Hopland Research and Extension Center

Granulocytic anaplasmosis is a potentially fatal rickettsial disease that occurs sporadically in northern and central California vectored primarily by Ixodes pacificus ticks. The ecology of western GA differs from the relatively simple 1-host-1 vector eastern disease cycle, although most aspects of the western ecology remain poorly understood. In particular, the contribution of lizards, one of the most important hosts for juvenile I. pacificus, has not been explored. Lizards could function as important reservoirs, or serve as zooprophylactic hosts to reduce disease incidence, analogously with western Lyme disease ecology. We evaluate two alternative hypotheses that Anaplasma phagocytophilum levels in western communities a) are increased by interactions of juvenile ticks with lizards or b) decreased because lizards function as zooprophylactic hosts. Our research intends to fulfill the following specific aims: To evaluate the contribution of lizards to the maintenance of juvenile I. pacificus ticks and A. phagocytophilum in nature, by 1) evaluating the incidence of A. phagocytophilum in marked S. occidentalis over two years, 2) quantifying the seasonal I. pacificus load on S. occidentalis, and 3) determining the prevalence of A. phagocytophilum in S. occidentalis and in ticks at HREC. Results will contribute to a mechanistic explanation for disease presence and emergence in the western US and ultimately be used to maximize the ability of public health workers to predict, detect, and ultimately manage the risk of A. phagocytophilum in the western US.

Host-parasite systems are a powerful arena for evolutionary research because the environment of many parasites is defined almost exclusively by its host.  In this way, factors governing evolution are easier to delineate for parasites than for free-living organisms.  This is particularly true for parasites which complete their entire life cycle on the body of a single host.  In some cases the phylogeny of these parasites is congruent with that of the host, indicating cospeciation between host and parasite.  Cospeciating host-parasite systems are unique in that they represent a long history of parallel evolution.  This is important because it provides a temporal framework for comparative analyses of the rates of evolution of the host and parasite.  Because the life histories of hosts and their parasites are generally extremely different, studying the molecular evolution in a host-parasite system can provide insight into questions relating to the possible effects of generation time, metabolic rate, and other life history parameters on rates of mutation and evolutionary change.

Malaria parasites infect a broad range of vertebrate hosts, including mammals (four species infect humans), birds, and reptiles.  Since 1978, a malaria system has been under study at the HREC: Plasmodium mexicanum in the western fence lizard, Sceloporus occidentalis.  This parasite-host association is now among the best known for any malaria parasite of wildlife hosts.  A central goal is a long-term study of the prevalence of the parasite over time at numerous locations at the Hopland site.  These 26 years of data reveal the parasite's prevalence seems to be following a long-term cycle, unexpected under standard epidemiological models for malaria parasites.  These data can now be compared and contrasted with those from our similar long-term studies at three sites in the Caribbean islands.  A second major goal is to understand the ecology and evolution of the life history of the parasite and relate those data to the evolution of parasite virulence.  Theory predicts that infections should follow different life histories depending on the genetic diversity of the parasites in the vertebrate host.  Over the past three years, a series of experiments were conducted to determine if clonal diversity of the infections is associated with differing behavior of the parasites.  More recently, a direct measure of clonal diversity has been developed:  variable microsatellite markers for P. mexicanum have been developed, the first for any malaria parasite of nonhumans.

This research is intended to clarify the role of the western gray squirrel (Sciurus griseus) as a keystone species for maintaining enzootic foci of the Lyme disease (LD) spirochete Borrelia burgdorferi sensu stricto (Bb ss) in the far-western USA; and to investigate the host-seeking behavior of Ixodes pacificus (Ip) nymphs in relation to environmental parameters and to risk of human exposure to Bb ss at the Hopland Research and Extension Center (HREC). The reservoir competence of S. griseus for Bb ss will be evaluated by determining the infectivity of naturally infected squirrels for uninfected Ip larvae; the capacity of fed larvae to pass the infection transstadially; the ability of infected nymphs to transmit infection to naïve squirrels; and the duration of infectivity in experimentally infected squirrels. Previous research has established that dense woodlands are primary biotopes of Ip nymphs, and that contact with either leaf litter and wood, but especially wood (e.g., logs), can elevate the risk of human exposure to nymphal ticks.  Therefore, host-seeking activities of Ip nymphs in relation to biotic and abiotic factors will be investigated quantitatively in oak/madrone woodlands.  These will include the diurnal questing cycle; the densities of host-seeking nymphs, and of Bb ss-infected nymphs, on logs and tree trunks versus adjacent leaf litter; and the movements of marked nymphal ticks.

