17 Medical Parasitology (81) Flashcards
What is a characteristic of the microbial agent causing her symptoms?
1 Flagellated protozoan
2 Produces cystic form under unfavorable conditions
3 Able to survive on fomite for 2-3 days
4 Undulating membrane extends the full length of the parasite
5 Ideal pH for growth is 3.8-4
Flagellated protozoan
The microorganism causing vaginitis in the patient is Trichomonas vaginalis, a flagellate protozoan parasite. For T. vaginalis, no cyst form is known. The parasite has four anterior flagella and a fifth flagellum along the undulating membrane. Flagella are the organs of motility. Cilia are absent. The undulating membrane of T. vaginalis is short and reaches up to the middle of the body, a differentiating feature from other trichomonads of humans in which the undulating membrane extends the full length of the parasite. The organism grows best under anaerobic conditions at 35-37°C, with an optimal pH 5.5-6. Normal acidic pH of 3.8-4 is detrimental to the growth of T. vaginalis.
The trophozoite, a protozoan in the metabolically active growth stage, cannot survive outside the body for long, so transmission has to be from person to person by close contact. Humans are the only natural hosts of the parasite. T. vaginalis lives mainly in the vagina and cervix in women and the anterior urethra of men. Sexual transmission is the typical mode of infection. The parasite divides by longitudinal binary fission. Infection is most common in sexually active women of reproductive age. The incubation period ranges from 4 days to 4 weeks. Infection may be asymptomatic or cause acute inflammatory disease of the vagina and cervix.
Laboratory diagnostic methods include microscopy, culture, antigen detection tests by enzyme-linked immunosorbent assay (ELISA), and molecular tests based on polymerase chain reaction (PCR). Microscopy for motile trichomonas in wet preparation of vaginal discharge should be performed within 10-20 minutes of collection to prevent the organisms from losing their viability. Nucleus, flagella, undulating membrane, and axostyle are prominent structures in stained smears.
The CDC recommends metronidazole 2 g orally as a single dose for treatment of T. vaginalis infection. Treatment of sexual partners is also recommended. Metronidazole-resistant infections have been reported, and Tinidazole, a 5-nitroimidazole, is useful for treatment of such cases.
What species of malarial parasite is the most likely cause of this patient’s illness?
1 Plasmodium falciparum
2 Plasmodium knowlesi
3 Plasmodium malariae
4 Plasmodium ovale
5 Plasmodium vivax
Plasmodium falciparum
The developmental forms of malarial parasite observed in the peripheral blood smear are characteristic of Plasmodium falciparum, which causes the most severe type of malaria. Small, delicate multiple rings and crescent-shape of the gametocytes are typical features of P. falciparum. Observation of gametocyte crescents helps with easy identification of this species, as gametocytes of all other species are round or oval. In P. falciparum infection, only ring forms and gametocytes are generally seen in the peripheral smear. Late trophozoites and schizonts are not ordinarily seen and the presence of falciparum schizonts in peripheral smear indicates grave prognosis of the disease. In infections by the other species of plasmodia, all forms of the asexual cycle (schizogony) as well as gametocytes are seen in peripheral blood smear. These differentiating points help to identify P. falciparum as the cause of malaria in the patient referred to in the question.
P. falciparum invades RBCs of all ages, including the erythropoetic stem cells of the bone marrow, resulting in high level of parasitemia. The parasitized cells develop numerous projecting knobs, which cause their adherence to blood vessel endothelium resulting in obstruction, thrombosis, and ischemia. P. falciparum infections may lead to severe and fatal complications like cerebral or algid malaria, hyper pyrexia, and black water fever. Renal damage causes appearance of protein, casts, and RBCs in urine.
Laboratory diagnosis is mainly by examination of thick and thin blood smears stained by Giemsa. For detection of malarial parasites, thick smear is helpful as the concentration of the parasites is higher. Characteristic changes in infected RBCs cannot be observed because of dehemoglobinization before staining. Thin smear helps species differentiation by observing appearance of the infected RBCs and different intraerythrocytic forms of the parasite.
Plasmodium knowlesi is a rare but emerging malarial infection, which more commonly affects children. It typically presents as a nonspecific febrile illness with associated thrombocytopenia.
In Plasmodium malariae, infected RBCs are not enlarged and no specific stipplings are observed. Ring forms are large with one chromatin dot and occupy 1/3 diameter of the infected cell. Older trophozoites may show band forms, mature schizonts contain merozoites arranged like rosettes, and gametocytes are round or oval.
In Plasmodium ovale, parasitized red cells are enlarged and pale and often oval-shaped with fimbriated or crenated appearance. Conspicuous stipplings are seen (Schuffner’s dots). Rings are large with 1 chromatin granule and older trophozoites are round. Mature schizonts and gametocytes show rosette forms and gametocytes are round or oval.
With Plasmodium vivax, infected RBCs are enlarged and pale and contain Schuffner’s dots. Ring trophozoites are large with single chromatin. Older trophozoites are very pleomorphic and mature schizonts contain a large number of merozoites (14-24). Gametocytes are round or oval.
What is the most likely causative agent of the patient’s illness?
1 Isospora belli
2 Cryptosporidium hominis
3 Giardia lamblia
4 Dientameba fragilis
5 Cyclospora cayetanensis
Cryptosporidium hominis
The correct choice is Cryptosporidium hominis.
All of the listed parasites are diarrheal agents. The description of small oocysts observed in the patient’s stool sample is typical of that of Cryptosporidium, a coccidian parasite that causes cryptosporidiosis.
Cryptosporidium hominis is a minute coccidian parasite of worldwide distribution. Besides humans, natural infection occurs in many species of animals and birds. The major reservoir is domestic livestock, predominantly cattle. Human infection is acquired mainly by ingestion of oocysts present in contaminated food or drink. The incubation period is 1-7 days. The parasite completes both sexual and asexual phases of its life-cycle in a single host. After ingestion, the 4 sporozoites in the oocyst are released in the small intestine and they infect the epithelial cells. They develop into trophozoites, multiply asexually (schizogony/merogony), and release merozoites. Merozoites infect the neighboring epithelial cells and repeat schizogony. Some of the merozoites develop into micro and macrogametes. The zygote formed by fertilization develops into oocyst, which is then shed in feces. The oocyst in freshly passed feces is fully mature and is infective. It is very resistant to disinfectants and temperatures up to 60°C, and it can remain viable in the environment. Waterborne outbreaks of cryptosporidiosis have occurred in many countries. In the US, a large outbreak involving >400,000 people occurred in 1993 in Milwaukee, Wisconsin.
