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1.34: Protozoan Parasites

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    Learning Objectives
    • Identify and name the following protozoan parasites in microscopic samples: Plasmodium sp., Trypanosoma cruzi, and Trichomonas vaginalis
    • Name the diseases caused by the following protozoan parasites: Plasmodium sp., Trypanosoma cruzi, and Trichomonas vaginalis
    • Explain how the following protozoan parasites are transmitted to humans: Plasmodium sp., Trypanosoma cruzi, and Trichomonas vaginalis
    • Interpret the life cycles of the following protozoan parasites: Plasmodium sp., Trypanosoma cruzi, and Trichomonas vaginalis
    • Examine the following protozoan parasites using a microscope and illustrate and label the specimens: Plasmodium sp., Trypanosoma cruzi, and Trichomonas vaginalis


    Introduction to Protozoan Parasites

    All protozoans are found in a life stage called trophozoites. The “troph” stage is actively feeding, mostly motile and responsible for symptoms in a host. Some protozoans can form a cyst stage – a resting, inactive stage that protects the organism because of a thick wall that is produced. These cysts enable the protozoan to survive harsh environmental conditions outside of a host and ensure the organism will be passed to other hosts. Ingestion of the cyst in contaminated food or water is a common method of acquiring many protozoan infections.

    Many protozoan parasites require vectors for transmission to occur. A vector is a transport mechanism. With parasites requiring a vector, the vector is a living thing, often an arthropod species (e.g. mosquito).

    The life cycles of parasites are very specific. Parasites require transmission to specific species of organisms (the hosts) in a specific order to continue spreading and reproducing. At times, a parasite may be transmitted to a host that is not in their life cycle. This accidental host can experience symptoms of parasite infection, but the parasite is unable to continue its life cycle and continue reproduction.

    In additional to an accidental host, host species can also be classified as either the definitive host or the intermediate host:

    • definitive host: a host species that can harbor either the adult form of a parasite or the sexual stage of a parasite
    • intermediate host: a host species that can harbor an immature form of a parasite in a non-sexual stage

    Protozoan parasites can undergo sexual reproduction, asexual reproduction, or both sexual and asexual reproduction depending on the species. Asexual reproduction will typically involve cell divisions to produce new individuals, whereas sexual reproduction will involve production and fusion of gametes ("male" and "female" cells that have half the DNA of the other cells and can fuse to produce genetically unique individuals).


    Plasmodium sp. (Cause of Malaria)


    Introduction to Plasmodium sp.

    Plasmodium (plaz-mo’dee-um) causes malaria, one of the common causes of hemolytic anemia worldwide. There are four species of PlasmodiumP. vivax (most common), P. ovale, P. malariae, and P. falciparum (most lethal). Typical symptoms are fever, chills, headache, muscle pain, and sweating. According to WHO, there were an estimated 229 million cases of malaria per year worldwide, mostly (94%) in the African Region. Cases of malaria diagnosed in the United States were mostly seen in travelers and immigrants returning to the U.S. from countries where malaria is endemic.

    Malaria is the number one cause of death by parasites in the world. To continue existing, it must alternate sexual and asexual cycles. Interruption of either life cycle will control the disease. Measures taken to interrupt the life cycle include attempts to eliminate the Anopheles mosquito, to protect the host from being bitten using chemical repellants and mosquito netting, to prophylactically treating travelers in high risk areas, to cure active cases with various antiparasitic drugs. The occurrence of drug resistant strains worldwide has dramatically increased in recent years. Contact with CDC will aid in determining the best prophylactic drug and drug of choice for treatment depending on the patient’s health and the area in which malaria was acquired.


