Unicellular Eukaryotic Parasites

Learning Objectives

  • Summarize the general characteristics of unicellular eukaryotic parasites
  • Describe the general life cycles and modes of reproduction in unicellular eukaryotic parasites
  • Identify challenges associated with classifying unicellular eukaryotes
  • Explain the taxonomic scheme used for unicellular eukaryotes
  • Give examples of infections caused by unicellular eukaryotes

Clinical Focus: Anthony, Part 1

Close-up photo of a raised, red ring on skin.

Figure 1. Ringworm presents as a raised, red ring on the skin. (credit: Centers for Disease Control and Prevention)

Upon arriving home from school, 7-year-old Anthony complains that a large spot on his arm will not stop itching. He keeps scratching at it, drawing the attention of his parents. Looking more closely, they see that it is a red circular spot with a raised red edge (Figure 1). The next day, Anthony’s parents take him to their doctor, who examines the spot using a Wood’s lamp. A Wood’s lamp produces ultraviolet light that causes the spot on Anthony’s arm to fluoresce, which confirms what the doctor already suspected: Anthony has a case of ringworm.

Anthony’s mother is mortified to hear that her son has a “worm.” How could this happen?

  • What are some likely ways that Anthony might have contracted ringworm?

We’ll return to Anthony’s example in later pages.

Eukaryotic microbes are an extraordinarily diverse group, including species with a wide range of life cycles, morphological specializations, and nutritional needs. Although more diseases are caused by viruses and bacteria than by microscopic eukaryotes, these eukaryotes are responsible for some diseases of great public health importance. For example, the protozoal disease malaria was responsible for 584,000 deaths worldwide (primarily children in Africa) in 2013, according to the World Health Organization (WHO). The protist parasite Giardia causes a diarrheal illness (giardiasis) that is easily transmitted through contaminated water supplies. In the United States, Giardia is the most common human intestinal parasite (Figure 2). Although it may seem surprising, parasitic worms are included within the study of microbiology because identification depends on observation of microscopic adult worms or eggs. Even in developed countries, these worms are important parasites of humans and of domestic animals. There are fewer fungal pathogens, but these are important causes of illness, as well. On the other hand, fungi have been important in producing antimicrobial substances such as penicillin. In this chapter, we will examine characteristics of protists, worms, and fungi while considering their roles in causing disease.

a) A micrograph of kite-shaped cells. B) a single triangular cell with multiple flagella.

Figure 2. (a) A scanning electron micrograph shows many Giardia parasites in the trophozoite, or feeding stage, in a gerbil intestine. (b) An individual trophozoite of G. lamblia, visualized here in a scanning electron micrograph. This waterborne protist causes severe diarrhea when ingested. (credit a, b: modification of work by Centers for Disease Control and Prevention)

Characteristics of Protists

The word protist is a historical term that is now used informally to refer to a diverse group of microscopic eukaryotic organisms. It is not considered a formal taxonomic term because the organisms it describes do not have a shared evolutionary origin. Historically, the protists were informally grouped into the “animal-like” protozoans, the “plant-like” algae, and the “fungus-like” protists such as water molds. These three groups of protists differ greatly in terms of their basic characteristics. For example, algae are photosynthetic organisms that can be unicellular or multicellular. Protozoa, on the other hand, are nonphotosynthetic, motile organisms that are always unicellular. Other informal terms may also be used to describe various groups of protists. For example, microorganisms that drift or float in water, moved by currents, are referred to as plankton. Types of plankton include zooplankton, which are motile and nonphotosynthetic, and phytoplankton, which are photosynthetic.

