Location: Animal Disease Research2013 Annual Report
1a. Objectives (from AD-416):
Objective 1: Improve control of bovine babesiosis and its tick vector through the identification of novel targets and development of new vaccine strategies. Sub-objective 1A: Identify surface exposed antigens expressed by Babesia bovis during its development within the tick. Sub-objective 1B: Test if antibody response in cattle immunized with tick midgut surface antigens enhances access of bovine antibodies to tick hemolymph. Sub-objective 1C: Determine if immunization with Babesia tick stage and midgut antigens blocks B. bovis transmission. Objective 2: Evaluate the risks of B. equi or B. caballi transmission by species of ticks indigenous to the United States. Sub-objective 2A: Identify tick species feeding on horses at the outbreak ranch in Texas that are capable of transmitting B. equi. Sub-objective 2B: Determine the B. equi transmission efficiency of vector-competent tick species from the outbreak ranch in Texas. Objective 3: Determine the impact of both new and current chemotherapeutic agents on the clearance of B. equi or B. caballi from persistently infected horses and on the risks of transmission. Sub-objective 3A: Assess the efficacy of imidocarb dipropionate to eliminate B. equi infection Sub-objective 3B: Develop a serological method that rapidly and accurately predicts elimination of B. equi infection following treatment. Sub-objective 3C: Assess tick-borne transmission risk of imidocarb dipropionate treated horses.
1b. Approach (from AD-416):
The discovery of chemical (acaricide) resistant ticks capable of transmitting protozoan parasites that cause bovine babesiosis and the reemergence of equine babesiosis within the United States are the catalysts for this research plan. National concerns are eradication of the currently emerging equine babesiosis, the possibility of reemergence of bovine babesiosis and the need for novel methods to control the causal parasites and their tick vectors. The current strategy in the U.S. for bovine babesiosis is control of ticks through acaricides. However, the development of ticks resistant to acaricides is resulting in geographic expansion of these vectors in the U.S. Since cattle entry is not monitored by serology, infected cattle are entering the U.S. The risk of bovine babesiosis is significantly increased by the presence and expanding range of ticks capable of transmission and the lack of serologic monitoring of cattle. To protect the U.S. cattle population from babesiosis, this research plan proposes to develop novel immunological interventions and to use these tools to test the hypothesis that tick transmission of Babesia bovis can be blocked. The primary goal is to disrupt the B. bovis-tick interface at the level of the tick midgut and hemolymph. In contrast to bovine babesiosis, the control strategy for equine babesiosis is based on serological restriction of infected horses from entering the country. Due to previous use of the complement fixation test (CFT), which lacked sensitivity, infected horses have been admitted into the U.S. which likely led, at least in part, to the recent reemergence of this foreign equine disease in the U.S. The goal of this research plan related to equine babesiosis is to assist the Animal Plant Health Inspection Service (APHIS), State Veterinarians and owners in eliminating Babesia infections, transmission risk, and potential endemicity by developing pharmacological interventions. Replacing 5348-32000-028-00D (October 2011).
3. Progress Report:
During FY 12-13 our laboratory has made significant progress toward the development Babesia bovis transmission blocking vaccines by identifying proteins expressed exclusively during the tick stage of B. bovis (Objective 1a). We have developed methods to synchronize feeding of cattle ticks (Rhipicephalus microplus) on calves during the peak of Babesia bovis parasitemia and to induce production of Babesia sexual stages in erythrocytes tissue culture. This has made it possible to collect of large numbers of B. bovis kinetes from tick hemolymph, allowing us to identify protein vaccine targets that are specific to this tick stage of the parasite. Production of parasite sexual stages in culture has allowed us to isolate pure Babesia sexual stages, which facilitates identification of protein vaccine targets that are specific to this stage of the parasite. These two methods have allowed us to study kinetes and sexual stage specific protein expression in Babesia in order to target vaccines to these critical parasite life stages. We have continued to develop the Babesia transfection system that can be used for development of live attenuated vaccine strains that deliver vaccine antigens, or can be used as a system to knockout specific genes for studies of gene function and strain attenuation. Progress includes development of a new transfection vector that can express two different exogenous genes at the same time. This system was used to transfect an attenuated strain of B. bovis and the transfected strain remained stable in infected cattle. Cattle infected with this transfected strain were protected from disease when challenged with a virulent strain of B. bovis, demonstrating that transfected attenuated strains can be used as live vaccines to protect against B. bovis infection (Objective 1). As part of our ongoing work on Babesia (Theileria) equi, we have determined that imidocarb dipropionate can eliminate Babesia equi from naturally persistently infected horses and we have shown that this treatment can remove the future risk of transmission (objective 3a). Because many infected animals that have been cleared remain seropositive on the commercially available regulatory approved cELISA assay we have also begun to identify other immunological markers that may be more useful for defining clearance of infection. We have done this by beginning to characterize the full repertoire of immunoreactive antigens in T. equi infected and treated/cleared horses (Objective 3b). Also, as part of our cooperative agreement between Texas A&M University and our laboratories we have identified tick species feeding on horses in the Equine piroplasmosis outbreak area of Texas and have begun vector competence studies to determine which of these species are capable of transmitting equine piroplasmosis (Objective 2).
1. Development of a treatment protocol for elimination of persistent equine piroplasmosis infection from horses. Equine piroplasmosis (EP) is a tick-borne disease of horses that was eradicated from the United States, but recent outbreaks have shown that there is a risk of reestablishment. Prior to this work the spread of Theileria equi, the agent of EP, has been controlled by euthanizing or permanently quarantining infected horses. ARS scientists at Pullman, Washington, in collaboration with scientists in the Department of Veterinary Microbiology and Pathology at Washington State University developed a method for treating naturally infected horses with imidocarb dipropionate and demonstrated that treatment can not only eliminate the parasite from the peripheral blood but also removes the risk of transmission to other horses. The success of this research has provided a tool to save infected horses and block the spread of disease; this will aid the equine industry both in the US and globally by removing the risk of transmission associated with infection.
2. Characterization of the repertoire of Theileria equi immunodominant antigens bound by equine antibody. The risk of transmission associated with persistent equine piroplasmosis infection severely limits ability to transport infected horses, which results in economic losses. Defining equine serological response to infection with T. equi is critical for development of markers for diagnosis and treatment. In this work ARS scientists at Pullman, Washington, in collaboration with scientists in the Department of Veterinary Microbiology and Pathology at Washington State University have characterized the five immunodominant T. equi antigens recognized by the antibodies in T. equi infected horses. This work will lead to the development of improved diagnostic and therapeutic methods for control of equine piroplasmosis, including possibly the development of an immunological assay that can distinguish between infected horses and horses that test seropositive but no longer pose a transmission risk because they have been treated and cleared of the infection.
Grause, J.F., Ueti, M.W., Nelsona, J.T., Knowles Jr, D.P., Kappmeyer, L.S., Bunn, T.O. 2012. Efficacy of imidocarb dipropionate in the elimination of Theileria equi in experimentally infected horses. The Veterinary Journal. 10.1016/j.tvjl.2012.10.025.