    Clinical Presentation of Malaria

    The symptoms of uncomplicated malaria can be rather non-specific and the diagnosis can be missed if health providers are not alert to the possibility of this disease. Since untreated malaria can progress to severe forms that may be rapidly (<24 hours) fatal, malaria should always be considered in patients who have a history of exposure (mostly: past travel or residence in disease-endemic areas). The most frequent symptoms include fever and chills, which can be accompanied by headache, myalgias, arthralgias, weakness, vomiting, and diarrhea. Other clinical features include splenomegaly, anemia, thrombocytopenia, hypoglycemia, pulmonary or renal dysfunction, and neurologic changes. The clinical presentation can vary substantially depending on the infecting species, the level of parasitemia, and the immune status of the patient. Infections caused by P. falciparum are the most likely to progress to severe, potentially fatal forms with central nervous system involvement (cerebral malaria), acute renal failure, severe anemia, or acute respiratory distress syndrome. Other species can also have severe manifestations. Complications of P. vivax malaria include splenomegaly (with, rarely, splenic rupture), and those of P. malariae include nephrotic syndrome.


    Asexual Stage of Plasmodium sp. in Humans

    One becomes infected with Plasmodium through the bite of the infected female Anopheles mosquito. The mosquito transmits Plasmodium sporozoites via her saliva when she inserts her proboscis into human skin to obtain a blood meal. The blood provides nourishment for the eggs she will lay. Sporozoites injected into the blood stream leave the blood vascular system within a period of forty minutes and invade the parenchymal cells of the liver. In liver cells, the sporozoites undergo asexual multiplication. They are then liberated and invade red blood cells, initiating the blood stream phase of the infection.

    An asexual cycle, known as schizogony, takes place within the red cells of the infected host. This process results in the formation of four to thirty-six new parasites in each red cell. Immature trophozoites, called a “ring” forms, develop which then enlarge to become mature trophozoites, filling most of the parasitized red blood cells. Asexual multiplication occurs when the trophozoites’ nuclear material and cytoplasm split. At the end of the schizogonic cycle the infected blood cells rupture, liberating merozoites, which, in turn, infect new red blood cells. Lysis of the red cells liberates products of metabolism of the parasites and the red cells. These toxic materials cause the symptoms of malaria – chills, fever, nausea, vomiting and headache. The fever spikes occur at varying intervals of 24, 48, or 72 hours depending on the species of Plasmodium present. The febrile period may last several hours, ending with a profuse sweating stage. This cycle is repeated many times.


    ring stage of Plasmodium inside of red blood cells in this microscopic image of infected human blood

    Figure 1: Plasmodium sp. infecting a human blood sample. The larger pink round-ish structures are all red blood cells from human blood. The purple-stained round structures found inside some of the red blood cells are the ring stage of Plasmodium. The ring stage is an intracellular stage, meaning the parasite lives inside of the host's cells. This phase is a common way malaria is diagnosed. Diagnosis involves identification of the Plasmodium sp. ring stage within red blood cells.


    Sexual Stage of Plasmodium sp. in Mosquitos

    Sexual stage male and female gametocytes may also appear in the red blood cells. When the mosquito bites an infected person, she draws blood into her stomach which may contain male and female gametocytes. In the mosquito’s gut, the male gametocytes form spermatozoa and the female forms an ovum. Fertilization takes place. The resulting zygote can invade the gut wall and produce numerous sporozoites. The sporozoites migrate through the tissue of the mosquito to the salivary glands where they will be injected into the next human host when the mosquito takes another blood meal. The asexual cycle then proceeds in the new host.


    Plasmodium sp. Life Cycle

    Blood parasites of the genus Plasmodium. There are approximately 156 named species of Plasmodium which infect various species of vertebrates. Four species are considered true parasites of humans, as they utilize humans almost exclusively as a natural intermediate host: P. falciparum, P. vivax, P. ovale and P. malariae.

    plasmodium life cycle

    Figure 2: Plasmodium sp. life cycle for species that infect humans. See the table below for details about the letters and numbers shown on this diagram.

    Plasmodium 1

    During a blood meal, a malaria-infected female Anopheles mosquito inoculates sporozoites into the human host.

    Plasmodium A

    Human Liver Stages (exo-erythrocytic schizogony)

    Plasmodium 2

    Sporozoites infect liver cells

    Plasmodium 3

    …and mature into schizonts

    Plasmodium 4

    … which rupture and release merozoites.


    (Of note, in P. vivax and P. ovale a dormant stage [hypnozoites] can persist in the liver and cause relapses by invading the bloodstream weeks, or even years later.)

