Parasitology MCQ

A malaria relapse is parasitemia developing from exoerythrocytic stages in the liver. These persistent stages are found in P ovale and P. vivax infections, and they may cause relapses up to 4 or 5 years after the primary infection. For infections caused by these species, treatment with primaquine is used to prevent relapses after clinical cure with chloroquine or an alternate drug.

Necator americanus, Ancylostoma duodenale, and Strongyloides stercoralis all pass through the human lung during their life cycles.

Although all the organisms listed have some degree of acid-fast positivity, only Cyclospora forms oocysts in the size range of 8-10 um. The oocysts of Cryptosporidium are generally 4—6 um in diameter and are generally strongly acid-fast positive. Oocysts of Isospora are much larger, approximately 25 X 18 um. Microsporidia are acid-fast variable, and this stain is not recommended for detecting microsporidia. The spores of microsporidia are generally 1-3 um in diameter.

Necator americanus rhabditiform larvae possess a long buccal cavity and a small genital primordium. Neither the rhabditiform or filariform larval stage has a notched tail. Strongyloides stercoralis rhabditiform larvae possess a short buccal cavity and a prominent genital primordium. Filariform larvae possess a notched tail.

Humans are infected with T. saginata by eating beef containing live cysticerci, the infectious larval stage of this parasite. Cattle become infected by ingesting viable eggs from human feces. Unlike Taenia solium, if humans ingest T. saginata ova, infection does not develop.

Strongyloides stercoralis is not often recovered in the ovum stage in feces. The rhabditiform larval stage is the primary diagnostic stage for strongyloidiasis.

A common sign of S. haematobium infection is the presence of blood in the urine. This is due to the damage caused when the eggs break out of the blood vessels of the vesicular plexus into the bladder. Falciparum malaria may also cause severe hematuria or "blackwater fever."

D. latum is the only one with operculated ova amng the medically important known human tapeworms include Dipylidium caninum, Diphyllobothrium latum, Hymenolepis diminuta, Hymenolepis nana, Taenia saginata, and Taenia solium. 

Polyvinyl alcohol (PVA) is a commonly used fixative for stool specimens. This preservative contains mercury and is used to fix fecal samples for making permanently stained smears. Formalin is commonly used to preserve stool samples in preparation for concentration procedures. Sodium acetate is used with formalin to preserve fecal specimens, but it does not contain mercury and is, therefore, less toxic. Buffered glycerol is sometimes used as a transport medium for stool samples when performing a bacterial culture.

Mature cysts of I. butschlii are usually ovoid, with a single nucleus with a large eccentric karyosome. The cytoplasm contains a compact mass of glycogen, which appears as a clear area in unstained or permanently stained preparations but stains dark brown with iodine. Chromatoid bodies are not present.

The fertilized ova of T. trichiura are unsegmented when released, and embryonic development occurs outside of the host. In moist, warm, shaded soil, the first-stage larva develops within the egg in about 2 weeks. This fully embryonated egg is infective when ingested by a susceptible host, and it hatches in the small intestine. During development from larva to adult, the worm usually passes to the cecum, where it embeds its slender anterior portion in the intestinal mucosa.

Acanthamoeba is a free-living ameba rarely causing human infections. This organism has been associated with granulomatous infections of the skin and lung, as well as meningoencephalitis. However, the most common presentation is keratitis, infection of the cornea. Most keratitis cases have been associated with contact lenses.

Cysts are the infective stage of most intestinal parasites. D. fragilis is currently classified as a flagellate, even though it does not produce a flagellum. However, D. fragilis does not have a cyst stage. G. lamblia is the only flagellate in the list of protozoa with a cyst stage. B. coli is a ciliate and Entamoeba coli is an ameba.

Because the eggs of E. vermicularis are usually deposited on the perianal area, cellulose tape slides are recommended for collecting the eggs. Recovery is best if specimens are collected late in the evening or before bathing or defecating in the morning. The gravid female worms usually migrate at night to the perianal region to deposit eggs. Because their migration is sporadic, several consecutive collections may be necessary to detect the infection.

The cysts of G. lamblia contain four nuclei and are passed in the stool of infected animals. The trophozoites are motile by flagella. G. lamblia does not possess an undulating membrane.

The trophozoite of E. histolytica has evenly arranged chromatin on the nuclear membrane; a central, compact karyosome in the nucleus; clear pseudopodia; and ingested RBCs in the cytoplasm.

When E. granulosus eggs are ingested by an intermediate herbivorous host, including humans, they usually develop into hydatid cysts in which invaginated larval scolices are produced. These cysts are most often in the liver or lung. Although clinical findings can provide a presumptive diagnosis, this is best confirmed by the finding, at surgery, of encysted larval scolices. Each scolex is capable of developing into an adult worm after ingestion by a dog or related animal, the definitive host.

T. gondii is a protozoan parasite of humans and a variety of lower animals. Human infections can be congenital or can result from ingestion of material containing oocysts from cat feces or from eating undercooked beef, lamb, or pork containing toxoplasma cysts. The life cycle of this parasite includes asexual multiplication in a number of hosts and sexual multiplication only in domestic cats and some closely related species, which then excrete potentially infectious oocysts.

E. histolytica and E. dispar cannot be morphologically differentiated. The cyst stage of both organisms has four nuclei with a centrally located karyosome. E. histolytica is a well recognized intestinal parasite, whereas E. dispar is considered nonpathogenic. Immunologic assays to detect antigens or molecular biology assays are necessary to differentiate these two species.

The life cycle of a number of human nematodes includes migration through the skin and peripheral bloodstream. In individuals who havesuffered several infections, allergic reactions can produce inflammation in the skin. S. stercoralis produces symptoms in the skin sometimes called larva migrans.

Babesia spp. are sporozoan parasites of RBCs that have been recognized as causing febrile illness in humans. B. microti has caused a number of tick-borne infections in the U.S. The parasites often appear as small rings within infected RBCs, resembling P. falciparum trophozoites. The pathognomic form of Babesia is the "Maltese cross," four ring forms inside a single RBC.

Identifying multiple parasites in fecal specimens is common, so a full examination of each slide is necessary.