Parasite classification and types

Definition

A parasite is a pathogenic organism that feeds and grows on another organism, known as a host, and causes what are known as parasitic diseases. Sometimes, a parasite produces little or no symptoms in the host. Often, however, a parasite is detrimental and even fatal to the host organism. Parasitic diseases are prevalent worldwide, but their incidence is significantly higher in developing nations.

Humans, as hosts, are susceptible to a number of parasites, namely protozoa (single-celled organisms), helminths (worms or wormlike organisms), and ectoparasites (such as fleas, lice, ticks, and mites). Ectoparasites infest the external (cutaneous) body surface (the skin and hair). Protozoa and helminths are endoparasites (parasites that invade the body). Endoparasites can infect the intestinal tract, the bloodstream, and internal organs (such as the brain, eyes, liver, and kidneys).

Some parasites are transmitted by an insect vector, an organism that transmits disease to another organism. For example, the Anopheles mosquito transmits malaria (Plasmodium species) to humans.

Parasites often have a life cycle in which one stage of development exists in another animal or environment. For example, a tapeworm infection begins when a person consumes water that has been contaminated with the eggs or larvae of tapeworms. The eggs hatch in the intestines and develop into larvae. Larvae, which have hatched before or after entering the intestines, develop into adult tapeworms. The adults attach to the intestinal wall, from which they derive nourishment. The adults lay eggs, which pass out in the feces. This fecal material can contaminate the water supply and, thus, complete the life cycle.

Classification

Parasites can be classified in several ways, such as by the location of the body affected—cutaneous (ectoparasite) or invasive (endoparasite)—and by type of parasite—protozoa, helminth, or ectoparasite. Endoparasites can be further classified by the area of the body they invade (intestines, brain, or liver).

Ectoparasitic infections, which most often cause skin irritation, are relatively easy to treat. Most endoparasites are protozoa or helminths; some are arachnids. Endoparasitic infections range from asymptomatic to fatal, and virtually every organ in the body is susceptible to more than one type of this infection. Helminths are further subdivided into platyhelminthes (flatworms) and nemathelminthes (roundworms).

Impact

Parasitic infections are a major health concern, and the risk of infection is present throughout the globe. However, these infections are of particular concern in subtropical and tropical regions of developing countries. At risk are not only residents of the regions but also travelers to these areas. In many cases, manifestations of the disease do not appear until a traveler has returned home to an area where the disease is not present; thus, a diagnosis may be missed. Parasitic diseases have a far greater impact on residents of endemic areas.

Many parasites cause serious, debilitating, and sometimes fatal illnesses. They affect people of all ages, including infants and children. In fact, some illnesses are more common in children.

Beyond the impact on health, the medical costs for pharmaceuticals, healthcare professionals, and hospitalization are significant. Many developing nations lack adequate resources to treat parasitic infections, relying on aid from developed countries and international support groups.

Contaminated water, food, and soil are major contributors to parasitic diseases. The eggs of many parasites are present in feces; thus, fecal-oral transmission is common. International groups, such as the World Health Organization (WHO), expend considerable resources educating people in developing nations about the importance of adequate sanitation and improved personal hygiene.

Some parasitic infections are responsible for significant mortality, while others rarely cause death. Even with this low mortality rate, some parasitic infections significantly affect societies, as they lead to lost wages and time away from schooling.

Parasitic infections that are common in developing nations are rare in the developed world. For example, about forty cases of trichinosis, which is caused by the beef or pork tapeworm, are reported annually in the United States. Malaria, too, is rare in the United States, and most cases are seen in immigrants from endemic countries or from US citizens who have traveled abroad. The Centers for Disease Control and Prevention (CDC) estimates that 249 million cases of malaria arise each year, and over 500,000 people die from the disease annually.

Malaria. Beginning in the 1940s, the pesticide dichloro-diphenyl-trichloroethane (DDT) was used to spray areas in which the Anopheles mosquito was present; this resulted in reduced rates (and even eradication) of malaria in many regions. However, the effect of DDT on wildlife, and its possible carcinogenic effects in humans, led to public outcry. In 1972, DDT was banned in the United States and, subsequently, worldwide. Subsequently, malaria reappeared, resulting in millions of deaths.

Malaria is one parasitic infection responsible for significant loss of life worldwide; there are many others, including elephantiasis (lymphatic filariasis), Loa loa filariasis (African eye worm), Chagas’ disease, trypanosomiasis (African sleeping sickness), and schistosomiasis.

Schistosomiasis. Schistosomiasis is a significant public health and socioeconomic concern in endemic areas, which include tropical and subtropical areas of Africa, Asia, and South America. The disease, caused by the parasitic worms of the genus Schistosoma, is prevalent in areas in which the local water supplies contain freshwater snails, which carry the parasite. More than 240 million people are infected, and more than 700 million people who live in rural and semi-urban areas are at risk. Although this disease has a low mortality rate, it causes chronic illnesses involving the intestines, bladder, kidneys, ureters, and lungs.