    Plasmodium B

    Human Blood Stages (erythrocytic schizogony)

    The parasites undergo asexual multiplication in the erythrocytes (red blood cells).

    Plasmodium 5

    Merozoites infect red blood cells.

    Plasmodium 6

    The ring stage trophozoites mature into schizonts, which rupture releasing merozoites.

    Plasmodium 7

    Some parasites differentiate into sexual erythrocytic stages (gametocytes).


    Blood stage parasites are responsible for the clinical manifestations of the disease.

    Plasmodium 8

    The gametocytes, male (microgametocytes) and female (macrogametocytes), are ingested by an Anopheles mosquito during a blood meal.

    Plasmodium C

    Mosquito Stages. The parasites’ multiplication in the mosquito is known as the sporogonic cycle.

    Plasmodium 9

    While in the mosquito's stomach, the microgametes penetrate the macrogametes generating zygotes.

    Plasmodium 10

    The zygotes in turn become motile and elongated (ookinetes)…

    Plasmodium 11

    … which invade the midgut wall of the mosquito where they develop into oocysts.

    Plasmodium 12

    The oocysts grow, rupture, and release sporozoites…

    Plasmodium 1

    …which make their way to the mosquito's salivary glands. Inoculation of the sporozoites into a new human host perpetuates the malaria life cycle.


    Laboratory Instructions: Plasmodium sp.

    1. Examine a blood smear infected with Plasmodium sp. and identify the ring-like immature trophozoites (will require 400x or 1000x magnification).
    2. Carefully illustrate the sample as you see it in the microscope. Label "red blood cells" and "Plasmodium sp. ring stage" in your illustration.


    Results & Questions: Plasmodium sp.

    Microscopic illustration location

    1. Carefully illustrate the sample as you see it in the microscope. Label "red blood cells" and "Plasmodium sp. ring stage" in your illustration.
    2. What stage of the Plasmodium sp. life cycle did you examine (human liver stage, human blood stage, or mosquito stage)?
    3. What disease does Plasmodium sp. cause in humans?
    4. What human tissues are inhabited by the Plasmodium sp. parasite in humans?
    5. How is Plasmodium sp. transmitted to humans?
    6. What species is the definitive host of Plasmodium sp.? How do you know?
    7. What species is the intermediate host of Plasmodium sp.? How do you know?
    8. What are the four Plasmodium species that cause malaria in humans?
    9. Where in the world is malaria present?
    10. How can people in the United States present with a malaria infection?
    11. Give at least four symptoms of malaria.


    Trypanosoma cruzi (Cause of Chagas Disease)


    Introduction to Trypanosoma cruzi

    Trypanosoma structure in microscopic image living between human red blood cells; Triatoma bugs that transmit Trypanosoma

    Figure 3: (Top) Human blood viewed under a light microscope. The normal human red blood cells (rbc) are round and pink. Trypanosoma cruzi inhabits the blood (purple). In this high magnification view of T. cruzi, visible are the flagellum wrapped around the cell and connected to the cell by its undulating membrane that waves to propel the parasite around. (Bottom left) Image of Triatoma sp., one of the types of reduviid bugs capable of spreading T. cruzi to humans. (Bottom right) Entry of T. cruzi from its vector to the human involves a break or opening in the skin (commonly from a bite from the bug) after which the bug defecates. T. cruzi passes from the feces into the skin wound to cause infection in the human.


    Trypanosoma cruzi (trip-an’o-so’muh/kroo’zye) is the protozoan responsible for causing Chagas disease. T. cruzi is limited to the western hemisphere, including California, the southern United States, Central and South America.It is most often seen in rural areas in Latin America where poverty is widespread. The vector for T. cruzi is reduviid bug species known as kissing bugs. The most common genera responsible for transmission of the disease are Triatoma, Rhodnius, and Panstrongylus. Infection usually occurs after bugs defecate on the bite site and are rubbed into the wound by the host scratching. Humans contract Chagas disease (South American Trypanosomiasis) when an infected bug bites the human skin and subsequently defecates in the wound. Infection may be mild or asymptomatic. There may be fever or swelling around the bug bite. Parasites may also be found in the circulating blood. Chagas disease occurs immediately after infection and may last a few weeks or months. In 20-30% of infected people, acute infection may result in severe inflammation of the heart muscle or the brain and lining around the brain.


