The intracellular pathogen Anaplasma marginale (Rickettsiales: Anaplasmataceae), described
by Sir Arnold Theiler in 1910, is endemic worldwide in tropical and subtropical areas.
Infection of cattle with A. marginale causes bovine anaplasmosis, a mild to severe
hemolytic disease that results in considerable economic loss to both dairy and beef
industries. Transmission of A. marginale to cattle occurs biologically by ticks and
mechanically by biting flies and by blood-contaminated fomites. Both male ticks and
cattle hosts become persistently infected with A. marginale and serve as reservoirs
of infection. While erythrocytes are the major site of infection in cattle, A. marginale
undergoes a complex developmental cycle in ticks that begins by infection of gut cells,
and transmission to susceptible hosts occurs from salivary glands during feeding.
Major surface proteins (MSPs) play a crucial role in the interaction of A. marginale
with host cells, and include adhesion proteins and MSPs from multigene families that
undergo antigenic change and selection in cattle, thus contributing to maintenance
of persistent infections. Many geographic strains of A. marginale have been identified
worldwide, which vary in genotype, antigenic composition, morphology and infectivity
for ticks. Isolates of A. marginale may be maintained by independent transmission
events and a mechanism of infection/exclusion in cattle and ticks. The increasing
numbers of A. marginale genotypes identified in some geographic regions most likely
resulted from intensive cattle movement. However, concurrent A. marginale strain infections
in cattle was reported, but these strains were more distantly related. Phylogenetic
studies of selected geographic isolates of A. marginale, using msp4 and msp1alpha,
provided information about the biogeography and evolution of A. marginale, and msp1alpha
genotypes appear to have evolved under positive selection pressure. Live and killed
vaccines have been used for control of anaplasmosis and both types of vaccines have
advantages and disadvantages. Vaccines have effectively prevented clinical anaplasmosis
in cattle but have failed to block A. marginale infection. Vaccines are needed that
can prevent clinical disease and, simultaneously, prevent infection in cattle and
ticks, thus eliminating these hosts as reservoirs of infection. Advances in genomics,
proteomics, immunology and biochemical and molecular technologies during the last
decade have been applied to research on A. marginale and related organisms, and the
recent development of a cell culture system for A. marginale has provided a format
for studying the pathogen/tick interface. Recent advancements and new research methodologies
should provide additional opportunities for development of new strategies for control
and prevention of bovine anaplasmosis.