Parasite Mcq

Exercise Pre-Test Attempt to answer the following questions before starting this exercise. Theywill serve as a guide to important concepts. Recheck your answers after you complete this exercise andanswer the laboratory report at the end of the exercise.

Exercise 9

1. The most environmentally resistant form of manyparasites is a stage of development called the:

a. vegetarium c. spore

b. cyst d. trophozoite

2. The main host of Toxoplasma gondii is:

a. pregnant women c. cats

b. mosquitoes d. rats

3. The parasite which “looks back at you” when seenunder the microscope is:

a. Toxoplasma gondii c. Plasmodium vivax

b. Giardia lamblia d. Plasmodium falciparum

4. Malarial parasites reproduce in:

a. lung tissue c. brain tissue

b. red blood cells d. placental tissue

5. Which parasitic infection is often associated withfecally contaminated water (fecal-oral) or food?

a. Cryptosporidium c. Toxoplasma

b. Giardia d. all of these

P A R A S I T O L O G Y 81

4. Explain why Toxoplasma gondii is dangerous tofetuses and people with damaged immune systems,but not as dangerous to people with normallyfunctioning immunity.

5. Describe the method of transmission of Giardiaand Cryptosporidium.

Objectives

After completing this lab, you should be able to:

1. Describe the relationship between a parasite and ahost.

2. Differentiate between protozoan cysts andtrophozoites.

3. Explain why the female Anopheles mosquito isconsidered a biological vector.

Any organism (or virus) that lives off another organismbecomes a parasite if that other organism, called thehost, suffers significant damage from this relationship.Parasitism is one of three forms of “living together,” orsymbiosis. Mutualism, in which both organisms benefit,and commensalism, in which one of the organisms ben-efits and the other is neither helped nor harmed, are theother two. Although bacteria, viruses, and fungi all havemembers that are quite capable of causing severe dam-

age to their respective hosts, a more narrow view lim-its the scope of parasitology to protozoa, helminths(worms), and arthropods (insects and arachnids).

The unappetizing thought of having one or even twoanimals using us as living quarters pales compared tothe conditions that allowed such living arrangementsin Europe during the Middle Ages. With virtually non-existent sanitation, lack of bathing facilities and no cen-tral heating, people wore several layers of clothing for

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82 M I C R O B I A L M O R P H O L O G Y , D I F F E R E N T I A L S T A I N S

*Malaria is typically thought of as a foreign disease, but it has oftenshown up in the United States. In 1988, two boys contracted this dis-ease while in a New York State Boy Scout camp.

weeks and months at a time. These conditions allowedfor a zoological garden of vermin to develop and thrivewithin the folds of clothing these people wore.

These conditions can be illustrated by the accountof the death of Thomas à Becket, the Archbishop ofCanterbury, who was murdered by agents of HenryII in December of 1170. At the time of his death, hewore no less than eight layers of clothing. As his bodycooled, lice, fleas, bedbugs, and various other six- andeight-legged fauna left the body. To quote a reportfrom the time: “The vermin boiled over like water ina simmering cauldron, and the onlookers burst intoalternate weeping and laughter.”

This exercise will consider several protozoanparasites.

Most protozoan parasites associated with humandiseases belong to the class Sporozoa. Characteristicsof this group include the alteration of sexual and asex-ual stages of development and a resistant sporelike stageof development called a cyst or an oocyst. The activelygrowing and motile forms of these microbes are calledtrophozoites. During this trophozoite stage, the proto-zoan reproduces, usually asexually, and invades the hosttissue. This is also the stage that is most easily killedby body defenses, changes in the environment, or med-ication. The cyst, or oocyst, can resist drying and harshchemicals for long periods of time. Cryptosporidiumcysts easily survive the levels of chlorination found inmunicipal water supplies.

Protozoan parasites are responsible for many dis-eases in tropical and subtropical areas of the world.Malaria is making a serious comeback as a major dis-ease after years of control via insecticides and drugs.Now resistance to both of these control methods is mak-ing malaria a killer of over 2 million people per year.*

Closer to home, protozoans such as Giardia,Cryptosporidium, and Toxoplasma cause serious dis-eases among various populations in the United States.Giardia lamblia is frequently found in untreated watersupplies such as rivers and streams. Hikers who pausefor that refreshing cool drink may wind up with severaldays of cramps, diarrhea, nausea, and flatulence. Evenmore prevalent now is Cryptosporidium parvum, whichhas been able to invade the water supplies of majorcities. The city of Milwaukee was placed at risk in 1993when its entire water supply became contaminated.

