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Organisms Borne by Ticks and Other Vectors 241

are classified as being of mononuclear and granulocytic
series and are produced in bone marrow.
Five types of white cells circulate in the vascular
systems of humans and of some other animals, although
the percentages of each of the types may vary among the
various animal species. Two types of white blood cells
are required for the Ehrlichia organism to survive, one of
which is a mononuclear cell called a lymphocyte and the
other is a segmented neutrophil (a granulocyte) which
are the most prevalent types of white blood cells useful in
fighting viral and bacterial infections, respectively. The dis-
ease was first diagnosed in 1987 and is now considered an
emerging disease that has the potential for perhaps explo-
sive growth in the numbers of cases. Ehrlichiosis is found
in the majority of the states within the United States.
Ehrlichiosis is characteristic of many other signifi-
cant parasitic and bloodborne bacterial infections as it is
a tick-borne bacterial infection caused by bacterial strains
of Anaplasmataceae. This is a family of bacteria in the
order Rickettsiales, which includes the genera Ehrlichia,
Anaplasma, Wolbachia, and Neorickettsia. The genera Source: Centers for Disease Control and Prevention (CDC)
of the causative bacterial organisms for ehrlichiosis in-
clude both Ehrlichia and Anaplasma (formerly known as
E. phagocytophila). The disease is known to be transmit-
ted by the Ixodes genus of tick vectors that include the
western blacklegged tick called I. pacificus for the strain
of bacteria called Anaplasma phagocytophila and by the
lone star tick, Amblyomma americanum, for the strain of FIGURE 11-16 Obvious white dot, or lone star,
the disease called Ehrichia chaffeensis (Figure 11-16). identifies this organism as an adult female of the
These bacteria are called obligate intracellular bacteria, species Amblyomma americanum
and are capable of infecting and killing the two types of
white blood cells (mononuclear lymphocytes and neu-
trophils), as previously listed.
Five species have been shown to cause human infec-
tion, including the three discussed here. Anaplasma phago-
cytophilum causes human granulocytic anaplasmosis and
was formerly known as human granulocytic ehrlichiosis
(HGE). Ehrlichia ewingii causes a human disease known
as ewingii ehrlichiosis, and E. chafeensis is responsible for Source: Centers for Disease Control and Prevention (CDC)
an infection called human monocytic ehrlichiosis (HME)
(Figure 11-17). E. canus and Neorickettsia sennetsu are
not as well studied, and in addition, the latter is found in
Japan. Recently, human infections have been traced to the
newly discovered Panoloa Mountain Ehrlichia species,
but still little is known of this species. Ehrlichiosis infec-
tions have been found to also affect dogs but to a lesser FIGURE 11-17 Human monocytic ehrlichiosis and
extent, apparently, than for humans. human granulocytic ehrlichiosis