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Parasites are organisms that live off other organisms, or hosts, to survive. Some parasites do not noticeably affect their hosts. Others grow, reproduce, or invade organ systems that make their hosts sick, resulting in parasitic infection.



What are Parasites

Parasitic diseases occur when an organism invades a person’s body and causes illness to the host. They are common in many tropical, subtropical, and rural parts of regions such as Africa, Asia, and Latin America. People who travel to these areas are at an increased risk of contracting a parasitic disease. Malaria is one of the deadliest parasitic diseases. It’s transmitted to humans by mosquitoes.

Each year more than half a million people die of malaria – a preventable and treatable disease. An estimated two-thirds of deaths are among children under the age of five. Parasitic diseases occur much less frequently in industrialized nations such as the United States. However, foodborne illness as a result of parasites is on the rise in the United States.

Malaria, which is caused by the parasitic protozoa Plasmodium, is the deadliest disease of all time. No disease, including the plague or smallpox, has killed more people. It has also killed more people than all wars, famines, and natural disasters combined.


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Parasitic infections can also occur in the United States. Common parasitic infections found in the United States include:

  • Trichomoniasis - about 7.4 million cases annually

  • Giardiasis - 2 million infections annually.

  • Cryptosporidiosis - about 300,000 infections per year

Common Parasitic

Parasites and Global Annual Deaths




Parasitic infections can be caused by three types of organisms:

What Causes Parasitic
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  • Protozoa

  • Helminths

  • Ectoparasites

Protozoa are single-celled organisms that can live and multiply inside your body. Some infections caused by protozoa include giardiasis. This is a serious infection that you can contract from drinking water infected with Giardia protozoa.
Helminth worms are multicellular and can often get quite large. Worms notoriously infect the digestive tract of untreated dogs and cats, but they can also infect humans. Ascaris, flatworms, and whipworms are the three common types of worms. They are each estimated to currently infect at least half a billion people, mostly in Africa. They include flatworms, tapeworms, thorny-headed worms, and roundworms.
Ectoparasites are multi-celled organisms that live on or feed off your skin. They include some insects and arachnids, such as mosquitos, fleas, ticks, and mites.



Parasites usually enter the body through the:

  • Mouth

  • Skin

Parasites that enter through the mouth are swallowed and can remain in the intestine or burrow through the intestinal wall and invade other organs. Often parasites enter the mouth through fecal-oral transmission.

Some parasites can enter directly through the skin. Others are transmitted by insect bites.

Rarely, parasites are spread through blood transfusions, in transplanted organs, through injections with a needle previously used by an infected person, or from a pregnant woman to her fetus.

There are known to be over 430 species of parasites that can and do live on or in the human body.


Fecal-oral transmission of parasites


Fecal-oral transmission is a common way to acquire a parasite. Fecal refers to feces or stool, and oral refers to the mouth, including things taken into the mouth. Infection that is spread through the fecal-oral route is acquired when a person somehow ingests something that is contaminated by feces from an infected person or animal, such as a dog or cat. Many parasites invade or live-in people's digestive tract. Thus, parasites or their eggs are often present in people's feces.

Roundworm is transferred by ingestion. The eggs hatch and quickly penetrate the intestinal wall, where they enter the bloodstream. From there, the worm enters the lungs, where it is coughed up, swallowed, and returned to the gut. Symptoms include fever, tiredness, an allergic rash, vomiting, diarrhea, nerve problems, and wheezing/coughing.

Infected people often spread their infection when they do not wash their hands adequately after using the toilet. Because their hands are contaminated, anything they touch afterward may be contaminated with parasites (or with bacteria or viruses that cause digestive tract disorders). If people with contaminated hands touch food—in restaurants, grocery stores, or homes—the food may become contaminated. Then, anyone who eats that food may get the infection.

Ingestion does not have to involve food. For example, if a person with contaminated hands touches an object, such as a restroom door, the door can become contaminated. Other people who touch the contaminated door and then touch their finger to their mouth can be infected through the fecal-oral route.

