What are 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.

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, health care professionals, and hospitalization are significant. Many developing nations do not possess adequate resources for the treatment of parasitic infections; thus, they must rely on aid from developed nations 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 effect 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 U.S. citizens who have traveled abroad.
The Centers for
Disease Control and Prevention (CDC) estimates that 300 to
500 million cases of malaria arise each year and that more than 1 million people
die from the disease annually.

Malaria. Beginning in the 1940’s, 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 ranked second behind
malaria in terms of public health and socioeconomic significance 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 200
million people are infected and more than 600 million people who live in rural and
semiurban 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, and more than 40 million of them are seriously
disfigured and incapacitated. Affected body parts include the arms, legs,
genitals, and breasts. Tremendously swollen legs and genitals make walking
difficult if not impossible. Kidney damage can cause serious health problems and
even death. More than 1 billion people in developing countries are at risk of
infection.

Amebiasis. Amebiasis is a gastrointestinal
infection caused by Entamoeba histolytica. The disease is
responsible for about seventy thousand 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 thirteen million people in western and central Africa are infected with
Loa loa filariasis, which is 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, is endemic to Latin American countries,
where it affects 8 to 10 million people; an additional 300,000 to 400,000 people
in nonendemic countries, including Spain and the United States, are affected.
Approximately 41,200 new cases occur annually in endemic countries, and more than
14,000 infants are born with congenital Chagas’ disease annually. The disease is
responsible for approximately 20,000 deaths each year.

Trypanosomiasis. Trypanosomiasis, which is caused by
T. brucei and transmitted by the tsetse fly, infects between 50,000 and 70,000 people in sub-Saharan
Africa. In 2008, 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.

Onchocerciasis. Onchocerciasis, also known as river
blindness, is caused by the nematode Onchocerca volvulus. Most
infections occur in sub-Saharan Africa;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 90 million people are at risk for
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.
More than 600 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 eleven million persons are infected
with trichinosis. Formerly, it was common in developed and
undeveloped nations; however, it now is rare in developed countries. 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 in death.

Combes, Claude, and Claude Simberloff. The Art of Being a Parasite. Chicago: University of Chicago Press, 2005. An extensive collection of stories that illuminate the ecology and evolution of interactions between species.

Fritsche, Thomas, and Rangaraj Selvarangan. “Medical Parasitology.” In Henry’s Clinical Diagnosis and Management by Laboratory Methods, edited by Richard McPherson and Matthew Pincus. 21st ed. Philadelphia: W. B. Saunders, 2007. A detailed discussion of parasitic infections that includes illustrations of the life cycles and characteristics of parasites.

Jong, Elaine C., and Russell McMullen, eds. Travel and Tropical Medicine Manual. 4th ed. Philadelphia: Saunders/Elsevier, 2008. A useful reference manual with advice on preventing, evaluating, and managing diseases that can be acquired in tropical environments and countries outside the United States.

Roberts, Larry S., and John Janovy, Jr. Gerald D. Schmidt and Larry S. Roberts’ Foundations of Parasitology. 8th ed. Boston: McGraw-Hill, 2009. A classic work focusing on the parasites of humans.