ch22 Flashcards
fungi
Most fungi exist as saprobes (absorbing
nutrients from dead organisms) and function as the major
decomposers of organic matter in the environment.
mycoses transmission
Mycoses are typically acquired via inhalation, trauma,
or ingestion; only very infrequently are fungi spread from person to prson.
mycoses transmission exception
One group of fungi that are contagious are dermatophytes, which live on the dead layers of skin and which
may be transmitted between people via fomites (inanimate objects).
Species of the genera Candida and Pneumocystis also appear to be transmitted at least some of
the time by contact among humans.
true fungal pathogens
Of all the fungi known to cause disease in humans, only four—
Blastomyces dermatitidis, Coccidioides immitis, Histoplasma capsulatum,
and Paracoccidioides brasiliensis—are considered true
pathogens; that is, they can cause disease in otherwise healthy
individuals.
opportunistic fungi
Other fungi, such as the common yeast, Candida
albicans (al´bi@kanz), are opportunistic fungi, which lack genes
for proteins that aid in colonizing body tissues, though they can
take advantage of some weakness in a host’s defenses to become
established and cause disease.
Four main factors increase an individual’s risk of experiencing
opportunistic mycoses:
invasive medical procedures, medical therapies, certain disease conditions, and specific lifestyle
factors
fungal pathogens and opportunists differ with
respect to geographical distribution:
Whereas the four pathogenic
fungi are endemic to certain regions, primarily in the Americas,
opportunistic fungi are distributed throughout the world.
dermatophytes
fungi that normally live on the skin,
nails, and hair—are the only fungi that do not fall comfortably
into either the pathogenic or the opportunistic grouping.
They are considered by some researchers to be “emerging”
pathogens. Dermatophytes can infect all individuals, not just
the immunocompromised, which makes them similar to the
pathogens. They are not, however, intrinsically invasive, being
limited to body surfaces. They also have a tendency to occur in
people with the same predisposing factors that allow access by
opportunistic fungi. For these reasons, dermatophytes will be
discussed as opportunists rather than as pathogens.
Fungal diseases are grouped into the following three categories
of clinical manifestations:
- fungal infections
- toxicoses
- allergies
fungal infections
the most common mycoses, are
caused by the presence in the body of either true pathogens
or opportunists.
toxicoses
Toxicoses (poisonings) are acquired through ingestion, as
occurs when poisonous mushrooms are eaten. rlatively rare
allergies
Allergies (hypersensitivity reactions) most commonly result
from the inhalation of fungal spores
Microbiologists culture fungi collected from patients on
Sabouraud dextrose agar, a medium that favors fungal growth
over bacterial growth
KOH preparations
Potassium hydroxide (KOH) preparations dissolve keratin in skin cells, leaving only fungal cells for examination
GMS stain
Gomori methenamine
silver (GMS) stain is used on tissue sections to stain fungal
cells, black (other cells remain unstained
direct immunofluorescence stain
used to detect fungal cells
in tissues; immunological tests are not always
useful for fungi
Diagnosis of opportunistic fungal infections is especially
challenging:
When a fungal opportunist infects tissues in which
it is normally not found, it may display abnormal morphology
that complicates identification.
- fungal masses resemble tumors
- symptoms andimaging profiles that resemble TB
Mycoses are among the most difficult diseases to heal for two
reasons.
First, fungi generally possess the biochemical ability to
resist T cells during cell-mediated immune responses. Second,
fungi are biochemically similar to human cells, which means
that most fungicides are toxic to human tissues.
The majority
of antifungal agents exploit one of the few differences between
human and fungal cells:
instead of cholesterol, the membranes
of fungal cells contain a related molecule, ergosterol. Antifungal
drugs target either ergosterol synthesis or its insertion into fungal
membranes. However, cholesterol and ergosterol are not
sufficiently different to prevent some damage to human tissues
by such antifungal agents
Serious side effects associated with
long-term use of almost all antifungal agents include
anemia,
headache, rashes, gastrointestinal upset, and serious liver and
kidney damage.