Protozoans inhabit a wide variety of habitats, both aquatic and terrestrial. Many are free-living, while others are parasitic, carrying out a life cycle within a host or hosts and potentially causing illness. There are also beneficial symbionts that provide metabolic services to their hosts. During the feeding and growth part of their life cycle, they are called trophozoites; these feed on small particulate food sources such as bacteria. While some types of protozoa exist exclusively in the trophozoite form, others can develop from trophozoite to an encapsulated cyst stage when environmental conditions are too harsh for the trophozoite. A cyst is a cell with a protective wall, and the process by which a trophozoite becomes a cyst is called encystment. When conditions become more favorable, these cysts are triggered by environmental cues to become active again through excystment.

One protozoan genus capable of encystment is Eimeria, which includes some human and animal pathogens. Figure 3 illustrates the life cycle of Eimeria.

Eimera life cycle. Envronment sporogony is the process of sporulation occruing outside the host; this requires several days and oxygen. A non-infectous unsporulated oocyst becomes an infectious sporulated oocyst. These enter the gut when swallowed and begin the proess of asexual schizogony. Oocsts realease sporocyts which release sporozoites. Sporozoites invate gut cells and form trophozoites. Trophozoites undergo schizogony (asexual reproduction) to form schizont which releases merozoites. Merozoites can reinfect and become trphozoites again or continue with sezual gametogon where the maerozoites form male and female gamets. The gamees undergo syngamy (sexual reproduction) to form a developing oocyst which mautres into an unsporulated non-infectious oocyst. This brings us back to the beginning of the environment sporogony stage of the cycle.

Figure 3. In the sexual/asexual life cycle of Eimeria, oocysts (inset) are shed in feces and may cause disease when ingested by a new host. (credit “life cycle,” “micrograph”: modification of work by USDA)

Protozoans have a variety of reproductive mechanisms. Some protozoans reproduce asexually and others reproduce sexually; still others are capable of both sexual and asexual reproduction. In protozoans, asexual reproduction occurs by binary fission, budding, or schizogony. In schizogony, the nucleus of a cell divides multiple times before the cell divides into many smaller cells. The products of schizogony are called merozoites and they are stored in structures known as schizonts. Protozoans may also reproduce sexually, which increases genetic diversity and can lead to complex life cycles. Protozoans can produce haploid gametes that fuse through syngamy. However, they can also exchange genetic material by joining to exchange DNA in a process called conjugation. This is a different process than the conjugation that occurs in bacteria. The term protist conjugation refers to a true form of eukaryotic sexual reproduction between two cells of different mating types. It is found in ciliates, a group of protozoans, and is described later in this subsection.

All protozoans have a plasma membrane, or plasmalemma, and some have bands of protein just inside the membrane that add rigidity, forming a structure called the pellicle. Some protists, including protozoans, have distinct layers of cytoplasm under the membrane. In these protists, the outer gel layer (with microfilaments of actin) is called the ectoplasm. Inside this layer is a sol (fluid) region of cytoplasm called the endoplasm. These structures contribute to complex cell shapes in some protozoans, whereas others (such as amoebas) have more flexible shapes (Figure 4).

Different groups of protozoans have specialized feeding structures. They may have a specialized structure for taking in food through phagocytosis, called a cytostome, and a specialized structure for the exocytosis of wastes called a cytoproct. Oral grooves leading to cytostomes are lined with hair-like cilia to sweep in food particles. Protozoans are heterotrophic. Protozoans that are holozoic ingest whole food particles through phagocytosis. Forms that are saprozoic ingest small, soluble food molecules.

Many protists have whip-like flagella or hair-like cilia made of microtubules that can be used for locomotion (Figure 4). Other protists use cytoplasmic extensions known as pseudopodia (“false feet”) to attach the cell to a surface; they then allow cytoplasm to flow into the extension, thus moving themselves forward.