Infection in immunocompetent persons may be asymptomatic or may result in self-limited febrile illness with watery diarrhea. In HIV-infected and other immunosuppressed individuals, the parasite may cause severe life-threatening illness. Extraintestinal infections with pulmonary and biliary tract involvement have been reported in patients with AIDS.
Diagnosis of cryptosporidium infection is made by microscopy of both unstained and stained preparations of stools. Wet mount with iodine is useful for screening fecal samples and the sporozoites can be visualized inside the oocysts. In addition to modified acid-fast stain, safranin also can be used to stain the oocysts. With trichrome stain the oocysts remain unstained. C. hominis oocysts can be demonstrated by fluorescent stains such as Auramine O and auramine- rhodamine. Definitive identification of the oocysts can be made by indirect immunofluorescence using specific antibody. An ELISA using monoclonal antibody is highly specific and sensitive for detection of cryptosporidium in stools.
Isospora belli oocyst usually contains a single sporoblast. It matures outside the body and develops 2 sporocysts containing 4 sporozoites each. Human infection occurs by ingestion of mature oocysts in food or drink. Schizogony and sporogony take place in the epithelial cells of the small intestine. In immunocompetent persons, the infection is asymptomatic or self-limited. HIV-infected individuals and other immunocompromised persons develop protracted diarrhea. Treatment with cotrimoxazole is effective.
Giardia lamblia, an intestinal flagellate, causes giardiasis. It is the only common pathogenic protozoan living in the duodenum and upper jejunum of humans. Infection can occur in immunocompetent and immunosuppressed persons. Giardiasis in immunosuppressed individuals is more prolonged, with severe clinical manifestations.
Dientameba fragilis is a protozoan parasite of humans that infects large intestinal mucosa and causes chronic or recurrent diarrhea. There is no evidence of natural hosts other than humans. It is now considered an ameboflagellate because of the similarities to trichomonads.
Cyclospora cayetanensis is a coccidian parasite and causes cyclosporiasis. The infection is found worldwide. Outbreaks of cyclosporiasis have been reported in the U.S. and Canada. A prodrome of flu-like symptoms may precede diarrhea. Oocysts in freshly passed stools are not infective. Sporulation and maturation occurs in the environment and mature oocyst is the infective form. Infection is confirmed by stool examination.
A resident of Egypt complained of painless terminal hematuria. His urine sample was received in the microbiology laboratory. Microscopic examination of centrifuged deposit of urine detected presence of embryonated eggs of a Trematode that is known to be endemic in most parts of Africa. What is likely to be a feature of this trematode?
1 It is a hermaphrodite
2 Eggs are operculated
3 Sheep are natural definitive hosts
4 A stage known as radia develops in snail
5 Infectious stage for humans is cercaria
6 Requires 2 intermediate hosts
Infectious stage for humans is cercaria
Infectious stage for humans is cercaria.
The embryonated eggs in the patient’s urine are of Schistosoma hematobium, a trematode that is endemic in most parts of Africa and Middle East.
Schistosomes (blood flukes) do not possess the other listed features.
Schistosomes are dioecious (sexes are separate). The adult male and female worms live in copula in the vesical and pelvic venous plexuses. The female worm releases numerous eggs per day. The egg of S.hematobium is ovoid, 150x50 microns, with transparent shell and having a terminal spine at one pole. Eggs pass into the lumen of the urinary bladder with some extravasated blood and are excreted in urine causing the typical manifestation of terminal hematuria. When released into fresh water, the eggs hatch out ciliated miracidia, which enter the intermediate host snail. Further development takes place in the snail and large numbers of cercariae are produced. The cercariae swarm out of the snail and, on coming in contact with human skin, enter through penetration of the unbroken skin. They get transformed into schistosomules, enter the peripheral venules, and after a long migration and sexual differentiation ultimately reach the vesical and pelvic venous plexuses where they mature, mate, and begin to lay eggs.
Chronic infection by S.hematobium can cause persistent cystitis, pyelonephritis, and obstructive renal disease and is found to be associated with increased incidence of bladder carcinoma. Neuroschistosomiasis, Katayama fever, and female genital schistosomiasis are other manifestations associated with S.hematobium infection. In women with chronic cervical schistosomiasis, the lesions may mimic cervical carcinoma and could become co-factors for viral infections such as HIV and HPV.
Laboratory diagnosis depends on microscopy for eggs in urine. Occasionally eggs can be demonstrated in stool samples of the infected person. In cases where urine and stool are negative, bladder and rectal biopsy specimens will be helpful for demonstration of ova.
Antibody detection is useful in persons who have traveled to endemic areas and in whom the microscopy is negative. Crude and purified antigens prepared from adult worm, cercaria, and eggs have been used for serological tests. Anti-schistosomal antibodies can be detected by FAST-ELISA (Falcon Assay Screening Test-Enzyme lnked immunosorbent assay). Species identification is not possible by this test. Immunoblot assay using adult worm microsomal antigen detects species-specific antibodies. Positive serology indicates only exposure to the parasite and cannot be correlated with active disease.
ELISA test developed for detection of circulating schistosomal antigen (CSA) using anti-S.hematobium monoclonal antibodies is reported to be specific and sensitive for diagnosing active infection.
Praziquantel is the drug of choice for treatment.
There is ongoing research in the field of S.hematobium vaccine development.
[Microscopic appearance of schistosome eggs and diagram of life cycle of the parasite is available in reference 2].
What is most likely causing this patient’s symptoms?
1 Cryptosporidiosis
2 Cyclosporiasis
3 Giardiasis
4 Isosporiasis
5 Microsporidiosis
Giardiasis
Diarrhea can manifest in all five diseases. Giardiasis is the only one diagnosed by microscopic detection of motile trophozoites of the etiological agent in stool.
