What are some other U.S.-associated tickborne spotted fevers?
In addition to Rickettsia rickettsia, the agent of Rocky Mountain spotted fever, there are other similar agents transmitted by ticks that have been shown to cause human infections. In the United States, these include Rickettsia parkeri, which is transmitted by Amblyomma maculatum and causes a maculatum infection (a relatively mild, eschar-associated illness). There are also other genetically similar Rickettsia that are thought to cause occasional human illness, including one called 364D (Rickettsia phillipi, proposed). Because these agents appear to exhibit serologic cross-reactivity with available tests for R. rickettsii, it is likely that some human illnesses currently being attributed to RMSF are actually caused by these distinct rickettsial species, and more research is needed to determine their prevalence and geographic distribution.