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United States Department of Agriculture

Agricultural Research Service

Research Project: DETERMINANTS OF ANAPLASMA MARGINALE TRANSMISSION AT THE VECTOR/PATHOGEN INTERFACE
2009 Annual Report


1a.Objectives (from AD-416)
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.


1b.Approach (from AD-416)
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. Formerly 5348-32000-023-00D (12/06).


3.Progress Report
This project addresses critical gaps in our understanding of the transmission and disease expression of Anaplasma marginale an economically important pathogen of cattle worldwide. Specifically through field studies in the United States the determinants of tick-borne (Dermacentor andersoni) transmission of A. marginale are being identified. Through our collaboration with Washington State University the relevant components of the pathogen and the cow’s immune response which lead to a preexisting protective immune response are being defined. We are continuing to collect the primary vector, Dermacentor andersoni, and to test cattle for A. marginale infection at our field study sites in four distinctly different geographic locations in the northwestern United States (north central Washington, south eastern Washington, east central Oregon and west central Montana). We have shown that the abundance of ticks at each site is correlated with prevalence of A. marginale infection in the cattle herd at each site. Anaplasma infections have also been strain typed from ranches in north central Mexico where the primary vector is Rhipicephalus microplus. In Mexico transmission pressure from R. microplus is much more intense and transmission occurs nearly year round, resulting in much higher infection prevalence, in sharp contrast to the situation in the US where the transmission season is short and infection prevalence is lower. Consequently, the strain diversity in Mexico is very high and a large proportion of the cattle are superinfected with more than one strain of A. marginale, whereas in the US the strain diversity is less and superinfection is uncommon. Differences in the intensity of vector-born transmission between the US and Mexico may play a significant role in the differences in infection prevalence and strain diversity. Using strains of A. marginale collected from study sites with higher and lower infection prevalence and D. andersoni collected from geographically separated populations, we have conducted a study to examine the relationship between tick populations and the geographically associated A. marginale strains they transmit. Data to date suggests that the vector capacity of the tick population and efficiency of transmission of A. marginale strains are characteristics that are intrinsic to the tick populations and/or pathogen strains. The current data confirmed that ticks from 2 of the sites (Burns and Stevensville) have significantly different levels of susceptibility to infection with A. marginale, and ticks from these populations have been colonized for future study. Defensins (DaD) are peptides produced in response to bacterial infection and we have hypothesized that these peptides may play a role in vector competence for A. marginale. Data indicates that the levels of expression of DaD are not significantly different between infected and uninfected ticks. Preliminary analysis from ongoing field studies suggests that there is a very large degree of genetic differentiation between geographically defined tick populations.


4.Accomplishments
1. Infection of cattle with the tick-transmitted pathogen Anaplasma marginale is not constrained by the interaction between pathogen strain and tick population. The ability of cattle producers to control disease caused by A. marginale infection is dependent on their ability to manage the parameters that impact transmission. Through studies on ranches within the northwestern United States where the wood tick is the primary vector of A. marginale, ARS scientists in Pullman, WA showed that ticks from widely separated locations are genetically distinct from one another, and yet, these tick populations are equally capable of becoming infected with different geographic isolates of A. marginale. Because factors affecting transmission appear to be intrinsic to the tick populations and pathogen strains rather than the result of interactions between geographically associated pathogens and vectors, producers in areas endemic for A. marginale infection should test newly acquired cattle before mixing animals from different herds. This will help avoid the risk of disease transmission from geographically distinct A. marginale strains.

2. Efficiency of Anaplasma marginale transmission is linked to ability of a strain to replicate to a certain level in the tick salivary gland. The ability of cattle producers to control disease caused by A. marginale infection is dependent on their ability to manage the parameters which impact transmission. By comparing how efficiently different strains of A. marginale are transmitted, ARS scientists in Pullman, WA were able to show that strains of A. marginale with different replication competencies in their tick vector have different transmission efficiencies. A. marginale strains that replicate to higher levels in the salivary glands of their tick vectors can be transmitted by substantially fewer ticks. These results make it clear that development of vaccines to control A. marginale transmission must take into account the presence of strains of A. marginale that have high transmission efficiencies.


Review Publications
Ueti, M.W., Knowles Jr, D.P., Davitt, C.M., Scoles, G.A., Baszler, T.V., Palmer, G.H. 2009. Quantitative Differences in Salivary Pathogen Load during Tick Transmission Underlie Strain-Specific Variation in Transmission Efficiency of Anaplasma marginale. Infection and Immunity. 77(1):70-75.

Galletti, M.F., Ueti, M.W., Knowles Jr, D.P., Brayton, K.A., Palmer, G.H. 2009. Independence of Anaplasma marginale Strains with High and Low Transmission Efficiencies in the Tick Vector following Simultaneous Acquisition by Feeding on a Superinfected Mammalian Reservoir Host. Infection and Immunity. 77(4):1459-1464.

Dark, M.J., Herndon, D.R., Kappmeyer, L.S., Gonzales, M.P., Nordeen, E., Palmer, G.H., Knowles Jr, D.P., Brayton, K.A. 2009. Conservation in the face of diversity: multistrain analysis of an intracellular bacterium. Biomed Central (BMC) Genomics. 10:16.

Leverich, C.K., Palmer, G.H., Knowles Jr, D.P., Brayton, K.A. 2008. Tick-Borne Transmission of Two Genetically Distinct Anaplasma marginale Strains following Superinfection of the Mammalian Reservoir Host. Infection and Immunity. 76(9):4066-4070.

Lysyk, T.J., Scoles, G.A. 2008. Reproductive Compatibility of Prairie and Montane Populations of Dermacentor andersoni. Journal of Medical Entomology. 45(6):1064-1070.

Baldridge, G.D., Scoles, G.A., Burkhardt, N.Y., Schloeder, B., Kurtti, T.J., Munderloh, U.G. 2009. Transovarial Transmission of Francisella-Like Endosymbionts and Anaplasma phagocytophilum Variants in Dermacentor albipictus (Acari: Ixodidae). Journal of Medical Entomology. 46(3):625-632.

Last Modified: 10/25/2014
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