    Trypanosoma in blood smear labeled

    Figure 4: Human blood smear infected with Trypanosoma cruzi trypomastigotes. The round pink structures are normal human red blood cells. The purple structures between the red blood cells is the T. cruzi parasite.


    Clinical Presentation of Chagas Disease

    Chagas disease has an acute phase and chronic phase. The acute phase is usually asymptomatic, but can present with nonspecific somatic symptoms. Rarely, the acute phase may be more severe with potential cardiac or neurologic symptoms and signs. Nodular lesions or furuncles, usually called chagomas, may develop around the vector’s feeding site. Chagomas occurring on the on the eyelids are commonly referred to as palpebral and periocular firm swelling. Most acute cases resolve over a period of a few weeks or months into a subclinical chronic form of the disease (“indeterminate form”). Reactivation of Chagas disease from this asymptomatic form may occur in patients with HIV or those receiving immunosuppressive drugs.

    The symptomatic chronic form (“determinate form”) may not occur for years or even decades after initial infection. This may include cardiac or gastrointestinal involvement, which occasionally occur together. The many complications of chronic Chagas disease can be fatal. Amastigote invasion of smooth muscle can lead to megaesophagus, megacolon, and dilated cardiomyopathy.



    Life Cycle of Trypanosoma cruzi


    Chagas Life Cycle

    Figure 5: Trypanosoma cruzi life cycle. See table below for details about each stage in the diagram.

    Trypanosoma 1 An infected triatomine insect vector (or “kissing” bug) takes a blood meal and releases trypomastigotes in its feces near the site of the bite wound. Trypomastigotes enter the host through the bite wound or intact mucosal membranes, such as the conjunctiva.
    Trypanosoma 2 Inside the host, the trypomastigotes invade cells near the site of inoculation, where they differentiate into intracellular amastigotes.
    Trypanosoma 3 The amastigotes multiply by binary fission…
    Trypanosoma 4 … and differentiate into trypomastigotes, and then are released into the circulation as bloodstream trypomastigotes.
      Trypomastigotes infect cells from a variety of tissues and transform into intracellular amastigotes in new infection sites. Clinical manifestations can result from this infective cycle. The bloodstream trypomastigotes do not replicate (different from the African trypanosomes). Replication resumes only when the parasites enter another cell or are ingested by another vector.
    Trypanosoma 5

    The “kissing” bug becomes infected by feeding on human or animal blood that contains circulating parasites.

    Trypanosoma 6

    The ingested trypomastigotes transform into epimastigotes in the vector’s midgut.

    Trypanosoma 7

    The parasites multiply and differentiate in the midgut…

    Trypanosoma 8

    … and differentiate into infective metacyclic trypomastigotes in the hindgut.


    Other less common routes of transmission include blood transfusions, organ transplantation, transplacental transmission, and foodborne transmission (via food/drink contaminated with the vector and/or its feces).


    Laboratory Instructions: Trypanosoma cruzi

    1. Examine a blood smear infected with Trypanosoma cruzi and identify the T. cruzi trophozoites (will require 400x or 1000x magnification).
    2. Carefully illustrate the sample as you see it in the microscope. Label the following in your illustration:
      • "red blood cells"
      • "Trypanosoma cruzi trypomastigote"
      • "flagellum"
      • "undulating membrane"
      • "nucleus"


    Results & Questions: Trypanosoma cruzi

    Microscopic illustration location

    1. Carefully illustrate the sample as you see it in the microscope. Label the following in your illustration:
      • "red blood cells"
      • "Trypanosoma cruzi trypomastigote"
      • "flagellum"
      • "undulating membrane"
      • "nucleus"
    2. Explain how humans become infected with T. cruzi.
    3. What disease is caused by T. cruzi infection in humans?
    4. What geographic regions of the world is T. cruzi infection possible?
    5. Give at least three symptoms of Chagas disease.
    6. Can Chagas disease be asymptomatic? Explain your answer.
    7. Carefully examine the life cycle of T. cruzi. Inside of the human host, is T. cruzi an intracellular parasites (lives inside host cells) or an intercellular parasite (lives outside of host cells)? Explain your answer.


