Microbiology and Immunology

Typhus WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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Chloramphenicol is the most effective drug treatment

for S. typhi, and symptoms begin to improve slightly after only

24–48 hours of receiving the medication. Another drug, ceftri-

axone, has been used recently, and is extremely effective, low-

ering fever fairly quickly.

Carriers of S. typhimust be treated even when asymp-

tomatic, as they are responsible for the majority of new cases

of typhoid fever. Eliminating the carrier state is actually a dif-

ficult task, and requires treatment with one or even two differ-

ent medications for four to six weeks. In the case of a carrier

with gall stones, surgery may need to be performed to remove

the gall bladder, because the S. typhibacteria are often housed

in the gall bladder, where they may survive despite antibiotic

treatment.

Hygienic sewage disposal systems in a community, as

well as hygienic personal practices, are the most important

factors in preventing typhoid fever. For travelers who expect

to go to countries where S. typhiis a known public health

problem, immunizations are available. Some of these immu-

nizations provide only short-term protection (for a few

months), while others may be protective for several years.

Immunizations that provide a longer period of protection, with

fewer side effects from the vaccineitself, are being developed.

See alsoAntibody formation and kinetics; Bacteria and

bacterial infection; Immunity, active, passive and delayed;

Immunochemistry; Immunogenetics; Immunology; Vacci-

nation

TTyphusYPHUS

Typhus is a disease caused by a group of bacteriacalled

Rickettsia. Three forms of typhus are recognized: epidemic

typhus, a serious disease that is fatal if not treated promptly;

rat-flea or endemic typhus, a milder form of the disease; and

scrub typhus, another fatal form. The Rickettsia species of

bacteria that cause all three forms of typhus are transmitted by

insects. The bacteria that cause epidemic typhus, for instance,

are transmitted by the human body louse; the bacteria that

cause endemic typhus are transmitted by the Oriental rat flea;

and bacteria causing scrub typhus are transmitted by chiggers.

Typhus takes its name from the Greek word typhos

meaning smoke, a description of the mental state of infected

persons. Typhus is marked by a severe stupor and delirium, as

well as headache, chills, and fever. A rash appears within four

to seven days after the onset of the disease. The rash starts on

the trunk and spreads to the extremities. In milder forms of

typhus, such as endemic typhus, the disease symptoms are not

severe. In epidemic and scrub typhus, however, the symptoms

are extreme, and death can result from complications such as

stroke, renal failure, and circulatory disturbances. Fatality can

be avoided in these forms of typhus with the prompt adminis-

tration of antibiotics.

Epidemic typhus is a disease that has played an impor-

tant role in history. Because typhus is transmitted by the

human body louse, epidemicsof this disease break out when

humans are in close contact with each other under conditions

in which the same clothing is worn for long periods of time.

Cold climates also favor typhus epidemics, as people will be

more likely to wear heavy clothing in colder conditions.

Typhus seems to be a disease of war, poverty, and famine. In

fact, according to one researcher, Napoleon’s retreat from

Moscow in the early nineteenth century was beset by typhus.

During World War I, more than three million Russians died of

typhus, and during the Vietnam war, sporadic epidemics killed

American soldiers.

Epidemic typhus is caused by Rickettsia prowazekii.

Humans play a role in the life cycle of the bacteria. Lice

become infected with the bacteria by biting an infected

human; these infected lice then bite other humans. A distin-

guishing feature of typhus disease transmission is that the

louse bite itself does not transmit the bacteria. The feces of the

lice are infected with bacteria; when a person scratches a louse

bite, the lice feces that have been deposited on the skin are

introduced into the bloodstream.

If not treated promptly, typhus is fatal. Interestingly, a

person who has had epidemic typhus can experience a relapse

of the disease years after they have been cured of their infec-

tion. Called Brill-Zinsser disease, after the researchers who

discovered it, the relapse is usually a milder form of typhus,

which is treated with antibiotics. However, a person with

Brill-Zinsser disease can infect lice, which can in turn infect

other humans. Controlling Brill-Zinsser relapses is important

in stopping epidemics of typhus before they start, especially in

areas where lice infestation is prominent.

Endemic typhus is caused by R. typhi. These bacteria

are transmitted by the Oriental rat flea, an insect that lives on

small rodents. Endemic typhus (sometimes called murine

typhus or rat-flea typhus) is found worldwide. The symptoms

of endemic typhus are mild compared to those of epidemic

typhus. In fact, many people do not seek treatment for their

symptoms, as the rash that accompanies the disease may be

short-lived. Deaths from endemic typhus have been docu-

mented, however; these deaths usually occur in the elderly and

in people who are already sick with other diseases.

Scrub typhus is caused by R. tsutsugamushi, which is

transmitted by chiggers. The term “scrub typhus” comes from

the observation that the disease is found in habitats with scrub

vegetation, but the name is somewhat of a misnomer. Scrub

typhus is found in beach areas, savannas, tropical rains forests,

deserts, or anywhere chiggers live. Scientists studying scrub

typhus label a habitat that contains all the elements that might

prompt an outbreak of the disease a “scrub typhus island.” A

scrub typhus island contains chiggers, rats, vegetation that will

sustain the chiggers, and, of course, a reservoir of R. tsutsug-

amushi. Scrub typhus islands are common in the geographic

area that includes Australia, Japan, Korea, India, and Vietnam.

The rash that occurs in scrub typhus sometimes includes

a lesion called an eschar. An eschar is a sore that develops

around the chigger bite. Scrub typhus symptoms of fever, rash,

and chills may evolve into stupor, pneumonia, and circulatory

failure if antibiotic treatment is not administered. Scrub

typhus, like epidemic typhus, is fatal if not treated.

Prevention of typhus outbreaks takes a two-pronged

approach. Eliminating the carriers and reservoirs of

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