Rickettsia

Rocky Mountain spotted fever is one of the most severe rickettsial diseases. First recognized in the Rocky Mountains, it has since been found to occur throughout the United States. The Centers for Disease Control report about 600-1,000 cases occurring annually, but this number may be underestimated due to underreporting. Rickettsia rickettsii are carried and transmitted by four species of the hard-shelled tick, all of which feed on humans, wild and domestic animals, and small rodents. When a tick feeds on an infected animal, the bacteria are transmitted to the tick, which can in turn infect other animals with its bite. Human-to-human transmission of R. rickettsii does not occur. Once inside the human bloodstream, the bacteria invade cells that line the small blood vessels.

The symptoms of Rocky Mountain spotted fever reflect the presence of bacteria inside blood vessel cells. Within two to 12 days of being bitten by an infected tick, the infected person experiences a severe headache, fever, and malaise. After about two to four days, a rash develops, first on the extremities, then the trunk. A characteristic sign of this disease is that the rash involves the soles of the feet and palms of the hands. If the disease is not treated with antibiotics, the infected blood vessel cells lyse, causing internal hemorrhage, blockage of the blood vessels, and eventual death of the cells. Shock, kidney failure, heart failure, and stroke may then occur. Rocky Mountain spotted fever is fatal if not treated.

A similar but milder disease is rickettsial pox, caused by R. akari. These bacteria are transmitted by mites which live preferentially on the common house mouse, only occasionally biting humans. Rickettsial pox is characterized by a rash that does not affect the palms or soles of the feet. The rash includes a lesion called an eschar-a sore that marks the spot of the infected mite bite. The mild course of this disease and the presence of the rash has sometimes led to its misdiagnosis as chicken pox, but the eschar clearly distinguishes rickettsial pox from chicken pox.

Outside of the United States, spotted fevers such as North Asian tick typhus, Queensland tick typhus, and boutonneuse fever are caused by other rickettsia species. As their names suggest, these diseases are found in Asia, Mongolia, and the Siberian region of Russia; in Australia; and in the Mediterranean region, Africa, and India, respectively. Symptoms of these spotted fevers resemble those of rickettsial pox. Although these spotted fevers share some of the symptoms of Rocky Mountain spotted fever, they are milder diseases and are usually not fatal.