Giardiasis is a parasitic infection caused by intestinal flagellate, Giardia lamblia, the most common intestinal protozoan pathogen of humans. Giardiasis is common in daycares and institutions. It is transmitted by ingestion of food or water contaminated with cysts of the organism. Most initially present with acute watery diarrhea associated with abdominal discomfort, bloating, and foul-smelling stools. Watery stools may alternate with fatty stools that tend to float. Giardia cysts can be detected in stool samples of symptomatic and asymptomatic infections. Immunosuppressed persons are susceptible to massive infection and severe clinical manifestations.
Cryptosporidiosis is a highly infectious disease caused by Cryptosporidium hominis. Outbreaks of diarrhea due to cryptosporidiosis are common in daycares, transmitted by ingestion of food or water contaminated with oocysts of the parasite. Diagnosis is established by demonstration of sporulated oocysts in the feces by modified acid-fast or other staining methods.
Cyclosporiasis is an infection caused by Cyclospora cayetanensis that manifests with diarrhea 2-11 days after consuming food or water contaminated with oocyst-laden feces. Diagnosis is by identification of oocysts in stool samples stained by modified safranin, acid-fast, or autofluorescence with UV-light microscopy.
Isosporiasis, caused by Isospora belli, is common in daycares and mental institutions. After incubation of 7-11 days, watery diarrhea can persist for several months. Diagnosis is by detection of oocysts in freshly passed stools.
Microsporidiosis is caused by microsporidia, minute intracellular parasites that reproduce by spores. They can cause a wide range of illness from diarrhea to involvement of the CNS, eyes, viscera, muscles, and disseminated disease. Diagnosis is established by visualization of spores of microsporidia in stools, body fluids, or tissues after appropriate staining or electron microscopy.
What is the most likely causative agent in this patient?
1 Sporothrix schenckii
2 Ancylostoma braziliense
3 Enterobius vermicularis
4 Bartonella henselae
5 Ixodes
Ancylostoma braziliense
The correct answer choice is Ancylostoma braziliense. This patient has cutaneous larva migrans, which most commonly occurs in tropical environments. This patient has a predisposition to this condition, as he has common contact with the sand. Other people who have contact with warm, moist, sandy soil are also at risk. The classic rash is described as an erythematous, serpiginous, pruritic, cutaneous eruption. The presentation of the rash, which expands a few mm per day, is also highly suggestive of Ancylostoma braziliense.
Sporothrix schenckii is commonly found in gardeners. This is a fungal disease and usually progresses slowly. The first symptom may appear 1 to 12 weeks (average 3 weeks) after the initial exposure to the fungus. The lesion starts off small and painless and ranges in color from pink to purple.
Enterobius vermicularis presents with anal pruritus.
Cat scratch disease (CSD) is a bacterial disease caused by Bartonella henselae. Most people with CSD have been bitten or scratched by a cat and develop a mild cutaneous infection at the point of injury.
Ixodes tick is responsible for Lyme disease. The classic lesion of Lyme disease is bulls-eye shaped with a red center, an area of central clearing, and a periphery of redness around the clearing.
What is the treatment of choice for this patient?
1 Vancomycin
2 Albendazole
3 Doxycycline
4 Metronidazole
5 Ceftriaxone
Metronidazole
Metronidazole is the drug of choice in the treatment of giardiasis. Giardiasis, an illness that affects the digestive tract, is caused by a microscopic parasite called Giardia lamblia. The parasite attaches itself to the lining of the small intestines in humans, where it frequently causes diarrhea and malabsorption. The patient’s history is suggestive, as she was likely exposed to contaminated water. Giardiasis is usually treated with metronidazole.
Vancomycin is indicated for the treatment of serious, life-threatening infections by Gram-positive bacteria that are unresponsive to other antibiotics. Oral vancomycin may also be used to treat Clostridioides difficile infection.
Albendazole is not first-line in the treatment of giardiasis. It is an antihelminthic drug used to treat hookworm, pinworm, and whipworm.
Doxycycline can be used to treat a variety of infections. Doxycycline is frequently used to treat chronic syphilis, chlamydia, acne, prostatitis, as well as many other infections.
Ceftriaxone is more commonly utilized in treating infections such as gonorrhea.
An outbreak of diarrhea occurred in a child day care center situated in Dhaka, Bangladesh. Freshly passed stool samples collected from the affected children were subjected to microscopy. Diagnosis was made based on detection of trophozoite and cystic forms of an intestinal flagellate protozoan parasite. What is most likely a feature of this infection?
1 Natural infection occurs only in humans
2 The disease is confined to developing countries
3 Trophozoites of the parasite are seen predominantly in the colon
4 Freshly passed cysts in feces are infective
5 Detection of specific antibodies is a routine diagnostic method
6 Trophozoites are excreted by asymptomatic carriers
Freshly passed cysts in feces are infective
Of the clinically important intestinal flagellate protozoan parasites, Giardia lamblia is the only one that affects gastrointestinal tract of humans. Infection may be asymptomatic or may result in diarrhea. Clinical disease caused by G.lamblia is known as Giardiasis. Outbreaks of giardiasis occur especially in children. G.lamblia has trophozoite and cystic forms, and the detection of these forms in stools helps diagnosis of giardiasis. Therefore, it is clear that the outbreak of diarrheal illness referred to in the question is Giardiasis.
Cyst is the infective form of G.lamblia. Freshly passed cysts in feces are infective. This characteristic helps direct person-to-person transmission of the infection by fecal-oral route. Direct transmission is more often seen in children, the mentally ill, and male homosexuals. Most common mode of acquiring infection is by ingestion of cysts in contaminated water or food. Cysts passed in stools remain viable in soil and water for several weeks.
Other features listed are not applicable to giardiasis.
Cysts of Giardia lamblia are oval 8-12 micronsx7-10 microns in size and surrounded by a tough hyaline cyst wall. The mature cyst contains 4 nuclei situated at 1 end. As few as 10 cysts are capable of initiating infection. Infection may result in asymptomatic carriage or clinical illness ranging from diarrhea with abdominal symptoms to fulminant diarrhea and malabsorption. Asymptomatic carriers excrete only cysts.They are important reservoirs of infection.
After being ingested, excystation of the parasite occurs in the duodenum. 2 trophozoites hatch out of a single cyst. The trophozoites multiply by binary fission and remain in the lumen of the small intestine, where they get attached to the mucosal epithelium. Encystation occurs as the trophozoites pass down the colon.
