1. The Parasites of Medical Importance

2. Parasitology The study of eucaryotic parasites, protozoa and helminths
Cause 20% of all infectious diseases
Less prevalent in industrialized countries; increasingly common in AIDS patients

3. Typical Protozoan Pathogens
Single-celled, animal-like microbes, most having some form of motility
Estimated 100,000 species, approximately 25 are important pathogens
Life cycles vary
Most propagate 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 and sexual phases.

5. Infective Amebas

6. Entamoeba histolytica and Amebiasis
Alternates between a large trophozoite, motile by means of pseudopods and a smaller nonmotile cyst
Trophozoite has a large nucleus and lacks most other organelles.
Humans are the primary hosts.
Ingested
Carried by 10% of world population

8. Entamoeba histolytica
Cysts are swallowed and arrive at the small intestine; alkaline pH and digestive juices stimulate cysts to release 4 trophozoites.
Trophozoites attach, multiply, actively move about and feed.
Asymptomatic in 90% of patients
Ameba may secrete enzymes that dissolve tissues and penetrate deeper layers of the mucosa.
Causing dysentery, abdominal pain, fever, diarrhea and weight loss

9. Entamoeba histolytica
Life-threatening manifestations are: hemorrhage, perforation, appendicitis, and tumorlike growths, amebomas.
May invade liver and lung
Severe forms of disease result in 10% fatality rate.
Effective drugs are iodoquinol, metronidazole, and chloroquine.

10. Amebic Infections of the Brain
Caused by Naegleria fowleri and 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.

11. An Intestinal Ciliate: Balantidium coli
An occupant of the intestines of domestic animals such as pigs and cattle
Acquired by ingesting cyst-containing food or water
Trophozoite erodes intestine and elicits intestinal symptoms.
Healthy humans are resistant.
Rarely penetrates intestine or enters blood
Treatment – tetracycline, iodoquinol, nitrimidazine or metronidazole

13. The Flagellates

14. Trichomonads: Trichomonas species
Small, pear-shaped
4 anterior flagella and an undulating membrane
Exist only in trophozoite form
3 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 cases yearly, a top STD
Female symptoms – foul-smelling, green-to-yellow discharge; vulvitis; cervicitis; urinary frequency and pain
Male symptoms – urethritis, thin, milky discharge, occasionally prostate infection
Metronidazole

17. Giardia lamblia and Giardiasis
Pathogenic flagellate
Unique symmetrical heart shape with concave ventral surface that acts like a suction cup
Cysts are small, compact, and multinucleate.
Reservoirs include beavers, cattle, coyotes, cats, and humans.
Cysts can survive for 2 months in environment.
Usually ingested with water and food
ID 10 to 100 cysts

19. Cysts enter duodenum, germinate, travel to jejunum to feed and 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, and iodine.

20. Hemoflagellates: Vector-Borne Blood Parasites
Obligate parasites that live in blood and tissues of human host
Cause life-threatening and debilitating zoonoses
Spread in specific tropical regions by blood-sucking insects that serve as intermediate hosts
Have complicated life cycles and undergo morphological changes
Categorized according to cellular and infective stages

22. Trypanosoma species and Tropanosomiasis
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

23. Trypanosoma brucei and African Sleeping Sickness
Spread by tsetse flies
Harbored by reservoir mammals
Two variants of disease caused by 2 subspecies:
T.b.gambiense – Gambian strain; West Africa
T.b. rhodesiense – Rhodesian strain; East Africa
Biting of fly inoculates skin with trypomastigotes, which multiplies in blood and damages spleen, lymph nodes and brain.

24. Chronic disease symptoms are sleep disturbances, tremors, paralysis and coma.
Trypanosomes are readily demonstrated in blood, spinal fluid or lymph nodes.
Treatment before neurological involvement melarsoprol, eflornithine
Control involves eliminating tsetse fly.

