Tick passage results in enhanced attenuation of Babesia bovis

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, but when retained, attenuated B. bovis can revert to virulence following tick passage. This study provides data showing that tick passage of the partially attenuated B. bovis T2Bo derivative strain further decreased virulence 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 naive, 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 those receiving 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 attenuated T2Bo strain, but also 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.

FIG 1

Sequestration and parasite levels in postmortem tissues of T2Bo_vir- and T2Bo_att-infected animals. (A) Sequestration determined by histologic analysis. Infected red blood cells are indicated by arrowheads. (B) Parasite level per gram of tissue, as determined by qPCR. vir, virulent; att, attenuated.

FIG 2

(A) Percentage of positive tick tissues after acquisition feeding on T2Bo_vir- or T2Bo_att-infected animals. (B) Level of parasites in tick tissues. The left y axis indicates parasite level per gram of tissue; the right y axis indicates parasite level per μl of hemolymph. An asterisk indicates statistical significance (P < 0.05). vir, virulent; att, attenuated.

FIG 3

Frequency of gene marker variants detected in acquisition-fed (AQ) and transmission feeding (TR) animals.