Giardia trophozoite is 9-21 microns long and 5-15 microns wide. It is pear-shaped, bilaterally symmetrical, and has 4 pairs of flagella. There are 2 nuclei, 1 on either side of the midline and have prominent central karyosomes. In iron hemotoxylin or trichrome-stained preparations the trophozoite creates a face-like image. The trophozoite firmly attaches to the epithelial surface of the duodenum and jejunum by means of a large concave sucking disc present on the ventral surface of its anterior portion.
A large number of parasites adhering to the mucosal cells may cause low-grade inflammation, resulting in clinical symptoms of acute diarrhea, abdominal pain, and flatulence. Usually the parasite does not invade or produce necrosis of the mucosal epithelium. In chronic giardiasis, flattening or atrophy of villi, epithelial cell damage, and loss of brush border enzyme activity occur. Resulting lactose intolerance and malabsorption cause a sprue-like syndrome. Occasionally Giardia trophozoites may colonize in the biliary tree and gall bladder, causing biliary colic. Children are more susceptible to giardiasis. Giardiasis is one of the common causes of traveler’s diarrhea.
Reactive arthritis has been reported as a rare sequel of giardiasis.
It has been shown that the immunodominant cysteine rich surface proteins of Giardia, known as variant surface proteins (VSPs), can undergo antigenic variation in the host and may play a role in the immune evasion by the parasite.
Laboratory diagnosis: microscopy of wet mounts of diarrheal stool samples is used to detect the trophozoite and cysts of the parasite. Samples should be freshly passed, as the trophozoites easily die outside the body. Trophozoites show typical dancing or swaying motility. Microscopy of stained preparations help to identify the structural details of the parasite. Only cysts are seen in asymptomatic carriers. Stool samples are tested after concentration if cysts are sparse. Direct Fluorescent Antibody (DFA) test is considered very specific and sensitive and is used in many laboratories for detection of the giardia cysts in stool samples (microscopic appearances of iron-hemotoxylin/trichrome stained trophozoite and cyst forms, and giardia cysts stained by DFA are available in Ref 5).
Detection of giardia antigens in stools by enzyme-linked immunosorbent assay (ELISA) and immunochromatographic-cartridge assay are sensitive and rapid diagnostic methods.
Sample collected by duodenal aspiration (Entero test) may be required for demonstration of the parasite, especially when the biliary symptoms predominate.
Biopsy of small intestine may be required for establishing the diagnosis in some cases.
Giardia can be cultured in vitro; cultures are used mainly for research purposes.
Since giardia trophozoites rarely invade the tissues, stimulation of systemic immune response is variable. Testing for serum antibodies for diagnosis is reported to be unreliable.
Giardiasis affects various animal species, including domestic animals like dogs, cats, and cattle, and wild animals, including beavers and bears.
Molecular typing of Giardia by PCR-based methods has helped to identify the genotypes associated with human infections. Some genotypes have been shown to cause giardiasis in both humans and animals, suggesting the zoonotic potential of giardia. Genotype determination can be helpful in epidemiological studies.
Giardiasishas worldwide distribution affecting developing and developed countries. The infection is endemic in most of the developing countries. In the U.S. and other industrialized countries, giardiasis is recognized as a re-emerging disease. Water borne epidemics and sporadic infections occur. In the US, giardiasis is a national notifiable disease since 2002.
Metronidazole and tinidazole are drugs of choice for treatment of giardiasis. Nitazoxanide is useful in children. Paromomycin can be used for treating giardiasis in pregnant women.
Which trophozoite ingests RBCs?
1 Entamoeba histolytica
2 Endolimax nana
3 Blastocystis hominis
4 Entamoeba coli
5 Iodamoeba butschlii
Entamoeba histolytica
Except Entamoeba histolytica, all the amebae mentioned above are nonpathogenic commensals found in humans.
Entamoeba histolytica is the major pathogen found in the large intestine and is associated with amebic dysentery in humans. The organism secretes proteolytic enzymes that aid in the disruption of the intestinal barrier. It invades the epithelial cells and causes lysis of the cells. The trophozoites feed on the red cells that appear either as a whole or on partially digested red cells in the cytoplasm of the trophozoites.
Entamoeba histolytica causes flask-shaped ulcers in the intestinal mucosa of the host. The organism shows resistance to phagocytosis and complement-mediated cell lysis.
The protozoa associated with primary amebic meningoencephalitis is:
1 Iodamoeba bütschlii
2 Naegleria fowleri
3 Giardia lamblia
4 Trichomonas vaginalis
5 Entamoeba coli
Naegleria fowleri
Naegleria fowleri is found in contaminated water environments and soil. Individuals acquire infection during summertime while swimming in contaminated water. The organism gains entry through the nasal passage. It invades the nasal mucosa and can reach the brain and cause destruction of brain tissue. This results in primary amebic meningoencephalitis, which is characterized by a rapid and often fatal course. The clinical symptoms are intense headache, sore throat, fever, blocked nose, and stiff neck. These symptoms are followed by irrational behavior, coma, and death.
Iodamoeba bütschlii and Entamoeba coli are nonpathogenic commensals.
Giardia lamblia is a flagellate associated with traveler’s diarrhea.
Trichomonas vaginalis is implicated in urogenital infections.
Which of the following infections is caused by Acanthamoeba in immunocompromised individuals?
1 Amebic dysentery
2 Malabsorption syndrome
3 Granulomatous amebic encephalitis
4 Chagas disease
5 Kala azar
Granulomatous amebic encephalitis
Acanthamoeba is associated with granulomatous amebic encephalitis, primarily in an immunocompromised host. The amebae spread hematogenously to the brain from primary infection sites (nasal mucosa, lungs, or skin). The infection may be characterized by brain abscesses, granulomas, thrombosis, or hemorrhage. Patients experience headaches, drowsiness, stiff neck, hemiparesis, and seizures. Death may result in 2-3 days if infection occurs directly through nasal mucosa. Brain biopsy may reveal cyst, as well as trophozoites stages of the organism. Acanthamoeba is also implicated in keratitis in contact lens users.
The following table shows the etiological agents associated with the clinical conditions given in the question.
Clinical Conditions Etiological Agents
Amebic dysentery Entamoeba histolytica
Malabsorption syndrome Giardia lamblia
Chagas disease Trypanosoma brucei
Kala azar Leishmania donovani
What trophozoite demonstrates “falling leaf” motility in the wet mount?