Protozoans have a variety of unique organelles and sometimes lack organelles found in other cells. Some have contractile vacuoles, organelles that can be used to move water out of the cell for osmotic regulation (salt and water balance) (Figure 4). Mitochondria may be absent in parasites or altered to kinetoplastids (modified mitochondria) or hydrogenosomes (see Unique Characteristics of Prokaryotic Cells for more discussion of these structures).

a) Paramecium cell with short strands on the outside labeled cilia. An indent in the outer layer is labeled cytostome. A sphere inside the cell at the base of the cytostome is labeled cytoproct. A star shaped structure inside the cell is labeled contractile vacuole. B) Amoeba cell with projections on the outside labeled pseudopods. The outer layer of the cell is labeled ectoplasm and the inner layer is labeled endoplasm. A sphere inside the cell is labeled contractile vacuole. C) Euglena with a single long flagellum on the outside. The outer layer of the cell is labeled etoplasm, the inner layer is labeled endoplasm. A star shaped structure is labeled contractile vacuole.

Figure 4. (a) Paramecium spp. have hair-like appendages called cilia for locomotion. (b) Amoeba spp. use lobe-like pseudopodia to anchor the cell to a solid surface and pull forward. (c) Euglena spp. use a whip-like structure called a flagellum to propel the cell.

Think about It

  • What is the sequence of events in reproduction by schizogony and what are the cells produced called?

Taxonomy of Protists

The protists are a polyphyletic group, meaning they lack a shared evolutionary origin. Since the current taxonomy is based on evolutionary history (as determined by biochemistry, morphology, and genetics), protists are scattered across many different taxonomic groups within the domain Eukarya. Eukarya is currently divided into six supergroups that are further divided into subgroups, as illustrated in (Figure 5). In this section, we will primarily be concerned with the supergroups Amoebozoa, Excavata, and Chromalveolata; these supergroups include many protozoans of clinical significance. The supergroups Opisthokonta and Rhizaria also include some protozoans, but few of clinical significance. In addition to protozoans, Opisthokonta also includes animals and fungi, some of which we will discuss in Parasitic Helminths and Fungi. Some examples of the Archaeplastida will be discussed in Algae. Tables 1 and 2 summarize the characteristics of each supergroup and subgroup and list representatives of each.

A branching tree diagram with common eukaryotic ancestor at the base. This leads to 5 branches. The top branch branches are classified as Excavata which is divided into 3 groups: diplomonads, parabasalids, and euglenozoans. The next branch splits into 2 branches: alveolates, and stramenopiles. The alveolates are divided into dinoflagellates, apicomplexans and ciliates. The stramenopiles are divided into diatoms, golden algae, brown algae and oomyces. All the alveotate and stramenopile groups are labeled Chromalveolata. The next branch divides into cercozoans, forams and radiolarians. These are all labeled rhizaria. The next branch divides into the red algae, chlorophytes (green algae), charophytes (green algae) and land plant. Thesea re all labeldd archaeplastidia. The next branch splits into 2. The top branch divides into slime molds, gymnamoebas and entamoebas. These are all labeled amoebozoa. The bottom branch divides into nucleariids, fungi, choanoflagellates, and animals. These are all labeled opisthokonta.

Figure 5. Click for a larger image. This tree shows a proposed classification of the domain Eukarya based on evolutionary relationships. Currently, the domain Eukarya is divided into six supergroups. Within each supergroup are multiple kingdoms. Dotted lines indicate suggested evolutionary relationships that remain under debate.

