Rickettsia rickettsii is unique to the Western Hemisphere. Although the organism was first detected in the Rocky Mountain states, it is found in all of the United States except Maine, Hawaii, and Alaska. It is especially common on the Atlantic seaboard. The disease occurs mainly from May to September, when adult ticks are active and people are likely to be in tick-infested areas. In the southern states, cases occur throughout the year.
Infected ticks transmit the rickettsiae to rabbits, squirrels, deer, bears, dogs, and humans.
The disease is not transmitted directly from person to person. The rickettsiae live and multiply in the cells lining blood vessels. Commonly infected are blood vessels in and under the skin and those in the brain, lungs, heart, kidneys, liver, and spleen. The blood vessels may become blocked by blood clots. Symptoms Symptoms begin suddenly 3 to 12 days after a tick bite.