1 Entamoeba histolytica
2 Endolimax nana
3 Giardia lamblia
4 Dientamoeba fragilis
5 Trichomonas vaginalis
Giardia lamblia
Giardia lamblia is an intestinal flagellate that has both a trophozoite and cyst form. The pear shaped trophozoite of Giardia lamblia exhibits characteristic “falling leaf” motility on a wet mount. Trophozoites are bilaterally symmetrical with 2 oval nuclei containing large central karyosomes on each side of midline. It has 4 pair of flagella, midline axonemes, 2 median bodies, and a large ventral sucking disk for attachment.
Trophozoites of Entamoeba histolytica have progressive directional motility while Endolimax nana and Dientamoeba fragilis trophozoites have non-directional motility. Trophozoites of Trichomonas vaginalis exhibit a non-directional, jerky motility.
Which of the following is an example of a pathogenic intestinal flagellate?
1 Dientamoeba fragilis
2 Trichomonas hominis
3 Chilomastix mesnili
4 Trichomonas vaginalis
Dientamoeba fragilis
Trichomonas hominis and Chilomastix mesnili are non-pathogenic intestinal flagellates.
Dientamoeba fragilis is pathogenic flagellate associated with gastrointestinal illness. Trichomonas vaginalis is a non-intestinal, pathogenic flagellate transmitted by sexual contact.
The etiological agent of malaria is
1 Babesia sp
2 Plasmodium sp
3 Leishmania sp
4 Trypanosoma sp
5 Plesiomonas sp
Plasmodium sp
Plasmodium sp. infects erythrocytes and causes malaria. It is the major cause of mortality in people in underdeveloped countries. Four organisms which are responsible for this disease are: Plasmodium vivax, Plasmodium falciparum, Plasmodium malariae, and Plasmodium ovale. Once established, Malaria can be transmitted via the Anopheles mosquito, blood transfusion, infected needles and across the placenta. Symptoms include fever, chills and the presence of parasites in the blood smear.
Plasmodium vivax has
1 quatrain life cycle
2 Crescent shaped gametocytes
3 The widest geographic distribution
4 Schizont with 6-12 merozites
The widest geographic distribution
Plasmodium vivax has the widest geographic distribution and is the major cause (48%) of malaria cases reported in the United States. Additional characteristics include the following: Tertian life cycle, (i.e. reproductive cycle of 48 hours), infects young red blood cells, schizont has 12-24 merozoites, and Schüffner stipplings are present on the cell. A peripheral blood smear may show a wide range of developmental stages. Plasmodium malariae has a quatrain life cycle (72 hours) and schizont with 6-12 merozoites. Crescent shaped gametocytes are characteristics of Plasmodium falciparum.
Which of the following Plasmodium sp. is/are characterized by presence of multiple ring forms and crescent shaped gametocytes in the peripheral blood smear?
1 Plasmodium vivax
2 Plasmodium ovale
3 Plasmodium falciparum
4 Plasmodium malariae
Plasmodium falciparum
Plasmodium falciparum has an asynchronous life cycle and red cell rupture takes place between 36 to 48 hours. It infects erythrocytes of all ages resulting in massive erythrolysis. This parasite causes electron-dense knobs on the surface of erythrocytes leading to alteration in their surface membrane. Peripheral blood smears demonstrate ring-form trophozoites and crescent-shaped gametocytes. Maturation of other stages takes place in the venules and capillaries of major organs.
Plasmodium vivax and Plasmodium ovale mainly target immature erythrocytes. Peripheral blood smears are characterized by the presence of enlarged red blood cells with Schüffner stipplings. Plasmodium malariae infects mature erythrocytes. Peripheral smears may show normal size red cells and compact trophozoites with a characteristic “band” appearance.
Which of the following is a vector for malaria?
1 Anopheles mosquito
2 Tsetse fly
3 Reduviid bug
4 Sandfly
5 Ixodes tick
Anopheles mosquito
The female Anopheles mosquito is a biological vector and definitive host for malaria.
The vector ingests the microgametocytes and macrogametocytes when she takes the blood meal from a human host. Sporogony takes place in the vector and results in the production of sporozoites, which are infective for human. The mosquito bites the human and injects the sporozoites present in the salivary gland. Once the sporozoites enter the human body, a primary exoerythrocytic cycle of asexual reproduction takes place in the liver, leading to production of merozoites. In the erythrocytic phase of asexual reproduction, merozoites invade red blood cells. The parasite feeds on the hemoglobin and matures into a trophozoite. The trophozoites develop into a schizont containing merozoites. Upon lysis of the red cells, merozoites are released and can then invade other red cells. Merozoites can develop into a gametocyte form, which is infective for mosquito.
The following table shows the association of the vector with the infections mentioned in the question.
Vector Parasitic Infection
Tsetse fly Sleeping sickness
Reduviid bug Chagas disease
Sandfly Leishmaniasis
Ixodes tick Babesiosis
Trypanosoma brucei gambiense is the etiological agent of
1 Rocky mountain spotted fever
2 Sleeping sickness
3 Babesiosis
4 American trypanosomiasis
5 Elephantiasis
Sleeping sickness
Trypanosoma brucei gambiense is a flagellate that causes west African sleeping sickness. Trypomastigote is ingested by the tsetse fly when it takes a blood meal from an infected human. Trypomastigote stage develops into an epimastigote stage in the insect’s gut and migrates to the salivary gland. This develops into infective metacyclic trypomastigote and is transmitted to humans through insect bite. This enters the blood and lymphatics of the host. The patient develops headache, fever, muscle pain and enlarged lymph nodes. The parasite can invade the central nervous system causing severe headaches, mental dullness and apathy, altered reflexes, and paralysis. This can result in convulsion, coma and death of the patient.
Chagas disease is caused by
1 Trypanosoma brucei
2 Trypanosoma cruzi
3 Leishmania donovani
4 Leishmania braziliensis
5 Babesia microti
Trypanosoma cruzi
Trypanosoma cruzi is the etiological agent of Chagas disease which is transmitted by the Reduviid bug. This parasite infects cells of the lymph system, macrophages, cardiac muscle and skeletal muscles. It causes severe forms of the disease in children and can result in death. Symptoms of acute Chagas disease includes fever, unilateral conjunctivitis, ulcerative skin lesion, lymphadenitis, hepatosplenomegaly and muscular pain. It can also cause acute myocarditis leading to coronary heart failure. Chronic disease results in the development of the organism in visceral organs. The amastigotes multiplies and destroys the host cells and can lead to megacolon, megastomata or megaesophagus.