Table 1. The Eukaryote Supergroups and Some Examples
Supergroup Subgroups Distinguishing Features Examples Clinical Notes
Excavata Fornicata Form cysts
Pair of equal nuclei
No mitochondria
Often parasitic
Four free flagella
Giardia lamblia Giardiasis
Parabasalids No mitochondria
Four free flagella
One attached flagellum
No cysts
Parasitic or symbiotic
Basal bodies
Trichomonas Trichomoniasis
Euglenozoans Photosynthetic or heterotrophic flagella Euglena N/A
Trypansoma African sleeping sickness, Chagas disease
Leishmania Leishmaniasis
Chromalveolata Dinoflagellates Cellulose theca
Two dissimilar flagella
Gonyaulax Red tides
Alexandrium Paralytic shellfish poisoning
Pfiesteria Harmful algal bloom
Apicomplexans Intracellular parasite
Apical organelles
Plasmodium Malaria
Cryptosporidium Cryptosporidiosis
Theileria (Babesia) Babesiosis
Toxoplasma Toxoplasmosis
Ciliates Cilia Balantidium Balantidiasis
Paramecium N/A
Stentor N/A
“Water molds”
Generally diploid
Cellulose cell walls
Phytophthora Diseases in crops
Table 2. The Eukaryote Supergroups and Some Examples
Supergroup Subgroups Distinguishing Features Examples Clinical Notes
Rhizaria Foraminifera Amoeboid
Threadlike pseudopodia
Calcium carbonate shells
Astrolonche N/A
Radiolaria Amoeboid
Threadlike pseudopodia
Silica shells
Actinomma N/A
Cercoza Amoeboid
Threadlike pseudopodia
Complex shells
Parasitic forms
Spongospora subterranea Powdery scab (potato disease)
Plasmodiophora brassicae Cabbage clubroot
Archaeplastida Red algae Chlorophyll a
Floridean starch
Agar in cell walls
Gelidum Source of agar
Gracilaria  Source of agar
Chlorophytes Chlorophyll a
Chlorophyll b
Cellulose cell walls
Starch storage
Acetabularia  N/A
Ulva  N/A
Amoebozoa Slime molds Plasmodial and cellular forms Dictyostelium  N/A
Entamoebas Trophozoites
Form cysts
Entamoeba Amoebiasis
Naegleria Primary amoebic meningoencephalitis
Acanthamoeba  Keratitis, granulomatous amoebic encephalitis
Opisthokonta Fungi Chitin cell walls
Unicellular or multicelluar
Often hyphae
Zygomycetes Zygomycosis
Ascomycetes Candidiasis
Basidiomycetes Cryptococcosis
Microsporidia Microsporidiosis
Animals Multicellular heterotrophs
No cell walls
Nematoda Trichinosis; hookworm and pinworm infections
Trematoda Schistosomiasis
Cestoda Tapeworm infections

Think about It

  • Which supergroups contain the clinically significant protists?


The supergroup Amoebozoa includes protozoans that use amoeboid movement. Actin microfilaments produce pseudopodia, into which the remainder of the protoplasm flows, thereby moving the organism. The genus Entamoeba includes commensal or parasitic species, including the medically important E. histolytica, which is transmitted by cysts in feces and is the primary cause of amoebic dysentery. The notorious “brain-eating amoeba,” Naegleria fowleri, is also classified within the Amoebozoa. This deadly parasite is found in warm, fresh water and causes primary amoebic meningoencephalitis (PAM). Another member of this group is Acanthamoeba, which can cause keratitis (corneal inflammation) and blindness.

The Eumycetozoa are an unusual group of organisms called slime molds, which have previously been classified as animals, fungi, and plants (Figure 6).

a) A micrograph wshwoing a circular dome with long branches emanating outward. B) A photograph showing a yellow structure that looks like foam on a branch.

Figure 6. (a) The cellular slime mold Dictyostelium discoideum can be grown on agar in a Petri dish. In this image, individual amoeboid cells (visible as small spheres) are streaming together to form an aggregation that is beginning to rise in the upper right corner of the image. The primitively multicellular aggregation consists of individual cells that each have their own nucleus. (b) Fuligo septica is a plasmodial slime mold. This brightly colored organism consists of a large cell with many nuclei.

Slime molds can be divided into two types: cellular slime molds and plasmodial slime molds. The cellular slime molds exist as individual amoeboid cells that periodically aggregate into a mobile slug. The aggregate then forms a fruiting body that produces haploid spores. Plasmodial slime molds exist as large, multinucleate amoeboid cells that form reproductive stalks to produce spores that divide into gametes. One cellular slime mold, Dictyostelium discoideum, has been an important study organism for understanding cell differentiation, because it has both single-celled and multicelled life stages, with the cells showing some degree of differentiation in the multicelled form. Figures 7 and 8 illustrate the life cycles of cellular and plasmodial slime molds, respectively.

