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When isolated and properly brought into the body, L-arginine has the ability to produce some remarkable results. A network of nerve fibers and ganglia located between the muscle layers of the alimentary canal ; control muscle movements of the alimentary canal [top]. They help in the clotting of blood, by producing the thrombin enzyme. In its pure form L-arginine tastes terrible and can have some serious side effects like:. For the patients of celiac disease, the safe foods include plain meats, vegetables, fruits, potato, corn flours, soybean, rice, most dairy products and a moderate amount of oats. Swelling; a response to injury or pathogenic invasion that results in fluid buildup due to increased permeability of capillaries in the area [top]. I would suggest getting professional advice if considering using formaldehyde or one of its relatives to disinfect for parvo.
Some taxa, such as the opossum , have the original number of teeth. In other groups the number of teeth is reduced. Marsupials in many cases have 40 to 50 teeth, significantly more than placental mammals.
The upper jaw has a high number of incisors, up to ten, and they have more molars than premolars. The second set of teeth grows in only at the 3rd premolar: Few general characteristics describe their skeleton.
In addition to details in the construction of the ankle, bones Ossa epubica are characteristic, two from the pubic bone of the pelvis, which is a forwardly projecting bone. Since these are present in males and pouchless species, it is believed that they originally had nothing to do with reproduction, but served in the muscular approach to the movement of the hind limbs. This could be explained by an original feature of mammals, as these epipubic bones are also found in monotremes. Marsupial reproductive organs differ from the placentalp mammals.
For them, the reproductive tract is doubled. The females have two uteri and two vaginas, and before birth, a birth canal forms between them, the median vagina. A pouch is present in most, but not all, species. Many marsupials have a permanent bag, whereas in others the pouch develops during gestation, as with the shrew opossum , where the young are hidden only by skin folds or in the fur of the mother.
The arrangement of the pouch is variable to allow the offspring to receive maximum protection. Locomotive kangaroos have a pouch opening at the front, while many others that walk or climb on all fours have the opening in the back. Usually, only females have a pouch, but the male water opossum has a pouch that is used to accommodate his genitalia while swimming or running.
Marsupials have adapted to many habitats, reflected in the wide variety in their build. The largest living marsupial, the red kangaroo , grows up to 1.
The smallest members of this group are the marsupial mice , which often reach only 5 centimetres 2. Some species resemble placental mammals and are examples of convergent evolution. The extinct Thylacine strongly resembled the placental wolf, hence its nickname "Tasmanian wolf". Flying and the associated ability to glide occurred both with marsupials as with sugar gliders and some placental mammals as with flying squirrels , which developed independently.
Other groups such as the kangaroo, however, do not have clear placental counterparts, though they share similarities in lifestyle and ecological niches with ruminants. Marsupials' reproductive systems differ markedly from those of placental mammals. In bandicoots , an additional chorioallantoic placenta forms, although it lacks the chorionic villi found in eutherian placentas. The evolution of reproduction in marsupials, and speculation about the ancestral state of mammalian reproduction , have engaged discussion since the end of the 19th century.
Both sexes possess a cloaca ,  which is connected to a urogenital sac used to store waste before expulsion. The bladder of marsupials functions as a site to concentrate urine and empties into the common urogenital sinus in both females and males.
Most male marsupials, except for macropods  and marsupial moles ,  have a bifurcated penis, separated into two columns, so that the penis has two ends corresponding to the females' two vaginas. The male thylacine had a pouch that acted as a protective sheath, covering his external reproductive organs while he ran through thick brush.
The shape of the urethral grooves of the males' genitalia is used to distinguish between Monodelphis brevicaudata , Monodelphis domestica , and Monodelphis americana. The grooves form 2 separate channels that form the ventral and dorsal folds of the erectile tissue. The only accessory sex glands marsupials possess are the prostate and bulbourethral glands.
However, there does not appear to be any seasonal difference in the weight of the testes. Female marsupials have two lateral vaginas , which lead to separate uteri , but both open externally through the same orifice. A third canal, the median vagina, is used for birth. This canal can be transitory or permanent. Marsupials give birth at a very early stage of development; after birth, newborn marsupials crawl up the bodies of their mothers and attach themselves to a nipple, which is located on the underside of the mother, either inside a pouch called the marsupium , or open to the environment.
There they remain for a number of weeks, attached to the nipple. The offspring are eventually able to leave the marsupium for short periods, returning to it for warmth, protection, and nourishment.
