Parasites 2847
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A parasite is an organism that depends on another organism, known as a host, for food and shelter. As an example, tapeworms live in the digestive system of a large variety of animals. The tapeworms have no digestive system of their own, but absorb nutrients through their skin from partially digested food as it passes through the host.

A parasite usually gains all the benefits of this relationship. In contrast, the host may suffer from various diseases, infections, and discomforts as a result of the parasitic attack. In some cases, however, the host may show no signs at all of infection by the parasite.

The life cycle of a typical parasite commonly includes several developmental stages. During these stages, the parasite may go through two or more changes in body structure as it lives and moves through the environment and one or more hosts.

Words to Know

Arthropod: A phylum of organisms characterized by exoskeletons and segmented bodies.

Definitive host: The organism in which a parasite reaches reproductive maturity.

Helminths: A variety of wormlike animals.

Intermediate host: An organism infected by a parasite while the parasite is in a developmental form, not sexually mature.

Nematodes: A type of helminth characterized by long, cylindrical bodies; commonly known as roundworms.

Protozoa: Single-celled animal-like microscopic organisms that must live in the presence of water.

Trematodes: A class of worms characterized by flat, oval-shaped bodies; commonly known as flukes.

Vector: Any agent, living or otherwise, that carries and transmits parasites and diseases.

Parasites that remain on a host's body surface to feed are called ectoparasites, while those that live inside a host's body are called endoparasites. Parasitism is a highly successful biological adaptation. More parasitic species are known than nonparasitic ones. Parasites affect just about every form of life, including nearly all animals, plants, and even bacteria.

The study of parasites

Parasitology is the study of parasites and their relationships with host organisms. Throughout history, people have coped with over 100 types of parasites affecting humans. Parasites have not, however, been systematically studied until the last few centuries. With his invention of the microscope in the late 1600s, the Dutch scientist Anton von Leeuwenhoek (1632–1723) was perhaps the first person to observe microscopic parasites. As Westerners began to travel and work more often in tropical parts of the world, medical researchers had to study and treat a variety of new infections, many of which were caused by parasites. By the early 1900s, parasitology had developed as a specialized field of study.

Typically, a parasitic infection does not directly kill a host. The stress placed on the organism's resources can affect its growth, ability to reproduce, and survival. This stress can sometimes lead to the host's premature death. Parasites, and the diseases they cause and transmit, have been responsible for tremendous human suffering and loss of life throughout history. The majority of parasitic infections occur within tropical regions and among low-income populations. However, almost all regions of the world sustain parasitic species, and all humans are susceptible to infection.

Infectious diseases

An infectious disease, or infection, is a condition that results when a parasitic organism attacks a host and begins to multiply. As the parasite multiplies, it interferes with the normal life functions of the host more and more. The host begins to feel ill as a symptom of the parasite's invasion and activities. In many cases, the host's immune system (which fights foreign bodies in the body) may be able to respond to the parasite and destroy it. In many other cases, however, the parasitic infection may over-whelm the immune system, resulting in serious disease and even death.

Until a century ago, infections were the primary means of human "population control" worldwide, often killing enormous numbers of people in epidemics of diseases such as bubonic plague and typhoid fever. Even today, infections actually cause more deaths during war and famine than do actual injuries and starvation. Fortunately, many infectious diseases can now be treated by means of antibiotics and other drugs and by a variety of preventative methods.

Almost all infections contracted by humans pass from other humans or animals. Some infections originate from outside the body, among them a cold from kissing someone with a cold; rabies from a dog bite; hepatitis B from a contaminated needle entering the bloodstream; hepatitis A from germs transferred from fingers to mouth after touching a dirty toilet seat; measles, mumps, and the flu from tiny moisture particles that exit the mouth and nose when a person sneezes, coughs, or talks; syphilis from an infected sex partner; tetanus from a soil-contaminated wound; salmonella from ingesting undercooked eggs, meat, and poultry; and many diseases ranging from the relatively innocent to the fatal—such as gastroenteritis, cholera, and dysentery—from drinking or bathing in contaminated water.

Endogenous (caused by factors within the organism) infections occur when the host's resistance is lowered, perhaps by malnutrition, illness, trauma, or immune depression. Weakening of the host's immune system may permit normally harmless organisms already present in or on the host or in the environment to cause illness.

Types of parasites

The major types of organisms that cause parasitic infections include species of protozoa, helminths or worms, and arthropods.

Protozoa. Protozoa are single-celled organisms that carry out most of the same physiological functions as more complex organisms. More than 45,000 species of protozoa are known, many of which are parasitic. As parasites of humans, this group of organisms has historically been the cause of more suffering and death than any other category of diseasecausing organisms.

