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Chapter Outline
23.1 The Parasites of Humans
A. Protozoa and Helminths (worms) living on the body of a host, spread by
humans, animal hosts and vectors
1. Some trophozoites (active feeding stage) only, others alternate between
trophozoite to cyst (dormant, resistant form)
2. Some have complex life cycles with sexual and asexual phases and more
than one host
3. Approximately 10 class of drugs used in treatments, many toxic to host
B. Parasitic disease exited in ancient times and public health concern today
1. International travel has spread parasites and their vectors
2. Increase of immunosuppressed people increases formerly obscure parasitic
diseases
23.2 Major Protozoan Pathogens
A. Infective Amoebas- Entamoeba histolytica, Naegleria fowleri, Acanthamoeba
1. Entamoeba histolytica
a. Epidemiology- amebic dysentery, affects 500 million people in the
tropics
b. Alternates between trophozoite and cyst, ingested cysts release
trophozoites that invade large intestine and cause ulceration and
dysentery
2. Amebic infections of the brain
a. Naegleria fowleri
b. Acanthamoeba
c. Free-living in natural waters, nasal contact or traumatic eye damage
d. Primary acute meningoencephalitis, infiltration to brain is usually
fatal
B. Intestinal Ciliate: Balantidium coli
1. Intestines of pigs and cattle
2. Ingesting cysts in food or water
3. Trophozoite erodes intestine, causing symptoms
23.3 Infectious Flagellates (Mastigophorans)
A. Trichomonas vaginalis-parasite or commensal
1. STD, trichomoniasis-infects vagina, cervix and urethra
2. Treatment- metronidazole
B. Giardia lamblia –intestinal flagellate
1. Reservoir- animal intestines, cyst contaminated fresh water and food
2. Severe diarrhea and cramps that may be chronic in immunocompromised
patients
3. Antiprotozoan drugs
C. Hemoflagellates: Vector-Borne Blood Parasites
1. Trypanosoma-complex life cycle with mastigote phases in insect and human
hosts (blood)
a. Trypanosoma brucei- African sleeping sickness
i. Vector-tsetse fly, reservoir- mammals
ii. Trypanosome multiplies in the blood, damages spleen, lymph
nodes and brain
iii. Chronic infection-sleep disturbances, tremors, paralysis and
coma
b. Trypanosoma cruzi- Chagas disease (Latin America)
i. Vector -reduviid bug (kissing bug), reservoir-mammals
ii. Trypanosome multiplies in the blood, targets heart muscle and
large intestine
iii. Treatment with antiprotozoan drugs and vector control
2. Leishmania species
a. Vector-phlebotomine fly (sand fly), zoonosis of wild animals
b. Multiplies in macrophages, cutaneous and systemic forms
c. Fever, enlarged organs and anemia
d. Treatment –pentavalent antimony and antiprotozoan drugs
23.4 Apicomplexan Parasites
A. Plasmodium: The agent of malaria
1. Vector- Anopheles mosquito, reservoir-salivary glands (mosquito), humans
and other primates
2. Life cycle: sporozoites, schizogony to form merozoites(in liver cells), some
merozoites differentiate into microgametocytes(male) and
macrogametocytes(female) in RBC.
3. Symptoms include chills, fever, anemia and organ enlargement
4. Treatment- antimalarial drugs, control of the vector, vaccine development
B. The Coccidian Parasites-zoonotic in animals and birds
1. Toxoplasma gondii-toxoplasmosis
a. Reservoir-cats have oocytes in GI tract
b. Ingesting raw/rare meats or accidental ingesting of oocysts from cat
litter box
c. Usually mild, flu-like, immunodepressed patients and fetuses suffer
brain damage and heart damage.
d. Treatment-avoid cat excrement, cook food, antiprotozoan drugs
2. Sarcocystis-parasites of cattle, swine and sheep
a. Transmission- undercooked meat
b. Diarrhea, nausea and abdominal pain peaking after 2 hours, lasting as
long as 2 weeks
c. Currently no treatment-supportive care
3. Cryptosporidium: newly recognized intestinal pathogen
a. Has both tissue and oocyte phases to life cycle
b. Transmission- handling infected animals, drinking contaminated
water
c. Symptoms- gastroenteritis with fever
d. Treatment-antiprotozoan drugs, supportive care
4. Isospora belli-coccidiosis
a. Fecally contaminated food and water
b. Distinct oocysts stage
c. Usually self-limiting malaise, nausea, vomiting, diarrhea, weight loss
5. Cyclospora cayetanensis
a. Emerging pathogen related to Isospora
b. Fecal contamination of fresh produce and water
c. Oocysts enter small intestine and colonize mucosa
d. GI symptoms
e. Treatment with antimicrobial drugs
6. Babesia species
a. Vector-tick, zoonosis in cattle and rats
b. Rare
23.5 A Survey of Helminth Parasites
A. General characteristics
1. Helminths are parasitic worms
2. Multicellular with specialized mouth parts and reduced organs, protective
cuticles, and complex life cycles
B. General life cycles
1. Worms mate and produce fertile eggs that hatch into larvae that mature in
stages to adults.
