Rickettsioses

Rickettsioses are zoonoses caused by obligate Gram-negative intracellular bacteria of the order Rickettsiales, comprising (1) rickettsioses due to bacteria of the genus Rickettsia, including spotted fever groups and typhus groups (Rickettsiaceae), (2) ehrlichioses and anaplasmoses due to bacteria of the Anaplasmataceae, and (3) scrub typhus due to Orientia tsutsugamushi (see Chapter 7.6.40).

Epidemiology, clinical features, and prognosis of particular rickettsioses

Tick-borne spotted fever group—20 species or subspecies of spotted fever group rickettsiae can infect humans following transmission from their natural vertebrate hosts by ixodid (hard) ticks, with many species having particular geographical restriction. Presentation is typically with fever, headache, muscle pain, rash, local lymphadenopathy, and—for some diseases—a typical inoculation eschar (the ‘tache noire’) at the tick bite site. These signs vary depending on the rickettsia involved and may allow distinction between different rickettsioses occurring at the same location. Diseases range in severity from mild to severe.

Murine (endemic) typhus—caused by Rickettsia typhi, whose natural host is rodents, between whom it is spread by the rat flea. Human infection usually results from contamination of disrupted skin or inhalation of flea faeces containing the organism. Disease is generally mild and self-limiting with non-specific features: less than 15% of cases present with the ‘classic’ triad of fever, headache, and rash.

Epidemic typhus—caused by R. prowazekii, for whom humans are the major (if not only) host, and transmitted by body lice, hence the disease is a particular problem during times of war, conflict, famine, and natural catastrophes. The most recent outbreak, the largest since the Second World War, occurred during the civil war in Burundi in the 1990s. Following a nonspecific prodrome, presentation is with fever, headache, myalgia and a wide range of other symptoms. Most patients develop a macular, maculopapular, or petechial rash. Mortality ranges from 4% (recent series) to 60% (without antibiotics).

Other rickettsioses—include (1) flea-borne spotted fever—cat flea typhus; (2) rickettsialpox—transmitted from mice by house mouse mites.

Diagnosis and treatment of rickettsioses

Diagnosis is by direct evidence of infection by culture or polymerase chain reaction (PCR), or by serological testing. Aside from supportive care, doxycycline remains the drug of choice for immediate empirical treatment of all rickettsioses on clinical suspicion, with many of these infections having high mortality if untreated.

Human ehrlichioses and anaplasmosis

These diseases are tick-borne zoonoses, whose causative agents are maintained through enzootic cycles between ticks and animals. Three species cause human diseases: (1) Ehrlichia chaffeensis—causes human monocytic ehrlichiosis; (2) Anaplasma phagocytophilum—causes human anaplasmosis; and (3) E. ewingii—causes granulocytic ehrlichiosis. These all present as undifferentiated seasonal febrile illnesses, ranging in severity from mild to severe, with multisystem organ failure. Diagnosis is by direct evidence of infection by culture or PCR, or (most commonly) by serological testing. Doxycycline is the antibiotic of choice.

Prevention

Prevention of rickettsioses in general is by (1) avoiding arthropod bites—by applying topical N,N-diethyl-m-toluamide (DEET) repellent to exposed skin, and treatment of clothing with permethrin; and (2) those staying in infested areas checking their bodies routinely for the presence of arthropods, and promptly removing ticks. In addition, (3) epidemic typhus—louse eradication is the most important preventive measure. No vaccines are available.