Transmission dynamics and control of Rickettsia rickettsii in populations of Hydrochoerus hydrochaeris and Amblyomma sculptum

Abstract

BackgroundBrazilian Spotted Fever (BSF), caused by the bacterium Rickettsia rickettsii, is the tick-borne disease that generates the largest number of human deaths in the world. In Brazil, the current increase of BSF human cases has been associated with the presence and expansion of capybaras Hydrochoerus hydrochaeris, which act as primary hosts for the tick Amblyomma sculptum, vector of the R. rickettsii in this area.MethodsWe proposed a semi-discrete-time stochastic model to evaluate the role of capybaras in the transmission dynamics of R. rickettsii. Through a sensitivity analysis, we identified the parameters with significant influence on the R. rickettsii establishment. Afterward, we implemented the Gillespie’s algorithm to simulate the impact of potential public health interventions to prevent BSF human cases.ResultsThe introduction of a single infected capybara with at least one infected attached tick is enough to trigger the disease in a non-endemic area. We found that to avoid the formation of new BSF-endemic areas, it is crucial to impede the emigration of capybaras from endemic areas by reducing their birth rate by more than 58%. Model results were corroborated by ex-situ data generated from field studies, and this supports our proposal to prevent BSF human cases by implementing control strategies focused on capybaras.ConclusionThe proposed stochastic model illustrates how strategies for the control and prevention of vector-borne infectious diseases can be focused on amplifier hosts management practices. This work provides a basis for future prevention strategies for other neglected vector-borne diseases.

Human-induced changes in habitat preference by capybaras (Hydrochoerus hydrochaeris) and their potential effect on zoonotic disease transmission

“…Capybaras are also linked to the amplification of rickettsial infection among A. sculptum populations, creating new cohorts of infected ticks during bacteremia periods (days or weeks), when they maintain R. rickettsii in their bloodstream [16]. In addition, A. sculptum populations are not able to sustain R. rickettsii for successive generations without the creation of new infected cohorts via horizontal transmission through vertebrate hosts [58, 59]. Therefore, we highlight the role of capybaras selecting disturbed forest in human-modified landscapes as an important factor in BSF spread.…”

Brazilian Spotted Fever Prevention through a Nonlethal Capybara Population Control Strategy

“…Capybaras (Hydrochoerus hydrochaeris), the world's largest rodent, are considered as potentially amplifying hosts for R. rickettsii, as they fulfill all the following necessary criteria: i) the potential host species must be abundant in the endemic area; ii) be a good host for ticks; iii) be susceptible to Rickettsia infection; iv) be highly proliferative, ensuring the introduction of susceptible animals; and v) have enough bacteremia to infect feeding ticks 5,6 . In Brazil, reduction in the natural predators, mainly in areas of anthropological impacts, and abundant water and food sources, largely due to agricultural activities, provide favorable conditions for this highly proliferative species to turn into superpopulations 4,5 .…”

“…Population control methods must be strategically employed to act as an effective BSF transmission management tool. As suggested by a transmission dynamic model of R. rickettsii involving H. hydrochaeris hosts and A. sculptum vectors 6 , to eliminate the etiological agent from an endemic area, a sustained reduction of 80% of the capybara's birth rate is necessary, which can result in the disappearance of the infected individuals by the 4 th year. Moreover, if a 90% birth rate decrease could be achieved, infected capybaras and ticks would also cease to exist by the 2 nd year 6 .…”

“…As suggested by a transmission dynamic model of R. rickettsii involving H. hydrochaeris hosts and A. sculptum vectors 6 , to eliminate the etiological agent from an endemic area, a sustained reduction of 80% of the capybara's birth rate is necessary, which can result in the disappearance of the infected individuals by the 4 th year. Moreover, if a 90% birth rate decrease could be achieved, infected capybaras and ticks would also cease to exist by the 2 nd year 6 . Furthermore, based on the nextgeneration matrix approach, elements corresponding to the number of infected attached nymphs and adult ticks produced by an infected capybara and vice versa have been identified as critical contributors to changes in the basic reproduction number (R 0 ≈1.7) 7 .…”

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“…Ambient and physical conditions influences the female's cyclicity, which is why contraceptive effects on the reproductive physiology can be complex and irregular (Berger et al, 2007;Clarke & Pompolo, 2005;Cunningham & Klein, 2007;McDonald, 2003;Mead et al, 2007;Reiter et al, 2009;Rotstein, 2011;Simpson et al, 1999;Tassigny & Colledge, 2010;Zouboulis, 2009). 6). Resulting in failure to produce an estrogen and LH peak, consequently, no ovulation occurs, with an added effect of preventing fertilization mechanically, through a thickening of the cervical mucus, interfering with sperm motility (Martin, 2016).…”