26. Trypanosoma cruzi Causes Chagas disease
Reduviid bug (kissing bug) is the vector.
Infection occurs when bug feces is inoculated into a cutaneous portal.
Local lesion, fever, and swelling of lymph nodes, spleen, and liver
Heart muscle and large intestine harbor masses of amastigotes.
Chronic inflammation occurs in the organs (especially heart and brain).
Treatment nifurtimox and benzonidazole

28. Leishmania species and Leishmaniasis
Leishmaniasis - zoonosis transmitted among mammalian hosts by female sand flies that require a blood meal to produce eggs
Endemic to equatorial regions
Promastigotes are injected with sand fly bite, convert to amastigote and multiply; if macrophage is fixed the infection is localized; systemic if macrophage migrates.

30. Cutaneous-oriental sore, Baghdad boil - localized ulcerated sore
Espunda – skin and mucous membrane infection of the head; chronic infection
Systemic-visceral - high intermittent fever; weight loss, enlarged spleen, liver, and lymph nodes
Kala azar is the most severe and fatal form if untreated.

31. Apicomplexan parasites
Sporozoans
Lack locomotor organelles in the trophozoite state
Alternate between sexual and asexual phases and between different animal hosts
Most form specialized infective bodies that are transmitted by arthropod vectors, food, water, or other means.
Plasmodium
Toxoplasma
Cryptosporidium

32. Plasmodium: The Agent of Malaria
Dominant protozoan disease
Obligate intracellular sporozoan
4 species: P. malariae, P. vivax,
P. falciparum and P. ovale
Female Anopheles mosquito is the primary vector; blood transfusions, mother to fetus
million new cases each year
2 million deaths each year

33. 2 distinct phases of malarial parasite development:
asexual phase – human host
Infected female mosquito injects asexual sporozoite which localizes in liver; it then undergoes schizogony generating 2,000-40,000 merozoites which enter circulation in 5-16 days depending on species.
Merozoites attach to and enter red blood cells, convert to trophozoites and multiply; red cell bursts releasing merozoites that differentiate into gametes.

34. Sexual phase – mosquito host
Mosquito draws infected RBCs; gametes fertilize forming diploid cell which forms sporozoites in stomach.
Sporozoites lodge in salivary glands; available to infect human host

36. Plasmodium
Symptoms include episodes of chills-fever-sweating, anemia, and organ enlargement.
Symptoms occur at hour intervals as RBCs rupture; interval depends on species.
P. falciparum most malignant type; highest death rate in children
Diagnosis by presence of trophozoite in RBCs, symptoms
Increasing drug resistance
Therapy is chloroquine, quinine, or primaquine.

37. Coccidian Parasites
Zoonotic in domestic animals and birds

38. Toxoplasma gondii and Toxoplasmosis
Intracelllular 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 and AIDS patients who can suffer brain and heart damage.
Treatment: pyrimethamine and sulfadiazine

40. Sarcocystis and Sarcocystosis
Sarcocystis – parasites of cattle, swine, and sheep
Domestic animals are intermediate hosts; they pick up infective cysts while grazing on grass contaminated with human feces.
Humans are infected when the meat is consumed.
Symptoms include diarrhea, nausea, and abdominal pain.
No specific treatment

41. Cryptosporidium: A Newly Recognized Intestinal Pathogen
An intestinal pathogen
Infects a variety of mammals, birds, and reptiles
Exists in tissue and oocyst phases
1990s – 370,000 cases in Milwaukee, WI due to contaminated water; filtration required for removal
Ingestion of oocysts which give rise to sporozoites that penetrate intestinal cells
Causes gastroenteritis, headache, sweating, vomiting, abdominal cramps, diarrhea
AIDS patients may suffer chronic persistent diarrhea.
No effective drugs

42. Isospora belli and Coccidiosis
Intracellular intestinal parasite with oocyst stage
Transmitted in fecally contaminated food or drink
Infection usually asymptomatic or self-limited
Symptoms include malaise, nausea and vomiting, diarrhea, fatty stools, abdominal cramping, and weight loss.
Treat with sulfadiazine and pyrimethamine, when required

43. Cyclospora cayetanensis and Cyclosporiasis
Emerging protozoan pathogen; causes cyclosporiasis
Oral-fecal transmission; fresh produce and water
Oocysts enter small intestine and invade the mucosa.
Symptoms of watery diarrhea, stomach cramps, bloating, fever, muscles aches
Diagnosis can be complicated.
Treatment: trimethoprim and sulfamethoxazole

44. Babesia species and Babesiosis
First protozoan found to cause a disease – redwater fever of cattle
First protozoan found to be associated with a vector - tick
Human babesiosis - relatively rare zoonosis
Associated with infected rodents
Infection resembles malaria.

