FIGURE 2.7 Life-reproductive cycle of a virus as illustrated by a T-even bacteriophage infecting a bacterial cell: (a) dormancy, (b) adsorption, (c) penetration, (d) replication of new proteins and nucleic acids, (e) maturation, and (f) release and bursting of hold cell.
Protozoa move through the use of the pseudopod or the use of the cilia and flagella. In pseudopodic movement, the ectoplasm flows inside the cell toward the direction of motion. As the mass of ectoplasm moves, the rear of the body is pulled forward. The flagellum is a whiplike prolongation of the protoplasm. It propels the organism forward by a lashing action. The cilium has a function similar to the flagellum except that it is more numerous, delicate, and shorter. Cilia are attached to the whole outer surface of the body forming a rimlike appearance.
One class of protozoa called Telosporidea does not have any organ of locomotion. The organisms belonging to this class live within cells, tissues, cavities, and fluids of the body. The Plasmodium, causing malaria, belongs to this class.
When adverse conditions prevail, the protozoa become inactive, transforming into a rounded form, and eventually developing a protective cell wall. Under the safety inside the cell wall, the organism may live for a long time and resist any destructive insult from the environment. This transformation is called a cyst.
Protozoa is capable of sexual and asexual reproduction. Some may have sexual reproduction in one host and asexual in another. Cells capable of sexual reproduction are called gametes. The cell formed by the union of two gametes is called zygote. Asexual reproduction is achieved by lengthwise or crosswise division of the cell as in the amebas and flagellates. Figure 2.8 shows sketches of some protozoa illustrating the vegetative and cyst forms.
Giardia lamblia. Giardia lamblia is a flagellate parasitic protozoan. This parasite is largely confined to the lining of the intestine. It can colonize the lining and feed and grow there. It is shed in feces in the form of cysts. The cyst, however, cannot multiply outside the host.
The vegetative form of G. lamblia is called a trophozoite. It is this form that lives and colonizes the lining of the intestine; in particular, the upper small intestine, the parasites attach to the intestinal wall by means of a disc-shaped suction pad on their ventral surface. It is here that the trophozoites actively feed and reproduce. At some time during the trophozoite's life, it releases its hold on the bowel wall and floats in the fecal stream through the intestine. As it makes its journey, it undergoes a morphologic transformation into an egg-like structure, the cyst, mentioned above. Figure 2.9 shows a picture of the trophozoites using a scanning
electron microscopy that reveals the parasite in the small intestine of an animal host. Figure 2.10 shows another picture of the parasite in the trophozoite stage.
Giardia cysts can be found in water, raw vegetables, and feces of contaminated individuals. Where adequate sanitation cannot be maintained, the cysts can also be found in institutions and day-care centers. The beaver has gained attention as a potential source of Giardia contamination of lakes, reservoirs, and streams;
however, human fecal wastes are equally important. Horses have also been implicated as sources of these pathogens. In addition, muskrats have been found to have high infection rates (30 to 40%) (Frost and Liechty, 1980). Studies have shown that they can be infected from cysts obtained from humans and beavers. Occasionally, coyotes, deer, elk, cattle, dogs, and cats have also been found infected. Because all these animals, including humans, can come in contact with surface waters, community water supplies that draw water from impounding reservoirs are a potential problem for diseases caused by the cyst.
The pathogen can cause a disease called giardiasis. Its symptoms include diarrhea, abdominal cramps, fatigue, weight loss, gas, anorexia, and nausea and may persist for 2 to 3 months if untreated. The diarrhea experienced may be mild or severe. Occasionally, some will have chronic diarrhea over several weeks or months, with significant weight loss. Cases may occur sporadically or in clusters or outbreaks. Upon exposure to the cyst, symptoms may appear within 5 to 25 days but usually within 10 days. As a method of treatment, doctors often prescribe antibiotics such as atabrine, metronidazole, or furizolidone to treat giardiasis. Some individuals, however, may recover on their own without medication; fever is rarely present.
The disease is contracted through the following means: The parasite is passed in the feces of an infected person or the feces of animals mentioned previously. The waste may then contaminate water or food. Or, person-to-person transmission may also occur as in day care centers or other settings where handwashing practices are poor.