A mature fruiting body produces a tall stalk with a sphere that generates spores via meiosis. The mature fruiting body releases spores. The haploid spores germinate and give rise to amoeba which divide to form more individual cells. Two amoeba fuse to form a zygote. The zygoe can grow and undergo meiosis and multiple rounds of mitosis. The new haploid amoeba are releases. Fertilization produces amoebas that aggregate into a structure called a slug. The slug migrates at a rate of 2 mm per hour. The migration stops the aggregate forms a fruiting body at the end of a stalk. This brings us back to the fruiting body in the life cycle.

Figure 7. The life cycle of the cellular slime mold Dictyostelium discoideum primarily involves individual amoebas but includes the formation of a multinucleate plasmodium formed from a uninucleate zygote (the result of the fusion of two individual amoeboid cells). The plasmodium is able to move and forms a fruiting body that generates haploid spores. (credit “photo”: modification of work by “thatredhead4″/Flickr)

A mature plasmodium (multinucleated free-flowing mass of protoplasm) can produce sclerotium (small cells) in a dry habitat. The mature plasmodium also produces diploid sporangia which produces haploid spores via meiosis. The mature sporangium releases mature spores which germinate. Germination gives rise to cells that can convert between ameboid and flagellated forms. Plasmogomy is the fusion of cytoplasm of two cells. Karyogamy is the fusion of nuclei and leads to the production of a diploid zygote. The zygote divides to form a multi-nucleated feeding plasmodium. This takes us back to the beginning of plasmodium stage of the life cycle.

Figure 8. Plasmodial slime molds exist as large multinucleate amoeboid cells that form reproductive stalks to produce spores that divide into gametes.


The supergroup Chromalveolata is united by similar origins of its members’ plastids and includes the apicomplexans, ciliates, diatoms, and dinoflagellates, among other groups (we will cover the diatoms and dinoflagellates in Algae). The apicomplexans are intra- or extracellular parasites that have an apical complex at one end of the cell. The apical complex is a concentration of organelles, vacuoles, and microtubules that allows the parasite to enter host cells (Figure 9). Apicomplexans have complex life cycles that include an infective sporozoite that undergoes schizogony to make many merozoites (see the example in Figure 3). Many are capable of infecting a variety of animal cells, from insects to livestock to humans, and their life cycles often depend on transmission between multiple hosts. The genus Plasmodium is an example of this group.

a) A diagram of an apicomlexan protist. The cell is a long oval with an apical complex at the apical end. B) A micrograph of the protist showing a long oval.

Figure 9. (a) Apicomplexans are parasitic protists. They have a characteristic apical complex that enables them to infect host cells. (b) A colorized electron microscope image of a Plasmodium sporozoite. (credit b: modification of work by Ute Frevert)

Other apicomplexans are also medically important. Cryptosporidium parvum causes intestinal symptoms and can cause epidemic diarrhea when the cysts contaminate drinking water. Theileria (Babesia) microti, transmitted by the tick Ixodes scapularis, causes recurring fever that can be fatal and is becoming a common transfusion-transmitted pathogen in the United States (Theileria and Babesia are closely related genera and there is some debate about the best classification). Finally, Toxoplasma gondii causes toxoplasmosis and can be transmitted from cat feces, unwashed fruit and vegetables, or from undercooked meat. Because toxoplasmosis can be associated with serious birth defects, pregnant women need to be aware of this risk and use caution if they are exposed to the feces of potentially infected cats. A national survey found the frequency of individuals with antibodies for toxoplasmosis (and thus who presumably have a current latent infection) in the United States to be 11%. Rates are much higher in other countries, including some developed countries.[1] There is also evidence and a good deal of theorizing that the parasite may be responsible for altering infected humans’ behavior and personality traits.[2]

A micrograph of an oval cell with many short projections.

