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United States Department of Agriculture

Agricultural Research Service

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

Location: Animal Diseases Research

2010 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
In this fourth year of our current research project we have continued to define the epidemiology of Dermacentor andersoni, by obtaining field ticks and testing geographically related cattle for Anaplasma marginale infection in four distinctly different geographic locations in the northwestern Unites States (north central Washington, south eastern Washington, east central Oregon and west central Montana). With this work we seek to define the relationship between tick population phenotype and risk of transmission. We have narrowed our lab studies to two tick populations with distinctly different phenotypes; the Burns population is more susceptible to infection with A. marginale whereas the Stevensville population is less susceptible to infection. These two populations also differ in the size of individual ticks with the Burns ticks being consistently smaller than the Stevensville ticks. Ticks reared for one generation in the lab do not appear to have the same population susceptibility phenotype as field collected ticks, suggesting that there may be an environmental component to vector competence. Crossing studies are underway to further define the genetics of these differential phenotypic tick-vector characteristics. In collaboration with Canadian scientists, genetic differences between prairie (Alberta) and mountain (British Colombia) populations of ticks have been demonstrated and the reproductive compatibility between populations documented. This same type of study is currently underway for the two U.S. populations described above (Burns and Stevensville), but with the added component of examining the effect of crossing on the phenotypic differences (susceptibility to A. marginale infection and tick size) between the two populations. We are also collaborating with scientists at Northern Arizona University on population genetic studies with D. andersoni and D. variabilis using Restriction Fragment Length Polymorphisms (RFLP), which provide a whole genome screen, in combination with analysis of mitochondrial 16s rDNA single nucleotide polymorphisms (SNP). Our preliminary data suggest that there is hybridization between the two tick species at sites where they are occur sympatrically. We do not yet fully understand the implications of hybridization, but since D. variabilis is a know vector of A. marginale in the southern U.S. and hybridization provides a mechanism for gene flow and for the movement of symbionts between the species it has implications for vector competence. In collaboration with scientists at Washington State University we are defining cattle immune responses to infection. Msp2 of A. marginale is a hypervariable protein that allows the pathogen to evade the host immune system and establish persistent infection. In contrast to persistence following infection, immunization with surface proteins, including Msp2, induces a response that prevents infection upon challenge. Future efforts in vaccine development will focus in the identification of broadly protective, conserved epitopes.


4.Accomplishments
1. Transmission efficiency of Anaplasma marginale. Defining the influence of the tick on the efficiency of pathogen transmission is necessary for the development of vaccines to decrease transmission and associated economic losses. ARS researchers in Pullman, WA in collaboration with their colleagues at Washington State University demonstrated that the efficiency of tick-borne transmission of A. marginale is determined in part by the pathogens’ ability to replicate within tick midgut (tick digestive tract) and tick salivary glands. These data provide direction to vaccine strategies intended to decrease or block transmission efficiency of A. marginale, an economically significant pathogen of cattle.

2. Identification of molecules produced by the cattle blood parasite Anaplasma marginale in a tick which transmits this pathogen. The lack of a vaccine for this blood parasite of cattle is an economic disadvantage for U. S. producers. ARS researchers in Pullman, WA in collaboration with their colleagues at Washington State University showed that a subset of A. marginale proteins is expressed at higher level in the tick digestive tract and salivary glands as compared to red blood cells during infection of cattle. Fifteen A. marginale proteins have been identified that are exclusively expressed or up-regulated during pathogen replication in the tick. These results provide data supporting development of a vaccine able to block tick infection and prevent or reduce the transmission of this pathogen to naïve cattle.

3. Testing of an Anaplasma marginale molecular complex vaccine. The lack of a safe and effective vaccine for anaplasmosis, an economically significant disease caused by the tick-borne pathogen A. marginale is a hindrance to efficient cattle production in many areas of the U. S. ARS researchers in Pullman, WA in collaboration with their colleagues from Washington State University showed that immunization with an A. marginale molecular complex induced protection upon A. marginale challenge of cattle. This molecular complex contained 11 surface-expressed A. marginale proteins. These data provide clear advancement toward defining the components of a safe and effective A. marginale vaccine.


Review Publications
Agnes, J.T., Herndon, D.R., Ueti, M.W., Ramabu, S., Evans, M., Brayton, K.A., Palmer, G.H. 2010. Association of pathogen strain-specific gene transcription and transmission efficiency phenotype of Anaplasma marginale. Infection and Immunity. Available: http://iai.asm.org/cgi/reprint/IAI.00108-10v1

Ramabu, S., Ueti, M.W., Brayton, K.A., Baszler, T.V., Palmer, G.H. 2010. Identification of Anaplasma marginale proteins specifically up-regulated during colonization of the tick vector. Infection and Immunity. Available: 10.1128/IAI.00300-10

Noh, S.M., Zhuang, Y., Futse, J.E., Brown, W.C., Brayton, K.A., Palmer, G.H. 2010. The immunization-induced antibody response to the Anaplasma marginale major surface protein 2 and its association with protective immunity. Vaccine. 28:3741-3747.

Herndon, D.R., Palmer, G.H., Shkap, V., Knowles Jr, D.P., Brayton, K.A. 2010. Complete Genome Sequence of Anaplasma marginale subsp. centrale. Journal of Bacteriology. 192(1):379-380.

Last Modified: 9/2/2014
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