure 1. C. parvum life cycle.
Cryptosporidiosis is mainly caused by C.parvum, an obligate intracellular protozoan parasite that infects the gastrointestinal tract of humans after ingestion of viable Cryrtosporidium oocysts. Oocysts are the cyst containing zygote, generally formed among parasitic protozoan species. After these oocysts are ingested, they release sporozoites, which invade epithelial cells of the gastrointestinal tract, primarily in the small intestine (Figure 1).

Temperature or presence of bile salt can lead the secretion of sporozoite, (5) after entry into the epithelial cells. Then the sporozoites mature into trophozoites and reproduce zygotes under two conditions: asexual and sexual cycles. In the asexual cycle, Type I merozoites are produced and secreted into the intestinal lumen, and infect other gastrointestinal epithelial cells. In the sexual cycle, Type II merozoites attach to the epithelial cells and mature into gametocytes, which are fertilized in the intestinal tract and form oocysts (Figure 1). At this point, the newly formed oocysts can either re-infect the intestinal epithelium or start the new life cycle from the beginning. Otherwise they stay in feces, capable of infecting other organisms (2).

Cryptosporidiosis outbreaks, caused by oocysts of Cryptosporidium species, have been associated with filtered and unfiltered surface water, groundwater, contaminated distribution systems, drinking water, recreational water, and chlorinated swimming pool water. It is recognized as a causative agent of watery diarrhea in individuals with compromised immune system, such as Acquired Immune Deficiency Syndrome (AIDS) or Severe Immune Deficiency Syndrome (SIDS). Within immunocompetent patients, who have complete immune system, rarely get severe or remain asymptomatic. Yet some immunocompetent patients can still have the watery diarrhea that lasts 10 to 14 days (2). In general, these patients who are asymptomatic or have complete immune system do not require treatment for Cryptosporidiosis, although be able to pass on the infection to the others (4).

In the case of immunodeficiency individuals who have HIV, cancer or AIDS, they display symptoms that are associated with significant weight loss, abdominal cramps, nausea, or low-grade fever. The effects of continued diarrhea and dehydration can be dangerous, especially for the very young, old, and frail individuals. Sometimes, Cryptosporidium spreads beyond the intestine in patients with AIDS, and reach lung, middle ear, pancreas, or stomach (4). The primary treatment for these AIDS patients with symptomatic Cryptosporidiosis is to restore the function of their immune system with effective anti-retroviral therapy (4). In many cases, cell-mediated immunity is essential for protection against Cryptosporidiosis, and increased amount of glycoprotein found in the surface of immune cells can result in resolution of symptoms. Several antimicrobial agents have been used to treat Cryptosporidiosis in HIV infected patients, including nitazoxanide, paromomycin, and azithromycin. Nonetheless, no antimicrobial agent has shown to be consistently effective against C.parvum in the absence of anti-retroviral therapy. When antimicrobial therapy is used in combination with anti-retroviral therapy, nitazoxanide is a recommended additional agent, but can’t remove C.parvum completely (4). Since patients with severe Cryptosporidiosis typically have significant dehydration, fluid rehydration and electrolyte repletion can be a main component use in therapy (4).

Even though C.parvum severely affect those immunodeficient individuals, none of the current exist therapies can completely remove C.parvum from an infected host. There is a recent study, which reveals the fact that oocyst’s survival is largely dependent on temperature in vitro (5). Under the ambient conditions, the period of oocyst infectivity decreases as the temperature increase from 4°C to 23°C (5). Since the oocyst wall is composed of various proteins, Pokorny et. al. (2002) suggest that the protein denaturation may disrupt oocyst wall at a high temperature and secrete the sporozoites under unfavorable conditions that will negatively affect their survivals (5). However, oocysts dependent on temperature has only been revealed in vitro, so there is no known mechanism yet to deal with oocysts in vivo.

Hence, physical removal of oocysts in the water before they get ingested to other organisms has proposed. However, due to their small size, C.parvum oocysts pass through the most conventional filters, making physical removal from water difficult. Also C.parvum oocysts are resistant to common disinfectants such as chlorine and monochloramine. Therefore, ozone has been suggested as one of the effective inactivation methods, which can produce free radicals that attack the oocysts’ wall and DNA. For that reason, Rennecker et. al. (2000) revealed the sequential disinfection of oocysts with different chemical agents resulted in greater inactivation. During oocysts inactivation mechanism using a sequential treatment, ozone is used as primary disinfectant, followed by free chlorine or monochloramine as a secondary disinfectant. This treatment measured 2.4-9.2 times faster rates to remove C.parvum organism, than one observed with monochloroamine disinfection alone, to demonstrate sequential treatment being more efficient than chemical control the C.parvum (3).

To detect C.parvum, Immunofluorescence assays (IFA) can be used. During the process of Immunofluorescence assays, chemical compounds that emit the light are covalently attach to the antibodies and become visualized. It is designed to bind a specific proteins or cell components to find where C.parvum gets attached to the host. Also, Enzyme-linked Immunosorbent Assays (ELISA) can be used to detect and diagnose C.parvum. ELISA is a popular way to visualize the protein interactions, and therefore, can be used to detect the binding between C.parvum surface protein and host cell surface protein to identify the infected individual.

As mentioned earlier, the risk of waterborne transmission of C.parvum is a global issue in drinking water safety. Recognition of C.parvum as a causative agent of Cryptosporidiosis and its result in public and environmental health issues have developed scientific investigation into its taxonomy, biology, epidemiology, genomic characterization, disinfection and detection of this organism (2). Therefore, further study to accurately detect the mechanism of C.parvum to find the Cryptosporidiosis treatment is crucial. Also, it would be beneficial to sequence the complete genome. This will allow researcher to better understand the host specificity and virulence determinants to develop an effective treatments.

(1)  Mauzy MJ., Enomoto S., Lancto CA., Abrahamsen MS., Rutherford, MS. (2012) The Cryptosporidium Parvum Transcriptome during In Vitro Development. PLoS ONE 7(3): e31715. doi:10.1371/journal.pone.0031715.
(2)  Hunter, Paul R., Hadfield, Stephen J., Wilkinson Dawn., Lake Iain R., Harrison,Florence C.D., Chalmers Rachel M. (2007) Subtypes of Cryptosporidium parvum in Humans and Disease Risk. Emerging Infectious Diseases. Vol. 13, No. 1, January.
(3)  Rennecker, Jason L., Driendger Amy M., Rubin Sara A., Marinas Benito J. (2000) Synergy in sequential Inactivation of Cryptosporidium Parvum with Ozone/free Chorine and Ozone/Monochloramine. Elsevier Science Ltd.Wat. Res. Vol. 34, No. 17, pp. 4121±4130.
(4)  Iqbal A., Lim YAL., Surin J., Sim BLH.. (2012) High Diversity of Cryptosporidium Subgenotypes Identified in Malaysian HIV/AIDS Individuals Targeting gp60 Gene. PLoS ONE 7(2): e31139. doi:10.1371/journal.pone.0031139.
(5)  Pokorny NJ., Weir SC., Carreno RA., Trevors JT., Lee H.. (2002) Influence of temperature on Cryptosporidium parvum oocyst infectivity in river water samples as detected by tissue culture assay. J Parasitol. 88(3): 641–3.


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