Transmitted trans-stadially (stage to stage) and trans-ovarially in ticks, as well as horizontally through vertebrate hosts
Trans-ovarial transmission uncommon for D. variabilis
Only the adult ticks feed on humans, during prolonged feeding of 1 to 2 weeks
Infection requires a minimum of 8 hours of tick attachment, though can theoretically be as quick as 10 minutes
Can also be infected during tick removal, when it is crushed between the fingers
As few as a single bacillus can cause disease
Pathophysiology
From cutaneous inoculation, bacilli spread via lymphatics and small blood vessels to the larger blood vessels
There, the OmpA, OmpB, Sca1, and Sca2 proteins induce phagocytosis by vascular endothelial cells
From there, they replicate and spread to adjacent cells
Epidemiology
Wide global distribution
Most cases in the US are in the south Atlantic and south-central regions
Highly endemic in North Carolina
Also occurs in Argentina, Brazil, Colombia, Panama, Costa Rica, and Mexico
Most cases occur in late spring and summer
Higher with occupational tick exposures
Other spotted fever species include R. conorii (Europe, Africa, and South Asia), R. sibirica (eastern Russia and Asia), R. africae (sub-Saharan African and West Indies), R. parkeri (North and South America), and R. slovaca (Europe), as well as R. felis (worldwide)