A significant risk to fetuses is Toxoplasma gondii.Infection from this microbe can be traced to under-cooked meat and cat litter boxes. Up to 50% of fetuses

whose mothers were infected during the first trimesterof pregnancy become infected themselves. (See Fig. 9.5for the life cycle of this microbe.) The possible conse-quences of such prenatal infection include miscarriage,stillbirth, and numerous congenital defects, includingmental retardation. The most common defect is symp-toms of retinitis, which involves pain, light sensitivity,and blurred vision. The parasite can remain latent in thebody for years before it becomes active. Depression ofthe host’s immune system acts as a trigger for this ac-tivation; therefore, Toxoplasma gondii is often one ofthe diseases found in full-blown or frank AIDS patients.

Materials

Prepared slides of Plasmodium vivax in blood;Toxoplasma gondii oocysts, pseudocysts, sexualand asexual forms, trophozoite; Giardia lambliatrophozoite, sporozoite; Cryptosporidium parvumoocyst, sporozoite with merozoites

Malaria has a complex life cycle that includes thesalivary glands of the Anopheles mosquito as well as thered blood cells of humans. The mosquito is consideredan example of a biological vector. The Plasmodium par-asite must spend part of its life cycle within the mos-quito in order to become infective to humans. Therefore,if you can control the vector, you can control the dis-ease. Although primarily a tropical disease, two species,Plasmodium falciparum and Plasmodium vivax, arefound in the United States. Plasmodium vivax will beused as an example of the life cycle shown in Figure 9.1.

Inside the intestine of the female Anopheles mos-quito, male and female forms of the parasite called macrogametocytes (female) and microgametocytes(male) combine to form a zygote, or fertilized egg.Unfortunately for some unsuspecting mammals, includ-ing humans, a blood meal is required for this process totake place. The zygote matures into a wormlike form,which then develops into an oocyst. Within the oocyst,hundreds of infective sporozoites develop. When theoocyst lyses, the sporozoites spread throughout the mos-quito, including its salivary glands and ducts.

When the mosquito feeds again, some saliva, act-ing as an anticoagulant, enters the feeding site of themammal. The sporozoites are carried to the liver wherethey invade the host’s liver cells, reproduce, leave, andthen infect red blood cells (Plate 29). Once inside thered blood cell, the sporozoite develops into a ringliketrophozoite (Plate 30) which now develops into thou-sands of infective merozoites. The red blood cell lyses,and the merozoites infect adjacent cells. During thisprocess, some trophozoites develop into the male andfemale gametocytes, which are also released into the

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FIG. 9.1. Life cycle of Plasmodium vivax. Note the dependence on the female Anopheles mosquito.

Mosquito ingestsgametophytes

and they mature

Merozoitesreproduce in

red blood cells

Merozoites producedin liver cells

INSIDE HUMAN

INSIDEHUMAN

INSIDEMOSQUITO

Infection ofred blood

cells

Liver acquiressporozoitesfrom blood

Gametes produced

Liver transmitssporozoites in mosquito'ssalivary glands to blood

Merozoitesreleased

Mosquito

Mosquitoto human

Tomosquito

Tohuman

Fertilization

Development ofsporozoites and

migration tosalivary glands


bloodstream. The cycle continues if the infectedindividual is once again bitten by a female Anophelesmosquito.

Observe the slide of P. vivax under oil immersion. Findand identify the forms shown in Figures 9.2, 9.3, and 9.4.

Toxoplasma gondii has a life cycle that invariablyincludes cats, which are its primary host (Fig. 9.5). Themicrobe can be found in undercooked meat such as pork(25%) and lamb (10%). From this reservoir, it can in-fect humans directly, or it can first infect a householdcat. Cats can also become infected by eating infectedmice and rats.* Since humans are not part of this para-site’s normal life cycle, we are considered accidentalhosts.

When reproducing in the intestinal tract of thecat, some of the microbes differentiate into male and

FIG. 9.2. Ringlike trophozoite. FIG. 9.3. Merozoites.

FIG. 9.4. Microgametocytes and macrogametocytes.

