Mapping the global distribution of spotted fever group rickettsiae: a systematic review with modelling analysis

Abstract

Background: Emerging and re-emerging spotted fever group (SFG) rickettsioses are increasingly recognised worldwide as threats to public health, yet their global distribution and associated risk burden remain poorly understood.

Methods: In this systematic review and modelling analysis, we mapped global distributions of all confirmed species of SFG rickettsiae (SFGR) detected in animals, vectors, and human beings, using data collected from the literature. We assessed ecological drivers for the distributions of 17 major SFGR species using machine learning algorithms, and mapped model-predicted risks.

Findings: Between Jan 1, 1906, and March 31, 2021, we found reports of 48 confirmed SFGR species, with 66 133 human infections worldwide, with a large spatial variation across the continents. 198 vector species were detected to carry 47 of these Rickettsia spp. (146 ticks, 24 fleas, 15 mosquitoes, six mites, four lice, two keds, and one bug). Based on model-predicted global distributions of the 17 major SFGR species, we found five spatial clusters aggregated by ecological similarity in terms of environmental and ecoclimatic features. Rickettsia felis is the leading SFGR species to which 4·4 billion (95% CI 3·8-5·3 billion) people are at risk, followed by Rickettsia conorii (3·7 billion) and Rickettsia africae (3·6 billion).

Interpretation: The wide spectrum of vectors is contributing substantially to the increasing incidence of SFGR infections among humans. Awareness, diagnosis, and surveillance of SFGR infections should be improved in the high-risk regions, especially in areas where human infections are underreported.

Funding: National Key Research and Development Program of China.

Figure 1:

Trial profile

Figure 2:. Publications about SFG rickettsiae in animals, vectors, and human beings (January, 1906, to March, 2021)

(A) Annual number of publications on SFGR stratified by host type. (B) Year of first reporting for each SFGR species. Dots indicate source of first detection. (C) Chord diagram between SFGR species and host types. (D) Overall number of publications on SFGR detection in vectors, animals, and humans. FGR=spotted fever group rickettsiae.

Figure 3:. Numbers of vector and animal species from which each SFGR species was detected

Vectors include ticks, fleas, mosquitoes, mites, louses, keds, and bugs. Ticks are further classified by genus. Animals include both wildlife and livestock, and wild animals are classified into orders. The total number of distinct host species harbouring SFGR in each host category is shown in the column header. The total number of rickettsia species detected in each host category is shown within the matrix. SFGR=spotted fever group rickettsiae.

Figure 4:. Global distributions of SFGR detection events

(A) Humans. (B) Vectors. (C) Animals. SFGR=spotted fever group rickettsiae.

Figure 5:. Global distributions of 17 predominant SFGR species in four continents

(A) Eurasia. (B) Zoomed-in area of Europe. (C) Africa. (D) Americas. (E) Oceania. (F) Number of detection locations (after deduplication) in four continents for each predominant SFGR species. SFGR=spotted fever group rickettsiae.