An early birth removes a developing marsupial from its mother's body much sooner than in placental mammals, thus marsupials have not developed a complex placenta to protect the embryo from its mother's immune system. Though early birth puts the tiny newborn marsupial at a greater environmental risk, it significantly reduces the dangers associated with long pregnancies, as there is no need to carry a large fetus to full term in bad seasons.
Marsupials are extremely altricial animals, needing to be intensely cared for immediately following birth cf. Because newborn marsupials must climb up to their mother's nipples, their front limbs and facial structures are much more developed than the rest of their bodies at the time of birth. Marsupials must develop grasping forepaws during their early youth, making the evolutive transition from these limbs into hooves , wings , or flippers , as some groups of placental mammals have done, more difficult.
However, several marsupials do possess atypical forelimb morphologies, such as the hooved forelimbs of the pig-footed bandicoot , suggesting that the range of forelimb specialization is not as limited as assumed. An infant marsupial is known as a joey.
Marsupials have a very short gestation period about four to five weeks , and the joey is born in an essentially fetal state. The blind, furless, miniature newborn, the size of a jelly bean ,  crawls across its mother's fur to make its way into the pouch , where it latches onto a teat for food. It will not re-emerge for several months, during which time it develops fully. After this period, the joey begins to spend increasing lengths of time out of the pouch, feeding and learning survival skills.
However, it returns to the pouch to sleep, and if danger threatens, it will seek refuge in its mother's pouch for safety. Joeys stay in the pouch for up to a year in some species, or until the next joey is born. A marsupial joey is unable to regulate its own body temperature and relies upon an external heat source.
Joeys are born with "oral shields". In species without pouches or with rudimentary pouches these are more developed than in forms with well-developed pouches, implying a role in maintaining the young attached to the mother's nipple.
The first American marsupial the Europeans encountered was the common opossum. He presented them to the Spanish monarchs, though by then the young were lost and the female had died.
The animal was noted for its strange pouch or "second belly", and how the offspring reached the pouch was a mystery.
On the other hand, it was the Portuguese who first described Australian marsupials. Some animals resemble ferrets, only a little bigger. They are called Kusus. They have a long tail with which they hang from the trees in which they live continuously, winding it once or twice around a branch.
On their belly they have a pocket like an intermediate balcony; as soon as they give birth to a young one they grow it inside there at a nipple until it does not need nursing anymore. As soon as she has borne and nourished it, the mother becomes pregnant again. From the start of the 17th century more accounts of marsupials arrived.
For instance, a record of an animal, killed on the southern coast of New Guinea, described it as "in the shape of a dog, smaller than a greyhound", with a snakelike "bare scaly tail" and hanging testicles. The meat tasted like venison , and the stomach contained ginger leaves. This description appears to closely resemble the dusky pademelon Thylogale brunii , in which case this would be the earliest European record of a member of the kangaroo family Macropodidae. The relationships among the three extant divisions of mammals monotremes , marsupials, and placentals were long a matter of debate among taxonomists.
The ancestors of marsupials, part of a larger group called metatherians , probably split from those of placental mammals eutherians during the mid- Jurassic period, though no fossil evidence of metatherians themselves are known from this time. The oldest metatherian fossils are found in present-day China. From there, metatherians spread westward into modern North America still attached to Eurasia , where the earliest true marsupials are found.
Marsupials are difficult to distinguish from other fossils, as they are characterized by aspects of the reproductive system which do not normally fossilize including pouches and by subtle changes in the bone and tooth structure that show a metatherian is part of the marsupial crown group the most exclusive group that contains all living marsupials.
The earliest definite marsupial fossil belongs to the species Peradectes minor , from the Paleocene of Montana , dated to about 65 million years ago. Marsurpials, Peradectes and the related Herpetotheriidae are nested within a clade of metatherians that also included a variety of Cretaceous North American taxa.
In South America, the opossums evolved and developed a strong presence, and the Paleogene also saw the evolution of shrew opossums Paucituberculata alongside non-marsupial metatherian predators such as the borhyaenids and the saber-toothed Thylacosmilus.
South American niches for mammalian carnivores were dominated by these marsupial and sparassodont metatherians. While placental predators were absent, the metatherians did have to contend with avian terror bird and terrestrial crocodylomorph competition. South America and Antarctica remained connected until 35 mya, as shown by the unique fossils found there. North and South America were disconnected until about three million years ago, when the Isthmus of Panama formed.
