Biomedical Engineering Reference
In-Depth Information
states. Ehrlichia canis is a disease of dogs, with sporadic human cases occur-
ring in the Southeast United States.
Neorickettsia contains one pathogenic species, Neorickettsia sennetsu,
formerly Ehrlichia sennetsu, the cause of Sennetsu ehrlichiosis. The disease is
said to mimic a mild case of infectious mononucleosis; both diseases produce
a mononucleosis and generalized systemic symptoms. Unique among the
Ehrlichioses, Neorickettsia sennetsu is transmitted by trematodes, harbored
within fish, and eaten undercooked or uncooked, by humans. Readers should
be aware that Neorickettsia, despite its name, is not a type of Rickettsia (i.e.,
not a member of Class Rickettsiaceae). Neorickettsia is a member of Class
Anaplasmataceae; hence, the disease it causes is an ehrlichiosis.
Wolbachia contains one, somewhat indirect, human pathogen: Wolbachia
pipientis.
Wolbachia pipientis happens to be an endosymbiont that infects most
members of the filarial Class Onchocercidae [27]. Onchocerca volvulus is the
filarial nematode that migrates to the eyes and causes river blindness, the sec-
ond most common infectious cause of blindness worldwide [28]. Wolobachia
pipientis lives within Onchocerca volvulus, and it is the Wolbachia organism
that is responsible for the inflammatory reaction that leads to blindness.
Members of Class Rickettsiaceae are obligate intracellular organisms.
Diseases are caused by species in Genus Rickettsia, transmitted by insects,
from a reservoir in animals or other humans. The various species of patho-
genic rickettsia are split into two groups: the typhus group and the spotted
fever group. In the typhus group, disease is louse-borne or flea-borne. In the
spotted fever group, disease is usually tick-borne, though some species are
transmitted by mites or fleas.
The typhus group contains two pathogenic organisms: Rickettsia typhi,the
cause of murine typhus or endemic typhus, which is transmitted by fleas that feed
on infected rats; and Rickettsia prowazekii, so-called epidemic typhus or
Brill
Zinsser disease and caused by the human body louse (Pediculus humanus),
feeding on infected humans. Typhus fever of either type can occur worldwide.
Like so many of the diseases caused by members of Class Alpha
Proteobacteria, symptoms are systemic. Typhus is characterized by a high
fever. Endemic typhus is usually a milder disease than epidemic typhus.
Between 1918 and 1922, epidemic, louse-borne, typhus (Rickettsia prowazekii)
infected 30 million people, in Eastern Europe and Russia, accounting for about
3 million deaths [29].
The second group of infections, the so-called spotted fever group, is
caused by many different rickettsial species. The diseases can be divided
into groups based on the transmission vector (i.e., tick, mite, or flea).
Tick-borne:
Rickettsia rickettsii (Rocky Mountain spotted fever, found in Western
continents, and not confined to the Rocky Mountains)