Although the ingestion of only one Giardia cyst is theoretically possible to cause the disease, the minimum number of cysts that can show the symptoms is found to be ten (Rendtorff, 1954). Every 12 hours, trophozoites divide by binary fission. This means that if a person swallowed only a single cyst, reproduction at this rate would result in more than 1 million parasites 10 days later, and 1 billion parasites 15 days later, using the binary-division equation explained earlier for the case of bacteria. From this arithmetic, it would seem that one cyst is enough to cause the disease. The exact mechanism by which Giardia causes illness is not yet well understood, however. From this simple calculation, it can be gleaned that contracting the disease is not necessarily related to the number of organisms present. Nearly all of the symptoms, however, are related to dysfunction of the gastrointestinal tract. Also, the parasite rarely invades other parts of the body, such as the gall bladder or pancreatic ducts. In addition, intestinal infection does not result in permanent damage.
The diagnosis of giardiasis is posed by observation of cysts and/or trophozoites in feces or duodenal aspirates. Because of the irregular shedding of parasites, multiple examinations are often required to diagnose the infection.
Measures to prevent transmission include proper disposal of feces, wastewater treatment, and a filtration step before chlorination in water treatment plants that draw from surface water sources. Cooking kills the cysts in contaminated foods and boiling will make water safe for use. For backpackers who walk through the wilderness, iodine has been shown to be a better disinfectant for giardia cyst than chlorine. It should be emphasized that of all the methods of preventing contamination from the cyst, a properly designed and operated water filtration plant is the best line of defense in drinking water supplies.
Analyzing water for Giardia involves collecting the cyst through filtration, purifying the collected cyst, detecting, and identifying. The cyst may be identified using a microscope with or without staining. Staining may be accomplished using a method called fluorogenic staining. In this method, live cysts are distinguished by two fluorescent dyes. One dye is fluorescein diacetate (FDA). When absorbed by cysts, FDA produces a fluorescent green only in live cysts. The second dye, either propidium iodide (PI) or ethidium bromide (EB), is excluded efficiently by live cysts but absorbed by dead cysts, resulting in red fluorescence.
Cryptosporidium parvum. Cryptosporidium parvum is a single-celled protozoan parasite that infects a wide variety of animals, including humans. It has a life cycle transmission where it can exist as an infective oocyst, usually 5-8 fim in diameter. When ingested, oocysts pass through the stomach into the small intestine where sporozoites are released. Sporozoites invade and penetrate the intestinal epithelial cell membrane and develop into merozoites, impairing the ability of the intestine to absorb water and nutrients. Microgametes and macrogametes are then formed. The former fertilizes the latter and zygotes result. Most of the zygotes pass through the host as very infective, environmentally hardy, thick walled oocysts. These oocysts, if finding a suitable host, release sporozoites, again, completing the cycle. Figures 2.11 and 2.12 show pictures of the oocysts, where the latter shows the parasite using a staining technique.
The oocysts can be found in water, raw vegetables, and feces of contaminated individuals. Where adequate sanitation cannot be maintained, the cysts can also be found in institutions and day care centers. Indeed, the occurrence of the organism is widespread and can be found in both domestic and farm animals, such as cattle, dogs and cats and turkeys; it can also be found in wild animals. The incidence of infection in cattle and sheep is especially high. All these animals when infected excrete vast numbers of oocysts, which may survive in water for up to one year. Because all
FIGURE 2.12 Cryptosporidium oocysts shown by staining.
FIGURE 2.12 Cryptosporidium oocysts shown by staining.
these animals, including humans, can come in contact with surface waters, as in the case of the Giardia, community water supplies that draw water from impounding reservoirs are a potential problem for diseases caused by the oocyst.
The pathogen can cause a disease called cryptosporidiosis. The symptoms may appear within 4 to 6 days, but may appear anytime from 2 to 10 days after infection. While some persons may not have symptoms, others have watery diarrhea, headache, abdominal cramps, nausea, stomach pain, vomiting, and low)grade, flu)like fever. As in giardiasis, these symptoms may lead to weight loss and dehydration. In healthy persons, these symptoms usually last for several days but rarely more than 2 weeks, at which time the immune system is able to stop the infection. In persons with suppressed immune systems, such as persons with cancer and AIDS and persons who have recently had an organ or bone marrow transplant, the infection may continue and become life)threatening. No known cure or effective treatment is available for Cryptosporidium infection, although an experimental drug called NTZ shows promise. An infected person will simply just recover as soon as the immune system stops the infection.
