Why does a parasite need a living host

Parasitic organisms are found almost everywhere, from tropical and sub-tropical regions to freezing places like Antarctica. Yet, what every parasite needs is an organism that provides food, shelter, and a place to reproduce. This organism is called a host. The host is eventually harmed by its interaction with the parasite.

Parasites can be classified into two groups, depending on where they live in their hosts. Parasites that live outside the host are called ectoparasites Figure 2A. For example, a flea is a parasite that lives on cats. In contrast, parasites that live inside their hosts are known as endoparasites Figure 2B.

For instance, Wuchereria bancrofti is a worm that lives in the human lymphatic system and causes a disease known as elephantiasis.

Among the endoparasites, some can live inside the host cells and are called intracellular parasites. Plasmodium falciparum is one such parasite, because it lives and grows inside the red blood cells of people who have malaria. Parasites also vary in the number of hosts they need to survive.

While certain parasites need only one host, other parasites use multiple hosts to complete their life cycles. The host in which the parasite completes its sexual reproduction is called the definitive host. Parasitism is a constant battle for survival between the parasite and its host. Because parasites depend on their host for food and shelter, they must not destroy the host until they are ready to move on to the next host. Meanwhile, the hosts must protect themselves from harm.

To protect themselves, the hosts constantly challenge parasitic organisms by creating unfavorable conditions. The host will try to deprive the parasites of nutrients, effectively starving them, or will attack the parasites with its strong immune defenses. To live in such a difficult environment, each parasite has evolved different survival strategies. Over millions of years of evolution, parasites have acquired lots of interesting features to help them adapt to specific environments [ 1 ]. Here are some of them.

They include heartworm, tapeworm , and flatworms. They include bacteria and viruses. Endoparasites rely on a third organism, known as the vector, or carrier. The vector transmits the endoparasite to the host. The mosquito is a vector for many parasites, including the protozoan known as Plasmodium, which causes malaria. These feed on other parasites in a relationship known as hyperparasitism. A flea lives on a dog, but the flea may have a protozoan in its digestive tract. The protozoan is the hyperparasite.

Protozoa : Examples include the single-celled organism known as Plasmodium. A protozoa can only multiply, or divide, within the host. Helminths : These are worm parasites. Schistosomiasis is caused by a helminth. Other examples include roundworm, pinworm, trichina spiralis, tapeworm, and fluke. Ectoparasites : These live on, rather than in their hosts. They include lice and fleas. There are many types of parasite, and symptoms can vary widely.

Sometimes these may resemble the symptoms of other conditions, such as a hormone deficiency, pneumonia , or food poisoning. Often there are no symptoms, or symptoms appear long after infection, but the parasite can still be transmitted to another person, who may develop symptoms.

Many types of parasites can affect humans. Here are some examples of parasites and the diseases they can cause. This tiny ameba can affect the eye, the skin, and the brain. It exists all over the world in water and soil. Individuals can become infected if they clean contact lenses with tap water. This disease that comes from parasites that are spread by ticks.

It affects the red blood cells. The risk is highest in summer in the Northeast and upper Midwest of the United States. This is passed on by Balatidium coli , a single-cell parasite that usually infects pigs but can, in rare cases, cause intestinal infection in humans. It can be spread through direct contact with pigs or by drinking contaminated water, usually in tropical regions. This affects the intestines. The blastocystis enters humans through the fecal-oral route.

A person can get it by eating food or drink contaminated with human or animal feces where the parasite is present. Coccidia is passed on through the fecal-oral route. It is found around the world. It can also affect dogs and cats, but these are different kinds.

Dogs, cats, and humans cannot normally infect each other. This is caused by the parasite Entamoeba histolytica. It affects the intestines. It is more likely in tropical regions and in areas with high population density and poor sanitation. It is transmitted through the fecal-oral route. If humans ingest food or water contaminated with feces, dormant cysts may infect the body.

This disease is caused by the Cystoisospora belli , previously known as Isospora belli. It affects the epithelial cells of the small intestine. It exists worldwide and is both treatable and preventable. It is passed on through the fecal-oral route. This is a disease that is passed on by parasites of the Leishmania family. It can affect the skin, the viscera, or the mucous membranes of the nose, mouth, and throat.

It can be fatal. The parasite is transmitted by types of sandflies. This is passed on through a free-living ameba known as Naegleria fowleri. It affects the brain and the nervous system, and it is nearly always fatal within 1 to 18 days.

It is transmitted through breathing in contaminated soil, swimming pools, and contaminated water, but not from drinking water. Different types of plasmodium affect the red blood cells. It exists in tropical regions and is transmitted by the Anopheles mosquito. This is caused by Rhinosporidium seeberi.

It mainly affects the mucous of the nose, conjunctiva, and urethra. Raffel, T. Parasites as predators: Unifying natural enemy ecology. Trends in Ecology and Evolution 23 , Schall, J. Parasite-mediated competition in Anolis lizards.

Oecologia 92 , Sinclair, A. The eruption of the ruminants. In Serengetti: Dynamics of an Ecosystem. Chicago: University of Chicago Press, : Sukhdeo, M. Food web patterns and the parasite's perspective. In Parasitism and Ecosystems.

Thomas, F, Renaud, F. New York: Oxford University Press, : Thomas, F. Parasitism, biodiversity and conservation. Tompkins, D. Ecological replacement of native red squirrels by invasive greys driven by disease. Predation, Herbivory, and Parasitism. Characterizing Communities. Species with a Large Impact on Community Structure. Successional Changes in Communities. Effects of Biogeography on Community Diversity.

Community Ecology Introduction. Avian Egg Coloration and Visual Ecology. Causes and Consequences of Biodiversity Declines. Disease Ecology. The Ecology of Avian Brood Parasitism. Elemental Defenses of Plants by Metals. The Maintenance of Species Diversity.

Neutral Theory of Species Diversity. Abiotically-Mediated Direct and Indirect Effects. Biological Stoichiometry. Sampling Biological Communities. Succession: A Closer Look. Dynamics of Predation. Ecological Consequences of Parasitism. Resource Partitioning and Why It Matters. Direct and Indirect Interactions. Keystone Species.

Citation: Preston, D. Nature Education Knowledge 3 10 Why are parasites ecologically important? Parasites can shape community structure through their effects on trophic interactions, food webs, competition, biodiversity, and keystone species.

Aa Aa Aa. Parasitism and Trophic Interactions. Figure 1: Parasites as prey. A Oligochaete worms Chaetogaster sp. Parasitism, Food Webs, and Ecosystem Energetics. Considering the prominent roles played by parasites in trophic interactions, we might expect parasites to strongly influence food web characteristics. Parasitic infections cause a tremendous burden of disease in both the tropics and subtropics as well as in more temperate climates.

Of all parasitic diseases, malaria causes the most deaths globally. Malaria kills more than , people each year, most of them young children in sub-Saharan Africa. The Neglected Tropical Diseases NTDs , which have suffered from a lack of attention by the public health community, include parasitic diseases such as lymphatic filariasis , onchocerciasis , and Guinea worm disease. The NTDs affect more than 1 billion people worldwide, largely in rural areas of low-income countries.

These diseases extract a large toll on endemic populations, including lost ability to attend school or work, stunting of growth in children, impairment of cognitive skills and development in young children, and the serious economic burden placed on entire countries.

However, parasitic infections also affect persons living in developed countries, including the United States. Contact Us. Skip directly to site content Skip directly to page options Skip directly to A-Z link.