Intestinal protozoa occur throughout the world. They are especially common in areas where food and water sources are subject to contamination from animal and human waste. Typically, protozoa that infect their host through water or food do so while in an inactive state, called a cyst. A cyst consists of a protozoan encased in a protective outer membrane. The membrane protects the organism as it travels through the digestive tract of a previous host. Once inside a new host, the parasite develops into a mature form that feeds and reproduces.

Amebic dysentery is one of the most common parasitic diseases. It often afflicts travelers who visit tropical and subtropical regions. The condition is characterized by diarrhea, vomiting and weakness. It is caused by a protozoan known as Entamoeba histolytica.

Another protozoan that causes severe diarrhea is Giardia lamblia. This organism was originally discovered by Leeuwenhoek and has been well-publicized as a parasite that can infect hikers who drink untreated water.

Other types of parasitic protozoa infect the blood or tissues of their hosts. These protozoa are typically transmitted through another organism, called a vector. A vector is an organism that carries a parasite from one host to another host. In many cases, the vector is an invertebrate, such as an insect that itself feeds on a host and then passes the protozoan on through the bite wound. Some of the most infamous of these protozoa are the ones that cause malaria and African sleeping sickness.

Helminths. Helminths are wormlike organisms including nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes). Leeches are also helminths and are considered ectoparasitic, since they attach themselves to the outside skin of their hosts.

One of the most infamous nematodes is Trichinella spiralis. At one stage of its life cycle, this nematode lives in the muscle tissue of animals, including swine. Eventually, these organisms make their way into the intestinal tissue of humans who happen to ingest infected, undercooked pork.

The largest parasitic roundworm, common among humans living in tropical developing countries, is Ascaris lumbricoides. This roundworm can grow to a length of 35 centimeters (15 inches) within the small intestine of its host.

A parasitic roundworm that affects dogs is Dirofilaria immitus, or heartworm. This worm infects the heart tissues and eventually weakens

A liver fluke seen from above. There are more than 6,000 species of parasitic flatworms. (Reproduced by permission of JLM Visuals.)
A liver fluke seen from above. There are more than 6,000 species of parasitic flatworms. (Reproduced by permission of
JLM Visuals

the cardiac (heart) muscles to the point of failure. If left untreated, heart-worm can kill a dog.

Tapeworms are a class of worms characterized by their flat, segmented bodies. The segments hold both male and female reproductive organs, allowing self-fertilization. Segments that contain fertilized eggs break off or dissolve, passing the eggs out of the host. Adult tapeworms typically reside in the intestinal tract of vertebrates, attaching themselves to the stomach lining with hooks or suckers on their head.

Common tapeworms that attack humans are Taenia saginata, Taenia solium, and Diphyllobothrium latum. These parasites use intermediate hosts, such as cattle, swine, and fish respectively, before entering the human body. Parasites such as these infect an intermediate host organism while in a early developmental form. But they do not grow to maturity until they have been transmitted to the final host.

In the case of Taenia species, for example, tapeworm eggs are passed into cattle or swine through infected soil. They develop into an intermediary

Entwined pair of male and female schistosomes. Adult pairs live in the small veins of human hosts, where the female produces about 3,500 eggs per day. (Reproduced by permission of Phototake.)
Entwined pair of male and female schistosomes. Adult pairs live in the small veins of human hosts, where the female produces about 3,500 eggs per day. (Reproduced by permission of

stage that embeds in the muscle and connective tissue of the animal. Infected animals that are processed for meat but improperly cooked still harbor the parasite, which are passed on when consumed by humans. The tapeworms develop into adults that attach to the intestinal lining of the host.

Trematodes, or flukes, are another class of helminths that have parasitic species. Adult flukes are typically flat, oval-shaped worms that have a layer of muscles just below the skin. These muscles allow the worm to expand and contract its shape and, thus, move its body. Flukes usually have an oral sucker, sometimes ringed with hooks. They use the sucker to attach themselves to the host's tissues.

Some of the most infamous flukes are species of the genus Schistosoma that cause the often-fatal disease known as schistosomiasis. These flukes infect human hosts directly by burrowing into the skin of a person wading or swimming in infected water. One species, S. mansoni, enters the bloodstream as an immature worm and can be carried through various organs, including the lungs and heart, before maturing in the liver.

Arthropods. Arthropods are organisms characterized by exterior skeletons and segmented bodies. Examples include the crustaceans, insects, and arachnids. The arthropods are the most diverse and widely distributed animals on the planet. Many arthropod species serve as carriers of bacterial and viral diseases, as intermediate hosts for protozoan and helminth parasites, and as parasites themselves.

