Bauman Chapter 22 Answers to Critical Thinking Questions
p. 624
Discuss the relationship between the following two facts: (1) A huge number of fungal
spores are present in the environment, and (2) fungal diseases are not typically acquired
via contact with infected individuals. How does not relying on a host for transmission
benefit a fungus?
Most fungal diseases result from environmental exposure to spores, which are abundant
in the environment and dispersed by air currents. Transmission by air rather than by a
host means that fungal pathogens are not dependent on encounters between
susceptible hosts, are not affected by host life-style (seasonal behavior, changing
population dynamics in response to other factors, e.g. changes in distribution), and are
not at risk of extinction if a host becomes rare or extinct.
p. 626
Suggest a way in which each of the risk factors listed in Table 22.1 can be reduced or
eliminated to limit opportunistic infections. What can the patient do? What can health
care providers do?
Abstinence or mutual monogamy prevent most cases of AIDS, making fungal infection
less likely in these people. Using minimally invasive surgical procedures and stringent
asepsis of medical devices can reduce nosocomial infections with fungi. Adjusting the
dose of immunosuppressive drugs to the lowest possible level (without affecting the
purpose for which they are given) may allow enough immune function to provide some
protection from infections. Fungal infections in immunocompromised persons (AIDS,
burn patients, etc.) can be reduced by maintaining an environment in which fungal
spores are filtered from the air or removed from fomites. Ensuring basic nutrition and
access to basic healthcare can help maintain immune function in poor people, making
them less susceptible to fungal infections. Needle exchange programs to provide sterile
needles to IV drug abusers can reduce exposure to fungal spores among drug abusers.
p. 629
Outbreaks of blastomycoses have occurred in Latin America even though the organism
itself is not normally found there. Based on what you have read, explain why a few cases
of blastomycosis might appear outside of endemic areas.
Fungal spores are hardy and light-weight enough to become suspended in the air on
dust particles. The spores can contaminate fomites and be carried from one part of the
world to another. Infected people might travel to nonendemic areas.
p. 630
Statistically, men are more likely than women to contract histoplasmosis. What might
explain this fact?
More men than women are engaged in occupations and activities that result in exposure
to soil, and particularly disturbed soil (e.g. farming, construction).
p. 631
All of the true fungal pathogens manifest initially as a pulmonary disease. Explain how
you could ascertain which of the four pathogens a patient has.
A KOH or GMS-stained sputum sample examined under the microscope is a first step in
identifying the fungal pathgens: Coccidioides immitis produces spherules containing
spores, while the yeast cells of Blastomyces dermatiditis, Histoplasma capsulatum, and
Paracoccidioides brasiliensis have distinctive appearances.
p. 635
Even though Candida species are not as virulent as some microbial pathogens, the
fungus can still invade every human tissue. Propose a possible explanation for this
observation.
Candida is part of the skin flora in a significant percentage of the population; therefore,
the fungus is already growing on the human body and can proliferate when the immune
system is compromised or the fungus is introduced into a novel environment, for
example, the blood.
p. 636
Compare and contrast the clinical features of cryptococcosis and PCP. Why are both of
these diseases so dangerous to AIDS patients?
Cryptococcosis and PCP both begin as respiratory infections with fever, cough chest
pain, and difficulty breathing, and both can result in respiratory failure. Frequently,
however, cryptococcosis progresses to central nervous system invasion to produce
meningitis or tumor-like cryptococcoma. On the other hand, PCP rarely establishes
extrapulmonary invasion. PCP usually results in death due to respiratory impairment.
Pneumocystis jiroveci is very common in the lungs of humans, and Cryptococcus
neoformans is very common in some geographic areas and is wind-dispersed, so
exposure to the fungi is very common. Immunocompetent individuals infected with
Pneumocystis jiroveci and Cryptococcus neoformans often have asymptomatic
infections; whereas, AIDS patients develop difficult to treat, often fatal infections.
p. 640
Explain why many superficial fungal infections are chronic, recurring problems.
Many superficial fungal infections are recurring infections because they are transmitted
from person to person by contact or even by contact with fomites. The infections can be
very difficult to treat, becoming chronic.
p. 643
Make a dichotomous key to distinguish among all the fungal mycoses covered in the
previous sections.
Basidium?
Yes
No
Lipophilic?
Yes
No
Malassesia
Ascus?
Cryptococcus
No
Yes
Mucor
Dimorphic?