The “gold standard” of antifungal agents is
the fungicidal
drug amphotericin B, considered the best drug for treating systemic
mycoses and other fungal infections that do not respond
to other drugs. Unfortunately, it is also one of the more toxic
antifungal agents to humans.
azoles
Some major anti-ergosterol alternatives
to amphotericin B are the azole drugs—ketoconazole,
itraconazole, and fluconazole—which are fungistatic (inhibitory)
rather than fungicidal and less toxic to humans.
With just a few antifungal drugs being used for long periods
and treating more and more patients, scientists predict that the
drugs should select drug-resistant strains from the fungal population.
Fortunately, this is rarely the case; naturally occurring
resistance, especially against amphotericin B, is extremely rare,
though researchers cannot explain why resistance does not develop
as it does in bacterial populations under similar conditions
of long-term use.
Treatment of opportunistic fungal infections in immunocompromised
patients involves
two steps: a high-dose treatment
to eliminate or reduce the number of fungal pathogens, followed
by long-term (usually lifelong) maintenance therapy involving
the administration of antifungal agents to control ongoing infections
and prevent new infections.
Three antifungal drugs that do not target ergosterol are
griseofulvin, 5-fluorocytosine, and echinocandins
griseofulvin
interferes with microtubule formation and chromosomal
separation in mitosis. Griseofulvin accumulates in the outer epidermal layers of the skin, preventing fungal penetration
and growth. Since these skin cells are scheduled to die as they
move toward the surface, griseofulvin’s toxicity does not permanently
damage humans.
5-Fluorocytosine
a nucleoside analog that inhibits RNA
and DNA synthesis
echinocandins
inhibit the synthesis of 1,3-d-glucan, which is
a sugar that makes up part of the cell wall of a fungus. This
sugar does not occur in mammals
mycoses prevention
Prevention of fungal infections generally entails avoiding
endemic areas and keeping one’s immune system healthy.
Vaccines against fungi have been difficult to develop because
fungal metabolism is similar to our own.
systemic mycoses
Systemic mycoses—those fungal infections that spread
throughout the body—result from infections by one of the
four pathogenic fungi: Histoplama, Blastomyces, Coccidioides,
or Paracoccidioides. All are in the fungal division Ascomycota.
These pathogenic fungi are uniformly acquired through inhalation,
and all begin as a generalized pulmonary infection that
then spreads via the blood to the rest of the body. dimorphic
dimorphic
the four pathogenic fungi
- In the environment, where the temperature is
typically below 30°C, they appear as mycelial thalli composed
of hyphae, whereas within the body (37°C) they grow as spherical
yeasts
Yeast forms
invasive bc they express a variety of enzymes and other proteins that aid
their growth and reproduction in the body. For example, they
are tolerant of higher temperatures and are relatively resistant
to phagocytic killing
histoplasmosis organism
histoplasma capsulatum; an
ascomycete and the most common fungal pathogen affecting
humans.
histoplasma capsulatum env
found in moist soils containing high levels of
nitrogen, such as from the droppings of bats and birds. Spores
may become airborne and inhaled when soil containing the
fungus is disturbed by wind or by human activities.
h capsulatum in the body
H. capsulatum is an intracellular parasite that survives inhalation
and subsequent phagocytosis by macrophages in air
sacs of the lungs. These macrophages then disperse the fungus
beyond the lungs via the blood and lymph. Cell-mediated immunity eventually develops, clearing the organism from
healthy patients
histoplasma disease
- 95p asymptomatic
- 5p clinical: chronic pulmonary, chronic cutaneous, systemic, ocular histoplasmosis
chronic pulmonary histoplasmosis
characterized by severe
coughing, blood-tinged sputum, night sweats, loss
of appetite, and weight loss. It is often seen in individuals
with preexisting lung disease. It can be mistaken for
tuberculosis
chronic cutaneous histoplasmosis
characterized by ulcerative
skin lesions, can follow the spread of infection from the
lungs.
systemic histoplasmosis
can also follow if infection spreads
from the lungs, but it is usually seen only in AIDS patients.
