City College of San Francisco
PROTOZOA
Unicellular, eukaryotic organisms, often with comples dividing forms and life histories.
Habitat: Occur wherever there is moisture (sea, fresh water, soil, cells and fluids of other organisms)
Nutrition: Heterotrophic
Gas exchange: Across cell membrane
Osmoregulation: By contractile vacuoles when in fresh water.
Locomotion: By flagella, cilia, or pseudopodia. Certain parasitic forms can depend on insect vectors to transport them from one host to another.
Reproduction: Sexual and asexual modes. For parasitic forms the host is called the final or definitive host if sexual reproduction of the parasite occurs within its body or tissues. A host is termed the intermediate host if asexual reproduction of the parasite occurs within its body. Many parasites use animal or reservoir hosts which are important sources of infection for humans. Many freshwater and parasitic forms form environmentally resistant cyst forms. The active feeding form is referred to as a trophozoite.
Many of these organisms take time to locate with a microscope. It is good to scan the field with the high dry objective and locate consistent forms and sized organisms with nuclei before looking at the organisms in detail with oil immersion. Focus on features of diagnostic significance and put the various forms into context with respect to their life histories. We will survey protozoa of medical importance.
Links to parasitology information
Phylum Zoomastigophora: the flagellates
Possess 1 to many flagella for motility and feeding.
Giardia lamblia
Disease: Giardiasis
Distribution. Worldwide.
Location in host: primarily, upper small intestine
Treatment: Metronidazole (note: pregnant women can not take this drug and it should never be taken with alcohol)
This is the most commonly diagnosed intestinal protozoan in the U.S; an estimated 2 million cases are acquired each year from contaminated water. Giardia infection classically produces a fatty/mucoid diarrhea without blood. Adherence to the upper part of the small intestine interferes with fat digestion and can lead to malabsorption. Trophozoites are seldom found in formed stools; they are most often recovered in diarrhoeic stools or in duodenal aspirates. Infection occurs by ingestion of cysts in fecally contaminated food or water. Cysts are known to resist normal levels of water chlorination.
Diagnostic features: Trophs are between 12-15 um and pear-shaped, with two nuclei containing distinctive nucleoli. Two rod-shaped bodies lie transversely in the cytoplasm. A depression called the sucking disk occurs in the upper part of the ventral surface and acts as the site of attachment to the intestinal epithelium. Trophs possess 8 flagella. Cysts are typically oval. Distinctive filaments called axonemes run their length. Immature cysts have two nuclei. Mature cysts have a total of four nuclei after one division. Each cyst can give rise to four trophs.
Trichomonas vaginalis
Disease: Trichomoniasis
Distribution: Worldwide
Location in host: vagina of females (vaginitis) and occasionally in prostate and seminal vesicles of males.
Treatment: metronidazole (all sexual partners should be treated).
This organism LACKS a cyst stage. Direct transmission of the troph occurs during sexual intercourse. An estimated 1-2 million cases of this STD are thought to occur each year in the USA. Vaginitis caused by this organism produces a burning itch of the vaginal mucosa and a characteristic fishy smelling, yellow-green discharge which contains organisms, eptithelial cells and blood cells.
Diagnostic features: Trophs are not visible in unstained wet mounts. Trophs are between 11-15 um and have a prominent nucleus. Four anterior flagella arise from a basal body. A fifth flagellum is directed along the edge of an undulating membrane which spans the bottom half of the body. A rigid filament called an axostyle projects from the base of the cell.
Hemoflagellates (vector-transmitted)
African trypanosomes
Disease: African trypanosomiasis or African sleeping sickness.
Distribution: A belt across Central Africa, south of the Sahara Desert from West Africa (Trypanosoma brucei gambiense) to East Africa (Trypanosoma brucei rhodesiense). An estimated 25,000 cases occur each year with many millions at risk.
