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Parasitology

Parasitology

Parasitology is the study of parasites, organisms that live, grow, and feed on or in other organisms. The prevention of parasite-infested consumption of raw (or undercooked) meat, fish, seafood, vegetables, and dairy products, as well as contaminated water, is a matter of public health. The United States Food and Drug Administration (FDA ), the Centers for Disease Control (CDC ), and several other local and state sanitary agencies are responsible for regulatory food safety measures and regular inspections of food and water quality to prevent the outbreak of epidemics caused by parasites and other pathogens .

When an epidemic outbreak occurs in a city or when several cases of food-related poisoning suddenly happen in an area, forensic pathologists or forensic parasitologists help epidemiologists to identify the source of the problem. For example, in 1980, 32 patients, including four physicians, reported to hospitals in Los Angeles within a short period of time complaining of abdominal distention, diarrhea, intermittent abdominal cramps, and flatulence. They were diagnosed as having been infested by a flatworm, Diphyllobothrium spp., a common parasite in freshwater and sea fish. All patients recalled that they had eaten sushi, a raw fish dish, ten days prior to the onset of symptoms. Alerted by hospitals, the CDC tracked the illness back to sushi made of salmon contaminated with the flatworm.

Another field where parasitology is also important is legal medicine , as some parasitic pathogens (disease-causing organisms) are transmitted through sexual contact and may constitute evidence of crime, especially in cases of child molestation.

Although some pathogenic (disease-causing) bacteria such as Chlamydia and Ricketsia can be thought of as obligate intracellular parasites (i.e., they can only be replicated inside living cells using the host cell's metabolic machinery) the strict definition of parasites refers to protozoa and helminthes or worms, also known as Metazoa. Pathogenic protozoa are unicellular (e.g., single-celled) organisms divided into four groups: Sarcodina (amoebas), Sporozoa (sporozoans), Mastigophora (flagellates), and Ciliata (ciliates). Metazoa or worms classified are divided in two groups, Platyhelminthes or flat worms, such as Trematoda (flukes) and Cestoda (tapeworms), and Nemathelminthes or roundworms. The most commonly occurring parasites in humans can be also grouped according to the areas of the body they infest, such as: 1) the intestinal tract (Giardia lamblia, Entamoeba histolytica, and Cryptosporidium); 2) urogenital tract (flagellate Trichomonas vaginalis ); 3) blood and tissues (flagellates Leishmania and Trypanosoma, protozoans Toxoplasma and Plasmodium).

Giardiasis, or infestation by Giardia lamblia, occurs in two forms: Giardia trophozoites (active Giardia) and cysts (latent, non-mobile Giardia). Water and food contaminated with fecal residues are the main means of transmission, with the cysts developing into Giardia trophozoites in the duodenum (upper part of the stomach). Giardia attaches to the duodenal mucosa where it competes for protein and fat nutrients, causing inflammation, flatulence, foul-smelly diarrhea, intestinal cramps, nausea, anorexia, and associated protein and fatty acid deficiency. Although 50% of the hosts do not present with symptoms, giardiasis is very common among children in daycare centers, and people who camp, hike, or drink unfiltered water directly from streams, with symptoms appearing especially in those with certain immune deficiencies. Giardiasis is an endemic infestation in the United States, affecting about 5% of the population.

Entamoeba histolytica have two life-cycle phases: trophozoites or mobile amoeba and cyst (non mobile) phases. They cause intestinal cramps, dysentery, and liver lesions, being transmitted by ingestion of cysts present in water or uncooked food, as well as through fecal-oral contact in sexual intercourse. Once inside the body, the cysts mature to the trophozoites phase, the active ameba. By causing necrosis (cell and tissue death and decay) of the intestinal epithelium, amebas invade the submucosa layers of the colonic tract and reach circulation, being transported to the liver where they cause systemic hepatic disease and liver abscesses. Approximately 2% of the American population suffers from amebiasis. Other types of amebiasis are rare, such as those caused by Acanthamoeba ssp. and Naegleria fowleri, which are pathogenic free-living amebas transmitted by water inhalation (while swimming) and by air. They can multiply in the tissues of the brain and spinal fluid, causing nerve damage and death if untreated. Naegleria causes primary amoebic meningoencephalitis (PAM) and Acanthamoeba leads to granulomatous amoebic encephalitis (GAE). If untreated, PAM can kill within a week of the onset of symptoms. GAE occurs in patients with immunodeficiencies and leads to death within several weeks to a year after the onset of disease. Both diseases cause eye infections that can lead to blindness. Between 1985 and 1986, 22 cases of amoeba-related ocular lesions were reported to the Centers for Disease Control. Investigators found out that the majority of the cases were associated with poor disinfection of contact lenses and homemade saline solutions.