Figure 10. This specimen of the ciliate Balantidium coli is a trophozoite form isolated from the gut of a primate. B. coli is the only ciliate capable of parasitizing humans. (credit: modification of work by Kouassi RYW, McGraw SW, Yao PK, Abou-Bacar A, Brunet J, Pesson B, Bonfoh B, N’goran EK & Candolfi E)

The ciliates (Ciliaphora), also within the Chromalveolata, are a large, very diverse group characterized by the presence of cilia on their cell surface. Although the cilia may be used for locomotion, they are often used for feeding, as well, and some forms are nonmotile. Balantidium coli (Figure 10) is the only parasitic ciliate that affects humans by causing intestinal illness, although it rarely causes serious medical issues except in the immunocompromised (those having a weakened immune system). Perhaps the most familiar ciliate is Paramecium, a motile organism with a clearly visible cytostome and cytoproct that is often studied in biology laboratories (Figure 11). Another ciliate, Stentor, is sessile and uses its cilia for feeding (Figure 12). Generally, these organisms have a micronucleus that is diploid, somatic, and used for sexual reproduction by conjugation. They also have a macronucleus that is derived from the micronucleus; the macronucleus becomes polyploid (multiple sets of duplicate chromosomes), and has a reduced set of metabolic genes.

Ciliates are able to reproduce through conjugation, in which two cells attach to each other. In each cell, the diploid micronuclei undergo meiosis, producing eight haploid nuclei each. Then, all but one of the haploid micronuclei and the macronucleus disintegrate; the remaining (haploid) micronucleus undergoes mitosis. The two cells then exchange one micronucleus each, which fuses with the remaining micronucleus present to form a new, genetically different, diploid micronucleus. The diploid micronucleus undergoes two mitotic divisions, so each cell has four micronuclei, and two of the four combine to form a new macronucleus. The chromosomes in the macronucleus then replicate repeatedly, the macronucleus reaches its polyploid state, and the two cells separate. The two cells are now genetically different from each other and from their previous versions.

Paramecium cell with short strands on the outside labeled cilia. An indent in the outer layer is labeled cytostome. The outside edge of the cytostome is an indent in the cell labeled oral groove. A sphere inside the cell at the base of the cytostome is labeled food vacuole, another nearby sphere is labeled cytoproct. A smaller opening in the cell is labeled anal pore. A star shaped structure inside the cell is labeled contractile vacuole. A large oval is labeled macronucluus and a smaller oval is labeled micronucleus.

Figure 11. Paramecium has a primitive mouth (called an oral groove) to ingest food, and an anal pore to excrete it. Contractile vacuoles allow the organism to excrete excess water. Cilia enable the organism to move.

A micrograph of long trumpet shaped cells. The wide part of the cell has an oval structure labeled cytostome and small projections labeled cilia.

Figure 12. This differential interference contrast micrograph (magnification: ×65) of Stentor roeselie shows cilia present on the margins of the structure surrounding the cytostome; the cilia move food particles. (credit: modification of work by “picturepest”/Flickr)

Öomycetes have similarities to fungi and were once classified with them. They are also called water molds. However, they differ from fungi in several important ways. Öomycetes have cell walls of cellulose (unlike the chitinous cell walls of fungi) and they are generally diploid, whereas the dominant life forms of fungi are typically haploid. Phytophthora, the plant pathogen found in the soil that caused the Irish potato famine, is classified within this group (Figure 13).

A photograph of an insect covered in white fuzz labeled water mold.

