The Parasites of Medical Importance Protozoa, Helminths, and Arthropod Vectors Chapter 23

Parasitology the study of eucaryotic parasites, protozoa and helminths cause 20% of all infectious diseases less prevalent in industrialized countries

Protozoans

Protozoa single-celled, animal-like microbes, usually having some form of motility life cycles vary most reproduce by simple asexual cell division of the active feeding cell (trophozoite) many undergo formation of a cyst others have a complex life cycle that includes asexual & sexual phases

Apicomplexans/sporozoans amoebas ciliates flagellates Apicomplexans/sporozoans

Entamoeba histolytica alternates between a large trophozoite, motile by means of pseudopods & a smaller nonmotile cyst trophozoite has a large nucleus and lacks most other organelles humans are the primary hosts Ingested (fecal-oral transmission) carried by 10% of world population

Entamoeba histolytica

Entamoeba histolytica cyst cysts are swallowed & arrive at the small intestine; alkaline pH & digestive juices stimulate cyst to release 4 trophozoites trophozoites attach, multiply, actively move about & feed asymptomatic in 90% of patients ameba may secrete enzymes that dissolve tissues & penetrate deeper layers of the mucosa causing dysentery, abdominal pain, fever, diarrhea & weight loss

Entamoeba histolytica

Entamoeba histolytica life-threatening manifestations are hemorrhage, perforation, appendicitis, & tumorlike growths (amebomas) may invade liver & lung severe forms of disease result in 10% fatality rate effective drugs are iodoquinol, metronidazole, & chloroquine

Amebic brain infections caused by Naegleria fowleri & Acanthamoeba ordinarily inhabit standing water primary acute meningoencephalitis is acquired though nasal contact with water or traumatic eye damage. infiltration of brain is usually fatal

Naegleria fowleri Phase-contrast of Naegleria feeding on bacteria. Organism is free-living in fresh water

Balantidium coli an occupant of the intestines of domestic animals such as pigs & cattle acquired by ingesting cyst-containing food or water trophozoite erodes intestine & elicits intestinal symptoms healthy humans are resistant rarely penetrates intestine or enters blood treatment – tetracycline, iodoquinol, nitrimidazine or metronidazole

Trichomonas Small, pear-shaped 4 anterior flagella & an undulating membrane Exist only in trophozoite form 3 species infect humans T. vaginalis T. tenax T. hominis

Trichomonas vaginalis causes an STD called trichomoniasis reservoir is human urogenital tract 50% of infected are asymptomatic strict parasite, cannot survive long outside of host 3 Million new cases a year in U.S., a top STD female symptoms – foul-smelling, green-to-yellow discharge; vulvitis; cervicitis; urinary frequency & pain male symptoms – urethritis, thin, milky discharge, occasionally prostate infection metronidazole

Trichomonas tenax small resides in oral cavity of 5-10% only flagellate in oral cavity not a true pathogen, but an opportunist in lesions of gingivitis & peridontal pockets

Trichomonas hominis a resident of the cecum of a small percentage of humans & great apes not associated with disease

Giardia lamblia pathogenic flagellate unique symmetrical heart shape cysts can survive for 2 months in environment cysts enter duodenum, germinate, & travel to jejunum to feed & multiply causes giardiasis – diarrhea, abdominal pain diagnosis is difficult because organism is shed in feces intermittently Treatment: quinacrine or metronidazole agent is killed by boiling, ozone, & iodine

hemoflagellates live in blood & tissues of human host obligate parasites cause life-threatening and debilitating zoonoses spread by blood-sucking insects that serve as intermediate hosts acquired in specific tropical regions have complicated life cycles & undergo morphological changes Trypanosoma Leishmania

Trypanosoma distinguished by their infective stage, trypomastigote is an elongate, spindle-shaped cell with tapered ends, eel-like motility 2 types of trypanosomiasis T. brucei – African sleeping sickness T. cruzi – Chagas disease – endemic to Central and South America

Trypanosoma brucei causes African Sleeping Sickness spread by tsetse flies harbored by reservoir mammals biting of fly inoculates skin with trypanosome, which multiplies in blood & damages spleen, lymph nodes & brain chronic disease symptoms are sleep disturbances, tremors, paralysis & coma treatment before neurological involvement melarsoprol, difluormethylornithine

Trypanosoma brucei

Trypanosoma cruzi causes Chagas disease reduviid bug (kissing) bug is the vector infection occurs when bug feces are inoculated into a cutaneous portal local lesion, fever, & swelling of lymph nodes, spleen, & liver heart muscle & large intestine harbor masses of amastigotes chronic inflammation occurs in the organs (especially heart & brain)

Heart pathology in Chagas disease Trypanosoma cruzi Heart pathology in Chagas disease

