Overview Of The Parasites

Cryptosporidium is a protozoan parasite that belongs to the subphylum Apicomplexa. Because of large and significant waterborne outbreaks associated with it, much has been learned about the epidemiology, immunology, and biology of this parasite. Cryptosporidium parvum was considered to be the only species of public health significance; however, studies involving pediatric populations, the immunocompromised, travelers, and endemic populations have demonstrated that humans can also be infected with C. meleagridis (of turkeys), C. canis (of dogs), C. felis (of cats), and C. muris

(of cattle and rodents). Molecular studies have also demonstrated that the previously described C. parvum is actually at least two different species: C. parvum (zoonotic) and C. hominis (considered anthroponotic, i.e., human-to-human). Both are morphologically similar [1,2].

The infective stage of cryptosporidium is the oocyst, which is excreted in the feces of infected hosts. These oocysts are immediately infectious to a susceptible host and contain four fully differentiated and infectious sporozoites. Once ingested, oocysts excyst in the gastrointestinal tract, releasing infective sporozoites. Sporozoites infect the epithelial cells of the ileum preferentially and may continue colonization of all the small intestine and bile ducts if the host is immunocompromised, thus making eradication of the parasite more difficult. Once parasites have entered the epithelial cells they are compartmentalized in a parasitic vacuole which is extracyto-plasmatic, but intracellular. This site localization is unique, and the parasite depends upon an elaboration of membrane surface at the cytoplasmic interface called the feeder organelle which allows for the transport of select nutrients to the parasite. Once established, the parasite multiplies asexually, producing type I and II meronts containing 8 and 4 merozoites, respectively. The first asexual meront generation can multiply indefinitely in the absence of immunity while those that have progressed to the second generation eventually differentiate and produce gamonts (macro- and microgametocytes). A microgametocyte fertilizes the macrogamont and leads to the formation of a zygote. When mature, the zygote will become either a thin- or thick-walled oocyst. The former constitutes approximately 20% of oocysts produced and is autoinfective, while the latter is environmentally resistant and fully infectious when excreted to the environment [3].

Cryptosporidiosis is characterized by abundant and persistent diarrhea, fever, and abdominal pain. There is no effective therapy but new drugs are being evaluated with promising results [4-6]. The immune system of the infected individual plays a crucial role in eradicating the parasite. In immunocompromised patients cryptosporidiosis can be fatal [7].

Cyclospora cayetanensis is also a parasite of the subphylum Apicomplexa. Fourteen different species of cyclospora have been described in rodents and insectivores [8]. Cyclospora cayetanensis is considered to be exclusively anthroponotic [9]. In 1999 three other species infecting nonhuman primates were described. These cyclospora species are morphologically similar to C. cayetanensis, but, based on their host specificity and 18S DNA sequence homology, are different [10,11].

The life cycle of cyclospora begins when oocysts are excreted in the feces of an infected individual. These oocysts are unsporulated and take approximately two weeks under optimal environmental conditions to sporulate fully and become infectious. Differentiated oocysts contain two sporocysts, each containing two sporozoites. When ingested and upon passage through the gastrointestinal tract, oocysts rupture and sporocysts are released. Enzymes and bile salts induce the release (excystation) of the sporozoites, which in turn invade the epithelial cells of the small intestine, forming an intracellular parasitic vacuole. Based on histological observations of biopsies of infected individuals, type I and II meronts are produced. Gametocytes have also been observed, suggesting that cyclospora has a life cycle similar to that of cryptosporidium and other coccidia. After zygote formation, oocysts are formed and excreted to the environment [12].

There are nonspecific fingerprinting tools for traceback studies; however, characterization of the internal transcribed spacers 1 (ITS1) sequences may be used for these purposes. The ITS1 sequences of clones of all five raspberry-associated isolates were identical, consistent with their origin from a single source. One of the two Guatemala isolates and two Peruvian isolates contained multiple ITS1 sequences [13,14].

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