This led to the Great American Interchange. Sparassodonts disappeared for unclear reasons — again, this has classically assumed as competition from carnivoran placentals, but the last sparassodonts co-existed with a few small carnivorans like procyonids and canines, and disappeared long before the arrival of macropredatory forms like felines,  while didelphimorphs opossums invaded Central America, with the Virginia opossum reaching as far north as Canada.
Marsupials reached Australia via Antarctica about 50 mya, shortly after Australia had split off. This suggests a single dispersion event of just one species, most likely a relative to South America's monito del monte a microbiothere , the only New World australidelphian.
This progenitor may have rafted across the widening, but still narrow, gap between Australia and Antarctica. In Australia, they radiated into the wide variety seen today. Modern marsupials appear to have reached the islands of New Guinea and Sulawesi relatively recently via Australia.
The branching sequence of marsupial orders indicated by the study puts Didelphimorphia in the most basal position, followed by Paucituberculata, then Microbiotheria, and ending with the radiation of Australian marsupials.
This indicates that Australidelphia arose in South America, and reached Australia after Microbiotheria split off. In Australia, terrestrial placental mammals disappeared early in the Cenozoic their most recent known fossils being 55 million-year-old teeth resembling those of condylarths for reasons that are not clear, allowing marsupials to dominate the Australian ecosystem.
A new hypothesis suggests that South American microbiotheres resulted from a back-dispersal from eastern Gondwana due to new cranial and post-cranial marsupial fossils from the Djarthia murgonensis from the early Eocene Tingamarra Local Fauna in Australia that indicate the Djarthia murgonensis is the most plesiomorphic, the oldest unequivocal australidelphian, and may be the ancestral morphotype of the Australian marsupial radiation.
From Wikipedia, the free encyclopedia. This article is about the mammals. For frogs, see Marsupial frog. This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed.
September Learn how and when to remove this template message. The sugar glider , a marsupial, left and flying squirrel , a rodent , right are examples of convergent evolution. This section may contain content that is repetitive or redundant of text elsewhere in the article.
These pathways are essentially prokaryotic and represent excellent drug targets. A photosynthetic alveolate, Chromera velia , that appears to be the earliest branching apicomplexan has also been identified 3.
The coccidia are characterized by a thick walled oocyst stage that is typically excreted with the feces. Some coccidia Cryptosporidium, Cyclospora, Isospora carry out their entire life cycle within the intestinal epithelial cells of the host and are transmitted by the fecal-oral route. Other coccidia Sarcocystis, Toxoplasma have a more complicated life cycle involving tissue cysts and multiple hosts ie, heteroxenous.
Since its initial identification in several Cryptosporidium species have been identified in a wide variety of animals ranging from fish to humans. The first human cases of cryptosporidiosis were reported in and were characterized as a diarrheal disease associated with immune suppression. Initially it was believed to be a rare and exotic disease.
However, it is now recognized that Cryptosporidium is a common cause of diarrhea in immunocompetent persons and has probably been a human pathogen since the beginning of humanity. Two species infecting humans have been identified: Cryptosporidium is often classified as a coccidian and exhibits a life cycle similar to other intestinal coccidia.
However, Cryptosporidium is more closely related to the gregarines and this is reflected in some aspects of its life cycle. The infection is acquired through the ingestion of sporulated oocysts Figure. Sporozoites Sz emerge from the oocyst and attach to intestinal epithelial cells. In contrast to other coccidia, Cryptosporidium sporozoites do not invade the enterocytes. Instead they induce the fusion and expansion of the microvilli resulting in the parasite becoming surrounded by a double membrane of host origin.
A junction, called the 'feeder organelle' or the 'adhesion zone', forms between the parasite and the host enterocyte. The parasite, now called a trophozoite Tr , likely derives nutrients from the host cell via this junction. For a review on the 'invasion' process see Borowski et al, Trophozoites undergo an asexual replication ie, merogony and produce merozoites Mz which are released into the intestinal lumen.
The merozoites infect new intestinal epithelial cells and undergo additional rounds of merogony. The increased severity of the disease in immunocompromised patients is due in part to their inability to limit these additional rounds of merogony.
As an alternative to merogony, the merozoites can develop into either macro- or microgametocytes following the infection of an enterocyte. Microgametogenesis involves several rounds of replication followed by the release of numerous microgametes into the intestinal lumen. The microgametes fertilize macrogametes still attached to the intestinal epithelial cells. The resulting zygote Zg undergoes sporogony and the sporulated oocysts Oo are excreted with the feces.