Figure 13. A saprobic oomycete, or water mold, engulfs a dead insect. (credit: modification of work by Thomas Bresson)

This video shows the feeding of Stentor:


The third and final supergroup to be considered in this section is the Excavata, which includes primitive eukaryotes and many parasites with limited metabolic abilities. These organisms have complex cell shapes and structures, often including a depression on the surface of the cell called an excavate. The group Excavata includes the subgroups Fornicata, Parabasalia, and Euglenozoa. The Fornicata lack mitochondria but have flagella. This group includes Giardia lamblia (also known as G. intestinalis or G. duodenalis), a widespread pathogen that causes diarrheal illness and can be spread through cysts from feces that contaminate water supplies (Figure 2). Parabasalia are frequent animal endosymbionts; they live in the guts of animals like termites and cockroaches. They have basal bodies and modified mitochondria (kinetoplastids). They also have a large, complex cell structure with an undulating membrane and often have many flagella. The trichomonads (a subgroup of the Parabasalia) include pathogens such as Trichomonas vaginalis, which causes the human sexually transmitted disease trichomoniasis. Trichomoniasis often does not cause symptoms in men, but men are able to transmit the infection. In women, it causes vaginal discomfort and discharge and may cause complications in pregnancy if left untreated.

The Euglenozoa are common in the environment and include photosynthetic and nonphotosynthetic species. Members of the genus Euglena are typically not pathogenic. Their cells have two flagella, a pellicle, a stigma (eyespot) to sense light, and chloroplasts for photosynthesis (Figure 14). The pellicle of Euglena is made of a series of protein bands surrounding the cell; it supports the cell membrane and gives the cell shape.

An oval cell with a long flagellum at one end near the photoreceptor (paraflagellar body). A large oval inside the cell is labeled nucleus and contains a smaller oval labeled nucleolus. Green structures are labeled chloroplasts. A red circle is labeled stigma (eyespot).Another sphere is labeled contractile vacuole and a large sphere is labeled pellicle bands. Gray stuructures are labeled polysaccharides stored by photosynthesis.

Figure 14. (a) This illustration of a Euglena shows the characteristic structures, such as the stigma and flagellum. (b) The pellicle, under the cell membrane, gives the cell its distinctive shape and is visible in this image as delicate parallel striations over the surface of the entire cell (especially visible over the grey contractile vacuole). (credit a: modification of work by Claudio Miklos; credit b: modification of work by David Shykind)

The Euglenozoa also include the trypanosomes, which are parasitic pathogens. The genus Trypanosoma includes T. brucei, which causes African trypanosomiasis (African sleeping sickness and T. cruzi, which causes American trypanosomiasis (Chagas disease). These tropical diseases are spread by insect bites. In African sleeping sickness, T. brucei colonizes the blood and the brain after being transmitted via the bite of a tsetse fly (Glossina spp.) (Figure 15). The early symptoms include confusion, difficulty sleeping, and lack of coordination. Left untreated, it is fatal.

The life cycle of Trypanosoma brucei takes place in both tsetse fly and humans. When the tsetse fly takes a blood meal it inject T. brucei into the bloodstream of a human. There the T. brucei multiplies by binary fission in blood, lymph, and spinal fluid. When another tsetse fly takes a blood meal it ingests T. brucei which multiplies by binary fission in the midgut of the fly. The T. brucei then transforms into an infectious stage which enters the salivary glands and multiplies. This can then be spread to another human.

Figure 15. Trypanosoma brucei, the causative agent of African trypanosomiasis, spends part of its life cycle in the tsetse fly and part in humans. (credit “illustration”: modification of work by Centers for Disease Control and Prevention; credit “photo”: DPDx/Centers for Disease Control and Prevention)

Chagas’ disease originated and is most common in Latin America. The disease is transmitted by Triatoma spp., insects often called “kissing bugs,” and affects either the heart tissue or tissues of the digestive system. Untreated cases can eventually lead to heart failure or significant digestive or neurological disorders.

The genus Leishmania includes trypanosomes that cause disfiguring skin disease and sometimes systemic illness as well.

