Tuesday, May 31, 2011

The Rickettsial Diseases

This week I am introducing a short series on the rickettsial diseases. These are caused by proteobacteria pathogens in the order known as Rickettsiales, which includes both families from which the pathogenic genera are derived: the Rickettsiaceae and the Anaplasmataceae. Those genera that are of importantce for human disease are the Rickettsia, the Anaplasma, and the Ehrlichia. Collectively, the pathogenic species of all of these are often called rickettsias, despite the fact that not all the pathogenic bacteria are within the Rickettsia genus. I will refer to these collectively as rickettsial diseases rather than rickettsia or rickettsias.

For the most part, the rickettsial diseases are transmitted by arthropod vectors, including hard ticks, mites, fleas and lice, depending on which pathogen is being transmitted. Two of the diseases that will be covered in this series, Rocky Mountain spotted fever and ehrlichiosis, are transmitted to humans primarily by ticks, though aerosolization is possible in some circumstances. The other two diseases, epidemic and murine typhus, are spread by lice and fleas, respectively.

The rickettsial infections are often categorized into three groups: the typhus group and the spotted fever group, both of which are infections of Rickettsia proper, and the "Other", Rickettsia-like infections, group, which includes infections whose genera and species are in the Anaplasmataceae.

For the most part, rickettsial diseases are zoonotic, meaning that human infections are derived from animal reservoir populations. The exception here being epidemic typhus, for which humans are the reservoir.

Another critical aspect of the rickettsial diseases is the pathogen biology in relation to the host. The Rickettsiales bacteria are obligate intracellular organisms. They cannot reproduce outside the host cell. Interestingly, our own mitochondria (and those of almost all eukaryotic cellular organisms) are thought to have once been bacteria of this order.

The diseases we will cover in this series are as follows:

1. Epidemic typhus, which is caused by Rickettsia prowazekii:

2, Murine typhus, which is caused by Rickettsia typhi:

3. Ehrlichiosis, which is caused by Ehrlichia chaffeensis and E. ewingii:

Ehrlichia chaffeensis

4. Rocky Mountain spotted fever, which is caused by Rickettsia rickettsii:


  1. What is the scientific evidence that would link the Rickettsiales bacteria to the mitochondria besides it being an intracellular obligate? Does this bacteria have biological functions that would hint to it having ancestry of forming a similar symbiotic relationship with the host cell?

  2. So it looks like the link was clarified by genetic sequencing. Mitochondria have their own genome, unique from the rest of our DNA, that we inherit from our mothers. The resulting greatest commonality is that ATP production in Rickettsia is the same as that in mitochondria, uniquely from the rest of our cells and other organelles.

    Hope this helps!


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  4. I am highly interested in the evolution and development of the eukaryotic cell. Although I am familiar with the architectural arguments (i.e. the composition of mitochondria, etc.) for the origin of the mitochondria as a bacteria that was, at one point, engulfed by larger organisms, I was not aware that the engulfed bacteria is thought to have been Rickettsiales bacteria.

    Interestingly, Rocky Mountain Spotted Fever is the most common rickettsial disease in the United States (400-700 cases per years, see: http://pathmicro.med.sc.edu/mayer/ricketsia.htm). Although this disesae was first discovered in Rocky Mountain states, it is most common in southern states, such as South Carolina. This regional diversity is obviously a testament to the potential for seeing a specific disease in multiple landscapes.


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