World of Microbiology and Immunology

Typhus is a disease caused by a group of bacteria called 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 administration of antibiotics.

Epidemic typhus is a disease that has played an important role in history. Because typhus is transmitted by the human body louse, epidemics of 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 distinguishing 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 infection. 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 documented, 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. tsutsugamushi. 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 Rickettsia is an important step in prevention. Spraying with insecticides, rodent control measures, and treating soil with insect-repellent chemicals have all been used successfully to prevent typhus outbreaks. In scrub typhus islands, cutting down vegetation has been shown to lessen the incidence of scrub typhus. The second preventative prong is protecting the body from insect bites. Wearing heavy clothing when venturing into potentially insect-laden areas is one way to protect against insect bites; applying insect repellent to the skin is another. Proper personal hygiene, such as frequent bathing and changing of clothes, will eliminate human body lice and thus prevent epidemic typhus. A typhus vaccine is also available; however, this vaccine only lessens the severity and shortens the course of the disease, and does not protect against infection.

World of Microbiology and Immunology | Typhus