This syndrome, characterized by enlargement of the spleen
and liver, can be rapid, severe, and fatal.
ocular histoplasmosis
a type I hypersensitivity reaction
against Histoplasma in the eye; it is characterized by inflammation
and redness.
histoplasmosis diagnosis
Diagnosis of histoplasmosis is based on the identification
of the distinctive budding yeast in KOH- or GMS-prepared
samples of skin scrapings, sputum, cerebrospinal fluid, or various
tissues. The diagnosis is confirmed by the observation of
dimorphism in cultures grown from such samples. Cultured
H. capsulatum produces distinctively spiny spores that are
also diagnostic
blastomycoses organism
caused by another ascomycete,
Blastomyces dermatitidis
Blastomyces dermatitidis env
B. dermatitidis normally
grows and sporulates in cool, damp soil rich in organic
material, such as decaying vegetation and animal wastes. In
humans, both recreational and occupational exposure occurs
when fungal spores in soil become airborne and are inhaled.
A relatively small inoculum can produce disease.
most common manifestation
of Blastomyces infection
Pulmonary blastomycosis is the most common manifestation
of Blastomyces infection. After spores enter the lungs,
they convert to yeast forms and multiply. Initial pulmonary
lesions are asymptomatic in most individuals. If symptoms
do develop, they are vague and include muscle aches,
cough, fever, chills, malaise, and weight loss. Purulent (pusfilled)
lesions develop and expand as the yeasts multiply,
resulting in death of tissues and cavity formation. In otherwise healthy people, pulmonary
blastomycosis typically resolves successfully, although
it may become chronic.
types of blastomycosis
pulmonary, cutaneous, osteoarticular
cutaneous blastomycosis
The fungus can spread beyond the lungs. Cutaneous blastomycosis
occurs in 60% to 70% of cases and consists of generally
painless lesions on the face and upper body (Figure 22.6). The
lesions can be raised and wartlike, or they may be craterlike if
tissue death occurs.
osteoarticular blastomycosis
In roughly 30% of cases, the fungus spreads
to the spine, pelvis, cranium, ribs, long bones, or subcutaneous
tissues surrounding joints, a condition called osteoarticular2
blastomycosis
Coccidioidomycosis organism
caused by
ascomycete Coccidioides immitis
c immitis cycle
In the warm and dry summer and fall months, particularly
in drought cycles, C. immitis grows as a mycelium and produces
sturdy chains of asexual spores called arthroconidia. When mature,
arthroconidia germinate into new mycelia in the environment,
but if inhaled, arthroconidia germinate in the lungs to produce
a parasitic form called a spherule. As each
spherule matures, it enlarges and generates a large number of
spores via multiple cleavages, until it ruptures and releases the
spores into the surrounding tissue. Each spore then forms a new spherule to continue the cycle of division and release. This type
of growth accounts for the seriousness of Coccidioides infection.
coccidioidomycosis symptoms
The major manifestation of coccidioidomycosis is pulmonary.
About 60% of patients experience either no symptoms or
mild, unremarkable symptoms that go unnoticed and typically resolve on their own. Other patients develop more severe infections
characterized by fever, cough, chest pain, difficulty breathing,
coughing up or spitting blood, headache, night sweats,
weight loss, and pneumonia; in some individuals a diffuse rash
may appear on the trunk.
- in those
who are severely immunocompromised, C. immitis spreads
from the lungs to various other sites. Invasion of the central
nervous system (CNS) may result in meningitis, headache,
nausea, and emotional disturbance. Infection can also spread
to the bones, joints, and painless subcutaneous tissues
paracoccidioidomycosis organism
Another ascomycete, Paracoccidioides brasiliensis
what is paracoccidioidomycosis
a chronic fungal disease similar to
blastomycosis and coccidioidomycosis.
