Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Anthrax

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The need to mirror the human situation has led to the

development of animal models that are specifically tailored

for certain diseases. One example is the so-called nude mouse,

which derives its name from the fact that it has no hair. Nude

mice lack a thymus, and so are immunodeficient in a number

of ways. Use of nude mice has been very useful in the study

of immunodeficiencydiseases in humans, such as acquired

immunodeficiency syndrome. As well, this animal model

lends itself to the study of opportunistic bacterial infections,

which typically occur in humans whose immune systemsare

compromised.

Depending on the infection and the focus of study, other

animals have proven to be useful in infectious disease

research. These animals include the rabbit, rat, guinea pig, pig,

dog, and monkey. The latter in particular has been utilized in

the study of AIDS, as primates are the genetically closest rela-

tives to humans.

The advent of molecular techniques of genetic alteration

has made the development of genetically tailored animal mod-

els possible. Thus, for example, mouse models exist in which

the activity of certain genes has been curtailed. These are

known as transgenic animals. The involvement of the gene

product in the infectious process is possible on a scale not pos-

sible without the use of the animal.

The data from animal models provides a means of indi-

cating the potential of a treatment. Furthermore, if a disease in

an animal does not exactly mimic the human’s condition, for

example cystic fibrosis in mice, the use of the animal model

provides a guide towards establishing the optimal treatment in

humans. In other words, the animal model can help screen and

eliminate the undesirable treatments, narrowing the successful

candidates for use in human studies. Further study, involving

humans, is always necessary before something such as a vac-

cine can be introduced for general use. Such human studies are

subject to rigorous control.

The use of animals in research has long been a con-

tentious issue, mainly due to questions of ethical treatment.

This climate has spawned much legislation concerning the

treatment of research animals. As well, in most institutions, an

evaluation committee must approve the use of animals. If the

research can be accomplished in some other way than through

the use of living animals, then approval for the animal study is

typically denied.

See alsoAIDS, recent advances in research and treatment;

Giardia and giardiasis; Immunodeficiency

ANIMALCULES•seeHISTORY OF MICROBIOLOGY

AAnthraxNTHRAX

Anthrax refers to a pulmonary disease that is caused by the

bacterium Bacillus anthracis. This disease has been present

since antiquity. It may be the sooty “morain” in the Book of

Exodus, and is probably the “burning wind of plague” that

begins Homer’s Iliad.Accounts by the Huns during their

sweep across Eurasia in 80 A.D. describe mass deaths among

their horse and cattle attributed to anthrax. These animals,

along with sheep, are the primary targets of anthrax. Indeed,

loss to European livestock in the eighteenth and nineteenth

centuries stimulated the search for a cure. In 1876, Robert

Kochidentified the causative agent of anthrax.

The use of anthrax as a weapon is not a new phenome-

non. In ancient times, diseased bodies were used to poison

wells, and were catapulted into cities under siege. In modern

times, research into the use of anthrax as a weapon was carried

out during World Wars I and II. In World War II, Japanese and

German prisoners were subjects of medical research, includ-

ing their susceptibility to anthrax. Allied efforts in Canada, the

U.S. and Britain to develop anthrax-based weapons were also

active. Britain actually produced five million anthrax cakes at

the Porton Down facility, to be dropped on Germany to infect

the food chain.

In non-deliberate settings, humans acquire anthrax from

exposure to the natural reservoirs of the microorganism; live-

stock such as sheep or cattle or wild animals. Anthrax has been

acquired by workers engaged in shearing sheep, for example.

Human anthrax can occur in three major forms.

Cutaneous anthrax refers to the entry of the organism through

a cut in the skin. Gastrointestinal anthrax occurs when the

organism is ingested in food or water. Finally, inhalation

anthrax occurs when the organism is inhaled.

All three forms of the infection are serious, even lethal,

if not treated. With prompt treatment, the cutaneous form is

often cured. Gastrointestinal anthrax, however, can still be

lethal in 25–75% of people who contract it. Inhalation anthrax

is almost always fatal.

The inhalation form of anthrax can occur because of the

changing state of the organism. Bacillus anthraciscan live as

a large “vegetative” cell, which undergoes cycles of growth

and division. Or, the bacterium can wait out the nutritionally

bad times by forming a spore and becoming dormant. The

spore is designed to protect the genetic material of the bac-

terium during hibernation. When conditions are conducive for

growth and reproduction the spore resuscitates and active life

goes on again. The spore form can be easily inhaled. Only

8,000 spores, hardly enough to cover a snowflake, are suffi-

cient to cause the pulmonary disease when they resuscitate in

the warm and humid conditions deep within the lung.

The dangers of an airborne release of anthrax spores is

well known. British open-air testing of anthrax weapons in

1941 on Gruinard Island in Scotland rendered the island unin-

habitable for five decades. In 1979, an accidental release of a

minute quantity of anthrax spores occurred at a bioweapons

facility near the Russian city of Sverdlovsk. At least 77 people

were sickened and 66 died. All the affected were some four

kilometers downwind of the facility. Sheep and cattle up to 50

kilometers downwind became ill.

Three components of Bacillus anthracisare the cause

of anthrax. First, the bacterium can form a capsule around

itself. The capsule helps shield the bacterium from being rec-

ognized by the body’s immune systemas an invader, and

helps fend off antibodies and immune cells that do try to deal

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