1.) More
important in tropical countries than in countries like the U. S. However, parasitic
diseases are becoming more prevalent in the U. S. as more infected people move here; also
because people with immune deficiencies such as AIDS are more susceptible to certain
parasites.
2.) How
the immune system responds to these parasites is a mystery.
An immune response is activated, but the immune system is seldom able to rid the
body of them.
3.) They
have more complex life cycles, with multiple hosts involved.
A. Some General Characteristics:
§ eukaryotic
cells
§ nonmotile
§ heterotrophic
(use organic compounds a carbon source; they can’t make their own sugars; no
photosynthesis)
§ prefer
more acidic conditions than bacteria
§ can
tolerate higher osmotic pressure and lower moisture than bacteria
§ larger
than bacteria and have more cellular and morphologic detail
§ cannot
tolerate the high temps. that bacteria can (fungal spores aren’t as resistant as
bacterial spores)
§ most
are aerobic; some are facultative anaerobes (ex. yeasts) & some are anaerobes
§ important
in ecosystems as decomposers (called saprophytes
- they obtain nutrients by decomposing dead & decaying matter); some are parasites,
causing disease (mycosis; mycoses is plural); some produce toxins that cause
disease (mycotoxicosis; mycotoxicoses is
plural).
§ major
cause of plant diseases
§ the
study of fungi is mycology
B. General Morphology:
§ most,
with exception of unicellular species, have a vegetative structure called a mycelium
(a multinucleate mass of cytoplasm enclosed within a system of rigid, branched, tube-like
filaments called hyphae).
§ hyphae
can be coenocytic (undivided network of
branching tubes) or have septa (cross walls).
§ cells
walls are composed of cellulose, chitin (contains nitrogen - also found in the
exoskeletons of insects, crayfish, etc.), or a combination of the two.
§ specific
morphology will be discussed later for each group of fungi
C. Reproduction
- Fungi are classified by how they reproduce (sexually or asexually).
[functions of spores include dissemination and reproduction]
1. Asexual
Reproduction
- Occurs by elongation of hyphae, budding, or asexual spore production.
Asexual
spores
are specialized cells that are dispersed & germinate in a favorable environment to
produce a new fungus; they are products of a type of cell division called mitosis (one cell divides to form 2 daughter cells
that are identical to one another and to the original parent cell). Types: sporangiospores , conidiospores.
2. Sexual
Reproduction
- Occurs by producing sexual spores, which form
following sexual fusion of gametes (similar to sperm & eggs). Types: zygospores, ascospores, & basidiospores.
[These
are descriptive terms, not taxonomic! These
organisms belong to many groups of fungi.]
1. Yeasts
- characteristics:
§ nonfilamentous,
unicellular
§ reproduce
asexually by budding
§ reproduce
sexually by producing various kinds of spores
§ aerobic
or facultative anaerobes
§ used
to prepare bread, wine, beer, etc. (fermentation
of carbohydrates produces ethanol & carbon dioxide)
ex. Saccharomyces cerevisiae (cervesa
means beer in Spanish)
§ some
are pathogenic; ex. Candida albicans (causes yeast infections, thrush; see below)
2. Molds
- characteristics:
§ filamentous,
multicellular
§ have
a vegetative structure called a mycelium (a
multinucleate mass of cytoplasm enclosed within a system of rigid, branched, tube-like
filaments called hyphae).
§ hyphae
can be coenocytic (undivided network of
branching tubes) or have septa (cross walls).
§ also
possess reproductive hyphae which produce different kinds of spores (discussed above and
below)
§ see
below for examples.
E. Classification of Some of the Lower Fungi:
1. Zygomycetes:
a. Characteristics: coenocytic hyphae, produce sporangiospores (asexual spores) & zygospores (sexual spores).
b. Ex. Rhizopus
nigricans - black mold that develops on
stale bread; the tiny black dots on the mold are the sporangia, which hold the sporangiospores;
sporangia look like tiny mushroom caps.
c. can
be opportunistic; some are pathogenic in the immnocompromised
F. Classification of Some of the Higher Fungi:
1. Ascomycetes (Sac Fungi)
a. Characteristics: includes molds with septate hyphae and some
yeasts; ascospores (sexual spores) develop
within sacs called asci (sing. ascus); also produce conidiospores
(asexual spores).
b. Examples:
1.) Saccharomyces
cerevisiae - yeast is used to make beer, bread, wine; cervesa means beer in Spanish.
2.) Trichophyton
-
causes athlete's foot (tinea pedis); ringworm
of the feet; other species infect different parts of the body (dandruff, nail fungus, jock
itch)
3.) Penicillium spp.