Neglected Parasites

The Centers for Disease Control and Prevention (CDC) is responsible for identifying public health priorities in the United States and developing strategies to address areas of concern. As part of this mandate, the CDC has officially identified five parasitic diseases it considers to have been neglected (i.e., not adequately studied). These neglected parasitic infections (NPIs) include toxoplasmosis, Chagas disease, toxocariasis (a nematode infection transmitted primarily by infected dogs), cysticercosis (a disease caused by a tissue infection of the tapeworm Taenia solium), and trichomoniasis (a sexually transmitted disease caused by the parabasalid Trichomonas vaginalis).

The decision to name these specific diseases as NPIs means that the CDC will devote resources toward improving awareness and developing better diagnostic testing and treatment through studies of available data. The CDC may also advise on treatment of these diseases and assist in the distribution of medications that might otherwise be difficult to obtain.[3]

Of course, the CDC does not have unlimited resources, so by prioritizing these five diseases, it is effectively deprioritizing others. Given that many Americans have never heard of many of these NPIs, it is fair to ask what criteria the CDC used in prioritizing diseases. According to the CDC, the factors considered were the number of people infected, the severity of the illness, and whether the illness can be treated or prevented. Although several of these NPIs may seem to be more common outside the United States, the CDC argues that many cases in the United States likely go undiagnosed and untreated because so little is known about these diseases.[4]

What criteria should be considered when prioritizing diseases for purposes of funding or research? Are those identified by the CDC reasonable? What other factors could be considered? Should government agencies like the CDC have the same criteria as private pharmaceutical research labs? What are the ethical implications of deprioritizing other potentially neglected parasitic diseases such as leishmaniasis?

Key Concepts and Summary

  • Protists are a diverse, polyphyletic group of eukaryotic organisms.
  • Protists may be unicellular or multicellular. They vary in how they get their nutrition, morphology, method of locomotion, and mode of reproduction.
  • Important structures of protists include contractile vacuoles, cilia, flagella, pellicles, and pseudopodia; some lack organelles such as mitochondria.
  • Taxonomy of protists is changing rapidly as relationships are reassessed using newer techniques.
  • The protists include important pathogens and parasites.

Multiple Choice

Which genus includes the causative agent for malaria?

  1. Euglena
  2. Paramecium
  3. Plasmodium
  4. Trypanosoma

Which protist is a concern because of its ability to contaminate water supplies and cause diarrheal illness?

  1. Plasmodium vivax
  2. Toxoplasma gondii
  3. Giardia lamblia
  4. Trichomonas vaginalis

Fill in the Blank

The plasma membrane of a protist is called the __________.

Animals belong to the same supergroup as the kingdom __________.

Think about It

  1. What are kinetoplastids?
  2. Aside from a risk of birth defects, what other effect might a toxoplasmosis infection have?
  3. What is the function of the ciliate macronucleus?
  4. Protist taxonomy has changed greatly in recent years as relationships have been re-examined using newer approaches. How do newer approaches differ from older approaches?
  5. What characteristics might make you think a protist could be pathogenic? Are certain nutritional characteristics, methods of locomotion, or morphological differences likely to be associated with the ability to cause disease?
  6. The protist shown has which of the following?
    1. pseudopodia
    2. flagella
    3. a shell
    4. cilia
A micrograph of an oval cell with a large sphere in the center and many short hair-like projections on the outside.

(credit: modification of work by Richard Robinson)

  1. J. Flegr et al. "Toxoplasmosis—A Global Threat. Correlation of Latent Toxoplasmosis With Specific Disease Burden in a Set of 88 Countries." PloS ONE 9 no. 3 (2014):e90203.
  2. J. Flegr. "Effects of Toxoplasma on Human Behavior." Schizophrenia Bull 33, no. 3 (2007):757–760.
  3. Centers for Disease Control and Prevention. "Neglected Parasitic Infections (NPIs) in the United States." http://www.cdc.gov/parasites/npi/. Last updated July 10, 2014.
  4. Centers for Disease Control and Prevention. "Fact Sheet: Neglected Parasitic Infections in the United States." http://www.cdc.gov/parasites/resources/pdf/npi_factsheet.pdf