|SONDGEROTH, KERRY - Washington State University|
|MCELWAIN, TERRY - Washington State University|
|LAU, AUDREY - Washington State University|
Submitted to: Infection and Immunity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2014
Publication Date: 8/11/2014
Citation: Sondgeroth, K.S., Mcelwain, T.F., Ueti, M.W., Scoles, G.A., Reif, K.E., Lau, A.0. 2014. Tick passage results in enhanced attenuation of babesia bovis. Infection and Immunity. 82(10):4426-4434.
Interpretive Summary: Bovine babesiosis is an infectious disease caused by the parasite Babesia bovis which is transmitted by ticks. This disease results in significant economic losses to the livestock industry. There are two strains of the parasite, one strain induces severe disease that may result in the death of animals. Another strain results in a milder form of the disease from which animals generally recover. This mild strain of B. bovis is generated by passing infected blood through several splenectomized cattle. There is limited information regarding the ability of ticks to transmit the mild strain of B. bovis. It is unknown if ticks are capable of transmitting the mild strain parasites and if transmission affects the severity of the disease induced by the mild strain. In this study, we compared severe and mild strains of B. bovis by tick transmission. Our results demonstrated that the mild strain was transmitted by ticks and, interestingly, tick transmission also made the mild strain less capable of inducing disease.
Technical Abstract: Serial blood passage of virulent Babesia bovis in splenectomized cattle results in attenuated derivatives that do not cause neurologic disease. Tick transmissibility can be lost with attenuation, and has been reported to result in a reversion to virulence following tick passage. This study provides data that tick passage of the partially attenuated B. bovis T2Bo derivative strain further decreased virulence when compared with intravenous inoculation of the same strain in infected animals. Ticks that acquired virulent or attenuated parasites by feeding on infected cattle were transmission fed on naïve, splenectomized animals. While there was no significant difference between groups in the number of parasites in the midgut, hemolymph or eggs of replete female ticks after acquisition feeding, animals infected with the attenuated parasites after tick transmission showed no clinical signs of babesiosis, unlike intravenous challenge with the same attenuated strain prior to tick passage. Additionally, there were significantly fewer parasites in blood and tissues of animals infected with tick passaged attenuated parasites. Sequencing analysis of select B. bovis genes before and after tick passage showed significant differences in parasite genotypes in both peripheral blood and cerebral samples. These results provide evidence that not only is tick transmissibility retained by the T2Bo attenuated strain, it results in enhanced attenuation, and is accompanied by expansion of parasite subpopulations during tick passage that may be associated with the change in disease phenotype.