- conidiospores form long chains on branching conidiophores, creating a brush-like
structure that looks like a broom (penicillus
means “brush"); some species produce the antibiotic penicillin.
4.) Aspergillus spp.
- form long chains on a globelike conidiophore; cause aspergillosis, a pulmonary disease of animals
& humans; infection is often secondary to tuberculosis, immunodeficiency, &
steroid therapy.
5.) Histoplasma
capsulatum -
causes Mississippi Valley fever (histoplasmosis); can get from bird droppings and
bat guano; endemic disease in this area; pulmonary disease.
6.) Candida
albicans - part of our natural flora; opportunistic;
becomes a problem when defenses are weakened or balance of microbes is upset (ex. from
antibiotic treatment); cause of vaginal & intestinal yeast infections & thrush in
the mouth ("cottage cheese patches") - called candidiasis.
a. Characteristics: many form basidiocarps
(mushrooms, puffballs, or shelflike bodies on trees); some are molds, a few are yeasts;
produce conidiospores; also produce basidiospores (sexual spores); basidiospores form
on the "gills" of mushroom basidiocarps.
b. Examples:
1.) Amanita - poisonous mushroom; toxin causes a mycotoxicosis
2.) Cryptococcus -
yeast cells surrounded by a capsule; causes fatal meningitis (cryptococcosis); transmission – inhalation of
contaminated dust; found in 8% of AIDS patients.
3. Deuteromycetes (Imperfect Fungi)
a. Characteristics: called the imperfect fungi because no sexual
stage has been observed; we put them in this group until a sexual stage is
observed; these fungi grow as yeasts or molds; identify on basis of shape &
arrangement of their conidiospores (asexual spores); some species are pathogenic; many of
these fungi have recently been placed in other phyla.
G. Dimorphic
Fungi - Some
fungi switch between a single-celled yeast phase of growth & a mycelial phase (called dimorphism); discovered by Pasteur; some species
will switch if oxygen supply decreases. Pathogenic
dimorphic fungi are mycelial outside of the host & single-celled inside the host. With pathogenic species, it is usually high body
temperature that causes the switch. Candida changes in response to the higher
nutrient concentrations found in the body. The
problem with dimorphism is that single cells are more readily spread in bloodstream,
leading to systemic infections.
H. Mycoses (Fungal Diseases)
§ Humans
usually acquire fungal disease from nature; they are not highly contagious.
§ mycotoxicosis
vs. opportunistic mycoses:
§ See
above for diseases
§ Some
produce toxins that are hallucinogenic; ex. muscarin
- produced by a mushroom
§ Some
produce toxins that are highly poisonous; ex.
1.) Claviceps
(rye
mold) - produces ergot; causes death to
anyone eating bread made from contaminated rye; LSD is made form fruiting structures
(causes hallucinations)
2.) Aspergillus
– produces
aflatoxin; which grows in many plant materials;
low levels of toxin can be carcinogenic.
3.) Amanita -
poisonous mushroom
I. Antibiotics:
Penicillins
produced by Penicillium ; Cephalosporins
produced by Cephalosporium.
A. General Characteristics:
§ Unicellular
eukaryotes.
§ The
protistan lineages continue into the kingdoms of plants, fungi, and animals.
§ Limited
to a moist environment because they lack a cell wall
§ Heterotrophs
§ Most
reproduce asexually by fission (one cell divides to form 2 identical daughter cells &
budding; some (ex. Plasmodium that causes
malaria) under go schizogony (multiple
fission). Sexual reproduction occurs by conjugation, the fusion of vegetative cells, or by
the fusion of specialized gametes called gametocytes.
§ Some
have complex life cycles, requiring multiple hosts and changing their morphology (ex. Plasmodium uses
the mosquito as an intermediate host)
§ Trophozoite
- active, motile, feeding stage of protozoans; parasitic stage that causes the disease in
the host.
§ Cyst
- resistant, inactive stage; how diseases are usually transmitted by the fecal-oral route;
usually more useful than trophozoites for lab identification.
B. Classification:
[based
on mode of locomotion or motility]
1. Mastigophora or Zoomastigophora (move by means of flagella)
a. Trypanosoma
gambiense
- infects the blood and tissue fluids; causes African sleeping sickness (it leads to the
loss of consciousness and death when it invades the CNS);
can also infect cattle; vector is the tsetse fly.
b. Giardia
lamblia - body has the appearance of a human face (4
“eyes” are nuclei); have 2-6 flagella; form cysts; causes a waterborne dysentery
(traveler's diarrhea); one of the “don’t drink the water” diseases; firs
sigh is usually an explosive, foul-smelling watery diarrhea followed by copious amounts of
campers are a high-risk group because of a sylvatic
cycle (parasite is found in mountain streams contaminated with human feces or animal
feces, especially beavers).
