The aim of this study is to impove the pharmacological treatment of Rocky mountain spotted fever, since is a very ancient disease with an antibiotic therapy that have not changed much the mortality rates, being compared with the natural curse of the disease.

Rocky mountain spotted fever is a potentially fatal infectious disease wich is transmitted trough vectors. This particular infection is caused by Rickettsia rickettsii wich is a gramnegative intracellular bacilli. The pathophysiological mechanism and clinical manifestation is mainly due to the vascular affectation, since this pathogen has a tropism for endothelial cells. As it has been mentioned, this is an obligate intracellular pathogen, so the main choice with the antibiotic therapy is tetracyclines because of its action mechanism in the inhibition of the subunit 30S. The objective of the study is to prove new strategies of treatment to improve the pharmacological aproach by implementing Azithromycin, wich is a macrolide that inhibits de 50S subunit of the bacteria. Azithromycin has shown effectivity against other rickettsial diseases.

Inclusion Criteria:

• Any gender
• Over 18 years old
• hospitalized in the internal medicine service, emergency department or intensive care unit of Hospital General de Mexicali (ISESALUD) or Hospital General Zona No. 30 (IMSS) with suspicion of RMSF that present fever and 2 or more of the next symptoms: headache, myalgia, rash, nausea, pharyngeal hyperemia, vomiting, abdominal pain, neurological disorders.
• In addition to at least one of the next epidemiologycal fators: presence of vectors in areas of residence or endemic areas visited two weeks prior to the onset of symptoms, history of visiting or residing in areas with RMSF transmission in the last two weeks, existence of confirmed cases in the locality of origin, history of vector bite or contact with dogs in the two weeks.

Exclusion Criteria:

• Other clinical suspicion rather than RMSF
• History of allergy to doxycycline, azithromycin, bean starch or bovine gelatin.
• Not signing the informed consent
• Pregnancy.

Patients will be randomized through the program ¨Randomizer for Clinical Trial¨ to be assigned in group of treatment A or B. One of the treatments will be azithromycin 500 mg orally every 24 hours for three dose plus doxycycline 100 mg orally every 12 hours for 7 days and the other schedule it will be doxycycline 100 mg orally every 12 hours for 7 days plus placebo 1 capsule every 24 hours, 3 doses.