Rickettsia parkeri
Rickettsia parkeri (abbreviated "R. parkeri") is a gram-negative intracellular bacterium. The organism is found in the Western Hemisphere and is transmitted via the bite of hard ticks of the genus "Amblyomma". "R. parkeri" causes mild spotted fever disease in humans, whose most common signs and symptoms are fever, an eschar at the site of tick attachment, rash, headache, and muscle aches. Doxycycline is the most common drug used to reduce the symptoms associated with disease. Biology. "R. parkeri" is classified in the spotted fever group of the genus "Rickettsia". Genetically, its close relatives include "R. africae", "R.  sibirica", "R. conorii", "R. rickettsii", "R. peacockii", and "R. honei". The organism has been isolated from numerous species of ticks in the genus "Amblyomma": "A. americanum" in the United States; "A. aureolatum" in Brazil; "A. maculatum" in Mexico, Peru, and the United States; "A. nodosum" in Brazil; "A. ovale" in Brazil and Mexico; "A. parvitarsum" in Argentina and Chile; "A. tigrinum" in Argentina, Bolivia, Brazil, and Uruguay; and "A. triste" in Argentina, Brazil, the United States, and Uruguay. Different ticks may carry different strains of the organism. "R. parkeri" sensu stricto ("in the strict sense") is found in "A. maculatum  and A. triste"; "R. parkeri" strain NOD, in "A. nodosum"; "R. parkeri" strain Parvitarsum, in "A. parvitarsum"; and "R. parkeri" strain Atlantic rainforest, in "A. aureolatum" and "A. ovale". Human infections. The first report of a confirmed human case of infection with "R. parkeri" was published in 2004. The person was infected in the state of Virginia in the United States. Other confirmed or probable human cases have been reported to have acquired infection elsewhere in the United States (e.g., Arizona, Georgia, and Mississippi), as well as in Argentina, Brazil, Colombia, Mexico, and Uruguay. Terms used to describe human infection  with "R. parkeri" include "American boutonneuse fever" because of its similarity to boutonneuse fever caused by "Rickettsia conorii"; "American tick bite fever" because of its similarity to African tick bite fever caused by "Rickettsia africae"; "Tidewater spotted fever," after the Tidewater region in the eastern United States; and ""Rickettsia parkeri" rickettsiosis" or ""R. parkeri" rickettsiosis." Epidemiology. Of all human cases documented in the medical literature, 87% were 18-64 years of age, and most cases were male. Brazil, Argentina, and the United States accounted for the majority of cases in the medical literature. In the United States, most of the  40 cases reported to the Centers for Disease Control and Prevention (CDC) as of 2016 became infected between the months of July and September. Diagnosis. The CDC recommends polymerase chain reaction (PCR) of a biopsy or swab of an eschar, or PCR of a biopsy of a rash, for diagnosis of "R. parkeri" infection. In addition, indirect immunofluorescence antibody (IFA) assays using paired acute and convalescent sera can be used. Clinical manifestations. A 2008 study compared 12 "R. parkeri" cases with 208 Rocky Mountain spotted fever cases caused by "R. rickettsii". Although both "R. parkeri" and "R. rickettsii" caused fever , rash, myalgia, and headache, "R. parkeri" caused eschars and "R. rickettsii" did not. Furthermore, the percentage of patients hospitalized was lower for "R. parkeri" than for "R. rickettsii" (33% vs 78%), and "R. parkeri" led to no deaths while "R. rickettsii" led to death in 7% of cases. A 2021 systematic review of 32 confirmed and 45 probable cases of human infection with "R. parkeri" determined that 94% of the confirmed cases had fever, 91% an eschar, 72% a rash, 56% headache, and 56% myalgia, with similar percentages among the probable cases. The rash was most frequently described as papular  or macular. Among the confirmed and probable cases, the most common treatment was doxycycline, followed by tetracycline. Although 9% of all the cases were hospitalized, there was a "100% rate of clinical recovery." History. In 1939, Ralph R. Parker, director of the Rocky Mountain Laboratory, and others published a paper on "a rickettsia-like infectious agent." The agent, found in "Amblyomma maculatum" ticks collected from cows in Texas, produced mild disease in guinea pigs. In 1965, Lackman and others named the rickettsial organism "R. parkeri" after Parker.