Microbiology- a clinical approach by Anthony
Strelkauskas et al. 2010
Chapter 14: Parasitic and fungal infections
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Over the last five chapters, we have looked
at the pathogenicity of bacteria and viruses.
In this chapter, we look at the last two
groups of infectious organisms, the parasites
and the fungi, which are both eukaryotic.
Fungal infections are usually opportunistic,
but parasitic infections affect billions of
people in the world.
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Parasites can be divided into two groups:
◦ Protozoans – microscopic, single-celled eukaryotes.
◦ Helminths – worms, macroscopic, multicellular
eukaryotes with organ systems.
Disease causing parasites depend on their
infected host for survival.
Often involve multiple hosts in which specific
developmental stages reached
◦ Definitive host – adults develop, sexual reproduction
◦ Intermediate host – asexual reproduction occurs
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Parasitic infections major problem worldwide.
More than 500 million people infected with
malaria.
More than 2 million (mostly children) die each
year from malaria.
Entamoeba are intestinal parasites that infect
10% of the world population.
Trypanosoma parasites infect 16 million people
in Latin America each year.
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Often the host response contributes to the
pathogenesis.
The host defense reaction can cause:
◦ Tissue damage
◦ Allergic or anaphylactic reactions
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Parasitic protozoans cause a wide variety of
infections.
Affect large number people throughout the world.
Parasitic protozoans vary in size.
Contain membrane-bound nuclei and cytoplasm.
Cytoplasm divided into:
◦ Inner form – endoplasm
◦ Outer form – ectoplasm
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Classified on the basis of their methods of
movement and reproduction.
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Most infectious protozoans:
◦ Facultative anaerobes
◦ Heterotrophs
◦ Highly developed reproductive system
Actively dividing form is called trophozoite
Some form cyst
◦ Way of protecting themselves.
◦ Mechanism of transmission from host to host.
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Intracellular parasites.
Alternate between sexual and asexual
reproduction.
Important diseases:
◦ Malaria
◦ Toxoplasmosis
◦ Together, affect one third of the world population.
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Caused by Plasmodium species
Febrile illness
Found throughout the world
Transmitted by bite of female Anopheles
mosquito
Mortality mainly in children and
immunocompromised adults.
Oocyst
Sporozoites
Merozoit
es
Zygote
Gametocytes
(female, male)
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Symptoms of malaria include:
◦ Fever (cold and hot periods, in some forms of malaria
synchronized)
◦ Anemia, jaundice
◦ Circulatory changes
Anemia is caused by the destruction of red blood
cells.
◦ Accompanied by depression of marrow function
◦ Enlarged spleen.
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Thrombocytopenia is common in malaria.
◦ Bleeding (hemorrhages)
Danger: cerebral malaria in infections with P.
falciparum
◦ Abnormal behavior, impairment of consciousness,
delusions
◦ Paralysis, seizures
◦ Coma and death
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Erythrocyte
aggregation in small
blood vessels
Coagulation disorder
Multiple petechial
hemorrhages
Inflammation
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P. ovale and P. vivax can develop into
hypnozoites in liver cell
These can go dormant for prolonged times
Can become reactivated and develop into
merozoites causing an acute malaria attack.
a)
b)
c)
d)
e)
Erythrocyte
aggregation in the
brain
Infection of
hepatocytes
Liberation of large
amounts of hemoglobin
when merozoits exit
erythrocytes
All of the above.
None of the above.
a)
b)
c)
d)
e)
Flagella
Pseudopodia
Cilia
Pili
None.
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Most primitive form of protozoans:
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Multiply by simple binary fission
Move by using pseudopodia
Produce a chitin wall for protection
Referred to as a cyst
Entamoeba histolytica is an obligate
intracellular parasite.
Passed from host to host as cysts:
◦ Fecal-oral route of infection
◦ Ingestion of a single cyst can cause infection
Third highest parasitic cause of deaths
worldwide:
◦ Only malaria and schistosomiasis are higher
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Amebiasis is on the rise in the US.
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Initial infection is via the
fecal-oral route.
Systemic amebiasis occurs
only after colon colonized.
Produces several virulence
factors and enzymes.
◦ Membrane lesions
◦ Cellular death.
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Infection usually mild and
asymptomatic.
◦ Lesions can open the intestine
for bacterial and viral
infection.
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Flagellates widespread
in nature.
Use flagella for
movement through host.
Multiply by binary
fission.
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Four flagellates cause
human disease:
◦ Trichomonas, Giardia
 Noninvasive
 Low morbidity rates
 No intermediate host
required
◦ Leishmania, Trypanosoma
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Invasive
High morbidity rates
Frequently lethal
Intermediate host required
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By the protozoan Trypanosoma.
