Epidemiology
Rickettsial infections are found wherever animals, including humans, live. They are an important cause of non-malarial febrile illness in Africa and Asia.[4][5]
Typhus group rickettsioses
The classic human rickettsial infection, epidemic typhus, is transmitted by the infected human body louse and is the only rickettsial infection for which humans are the natural vertebrate host. This disease is now rare, but may be found in cold, impoverished regions, often beset by economic hardship, famine, war, and social dislocation (e.g., refugee camps). The epidemic typhus bacterium is also present in fleas of flying squirrels in parts of the US, and this has led to human infections.[6] Occurs in Central Africa, Asia, and North, Central, and South America.[2]
Murine (endemic) typhus is a flea-borne typhus. Infections are found worldwide, where humans are exposed to rodents (rats and mice), the rodents' infected fleas, and the fleas' infectious faeces. People in poor economic circumstances, being most likely to live in rodent-infected conditions, are at greatest risk. Occurs in temperate, tropical and subtropical areas worldwide.[2]
Spotted fever group rickettsioses
Spotted fever group infections occur when people are bitten by infected ticks. In each country of the world these diseases are given a different name and the rickettsia a different species designation. The three leading species to which people are at risk globally are Rickettsia felis, R conorii, and R africae.[7] Cat-flea rickettsiosis is found worldwide and associated with cat and dog fleas.
Passive surveillance in the US between 2010 and 2018 indicates an average annual incidence of 12.7 cases per million persons for spotted fever group infections, and the annual incidence has increased during the past two decades.[8] In 2018 and 2019, 5544 and 5207 cases of spotted fever rickettsiosis were reported in the US, respectively, after a peak of 6248 cases in 2017.[9] African tick-bite fever (caused by R africae) is the most commonly reported travel-associated rickettsial infection in the US.[2]
In Europe, Mediterranean spotted fever (caused by R conorii) is the most commonly reported rickettsial infection. Italy, Portugal, and Spain report the highest number of cases.[10] Cases have been reported rarely in the UK in returning travellers.[11]
Scrub typhus group rickettsioses
Scrub typhus is endemic in East Asia, Southeast Asia, the Indian subcontinent, Oceania, and northern, tropical Australia, where the mite Leptotrombidium species lives. There have been case reports of scrub typhus in Africa, South America, and the United Arab Emirates.[12][13][14][15][16][17] Scrub typhus reported from the United Arab Emirates has been classified as a new species (Orientia chuto).[15] Only the larval form ('chigger') bites humans, to transmit the rickettsiae.
Globally, more than 1 million cases of scrub typhus are reported annually.[2] Reliable incidence estimates are difficult to obtain due to heterogeneity in surveillance and diagnostic methods. In one systematic review, reported incidence in Asia ranged from 1.22/100,000 in China to 17.7/100,000 in South Korea.[5]
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