Elephantiasis. More than 120 million people have elephantiasis. Affected body parts include the arms, legs, genitals, and breasts. Tremendously swollen legs and genitals make walking difficult or impossible. Kidney damage can cause serious health problems and even death. More than 657 million people in developing countries are at risk of infection. The World Health Organization’s Global Programme to Eliminate Lymphatic Filariasis began in 2000, reducing the rate of the disease by nearly 75 percent in two decades.

Amebiasis. Amebiasis is a gastrointestinal infection caused by Entamoeba histolytica. The disease is responsible for 40,000 to 100,000 deaths annually. The usual symptom is diarrhea, which ranges from mild to severe. If untreated, the infection can remain in the gastrointestinal tract for years. Asymptomatic persons can infect others through poor hygienic practices. Sometimes, the infection invades the bloodstream and can form liver abscesses.

Loa loa filariasis. Approximately ten million people in western and central Africa are infected with Loa loa filariasis, caused by a bite from the deer fly or mango fly. The nematode invades the subcutaneous layers of the skin and the subconjunctival layers of the eyes, where it can be readily observed. The disease is rarely fatal.

Chagas’ disease. Chagas’ disease, which is caused by Trypanosoma cruzi and transmitted to humans by the triatomine bug, is endemic to Latin America. Globally, it affects 6 to 7 million people, including around 280,000 in the United States. Around 75 million people are said to be at risk for contracting the disease. The disease is responsible for 10,000 to 20,000 deaths each year.

Trypanosomiasis. Trypanosomiasis, which is caused by T. brucei and transmitted by the tsetse fly, infected between 50,000 and 70,000 people in sub-Saharan Africa through the late 1990s. In 2008 alone, the parasite led to about 48,000 deaths. Since the late nineteenth century, four major epidemics have occurred: 1896 through 1906, primarily in Uganda and the Congo Basin; 1920 and 1970, in several African countries; and 2008, in Uganda. However, the incidence rate of trypanosomiasis declined significantly in the first decades of the twenty-first century. By 2017, less than 2,000 cases were reported worldwide, and by 2018, there were less than 1,000 cases. In 2020, the World Health Organization reported fewer than 700 cases, primarily in Angola, Chad, Central African Republic, Congo, Gabon, Malawi, Guinea, and South Sudan.

Onchocerciasis. Onchocerciasis, also known as river blindness, is caused by the nematode Onchocerca volvulus. Most infections occur in sub-Saharan Africa in individuals bitten by blackflies of the genus Simulium; however, cases have been reported in Central America, South America, and Yemen. WHO estimates that worldwide, 37 million people are infected with the parasite; of these persons, 270,000 have been blinded, and 500,000 have impaired vision. About 205 million people are at risk of becoming infected with the parasite.

Hookworms. Two hookworm species frequently infect humans: Ancylostoma duodenale, which is present in India, the Middle East, and North Africa, and Necator americanus, which is found in the Americas, China, Indonesia, Southeast Asia, and sub-Saharan Africa. Between 570 and 740 million people are believed to be infected worldwide. Although the infection can be asymptomatic for more than one year, it can be extremely harmful to its host. It causes iron deficiency anemia, intestinal blood loss, and malnutrition.

Trichinosis. Worldwide, about 10,000 persons are infected with trichinosis annually, though only around fifteen people are diagnosed each year in the United States. Formerly, it was common in developed and undeveloped nations, but in the twenty-first century, it is rare in developed nations. Most infections are caused by Trichinella spiralis and arise from eating raw or undercooked pork. Infections are often asymptomatic or produce mild muscular pain, which disappears over time. Occasionally, trichinosis invades the lungs, heart, and brain, resulting in severe illness or death.

Bibliography

"About Parasites." Centers for Disease Control, 11 Sept. 2024, www.cdc.gov/parasites/about/index.html. Accessed 1 Nov. 2024.

Combes, Claude, and Claude Simberloff. The Art of Being a Parasite. U of Chicago P, 2005.

Fritsche, Thomas, and Rangaraj Selvarangan. “Medical Parasitology.” Henry’s Clinical Diagnosis and Management by Laboratory Methods, edited by Richard McPherson and Matthew Pincus, 24th ed., W. B. Saunders, 2022.

Jong, Elaine C., and Russell McMullen, editors. Travel and Tropical Medicine Manual. 5th ed., Saunders/Elsevier, 2017.

"Parasites." Centers for Disease Control and Prevention, US Dept. of Health and Human Services, www.cdc.gov/parasites. Accessed 1 Nov. 2024.

"Parasites." Cleveland Clinic, 14 Apr. 2023, my.clevelandclinic.org/health/diseases/24911-parasites. Accessed 1 Nov. 2024.

Roberts, Larry S., and John Janovy, Jr. Gerald D. Schmidt and Larry S. Roberts’ Foundations of Parasitology. 9th ed., McGraw-Hill, 2013.