An autoinfection is also possible and this too may contribute to the increased disease severity in immunocompromised patients. The risk factors of transmission for Cryptosporidium are similar to other fecal-oral diseases.
However, waterborne cryptosporidiosis outbreaks have been especially notable. The most infamous is an outbreak in Milwaukee during the spring of in which an estimated , people developed symptomatic cryptosporidiosis MacKenzie et al, New Eng.
Factors that contribute to the increased risks of Cryptosporidium waterborne outbreaks are:. Despite the impressiveness of some waterborne outbreaks, human-to-human transmission appears to predominate. For example, asymptomatic infected children are common, secondary cases in households are high, and outbreaks tend to occur in hospitals, institutions and day care centers--situations typical for fecal-oral transmission.
Molecular studies have revealed two primary genotypes isolated from humans. Genotype 1 has only been isolated from human sources and is non-infective for mice and calves. Genotype 2 has been isolated from both animal bovine and ovine and human sources and is infective for mice and calves. Based upon these and other biological differences it has been proposed to rename genotype 1 as Cryptosporidium hominis Morgan-Ryan et al, J.
Other species and genotypes of Cryptosporidium eg. A third species from the Indian subcontinent, C. Genetic data imply that there are two distinct transmission cycles in humans involving two different populations of Cryptosporidium: The zoonotic cycle would initially involve transmission from animals eg, cattle or sheep to humans and then subsequently human-to-human transmission and possibly a human-to-animal transmission.
Both genotypes have been demonstrated to be the etiological agent in waterbourne outbreaks. Waterborne outbreaks linked to C.
The most common clinical manifestation of cryptosporidiosis is a mild to profuse watery diarrhea. This diarrhea is generally self-limiting and persists from several days up to one month. Abdominal cramps, anorexia, nausea, weight loss and vomiting are additional manifestations which may occur during the acute stage.
The disease can be much more severe for persons with AIDS which manifests as a chronic diarrhea lasting for months or even years. Some AIDS patients exhibit a fulminant cholera-like illness which requires intravenous rehydration therapy.
The fatality rate can be quite high in these fulminant cases. Diarrhea can have osmotic, inflammatory, or secretory components see Box. The watery nature of the diarrhea associated with Cryptosporidium infections has suggested the presence of an enterotoxin.
However, there is no evidence for a toxin-mediated secretory diarrhea despite efforts to identify such a toxin. Therefore, the diarrhea associated with Cryptosporidium appears to be primarily osmotic in nature see Figure. Associated with this disruption of enterocyte i.
A possible mechanism of pathogenesis is that the infection of intestinal epithelial cells with Cryptosporidium damages the enterocytes and eventually leads to their death. This triggers cell division in the crypt region i. The combination of destruction of absorbtive cells at the tips of the villi and the increase in the Cl - -secreting crypt leads to an overall enhanced secretion. In addition, an increased intercellular permeability and inflammation in the submucosal layer aka, lamina propria has been associated with Cryptosporidium infection.
These phenomenon could also contribute to the secretory process via cytokines and neurohormones. These products may then promote secretion and impair absorption. The parasite exhibits a trophism for the jejunum and ileum in immunocompetent persons, whereas the infection is more wide spread in AIDS patients and can include the stomach, duodenum, colon and biliary tract. This more extensive anatomical range in AIDS patients is presumably due to the inability of the immune system to control and limit the infection.
Interferon-gamma, interleukin, and tumor necrosis factor-alpha are involved in protection against Cryptosporidium infection.
Isospora belli is believed to be a valid species which only infects humans. It has a worldwide distribution but is more common in tropical regions and areas with poor sanitation. Infections are often asymptomatic and those with symptoms tend to be self limiting with a duration of a few weeks. Infections are more common and the symptoms more severe in AIDS patients that in immunocompetent persons. The infection is acquired through the ingestion of sporulated oocysts sOo.
Sporozoites Sz are release in the intestinal lumen and invade intestinal epithelial cells. Within the epithelial cells the parasite undergoes a round of merogony leading to the production of merozoites Mz. The released merozoites reinvade intestinal epithelial cells and can undergo additional rounds of merogony or develop into either micro- or macrogamonts.
