Phylogeographic Analysis of Tick-borne Pathogen - Recent Advances and Issues in Environmental Science - page 77

vacuoles in the cytoplasm of both vertebrate and invertebrate host cells [1, 2]. The genus
Anaplasma includes pathogens of ruminants, A. marginale, A. centrale, A. bovis , and A.
ovis . Also included in this genus are A. phagocytophilum , which infects a wide range of
hosts including humans and wild and domesticated animals, and A. platys that infects dogs.
To date, most research has been reported for A. marginale , the type species for
the genus Anaplasma [3]. Both cattle and ticks develop persistent infections with A.
marginale and therefore can serve as reservoirs of infection. A. marginale is transmit-
ted horizontally by ixodid ticks including Rhipicephalus spp . and Demacentor spp.
Rhipicephalus (Boophilus) microplus is considered the most important biological vec-
tor in tropical and subtropical regions of the world [4]. Transfer of infected blood
by biting fl ies or blood-contaminated fomites effects mechanical transmission of A.
marginale . The complex developmental cycle of A. marginale has been described and
shown to be coordinated with the tick feeding cycle [3]. The midgut is the fi rst site of
infection, where membrane-bound vacuoles or colonies initially contain reticulated
forms that divide by binary fi ssion and subsequently transform into dense forms. In-
fection of salivary glands and other tissues then occurs which completes the devel-
opmental cycle and allows for transmission to susceptible hosts during tick feeding.
Vector-pathogen interactions involve traits from both the vector and the pathogen
[5]. Several MSPs have been identifi ed and characterized in A. marginale [3, 5]. The
MSPs are involved in interactions with both vertebrate and invertebrate hosts [2, 3,
6-9], and therefore are likely to evolve more rapidly than other genes because they
are subjected to selective pressures exerted by host immune systems. The MSP1a of
A. marginale geographic strains differs in molecular weight due to a variable number
of tandem 23-31 amino acid repeats, and the sequence of MSP1a has been shown
to be a stable marker for identifi cation of geographic strains [10, 11]. Functionally,
MSP1a was shown to be an adhesin for bovine erythrocytes and tick cells [12-14].
Tick molecules involved in vector- A. marginale interactions were recently identifi ed
and functionally characterized [15].
The geographic strains of A. marginale are highly variable, as demonstrated by
the analysis of MSP1a sequences [7, 11]. Such genetic heterogeneity observed among
A. marginale strains in endemic regions could be explained by cattle movement and
maintenance of different genotypes by independent transmission events, due to in-
fection exclusion of A. marginale in cattle and ticks which commonly results in the
establishment of only one genotype per animal [16-18]. Due to the high degree of se-
quence variation within most endemic areas, MSP1a sequences have failed to provide
phylogeographic information on a global scale [7]. These studies also suggested that
multiple introductions of A. marginale strains from different geographic locations had
occurred in many regions.
The evolutionary history of vector-pathogen interactions can be refl ected in the
sequence variation of Anaplasma MSPs. Previous studies demonstrated that A. marginale
MSP1a evolved under positive selection [19]. Analysis of A. marginale MSP1a re-
peats provided evidence of tick-pathogen co-evolution [5, 6, 20], a result that is con-
sistent with the biological function of MSP1a in pathogen transmission by ticks [14].
However, the study of A. marginale evolutionary history and tick-pathogen co-evolution
has remained elusive because of the extensive genetic diversity of MSP1a sequences.