Rocky Mountain spotted fever (RMSF) is caused by a bacterium called *Rickettsia rickettsii*. This bacterium is transmitted to humans primarily through the bite of infected ticks. The ticks act as carriers, or vectors, of the bacteria, passing it on when they feed on human blood. The main tick species responsible for spreading RMSF include the American dog tick, the Rocky Mountain wood tick, and the brown dog tick. These ticks are commonly found in grassy, brushy, or wooded areas where they wait to latch onto a passing host.
The process begins when an infected tick bites a person, usually attaching itself firmly to the skin to feed. During this feeding, *Rickettsia rickettsii* bacteria enter the bloodstream through the tick’s saliva. Once inside the body, the bacteria invade the cells that line the small blood vessels, called endothelial cells. This invasion causes inflammation and damage to the blood vessels, which leads to the characteristic symptoms of RMSF, such as fever, headache, muscle pain, and a distinctive spotted rash.
Ticks become infected with *Rickettsia rickettsii* by feeding on small mammals or birds that carry the bacteria. The bacteria can then be passed from one tick generation to the next through their eggs, allowing the infection to persist in tick populations over time. This means that even young ticks, called larvae or nymphs, can transmit the disease if they are infected.
The risk of contracting RMSF is highest during the warmer months, typically from spring through early fall, when ticks are most active. People who spend time outdoors in tick-infested areas—such as campers, hikers, hunters, or those working in fields or forests—are more likely to encounter infected ticks. The bacteria do not spread directly from person to person; transmission requires a tick bite.
In addition to *Rickettsia rickettsii*, other related bacteria in the same family can cause similar illnesses, but RMSF is the most severe and common rickettsial disease in the United States. The bacteria’s ability to infect the lining of blood vessels throughout the body explains why the disease can affect multiple organs and lead to serious complications if not treated promptly.
Preventing RMSF involves avoiding tick bites by wearing protective clothing, using tick repellents, performing thorough tick checks after outdoor activities, and promptly removing any attached ticks. Understanding that the root cause is the bite of an infected tick carrying *Rickettsia rickettsii* helps guide both prevention and treatment efforts to reduce the impact of this potentially deadly disease.