- Because this fungus is far more
geographically limited than the other true fungal pathogens,
paracoccidioidomycosis is not a common disease.
paracoccidioidomycosis organism env
Paracoccidioides brasiliensis is found
in cool, damp soil
paracoccidioidomycosis symtptoms
Infections range from asymptomatic to systemic, and disease
first becomes apparent as a pulmonary form that is slow
to develop but manifests as chronic cough, fever, night sweats,
malaise, and weight loss. The fungus almost always spreads,
producing a chronic inflammatory disease of mucous membranes.
Painful ulcerated lesions of the gums, tongue, lips,
and palate progressively worsen over the course of weeks to
months
paracoccidioidomycosis diagnosis
KOH or GMS preparations of tissue samples reveal yeast
cells with multiple buds in a “steering wheel” formation that is
diagnostic for this organism
Opportunistic mycoses do not typically affect healthy humans
because
the fungi involved lack genes for virulence factors that
make them actively invasive.
Even though any fungus can become an opportunist, five genera of fungi are routinely encountered:
Pneumocystis,
Candida, Aspergillus, Cryptococcus, and Mucor.
Opportunistic infections present a formidable challenge to
clinicians.
Because they appear only when their hosts are weakened,
they often display “odd” clinical signatures; that is, their
symptoms are often atypical, or they occur in individuals not
residing in endemic areas for a particular fungus.
PCP organism
Pneumocystis pneumonia; ascomycete Pneumocystis pneumonia
- formerly known as P. carinii n classified as protozoan (reclassified as a fungus based on rRNA nucleotide sequences
and biochemistry. Its morphological and developmental characteristics,
however, still resemble those of protozoa more than
those of fungi.)
- Because P. jiroveci is an obligate parasite and cannot
survive on its own in the environment, transmission most
likely occurs through inhalation
PCP how widespread
Prior to the AIDS epidemic,
disease caused by Pneumocystis was extremely rare. Now, the disease is almost diagnostic
for AIDS.
- P. jiroveci is distributed worldwide in humans; based on
serological confirmation of antibodies, the majority of healthy
children (75%) have been exposed to the fungus by the age of
five
PCP symptoms
Once the fungus enters the lungs of an AIDS patient, it multiplies
rapidly, extensively colonizing the lungs because the patient’s
defenses are impaired. Widespread inflammation, fever,
difficulty in breathing, and a nonproductive cough are characteristic.
If left untreated, PCP involves more and more lung tissue
until death occurs.
PCP opportunistic aspect
In immunocompetent people, infection is asymptomatic,
and generally clearance of the fungus from the body is followed
by lasting immunity
candidiasis organism
any opportunistic fungal infection
or disease caused by various species of the genus Candida (dimorphic ascomycetes)—
most commonly Candida albicans
candida env
common members of the
microbiota of the skin and mucous membranes; for example,
the digestive tracts of 40% to 80% of all healthy individuals harbor
Candida species.
candidiasis diagnosis
Physicians diagnose candidiasis on the basis of signs and
demonstration of clusters of budding yeasts (see Figure 22.1b)
and pseudohyphae, which are series of buds remaining attached
to the parent cell and appearing as a filamentous hypha
candidiasis in immunocompetent patients
In immunocompetent patients for whom excessive friction
and body moisture in skin folds or preexisting diseases
are the reasons for fungal colonization, resolution involves
treating these underlying problems in addition to administering
topical antifungal agents
candidiasis opportunistic aspect
- It is Candida that causes vaginal
yeast infections, as the fungus grows prolifically when
normal bacterial microbiota are inhibited due to changes in
vaginal pH or use of antibacterial drugs - Systemic disease is seen almost universally in immunocompromised
individuals
thrush
oral candidiasis
aspergillosis organism
not a single disease but instead
a term for several diseases resulting from the inhalation of spores
of fungi in the genus Aspergillus
aspergillosis organism env
Aspergillus is found in soil, food, compost, agricultural
buildings, and air vents of homes and offices worldwide.