c. Trichomonas vaginalis - causes vulvovaginitis; numerous flagella
2. Sarcodina
(move
by means of pseudopodia or "false
feet" - temporary extensions of the cell body caused by protein filaments of the
cytoskeleton pushing on the cell membrane); feed on algae, bacteria, and other protozoans
by phagocytosis.
a. Amoeba
proteus
- freshwater; not pathogenic
b. Entamoeba
histolytica
- causes amoebic dysentery; usually acquired by
consuming fecally contaminated water or food; flies and cockroaches can also be mechanical
vectors; produce cysts; first protozoan to be shown to be a pathogen (1875); one of the
“don’t drink the water” diseases; trophozoites may invade the intestinal
mucosa where they can cause ulceration and escape into the blood vessels; they may allow
bacteria in fecal material to enter the body cavity and cause peritonitis.
c. Naegleria
fowleri -
causes amoebic meningioencephalitis; usually seen in swimmers.
d. Acanthamoeba
polyphaga
– accumulates on the water surface of contaminated hot tubs when tubs are covered;
cause ulceration of the eyes and skin; can invade the central nervous system and cause
meningioencephalitis.
3. Ciliophora (move by means of cilia)
a. Paramecium
caudatum - freshwater; not pathogenic
b. Balantidium
coli - only ciliophoran that causes disease; produces
cysts; causes diarrhea of large intestine; rare except in the Philippines; symptoms are
similar to those of amoebic dysentery.
4. Apicomplexa
or Sporozoa or Haemosporina - Basically nonmotile.
All have an infectious, sporelike stage (sporozoite)
that is often transmitted to new hosts by an insect vector.
All are parasitic (obligate parasites - cannot live apart from the host). Some have elaborate life cycles, changing body
form (trophozoite, sporozoite, merozoite);
life cycle includes schizogony (multiple
fission). Examples
a.
Plasmodium
vivax - causes malaria;
vector is the mosquito; kills 1-3.5 million people each year; malaria = bad air; used to infect people with
malaria to stop the progression of syphilis (fevers would kill the bacteria).
b. Toxoplasma
gondii - causes toxoplasmosis;
humans acquire the disease by consuming cysts in the meat of infected animals or ingesting
material contaminated by cat feces containing the parasite (can get it from cleaning the
litter box - doctors warn pregnant women not to do this).
c.
Cryptosporidium
- form cysts; cause enteritis & diarrhea; can occur in water supplies; can also be
transmitted by fecal-oral transmission from kittens/puppies; resistant to chlorine (it can
survive full-strength Chlorox!); threat only AIDS patients and those immunocompromised; no
effective treatment found.
d. Pneumocystis
carinii - may be a fungus!!; causes pneumocystis pneumonia; spread in respiratory
droplets; common in AIDS patients.
III. HELMINTHS – Flatworms & Roundworms
General
Characteristics:
¨ Animals
¨ Cephalization
- concentration of sensory receptors toward the anterior end.
¨ Organ/system
level or organization/
¨ Sexual
reproduction. Most flatworms are monoecious (male & female reproductive organs
in same animal). Roundworms are dioceious
(separate sexes).
A. Platyhelminthes
(Flatworms = Trematodes + Cestodes) - most are free-living; marine and freshwater;
predators, scavengers, or parasitic; some have regenerative capabilities.
a.
Clonorchis
sinensis (Chinese or Human Liver Fluke)
¨ Adults
live in bile ducts (in the liver) of humans (definitive host)
¨ Intermediate
hosts: snail (first) and fish (second)
¨ Life
cycle: a snail ingests the eggs; the eggs
hatch & release a larval stage which goes through several transformation before
finally forming a tadpole-like cercariae; the
cercariae bore through the flesh of the snail, & escape into the water; they swim
until they find the appropriate species of fish; they encyst in the muscle tissues of the
fish (forming metacercariae); the adult flukes
develop in livers of humans who eat raw, infected fish; eggs of the parasite are excreted
in the feces; when human feces end up in ponds, etc., snails ingest the eggs & the
cycle repeats itself.]
b.
Schistosoma mansoni (Schistosomes
or Blood Flukes) - adults live in circulatory system; spiny eggs break through the blood
vessel wall and through the gut wall to be expelled in feces; eggs hatch into cercaria in
water; cercaria then penetrate skin when a person is bathing or swimming; cause spleen and
liver enlargement, dysentery, and cirrhosis of the liver
Morphology: scolex
(head) with suckers and/or hooks (for attachment), proglottids (body units - each one has male and
female reproductive organs): immature proglottids (closest to the scolex), mature proglottids (next closest to the scolex),
and gravid proglottids (furthest from the
scolex - in these proglottids, the uterus is filled with eggs).
