Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Sleeping sickness

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mata, which may increase the risk for cervical or penile can-

cer. Many methods are used in attempts to remove warts, with

varying degrees of success. These include cryotherapy, antivi-

ral agents, application of salicylic acid, surgical removal, and

laser treatment.

Skin infections caused by fungi, including yeast, are

called dermatomycoses. A common subcategory consists of

the dermatophytoses, caused by Trichophytonspecies. These

infections include tinea capitis (“cradle cap”), tinea corporis

(“ringworm”), tinea cruris (“jock itch”), and tinea pedis (“ath-

lete’s foot”). Candida,which often affects the mucous mem-

branes, may also be responsible for skin infections. Obese

patients are prone to fungal infections in skin folds, as are

uncircumcised men. Candida is also involved in some cases of

diaper rash. Fungal infections are typically treated with topi-

cal imidazole creams or sprays.

See also Bacteria and bacterial infection; Candidiasis;

Infection and resistance; Viruses and responses to viral infec-

tion; Yeast, infectious

SSleeping sicknessLEEPING SICKNESS

Sleeping sickness (trypanosomiasis) is a protozoan infection

passed to humans through the bite of the tsetse fly. It pro-

gresses to death within months or years if left untreated. Near-

control of trypanosomiasis was achieved in the 1960s, but the

disease has since re-emerged in Sub-Saharan Africa, where

political instability and war have hampered public health

efforts. As of 2002, the World Health Organization, in con-

junction with Médicines Sans Frontièrs (Doctors Without

Borders) and major pharmaceutical companies were in the

midst of a five-year major effort to halt the spread of try-

panosomiasis and treat its victims.

Protozoaare single-celled organisms considered to be

the simplest animal life form. The protozoa responsible for

sleeping sickness are a flagellated variety (flagella are hair-

like projections from the cell which aid in mobility) which

exist only in Africa. The type of protozoa causing sleeping

sickness in humans is referred to as the Trypanosoma brucei

complex. It is divided further into Rhodesian (Central and East

Africa) and Gambian (Central and West Africa) subspecies.

The Rhodesian variety live within antelopes in savanna

and woodland areas, causing no disruption to the antelope’s

health. (While the protozoa cause no illness in antelopes, they

are lethal to cattle who may become infected.) The protozoa

are acquired by tsetse flies who bite and suck the blood of an

infected antelope or cow. Within the tsetse fly, the protozoa

cycle through several different life forms, ultimately migrating

to the salivary glands of the tsetse fly. Once the protozoa are

harbored in the salivary glands, they can be deposited into the

bloodstream of the fly’s next blood meal.

Humans most likely to become infected by Rhodesian

trypanosomes are game wardens or visitors to game parks in

East Africa. The Rhodesian variety of sleeping sickness causes

a much more severe illness with a greater likelihood of even-

tual death. The Gambian variety of Trypanosoma thrives in

tropical rain forests throughout Central and West Africa, does

not infect game or cattle, and is primarily a threat to people

dwelling in such areas. It rarely infects visitors.

The first sign of sleeping sickness may be a sore appear-

ing at the tsetse fly bite spot about two to three days after hav-

ing been bitten. Redness, pain, and swelling occur. Two to

three weeks later, Stage I disease develops as a result of the

protozoa being carried through the blood and lymphatic circu-

lations. This systemic (meaning that symptoms affect the

whole body) phase of the illness is characterized by a high

fever that falls to normal then re-spikes. A rash with intense

itching may be present, and headache and mental confusion

may occur. The Gambian form includes extreme swelling of

lymph tissue, enlargement of the spleen and liver, and swollen

lymph nodes. Winterbottom’s sign is classic of Gambian

sleeping sickness; it consists of a visibly swollen area of

lymph nodes located behind the ear and just above the base of

the neck. During this stage, the heart may be affected by a

severe inflammatory reaction, particularly when the infection

is caused by the Rhodesian form.

Many of the symptoms of sleeping sickness are actually

the result of attempts by the patient’s immune systemto get rid

of the invading organism. The overly exuberant cells of the

immune system damage the patient’s organs, causing anemia

and leaky blood vessels. These leaky blood vessels help to

spread the protozoa throughout the patient’s body.

One reason for the immune system’s intense reaction to

the Trypanosomes is also the reason why the Trypanosomes

survive so effectively. The protozoa are able to change rapidly

specific markers on their outer coats. These kinds of markers

usually stimulate the host’s immune system to produce

immune cells specifically to target the markers and allow

quick destruction of these invading cells. Trypanosomes are

able to express new markers at such a high rate of change that

the host’s immune system cannot catch up.

Stage II sleeping sickness involves the nervous system.

The Gambian strain has a clearly delineated phase in which

the predominant symptomatology involves the brain. The

patient’s speech becomes slurred, mental processes slow, and

he or she sits and stares or sleeps for long periods of time.

Skin infection caused by tinea.

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