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

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

3. Protozoans

4. 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

5. Apicomplexans/sporozoans
amoebas
ciliates
flagellates
Apicomplexans/sporozoans

6. 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

7. Entamoeba histolytica

8. 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

9. Entamoeba histolytica

10. 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

11. 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

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

13. 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

14. 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

16. 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

17. 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

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

19. 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

20. 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

21. 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

22. 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

23. Trypanosoma brucei

24. 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)

25. Heart pathology in Chagas disease
Trypanosoma cruzi
Heart pathology in Chagas disease

26. 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)

27. Life cycle of Leishmania
Disseminated leishmaniasis

28. 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

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

30. 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

31. 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

32. Plasmodium falciparum infection of RBCs showing ring form

33. 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

34. Life cycle of Toxoplasma gondii

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

36. 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

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

38. 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

39. 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

40. 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.

41. Ascaris lumbricoides

42. 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

43. 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).

44. 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.

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

46. 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

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

48. Enterobius vermicularis pinworm

49. 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

50. Hookworms

51. 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

52. Strongyloides stercoralis
Worm migration tracks under the skin

53. 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

55. 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

56. Heartworm in dogs

57. 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

58. 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

59. 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

60. 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

61. Beef Tapeworm
Tanenia saginata

62. 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

63. 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

64. Tick
Mite
Flea
Lice
Mosquito
Kissing Bug
Fly

65. 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

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

67. 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

68. 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