Cryptosporidium is another pathogen that induces diarrhea, which is more severe in small children, senior patients, and those with immunodeficiencies such as HIV. Transmission is generally under the form of oocysts present in water and may cause collective outbreaks of watery diarrhea with risk of severe dehydration, particularly to those belonging to the more vulnerable groups. Water filtration is the most effective way of preventing both giardiasis and Cryptosporidium-related diarrhea because these two parasites are resistant to water chlorination.

Almost two billion people live in parts of the world where malaria is an endemic (naturally occurring in the environment) disease. Malaria is a parasitic disease caused by four different species of the Plasmodium parasite, and is transmitted by the bite of infected mosquitoes. The worldwide use of pesticides containing DDT greatly reduced the incidence of malaria, but since DDT was found to contain possibly carcinogenic (cancer-causing) chemicals in the late 1960s, its use has declined greatly, and in turn, the incidence of malaria has increased sharply around the world. As of 2005, malaria is estimated to have killed more than 300500 million people over the centuries and still kills an estimated 2.5 million people per year (including 1 million children) in Africa and the world's tropical areas. Many countries in these regions are returning to the use of DDT to control the mosquitoes carrying the parasite that causes malaria.

Trichomonas vaginalis is a sexually transmitted parasite that exists only as trophozoites, causing genital itching and smelly-greenish vaginal secretions as well as urethritis (a burning sensation when urinating). In men, the only symptom is urethritis, although the parasite is transmitted in the prostatic secretions (secretions of the prostate gland). The use of condoms prevents infection. When found in a child, this and other sexually transmitted diseases may suggest a case of child molestation. Some rare cases of trichomoniasis appear to be associated with contact with wet toilet seats.

Toxoplasma gondii, a blood parasite, may be transmitted through the contact with infected feces of cats and other mammals, or by consumption of raw or undercooked meat or contaminated water, causing toxoplasmosis. It can be also transmitted from mother to the fetus, in what is known as congenital toxoplasmosis. Congenital infection favors miscarriage, neonatal mental retardation, or chorioretinitis (inflammation of the choroids portion of the eye), which leads to blindness during childhood. In immunodepressed adults, toxoplasmosis may cause encephalitis, although most of the infected population remains asymptomatic, due to the action of the immune system . However, T. gondii passes from the intestinal tract to other tissues of the body, such as brain, liver, lungs, and eyes, where it remains as cysts for years. As long as the infected individual's immune system is healthy, antibodies and the immune cells will keep the infection at bay, preventing disease progression.

Diarrheal parasites and other pathogens account for 4% of deaths worldwide each year. Periodical tests for these and other parasitic infestations are a valuable preventive measure that can avert serious and unnecessary diseases and even death.

see also Air and water purity; Antibiotics; Antibody; FDA (United States Food and Drug Administration); Hemoglobin; Immune system; Medical examiner.

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parasitology

parasitology The study of small organisms (parasites) living on or in other organisms (hosts), regardless of whether the effect on the hosts is beneficial, neutral, or harmful. The study uses the term ‘parasite’ in a wider sense than is usually associated with parasitism.

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parasitology

parasitology The study of small organisms (parasites) living on or in other organisms (hosts), regardless of whether the effect on the hosts is beneficial, neutral, or harmful. The study uses the term ‘parasite’ in a wider sense than is usually associated with parasitism.

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parasitology

parasitology (pa-ră-sit-ol-ŏji) n. the study and science of parasites.

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parasitology

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