Rickettsial and mycoplasma infections

Published on 19/03/2015 by admin

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Last modified 19/03/2015

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Rickettsial and mycoplasma infections

RICKETTSIAE

Although historically an important cause of human infection (epidemic typhus was responsible for over three million deaths in Russia between 1915 and 1925), rickettsiae are rarely encountered in neuropathological practice, even in regions where systemic rickettsial infection itself is comparatively common.

Rickettsiae are obligate intracellular microorganisms measuring 1–2 μm in length and 0.3 μm in diameter. Their appearance and staining characteristics are those of Gram-negative coccobacilli. They usually infect animals and insects, and only infrequently cause human disease.

image PATHOGENESIS OF RICKETTSIAL INFECTIONS

image Rickettsiae (apart from C. burnetii) are inoculated into the bloodstream by feeding ticks or mites, or when skin onto which infected flea or louse feces have been deposited is scratched.

image The rickettsiae are disseminated hematogenously. From the bloodstream they enter, proliferate within, and cause damage to vascular endothelial and smooth muscle cells.

image Most of the manifestations of human disease (including the spotted fever rash) are a consequence of vasculitis or endothelial involvement with variably severe inflammation (Fig. 14.1).

image Though vessel wall involvement is considered key to the pathogenesis of rickettsial infections, there is experimental evidence that they can also infect macrophages. Different isolates of even a single microorganism (e.g. R. rickettsii) may induce widely differing degrees of injury in cultured endothelial cells.

image C. burnetii is present in the urine, feces, milk, and placentas of infected sheep, cattle, and goats. Humans are infected by inhalation of the microorganisms in contaminated aerosols or dust. This results in lung and systemic infection but usually not vasculitis.

image EPIDEMIOLOGY OF RICKETTSIAL INFECTION

image HUMAN RICKETTSIAL DISEASES

image Human rickettsial diseases fall into the following four main groups:

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