Yes
No
Spherules?
Yes
Pseudohyphae?
No
No
Coccidioides
Yes
Spiny
spores?
Candida
Yes
No
Histoplasma
Yeast?
Yes
Steering
wheel
budding?
Yes
Brown
pigment?
No
Paracoccidioides
Pulmonary?
Yes
Yes
No
Yes
Fonsecaea
No
Cladophialphora
Sclerotic
bodies?
Blastomyces
No
Pneumocystis
Aspergillus
No
Sporothrix
p. 644
Despite the fact that fungi are everywhere, fungal allergies are still not as common as
allergies to pollen or dust mites. Propose an explanation to explain this observation.
Exposure to fungal antigens begins early in life and at exposure levels which allow the
development of immunity mediated by IgG or cytotoxic T cells, and not by IgE, which is
the primary factor in allergic reactions.
p. 648
1. Correlate the observation that the majority of fungal infections are caused by
opportunists with the fact that most infections are acquired from the environment.
How does this make the control and diagnosis of fungal infections difficult?
Fungi are so ubiquitous that environmental exposure to opportunistic fungal pathogens
is not preventable. Many of the fungal pathogens use the same portals of entry and
cause similar symptoms, making diagnosis difficult.
2.
The four pathogenic fungi (Blastomyces, Coccidioides, Histoplasma, and
Paracoccidioides) are all dimorphic. Antifungal agents, like all antimicrobials, are
generally designs to target something specific about the organism to avoid harming
human tissues. Propose how you could use the idea of dimorphism to produce new
antifungal agents.
Dimorphism suggests that environmental cues are inducing changes in gene expression
in fungal pathogens. Characterization of the genes being expressed in the pathogenic
yeast phase identifies functions critical to pathogenesis that may be targeted by
chemical analogs of their metabolic intermediates.
3.
What actions could be taken to limit the contamination of foods with fungal toxins?
Would these actions differ depending on where in the world the food is grown?
Explain.
Reduction of fungal toxins requires reducing fungal growth in and on foods. Using
fungicides in fields where crops are grown can reduce the amount of fungi growing along
with the crops and therefore also the amount of fungal spore contamination while the
crops are in the field. Washing with detergents after harvest, irradiation or cooking can
reduce fungal growth on foods during storage. Many of these precautions can be used
worldwide, but toxic fungi with discrete geographic distributions may require specific
precautions tailored to local conditions.
4.
Onychomycoses are nail infections that can be caused by several species of fungi.
Explain why these mycoses are so difficult to treat, and why it is generally
necessary to treat patients with oral antifungal agents for long periods of time.
Nail beds do not have direct blood supply and the nails grow slowly. The slow growth
and the fact that antifungal agents diffuse slowly to the nail beds requires a constant
level of medication over a long period of time for the antifungal to penetrate to the
location of the infection.
5.
What factors contribute to the pathogenicity of Cryptococcus infections?
Cryptococcus has a polysaccharide capsule that makes it difficult for white blood cells to
phagocytose the cells, and the fungus produces melanin that has an inhibitory affect on
phagocytes. The net effect is that the immune system has difficulty killing Cryptococcus
cells.
6.
Clinically, all fungi that cause subcutaneous mycoses produce lesions on the skin
around the site of inoculation. What are some of the things you would look for when
attempting to distinguish among them?
Characteristics of the fungi that cause subcutaneous mycoses useful in distinguishing
one from another include the presence of pigments (e.g. golden brown Fonsecaea vs.
brown Cladophiolaphora), dimorphism, and whether the form is yeast or filamentous
(e.g. Sporothrix yeast cells, hyphal Madurella and Pseudoallescheria). The geographic
local in which the infection was acquired can also be useful in determining the causative
agent.
7.
Given the various predisposing factors that make humans susceptible to
opportunistic infections, how can health care providers curtail the rising incidence of
such infections?
Health care providers have limited options to try to slow the rising incidence of
opportunistic fungal infections because of the ubiquity of fungi and the difficulty in
reducing the rate of increase of immunocompromised individuals. Rigorous aseptic
techniques can reduce the presence of fungi and spores and filtration of air supplies in
hospitals can reduce some infections. Careful use of immunosuppressive drugs can
reduce patient susceptibility. Using the least invasive procedural option can reduce
nosocomial fungal infections associated with surgery. Better patient education about
risks and sources of infection may also help prevent some infections.