Bauman Chapter 23 Answers to Critical Thinking Questions
p. 650
Why are diseases such as balantidiasis generally more severe in those who are already
in poor health?
Immune system responses prevent the population of a parasite like Balantidium coli from
growing to the point of tissue damage (symptomatic disease). A person in poor health
has an immune system that is not functioning at normal levels and therefore does not
control infection and prevent symptomatic damage.
p. 658
How does the visually distinctive appearance of Giardia trophozoites improve the
success of medical treatment of giardiasis as compared to that for amoebic infections?
A moderate level of skill with the microscope is sufficient for a technician to be able to
recognize Giardia trophozoites in fecal samples, and living trophozoites are not required
for recognition. Parasitic amoeba do not have fixed shapes and have few features that
are constant enough for reliable recognition, and since their motility is one of the things
that makes them distinctive, dead amoeba trophozoites are very hard for even for a
skilled microscopist to differentiate from cell debris and fecal material.
p. 665
How might humans become intermediate hosts instead of definitive hosts?
Humans can become intermediate hosts for some parasites when they ingest the eggs
of the parasite.
p. 667
Compare the general tapeworm life cycle to the general fluke life cycle. What is similar?
What is different?
Both tapeworm and fluke life cycles generally involve at least one intermediate host, and
a dormant stage which is ingested by the definitive host, where the worm matures into a
monoecious adult that self-fertilizes and produces eggs.
The tapeworm egg must be ingested by the intermediate host to infect it, while the fluke
egg hatches in fresh water to release a free-swimming larva that seeks out the
intermediate host. Tapeworm adults reside in the intestines where their cysts excyst,
whereas flukes usually migrate from the intestines of the definitive host to their normal
site of residence (liver, lungs).
p. 668
Propose some additional methods that could be used to prevent transmission of
Schistosoma infections to humans.
Prompt treatment of infected individuals reduces the number of eggs released into the
environment, and discouraging the use of human feces as fertilizer reduces the
contamination of bodies of fresh water. Treating bodies of water near human habitation
with chemicals to kill the snail intermediate stops the development of the parasite, but
since this may result in unacceptable damage to ecosystems, it should only be
considered for manmade ponds and rice paddies. Development of a chemical that is
specifically toxic to the miracidium and can safely be distributed in water is a less
damaging alternative to killing the intermediate host.
p. 669
Thoroughly cooking raw vegetables prior to consumption would kill the metacercariae of
Fasciola and prevent infection. Why is this method of prevention not practical?
Cooking is not practical for some of the vegetables that Fasciola metacercariae may be
found on (water cress, lettuce) because thorough cooking makes the vegetable
unpalatable.
p. 678
1. Compare Entamoeba, Acanthamoeba, and Naegleria. Based on incidence, which
of these parasites is a threat to more people? Clinically, which is the most serious?
Explain why your answer is not the same for both questions.
Entamoeba is a global parasite of humans that can be carried asymptomatically and
spread easily by the fecal-oral route and thus threatens more people than the other two
amoebae. Naegleria is the most serious of the three pathogenic amoebae.
Acanthamoeba and Naegleria are accidental parasites. Acanthamoeba causes keratitis
and other localized infections in healthy individuals, and generally causes more serious
neurologic disease in immunocompromised persons. In addition, Acanthamoeba
encephalitis lasts weeks, making it possible to diagnose and treat the disease before it is
fatal. Naegleria causes difficult to treat, usually fatal neurologic infections in otherwise
healthy persons and disease progresses so rapidly that a patient may die before a
diagnosis is made and treatment begun. Fortunately, intranasal inoculation (necessary
for infection) with Naegleria is rare.
2.
Explain how each of the following could lead to the reemergence of malaria in the
United States: (a) global warming, (b) increased travel of individuals from endemic
regions to the United States, (c) increased immigration of individuals from endemic
regions to the United States, (d) regulations against the use of insecticides, and (e)
laws protecting wetlands.
Global warming results in the mild winter weather necessary for the survival of mosquito
vectors of malaria becoming more widespread in US territory. Increased travel or
immigration of people from endemic areas to the U. S. increases the number of imported
cases of malaria, thereby increasing the odds of the U. S. mosquito population becoming
infected with Plasmodium. Restrictions on the use of pesticides make mosquito
population control more difficult, and protecting the wetlands increases the extent of
potential breeding sites for the mosquitoes.
3.
People who suffer from AIDS are severely immunocompromised because HIV
destroys immune cells. Thus all the diseases in this chapter could be a threat to an
AUDS patient. Which ones, however, would specifically exacerbate the immune
dysfunction of AIDS? Why?
In Chagas’ disease and leshmaniasis macrophages are infected and destroyed,
exacerbating immune dysfunction. Chagas’ disease and African sleeping sickness,
which are both cause by trypanosomes, are caused by parasites capable of changing
surface antigens, resulting in the immune system continuously functioning at a high level
of activity, which increases the rate of damage by HIV. Lymphatic filariasis (Wuchereria)
results when nematodes take up residence in lymph nodes, impairing the function of the
nodes and reducing immune function, although not as severely as when leukocytes are
directly killed.
4.
Discuss why sickle-cell trait is advantageous to people living in malaria endemic
areas, but is not advantageous in malaria-free areas.
The red blood cells of a person with the sickle-cell trait contain a form of hemoglobin that
makes the RBCs resistant to infection by Plasmodium, thereby reducing the severity of
malaria. The sickle-cell trait also causes RBCs to take on an abnormal (“sickle”) shape
that makes the cells both fragile and likely to get stuck in capillaries. Where malaria is
endemic the increased resistance increases a person’s survival, but in the absence of
the selective pressure of malaria, damage to RBCs and blood vessels damages a
patient’s health.
5.
Present a logical argument to explain the differences between the clinical
manifestations of Trypanosoma brucei and T. cruzi. Relate your argument to the
respective life cycles.
Trypanosoma brucei resides in the fluid compartments of the body (blood, lymph and
eventually cerebrospinal fluid): the early symptoms are the result of immune responses
to the antigens in the blood and lymph, and then inflammation of the brain when T.
brucei has invaded the central nervous system. T. cruzi is an intracellular parasite that
invades a variety of cell types, so the immune response is directed against infected cells
and damaged tissues, a fundamentally different response than that directed against
antigen in body fluids.
6.
What feature of the life cycle of T. brucei makes it difficult to create a successful
vaccine?
Trypanosoma brucei is capable of changing at frequent intervals between a large
number of possible variations of the antigens on its cell surface. The antigens are
distinct enough that antibodies to one are not effective against the others. A vaccine
against T. brucei is difficult to develop because a vaccine against so many antigens is
not feasible.