- Because the fungi are so common in the environment,
little can be done to prevent exposure to the spores.
aspergillosis manifestation
Although
exposure to Aspergillus most commonly causes only allergies, more
serious diseases can occur, and aspergillosis is a growing problem
for AIDS patients.
- can be opportunistic pathogens
of almost all body tissues
- 3 clinical pulmonary diseases: hypersensitivity, noninvasive, acute invasive pulmonary aspergillosis
Hypersensitivity aspergillosis
manifests as asthma or other
allergic symptoms and results most commonly from inhalation
of Aspergillus spores. Symptoms may be mild and
result in no damage, or they may become chronic, with recurrent
episodes leading to permanent damage.
nonivasive aspergillosis
ball-like masses of fungal
hyphae—can form in the cavities resulting from a previous
case of pulmonary tuberculosis. Most cases are asymptomatic,
though coughing of blood-tinged sputum may occur.
Acute invasive pulmonary aspergillosis
more serious. Signs
and symptoms, which include fever, cough, and pain, may
present as pneumonia. Death of lung tissue can lead to significant
respiratory impairment.
Aspergillus also causes
nonpulmonary disease when aspergillomas
form in paranasal sinuses, ear canals, eyelids (Figure 22.14),
the conjunctivas, eye sockets, or brain
cryptococcosis organism
A basidiomycete, Cryptococcus neoformans
- C neoformans gattii
- C. neoformans neoformans is found
worldwide and infects mainly AIDS patients. Approximately
50% of all cryptococcal infections reported each year are due to
strain neoformans.
cryptocossosis transmission
Human infections follow the inhalation of spores or dried
yeast in aerosols from the droppings of birds.
The pathogenesis of Cryptococcus is enhanced by several characteristics
of this fungus:
the presence of a phagocytosis-resistant
capsule surrounding the yeast form; the ability of the yeast to produce
melanin, which further inhibits phagocytosis; and the organism’s
predilection for the central nervous system (CNS), which is
isolated from the immune system by the so-called blood-brain barrier.
Cryptococcal infections also tend to appear in terminal AIDS
patients when little immune function remains.
list of cryptococcoses
- primary pulmonary cryptococcosis
- cryptococcal meningitisi
- cryptococcoma
- cutaneous cryptococcosis
Primary pulmonary cryptococcosis
is asymptomatic in most individuals,
although some individuals experience a low-grade fever,
cough, and mild chest pain
Cryptococcal meningitis
the most common clinical form of
cryptococcal infection, follows dissemination of the fungus to
the CNS. Symptoms develop slowly and include headache, dizziness,
drowsiness, irritability, confusion, nausea, vomiting, and
neck stiffness. In late stages of the disease, loss of vision and
coma occur. Acute onset of rapidly fatal cryptococcal meningitis
occurs in individuals with widespread infection
cryptococcoma
a very rare condition
in which solid fungal masses form in the cerebral hemispheres,
cerebellum, or (rarely) in the spinal cord. The symptoms of this
condition, which can be mistaken for cerebral tumors, are similar
to those of cryptococcal meningitis but also include motor
and neurological impairment.
Cutaneous cryptococcosis
Primary infections
manifest as ulcerated skin lesions or as inflammation of subcutaneous
tissues. Infection may resolve on its own, but patients should
be monitored for infection spreading to the CNS. Secondary
cutaneous lesions occur following spread of Cryptococcus to other
areas of the body. In AIDS patients, cutaneous cryptococcosis is the
second most common manifestation of Cryptococcus infection (after
meningitis). Lesions are common on the head and neck.
zygomycoses organisms
various genera of fungi classified in the division
Zygomycota, especially Mucor (sometimes rhizopus, absidia)
zygomycoses organism env
extremely common in soil, on decaying
organic matter, or as contaminants that cause food spoilage
zygomycoses oopportuistic aspect
Zygomycoses are commonly seen in patients with uncontrolled
diabetes, in people who inject illegal drugs, in some
cancer patients, and in some patients receiving antimicrobial
agents.
