Submitted to: Journal of Clinical Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 29, 2003
Publication Date: December 12, 2003
Citation: Ueti, M.W., G.H. Palmer, L.S. Kappmeyer, G.A. Scoles and D.P. Knowles. 2003. Expression of Equi Merozoite Antigen 2 during Development of Babesia equi in the Midgut and Salivary Gland of the vector tick Boophilus microplus. J. Clin. Micro. 41(12):5803-5809. Interpretive Summary: Babesia equi is a protozoan parasite that infects horses, it is transmitted from one horse to another by Boophilus microplus ticks. Two proteins called equi merozoite antigen-1 and 2 (EMA-1 and EMA-2) are present on the surface of the form of Babesia equi that is found in the blood of infected horses (the merozoite). These proteins are similar to proteins found on the surface of the blood stage of Theileria, which is another similar tick transmitted pathogen. We wanted to find out if EMA-1 and EMA-2 were also present on the surface of the stages of Babesia equi that grow in the tick which transmits it. In ticks Babesia equi goes through several stages of development; each of these stages looks different and infects different tissues of the tick. These developmental stages include sexually reproducing forms in the blood inside the tick gut, stages which infect the cells of the gut, and stages which infect the salivary glands of the tick. The developmental stages in the salivary glands are called sporozoites, and they are what the tick transmits when it feeds on another horse. There are monoclonal antibodies that can identify both EMA-1 and EMA-2 proteins by binding to them. The antibody for EMA-2 binds to Babesia equi in the gut of the tick (the sexual stages), within the cells of the gut, and in the salivary glands (the sporozoites); this shows that the EMA-2 protein is present on the surface of all of these developmental stages of Babesia equi. By using the antibody to EMA-2 to identify Babesia equi in the gut of the tick we were able to show that the amount of Babesia equi in the blood of the horse during the time that the ticks are feeding affects the number of ticks that have Babesia equi infecting their gut cells. The antibody for EMA-1 did not bind to any of the stages of Babesia equi in the tick; this is evidence that the EMA-1 protein is not present on the surface of the tick stages of Babesia equi. One possible method for the control of tick transmitted pathogens is to make vaccines that prevent the ticks from being able to transmit them (transmission blocking vaccines). Knowing that the EMA-2 protein is present on the surfaces of all of the tick stages of Babesia equi and being able to detect it with a monoclonal antibody will make it a useful tool for detecting Babesia equi infection in ticks. This will help us to find proteins that are found only on the tick stages of Babesia equi, and it will be important for testing transmission blocking vaccines.
Technical Abstract: Equi merozoite antigen-1 and 2 (EMA-1 and EMA-2) are Babesia equi proteins expressed on the parasite surface during infection in horses and are orthologues of proteins in Theileria spp, which are also tick-transmitted protozoal pathogens. We determined in this study whether EMA-1 and EMA-2 were expressed within the vector tick Boophilus microplus. B. equi transitions through multiple, morphologically distinct stages, including sexual stages, and these transitions culminate in the formation of infectious sporozoites in the tick salivary gland. EMA-2 positive B. equi stages in the midgut lumen and midgut epithelial cells of Boophilus microplus nymphs were identified by reactivity with monoclonal antibody 36/253.21. This monoclonal antibody also recognized B. equi in salivary glands of adult Boophilus microplus. In addition, quantification of B. equi in the mammalian host and vector tick indicated that the duration of tick feeding and parasitemia levels affected the percentage of nymphs that contained morphologically distinct B. equi in the midgut. In contrast, there was no conclusive evidence that B. equi EMA-1 was expressed in either Boophilus microplus midgut or salivary gland when monoclonal antibody 36/18.57 was used. The expression of B. equi EMA-2 in Boophilus microplus provides a marker for detecting the various development stages and facilitates the identification of novel stage-specific Babesia proteins for testing transmission blocking immunity.