On the possible role of ticks in the eco-epidemiology of Coxiella burnetii in a Mediterranean ecosystem

Ruminant livestock is the main reservoir of Coxiella burnetii (Cb), but little is known about the role of wildlife and ticks in its epidemiology. The Iberian ibex (Capra pyrenaica, Schinz 1838) population of “Ports de Tortosa i Beseit” (NE Spain) suffers intense tick infestations and low reproduction rates. This study aims to (1) assess the relationship between infection in ibexes (detection of serum antibodies and/or of Cb DNA in tissues) and Cb DNA presence in ticks hosted by the same ibexes; and (2) identify Cb associated risk factors. Between 2011 and 2015, serum (n=130), spleen (n=72), lymph node (n=89) and tick (n=669) samples from 134 hunter-harvested ibexes were collected. Antibody detection was performed by ELISA and Cb DNA presence was assessed by PCR. Potential risk factors were assessed with regression tree models. Although 30% of the ibexes (39/130; 95%CI, [10%-29.8%]) had antibodies, Cb DNA was detected in only 9.8% of the ibexes (11/112; 95%CI [7.6% - 27.25%]). The prevalence of Cb-carrier ticks averaged 10% and exceeded 20% for the genus Haemaphysalis. However, lacking correlation between infection in ibexes and their ticks does not support tick-to-ibex transmission or vice versa. Tree modelling points to host, population and environmental factors as drivers of Cb infection in ticks and suggests connections with the domestic cycle. The percentage of Cb-carrier ticks detected is noteworthy. Along with heavy tick infestations, it suggests vector potential for these tick species, especially for the genera Rhipicephalus and Haemaphysalis. Since vector competence has not been assessed in these tick species, a classic vector role cannot be proposed nor discarded, but promoter factors of vector capacity occur. In addition, the risk of tick-borne infection through tick excreta should not be neglected. While the airborne route is the preeminent route for Cb infection, ticks' contribution to Cb epidemiology deserves further attention.

Emmanuel Serrano was funded by the Spanish Ministerio de Economía y Competitividad (MINECO) through a Ramón y Cajal agreement (RYC-2016-21120). João Carvalho was supported by a PhD grant (SFRH/BD/98387/2013) co-financed by Fundação para a Ciência e a Tecnologia (FCT), European Social Fund (ESF) and Ministério da Educação e Ciência (MEC) National Funds. Raquel Castillo-Contreras was supported by a PhD grant (2016FI_B 00425) co-financed by Generalitat de Catalunya (Secretaria d’Universitats i Recerca del Departament d’Economia i Coneixement) and European Social Fund (ESF). We would like to thank the University of Aveiro (Department of Biology) and FCT/MEC for the financial support to CESAM RU (UID/AMB/50017) through national funds and, where applicable, co-financed by the FEDER, within the PT2020 Partnership Agreement. We express our gratitude to the Catalonia Government for promoting active disease surveillance of wildlife. We are also very grateful to the staff of the National Game Reserve and Natural Park “Els Ports de Tortosa i Beseit” for their interest and involvement.