In acute infections, Trypanosoma cruzi appears in peripheral blood as C or U shaped trypomastigotes with a single large nucleus midbody and posterior kinetoplast with an undulating membrane attached to it. An Enzyme Linked Immunoassay can be used to diagnose chronic cases of Chagas disease. Nifurtimox can be used to eliminate trypomastigotes from circulation and improve the patient’s clinical condition.
Sandflies are vectors for
1 Malaria
2 Sleeping Sickness
3 Leishmaniasis
4 Chagas disease
5 Lyme disease
Leishmaniasis
Leishmaniasis is transmitted by the bite of the Sandfly among animal and human reservoirs. When sandflies take a blood meal, the organism is ingested as an amastigote. This develops into a promastigote in the insect gut and migrates to its salivary glands. When the sandfly subsequently bites another reservoir, the promastigote is ingested by the reservoir’s macrophages and is transformed to the amastigote stage and multiplies within the cell.
The following table shows the association of the vector with the diseases mentioned in the question.
Disease Vectors
Malaria Anopheles Mosquito
Sleeping Sickness Tsetse fly
Chagas Disease Reduviid bug
Lyme Disease Tick
Which parasite is acquired by ingestion of raw or improperly cooked fish?
1 Diphyllobothrium latum
2 Taenia solium
3 Taenia saginata
4 Hymenolepis diminuta
5 Dipylidium caninum
Diphyllobothrium latum
Diphyllobothrium latum is acquired by ingesting raw or improperly cooked fish. It is the largest of the human tapeworms and is also known as the broad or fish tapeworm. It can cause abdominal discomfort, nausea, vomiting, diarrhea, and weight loss. Large numbers of this parasite can cause blockage of the intestine. It also can compete with the host for vitamin B12 and cause pernicious anemia.
The following table shows the association of parasites with the diseases mentioned above.
Parasites Acquired by ingestion of
Taenia solium Undercooked Pork
Taenia saginata Undercooked Beef
Hymenolepis diminuta Flea containing cysticercoid
Dipylidium caninum Flea containing larval stage
Which of the following is the most common human tapeworm in the United States?
1 Diphyllobothrium latum
2 Taenia solium
3 Taenia saginata
4 Hymenolepis diminuta
5 Hymenolepis nana
Hymenolepis nana
Hymenolepis nana is the most common tapeworm in the United States and is prevalent in the Southeastern United States. It is most commonly seen in children younger than 8 years of age. Hymenolepis nana does not require an intermediate host and can cause direct infection upon ingestion of eggs. It can be transmitted hand to mouth or by contaminated foods or fluids. Mice, fleas and beetles can also act as a transport host and produce cysticercoid larvae that can infect humans and rodents. Adult worms live in the intestine and can cause abdominal discomfort, diarrhea, and headaches if present in large numbers. Infection can be diagnosed by recovery of the parasite’s eggs.
Cysticercosis is caused by the tissue stage of
1 Taenia saginata
2 Taenia solium
3 Diphyllobothrium latum
4 Ascaris lumbricoides
5 Hymenolepis nana
Taenia solium
Cysticercosis is a serious disease caused by the larvae or tissue stage of Taenia solium. Humans accidentally become the intermediate host and harbor the larvae in their tissues. The egg hatches in the small intestine and enters the circulation developing into a cysticercus in the tissue or organ. This larva has predilection for skeletal tissue and the nervous system. It can lead to ocular cysticercosis or cerebral cysticercosis. The organism elicits host-tissue reaction and produces a fibrous capsule. Dead larvae can cause increased inflammatory reaction and can lead to calcification.
Cysticercus are oval, translucent and contain an invaginated scolex with 4 suckers and a circle of hooklets on the rostellum.
Which parasitic infection in humans is acquired by ingestion of beef?
1 Taenia saginata
2 Taenia solium
3 Diphyllobothrium latum
4 Leishmania donovani
5 Trypanosoma cruzi
Taenia saginata
Taenia saginata is a beef tapeworm. It is prevalent in Ethiopia, Iran, Taiwan, Kenya and other beef eating countries in the world. Human infection results from the ingestion of larvae in the beef. Larva attach to the ileum and mature into an adult worm. It attaches by means of the scolex to the mucosa of the small intestine. The infection can cause abdominal discomfort, hunger pain or loss of appetite, weight loss, etc.
The infection is detected when a person passes the proglottids in the stool or detection of eggs through the microscopic examination of stool. The infection can be treated by niclosamide or praziquantel.
Taenia solium infection results when humans ingest insufficiently cooked pork containing cysticercus. Diphyllobothrium latum infection is acquired from ingestion of fish containing plerocercoid larva. Sandflies are vectors for Leishmania donovani. Reduviid bug is a vector for Trypanosoma cruzi.
Which Trematode, or giant intestinal fluke, is associated with the ingestion of water chestnuts?
1 Fasciolopsis buski
2 Opisthorchis sinensis
3 Paragonimus westermani
4 Schistosoma mansoni
Fasciolopsis buski
Fasciolopsis buski is an intestinal fluke that is common in Vietnam, India, and China. Humans become infected by ingesting metacercaria on freshwater vegetation such as water chestnuts and bamboo shoots. The adult fluke lives in the duodenum and causes mechanical and toxic damage leading to inflammation and ulceration. Clinical symptoms of heavy infection include anorexia, nausea, vomiting, edema, and ascites. Recovery of adult flukes or eggs of the organism in feces is diagnostic of infection.
Paragonimus westermani is a
1 Liver fluke
2 Lung fluke
3 Intestinal fluke
4 Bladder fluke
5 Blood fluke
Lung fluke
Paragonimus westermani is lung fluke primarily found in the Far East, India and parts of Africa. Ingestion of crabs or crayfish containing metacercaria can result in human infection. The metacercaria penetrates the intestinal wall and diaphragm on its way to the lung. Adult flukes live within capsules in the bronchioles which can lead to an inflammatory response, persistent cough, chest pain and hemoptysis. A wet mount of sputum can reveal presence of eggs in the infected individual.