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Transmitted by vectors:
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Chronic infection
◦ Motile
◦ Fusiform
◦ Moves in a spiral fashion
◦ African form via tse tse fly – causes sleeping sickness
(encephalitis)
◦ American form via kissing bug – causes Chagas’
disease (organ megaly)
◦ Invade endothelial cells in brain (sleeping disease) or
organ tissue (Chagas disease)
◦ Escape immune response by changing their change
antigens frequently
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Protozoa that move with cilia
Example : Balantidium coli cause dystentery.
Image: Left: Balantidium coli cyst.
Right: Balantidium coli trophozoite
in a wet mount at 1000x magnification.
Credit: PHIL, Oregon Public Health Laboratory.
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Macroscopic multicellular parasites.
Body covered by a tough acellular cuticle.
Some have suckers, hooks, or plates used for
attachment.
Helminths have:
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Differentiated organs
Primitive nervous systems
Primitive excretory systems
Highly developed reproductive systems
Do not have a circulatory system.
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Three types of parasitic helminths:
Nematodes
(round worm)
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Cestodes
(tape worm)
Trematodes
(fluke)
Differentiated based on alimentary tract and
reproduction
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Roundworms.
Complete
intestinal
tract.
Diecious (male
and female
individuals).
Two
subgroups:
◦ Intestinal
nematodes
◦ Tissue
nematodes
sharonapbio-taxonomy.wikispaces.com
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Characteristics:
Fusiform body shape
Tough outer cuticle
Male and female forms
Thousands offspring
produced
◦ Eggs must incubate outside
the host to become
infective
◦ There is a larval form.
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Several types of
intestinal nematodes:
◦ Pinworms (Enterobius
vermicularis)
Large roundworms (Ascaris
lumbricoides)
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Intestinal nematode infection produce:
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Malnutrition
Discomfort
Anemia
Occasionally death
Symptoms depend on number of
worms:
◦ Low numbers may be asymptomatic
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Pinworm, Enterobius vermicularis, ubiquitous
parasite of humans.
More than 200 million people infected each
year.
Most of infections in children.
Pinworms attach to mucosa of cecum.
Females migrate down to perianal tissue to lay
eggs.
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Eggs stick to tissue, bedding, towels or fingers.
Eggs can be inhaled or swallowed.
Eggs hatch in the upper intestine.
Larvae migrate down to the cecum
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Largest and most common intestinal nematode.
Adult worms live in small intestine.
Female parasites lay 250,000-500,000 eggs per
day.
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Eggs passed into feces.
Eggs embryonate in soil for 3 weeks.
Very resistant to environmental pressure.
Viable for up to 6 years.
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Once ingested, eggs produce a larval stage.
Larvae penetrate intestinal mucosa and invade liver.
Exit from hepatic vein, enter the heart, progress
to the lung.
Rupture in the alveolar spaces.
Can be coughed up and swallowed.
Symptoms include
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Worms pass out of the body in a variety of ways:
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◦ Fever, coughing, wheezing, shortness of breath
◦ Vomiting, in stool, crawl out of the anus, nose, mouth, and
ears.
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Tissue nematodes can induce disease in:
◦ Tissues
◦ Blood
◦ Lymph system
They can live for years in subcutaneous tissues and
lymph vessels.
Tissue nematodes can discharge live offspring
called microfilariae.
◦ Circulate through the blood or tissue
◦ Can be ingested by blood sucking insects.
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Examples include Wucheria and Trichinella
Image: Left: Microfilaria of Wuchereria bancrofti in thick blood smear
stained with Giemsa. Center: Photograph of a female Aedes aegypti
mosquito as she was in the process of obtaining a "blood meal."
Laboratory strains of Aedes aegypti can be infected with Brugia. Credit:
DPDx, PHIL
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Lives in the duodenum and jejunum of flesh
eating mammals.
Particularly found in swine and bears.
Trichinella larvae enter through the host
vascular system and are distributed widely.
Larvae that penetrate the skeletal muscle
survive.
◦ Can become encapsulated in muscle (skeletal, heart)
◦ Can remain viable for 5-10 years.
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Some larvae enter the central nervous system
and cause encephalitis.
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Cestodes are commonly
called tapeworms.
They are the largest
of the intestinal
parasites.
They do not have an
alimentary tract.
◦ Nutrients are absorbed
across the cuticle.
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Monoecious
(hermaphrodites)
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The adult body has
three sections:
◦ Head – the scolex which
may have a rostellum
(hooks) and has suckers
to adhere to the
intestinal wall
◦ Regenerative neck
◦ Segmented body with
proglottides that
contain a
hermaphroditic unit
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Have a bilateral
symmetry
Have two deep suckers:
◦ One in the oral cavity for
food uptake
◦ One on the ventral side of
the worm for attachment
Have incomplete
alimentary tract (mouth,
intestine, but no outlet)
Almost all are
monoecious (exception:
Schistosoma, these are
also round)
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Trematodes can live for decades in human
tissue and blood vessels.
◦ They produce progressive damage to vital organs.
