Rickettsial diseases

Rickettsial infections occur worldwide and are associated with the patient having been bitten by an ectoparasite such as a louse, mite, flea, mosquito, or tick.[18]Drexler NA, Dahlgren FS, Heitman KN, et al. National surveillance of spotted fever group rickettsioses in the United States, 2008-2012. Am J Trop Med Hyg. 2016 Jan;94(1):26-34. http://www.ncbi.nlm.nih.gov/pubmed/26324732?tool=bestpractice.com [19]Botelho-Nevers E, Raoult D. Host, pathogen and treatment-related prognostic factors in rickettsioses. Eur J Clin Microbiol Infect Dis. 2011 Oct;30(10):1139-50. http://www.ncbi.nlm.nih.gov/pubmed/21519943?tool=bestpractice.com [20]Aung AK, Spelman DW, Murray RJ, et al. Rickettsial infections in Southeast Asia: implications for local populace and febrile returned travelers. Am J Trop Med Hyg. 2014 Sep;91(3):451-60. http://www.ncbi.nlm.nih.gov/pubmed/24957537?tool=bestpractice.com ​​ Around 200 vector species have been detected, carrying 47 Rickettsia spp, with ticks big the most common vector.​[7]Zhang YY, Sun YQ, Chen JJ, et al. Mapping the global distribution of spotted fever group rickettsiae: a systematic review with modelling analysis. Lancet Digit Health. 2023 Jan;5(1):e5-15. https://www.thelancet.com/journals/landig/article/PIIS2589-7500(22)00212-6/fulltext http://www.ncbi.nlm.nih.gov/pubmed/36424337?tool=bestpractice.com ​ These small arthropods contain the rickettsiae living inside them, usually as part of their normal microbial flora, and they are transmitted to the patient by the bite of the ectoparasite or via its infected faeces, either when rubbed into the bite site or when the ectoparasite faeces are inhaled by the patient. CDC: tick ID Opens in new window

Rickettsial infections in humans are caused by several related genera of bacteria including Rickettsia species, Orientia tsutsugamushi and O chuto, Anaplasma species, Ehrlichia species, and Neoehrlichia species. Sometimes Coxiella burnetii (which causes Q fever) is included as a rickettsiosis, but strictly speaking it is not as it belongs to a phylogenetically different clade of bacteria and is more closely related to Legionella species. See the Classification section for a list of types of rickettsioses, causative organisms, and vectors. 

Rickettsia have two common features: they are obligate intracellular bacteria and will only grow inside a host animal eukaryotic cell, and they live in the cells of both vertebrate (e.g., humans, rats) and invertebrate animals (e.g., lice, fleas, mites, and ticks). While the invertebrate is the main host animal for the rickettsia, it is also the vector for transmission of the rickettsia to humans and other vertebrate animals.[Figure caption and citation for the preceding image starts]: Male yellow dog tick (Amblyomma aureolatum)CDC/Dr Christopher Paddock and James Gathany (photographer) [Citation ends].

Rickettsia are taken up by human cells that are not professional phagocytes. The cell type most favoured is the endothelial cell, leading to leaking tight junctions, oedema, capillary destruction, and petechiae. This pathology can occur in any end organ and lead to thrombosis, infarction, and tissue necrosis. There is some variation between the different rickettsial pathogens, but it is variation on a theme. Brain, lung, spleen, liver, and kidney may all be damaged by the intracellular growth of the rickettsia. There is not thought to be a rickettsial toxin involved, but simply the destruction of the patient's endothelial cells by the intracellular growth of the rickettsiae.[21]Walker DH, Valbuena GA, Olano JP. Pathogenic mechanisms of diseases caused by Rickettsia. Ann N Y Acad Sci. 2003;990:1-11. http://www.ncbi.nlm.nih.gov/pubmed/12860594?tool=bestpractice.com [Figure caption and citation for the preceding image starts]: Life cycle of ticksCDC [Citation ends].

Centers for Disease Control and Prevention classification[2]Centers for Disease Control and Prevention. CDC Yellow Book: health information for international travel. Section 5: travel-associated infections & diseases - bacterial (rickettsial diseases) May 2023 [internet publication]. https://wwwnc.cdc.gov/travel/yellowbook/2024/infections-diseases/rickettsial-diseases

• Scrub typhus group rickettsioses

  • Due to mite-transmitted infection with the rickettsia Orientia tsutsugamushi, O chuto, or O chiloensis.

• Typhus group rickettsioses

  • Epidemic typhus, sylvatic typhus: due to infection by human body lice themselves infected with Rickettsia prowazekii. Now rare, but may occur in situations of mass crowding, poverty, famine, and war.

  • Murine typhus: due to infection with R typhi from the infectious faeces of rodent fleas, living on rodents in close proximity to humans.

• Spotted fever group rickettsioses

  • Different species occur in geographically different parts of the world. About 20 different infections are classified in this group, including:

    • African tick-bite fever (tick transmitted) due to R africae

    • Aneruptive fever (tick transmitted) due to R helvetica

    • Astrakhan spotted fever (tick transmitted) due to R conorii (subspecies caspiae - proposed)

    • Cat-flea rickettsiosis (flea transmitted) due to R felis

    • Far Eastern spotted fever (tick transmitted) due to R heilong-jiangensis

    • Flinders Island spotted fever or Thai tick typhus (tick transmitted) due to R honei (including marmionii strain)​

    • Indian tick typhus (tick transmitted) due to R conorii (subspecies indica - proposed)

    • Israeli tick typhus (tick transmitted) due to R conorii (subspecies israelensis - proposed)

    • Japanese spotted fever (tick transmitted) due to R japonica

    • Lymphangitis-associated rickettsiosis (tick transmitted) due to R siberica mongolotimonae

    • Maculatum infection (tick transmitted) due to R parkeri

    • Mediterranean spotted fever or Boutonneuse fever (tick transmitted) due to R conorii (subspecies conorii - proposed)

    • Mediterranean spotted fever-like disease (tick transmitted) due to R massiliae or R monacensis

    • North Asian tick typhus or Siberian tick typhus (tick transmitted) due to R sibirica

    • Queensland tick typhus (tick transmitted) due to R australis

    • Rickettsialpox (mite transmitted) due to R akari

    • Rickettsiosis (tick transmitted) due to R aeschlimannii

    • Rocky Mountain spotted fever (tick transmitted) due to R rickettsii

    • Tickborne lymphadenopathy or Dermcentor-borne necrosis and lymphadenopathy (tick transmitted) due to R raoultii or R slovaca.