Etiology

• Bartonella spp are fastidious hemotropic gram negative bacteria that are vector transmitted, and, through adaptation to their host, may cause chronic bacteremia without disease.
• Cats may be infected with B. henselae, B. clarridgeiae, B. koehlerae or B. bovis. Serologic evidence shows that pet, feral and wild cats living in temperate climates worldwide may be infected with these organisms with B. henselae being the most common species cats are exposed to. 
• Studies in the United States have shown serologic prevalence rates of B. henselae between 5 and 40% with the higher exposure rates being in the warmer, more humid regions. In the United States, France and the Philippines, B. clarridgeiae seropositivity ranges between 10 to 31%. While cats are the main reservoir for these organisms, fleas (Ctenocephalides felis felis), ticks (Ixodes ricinus) and biting flies have also been found to be infected with Bartonella spp. 
• Cat fleas, transmit B. henselae between cats making flea control the most important method for the prevention of the possibility of zoonotic disease. 

Specific

• Spreads from cat to cat via cat flea.
• Requires cat-induced trauma for transmission to man.
• No evidence of transmission from cat to man via fleas.

Pathophysiology

• Cats usually asymptomatic.
• Bacteremia in cats is typically chronic with a waxing and waning nature. Bacteremia is undetectable by culture or PCR testing intermittently and IL-4 levels as well as serum antibody titers decrease or disappear for periods of time. Naturally infected cats have been shown to remain infected for up to three years; reinfection by fleas of an individual cat may occur even with different strains of the same species. This is likely due to the protective intracellular location of the parasite making it difficult for the immune system to detect and eliminate. The organisms may be found inside erythrocytes or vascular endothelial cells.