*Recent research suggests that this parasite enhances its chance ofsurvival while in one of its intermediate hosts, the rat. When the ratis infected, usually by contacting the cyst form of Toxoplasma fromsoil or food, the brain becomes damaged. Such brain damage causesthe rat to be less aware of its surroundings, including the scent ofcats and the odor of its urine. This phenomenon allows the rat to bemore likely eaten by cats, thus continuing the life cycle of the parasite.

Red blood cell

Ringforms

Red blood cell

Malegametocyte

Femalegametocyte

Red blood cell

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FIG. 9.10. Life cycle of Giardia lamblia.

FIG. 9.8. Trophozoites or tachyzoites within pseudocystin liver.

FIG. 9.9. Pseudocysts in brain.female gametes, called micro- and macrogametocytes,the equivalent of eggs and sperm (Fig. 9.6). When thesegametes unite, a thick-walled oocyst is formed, whichis then expelled in the feces by the millions. In the soil,each oocyst develops into two oocysts, each containingfour sporozoites (Fig. 9.7). Once the sporozoites form,the microbe is infectious. Since the oocysts are resist-ant to drying and remain viable for up to a year, theycan waft into the air when cat litter is changed. Theyundergo further maturation and eventually become in-gested or inhaled.


FIG. 9.7. Diagram of oocysts and sporozoite forms.

FIG. 9.6. Small intestine of cat showing sexual and asex-ual forms.

FIG. 9.5. Life cycle of Toxoplasma gondii.

Immature oocyst

Changinglitter box

Infectedraw meat

Congenital infection

Litter boxOocyst

Sexualform

Asexualform

Once the oocysts and sporozoites are ingested byother animals, including humans, they invade the cellsof the intestines, and spread to the cells of the heart,brain, and muscle tissue (Plate 31). In these other ani-mals, there is no sexual stage as in the cat. As host im-munity is stimulated, large numbers of tropozoites (Plate32) (also called tachyzoites) become contained withinprotective coverings produced by the host called pseudo-cysts (Fig. 9.8). As long as the immune system remainsefficient, these pseudocysts remain intact, and furtherspread of the trophozoites is blocked. If a person withpseudocysts has a severe immunodeficiency such asAIDS or has a poorly developed immune system, suchas a fetus, the pseudocysts either never develop or theybreak open, allowing the parasite to spread (Fig. 9.9).

Giardia lamblia was first described byLeeuwenhoek over 300 years ago. (See Fig. 9.10 for lifecycle.) Students are often startled at their first look atthis parasite whose arrangement of paired nuclei in thetrophozoite form gives the appearance of them lookingback (Fig. 9.11). The trophozoite parasitizes the upperportion of the small intestine where it holds on to theintestinal wall by way of an adhesive disk, much like a

T. gondii

10 µm

Cyst wall

Development intotrophozoite insmall Intestine

Cyst inlarge Intestine

Ingestionvia drinking

Excretion intowater supply

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suction cup. If they break loose, they are carried towardthe colon where many of them develop into the inactivecyst form (Fig. 9.12). It is this cyst that causes diseasewhen ingested. If the trophozoite form is swallowed, itwill not survive the acidity of the stomach. (See Fig.9.10.) (Plate 33)

Cryptosporidium is also an intestinal parasite. Itsmethod of transmission is similar to that of Giardia(fecal-oral), but it is even more resistant to control meth-ods such as chlorination. Its life cycle is similar to thatof Toxoplasma, with various animals acting as interme-diate hosts (Fig. 9.13). While only annoying to healthypeople, severe, uncontrollable diarrhea and death can bethe result in AIDS patients. Young children and other

FIG. 9.11. Trophozoite of G. lamblia.

FIG. 9.12. Sporozoite of G. lamblia.

FIG. 9.13. Life cycle of Cryptosporidia.

FIG. 9.15. Oocyst of Cryptosporidium.

FIG. 9.14. Late stage of sporozoite development witheight banana-shaped merozoites.

Flagella

Nuclei

Flagella

Nuclei

Medianbodies

Cyst wall

Contaminatedfeces

Male andfemale

gametocytesproduceoocysts

Sporozoitesreleased,

merozoites produced

Intestine Human ingestingoocyst

Contaminatedwater

Oocyst

Cattle

HUMAN

Sporozoiteswithin oocyst

immunocompromised people are also at high risk forsevere symptoms.