Microgametes ga will fertilize the macrogametes ga to form a zygote Zy which develops into the oocyst Oo. Immature oocysts are passed in the feces and maturation into infectious sporulated oocysts occurs in the environment. Recognizable stages during this maturation ie, sporogony include oocysts with a single sporoblast, oocysts with two sporoblasts, and the mature oocyst with two sporocysts, each of which contains four sporozoites.
Symptoms associated with I. Blood is rarely present in the feces. In general, the symptoms are similar to those of cryptosporidiosis. The disease is often self-limiting. However, it can become chronic with oocysts being detected in the feces for months to years and recrudescences of the symptoms. The disease tends to be more severe in infants and young children than adults. Pathology associated with I. The diarrhea in AIDS patients is often very watery and can lead to dehydration requiring hospitalization.
Fever and weight loss are also a common finding. Another common finding among AIDS patients is a chronic intermittent diarrhea lasting for months to years.
The resulting excessive weight loss and electrolyte imbalance can lead to wasting and even death. There have also been a few reports of disseminated extraintestinal isosporiasis in AIDS patients.
The first human cases of Cyclospora cayetanensis were reported in It was originally referred to as cyanobacteria-like bodies or coccidian-like bodies CLB. The organism was confirmed to be a coccidian parasite with an oocyst structure similar to the genus Cyclospora and then named in after the Universidad Peruana Cayetano Heredia in Peru where most of the early studies had been carried out.
Molecular studies indicate a close relationship to Eimeria, an important veterinary parasite of poultry and other livestock. Life cycle and transmission. The life cycle of Cylcospora is similar to Isospora see above.
The infection is acquired through the ingestion of oocysts. Sprorozoites are released and infect epithelial cells of the upper small intestine. The parasite undergoes merogony and the merozoites reinfect enterocytes and several more rounds of merogony can occur. Some of the merozoites undergo a sexual development resulting in the production of micro- and macrogametes. Fertilization of the macrogamete by the microgamete initiates sporogony and the formation of the oocyst.
Like Isospora , sporulation is completed in the environment and immature non-infectious oocysts are excreted in the feces. The maturation of the oocysts to sporulated infectious oocysts probably takes days to weeks. In addition, the structure of the Cyclospora oocyst is different from that of Isospora. The oocyst contains two sporocysts which each contain two sporozoites.
Several outbreaks in the United States and Canada have been associated with fresh produce imported from South and Central America Table. In particular, berries and leafy vegetables have been identified as the probable contaminated item. These are food items that are typically eaten raw and are only rinsed. No outbreaks have been associated with frozen, processed, or peeled fruits or vegetables. A seasonality in the outbreaks has also been observed with most of the cases occurring in the spring and early summer.
A similar seasonality has been observed in endemic countries also. In contrast to the United States and Canada, where foodborne transmission dominates, the majority of the cases in Europe and Australia have been associated with travel to endemic countries. As a result of the high number of outbreaks associated with raspberries from Guatemala the United States restricted the import of raspberries and required inspection of farms.
This resulted in a subsequent drop in the number of outbreaks in the United States. Canada, which did not restrict importations, did not experience a drop in the number of outbreaks during this period. Subsequent case controlled studies carried out in Guatemala revealed that infections were most common in children with the prevalence peaking in June. The major risk factor associated with the infection was drinking untreated water. In Peru contact with soil was identified as another risk factor, especially among children less than two years old.
Inadequate water treatment in these endemic countries may lead to contamination of the ground water and thus maintain the transmission cycle. Presumably foodborne transmission is due to irrigation or applying fertilizer with contaminated water or washing and processing foods with poorly treated water. Cyclospora primarily infects epithelial cells in the upper portion of the small intestine. The incubation period is generally one-two weeks.
Symptoms are similar to the gastroenteritis caused by Isospora and Cryptosporidium which typically includes cycles of watery diarrhea and periods of apparent remission. The diarrhea is characterized by frequent stools and can persist for up to six weeks, but is generally self-limiting in immunocompent persons. Anorexia, malaise, nausea and cramping are other frequent symptoms associated with cyclosporiasis.
In some cases patients may experience vomiting, muscle aches, substantial weight loss, and explosive diarrhea. Previous exposure to Cyclospora appears to confer some resistance to infection with a lessening of the symptoms. Over time adults appear to develop immunity and asymptomatic carriers can be found in endemic areas. As is also the case for Cryptosporidium and Isospora , the diarrhea caused by Cyclospora in AIDS patients is much more severe than in immunocompetent persons.