45. A Survey of Helminth Parasites
Adults are large, multicellular animals with specialized tissues and organs.
Adult worms mate and produce fertilized eggs that hatch; larvae then mature in several stages to adults.
The sexes may separate or are hermaphroditic.
Adults live in the definitive host.
Eggs and larvae may develop in the same host, the external environment of the intermediate host.
A transport host experiences no parasitic development.
Four basic patterns of life and transmission

47. Helminths
Pathology arises from worms feeding on and migrating through tissues, accumulation of worms and worm products.
Diagnosis based on blood cell count (eosinophilia), serological tests; eggs, larvae, or adult worms in feces; sputum, urine, blood, or tissue biopsies.
Antihelminthic drugs suppress a helminthic metabolic process that differs from the human process, inhibit the worm’s movement, prevent it from holding position, and act locally in the intestine.

49. Nematode (Roundworm) Infestations
Most abundant animal groups; 50 species that affect humans
Enlongated, cylindrical worms with protective cuticles, circular muscles, a complete digestive tract, and separate sexes
Ascaris lumbricoides, Trichuris trichiura, Enterobius vermicularis, hookworms, Strongyloides stercoralis, Trichinella spiralis, filarial worms

50. Ascaris lumbricoides A large intestinal roundworm
Most cases in the U.S. occur in the southeastern states
Indigenous to humans
Ascaris spends its larval and adult stages in humans; release embryonic eggs in feces, and are spread to other humans; food, drink, or contaminated objects
Ingested eggs hatch into larvae and burrow through the intestine into circulation and travel to the lungs and pharynx and are swallowed.
Adult worms complete cycle in intestines and reproduce – 200,000 eggs/day.

51. 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 and mental development.

52. Trichuris trichiura and Whipworm Infection
Humans sole host
Trichuriasis has its highest incidence in the tropics.
Eggs hatch in intestines, larvae attach, penetrate the outer wall and develop into adults.
Females lay 3,000-5,000 eggs daily.
Worms can pierce capillaries, cause localized hemorrhage, and allow bacteria to leave intestine.
Heavy infestations can cause dysentery, rectal prolapse – can be fatal in children.

53. Enterobius vermicularis and Pinworm Infection
Pinworm or seatworm
Enterobiasis most common worm disease of children in temperate zones
Eggs are picked up from surroundings and 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

54. Hookworms Characteristic curved ends and hooked mouths
Necator americanus and Ancylostoma duodenale
Humans shed eggs in feces, which hatch into filariform larvae and burrow into the skin of the lower legs.
Larvae travel from blood to lungs, proceed up bronchi and throat and are swallowed.
Worms mature and reproduce in small intestine and complete, the cycle.
May cause pneumonia, nausea, vomiting, cramps and bloody diarrhea
Blood loss is significant – anemia.

56. Strongyloides stercoralis and Strongyloidiasis
Threadworm
Tiny roundworms completes life cycle in humans or moist soil.
Larvae penetrate skin and migrate to lungs, are swallowed and complete development in the intestine.
Can reinfect the same host without leaving the body
Heavy worm loads can cause pneumonitis and eosinophilia, bloody diarrhea, liver enlargement, bowel obstruction and malabsorption.

57. Trichinella spiralis and Trichinosis
Life cycle entirely within mammalian host
Acquired from eating undercooked pork or bear meat
Larvae migrate from intestine to blood vessels, muscle, heart, and brain, where it forms cysts
First symptoms –flulike, diarrhea
Second symptoms – muscle and joint pain, shortness of breath, pronounced eosinophilia
No cure after larva have encysted

59. Tissue Nematodes
Complete their life cycle in human blood, lymphatics, or skin
Filarial worms; elongate, filamentous bodies, spread by biting arthropods
Cause chronic, deforming disease
Wuchereria bancrofti – elephantiasis
Onchocerca volvulus – river blindness
Loa loa – eye worm

60. Wucherereia bancrofti and Bancroftian Filariasis
Tropical infection spread by mosquitoes
Vector deposits larvae which move into lymphatics and develop into adults.
Chronic infection causes blockage of lymphatic circulation and elephantiasis, massive swelling in the extremities.