Fasciola hepatica - Sheep liver fluke
Opisthorchis sinensis - Chinese liver fluke
Fasciolopsis buski - Large intestine fluke
Schistosoma japonicum - Oriental blood fluke
Schistosoma mansoni - Mansion’s blood fluke
Schistosoma haematobium - Bladder fluke
The adult fluke of Opisthorchis sinensis lives in the
1 Bladder
2 Heart
3 Stomach
4 Distal bile ducts
5 Lymphatics
Distal bile ducts
Adult Opisthorchis sinensis or Chinese liver fluke live in the distal bile duct of the liver. Ingestion of metacercaria in raw undercooked or pickled fish can cause infection in humans. Heavy infection can lead to inflammation, fever, diarrhea, pain, fibrosis and bile duct obstruction. Demonstration of eggs in stool specimen can be used for diagnosis. The embryonated eggs are vase shaped with a domed operculation with prominent shoulder, and a knob at the end opposite to operculum.
The etiological agent of Elephantiasis is
1 Enterobius vermicularis
2 Wuchereria bancrofti
3 Trichinella spiralis
4 Trypanosoma cruzi
5 Leishmania braziliensis
Wuchereria bancrofti
Wuchereria bancrofti causes bancroftian filariasis and elephantiasis. Elephantiasis is a debilitating and deforming condition occurring in patients with many years of continuous filarial infection. It is transmitted by mosquitoes of Culex , Aedes and Anopheles spp. Infective filariform larval stages are introduced into human circulation through insect bites. Wuchereria bancrofti invades the lymphatics and causes granulomatous lesions, chills fever, and eventually elephantiasis. Lymphatics and lymph nodes of the lower extremities are mostly involved. The adult filarial worm initiates the cellular reaction; edema and hyperplasia. Presence of a dead worm causes granulomatous reaction leading to formation of fibrous tissue. This causes blockage of lymphatics with development of collateral lymphatics. Many years of filarial infection can lead to elephantiasis.
Onchocerca volvulus is the etiological agent of
1 Rectal prolapse
2 Kala azar
3 Sleeping sickness
4 Black water fever
5 River blindness
River blindness
Onchocerca volvulus is a roundworm and a blood parasite associated with river blindness. The parasite is transmitted by blackflies. Adult worms live in subcutaneous tissues encapsulated within fibrous nodules. The nodule forms as a result of an inflammatory and granulomatous reaction around the worm. Microfilariae can migrate to other parts of the body including the eye. Blindness is the serious complication of infection with Onchocerca volvulus . It can be lodged in the cornea and iris leading to keratitis and atrophy of the iris. Sclerotic keratitis can lead to following conditions:
Blindness
Cataracts
Iridocyclitis
Secondary glaucoma
The etiological agent of cutaneous larval migrans is
1 Fasciola hepatica
2 Fasciolopsis buski
3 Trichuris trichiura
4 Ancylostoma braziliense
5 Toxocara canis
Ancylostoma braziliense
Dog and cat hookworms like Ancylostoma braziliense and Ancylostoma caninum are associated with cutaneous larval migrans. When Ancylostoma braziliense penetrates the human skin, it can not enter the circulation to complete the life cycle. It causes pruritic eruption by creating long winding tunnels when it moves through subcutaneous tissue. The clinical symptoms include itching, reddish papule at the point of entry, edema and inflammatory trait with crusty opening and eosinophilia. The infection resolves on its own when the larva dies. Toxocara canis is a dog roundworm and is associated with visceral larval migrans.
Toxocara canis is associated with
1 Visceral larval migrans
2 Cutaneous larval migrans
3 Primary amebic encephalitis
4 Granulomatous amebic encephalitis
5 Toxoplasmosis
Visceral larval migrans
Visceral larval migrans is caused by Toxocara canis and Toxocara cati. Toxocara canis is a dog roundworm. Toxocara cati is a cat roundworm. When the human accidentally ingests the eggs, the larva hatches in the intestine. It penetrates the gut and wanders through the abdominal cavity to other organs of the body like liver, lungs, eyes and brain. Infection is common in children between 1 to 4 years of age. Fever, pneumonitis, hepatomegaly and eosinophilia are the symptoms associated with the infection. Complications of central nervous system are seen when the larva invades the brain.
Ancylostoma braziliense and Ancylostoma caninum are associated with cutaneous larval migrans.
Naegleria fowleri is the etiological agent of primary amebic encephalitis.
Acanthamoeba spp. causes granulomatous amebic encephalitis primarily in immunocompromised patients.
Toxoplasmosis is caused by Toxoplasma gondii .
Which parasite is associated with anal pruritus?
1 Trichuris trichiura
2 Enterobius vermicularis
3 Wuchereria bancrofti
4 Paragonimus westermani
5 Schistosoma haematobium
Enterobius vermicularis
Enterobius vermicularis is commonly known as “pinworm” and is mostly found in children or families in crowded condition. The eggs of Enterobius vermicularis are resistant to drying, and direct feco-oral transmission is common in children. The adult worm lives in the large intestine. Adult females migrate outside the body to lay eggs in the perianal area. The crawling of the female adult worm and the eggs causes intense itching resulting in anal pruritus. Enterobius vermicularis can be asymptomatic or can cause nausea, vomiting, abdominal pain, insomnia and restlessness. Treatment includes pyrantel pamoate, mebendazole, or piperazine citrate.
From what disease (parasitic infestation) is he most likely suffering?
1 Schistosomiasis
2 Trypanosomiasis
3 Leishmaniasis
4 Kala-azar
5 Fascioliasis
Schistosomiasis
The image included here is a scanning electron microscopy image (SEM) of a schistosome parasite magnified 256X.
Blood flukes cause schistosomiasis (bilharziasis). The intermediate host for blood flukes is a fresh water snail. Cercariae (the larva) are able to get through human skin. The freshwater swimming this man did on his vacation exposed him to the infection.
Trypanosomiasis is caused by a protozoa. Chagas disease, or American trypanosomiasis usually causes a mild febrile illness but may cause cardiac or gastrointestinal symptoms in patients who are chronically infected. Sleeping sickness or African trypanosomiasis is caused by T. brucei is transmitted by the tsetse fly. It is characterized by fever, daytime somnolence, anemia, thrombocytopenia, and other central nervous system effects.