Tapeworms can survive up to 25 years in humans. They are transmitted through infected food and attach themselves to the host’s intestines with hooks on their “head” or scolex

Other ways infection can be spread through the fecal-oral route include

  • Drinking water contaminated with raw sewage (in areas with poor sanitation)

  • Eating raw shellfish (such as oysters and clams) that have been cultivated in contaminated                                                        water

  • Eating raw fruits or vegetables washed in contaminated water

  • Engaging in sexual activity that involves mouth-to-anus contact

  • Swimming in pools that have not been adequately disinfected or in lakes or parts of the ocean that are contaminated with sewage

Skin transmission of parasites


Some parasites live inside the body and enter through the skin. They may

  • Bore directly through the skin

  • Be introduced by the bite of an infected insect

Some parasites, such as hookworms, enter through the skin on the soles of the feet when a person walks barefoot on contaminated soil. Others, such as schistosomes, which are flukes, enter through the skin when a person swims or bathes in water containing the parasites.
Insects that carry and transmit organisms that cause disease are called vectors. Some insect vectors transmit parasites called protozoa (such as those that cause malaria) and some helminths (such as those that cause river blindness). Many of these parasites have very complex life cycles.

Researchers found that 9- and 10-year-old girls are most likely to get head lice because they like to do group hugs.

The Naegleria fowleri ameba, or the “brain-eating amoeba,” makes its home in people’s brains. The infection causes brain inflammation, extensive destruction of brain tissue, vomiting, stiff neck, hallucinations, and seizures. After the onset of symptoms, the disease progresses rapidly and causes death within 3–7 days.

Insects (for example, lice) and mites (for example, scabies) that live on or burrow into the skin are known as ectoparasites. They are transmitted by having close contact with an infected person or their belongings.

Onchocerca volvulus is a parasitic worm that causes “river blindness,” the world’s second-leading infectious cause of blindness. It can live up to 15 years in the human body and is transmitted by the bite of the black fly. The worms spread throughout the body, and when they die, they cause intense itching and an immune response that can destroy tissue, such as eye tissue.



The symptoms of parasitic infections depend on the type of parasite involved and wherein the body it lives and multiplies. Parasites that live in the intestines usually cause abdominal pain and diarrhea. Parasites that live in the bloodstream usually cause fever and swelling.



For some parasitic infections, no treatment is needed. The infection may disappear on its own.
Some medications (antiparasitic drugs) are designed particularly to eliminate parasites or, in the case of some worm infections, reduce the number of worms enough so that symptoms clear up. 


Parasitic body lice may carry a parasite belonging to the group of bacteria called Rickettsia, which causes typhus. Next to malaria and the plague, typhus is the greatest killer of people in history. Unless treated with antibiotics, 4 in 10 victims die.

Antiparasitic medications are usually well-tolerated by patients and cause few side effects. Adverse reactions that do occur tend to be minor.  Most people can use these drugs safely and effectively regardless of any other medical conditions they might have. Antiparasitic agents are available in the form of pills, creams, lotions, and shampoos. Some of these drugs are available in over-the-counter formulations, whereas others can only be obtained with a physician’s prescription.  Also, certain antibiotics and antifungal drugs are effective against some parasitic infections/. No single drug is effective against all parasites. For some parasitic infections, no drug is effective.

The key to the recognition of parasitic infection is the knowledge of epidemiologic risk factors such as parasites’ geographical distribution, and the most common modes of clinical presentation. Given present levels of travel, changing immigration patterns, and immunosuppressive effects of infection, Apex physicians have a heightened awareness of radical changes in parasite diagnostic techniques as microscopy is replaced with more accurate and sensitive techniques such as antigen detection and polymerase chain reaction assays, thus increasing the understanding of pathogenesis as host, parasite and environmental factors impact disease.  Apex Physicians are frequently called on to diagnose, manage and or treat parasitic (protozoal) infections. 

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