Location in the host: trypomastigote stages are found in the blood, lymph nodes, CSF, and the brain where they can cause severe meningoencephalitis, often fatal if untreated. Sleeping sickness refers to the coma induced by invasion of the CNS. The parasites are transmitted by the bite of tsetste flies. Trypanosomes are able to escape the immune system by continually switching the antigens on their cell surface. The antibodies formed against one set of antigens are useless against new set of antigens.
Treatment: Suramin and pentamidine are effective in treating most early cases. A drug known as DFMO is the drug of choice for CNS involvement. Interestingly, adequate dietary lipids probably protect people against this disease.
Diagnostic features: The African species are indistinguishable. The trypomastigote has a prominent central nucleus and a small kinetoplast (with its own DNA) at the posterior end. Trypomastigotes have a very wavy undulating membrane and a prominent anterior flagellum. Dividing forms are found in the blood.
New World Trypanosomes
Trypanosoma cruzi
Disease: South American trypanosomiasis or more commonly known as Chagas Disease.
Distribution: Most of Central and South America. An estimated 16-19 million persons are infected and many more are exposed.
Location in the host: Trypomastigotes are found in the blood and intracellular amastigote forms are found in pseudocysts in cardiac and smooth muscle cells, macrophages and glial cells of the brain.
Treatment: Many drugs are ineffective against intracellular forms.Disease often becomes chronic and incurable.
The disease is named after the Brazilian physician that identified and described the disease in 1909. The disease is transmitted by "kissing" or "assassin bugs"t hat commonly live in the cracks of poor quality housing. The parasite is not transmitted by bite of the kissing bug but in infected feces. A person will scratch them self after a bite and rub infected feces into their skin. The parasite can also be transmitted by blood transfusion. Many wild and domestic animals are reservoirs.
The disease has an early acute stage which is most common and severe in children under 5 years. In adults often becomes chronic without symptoms. Over many years the infection can damage heart tissue and autonomic ganglia of the esophagus or colon. Some believe that the damage to the heart is an autoimmune reaction. The disease is a leading cause of heart failure in Latin America.
Diagnostic features: T. cruzi trypomastigotes in the blood have a characteristic "C" or question-mark shape with a large kinetoplast at the posterior end. The undulating membrane is hard to see which heps to differentiate it from the African trypanosomes. Confirmation of active infection is carried out by demonstrating that the patient can infect a clean vector (xenodiagnosis). Trypomastigotes invade cells and transform into small round amastigotes which lack flagella. They possess a nucleus and a bar-shaped kinetoplast. Amastigotes multiply and form pseudocysts in tissues. They have been associated with fatal encephalitis in the brains of AIDS patients.
Leishmania donovani
Disease: Visceral leishmaniasis also known as Kala-azar
Distribution: The disease occurs worldwide in the tropics, middle east and Mediterranean and is transmitted by the bite of sand flies. Sand flies are very small and can penetrate screens and mosquito nets. Dogs are reservoirs. The old world disease is caused by L. donovani: about half a million cases are reported each year.
About 3% of infants are infected in endemic areas. VL is being increasingly reported as as opportunistic infection (reactivation disease) in AIDS patients in endemic regions. Location in the host: organisms invade macrophages of the skin and go on to invade the liver, spleen bone marrow and lymph nodes. The disease causes a characteristic undulant fever, marked swelling of liver and spleen, extreme wasting and is fatal if untreated.
In the new world L. braziliensis causes mucocutaneous leismaniasis. The disease is not visceral but often develops a secondary lesion on some region of the body. This often involves destruction of the cartilage and soft tissues of the nasal and buccal mucosa.
Treatment: Injections of pentavalent antimonials. There is a 5-10% mortality rate with Kala-Azar despite treatment. Secondary bacterial infections are treated with antibiotics.
Diagnostic features: An infective flagellated promastigote stage occurs in the sand fly vector which infects humans and animal reservoirs. The promastigote penetrate local tissue macrophages and transform into amastigotes (lacking flagella). Amastigotes reproduce by binary fission forming "cell nests." Characteristic amastigotes with a nucleus and bar-shaped kinetoplast are typically isolated from bone marrow or splenic aspirates. Newer serologic techniques (detect antibodies) and PCR (detect parasite DNA) avoid the use of invasive techniques.
