Exam 5 - (CH 28) Diagnostic Parasitology Textbook Review Questions Flashcards

Question

Describe the microscopic characteristics you would use to differentiate the oocysts of Cyclospora cayetanensis and Cryptosporidium parvum. Include size, appearance on routine wet mount or trichrome stain, and appearance with special stains.

Answer 1

C. cayetanensis oocysts are round and average 8 to 10 µm in diameter. On a wet mount, they appear nonrefractile, with a cluster of granules. The organisms are not readily distinguishable on trichrome-stained smears and may appear as round, wrinkled, clear objects. With modified acid-fast stain, the organisms stain variably from dark pink to almost colorless and may have a wrinkled or distorted shape. Oocysts of Cyclospora autofluoresce blue under ultraviolet light, at a wavelength of 365 nm, and green at 450 nm. C. parvum oocysts are round and approximately 4 to 6 µm in diameter. Because of their small size, they are not distinguishable on wet mounts or in most permanent-stained smears. With a modified acid-fast stain, they are dark pink and round. Direct fluorescent antibody techniques that detect the oocyst in the feces are available.

Answer 2

The most likely identification is E. histolytica trophozoite. If no ingested erythrocytes had been seen, the organism would have been reported as E. histolytica– E. dispar because the organisms are morphologically identical. The presence of ingested erythrocytes indicates that it is E. histolytica. This parasite is considered pathogenic for humans. Patients with E. histolytica infection can suffer from amebic dysentery, which can manifest as fever and intense colicky abdominal pain.

Answer 3

C. parvum produces two types of oocysts, thin-walled and thick-walled. The thick-walled oocysts are passed in the feces and are infective. The thin-walled oocysts are involved in autoinfection. These structures rupture in the intestine and release sporozoites that reinfect intestinal cells. With S. stercoralis infection, the eggs rupture in the intestine and release rhabditiform larva that mature to filariform larva in the intestine. The filariform larvae burrow into the intestinal wall, gain access to the circulation, and then complete the life cycle. Autoinfections result in persistent infections with large numbers of parasites.

Answer 4

The organism belongs to the genus Trypanosoma. The morphologic form is the trypomastigote. Members of the genus Trypanosoma cause African sleeping sickness, transmitted by tsetse flies, and Chagas disease, transmitted by kissing or reduviid bugs.

Answer 5

Primary amebic meningoencephalitis is caused by Naegleria fowleri. Young, healthy individuals are usually infected and present with a history of swimming in warm, stagnant water. The trophozoite gains direct access to the brain via the nasal olfactory nerve when water is forced into the nose. The infection has a rapid onset (within 2 to 7 days) after exposure, and symptoms resemble those of bacterial meningitis— photophobia, headache, vomiting, and stiff neck. Death usually occurs within 1 week of onset of symptoms. Diagnosis can be made by observing the ameboid trophozoite in a wet mount of the CSF sediment. Trophozoites may be found in biopsy tissue of the brain. Granulomatous amebic encephalitis is a chronic infection caused by Acanthamoeba spp. and is usually found in individuals with underlying conditions such as lymphoma or diabetes. The organism may enter through the skin or be inhaled, and it spreads hematogenously to the brain. It may take years for symptoms to develop. Symptoms include headache, dizziness, mental confusion, and seizures. Often, the infection is detected at autopsy. Trophozoite and cyst forms can be identified on biopsy specimens.

Answer 6

Toxoplasmosis in the immunocompetent host is usually asymptomatic or presents as a transient, flulike illness. The immunocompromised patient often develops encephalitis or pneumonia. Cryptosporidium infection in the immunocompetent host causes self-limiting diarrhea. In the immunocompromised host, the diarrhea is prolonged and severe (partially because of the autoinfective part of the life cycle). The patient may lose as much as 10 L/ day of fluid and develop electrolyte imbalance. Long-term infection may lead to malabsorption syndrome. Strongyloides infection in the immunocompetent host also may be asymptomatic or present with vague abdominal symptoms that can mimic those of an ulcer. In the immunocompromised host, hyperinfection (dissemination) may result, with filariform larvae migrating to multiple body organs.