Certain insect species are the carriers of some of humanity's most dreaded diseases, including malaria, typhus, and plague. As consumers of agricultural crops and parasites of our livestock, insects are also humankind's number-one competitor for resources.

Mosquitoes are the most notorious carriers of disease and parasites. Female mosquitoes rely on warm-blooded hosts to serve as a blood meal to nourish their eggs. During the process of penetrating a host's skin with their long, sucking mouth parts, saliva from the mosquito is transferred into the bite area. Any viral, protozoan, or helminth infections carried in the biting mosquito can be transferred directly into the blood stream of its host. Among these diseases are malaria, yellow fever, filariasis, elephantiasis, and heartworm.

Flies also harbor diseases that can be transmitted to humans and other mammals when they bite to obtain a blood meal for themselves. For example, black flies can carry Onchocerciasis (which causes river blindness), sandflies can carry leishmaniasis and kala-azar, and tsetse flies can carry the trypanosomes that cause sleeping sickness. Livestock, such as horses and cattle, can be infected with a variety of botflies and warbles that infest and feed on the skin, throat, nasal passages, and stomachs of their hosts.

Fleas and lice are two of the most common and irritating parasitic insects of humans and livestock. Lice commonly live among the hairs of their hosts, feeding on blood. Some species are carriers of typhus fever. Fleas usually infest birds and mammals, and can feed on humans when they are transferred from pets or livestock. Fleas are known to carry a variety of devastating diseases, including the plague.

Another prominent class of arthropods that contains parasitic species is the arachnids. Included in this group are spiders, scorpions, ticks, and mites.

Mites are very small arachnids that infest both plants and animals. One common type of mite is the chigger, which lives in grasses. As larvae, they may grab onto passing animals and attach themselves to the skin, often leading to irritating rashes or bite wounds. Scabies are another

A photo of a flea magnified 50 times. Fleas are common parasitic insects that are known to carry a variety of devastating diseases, including the plague. (Reproduced by permission of The Stock Market.)
A photo of a flea magnified 50 times. Fleas are common parasitic insects that are known to carry a variety of devastating diseases, including the plague. (Reproduced by permission of
The Stock Market

mite that causes mange in some mammals by burrowing into the skin and producing severe scabs, lesions, and loss of hair.

Ticks also live their adult lives among grasses and short shrubs. They are typically larger than mites. The adult female tick attaches itself to an animal host for a blood meal. Tick bites themselves can be painful and irritating. More importantly, ticks can carry a number of diseases that affect humans. The most common of these diseases include Rocky Mountain spotted fever, Colorado tick fever, and Lyme disease.

Control of parasites

Many parasitic infections can be treated by a variety of medical procedures, such as the use of antibiotics. The best way of controlling infection, however, is prevention. Scientists have developed and continue to test a number of drugs that can be taken as a barrier to certain parasites. Other measures of control include improving sanitary conditions of water and food sources, proper cooking techniques, education about personal hygiene, and control of intermediate and vector host organisms.

[ See also Arachnids ; Arthropods ; Plague ; Protozoa ]

Also read article about Parasites from Wikipedia

User Contributions:

Looking for information to help a friend. He worked in the jungles of Venezeula this was about 10 years ago. Came home and within a couple of months, had a hole the size of a dime in the bottom of his foot. His wife removed a weblike substance from the wound. Cleaned it out and it healed up. Now he is suffering as a somewhat diagnosis (because the various doctors don't know what is going on with him). He has been diagnosised with Multiple System Atrophy. When hee takes antibiotics he feels better.

I need help, does anyone know what could have burrow into his foot. It was said it was a sand mite.