Rhinocerebral zygomycosis
begins with infection of the nasal
sinuses following inhalation of spores. The fungus spreads
to the mouth and nose, producing macroscopic cottonlike
growths. Mucor can subsequently invade blood vessels,
where it produces fibrous clots, causes tissue death, and
subsequently invades the brain, which is fatal within days,
even with treatment.
pulmonary zygomycosis
follows inhalation of spores (as from
moldy foods). The fungus kills lung tissue, resulting in the
formation of cavities.
gastrointestinal zygomycosis
involves ulcers in the intestinal tract
Cutaneous zygomycosis
results from the introduction of
fungi through the skin after trauma (such as burns or needle
punctures). Lesions range from pustules and ulcers to
abscesses and dead patches of skin.
Over the course of the past decade,
several new fungal opportunists have been identified
Fusarium spp. and Penicillium marneffei, which are ascomycetes,
and Trichosporon beigelii, which is a basidiomycete.
fusarium spp
cause respiratory distress,
disseminated infections, and fungemia (fungi in the bloodstream).
These species also produce toxins that can accumulate
to dangerous levels when ingested in food. Fusarium spp. are
resistant to most antifungal agents.
penicillium marneffei
a dimorphic,
invasive fungus that causes pulmonary disease upon inhalation.
trichosporon beigelii
When Trichosporon beigelii enters
an AIDS patient through the lungs, the gastrointestinal tract, or
catheters, it causes a drug-resistant systemic disease that is typically
fatal.
Superficial, Cutaneous,
and Subcutaneous Mycoses
localized
at sites at or near the surface of the body. They are the most commonly
reported fungal diseases. All are opportunistic infections,
but unlike those we have just discussed, they can be acquired both
through environmental exposure and more frequently via personto-person
contact. Most of these fungi are not life threatening, but
they often cause chronic, recurring infections and diseases.
Superficial, Cutaneous,
and Subcutaneous Mycoses
localized
at sites at or near the surface of the body. They are the most commonly
reported fungal diseases. All are opportunistic infections,
but unlike those we have just discussed, they can be acquired both
through environmental exposure and more frequently via personto-person
contact. Most of these fungi are not life threatening, but
they often cause chronic, recurring infections and diseases.
superficial mycoses
Superficial mycoses are the most common fungal infections.
They are confined to the outer, dead layers of the skin, nails, or
hair, all of which are composed of dead cells filled with a protein
called keratin—the primary food of these fungi. In AIDS patients,
superficial mycoses can spread to cover significant areas
of skin or become systemic.
ringworms
dermatophytoses were called ringworms because
dermatophytes produce circular, scaly patches that resemble
a worm lying just below the surface of the skin.
dermatophytes
Dermatophytes use keratin as a nutrient
source and thus colonize only dead tissue. The fungi may
provoke cell-mediated immune responses, which can damage
living tissues. Dermatophytes are among the few contagious
fungi; that is, fungi that spread from person to person. Spores
and bits of hyphae are constantly shed from infected individuals,
making recurrent infections common.
