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Important points for parasites• Life cycle• Definitive host• Intermediate host• Reservoir hosts• Vectors• How humans are infected• Migration pattern and where adults live in
humans• Diagnostic form in humans• Geographical distribution
Classification of Parasites• Protozoa - single celled eukaryotes
– mastigophora - Giardia lamblia– sarcodina - Entamoeba histolytica– sporozoa - Toxoplasma gondii, Cryptosporidium sp.
Plasmodium sp. (malaria)
• Helminths - multi-cell eukaryotes (Worms)– nematodes - round worms (Ascaris lumbricoides)– platyhelminths - flat worms
• trematodes - (flukes) Schistosoma sp.• Cestodes - (tapeworms) - Taenia sp. (pork tapeworm.)
Entamoeba histolytica/dispar• Humans are the only host• world wide distribution• fecal oral or water borne - cysts are the infectious
form• diagnostic forms are cysts or trophozoites in
stool or serology• Causes amebic dysentery • invade and lyse tissue either in the colon or
extraintestinal sites usually liver or lung
Non pathogenic ameba
• Entamoeba dispar - morphologically identical to E. histolytica
• Entamoeba hartmanni - small race E. histolytica• Entamoeba coli• Iodamoeba butschlii• Endolimax nana
Giardia lamblia• The most commonly diagnosed and important
parasite of humans in the US• Transmission is fecal oral or water borne• Daycare centers• Fifty percent asymptomatic - 2 - 15% incidence• Symptoms - frothy fatty diarrhea - malabsorption,
osmotic diarrhea • Reservoir hosts - beaver and other animals• Diagnostic forms are cysts and trophozoites in stool
- noninvasive
1st trimester
2nd trimester
3rd trimester
Transmission rate
Rate of Severe sequelae/stillbirth
Among infected infants
14%
8%
41%
0%59%
29%
Congenital Toxoplasmosis400 – 4000/ year750 deaths from toxoplasmosis/ year 50% food-borne
Cryptosporidium parvum• Zoonosis - first human cases in 1979-1982 Auburn• Worldwide distribution• Cysts are infective - resistant to chlorine
– Milwaukee - 1993 370,000 cases
• Common in daycare centers• Intracellular in gut epithelium• Self limiting watery diarrhea in adults and children• Severe untreatable disease in AIDS patients
The little blue organisms lined up along the brush border of the small intestinal epithelium are Cryptosporidia. This infection causes diarrhea in immunocompromised hosts. medlib.med.utah.edu
Overview of Malaria Epidemiology
• 2.7 million deaths/year mostly children in Africa
• 300 to 500 million cases annually• 90 percent of them in sub-Saharan Africa• More than 40 percent of the world's
population lives in areas where malaria naturally occurs
• 1000 cases/year in the US
Plasmodium vivax
Trophozoites: ameboid; deforms the erythrocyte
Gametocytes: round-oval Schizonts: 12-24 merozoites
Rings
Infected erythrocytes: enlarged up to 2X; deformed; (Schüffner’s dots)
Malaria prevention decreased contact with mosquito
• Night biting Anopheles mosquitos are the only intermediate host
• Insecticide-treated nets – cheap and, if used correctly, an effective means of
preventing malaria.• Indoor house-spraying with residual insecticides
(e.g. DDT) – Mosquito resistance– Ecological concerns
• wearing of protective clothing and insect repellents at dawn and dusk
US Cases of Cyclospora• Guatemalan spring raspberries
– 1995 retrospective cases in New York and Florida– 1996 1465 cases in 14 states and 2 provinces– 1997 90 cases before importation of raspberries stopped
• Sweet Basil– Washington DC and Virginia 61 events with at least 1700
cases
• Lettuce - specialty baby leaves – 2 events, 1 cruise ship
Cyclospora cayetanensis
• First found in Nepal, Peru and Cook County Hospital 1989 - 1990
• Found to be a coccidian and named 1993• Reservoir unknown• Infects small intestine epithelial cells• Profuse watery diarrhea for 4 - 7 days. Multiple