This is killing him, he is unable to walk now, please if anyone can give me any information, please help. Thanks
If a person were to see something after a stool that looks loke the first pic. Could the person have a fluke? Symptoms: Night sweat, fever 99-102 F sometimes, chest pain, forgetting things, pain/ swollen around the tummy, sensation of fire on the spinal chord and sometimes nubness on left side of body???
sandy collier
My mom had a bad case of diarhea for about a month and the doctor finally got it to stop, but now her hands have severe pain periodically. She is 78 and generally in good health except for this. When her hands act up she cant even dress herself. One doctor said it sounded like the parasite had moved to her hands when they were treating the diarhea, and actually gave it a name. However, this doctor has left the practice and there is nothing written in my mom's medical record about this. She is desperate to find a cure. The symptoms sound alot like arthritis: pain, swelling, difficulty in movement, weakness. Can you help?
I have seen five different doctors for parasites in my face,mouth, stool. They have told me everything from No, you are not seeing any thing, to mites that affect humans like I have described
just do not exist. So Frustrating. I have lived in the same building for 12 years. There is loft above me that housed pigeons until last year when the owner closed off the entry. Pretty much exactly what I have read on many sites that I have looked at about bird mites. Does any body know the best site to send my Dr to. If there is info on this site, I guess I have not gotten that far yet. This ordeal is making me crazy. Please help?
thanks Katiej
the organism that causes scalp infection,athletes foot and ringworm
this stuff is vital in terms of parasitic informations and there is a need for more of it kind so that students can benefit from it. thanks for your contribution and supports
because it provide more information for us and every children in our house
i am very interested in this article and i was wondering if i can make use of this in order to do an activity for some language students. I would be very glad if i receive a positive answer on this e-mail:

please send new information or atlas for diagnosis of parasitology
Tell him to go see an Animal Scientist, the best Vet Tech you can find, a parasite specialist or if near a military base go to their contagious/infectious disease specialist. This may be too little too late but I hope someone can help. If there is a problem with payment have them contact CDC or the people he used to go to Venezuela with becuase he should have been taken care of before he got home...
Note to AMy. I am beginning to feel that there are many things we don't know, especially about parasites. Frankly, if your friend feels better after taking antibiotics, then he should go for it. I know that people are all concerned that we take too many, but I actually think that we take too few. If an organism has a long reproduction cycle, say 6 months, and we consider the job done after taking them for 2 weeks, we could be wrong and just encouraging resistance. Even if the parasite is not a bacteria, many worms, etc. are killed by antibiotics because they kill the bacteria inside their stomachs so they can't digest food. The antibiotic family that I think is best is the macrolides (anything ending in mycin). I personally like clarythromycin. I have been taking it for years and will continue until I am completely well. If he hasn't died yet, and he can tolerate it (sometimes if you are too sick, you can't handle it). I would take it and not stop (even if he takes only a small amount per day, as little as 1/20th of a pill). Good luck
I have a sister that has some sorta of worm in her body. She is waiting for her to tell her what kind. She vomited them up. I am wondering can her husband or any one been at risk to have gotten these from her, She worked in Africa years ago. I my self am not concern about exposure .But she is worried so, that she could have given this to someone .If you have any info could you please let me know. Perhaps I could put her mind at ease. Thank you
I have been treated since 1998 with enbrel for RA I need to know if you had a adverse reaction would you have rashes like a psoriasis and skin ailments that look like cysts . I have told my doctor I think that there are some sort of worms in me because of the foreign places I have visited and a lot of the symptoms I have are the same as worm infestation of some sort. I understand some worms can lie dormant for years. I have used black walnut hull and wormwood and other cognotions too, over the years I have been on antibiotics and I feel better(always short lived) . What treatment would you take next. I am on so many drugs now from steroids to blood pressure to thyroid and skin medications. I need help this is debilitating. Any answers will be considered and appreciated, I am ready for a radical detoxing or something. By the way I am a research give it your best shot!!
Sylvia Dudek
I recently traveled to Guyana, South America. I stayed in the rainforest and I learned from my amerindian guides about a parasite that lives inside dogs or pigs' feet. This parasite is about half a centimetre in size and is not a mite or tick. Any ideas what it can be?
elizabeth gagliano
I have your answer. It is a parasite called strongyloides. They are not common and when in areas that are somstimes considered edemic you should were shoes at all times. It will get into the soft tissue of your intestines. there is a blood test to see if you have the strongyloides antibodies. It is not the IGg type of blood test,although if yu have a work-up and your bloodwork indicates you have IGg antibodies your on the right track. Good Luck
We have to be careful in every thing we put near to us. Right???
AMy, I'm sorry to hear about your friend. Please, could you tell me what the neurologists say is the relationship between Multiple System Atrophy and this infection/parasite. Thank you. Mark
I have just returned from India (3week visit) didn't have any problems until the last day at the airport I purchased a cucumber and tomato sandwich. The stomach cramps and diahorrea were almost immediate. It has now been 4 days and it still hasn't settled? I also have lethargy and because of the frequency not sleeping . I have been drinking electrolytes and taking psyllium powder. I am concerned that I may have picked up a parasite. I would welcome any recommendations.
Tnx a lot pipz diz research has actually helped me in my sec, school assignment. Really appreciate it.
I literally just wanted to say that I love this site for science information.
It's the steps for the Laboratory Students to follow

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