dermatophytoses organisms
3 genera of ascomycetes r responsiblefor most:
- trichophyton spp
- epidermophyto floccosum
- microsporum spp
athletes foot
tinea pedis - Red, raised lesions on and around the toes and soles of the feet; webbing between the toes is heavily infected
jock itch
tinea cruris
- Red, raised lesions on and
around the groin and buttocks
tinea unguium
onychomycosis - superficial white: patches or pits on the nail surface - invasive: yellowing and thickening of the distal nail plate, often leading to loss of the nail
tinea corporis
Red, raised, ringlike lesions occurring
on various skin surfaces
tinea capitis
ectothrix invasion, endothrix invasion, favus
ectothrix invasion
fungus
develops arthroconidia on the
outside of the hair shafts,
destroying the cuticle
endothrix invasion
fungus develops
arthroconidia inside the
hair shaft without destruction
favus
crusts form on the scalp,
with associated hair loss
pityriasis organism
the dimorphic basidiomycete malassezia furfur
m furfur env
a normal member of the microbiota of the skin of
humans worldwide. It feeds on the skin’s oil and causes common,
chronic superficial infections.
pityriasis symptoms
characterized by depigmented or hyperpigmented patches of
scaly skin resulting from fungal interference with melanin production
(Figure 22.16). This condition typically occurs on the
trunk, shoulders, and arms, and rarely on the face and neck.
sub, cutaneous mycoses
The fungi involved in cutaneous and subcutaneous mycoses are
common soil saprobes (organisms that live on dead organisms),
but the diseases they produce are not as common as superficial
mycoses because infection requires traumatic introduction
of fungi through the dead outer layers of skin into the deeper,
living tissue. Most lesions remain localized just below the skin,
though infections may rarely become systemic.
chromoblastomycosis, phaeohyphomycosis orgaisms
caused by dark-pigmented
ascomycetes. Despite the worldwide occurrence of the relevant fungi,
the overall incidence of infection is relatively low. People who
work daily in the soil with bare feet are at risk if they incur foot
wounds. prevention by wearing shoes
chromoblastomycosis symptoms
Initially, chromoblastomycosis uniformly presents as small,
scaly, itchy, but painless lesions on the skin surface resulting
from fungal growth in subcutaneous tissues near the site of
inoculation. Over the course of years and decades, the lesions
progressively worsen, becoming large, flat to thick, tough, and
wartlike. They become tumorlike and extensive if not treated
(Figure 22.17). Inflammation, fibrosis, and abscess formation
occur in surrounding tissues. The fungus can spread throughout
the body
Phaeohyphomycoses
more variable in presentation, involving
colonization of the nasal passages and sinuses in allergy
sufferers and AIDS patients or of the brains of AIDS patients.
Fortunately, brain infection is the rarest form of phaeohyphomycosis
and occurs only in the severely immunocompromised
Phaeohyphomycoses vs chromoblastomycosis
key distinguishing
feature between the two diseases: the microscopic morphology
of the fungal cells within tissues. Tissue sections from
chromoblastomycosis cases contain golden brown sclerotic bodies
that are distinctive and distinguishable from budding yeast
forms (Figure 22.18a), whereas tissues from phaeohyphomycosis
cases contain brown-pigmented hyphae.
mycetomas organisms
caused by fungi of several genera in the division Ascomycota. The cases
that occur in the United States are almost always caused by
Pseudallescheria or Exophiala
mycetoma organism env
Mycetoma-producing fungi live in the soil and are introduced
into humans via wounds caused by twigs, thorns, or
leaves contaminated with fungi. As with chromoblastomycosis
and phaeohyphomycosis, those who work barefoot in soil
are most at risk, and wearing protective shoes or clothing can
greatly reduce incidence.
mycetoma symptoms
Infection begins near the site of inoculation with the formation
of small, hard, subsurface nodules that slowly worsen and
spread as time passes. Local swelling occurs, and ulcerated lesions
begin to produce pus. Infected areas release an oily fluid
containing fungal “granules” (spores and fungal elements). The
fungi spread to more tissues, destroying bone and causing permanent
deformity.
sporotrichosis organism
dimorphic ascomycte sporothrix schenckii
sporotrichosis organism env
S. schenckii resides in the soil and is most commonly introduced by thorn pricks or wood splinters. Avid gardeners, farmers, and
artisans who work with natural plant materials have the highest
incidence of sporotrichosis.