Interactions between human-induced environmental change and pathogen dynamics are one of the most pressing and poorly understood issues facing scientists this century.  Environmental change can alter pathogen dynamics in ways that increase human disease risk, intensify pathogen pressure on imperiled species, degrade ecosystem services, and endanger agricultural systems.  It is crucial to understand the two major mechanisms by which anthropogenic activity affects pathogens: 1. alteration of host-community structure and 2. alteration of the abiotic environment.  Vector-transmitted generalist pathogens are of particular concern as leading causes of emerging diseases.  Our ability to predict how this group of pathogens will respond to human activity is limited because most theory focuses on specialists but does not explicitly incorporate the abiotic environment, and it is often logistically impossible or unethical to conduct experiments to directly test causation of processes thought to control pathogen transmission. To address these limitations, we will develop mathematical theory of generalist vector-transmitted pathogens that explicitly incorporates host competition for abiotic resources.  We will test the predictions of this theory using geographic-scale field experiments with an aphid-vectored plant virus, barley yellow dwarf (BYDV).  We will test how changes in moisture and nitrogen availability interact with changes in host community diversity and composition to control disease dynamics. The BYDV system is unique in that it allows us to create spatially- and phylogenetically-replicated experimental communities that mimic five important types of disease systems, thus allowing us to predict the effects of human activity on a variety of host communities and pathogens using a single general theoretical framework.

Pierce's disease (PD) of grapevines,  caused by the bacterium, Xylella fastidiosa (Xf), has caused economic losses to California vineyards for over 100 years. The introduction of a new insect vector, the glassy-winged sharpshooter, in many areas of CA, will likely increase PD losses. The historical distribution of PD in North and Central America, as well as California research conducted by Purcell at UC Berkeley has documented that PD-affected vines that are exposed to relatively severe winter temperatures are often cured of PD.  However, the physiological/bichemical basis for the cold curing phenomenon is unknown and the focus of our CDFA-funded research project. Part of the project involves subjecting healthy and PD-affected grapevines, growing in 4 gallon pots, to various cold temperature regimes during the winter. Hopland was selected as one of the test sites because Purcell's previous work showed vines were cured of PD at this site. The potted vines will be planted in holes in the ground sometime in October and removed from the site when the buds begin to push in the spring. Previous research has shown that the infected vines cannot act as reservoirs of the pathogen during the dormant season. CDFA has reviewed the proposed project and signed a permit allowing the research to be conducted. Canes will be removed from the vines during various times of the winter and xylem sap will be extracted and tested for several physiological/biochemical parameters. A full description of the various things we will be testing for is described in our funded project proposal which is available upon request. We hope that by better understanding and identifying the physiological/biochemical changes that occur in cold-stressed grapevines we may be able to devise novel techniques to induce these changes under non-freezing temperatures and possibly develop novel approaches for managing PD.

The objective of this research is to determine the epidemiological role of seasonal changes in concentrations of Xylella fastidiosa (Xf), the bacterium that causes Pierce's Disease (PD) of grapevine, in riparian hosts in North-coastal California.  A preliminary field experiment was conducted at three vineyards in Napa Co. to measure Xf concentrations in five riparian hosts: Rubus discolor (Himalayan blackberry), Rubus ursinus (California blackberry), Sambucus mexicana (blue elderberry), Vinca major (periwinkle) and Vitis californica (California grape).  All five species are hosts of Graphocephala atropunctata (blue-green sharpshooter, BGSS), the most efficient vector of PD in North-coastal California.  In spring, 30 individuals per species were mechanically inoculated with Xf.  In summer, fall, and winter following inoculation, Xf concentrations were estimated in petioles distal to stem inoculation sites by dilution plating.  Xf concentrations reached detectable levels in California blackberry, blue elderberry, and California grape in summer and increased through fall.  Xf was not detected in periwinkle until early fall, when populations were found to be as high as that of California blackberry, blue elderberry, and California grape.  Unfortunately, too few plants developed infections to obtain an adequate sample size for statistical analysis of Xf concentrations among riparian hosts and seasons.  Methodology was modified to improve inoculation success by inoculating potted plants in the greenhouse.  After inoculated plants are confirmed infected, they will be transferred to sites in Mendocino County and Napa County, where they will be exposed to two different grape-growing climates.  Seasonal Xf concentrations in petioles will be measured using dilution plating.