    Trichomonas vaginalis (Cause of Trichomoniasis)


    Introduction to Trichomonas vaginalis

    Trichomoniasis a common, treatable, sexually transmitted disease (STD). Most people who have trichomoniasis do not have any symptoms.

    Trichomonas vaginalis (trick’o-mo’nas/vadj-i-nay’lis) has no cyst stage so it cannot survive outside its host very long. It is transmitted by intimate contact between individuals. It is the cause of trichomoniasis, commonly called “ping-pong vaginitis” because it may be passed back and forth between sexual partners. In the United States, an estimated 2 million people have the infection, but only about 30% develop any symptoms of trichomoniasis. While most infections are asymptomatic, it can cause prostate and epididymis infections in men. Females report frequent urination, itching, burning, and a vaginal discharge. Diagnosis often occurs when people seek medical help for what they believe is a urinary tract infection. Flagyl is a medication used to treat infections. Diagnosis is usually made by microscopically identifying the tear-dropped shaped trophozoites in a wet preparation of a vaginal or urethral discharge. Three to five flagella may be visible as a tuft at the anterior end of the cell. T. vaginalis will exhibit a quick, jerky, darting motility as it zooms around the field of vision.



    In the United States, CDC estimates that there were more than two million trichomoniasis infections in 2018. However, only about 30% develop any symptoms of trich. Infection is more common in women than in men. Older women are more likely than younger women to have the infection.


    Trichomonas vaginalis cell structure shown along side a microscopic image of T. vaginalis

    Figure 6: (Left) Illustration of a Trichomonas vaginalis trophozoite with its undulating membrane, axostyle, nucleus, and flagella labeled. (Right) Microscopic image of a T. vaginalis trophozoite.


    Clinical Presentation of Trichomoniasis

    About 70% of people with the infection do not have any signs or symptoms. When trich does cause symptoms, they can range from mild irritation to severe inflammation. Some people get symptoms within 5 to 28 days after getting the infection. Others do not develop symptoms until much later. Symptoms can come and go.

    Men with trich may notice:

    • Itching or irritation inside the penis;
    • Burning after peeing or ejaculating; and
    • Discharge from the penis.

    Women with trich may notice:

    • Itching, burning, redness or soreness of the genitals;
    • Discomfort when peeing; and
    • A clear, white, yellowish, or greenish vaginal discharge (i.e., thin discharge or increased volume) with a fishy smell.

    Having trich can make sex feel unpleasant. Without treatment, the infection can last for months or even years.

    Pregnant people with trich are more likely to have their babies early. Also, their babies are more likely to have a low birth weight (less than 5.5 pounds).


    Life Cycle of Trichomonas vaginalis


    Trichomonas vaginalis life cycle

    Figure 7: Life cycle of T. vaginalis.


    Laboratory Instructions: Trichomonas vaginalis

    1. Examine the sample of T. vaginalis and identify the T. vaginalis trophozoites (use 400x or 1000x magnification).
    2. Carefully illustrate the sample as you see it in the microscope. Label the following in your illustration:
      • "Trichomonas vaginalis"
      • "flagella"
      • "undulating membrane"
      • "nucleus"
      • "axostyle"
      • "posterior axostyle"


    Results & Questions: Trichomonas vaginalis

    Microscopic illustration location

    1. Carefully illustrate the sample as you see it in the microscope. Label the following in your illustration:
      • "Trichomonas vaginalis"
      • "flagella"
      • "undulating membrane"
      • "nucleus"
      • "axostyle"
      • "posterior axostyle"
    2. What disease is caused by T. vaginalis?
    3. Give at least two common symptoms of trichomoniasis in men.
    4. Give at least two common symptoms of trichomoniasis in women.
    5. How frequently is trichomoniasis asymptomatic?
    6. How is trichomoniasis spread?
    7. How many host species does T. vaginalis have?
    8. How is trichomoniasis diagnosed?



    This page titled 1.34: Protozoan Parasites is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Rosanna Hartline.

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