General
life cycle of tapeworms:
the gravid proglottids break off and are passed in the definitive host's feces; larval
forms hatch when the eggs are ingested by the intermediate host; larvae then encyst in the
intermediate host (called a cysticercus or
bladder worm); adult worms usually develop in the definitive host when raw or poorly
cooked infected meat is eaten. Examples:
a.
Taenia
solium (pork tapeworm) – reaches a length of 2-7
meters; primary host: humans, etc.; intermediate host: swine
¨ Humans
can be infected with the adults by consuming rare pork containing cysticerci larvae;
larvae then develop into adults in digestive tract of the human.
¨ Humans
can also be infected with larval forms when they accidentally ingest eggs (they get them
from other infected humans who contaminate food, etc. with the eggs when they don’t
use proper hygiene after going to the bathroom). In
this case every organ in the body may harbor cysticerci. When a cysticercus dies, it releases toxins and
usually causes a severe allergic reaction, which is sometimes fatal.
b. Taenia
saginata (beef tapeworm) – reaches a length of 5-25
meters; primary host: humans, etc.; intermediate host:
cattle, sheep, etc.; life cycle similar to that of
T. solium above; beef riddles with
encysted larvae is called “measly beef.”
c.
Echinococcus
granulosus (dog tapeworm); small - only 3 proglottids long;
typical life cycle:
¨ dogs
are infected by adults when they eat raw butchered livestock containing larvae (ex. raw
bones, etc.)
¨ eggs
are passed in feces of dog; livestock eat vegetation with eggs when grazing
¨ larvae
hatch and encyst in the muscle tissue of livestock
Humans
can get hydatid cysts (larvae) from ingesting
the eggs (the eggs are passed in feces of dog, dog licks himself, then dog licks your
face). These cysts develop in the liver,
lungs, and brain. Each fluid-filledcyst,
containing many larvae, can reach the size of a grapefruit.
d. Dipylidium
caninum (dog & cat tapeworm) - often seen in children;
flea is the intermediate host - it eats the eggs on an animal; larvae develop in flea; if
a dog, cat, or human ingests the flea, the adult will develop. Note: Larvae
are not transmitted through the bite of the flea!!
e.
Hymenolepis
nana
(dwarf tapeworm) - most common tapeworm of humans in the world; intermediate host is a
grain beetle; humans can ingest the eggs in cereals and other foods that contain parts of
the insects; intermediate host is optional (meaning that if you ingest the eggs of this
worm, you get an adult infection).
B. Nematoda
(Nematodes)
General characteristics:
¨ Nematodes
are everywhere!!!! They are freeliving in
soil, fresh & salt water, & are parasitic in plants and animals.
¨ Dioecious
(separate sexes).
¨ Possess
a nonliving cuticle, which is secreted by the epidermis and is resistant to the digestive
enzymes of the hosts.
¨ More
highly developed than flatworms.
¨ Adults
do not latch onto the host like the tapeworms.
1.
Ascaris
lumbricoides
(intestinal roundworm of pigs, horses, humans)
¨ largest
intestinal nematode in humans.
¨ disease
is called ascariasis
¨ Eggs
can remain viable even in preservative!
¨ Females
are longer; males are shorter & have a hooked posterior end.
2.
Enterobius
vermicularis (pinworm)
¨ parasitizes
large intestine of humans (especially children)
¨ at
night the females migrate to the perianal region to lay eggs
¨ their
presence there causes itching
¨ hosts
become infected by accidentally ingesting the eggs.
3.
Ancylostoma
caninum & Necator americanus (hook
worms)
¨ larvae
can penetrate the skin of a barefoot person
¨ once
inside, the larvae travels through the bloodstream to the lungs, moves up the trachea, is
coughed up & swallowed; it matures in the small intestine.
¨ Ancyclostoma
has “fangs”; Necator has cutting
plates.
4.
Trichinella
spiralis (pork roundworm)
¨ humans
usually become infected by eating insufficiently cooked pork
¨ larvae
are encysted in the muscle tissues of the pig
¨ causes
trichinosis; larval migration can cause death.
5.
Wucheria
bancrofti
¨ adults
live in lymph nodes where obstruction of lymph vessels leads to grotesque enlargement of
these nodes & to a condition called elephantiasis
¨ a
mosquito is the intermediate host.
6.
Dirofilaria
immitis - causes heartworm
disease in dogs; a few cases in humans &
cats; mosquito is intermediate host.
7.
Trichuris
trichiura
- (whipworm) - adults partially embed in the mucosa of the large intestine; each adult
produces 1,000-7,000 eggs/day!