relapses over 3 - 4 months• Treatment trimethoprim - sulfamethoxazole usually
more than one course required
Helminth species* x 106
Ascaris lumbricoides 1472
Ancylostoma Duodenale and Necator americanus 1298
Trichuris trichiura 1049
Schistosoma haematobium 114
Wuchereria bancrofti 107
Schistosoma mansoni 83
Taenia saginata 77
Hymenolepis nana 75
Strongyloides stercoralis 70Paragonimus westermani and related species 21
Onchocerca volvulus 18Brugia malayi and Brugia timori 13
Loa loa 13Opisthorchis viverrini and Opisthorchis felineus 10
Taenia solium 10
Diphyllobothrium latum 9
Clonorchis sinensis 7
Helminth species* x 106
Echinococcus granulosus and E. multilocularis 2.70Fasciola gigantica and Fasciola hepatica 2.40Schistosoma intercalatum 1.73Schistosoma japonicum 1.55Schistosoma mekongi 0.91Metagonimus yokogawai and related species 0.66Heterophyes heterophyes and related species 0.24Fasciolopsis buski 0.21Echinostoma ilocanum and related species 0.15Dracunculus medinensis 0.08
58,000 specimens from 601 US laboratories
Parasite PatientsGiardia lamblia 723Cryptosporidium sp. 81Dientamoeba fragilis 77Ascaris lumbricoides 59Entamoeba histolytica 52Trichuris trichiura 44Hookworm 45Enterobius vermicularis 49Strongyloides stercoralis 36Hymenolepis nana 17Isospora sp. 7Microsporidia 6Clonorchis sp. 4Others 241224 positive patients 58,000 specimens
Ascaris lumbricoideslarge roundworm
• World wide distribution• Humans are the only definitive host• Embryonated eggs are infective (not food handler)• Eggs hatch in duodenum, larvae penetrate the
intestine and migrate through the lungs, swallowed• Large round worm - 20 - 35 cm long• Adults live in small intestine• life span is 6 months to 1 year• Diagnostic form is eggs in stool or passed adult• Erratic adult worms
Strongyloides stercoralis
• Mainly tropical distribution• Can have free living adult forms• Infection is by larvae boring through intact skin• Adult females are small (2 mm long) and live in
the mucosa of the small intestine• Autoinfection can lead to hyperinfection syndrome • Infections can be subclinical for decades• Eosinophilia is common• Diagnostic forms are larvae in stool or sputum
Schistosoma sp. (blood flukes)• S. mansoni, S. haematobium and S. japonicum• Geographic distribution - tropics• Adults (1 - 2.5 cm long) reside in conjugal bliss in
venules• Infection is by cercaria penetrating intact skin• Snails are the intermediate host - humans acquire
infection in snail infested water • Zoonotic - except S. haematobium• Diagnostic form is ova in stool or urine or serology
Taenia soliumpork tape worm
• Found where ever people eat uncooked pork and allow pigs to scavenge human waste
• Humans can be both definitive host and intermediate host
• Infective forms for humans– cysticerci in pork - adult tape worm (10 years)– ova from human stool - cysticerci in humans
• Adult tapeworms are 2 - 7 meters long• Diagnostic forms are ova in stool or passed worm
segments. CAT/MRI scans for cysticerci
Other cestodes - tapeworms
• Taenia saginata - beef tapeworm• Diphyllobothrium latum - broad fish tapeworm• Hymenolepis nana - dwarf tapeworm• Hymenolepis diminuta - rat tapeworm
Cuterebra sp.• Adult flies do not bite or feed• Deposit eggs where hosts will be exposed • Eggs hatch in response to body heat gain
entrance through nose or mouth migrate to the subcutaneous area and complete development in 30 days
Acanthamoeba sp.
• No flagella• Cyst form often in tissue• Can cause amoebic
meningoencephalitis can be more chronic because of cysts
• Contact lens wearers are at risk of corneal infections
• Resistant to chlorine • More susceptible to NaCl• Somewhat more treatable
than Nagleria.