Answer 7

■ Protozoan cysts, helminth eggs, and helminth larvae can be identified on a wet mount preparation of a fecal concentrate. The identification of protozoan trophozoites and cysts is confirmed on a permanently stained smear. ■ Rapid tests (immunochromatographic) are routinely used for detection of Giardia duodenalis, Cryptosporidium spp., and Entamoeba histolytica. ■ Multiplex molecular assays for intestinal pathogens include several intestinal organisms— usually G. intestinalis, Cryptosporidium spp., and E. histolytica. ■ Fecal-oral transmission is the route of infection for enteric protozoa. The cyst is the infective stage, and the trophozoite is the stage that causes tissue damage. ■ The major intestinal protozoan pathogens include E. histolytica and G. duodenalis. D. fragilis and B. hominis cause symptomatic infections in some patients. ■ E. histolytica, E. dispar, and E. moshkovkii are morphologically identical, and specific immunoassay techniques must be performed to differentiate the pathogenic E. histolytica from the nonpathogenic E. dispar and E. moshkovkii. If the organism is ingested, RBCs are present in the trophozoite, the organism can be reported as E. histolytica. ■ N. fowleri, Acanthamoeba spp., and Balamuthia mandrillaris are free-living amebae that can infect humans. N. fowleri causes an acute condition called primary amebic meningoencephalitis, which is rapidly fatal. Acanthamoeba spp. cause keratitis, skin infections, or granulomatous amebic encephalitis, whereas Balamuthia is associated with GAE and skin infections. ■ The genera Leishmania and Trypanosoma are blood flagellates of humans transmitted by insects. ■ Human malaria can be caused by four different Plasmodium spp.— P. vivax, P. ovale, P. malariae, and P. falciparum. ■ The life cycle of malaria is complex, with asexual reproduction taking place in human RBCs and sexual reproduction occurring in the gut of the mosquito. ■ Identification of Plasmodium spp. is made by observing characteristics of the infected RBCs and the malarial organism on a Wright- or Giemsa-stained peripheral blood smear. ■ One rapid detection test (P. vivax and P. falciparum) is approved for use in the United States for diagnosing malaria. Microscopic confirmation is still performed. ■ B. microti, an intraerythrocytic parasite, morphologically resembles P. falciparum on blood smears. ■ The intestinal Apicomplexa species include C. parvum, C. belli, and C. cayetanensis; the infective stage for humans is the acid-fast– positive oocyst. ■ T. gondii causes a tissue infection that is usually asymptomatic in immunocompetent hosts. In patients with AIDS, a latent infection can reactivate and cause encephalitis or pneumonia. Congenital transmission can result in serious complications. ■ For most flukes, diagnosis is made by finding the egg in a fecal specimen. In the case of the lung fluke (P. westermani), the egg may be found in sputum; in the case of S. haematobium, the egg is found in urine. ■ Humans are the definitive hosts, and animals or insects serve as intermediate hosts for most tapeworms infecting humans. ■ Eggs of the beef tapeworm, T. saginata, and eggs of the pork tapeworm, T. solium, are identical and must be reported as Taenia sp. ■ The diagnostic stage for a roundworm may be an egg or a larval form, depending on the species. ■ Pinworm, E. vermicularis, infection is common in children and is diagnosed by finding eggs on a cellophane tape preparation or pinworm paddle. Eggs are laid on the perianal area when the female migrates out through the anus at night. ■ Human hookworm infection may be caused by N. americanus and A. duodenale. The eggs are identical and are reported as “hookworm eggs.” ■ Eggs of S. stercoralis are not usually present in the stool; the typical diagnostic stage is the rhabditiform larva. ■ T. spiralis is acquired when humans ingest raw or undercooked pork containing the larval form. ■ Diagnosis of trichinosis is made through biopsy of tissue to identify the coiled larval stage. ■ Diagnosis of filarial worm infection is made by observing the microfilariae in blood or tissue specimens. Larval characteristics include the presence or absence of a sheath and the location and arrangement of nuclei in the tail of the microfilariae.