2. Some sexes are separate others hermaphroditic
3. Adults live in definitive host
4. Eggs and larvae may develop in same host, intermediate host, or external
environment
5. Four basic life and transmission cycles- see text book
C. Pathology of helminth infestation
1. Worms feeding on host tissue (damage and inflammation)
2. Migrating through host tissues
3. Accumulation of worms and worm secretions
D. Control and treatment
1. Antihelminthic drugs
a. Paralyzing worms which causes them to be shed
b. Interfering with metabolism
2. Control the incidence of infection
a. Improved sanitation of food, water
b. Protective clothing
c. Cooking meat
d. Controlling vector populations
23.6 Nematode (Roundworm) – protective cuticles, complete digestive tract, sexes
separate
A. Ascaris lumbricoides
1. Transmission is eggs ingested in food, drink or soiled objects
2. Intestine burrow into circulation, lungs, pharynx where they are swallowed
and return to intestine to complete reproduction and host to shed eggs
B. Trichuris trichiura –whipworm
1. Transmission is eggs ingested via fecal contamination of soil
2. Life cycle 90 days, untreated infections can last up to 2 years,
3. Secondary infections of rectal area
C. Enterobius vermicularis- pinworm
1. Transmission is via soil contaminated with eggs stick to fingers and fomites
2. Eggs hatch in small intestine, mate in large intestine and female emerge from
anus to lay eggs-itching
3. Childhood illness, diagnosis with adhesive tape
D. Intestinal Helminths- hookworms (curved ends and hooked mouths)
1. Necator americanus-New World
a. Larvae hatch outside the host and infect by penetrating the skin
b. Lymphatic vessels or circulation takes them to the heart and lungs,
throat where they are swallowed and then head to small intestine to
mature and reproduce and shed eggs
2. Ancylostoma duodenale- Old World same as Necator and both overlap in
Latin America
E. Strongyloides stercoralis- threadworm
1. Larvae emerge from soil-borne eggs that penetrate the skin and migrate to
lungs, then swallowed and complete development in intesting
2. Treatment needed to prevent death
F. Trichinella spiralis-
1. Zoonosis, humans are dead end host
2. Ingesting undercooked pork or bear meat with encysted larvae
3. Larvae migrate from intestine- blood vessels-heart and muscles-dormant
4. Symptoms include fever, diarrhea, abdominal pain, muscle and joint pain
G. Filarial Nematodes-long, threadlike worms with tiny larvae (microfilariae) that
circulate in the blood, and reside in various organs, spread by vectors
1. Wuchereria bancrofti -bancroftian filariasis
a. Mosquito vector
b. Lymph site of maturation
c. Chronic infection blocks lymph circulation causing elephantiasis
2. Onchocerca volvulus -river blindness
a. Vector small biting black fly- Africa and Americas
b. Similar symptoms to bancroftian filariasis
c. Coinfection with bacteria Wolbachia causes the most serious eye
damage
d. Antibacterial and antiprotozoan medication, control of the vector
e. Loa Loa: African eye worm-temperature sensitive, visible in the eye,
can resolve in time or be removed by hand from small hole in
conjunctiva
3. Dracunculus medinensis-guinea worm
a. Vector- arthropod that lives in still water
b. Contaminated open pond -reservoir
c. Convert to well water, water filtration and insecticide treatment
d. 2008 WHO close to eradication
23.7 The Trematodes- flukes, flatworms with leaflike bodies and suckers, most
hermaphroditic
A. Schistosoma-tropical disease
1. Adults live in humans, eggs released into water- micracidium develop in
freshwater snails, into a cercaria that penetrates human skin, liver to mature,
migrate to liver or bladder to shed eggs
2. Chronic organ enlargement
3. Biological control of intermediate host by introducing fish that eat the snails
B. Zoonotic flukes
1. Opisthorchis sinesis- Chinese liver fluke
a. Cycles between mammals and snails or fish
b. Humans eat undercooked fish/crustaceans contaminated with larvaemature in the bile duct and are shed in feces into standing water
2. Fasciola hepatica-mammals and humans are accidental host
a. Cercariae encyst on water plants, ingested by humans
b. Vomiting, diarrhea, bile duct abstruction(chronic infections)
3. Paragonimus westermani- pulmonary tissue of mammals and intermediate
hosts in snails and crustaceans
a. Humans are accidental host from eating infected, undercooked
crustaceans
b. Worms migrate to lungs and cause cough, pleural pain and abscesses
23.8 Cestode (Tapeworm)- flatworms with long, thin, ribbonlike bodies composed of
sacs(proglottids) and a scolex that grips the host intestine. Each proglottid is
independent unit capable of absorbing food, making and producing eggs
A. Taenia saginata- beef tapeworm
1. Humans definitive host
2. Animals infected by grazing land contaminated with human feces, humans
ingest raw beef with the larval form encysted, larva attaches to small intestine
and matures to adult
3. Proglottids in stool, treatment with antihelminthic drugs
B. Taenia solium- pork tapeworm
1. Ingesting food or drink contaminated with cysticerci
2. Larvae hatch and encyst in many organs causing mild to life threatening
damage
23.9 The Arthropod Vectors of Infectious Disease
A. Mosquitoes
1. Aquatic habitat necessary
2. Only females take blood meals to complete egg development
B. Fleas
1. Tiny, highly mobile with flattened bodies that feed on warm blooded
mammals
2. Not host specific so carry zoonotic diseases
C. Lice
1. Small, soft insects that attach to hair feeding on blood and tissue, while
releasing feces that contaminate the wound
D. Other insect vectors
1. Flies- tsetse fly, sand fly
2. Reduviid bug
E. Ticks
1. Hard ticks( Dermacentor, Ixodes)- small compact, rigid bodies
2. Soft ticks( Ornithodoros)- more flexible bodies