The physical space of the MSK Library is permanently closed to visitors as of Friday, May 17, 2024. Please visit this guide for more information.
The physical space of the MSK Library is permanently closed to visitors as of Friday, May 17, 2024. Please visit this guide for more information.
A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host. Parasitism is a kind of symbiosis, a close and persistent long-term biological interaction between a parasite and its host; but unlike commensalism and mutualism, the parasitic relationship harms the host, either feeding on it or, as in the case of intestinal parasites, consuming some of its food. Because parasites interact with other species, they can readily act as vectors of pathogens, causing disease.
There are three main classes of parasites that can cause disease in humans: protozoa, helminths, and ectoparasites.
Protozoa are microscopic, single-celled organisms that can be free-living or parasitic in nature. They are able to multiply in humans, which contributes to their survival and also permits serious infections to develop from just a single organism.
Transmission of protozoa that live in a human’s intestine to another human typically occurs through a fecal-oral route (for example, contaminated food or water or person-to-person contact). Protozoa that live in the blood or tissue of humans are transmitted to other humans by an arthropod vector (for example, through the bite of a mosquito or sand fly).
The protozoa that are infectious to humans can be classified into four groups based on their mode of movement:
Helminths are large, multicellular organisms that are generally visible to the naked eye in their adult stages. Like protozoa, helminths can be either free-living or parasitic in nature. In their adult form, helminths cannot multiply in humans.
There are three main groups of helminths (derived from the Greek word for worms) that are human parasites:
Although the term ectoparasites can broadly include blood-sucking arthropods such as mosquitoes (because they are dependent on a blood meal from a human host for their survival), this term is generally used more narrowly to refer to organisms such as ticks, fleas, lice, and mites that attach or burrow into the skin and remain there for relatively long periods of time (e.g., weeks to months). Arthropods are important in causing diseases in their own right, but are even more important as vectors, or transmitters, of many different pathogens that in turn cause tremendous morbidity and mortality from the diseases they cause.
Parasites can be transmitted in several ways, including via animals (often pets), blood, insects, food, and water.
Pets can carry parasites and pass parasites to people. Proper handwashing can greatly reduce risk. Some dog and cat parasites can infect people. Young animals, such as puppies and kittens, are more likely to be infected with roundworms and hookworms.
What Every Pet Owner Should Know About Roundworms & Hookworms
Wild animals can also be infected with parasites that can infect people. For example, people can be infected by the raccoon parasite Baylisascaris if they accidentally swallow soil that is contaminated with infected raccoon feces.
Some parasites can be bloodborne. This means:
the parasite can be found in the bloodstream of infected people; and
the parasite might be spread to other people through exposure to an infected person’s blood (for example, by blood transfusion or by sharing needles or syringes contaminated with blood).
Examples of parasitic diseases that can be bloodborne include:
In nature, many bloodborne parasites are spread by insects (vectors), so they are also referred to as vector-borne diseases. Toxoplasma gondii is not transmitted by an insect (vector). In the United States, the risk for vector-borne transmission is very low for these parasites except for some Babesia species.
An insect that transmits a disease is known as a vector, and the disease is referred to as a vector-borne disease. Insects can act as mechanical vectors, meaning that the insect can carry an organism but the insect is not essential to the organism’s life cycle, such as when house flies carry organisms on the outside of their bodies that cause diarrhea in people. Insects can also serve as obligatory hosts where the disease-causing organism must undergo development before being transmitted (as in the case with malaria parasites).
Numerous parasites can be transmitted by food including many protozoa and helminths. In the United States, the most common foodborne parasites are:
Protozoa such as:
Roundworms such as:
Trichinella spp.
Anisakis spp.
Tapeworms such as:
Diphyllobothrium spp.
Taenia spp.
Many of these organisms can also be transmitted by water, soil, or person-to-person contact. Occasionally in the U.S., but often in developing countries, a wide variety of helminthic roundworms, tapeworms, and flukes are transmitted in foods such as
undercooked fish, crabs, and mollusks;
undercooked meat;
raw aquatic plants, such as watercress; and
raw vegetables that have been contaminated by human or animal feces.
Some foods are contaminated by food service workers who practice poor hygiene or who work in unsanitary facilities.
Parasites can live in natural water sources. When outdoors, treat your water before drinking it to avoid getting sick. Globally, contaminated water is a serious problem that can cause severe pain, disability and even death. Common global water-related diseases caused by parasites include:
People become infected with these diseases when they swallow or have contact with water that has been contaminated by certain parasites. For example, individuals drinking water contaminated with fecal matter containing the ameba Entamoeba histolytica can get amebic dysentery (amebiasis).