sporotrichosis symptoms
a subcutaneous infection usually limited to the arms or legs. Sporotrichosis initially appears as painless, nodular lesions that
form around the site of inoculation. With time, these lesions produce
a pus-filled discharge, but they remain localized and do not
spread. If the fungus enters the lymphatic system from a primary
lesion, it gives rise to secondary lesions on the skin surface along
the course of lymphatic vessels (Figure 22.20). The fungus remains
restricted to subcutaneous tissues and does not enter the blood.
mycotoxins
fungal toxins. low molcular weight metabolites that can harm humans and animals that ingest them,
causing toxicosis
fungal allergens
Fungal allergens are usually proteins or
glycoproteins that elicit hypersensitivity reactions in sensitive
people who contact them.
Mycotoxicosis
caused by eating mycotoxins; the fungus itself is not present.
Mycetismus
is mushroom poisoning resulting
from eating the fungus.
mycotins env
Fungi produce mycotoxins during their normal metabolic activities.
People most commonly consume mycotoxins in grains
or vegetables that have become contaminated with fungi. Up to 25% of the world’s food
supply is contaminated with mycotoxins, but only 20 of the 300
or so known toxins are ever present at dangerous levels.
- Longterm
ingestion of mycotoxins can cause liver and kidney damage,
gastrointestinal or gynecological disturbances, or cancers;
each mycotoxin produces a specific clinical manifestation.
aflatoxins organism
ascomycete Aspergillus
aflatoxins
the best-known mycotoxins. Aflatoxins are fatal to
many vertebrates and are carcinogenic at low levels when consumed
continually. Aflatoxins cause liver damage and liver cancer
throughout the world
aflatoxins env
aflatoxicosis is most prevalent in the tropics,
where mycotoxins are more common because of subsistence
farming, poor food-storage conditions, and warm, moist conditions
that foster the growth of Aspergillus in harvested foods
useful mycotoxins
Among them are
ergot alkaloids, produced by some strains of another ascomycete,
Claviceps purpurea
ergometrine
used to stimulate labor contractions and is used to
constrict the mother’s blood vessels after birth (when she is at
risk of bleeding excessively)
ergotamine
used to treat migraine headaches.
mushrooms
the spore-bearing structures of certain basidiomycetes
must mushrooms arent toxic, though some
produce extremely dangerous
poisons capable of causing neurological dysfunction or
hallucinations, organ damage, or even death.
toadstools
poisonous mushrooms
myctismus treatment
Treatment involves inducing vomiting followed by oral
administration of activated charcoal to absorb toxins. Severe
mushroom poisoning may necessitate a liver transplant.
deadliest mushroom toxin
produced by the “death
cap” mushroom, Amanita phalloides. The death cap contains two related polypeptide
toxins: phalloidin, which irreversibly binds actin within cells,
disrupting cell structure, and alpha-amanitin, which inhibits
mRNA synthesis. Both toxins cause liver damage.
hallucinogenic mycotoxins
Psilocybe cubensis produces hallucinogenic
psilocybin, and Amanita muscaria produces
two hallucinogenic toxins—ibotenic acid and muscimol. These
toxins may also cause convulsions in children.
Fungal allergens
typically cause
type I hypersensitivities in which immunoglobulin
E binds the allergen, triggering responses such as asthma,
eczema, hay fever, and watery eyes and nose.
Fungal allergens
typically cause
type I hypersensitivities in which immunoglobulin
E binds the allergen, triggering responses such as asthma,
eczema, hay fever, and watery eyes and nose.
type III hypersensitivities
Less frequently, type III hypersensitivities result from
chronic inhalation of particular fungal allergens. In these cases,
allergens that have penetrated deep into the lungs encounter complementary antibodies and form immune complexes in the
alveoli that lead to inflammation, fibrosis, and in some cases
death. Type III fungal hypersensitivities are associated with certain
occupations, such as farming, in which workers are constantly
exposed to fungal spores in moldy vegetation.
