Location: Animal Disease Research2012 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:
The current research plan addressed critical gaps in our knowledge concerning transmissibility, pathogenesis, and chemotherapy of tick-borne pathogens that affect livestock including cattle and horses. These pathogens transmitted by ticks cause significant economic losses to the livestock industries worldwide. In the United States, a significant economic loss has been observed recently caused by the outbreaks of equine piroplasmosis. The major concerns to the US livestock industry are: 1) the reestablishment of endemicity of equine piroplasmosis following the recent outbreaks in Florida and Texas and 2) the risk of outbreaks of bovine babesiosis due to increasing resistance of Rhipicephalus tick populations to acaricides in the southern border of United States. The current report encompasses the progress of project plan 5348-32000-034-00D throughout FY 11-12. During the past year our laboratory has made significant progress toward the development of blocking Babesia bovis transmission vaccine by identifying B. bovis tick stage specific proteins. We were able to synchronize Rhipicephalus microplus ticks acquisition feeding and Babesia bovis peak parasitemia to enhance B. bovis kinete stage formation in the tick hemolymphs. The synchronization has facilitated the isolation of kinete stage and has assisted to identify unique surface expressed proteins on B. bovis kinete stage. In addition, in a collaboration with Washington State University (research project #5348-32000-034-01S/Specific Cooperative Agreement: 58-5348-7-528), ARS scientists from Pullman, Washington, have developed transfection Babesia delivery vaccine system. The gfp-bsd-transfected B. bovis was able to infect naïve calves and causes acute and persistent infection. These gfp-transfected parasites remained stably integrated in the genome of parasites recovered from experimentally infected cattle. Our laboratory also completed risk assessment for understanding the ability of native ticks in the United States capable of transmitting Babesia equi and initiated work testing the ability of imidocarb dipropionate in the elimination of B. equi from naturally, persistently infected horses in the United States. We have identified tick species feeding on horses at the outbreak capable of transmitting equine piroplasmosis. In addition, we have determined the impact of chemotherapeutic agent, imidocarb dipropionate, on the elimination of Babesia equi from naturally, persistently infected horses and on the removal of the risk of transmission.
1. Identification of tick stage-specific proteins of Babesia bovis. Bovine babesiosis is one of the most important tick-borne diseases that cause high morbidity and mortality worldwide. The lack of vaccine to control infection and/or disease in the United States and the emerging of tick population resistance of acaricides in the US-Mexico border are the major concerns for the US cattle industry. ARS scientists at Pullman, Washington, are investigating differential surface proteins expression on distinct stage of Babesia bovis parasites during infection of mammalian and invertebrate hosts. We demonstrated that unique proteins are expressed by B bovis kinete as compared with the blood stages and identified conserved sexual stage proteins among Apicomplexan parasites transmitted by arthropod vectors. These results provided the basis for the development of a novel strategy to control the spread of B bovis through its tick-vector, which would offer a strategy to prevent B bovis dissemination in case of outbreaks within the United States.
2. Identification of tick vector responsible for the spread of Babesia equi within the United States. Equine piroplasmosis is considered a foreign disease in the United States since 1980's. However, the U.S. horse industry suffered a significant economic loss in the recent outbreaks in Florida and Texas. Persistently infected horses are the reservoirs for tick acquisition and transmission of the pathogens. ARS scientists at Pullman, Washington in collaboration with APHIS-National Veterinary Service Laboratory, Ames, Iowa and APHIS-Western Regional Office, Fort Collins, Colorado identified native tick in the U.S. Amblyomma cajennense, is the predominant tick species found on horses at the Texas outbreak and is capable of transmitting B. equi. In addition, our laboratory was able to acquire tick transmitted B. equi isolates from the Texas outbreak of equine piroplasmosis. Understanding vectorial competency of the U.S. native ticks is critical to design an efficient control strategy to prevent the dissemination of equine piroplasmosis within the United States and avoid great economic burdens to the U.S. horse industry.
3. Elimination of Babesia equi from naturally infected horses using imidocarb dipropionate. Control of the spread of equine piroplasmosis is based on permanent quarantine or euthanasia of infected horses. The euthanasia had been applied to eradicate equine piroplasmosis since the 1980's. However, the use of euthanasia as the sole control method is problematic for the following reasons: 1) euthanasia is costly to the owner due to loss of commerce and in some cases loss of valuable and irreplaceable genetics; 2) costs to the government associated with indemnity payments, long-term surveillance to assure our trading partners of Babesia-free status, and in order for euthanasia to work, all U.S. horses must be tested and positive horses euthanized; and 3) euthanasia is controversial because of the emotional impact on owners and families. This research investigates the ability of anti-protozoan drugs to eliminate B. equi infection as an alternative to euthanasia. ARS scientists at Pullman, Washington in collaboration with Department of Veterinary Microbiology and Pathology, Washington State University initiated treatment of naturally infected horses to determine imidocarb dipropionate efficacy against B. equi parasites. The results are promising and could facilitate the use of imidocarb in the elimination of parasites and risk of transmission. Meanwhile, we were able to collect serum samples from the treated horses during multiple time points to develop new diagnostic test for confirmation of B. equi elimination.
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