Leishmaniasis is caused by protozoa of the species Leishmania. Kala-azar is caused by Leishmania donovani. Leishmania species produce a wide variety of clinical syndromes from cutaneous ulcers to fatal visceral disease.
Fascioliasis is caused by the liver fluke, Fasciola hepatica. In its acute phase, it causes fever, malaise, abdominal pain, gastrointestinal symptoms, urticaria,
anemia, jaundice, and respiratory symptoms.
What is the most likely diagnosis?
1 Plasmodium vivax malaria
2 Plasmodium falciparum malaria
3 Plasmodium ovale malaria
4 Plasmodium malariae malaria
5 Babesiosis
Plasmodium falciparum malaria
Malaria is an acute and sometimes chronic infection of the blood stream caused by a parasite of the genus plasmodium.
The 4 species of plasmodium causing human malaria include:
Plasmodium falciparum
Plasmodium vivax
Plasmodium ovale
Plasmodium malariae
P. falciparum causes most of the deaths and severe disease related to malaria. Anemia and liver dysfunction and more common in children. Crescent-shaped gametocytes on blood smear are characteristic of the P. falciparum species. Other common presenting symptoms of malaria include chills and fever, which are often associated with splenomegaly. In the early stages of the disease, the febrile episodes occur irregularly but eventually become more synchronous, assuming the usual tertian (P. vivax, P. Falciparum, P. ovale) and quartan (P. malariae) periodicity. Other manifestations include diarrhea, abdominal pain, headache, and muscle aches and pains.
P. falciparum malaria can result in high parasitemias, which can lead to severe hemolysis with hemoglobinuria and profound anemia. Erythrocytes infected with growing trophozoites, schizonts of P. falciparum, become sequestrated in small vessels of the body, and they may lead to occlusion of these vessels, causing symptoms related to capillary obstruction and tissue anoxia. Involvement of the brain is known as cerebral malaria, in which the patient becomes disoriented, progressing to delirium, coma, and often death.
Malarial parasites undergo sexual phase (sporogony) in anopheles mosquitoes and an asexual stage (schizogony) in humans that results in the production of schizonts and merozoites. In the blood stream, some merozoites eventually differentiate into gametocytes (gametogony), which when ingested by female anopheline mosquitoes, mature into male microgametes and female macrogametes. Fusion of a microgamete and a macrogamete results in the formation of the motile ookinete, which migrates to the outside of the stomach wall and forms an oocyst. Within the oocyst, numerous spindle-shaped sporozoites are formed. The mature oocyst ruptures into the body cavity, releasing the sporozoites, which then migrate through the tissues to the salivary glands, from which they are injected into the vertebral host as the mosquito feeds. The time required for the development in the mosquito ranges from 8 to 21 days.
The sporozoites injected into the vertebrate host reach the hepatic parenchymal cells within minutes and initiate the proliferative phase known as exoerythrocytic schizogony. Release of merozoites from ruptured hepatic schizonts initiates the blood stream infection or erythrocytic schizogony and eventually the clinical symptoms of malaria.
P. vivax and P. ovale differ from P. falciparum and P. malariae in that true disease relapses of the former species may occur weeks to months following subsidence of previous attacks. This occurs because of renewed exoerythrocytic and eventually erythrocytic schizogony from latent hepatic sporozoites, which are known as hypnozoites. Recrudescences of disease due to P. falciparum or P. malariae arise from an increase in the number of persisting blood stage forms to clinically detectable levels, not from persisting liver stage forms. P. vivax and P. ovale parasites primarily infect young erythrocytes, whereas P. malariae affects older erythrocytes, and P. falciparum infects erythrocytes of all stages.
What is the most likely cause of his symptoms?
1 Giardia lamblia
2 Pneumocystis jiroveci
3 Babesia microti
4 Toxoplasma gondii
5 Trypanosoma cruzi
Giardia lamblia
Giardia lamblia is a protozoan that can cause gastrointestinal discomfort, malabsorption, and diarrhea. It is transmitted by the fecal-oral route and by drinking infected water.
Pneumocystis jiroveci (Pneumocystis carinii) is a fungus that can cause pneumonia in an immunosuppressed host. It is transmitted by inhalation.
Babesia microti is a protozoan that can cause fever. In a vulnerable host, it occasionally causes a more severe illness. It is transmitted by ticks.
Toxoplasma gondii is a protozoan that can be congenital or acquired. It can produce both mild and severe symptoms. When the symptoms are severe, it can cause encephalitis, fever, hepatitis, pneumonitis, myocarditis, retinochoroiditis, or a brain abscess. It is transmitted by cat feces or by eating raw meat. On CT scan, there can be contrast-enhancing lesions.
Trypanosoma cruzi is a protozoan that causes Chagas disease. It is transmitted by triatomine insects.
What is the causative agent?
1 Trypanosoma brucei gambiense
2 Leishmania donovani
3 Trypanosoma cruzi
4 Leishmania tropica
5 Trypanosoma brucei rhodesiense
Trypanosoma cruzi
Chagas disease is caused by the protozoan Trypanosoma cruzi. The image shows a micrograph of Trypanosoma cruzi in a blood smear (using Giemsa staining technique).
Chagas disease is also referred to as American trypanosomiasis; it is seen in the Western Hemisphere, primarily in South America, Central America, and Mexico. Transmission of Chagas disease is by contact with the feces of an infected insect. There is an acute stage and a chronic stage. The chronic infection can lead to cardiomyopathy. The cardiac changes seen with Chagas disease include an enlarged heart and EKG findings as seen in this patient (i.e., right bundle branch block and premature ventricular contractions). The chronic infection can also lead to megaureter, megaesophagus, and megacolon.
West African sleeping sickness is also called West African trypanosomiasis; it is caused by Trypanosoma brucei gambiense.
Leishmaniasis is a disease caused by the protozoan parasite of the genus Leishmania, which includes Leishamnia donovani and Leishmania tropica. Leishmania donovani produces visceral leishmaniasis, which is marked by fever, lymphadenopathy, and splenic enlargement and hepatomegaly. Leishmaniasis tropica produces cutaneous leishmaniasis, which produces a painless ulcer.
Rhodesian trypanosomiasis is caused by Trypanosoma brucei rhodesiense. It is also called East African sleeping sickness.
