Research Article: Warmer Weather Linked to Tick Attack and Emergence of Severe Rickettsioses

Date Published: November 18, 2008

Publisher: Public Library of Science

Author(s): Philippe Parola, Cristina Socolovschi, Luc Jeanjean, Idir Bitam, Pierre-Edouard Fournier, Albert Sotto, Pierre Labauge, Didier Raoult, Peter J. Hotez

Abstract: The impact of climate on the vector behaviour of the worldwide dog tick Rhipicephalus sanguineus is a cause of concern. This tick is a vector for life-threatening organisms including Rickettsia rickettsii, the agent of Rocky Mountain spotted fever, R. conorii, the agent of Mediterranean spotted fever, and the ubiquitous emerging pathogen R. massiliae. A focus of spotted fever was investigated in France in May 2007. Blood and tissue samples from two patients were tested. An entomological survey was organised with the study of climatic conditions. An experimental model was designed to test the affinity of Rh. sanguineus for biting humans in variable temperature conditions. Serological and/or molecular tools confirmed that one patient was infected by R. conorii, whereas the other was infected by R. massiliae. Dense populations of Rh. sanguineus were found. They were infected with new genotypes of clonal populations of either R. conorii (24/133; 18%) or R. massiliae (13/133; 10%). April 2007 was the warmest since 1950, with summer-like temperatures. We show herein that the human affinity of Rh. sanguineus was increased in warmer temperatures. In addition to the originality of theses cases (ophthalmic involvements, the second reported case of R. massiliae infection), we provide evidence that this cluster of cases was related to a warming-mediated increase in the aggressiveness of Rh. sanguineus, leading to increased human attacks. From a global perspective, we predict that as a result of globalisation and warming, more pathogens transmitted by the brown dog tick may emerge in the future.

Partial Text: The recent outbreak of Chikungunya virus in the Indian Ocean Islands and India [1] that reached Europe in 2007 [2], has illustrated the current medical importance of globalising vector-transmitted infections [3]. The impact of ticks on human public health was recognised with the emergence of Lyme disease 25 years ago [4]. Since then, around 15 emerging tick-borne rickettsioses have emerged [5].

When investigating these grouped cases of severe spotted fevers first presumed to be MSF caused by R. conorii, two rickettsial pathogens were in fact identified. This report describes the second human case of R. massiliae infection and was documented using the IF reference serology assays [5], completed by western blot and cross absorption studies and definitely confirmed with the use of molecular tools. R. massiliae is a worldwide rickettsia that was isolated in 1992 and thereafter detected in Rhipicephalus spp. in Europe and Africa [5], Argentina [27], and recently in Arizona, USA [15]. The recognition of the pathogenicity of R. massiliae occurred in 2005 when molecular tools were used to identify a rickettsial isolate obtained 20 years before from a man hospitalised in Italy with fever, an eschar, and a maculopapular rash [16]. In fact, R. massiliae is the sole pathogenic rickettsia known to be prevalent in America, Africa and Europe. In the present report, the predominant symptom was acute visual loss, and both diagnosis and treatment were delayed. Although eye involvement has been reported in spotted fever group rickettsioses, these manifestations are underdiagnosed or frequently misdiagnosed [28]–[30]. Clinicians should suspect rickettsioses in patients with febrile acute visual loss, particularly during the warmest and most common months for Rh. sanguineus-transmitted diseases.



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