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Eggs are excreted from the human host.
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Hatching releases larvae called miracidia.
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Miracidia develop into cercariae.
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◦ They must reach water in order to hatch.
◦ Miracidia penetrate snails or fish, the intermediate
host.
◦ Cercariae are released from the snail or fish.
Cercariae enter the definitive host and develop
into adults that lay eggs
◦ Schistosoma cercariae invade skin of humans.
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Three major groups of flukes invade humans:
◦ Lung flukes – Paragonimus species
◦ Liver flukes – Clonorchis species
◦ Blood flukes – Schistosoma species
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Two to three hundred million people are infected
worldwide and up to one million people die each year.
There are male and female forms.
Schistosome couples mate in the portal vein.
◦ They stay conjoined for life.
◦ They use suckers to ascend the mesenteric vessels.
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They lay eggs in the submucosal veins
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Eggs penetrate the veins and enter hollow organs
◦ 300 to 3000 eggs can be laid per day for as long as 35 years.
◦ Intestine
◦ Bladder
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Eggs induce inflammation
◦ Bladder cancer can develop
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Mycology is the study of fungi.
Fungi are important for the environment.
Fungi are commensal organisms.
◦ They are normally harmless to humans.
◦ Fungi can be opportunistic pathogens.
◦ Most fungal infections are opportunistic.
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Fungi are eukaryotes with cell walls.
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Fungi use heterotrophic metabolism.
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◦ The cell wall contains glucan, mannan, and chitin.
◦ The cell membrane contains ergosterol.
◦ They obtain carbon from decaying organic matter.
Most are obligate aerobes but some are
facultative anaerobes.
◦ No fungi are obligate anaerobes.
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There are two forms:
◦ Molds – multicellular, form hyphae, distinct
morphology used for diagnostics
◦ Yeasts – unicellular, budding
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Fungi reproduce either sexually or asexually.
Asexual reproduction:
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Sexual reproduction:
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◦ Through conidia
◦ Involves mitotic division and budding
◦ Involves spores - ascospores, zygospores, or
basidiospores
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Some fungi can grow in mold or yeast form.
The yeast form requires environmental
conditions similar to in vivo.
◦ Proper temperature
◦ Increased nutrients
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The mold form requires:
◦ Ambient temperatures
◦ Minimal nutrients
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Classification is based on:
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Nature of the sexual spores
Septation of the hyphae
rRNA
The tissue types they parasitize
The diseases they produce
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Fungal diseases are classified into 4 groups:
◦ Superficial mycoses
 Top layer of skin or hair shafts, no tissue response
◦ Cutaneous and mucocutaneous mycoses
 Associated with the skin and mucosa
 Ringworm
 Onchomycosis
◦ Subcutaneous mycoses
◦ Deep mycoses
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Usually seen in immunosuppressed patients
with:
◦ AIDS, cancer, diabetes
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Can be acquired by:
◦ Inhalation of fungi or fungal spores
◦ Coccidiomycosis, histoplasmosis, aspergillosis
◦ Use of contaminated medical equipment
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Deep mycoses can cause a systemic infection –
disseminated mycoses
◦ Can spread to the skin
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Systemic dissemination
of the fungi into the
skin
Surface lesions are
characteristic
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Thousands of fungal spores are inhaled every
day.
Fungi are part of normal microbial flora.
Fungal infections very uncommon in
immunocompetent individuals.
Fungal infections are more common in
immunodeficient patients.
◦ Candida, Aspergillus, Mucor and Rhizopus
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Disseminated mycoses are very hard to treat.
Pathogenesis of fungal infection is divided into
three stages:
◦ Adherence
◦ Invasion
◦ Tissue injury
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Adherence
Several fungal species, particularly yeasts, can
adhere to mucosal surfaces.
Usually involves an adhesion molecule on the
fungus and host cell receptor.
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Invasion
Some fungi are
introduced through
breaks in the skin.
The small size of spores
can get past host
defenses in the lungs.
Some dimorphic fungi
can become invasive.
◦ They switch from yeast
form to mold form.
◦ The hyphae invade tissues
and disseminate.
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Tissue injury
Fungi do not produce exotoxins like bacteria in
vivo in humans.
Primary tissue injury is due to host
inflammatory response.
Phagocytes and the adaptive immune response
are important to protect against fungal
infections
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Parasitic infections in humans can be caused by
protozoans and helminths.
These infections affect hundreds of millions of
people around the world, and millions die from
these infections each year.
Protozoan parasites include rhizopods, ciliates,
and flagellates.
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Parasitic helminths are found in three classes:
nematodes, cestodes, and trematodes.
Medically important fungi can be divided on the
basis of the types of infection they cause:
superficial mycoses, subcutaneous mycoses,
mucocutaneous mycoses, or deep mycoses.
Deep mycoses are the most serious fungal
infections and can be either localized or
systemic.