Cryptosporidiosis is contracted in practically the same way as giardiasis. That is, the parasite is passed in the feces of an infected person or the feces of animals. The waste may then contaminate water or food. Direct person-to-person transmission may also occur in day)care centers or other settings where handwashing practices are poor. It is estimated that as few as one to 10 oocysts constitute an infective dose. In 1993, waterborne Cryptosporidium sickened 403,000 Milwaukee residents and killed more than 100 people, many with HIV/AIDS.
The diagnosis of cryptosporidiosis is posed by observation of the oocysts in the feces of infected individuals. Routine stool examination used for most parasites usually fails to detect Cryptosporidium, however. Thus, a stool specimen is examined using staining techniques available for this parasite. See Figure 2.12.
As in giardiasis, measures to prevent transmission include proper disposal of feces, wastewater treatment, and a filtration step before chlorination in water treatment plants that draw from surface water sources. Cooking kills the oocysts in contaminated foods and boiling for at least one minute will make water safe for use. Heating to 160°F is known to kill the oocysts. Chlorination is ineffective in killing the parasites. For municipal water systems, the most practical and effective way of getting rid of the parasite is the use of a properly operated and maintained filtration plant. The turbidity of the effluent should be brought down to around 0.1 NTU.
Analysis of water for Cryptosporidium uses similar techniques as those used for the analysis of Giardia. It involves filtering large quantities of water through a one-micron filter, removing the trapped particles, extracting the oocysts, and refiltering the extract. The sample is then analyzed using an indirect fluorescent antibody procedure that relies heavily on the skill of an experienced microscopist. At this time, there is no accepted standard method for Cryptosporidium and the oocyst recovery efficiency and sensitivity are variable.
Entamoeba histolytica. Entamoeba histolytica is a single celled parasitic ameba, a protozoan that infects predominantly humans and other primates all around the world. Dogs and cats can become infected but they usually do not shed cysts with their feces, thus do not contribute significantly to transmission. The infectious trophozoite stage exists only in the host and in fresh feces; cysts survive outside the host in water and moist soils and on foods. The ameba are cytotoxic when in contact with human cells. With a rapid influx of calcium into the contacted cell, all membrane movement stops, save for some surface blebbing. Internal organization is disrupted, organelles lyse, and the cell dies. The ameba may eat the dead cell directly or absorb nutrients released from the cell. Following malaria and schistosomiasis, Entamoeba histolytica is the third leading cause of morbidity and mortality due to parasitic disease in humans and is estimated to be responsible for infecting one tenth of the world population. Figure 2.13 shows the life cycle of the parasite. Figures 2.14 and 2.15 show pictures of the tropho-zoites, the latter being shown in a section of an intestine.
As depicted in its life cycle in Figure 2.13, the sources of the parasites are the infected humans and other primates. Anyone can harbor this parasite, but it is more frequently found in individuals who live or have visited areas of the world with poor sanitation. It is also more commonly found in individuals living in institutions for the developmentally disabled and in nursing care institutions with infected residents.
Conceptually, the ingestion of only one viable cyst can cause an infection. When swallowed, it causes infections by excysting to the trophozoite stage in the digestive tract and causes the disease called amebiasis. An individual with amebiasis may have no symptoms, or may have a range of symptoms from mild to severe. Mild symptoms, which is the most common and may last for years, include abdominal discomfort and diarrhea that may include blood or mucus alternating with periods of constipation or remission. Other symptoms, such as nausea, weight loss, fever, and chills may also occur. Rarely does the parasite invade other organs of the body such as the liver, lung, and brain. If it does invade these areas, it may cause abscesses there. The symptoms usually appear within 2 to 4 weeks after swallowing the cyst, but they may appear as quickly as a few days, or take as long as several months or even years. Several antibiotics are available through prescription by physicians to treat amebiasis. No over-the-counter medications are available to cure this disease.
As Figure 2.13 shows, the cysts are excreted in feces that may survive in moist environments, which then, again, become available to infect another individual. Thus, the disease is contracted in practically the same way as giardiasis and cryptosporidiosis
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