Leishmania Leishmaniasis is a zoonosis transmitted among mammalian hosts by female sand flies that require a blood meal to produce eggs infected macrophages carry the pathogen into the skin (cutaneous leishmaniasis) & bloodstream (systemic or visceral leishmaniasis), giving rise to fever, enlarged organs & anemia (Kala azar is the most severe & fatal form)

Life cycle of Leishmania Disseminated leishmaniasis

Apicomplexan parasites sporozoans lack locomotor organelles in the mature state alternate between sexual & asexual phases & between different animal hosts most form specialized infective bodies that are transmitted by arthropod vectors, food, water, or other means Plasmodium Toxoplasma Cryptosporidium

Plasmodium causes malaria female Anopheles mosquito is the vector obligate intracellular sporozoan 4 species: P. malariae, P. vivax, P. falciparum & P. ovale 300-500 Million new cases each year 2 Million deaths each year

Plasmodium infective forms for humans (sporozoites) enter blood with mosquito saliva, penetrate liver cells, multiply, and form hundreds of merozoites, which multiply in & lyse RBCs. symptoms include episodes of chills-fever-sweating, anemia, and organ enlargement. therapy is chloroquine, quinine, or primaquine

RBCs infected by merozoites from the liver Sexual phase with sporozoite formation in the mosquito) Asexual phase in humans as sporozoite enters Gametocytes produced and ingested by mosquito Liver phase (sporozoites invade & merozoites produced) RBCs infected by merozoites from the liver

Plasmodium falciparum infection of RBCs showing ring form

Toxoplasma gondii obligate apicomplexan parasite with extensive distribution lives naturally in cats that harbor oocysts in the GI tract acquired by ingesting raw meats or substances contaminated by cat feces most cases of toxoplasmosis go unnoticed except in fetus & AIDS patients which can suffer brain & heart damage treatment: pyrimethamine & sulfadiazine

Life cycle of Toxoplasma gondii

Biopsy showing clusters of intracelluar T. gondii CAT scan of brain showing lesion Toxoplasmosis in an AIDS patient

Cryptosporidium an intestinal pathogen infects a variety of animals Oocysts shed in feces an intestinal pathogen infects a variety of animals exists in tissue & oocyst (passed in feces) phases 1990s – 370,000 cases in Milwaukee, WI due to contaminated water causes enteric symptoms AIDS patients may suffer chronic persistent diarrhea no real effective drugs

Babesia causes babesiosis first protozoan found to cause a disease first protozoan found to be associated with a tick infection resembles malaria

Helminths adults are large, multicellular animals with specialized tissues & organs adult worms mate & produce fertilized eggs that hatch into larvae that mature in several stages to adults (the sexes may separate or hermaphroditic) Hosts adults live in the definitive host eggs & larvae may develop in the same host, external environment, or intermediate host a transport host experiences no parasitic development pathology arises from worms feeding on & migrating through tissues and accumulation of worms & worm products

Nematodes - roundworms Filamentous with protective cuticles, a complete digestive tract, & separate sexes Ascaris lumbricoides Trichuris trichiura (whipworm) Enterobius vermicularis (pinworm) Hookworms Strongyloides stercoralis Trichinella spiralis Filarial worms

Ascaris lumbricoides a large (up to a foot long) intestinal roundworm 1 Billion cases worldwide Most cases in the US occur in the southeastern states indigenous to humans Ascaris spends its larval & adult stages in humans & releases embryonic eggs in feces, which are spread to other humans ingested eggs hatch into larvae & burrow through the intestine into circulation & travel to the lungs & pharynx & are swallowed. Adult worms complete cycle in intestines.

Ascaris lumbricoides

Ascaris lumbricoides worms retain motility, do not attach severe inflammatory reactions mark the migratory route, allergic reactions can occur heavy worm loads can retard physical & mental development

Roundworm Zoonoses-1 Don’t play with the animal feces ! Don’t eat dirt ! Dog and Cat Roundworms (toxocariasis) estimated 10,000 new cases of roundworm infection occur in children every year, most often as a result of eating dirt contaminated with animal feces. Most human infections are mild enough to go unnoticed and apparently produce no permanent damage……But…. Visceral Larval Migrans (VLM) Ocular Larval Migrans (OLM).

Roundworm Zoonoses-2 Raccoon roundworm (Baylisascaris procyonis) Raccoons are very common in the urban/suburban environment, and perhaps 80%+ carry the roundworm. Worm eggs are very hardy, surviving years outdoors. When ingested by humans the larvae migrate throughout the body. Some enter the brain and death my ensue.

