Rickettsial Diseases

The rickettsial diseases are a group of related maladies with common characteristics such as arthropod vectors, obligate intracellular etiologic agents, and similar symptoms, including skin rashes, high fever, and headache. The prototype is classic, epidemic, louse-borne typhus fever. Most other rickettsial diseases were originally described as "typhus-like" and were differentiated from the classic disease during the twentieth century.

Those whose etiologic agents share the Rickettsia genus with the historic disease are murine, or flea-borne typhus, Rocky Mountain spotted fever and other members of the spotted fever group of diseases, and scrub typhus or tsutsugamushi. Two other diseases, Q fever and trench fever, are also known as rickettsial diseases. In recent decades, however, key differences in the clinical manifestations, in mode of transmission, and in the physiology of the etiologic agents of these two diseases have caused them to be placed in separate genera.

Pathological rickettsiae were discovered early in the twentieth century and named after Howard Taylor Ricketts, a University of Chicago investigator, who lost his life in research on typhus in Mexico after several years of fruitful research on Rocky Mountain spotted fever. Although smaller than most bacteria, rickettsiae are visible under the light microscope. Unlike common bacteria, they are obligate intracellular parasites - that is, they metabolize and multiply only inside living cells, a characteristic shared with the viruses. This peculiar combination of traits caused the rickettsiae to be classified for several decades as organisms midway between bacteria and viruses. By the late 1960s, however, research revealed that they were true, if highly fastidious, bacteria.

Most of the rickettsial maladies are "diseases of nature," normally existing as infections of arthropods (insects, ticks, and mites) and their mammalian hosts. Humans are accidental intruders into the natural cycle. Like bubonic plague and yellow fever, the manifestations of infection are often more severe in humans than in the arthropods and mammals to which the organisms have adapted over eons. The geographic distribution of the rickettsial diseases is linked to environments favorable to host arthropods. In addition, these diseases tend to be found in "islands of infection" within favorable environments. This phenomenon has been attributed to ecological conditions in the case of scrub typhus or tsutsugamushi and to antigenic incompatibility between pathogenic and nonpathogenic rickettsiae seeking to establish themselves in the ovaries of female ticks in Rocky Mountain spotted fever.

The common clinical manifestations of these diseases reflect their pathological physiology as infections of the human circulatory system. Their etiologic agents multiply inside endothelial cells lining small blood vessels. Affected cells become swollen and may impede blood flow. Electrolyte imbalance and capillary permeability establish a vicious circle that progresses to circulatory collapse in fatal cases. Blood seeping from the capillaries into the skin causes the typical rash, and capillary blockage in the brain contributes to the neurological symptoms. Since the introduction of broad-spectrum antibiotics in 1948, the rickettsial diseases have been curable, if diagnosed before the diseases have progressed too far.

Victoria A. Harden

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