Subphylum Sarcodina
Naked and shelled amoebae with broad pseudopodia for locomation and feeding.
Entamoeba histolytica
Disease: Amoebic dysentery
Distribution: Worldwide: up to 500 million infections occur at any one time, with up to 100,000 deaths per year.
Location in the host: colon and cecum. If immune system is depressed trophozoites can invade tissue of colon and spread to liver, lung brain and other tissues
Treatment: Metronidazole is the drug of choice. Tetracycline in combination with diiodohydroxyquin also have high rates of success.
Infection occurs by ingesting cysts in fecally contaminated material. Excystment occurs in small intestine and gives rise to 4 uninucleate trophs which colonize the colon. Some trophs are associated with tissue invasion and often ingested RBCs can be seen within their cytoplasm. They produce characteristic flask-shaped ulcers in the colon. The number one invasion site is the liver, where trophs can produce a sterile abscess (no bacteria) filled with blood and chocolate-looking liquid. ELISA tests can now distinguish between invasive E. histolytica and a non-invasive commensal known as E. dispar.
Diagnostic features: Trophs have a granular-looking cytoplasm and often exhibit pseudopodia. The nucleus is surrounded by an even layer of chromatin and has a distinct central nucleolus. WBCs can be mistaken for trophs by inexperienced microscopists. RBCs within the cytoplasm of a troph is considered diagnostic for E. histolytica.
Cysts have up to 4 nuclei and have distinctive central nucleoli. Cysts contain distinctive bar-shaped 'chromatoidal bodies.' The organism must be differentiated from Entamoeba coli which is considered non pathogenic. E. coli cysts contain up to 8 nuclei and their nucleoli are off-center.
Phylum Apicomplexa
The organisms in this phyla are classified according to structures collectively called an apical complex.This complex is associated with stages that are involved with invading host cells and is the basis of much vaccine development against these parasites.
Many medically important apicomplexan parasites belong to the class Sporozoa
Plasmodium species
Four species infect humans. They are:
The most significant and widespread species with respect to morbidity and mortality are P. falciparum and P.vivax
Disease: Malaria
Distribution: Tropics and subtropics worldwide. An estimated 2-3 million deaths occur each year due to P. falciparum alone, with many more millions at risk!. Malaria is once more on the rise due to drug-resistant parasites and insecticide-resistant mosquitoes.
Treatment: Mefloquine (replaces chloroquine in drug-resistant areas) and primaquine to prevent relapses due to P. vivax (there are other drugs such as fansidar). THERE IS NO VACCINE!
The life cycle of the malaria parasite is extremely complex but can be thought of as a large amplification cycle. Malaria is transmitted by the bite of a female Anopheles mosquito. The infective stage soon travels to and infects liver cells where it multiplies for a week or two. There are no symptoms until the parasite leaves the liver and invades RBCs. The parasite cyclically invades, multiplies and ruptures RBCs causing characteristic fever, chills and anaemia. Sexual stages of the parasite are found in the mosquito. Humans are the intermediate host as they carry asexual forms of the parasite. True relapses can occur due to activation of dormant liver stages (hypnozoites) of P. vivax but not P. falciparum.
Diagnostic features: Each species of malaria parasite has unique features. We will focus on P. falciparum. Only two stages of the parasite are typically found circulating in the blood. One stage is an early troph form called a ring stage. Note the characteristic nucleus and thin strand of pale blue cytoplasm that looks like a signet ring. Some RBCs may contain several rings. The other stage is called a gametocyte, which is picked up by a mosquito during a blood meal. Gametocytes form male and female gametes and are typically crescent shaped. You may see dots of pigment in the gametocytes which are formed when the parasite digests hemoglobin. The other blood stages sequester in deeper tissues and organs. In these locations infected RBCs express knob proteins which adhere to the endothelial lining of blood vessels. Unfortunately, infected cells tend to block blood vessels in the brain (causing coma and death = cerebral malaria) and placenta (lead to stillbirths or miscarriage).