Trichuris trichiura whipworm humans sole host trichuriasis has its highest incidence in tropics remains in intestine Egg in fecal sample

Enterobius vermicularis pinworm enterobiasis most common worm disease of children in temperate zones eggs are picked up from surroundings & swallowed. after hatching in the small intestine, they develop into adults anal itching occurs when mature females emerge from intestine to release eggs self-inoculation is common tape test

Tape test for pinworms Flashlight Test: At night, the female adult worms deposit their eggs outside the rectum or anal area.

Enterobius vermicularis pinworm

Hookworms characteristic curved ends & hooked mouths Endemic to tropics and subtropics Necator americanus & Ancylostoma duodenale humans shed eggs in feces, which hatch into filariform (threadlike) larvae & burrow into the skin of lower legs (may cause a local irritation…”ground itch”) larvae travel from blood to lungs, proceed up bronchi & throat & are swallowed. worms mature & reproduce in small intestine & complete cycle may cause pneumonia, nausea, vomiting, cramps & bloody diarrhea…blood loss can be significant - anemia

Hookworms

Strongyloides stercoralis threadworm tiny roundworm completes life cycle in humans or moist soil larvae penetrate skin & migrate to lungs, are swallowed & complete development in intestine can reinfect the same host without leaving the body heavy worm loads can cause pneumonitis & eosinophilia, bloody diarrhea, liver enlargement, bowel obstruction & malabsorption

Strongyloides stercoralis Worm migration tracks under the skin

Trichinella spiralis causes trichinosis life cycle entirely within mammalian host acquired from eating undercooked pork (also bear) larvae migrate from intestine to blood vessels, muscle, heart, & brain, where it forms cysts first symptoms –flulike, diarrhea second symptoms – muscle & joint pain, shortness of breath, pronounced eosinophilia no cure after larva have encysted

Edema from blockage of lymphatics Filarial worms long threadlike worms with tiny larvae that circulate in blood & reside in various organs spread by biting insects Wuchereia bancrofti – elephantitis Onchocerca volvulus – river blindness Loa loa – eye worm Edema from blockage of lymphatics

Heartworm in dogs

Trematodes (flukes) flatworms with ovoid leaflike bodies have digestive, excretory, neuromuscular, & reproductive systems lack circulatory & respiratory systems animals such as snails or fish are usually the intermediate hosts & humans are the definitive hosts

Blood flukes: Schistosomes schistomiasis is the 2nd most prominent parasitic disease (after malaria) adult flukes live in humans & release eggs into water. The early larva develops in freshwater snail into a 2nd larva this larva penetrates human skin & moves into the liver to mature; adults migrate to intestine or bladder & shed eggs, giving rise to chronic organ enlargement

Tapeworms flatworms long, very thin, ribbonlike bodies composed of sacs (proglottids) & a scolex that grips the intestine each proglottid is an independent unit adapted to absorbing food & making & releasing eggs Taenia saginata Taenia solium

Taenia saginata beef tapeworm very large, up to 2,000 proglottids humans are the definitive host animals are infected by grazing on land contaminated with human feces infection occurs from eating raw beef in which the larval form has encysted larva attaches to the small intestine & becomes an adult causes few symptoms

Beef Tapeworm Tanenia saginata

Taenia solium pork tapeworm infects humans through ingesting cysts or eggs eggs hatch in intestine, releasing tapeworm larva that migrate to all tissues & encyst most damaging if they lodge in heart muscle, eye, or brain may cause seizures, psychiatric disturbances

Arthropod Vectors Vectors are animals that carry microbial pathogens Arthropods are a common vector Some arthropods also serve as hosts for the pathogens they transmit Disease vectors belong to two classes of arthropods Arachnida Insecta

Tick Mite Flea Lice Mosquito Kissing Bug Fly

Arachnids Adult arachnids have four pairs of legs Ticks and mites resemble each other morphologically Ticks are the most important arachnid vectors Serve as vectors for bacterial, viral, and protozoan diseases Second only to mosquitoes in the number of diseases they transmit Hard ticks are the most prominent disease vector Transmit Lyme disease, Rocky Mountain spotted fever, tularemia, relapsing fever, and tick-borne encephalitis

Arachnids Parasitic mites are found wherever humans and animals coexist Transmit rickettsial diseases among animals and humans

Insects Adults have three pairs of legs as well as a head, thorax, and abdomen Fleas Most fleas are not associated with humans but a few do feed on humans Plague is the most significant disease transmitted by fleas Body lice Parasites that can also transmit disease Most common among poor or overcrowded communities

Insects Flies Among the most common insects Those that transmit disease are generally bloodsuckers Mosquitoes Most important arthropod vector of disease Carry some of the world’s most devastating diseases Kissing bugs Often take blood meals near the mouth of their human hosts Feed on blood nocturnally while the host sleeps