Typhus (cont.)

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What causes typhus? How is typhus transmitted?

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The causes of typhus are small Gram-negative coccobacilli-shaped bacteria, members of the genus Rickettsia that are intracellular parasites of many animals and utilize the components within the cell to survive and multiply. They are difficult to cultivate because they usually only grow within cells they infect. Occasionally, the bacteria may become dormant in infected cells, and years later, again begin to multiply (causing Brill-Zinsser disease). Generally, typhus follows an animal (rat, mouse) to vector (louse, flea) cycle. Humans are incidentally infected usually when the vectors come in close proximity to humans. The two Rickettsia spp. responsible for the two main types of typhus are R. prowazekii, the cause of epidemic typhus, and R. typhi, the cause of endemic typhus. However, R. felis, another species usually found in cat and cat fleas, has been linked to people with endemic typhus also. Epidemic typhus is usually spread or transmitted to humans from body lice (Figure 1) feces contaminated with R. prowazekii or occasionally from animal droppings contaminated with these bacteria. Endemic typhus is usually transmitted to humans by flea feces or animal droppings containing R. typhi or R. felis. The flea or louse bite causes itching and scratching and may allow the bacteria to enter the scratch or bite area in the skin. Indirect person-to-person transmission can occur if lice or fleas infect one person who develops the disease and then the infected lice or fleas move from person to person by direct contact or via shared clothing. In general, head lice that differ from body lice do not transmit Rickettsia.

R. prowazekii, R. typhi, and R. felis differ from other Rickettsia spp. For example, R. rickettsii and many other R. spp. are considered in the medical literature as a separate group of bacteria and are transmitted by ticks, cause Rocky Mountain spotted fever (RMSF), and preferentially infect and spread through endothelial cells after tick bites.

Photo of a body louse and larvae
Fig. 1: Photo of a body louse and larvae; SOURCE: World Health Organization

Orientia tsutsugamushi, a bacterial species originally named Rickettsia tsutsugamushi, adds complexity to the typhus terminology because the disease it causes is termed scrub typhus. This change in name of the bacteria occurred because the bacteria were found to be genetically distinct enough to be termed a separate genus named Orientia. Also, scrub typhus is transmitted, in general, by a different vector: mites or "chiggers." Scrub typhus is found mainly in Asia and Australia. Many investigators consider scrub typhus as a different disease, in terms of the bacterial agent, vector, and localization, that is only remotely related to the two major types of typhus seen worldwide (endemic and epidemic typhus). For additional details about scrub typhus, we refer the reader to the last reference in the additional information section.

There are two other aspects readers may discover about these interesting Rickettsia bacteria. First, recent research has implicated that intracellular structures that produce energy for all animal cells, termed mitochondria, arose from primitive ancestors of Rickettsia bacteria. Genetic studies show that the many DNA sequences in Rickettsia bacteria are more closely related to the DNA sequences found in mitochondria than to DNA found in other bacterial genera. The other aspect is disturbing because Rickettsia (mainly R. prowazekii) have been studied and found to be possible agents for biological warfare since the bacteria can artificially be spread by aerosol. However, highly skilled personnel and technical expertise is required to develop these organisms into a weapon and, because the diseases caused by these organisms are treatable with antibiotics, some experts suggest that the organisms will not be developed beyond lab experimentation in most countries.


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