Effects of exposure to <i>Rickettsia bellii</i> on reservoir competence of guinea pigs for <i>Rickettsia rickettsii</i>

Binder, Lina de Campos; Fanchini, Carlos Eduardo Camargo; Soares, Herbert Sousa; Santos, Talita Beck Strabelli dos; Taniwaki, Sueli Akemi; Nassar, Marcelly Bastos; Fogaça, Andrea Cristina; Serpa, Maria Carolina de Azevedo; Martins, Matheus Pasini; Labruna, Marcelo Bahia

doi:10.1590/0074-02760250195

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RESEARCH ARTICLE • Mem. Inst. Oswaldo Cruz 121 • 2026 • https://doi.org/10.1590/0074-02760250195 linkcopy

Effects of exposure to Rickettsia bellii on reservoir competence of guinea pigs for Rickettsia rickettsii

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract

BACKGROUND Amblyomma sculptum is the main vector implicated in the transmission of Rickettsia rickettsii in southeastern Brazil, where capybaras are known to be the main amplifying hosts of this pathogen. Interestingly, many areas inhabited by large populations of capybaras and A. sculptum remain free of R. rickettsii, suggesting that other drivers may be involved in the pathogen perpetuation. A. sculptum frequently occurs in sympatry with Amblyomma dubitatum ticks, sharing the same hosts with them. Considering that A. dubitatum ticks are frequently infected with Rickettsia bellii, a possible interference of R. bellii in the reservoir competence of capybaras for R. rickettsii could explain the heterogeneous distribution of R. rickettsii in A. sculptum populations in southeastern Brazil.

OBJECTIVES The present study aimed to evaluate the effect of primary exposure to R. bellii-infected A. dubitatum on the reservoir competence for R. rickettsii using guinea pigs as experimental models.

METHODS Three guinea pigs were infested with R. bellii-infected A. dubitatum (Group GB), four guinea pigs with uninfected A. dubitatum (Group GD), and three guinea pigs were not exposed to A. dubitatum (Group GC). After infestation with A. dubitatum, all guinea pigs were exposed to one single R. rickettsii-infected A. sculptum female and then were infested with uninfected A. sculptum larvae. After ecdysis, nymphs were tested for detection of spotted fever group rickettsiae.

FINDINGS Five out of six guinea pigs from GD and GC groups died, while one out of three guinea pigs in the GB group had a fatal outcome. R. rickettsii infection rate among ticks fed on animals from GB group was 21% (17/80), significantly lower than the rate of 54% (60/111) recorded in ticks fed on animals from GD and GC groups.

MAIN CONCLUSIONS Prior exposure of guinea pigs to R. bellii-infected A. dubitatum ticks reduced their reservoir competence for R. rickettsii.

Key words:
Amblyomma dubitatum ; Amblyomma sculptum ; spotted fever; capybara; rickettsiosis

Experimental group	Guinea pig ID	Endpoint titre to R. belliia	Endpoint titer to R. rickettsiia	Febrile period (DAI)	Death (DAI)	R. rickettsii DNA detected in blood (DAI)	No. of bacteria/ng of extracted DNA*
GB	C10	4096	16184	7 - 11	8, 11	10.0 (7.0-12.9)	NO
	C11	NA	NA	7 - 11	10, 11	235.7 (26.9-444.5)	13
	C12	4096	8192	6 - 14	9, 11, 12, 14	35.1 (4.1-129.0)	NO
GD	C16	4096	4096	7 - 13	12, 13	57.8 (44.6-70.9)	NO
	C17	512	1024	12 - 18	14, 15	81.8 (29-134.6)	21
	C18	NA	NA	7 - 10	9	6.4	13
	C19	NA	NA	6 - 10	8, 10	285.2 (34.2-536.2)	12
GC	C30	NA	NA	5 - 8	5, 6, 7, 8	42.0 (2.6-146.7)	10
	C31	<64	<64	NO	NO	NO	NO
	C32	NA	NA	6 - 10	7, 9	58.45 (13.3-103.6)	12

Experimental group	Guinea pig ID	Infection rate (No. of infected nymphs/No. of tested nymphs)*+
GB	C10	NA
	C11	NA
	C12	21% (17/80)
	Total	21% (17/80)^a
GD	C16	0% (0/4)
	C17	NA
	C18	41% (7/17)
	C19	55% (42/76)
	Total	50% (49/97)^b
GC	C30	NA
	C32	79% (11/14)
	Total	79% (11/14)^b
GD + GC	Total	54% (60/111)^b

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[1] GB: guinea pigs previously exposed to Rickettsia bellii-infected Amblyomma dubitatum ticks; GD: guinea pigs previously exposed to R. bellii-uninfected A. dubitatum ticks; GC: guinea pigs not exposed to A. dubitatum ticks; DAI: days after infestation with R. rickettsii-infected tick; NA: not analysed (animal died before 21 DAI); NO: not observed. a: Endpoint titre to R. rickettsii or R. bellii at 21 DAI; all guinea pigs were seronegative (endpoint titer < 64) at 0 DAI; *median (range).