Apicomplexan Parasites

Life cycle of Cyclospora cayetanensis When freshly passed in stools, the oocysts of Cyclospora cayetanensis are not infective (1), so direct fecal-oral transmission cannot occur (this is in contrast to what is seen with another important coccidian parasite, Cryptosporidium). In the environment (2), sporulation occurs after days or weeks at temperatures between 22 C and 32 C, resulting in the division of the sporont into two sporocysts, each containing two elongate sporozoites (3). Fresh produce and water can serve as vehicles for transmission (4) and the sporulated oocysts are ingested (in contaminated food or water) (5). The oocysts excyst in the gastrointestinal tract, freeing the sporozoites, which invade the epithelial cells of the small intestine (6). Inside the cells, they undergo asexual multiplication and sexual development to mature into oocysts, which will be shed in stools (7). Potential mechanisms of contamination of food and water are still under investigation.From Centers for Disease Control Parasites and Health website.

Life cycle of Cystoisospora belli At the time of excretion, the immature oocyst usually contains one sporoblast (more rarely, two) (1). After excretion, maturation continues, with the sporoblast dividing in two (the typical oocyst now contains two sporoblasts). The sporoblasts secrete a cyst wall, thus becoming sporocysts, and these sporocysts divide twice, producing four sporozoites each (2). Infection occurs by ingestion of sporocyst-containing oocysts: the sporocysts excyst in the small intestine and release their sporozoites, which invade the epithelial cells and initiate schizogony (3). Upon rupture of the schizonts, the merozoites are released, invade new epithelial cells, and continue the cycle of asexual multiplication (4). Trophozoites develop into schizonts, which contain multiple merozoites. After a minimum of one week, the sexual stage begins with the development of male and female gametocytes (5). Fertilization results in the development of oocysts that are excreted in the stool (1).From Centers for Disease Control Parasites and Health website.

Cystoisospora belliAn oocyst of Cystoisospora belli in the epithelial cell of a mammalian host stained with hematoxylin and eosin. The oocyst is marked with yellow arrow.Cystoisosporiasis. Microscopy Findings on the website of the Division of Parasitic Diseases and Malaria (Centers for Disease Control and Prevention)This image is a work of the Centers for Disease Control and Prevention, part of the United States Department of Health and Human Services,From Wikimedia Commons

Cystoisospora belliFigure 1: Microscopic photograph showing acid fast I. belli oocysts in stool smear (×400)From: Mudholkar VG, Namey RD. Heavy infestation of Isospora belli causing severe watery diarrhea. Indian J Pathol Microbiol [serial online] 2010 [cited 2014 Oct 10];53:824-5. Available from: http://www.ijpmonline.org/text.asp?2010/53/4/824/72091

Life cycle of Sarcocystic hominis and S. suihominisBoth sporulated oocysts (containing two sporocysts) and individual sporocysts can be passed in stool (1). Sporocysts contain four sporozoites and a refractile residual body. Sporocysts ingested by the intermediate host (cattle for S. hominis and pigs for S. suihominis) rupture, releasing sporozoites. Sporozoites enter endothelial cells of blood vessels and undergo schizogony, resulting in first-generation schizonts. Merozoites derived from the first-generation invade small capillaries and blood vessels, becoming second-generation schizonts. The second generation merozoites invade muscle cells and develop into sarcocysts containing bradyzoites, which are the infective stage for the definitive host (2). Humans become infected when they eat undercooked meat containing these sarcocysts. Bradyzoites are released from ruptured cysts in the small intestine (3) and invade the lamina propria of the intestinal epithelium (4). There, they differentiate into macro- and microgametocytes. Fusion of male and female gametes (5) results in the formation of oocysts (6). Oocysts sporulate in the intestinal epithelium and are shed from the host in feces (7). Due to the fragile nature of the oocyst wall, individual sporocysts may also be detected in feces.From Centers for Disease Control Parasites and Health website

Life cycle of Babesia microtiThe normal Babesia microti life cycle involves two hosts, a rodent (often the White-footed Mouse, Peromyscus leucopus) and a tick in the genus Ixodes. During a blood meal, a Babesia-infected tick introduces sporozoites (an active, infective developmental stage) into the mouse host (1). Sporozoites enter erythrocytes (red blood cells) and undergo asexual reproduction (budding) (2). In the blood, some parasites differentiate into male and female gametes, although these cannot be distinguished at the resolution of a light microscope (3). The definitive host (i.e., the host in which sexual reproduction occurs) is the tick. Once ingested by an appropriate tick (4), gametes unite and undergo a sporogonic cycle resulting in sporozoites (5), which can then be transmitted back to a rodent fed on by the tick. Transovarial transmission ("vertical" transmission across host generations) has been documented for “large” Babesia species but not for the “small” babesiae, such as B. microti. Humans enter the cycle when infected ticks bite them in lieu of a rodent host . During a blood meal, a Babesia-infected tick introduces sporozoites into the human host (6). Sporozoites enter erythrocytes and undergo asexual replication (budding) (7), as in a rodent host. Multiplication of the blood stage parasites is responsible for the clinical manifestations of the disease. However, humans are, for all practical purposes, dead-end hosts and there is probably little, if any, subsequent transmission that occurs from ticks feeding on infected persons. However, human-to-human transmission can occur through blood transfusions (8).From Centers for Disease Control Parasites and Health website.