The diarrhea can last for months and produce a syndrome that is debilitating and life threatening. Sterling and Ortega Clycospora: BL Herwaldt Cyclospora cayetanensis: A review focusing on the outbreaks of clyclosporiasis in the s. International Journal for Parasitology 33, Coccidiosis is diagnosed by demonstrating oocysts in the feces. Acid-fast staining is the preferred method for coccidia which stain bright red.
Cryptosporidium, Cyclospora, and Isospora are distinguished by size and oocyst structure Table. Cyclospora and Isospora do not uniformly take up the stain resulting in a mixture of unstained, partially stained and completely stained oocysts.
Cyclospora and Isospora can also be detected via an autofluorescence associated with the cyst wall. Because of its relatively large size, Isospora is readily detected in unstained specimens. Sarcocystis is a rare human infection see below with oocysts similar to Isospora except that the sporocysts are generally released from the oocysts while still in the intestinal lumen. The recommended treatment for Cyclospora and Isospora is the combination of trimethoprim-sulfamethoxazole Bactrim.
There is no completely satisfactory treatment for Cryptosporidium. It is hypothesized that the 'extracytoplasmic' location of Cryptosporidium shelters it from drugs. Paromomycin has been used for the treatment of cryptosporidiosis, however, its efficacy is debated.
Controlled studies indicate that paromomycin modestly suppresses parasitemia in immunocompromised individuals. Treatment of severe cryptosporidiosis should include supportive care rehydration and nutritional support and anti-motility agents.
Preventive measures will be similar to other diseases transmitted by the fecal-oral route see risk factors or Giardia control. Some coccidian species exhibit a heteroxenous life cycle in which merogony takes place in tissues of the intermediate host prey and gametogony takes place in the intestinal epithelium of the definitive host predator.
The life cycle see Figure of Sarcocystis within the predator ie, carnivore is similar to the life cycles of intestinal coccidia, such as Isospora , involving a sexual cycle gametogony within the intestinal epithelial cells. One difference in the Sarcocystis life cycle is the lack of merogony in the intestinal epithelial cells. In other words, the merozoites acquired by ingesting an infected prey will only produce gametes following the invasion of intestinal epithelial cells.
Fusion of the gametes leads to the production of oocysts. In addition, the sporocysts are usually released from the oocysts within the intestine of the host and therefore infectious sporocysts are found in the feces. The intermediate hosts herbivore acquire the infection by ingesting the sporulated sporocysts. Sporozoites are released, invade intestinal epithelial cells, and undergo merogony as is typical of intestinal coccidia.
In contrast to intestinal coccidia, the merozoites will invade endothelial cells and produce a systemic infection. Quite often there is a tropism for particular tissues such as brain or muscle. The meronts or schizonts in these tissues are often often encapsulated and referred to as 'tissue cysts'.
These tissue cysts, or sarcocysts in the case of muscle, often exhibit a lower level of replication during merogony and are somewhat dormant. Ingestion of the infected animal by a carnivore will release the merozoites which will invade intestinal epitheial cells and thus completing the life cycle. This predator-prey life cycle was not worked out until the 's. Previously, the intestinal infections in the predator were usually designated as Isospora species and the tissue infections in the prey were usually designated as Sarcocystis species.
In many cases the definitive hosts have not been positively identified and the taxonomy of many Sarcocystis species is uncertain.
Sarcocystis infections in humans have been documented, but are rare. Humans are the definitive host for S. Ingestion of undercooked beef or pork from infected animals will produce an enteric infection which can produce acute intestinal symptoms abdominal discomfort, nausea, diarrhea. However, most infections are believed to be asymptomatic. Infected individuals can shed sporocysts in the feces for weeks to months following the infection.
Sporocysts from human feces are infective to cows, pigs and deer. Humans can also serve as the intermediate host to at least some of the Sarcocystis species found in nature. Ingestion of the sporocysts by humans can result in the tissue stage of the infection and the formation of sarcocysts. Clinical symptoms can include muscle tenderness or episodic painful inflammatory swellings. One study noted that the sarcocysts in humans tended to resemble Sarcocystis species commonly found in local monkeys Beaver et al, Am.
