Project Number: 5348-32000-027-00
Project Type: Appropriated

Start Date: Dec 01, 2006
End Date: Nov 30, 2011

Objective:
Our objective in this project is to investigate the factors influencing transmission of Anaplasma marginale by Dermacentor andersoni. We hypothesize that there are interactions between the vector and the pathogen that are determinants of transmission. Our first objective is to set up a field study to examine the relationship between tick vector competence and transmissibility of Anaplasma marginale strains at field sites selected for differences in vector abundance and pathogen strain composition. We will collect ticks annually and determine the susceptibility phenotype of the population at each site by determining the proportion of ticks that are susceptible to midgut infection with A. marginale; midgut susceptibility is a surrogate marker for vector competence. Using a longitudinal survey of a cohort of cattle at each site we will test the hypothesis that some strains of A. marginale are more highly transmissible than others. The first objective will also provide ticks and A. marginale isolates for study in the subsequent objectives. Our next 2 objectives target vector competence of the tick population. First we will attempt to establish if vector competence is a stable genetic characteristic of the tick populations at our field sites by testing the hypotheses that: 1) the proportion of ticks that are susceptible to midgut infection with A. marginale within each population (i.e. the population susceptibility phenotype) is stable characteristic of the population from one year to the next, and 2) that there is limited gene flow between populations of D. andersoni. Secondly, we will determine if tick innate immune responses regulate vector competence by testing the hypotheses that 1) there are differences between tick populations in sequence or expression of tick defensins, and 2) that these differences correlate with phenotypes which are associated with vector competence for A. marginale. Our final two objectives target A. marginale strain transmissibility. First, we will identify common genetic markers of highly transmissible A. marginale strains collected in our field study, and test the hypothesis that these strains share genetic determinants that are associated with, and are predictive of, more efficient transmission by ticks. We will then identify the outer membrane protein (OMP) structure of these highly transmissible A. marginale strains and test the hypotheses that 1) highly transmissible A. marginale strains share conserved OMPs, and 2) that immunization with conserved cross-linked OMPs will induce protection against challenge by heterologous A. marginale strains. By simultaneously approaching studies of the determinates of transmission of A. marginale from the prospective of tick vector competence and from the prospective of strain transmissibility we can begin to define the parameters that influence transmission, including parameters relating to the vector, the pathogen, and to their interaction.

Approach:
Our objective in this project is to investigate the factors influencing transmission of Anaplasma marginale by Dermacentor andersoni. Our first approach is to set up a field study to examine the relationship between tick vector competence and transmissibility of Anaplasma marginale strains at field sites selected for differences in vector abundance and pathogen strain composition. We will collect ticks annually and determine the susceptibility phenotype of the population at each site by determining the proportion of ticks that are susceptible to midgut infection with A. marginale; midgut susceptibility is a surrogate marker for vector competence. Our next approach is to target vector competence of the tick population. We will establish if vector competence is a stable genetic characteristic of the tick populations at our field sites. And secondly, we will determine if tick innate immune responses regulate vector competence by testing if there are differences between tick populations in sequence or expression of tick defensins, and whether these differences correlate with phenotypes which are associated with vector competence for A. marginale. Our final approachs target A. marginale strain transmissibility. First, we will identify common genetic markers of highly transmissible A. marginale strains collected in our field study, and test the hypothesis that these strains share genetic determinants that are associated with, and are predictive of, more efficient transmission by ticks. We will then identify the outer membrane protein (OMP) structure of these highly transmissible A. marginale strains and test the hypotheses that 1) highly transmissible A. marginale strains share conserved OMPs, and 2) that immunization with conserved cross-linked OMPs will induce protection against challenge by heterologous A. marginale strains. IBC approved 9-04-06; BSL 1. Formerly 5348-32000-023-00D (12/06).