Life cycle of plasmodium parasites causing malaria in humansThe malaria parasite life cycle involves two hosts. During a blood meal, a plasmodium-infected female Anopheles mosquito inoculates sporozoites into the human host (1). Sporozoites infect liver cells (2) and mature into schizonts (3), which rupture and release merozoites (4). (Note that in P. vivax and P. ovale, a dormant stage [hypnozoite] can persist in the liver and cause relapses by invading the bloodstream weeks, or even years, later.) After this initial replication in the liver, a process known as exo-erythrocytic schizogony (A), the parasites undergo asexual multiplication in the erythrocytes (red blood cells), a process known as erythrocytic schizogony (B). Merozoites infect red blood cells (5). The ring stage trophozoites mature into schizonts, which rupture and release merozoites (6). Some parasites differentiate into sexual erythrocytic stages (gametocytes) (7). Blood stage parasites are responsible for the clinical manifestations of the disease. The gametocytes, both male (microgametocytes) and female (macrogametocytes), are ingested by an Anopheles mosquito during a blood meal (8). The parasites’ multiplication in the mosquito is known as the sporogonic cycle (C). While in the mosquito's stomach, the haploid microgametes penetrate the haploid macrogametes, generating diploid zygotes (9). The zygotes in turn become motile and elongated stages known as ookinetes (10), which invade the midgut wall of the mosquito, where they develop into oocysts (11). The oocysts grow, rupture, and release sporozoites (12), which make their way to the mosquito's salivary glands. Inoculation of the sporozoites into a new human host perpetuates the malaria life cycle (1).From Centers for Disease Control Parasites and Health website.

Life cycle of Toxoplasma gondii, the cause of toxoplasmosis in humansThe only known definitive hosts for the apicomplexan protozoan parasite Toxoplasma gondii are members of family Felidae (domestic cats and their relatives). Unsporulated oocysts are shed in the cat’s feces (1). Although oocysts are usually only shed for 1-2 weeks, large numbers may be shed. Oocysts take 1-5 days to sporulate in the environment and become infective. Intermediate hosts in nature (including birds and rodents) become infected after ingesting soil, water or plant material contaminated with oocysts (2). Oocysts transform into tachyzoites shortly after ingestion. These tachyzoites localize in neural and muscle tissue and develop into tissue cyst bradyzoites (3). Cats become infected after consuming intermediate hosts harboring tissue cysts (4). Cats may also become infected directly by ingestion of sporulated oocysts. Animals bred for human consumption and wild game may also become infected with tissue cysts after ingestion of sporulated oocysts in the environment (5). Humans can become infected by any of several routes: eating undercooked meat of animals harboring tissue cysts (6). consuming food or water contaminated with cat feces or by contaminated environmental samples (such as fecal-contaminated soil or changing the litter box of a pet cat) (7). blood transfusion or organ transplantation (8). transplacentally from mother to fetus (9). In the human host, the parasites form tissue cysts, most commonly in skeletal muscle, myocardium, brain, and eyes; these cysts may remain throughout the life of the host. Diagnosis is usually achieved by serology, although tissue cysts may be observed in stained biopsy specimens (10). Diagnosis of congenital infections can be achieved by detecting T. gondii DNA in amniotic fluid using molecular methods such as PCR (11).From Centers for Disease Control Parasites and Health website

This is an illustration of the life cycle of Isospora belli, the causal agent of Isosporiasis.Created: 2002

This is an illustration of the life cycle of Toxoplasma gondii, the causal agent of Toxoplasmosis.Created: 2002

This illustration depicts the life cycle of different species of Cryptosporidium, the causal agents of Cryptosporidiosis.Created: 2002

Note the developmental tetrad configuration of these Babesia sp. trophozoites, which resemble P. falciparum.Created: 1973

Note the developmental tetrad configuration of these Babesia sp. trophozoites, which resemble P. falciparum.Created: 1973

Magnified 1000X, this photomicrograph revealed the presence of what were determined to be numbers of intraerythrocytic Babesia sp. ring-form parasites.Created: 1974

This photomicrograph revealed the morphologic details of Cryptosporidium parvum oocysts, i.e.,encapsulated zygotes, which had been stained using the modified acid-fast method. These oocysts exhibit a bright red coloration when using this staining technique, and in this case, you’ll note the sporozoites that were made visible inside the two oocysts on the right. Sporozoites are the nucleated, motile stage of development through which many protozoans pass such as C. parvum, on their way to becoming adults, and represent a very infectious form of these organisms. The sporozoites will be released from these C. parvum oocysts.Created:

This photomicrograph of a fresh stool sample, which had been prepared using a 10% formalin solution, and stained with safranin, revealed the presence of three uniformly stained Cyclospora cayetanensis oocysts in the field of view.Created:

This photomicrograph of a fresh stool sample, which had been prepared using a 10% formalin solution, and stained with modified acid-fast stain, revealed the presence of four Cyclospora cayetanensis oocysts in the field of view. Compared to wet mount preparations, the oocysts are less perfectly round and have a wrinkled appearance due to this method of fixation. Most importantly, the staining is variable among the four oocysts.Created:

IZ 1507371 - Goussia bayae 5, 1000x magnification formalin preserved, hematoxylin and eosin stained specimens in host gallbladder

IZ 1490788 - Goussia bayae 2, 1000x magnification formalin preserved, hematoxylin and eosin stained specimens in host bile duct

IZ 1490789 - Goussia bayae 3, 1000x magnification formalin preserved, hematoxylin and eosin stained specimens in host bile duct

IZ 1507371 - Goussia bayae 4, 100x magnification formalin preserved, hematoxylin and eosin stained specimens in host gallbladder

IZ 1490790 - Goussia bayae 6, 1000x magnification Fresh, unstained specimens in host bile sample, Nomarski DIC imaging

IZ 1490787 - Goussia bayae 1, 200x magnification formalin preserved, hematoxylin and eosin stained specimens in host bile duct