Association of Ixodes pacificus (Acari: Ixodidae) with the Spatial and Temporal Distribution of Equine Granulocytic Ehrlichiosis in California

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This study was conducted to determine if the biology of certain ticks associated with horses regulates the spatial and temporal distribution of equine granulocytic ehrlichiosis (EGE) in California north of Monterey County. We compared the spatial and temporal distribution of EGE cases with the seasons of activity and life histories of ticks that infest horses. Spatially, cases collected from equine veterinarians clustered around each other in a manner different from the way in which control cities of practice were distributed, with foci limited to the Sierra Nevada and coastal foothills. Cases also clustered seasonally; most were diagnosed between November and April. The spatial and temporal pattern of EGE cases closely parallels the well-characterized life history and distribution of Ixodes pacificus Cooley & Kohls, but not other ticks commonly associated with horses. Building on previous studies, there is compelling evidence that this tick has the vectorial capacity to transmit Ehrlichia equi to horses. Based on the life history and distribution of I. pacificus in relation to EGE cases, we reason that this tick is the only biologically plausible vector of E. equi in California, and provide evidence for a tightly linked association between I. pacificus and the epidemiology of EGE.

Keywords: Ehrlichia; Ehrlichia equi; Ixodes pacificus; equine ehrlichiosis; tick; vectorial capacity

Document Type: Research Article

Publication date: September 1, 1999

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  • Journal of Medical Entomology is published bimonthly in January, March, May, July, September, and November. The journal publishes reports on all phases of medical entomology and medical acarology, including the systematics and biology of insects, acarines, and other arthropods of public health and veterinary significance.

    The journal is divided into the following sections: Morphology, Systematics, Evolution; Sampling, Distribution, Dispersal; Development, Life History; Population and Community Ecology; Behavior, Chemical Ecology; Population Biology/Genetics; Molecular Biology/Genomics; Neurobiology, Physiology, Biochemistry; Vector Control, Pest Management, Resistance, Repellents; Arthropod/Host Interaction, Immunity; Vector/Pathogen/Host Interaction, Transmission; Vector-Borne Diseases, Surveillance, Prevention; Direct Injury, Myiasis, Forensics; Modeling/GIS, Risk Assessment, Economic Impact. In addition to full-length research articles, the journal publishes interpretive articles in a Forum section, Short Communications, Rapid Communications, and Book Reviews.
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