Mike Teglas, Coevolutionary Relationship between Ixodes spp. tick and Anaplasma phagocytophilia

Ticks are second only to mosquitoes in their importance as vectors of bacterial, viral and protozoal diseases worldwide. Ticks and their pathogens are a good system in which to study the evolution of virulence (deadliness), transmissibility (tendency of an infected host to infect more hosts), and a variety of other aspects of host/ pathogen population dynamics. This is because many tick-vectored disease agents are highly specialized for survival in specific vertebrate and invertebrate hosts. Theory predicts that while virulence in such a system may evolve either upward or downward, transmissibility should always increase because this maximizes the pathogen’s reproductive success.

My work involves the bacterium Anaplasma phagocytophilum, which causes the disease known as granulocytic ehrlichiosis in humans and domestic animals, and the deer tick, Ixodes spp., which carries Anaplasma. The Ixodes-Anaplasma shows a great deal of geographical variation. Different species of Ixodes carry Anaplasma in different places: Ixodes scapularis in the eastern U.S., I. ricinus in Europe and I. pacificus in California. Interestingly, while over 400 human cases of granulocytic ehrlichiosis have been diagnosed in the upper Midwest and northeastern United States since 1994, only 8 cases have been reported in California. In dogs, the disease is manifested in a less severe form in California than it is in the eastern U.S but it is much more common in California than is human disease.

I hypothesize that, in this system,tightly coevolved tick-pathogen pairs exhibit higher transmissibility compared with poorly coevolved pairs. To test this, I am using molecular methods to analyze the evolutionary relationship between distinct Anaplasma strains and their various Ixodes hosts across the U.S. and Europe. It appears that Ixodes spp. may have arisen in Australia, migrated to Asia, and then moved to North America. Preliminary analyses suggest that the California form of Anaplasma is a recent branch from European strains. The coupling of European-derived strains of A. phagocytophila with a possibly Asian-derived species of tick, I. pacificus, in the West may have been an evolutionarily recent event. Thus, the weak (young) coevolutionary relationship between A. phagocytophila, its California vector I. pacificus, and the only identified reservoir, N. fuscipes, the dusky-footed woodrat, may be an important factor in the low prevalence of granulocytic ehrlichiosis in this state.

My work will test basic predictions about the evolution of transmissibility in a natural host-pathogen system. Many important studies of wildlife and associated pathogens rely on secure long-term access to highquality study sites. The Quail Ridge Reserve provides us a place to collect tick vectors and small mammals in close proximity to our laboratory, with the assurance that we can keep returning to monitor our study system over time.

Photo Credits: Title, Research (Mike Benard), Ticks (http://www.cdc.gov/ncidod/dvbid/lyme/)

This page last updated: June 23, 2005  

Contact: Dr. Virginia Boucher
John Muir Institute of the Environment
109 The Barn, University of California, Davis, CA 95616
Phone: 530-752-6949; email: vlboucher@ucdavis.edu

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