Toxoplasma gondii
Disease: Toxoplasmosis
Distribution: Worldwide
Location in the host: Trophs are found in various cells, fluids and tissues. Cysts occur in the CNS, cardiac and skeletal muscle and viscera.
A complex life cycle. Sexual stages occur in the intestines of cats which pass infective oocysts in their feces. Human infection can occur in various ways:
In immunosuppressed patients toxoplasmosis is due to reactivation of a latent infection. In AIDS patients toxoplasmic encephalitis is the most common parasitic infection of the CNS.
Diagnostic features: Trophs are crescent shaped organisms with a prominent nucleus. When actively dividing and not encysted they are called tachyzoites. Tissue cysts vary in size and are usually spherical in the brain and contain few to many organisms. They are considered slow-growing and termed bradyzoites.
Treatment: pyrimethamine, TMP-SMZ (Brand names: Septra and Bactrim)
Cryptosporidium parvum
Disease: Cryptosporidiosis
Distribution: Worldwide, many animal reservoirs, especially young calves.
Location in the host: Cryptosporidium invades the brush border of intestinal epithelial cells and causes little obvious tissue inflammation.
Treatment: The antibiotics paromomycin and azithromycin alleviate but do not cure crypto diarrhea. Nitazoxanide (NTZ) was granted orphan drug status by the FDA to alleviate this diarrhea. Other treatments are largely experimental and supportive (rehydration). HAART shows reduction in opportunistic infections.
The parasite produces a severe, watery diarrhea which resolves after a week or two in immunocompetent individuals. Immunosuppressed patients are unable to control the infection and the diarrhea can become life-threatening. Cryptosporidium can contribute to AIDS wasting syndrome. Infection is acquired by ingestion of oocysts in fecally-contaminated material.
Diagnostic features: Diagnosis is made by identifying oocysts in the feces. In acid-fast smears, oval oocysts stain pink ( about 4 um) in a blue background. Oocysts contain 4 infective sporozoite stages. Oocysts must now be differentiated from those of another newly reported parasite known as Cyclospora. This parasite causes a diarrheal disease which has been associated with eating raspberries (of Guatemalan origin)imported into the U.S. The oocysts of this parasite are larger (8-10 um) than those of Cryptosporidium. Fortunately, Cyclospora is treatable with TMP-SMZ.
Phylum Microspora
The medical significance of these parasites did not become clear until the advent of the HIV/AIDS epidemic. Not all sources of infection are known making prevention difficult.These organisms produce environmentally resistant spores (1.0-2.0 um) which infect host cells. Ten species have been reported in humans.
Enterocytozoon bienusi produces diarrhea and malabsorption in 6-30% of all AIDS patients with chronic diarrhea (the higher number generally refers to developing countries). It is an important contributor to AIDS-wasting syndrome. Infective spores are shed in feces, urine and/or mucus secretions.
Other species of microsporidia infect the eyes and brain and may become systemic.
Treatment: albendazole (not effective for AIDS-related species) fumagillin (topical application only) for keratitis HAART : reduces OIs.
Phylum Ciliophora
Balantidium coli
Disease: Balantidiasis
Distribution: Wide in temperate and warm climates, particularly the Phillipines
Location in the host: Colon of humans, pigs and monkeys
Treatment: Tetracycline
The largest protozoan and only parasitic ciliate of humans: produces a diarrheal illness. Infection acquired by ingesting cysts (fecal-oral route).
Diagnostic features: Trophs: covered with cilia. Large 50-200 um. Large kidney-shaped macronucleus, small micronuclei. Has a mouth called a cytostome and carries out osmoregulation with a contractile vacuole.
Note: sites with many images may be slow to load.
This is my favorite so far!
The World Health Organization on Tropical Diseases
The American Society of Parasitologists
Yet more parasite images from our friends "down under."
Medical information including travel medicine