Humans become infected by drinking water con-taminated with Cryptosporidium oocysts or by eatingfood prepared with such water. The usual host for thisparasite includes cattle, with up to an 80% infection rate(Plate 34). Poultry, sheep, even puppies and kittens, alsoshow significant rates of infection. Once in the intes-tine, the oocyst releases sporozoites, which then invadethe intestinal wall. The sporozoites divide into mero-zoites, which continues the invasive process (Fig. 9.14).Some merozoites develop into male and female gametes.When these gametes combine, they then produce theresistant oocysts. The oocysts then leave via the fecesand the cycle continues (Fig. 9.15).

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Biological vector An animal, such as a mosquito whichallows a parasite to spread, in which the parasite mustspend part of its life cycle and where it is able to re-produce.

Host The organism in or on which a parasite lives, of-ten causing harm or disease.

Intermediate host An animal in which the parasitegoes through a developmental stage.

Macrogametocyte The female gametocyte of the sex-ual stage of protozoan reproduction.

Merozoite The motile, infective stage of sporozoanprotozoa.

Microgametocyte The male gametocyte of the sexualstage of protozoan reproduction.

Oocyst The encysted form of a fertilized zygote or egg.The oocyst tends to be resistant to disinfection and re-leases large numbers of infectious sporozoites.

Parasite An organism that lives on or in another, de-rives nourishment, and often causes harm or disease.

Trophozoite The ameboid, asexual form of certain single-celled parasites.

WORKING DEFINITIONS AND TERMS

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A. CRITICAL THINKING

1. What organisms other than protozoa would be considered parasites?

2. Why would proper uses of insecticides, proper water purification methods, and proper meat and poultry inspec-tion and handling significantly reduce the numbers of parasitic infections or infestations?

3. Differentiate between the asexual stage of parasitic infections and the sexual stage.

4. Why would changing kitty litter be a possible danger to a pregnant woman?

5. Why are parasitic diseases a greater threat to the immunocompromised than to those with a normal immunesystem?

6. Differentiate between an intermediate host and a definitive host.

NAME _________________________________________ DATE ____________ SECTION ____________________

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B. MATCHING

1. a. accidental host of T. gondii

b. zygote

c. cyst

d. biological vector

e. definitive host of T. gondii

f. pseudocyst

g. gametocyte

h. trophozoite

i. Giardia lamblia

j. Cryptosporidium parvum

2.

_____ active form of protozoan parasite

_____ resistant form of protozoan parasite

_____ intermediate animal or host needed for a parasite to complete its lifecycle

_____ fertilized egg

_____ male and/or female form of a parasite

_____ humans

_____ cats

_____ “eyelike” paired nuclei

(Answers may be used more than once.)

Match the diagram with the parasite

a. malarial merozoite

b. malarial sporozoite

c. malarial trophozoite

d. malarial microgametocyte

e. Toxoplasma gondii sexual form

f. Toxoplasma gondii asexual form

g. Toxoplasma gondii pseudocysts

h. Giardia lamblia trophozoite

i. Giardia lamblia sporozoite

j. Cryptosporidium sporozoite

k. Cryptosporidium oocyst

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C. MULTIPLE CHOICE

1. The sexual reproductive stage of Plasmodium takes place:

a. in the human liver c. in a mosquito’s intestineb. in red blood cells d. in blood plasma

2. The first site to be infected in a human by Plasmodium vivax is:

a. the liver c. nervous tissueb. red blood cells d. blood plasma

3. The merozoite form of Plasmodium vivax is found:

a. in the human liver c. in a mosquito’s intestineb. in red blood cells d. in blood plasma

4. The oocyst of T. gondii can be found:

a. in cat litter boxes c. in the airb. in the soil d. in all of these

5. The definitive or final host of T. gondii is:

a. cat b. human c. cow d. rat

6. Which of the following can cause fetal damage?

a. Plasmodium b. Toxoplasma c. Giardia d. Cryptosporidium

7. Which of the following is found in contaminated water?

a. Plasmodium b. Toxoplasma c. Giardia d. all of these

8. Eight banana-shaped merozoites can be observed:

a. in the macrogametocyte of Plasmodiumb. in the sporozoite of Cryptosporidiumc. in the trophozoite of Giardiad. in the pseudocyst of Toxoplasma

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Parasite Mcq