61. Onchocerca volvulus and River Blindness
Transmitted by biting black flies
Larvae develop into adults in subcutaneous tissues.
Adult females migrate via the blood to the eyes, provoking inflammatory reactions.
Coinfection with Wolbachia bacteria causes river blindness.
Treatment: tetracycline and ivermectin

62. Loa loa: The African Eye Worm
Spread by bite of small flies
Temperature-sensitive worm migrates around/under the skin and may enter the eye.
Treatment – pull worm from a small hole in conjunctiva or diethylcarbamazine

63. Trematodes or Flukes Flatworms with ovoid leaflike bodies
Have digestive, excretory, neuromuscular, and reproductive systems
Lack circulatory and respiratory systems
Animals such as snails or fish are usually the intermediate hosts and humans are the definitive hosts.

64. Blood Flukes: Schistosomes
Schistosomiasis - prominent parasitic disease
Schistosoma mansoni, S. japonicum,
S. haematobium
Adult flukes live in humans who release eggs into water; early larva (miracidium) develops in freshwater snail into a 2nd larva (cercaria).
This larva penetrates human skin and moves into the liver to mature; adults migrate to intestine or bladder and shed eggs, giving rise to chronic organ enlargement.

65. Liver and Lung Flukes Zoonotic Liver flukes:
Opisthorchis (Chlonorchis) sinesis - cycles between mammals and snails and fish; humans are infected by eating inadequately cooked fish containing cercaria, larvae crawl into bile duct, mature and shed eggs into feces; snail are infected.
Fasciola hepatica-cycles between herbivores, snails, and aquatic plants; humans are infected by eating raw aquatic plants; fluke lodges in liver.

66. Lung fluke:
Paragonimus westermani – cycles between carnivorous animals, snails, and crustaceans; humans infected by eating undercooked crustaceans; intestinal worms migrate to lungs.

67. Cestode (Tapeworm) Infestations
Flatworms
Long, very thin, ribbonlike bodies composed of sacs (proglottids) and a scolex that grips the intestine
Each proglottid is an independent unit adapted to absorbing food and making and releasing eggs.
Taenia saginata
Taenia solium

68. 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.
In humans, larva attaches to the small intestine and becomes an adult.
Causes few symptoms; vague abdominal pain and nausea; proglottids in stool

70. Taenia solium Pork tapeworm
Infects humans through ingesting cysts or eggs
Eggs hatch in intestine, releasing tapeworm larva that migrate to all tissues and encyst.
Most damaging if they lodge in heart muscle, eye, or brain
May cause seizures, psychiatric disturbances

71. The Arthropod Vectors of Infectious Disease
Arthropods – exoskeleton and jointed legs; includes arachnids and crustaceans; many must feed on blood and tissue fluid of host during life cycle; ectoparasites
Those of medical importance transmit infectious microbes in the process of feeding – biological vectors

72. Insects
Mosquitoes – require an aquatic habitat; females take blood meal transmitting disease: malaria, filariasis, zoonoses
Fleas – highly motile, flattened bodies; feed on warm-blooded animals; carry zoonotic diseases: plague, murine typhus
Lice – small, soft; attach to head and body hair feeding inconspicuously on blood and tissue fluid; release feces that contaminate wound; epidemic typhus, relapsing fever
Flies – tsetse fly, sand fly

73. Arachnids
Ticks – cling on vegetation and attach to host on contact; larvae, nymph and adults get blood meal by piercing skin of host
hard ticks – Dermacentor, Ixodes – small compact, rigid bodies; transmit rickettsial, borrelial, and viral diseases
soft or argasid ticks – Ornithodoros- flexible outer bodies; transmit relapsing fever