Infection of Reservoir and Incidental Host

Bartonella species are usually inoculated through feeding of the arthropod vector on the mammal host. In the reservoir host, Bartonella species occupy a unique niche in the blood [27, 28] where they invade and persistently colonize mature erythrocytes. The intraerythrocyte habitat is thought to protect the Bartonella organisms from the host immune response, and also guarantees transmission by the vector. Experiments with animal models have shown that before the invasion of erythrocytes, the bacteria colonize a yet unknown primary niche [29], where they replicate during the first days of infection. There is some evidence suggesting that endothelial cells represent the primary niche [30]. Approximately 5 days after infection the bacteria reappear in the bloodstream and adhere to and invade ery-throcytes, where they remain during the lifespan of the erythrocyte [30]. Bacterial infections in the blood are normally associated with severe disease, but Bartonella species infection in the reservoir host is in most cases asymptomatic.

B. henselae infections in humans may cause cat-scratch disease, typically manifested as regional swollen lymph nodes. The disease is spread from cats to humans indirectly by the cat flea or directly by a cat bite or scratch. In the United States, B. henselae causes 22 000 diagnosed cases of cat-scratch disease each year. Bartonella species that incidentally infect humans, like B. henselae, do not colonize erythrocytes but vascular endothelial cells. Studies of Bartonella infection of cultured human umbilical vein endothelial cells (HUVEC) have revealed two distinct routes of entry, both induced by the bacterium. The classical route of uptake is reminiscent of bacterium-directed phagocytosis. In B.bacilliformis, uptake is triggered by key regulators of actin reorganization [31] . B. henselae uses an additional uptake route, where aggregates are formed on the cell surface and are subsequently engulfed [32]. Both processes are dependent on actin rearrangements.

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