Most of the cases have been reported from tropical and subtropical Asia, including an outbreak of muscular sarcocystosis among travelers returning from Malaysia MMWR Fayer, R Sarcocystis spp. Toxoplasma gondii is a coccidian parasite which infects humans as well as a wide variety of mammals and birds. It exhibits a predator-prey type life cycle as discussed above for Sarcocystis and felines are the only definitive host. Toxoplasmosis is found throughout the world except extremely cold or dry climates and tends to be more prevalent in tropical climates.
In the United States an estimated 0. Toxoplasmosis is most often a benign disease. Noted exceptions are in the cases of congenital infection or immunocompromised individuals.
Toxoplasma has a complex life cycle consisting of intestinal and tissue phases. Although the organism was first discovered in as a tissue parasite of the gondi an African rodent , its complete life cycle was not determined until The intestinal phase of the infection occurs only in felines and exhibits a typical intestinal coccidian life cycle consisting of merogony and gamogony see Isospora life cycle. Cats acquire the infection by eating animals infected with the tissue stage of the parasite.
The parasites invade intestinal epithelial cells and undergo merogony. The resulting merozoites can then either undergo additional rounds of merogony or undergo gametogony. As is similar to other apicomplexa see general apicomplexan life cycle macro- and microgametes are produced. Thus, the cat is considered the definitive host since this is the host in which the sexual cycle occurs.
The bi-flagellated microgametes are released into the lumen of the intestine and fertilize the macrogametes within the host epithelial cells. Secretion of the oocyst wall begins shortly after fertilization. This sexual cycle culminates in the production of oocysts which are excreted in the feces.
These immature oocysts undergo sporogony at ambient temperature resulting in mature oocysts containing two sporocysts, each with four sporozoites. Sporulation generally takes days and the oocysts remain infective for months in shaded moist soil.
It has been hypothesized that unfertilized macrogametes may also be capable of forming mature oocysts Ferguson, Tr. Intermediate hosts, such as rodents and birds, become infected through the ingestion of sporulated oocysts.
Sporozoites are released, penetrate the intestinal epithelium, and invade macrophages and other types of cells. The invasion process is typical of apicomplexa and the parasite lies within a parasitophorous vacuole. Within the vacuole the parasite undergoes binary fission i. Endodyogeny is a specialized type of division in which the two daughter cells form within the mother cell. These trophic forms are called tachyzoites tachy means rapid in reference to their high level of replication.
The host cell will rupture and release the tachyzoites which will invade new host cells and repeat the replicative cycle. Infected macrophages will disseminate the tachyzoites throughout the host during this acute infection. As the host develops immunity the replication rate will slow and the infected host cells will become encapsulated ie, tissue cysts. These slowly replicating forms are called bradyzoites brady means slow and represent a dormant or resting stage.
Bradyzoites are considered to be metabolically quiescent, but remain viable Dubey et al, Clin. Other changes that occur when tachyzoites convert to bradyzoites include secretion of chitin and other components to form a cyst wall and the accumulation of amylopectin granules reflecting glucose storage. The bradyzoites are primarily found in brain and muscle tissue, whereas the tachyzoites tend to be in reticuloendothelial cells. The tissue phase of the infection can also be transmitted congenitally to offspring and to other intermediate hosts through carnivorism.
Ingestion of an infected animal will release the bradyzoites from the tissue cysts which then infect cells in the new host. Possibly all mammals, including humans, can become infected with Toxoplasma. As in the case of acquiring the infection through the ingestion of oocysts, the parasites will go through an acute phase characterized by rapid replication followed by a chronic phase characterized by dormant tissue cysts.
Ingestion of an infected intermediate host by the cat will initiate the intestinal stage of the life cycle involving merogony and gamogony in the intestinal epithial cells. Cats can also support the tissue stage of the infection. Obviously humans are not a natural part of the predator-prey life cycle and represent an accidental host that does not participate in the continuation of the transmission cycle. One source of infection is the ingestion of material contaminated with sporulated oocysts excreted by cats.
This implies some association with cats. However, since the oocysts need to mature in the environment before becoming infectious, transmission will include features of soil transmission similar to Isospora and Cyclospora. For example, children of crawling and dirt-eating ages are believed to be at higher risk for infection.
Oocysts can also be acquired through gardening activities or unwashed fruits or vegetables. In addition a few water-borne outbreaks have been documented. The high prevalence of toxoplasmosis in South and Central America is believed to be due to high levels of contamination of the environment with oocysts.
Ironically, contact with dogs is more of a risk factor for becoming infected with Toxoplasma than contact with cats.