Differences in the Life Cycles Between a Vaccine Strain and an Unmodified Strain of Babesia bovis (Babes, 1889) in the Tick Boophilus microplus (Canestrini)*

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Abstract
Developmental forms of 2 strains of Babesia bovis were studied in the tick vector Boophilus microplus. One strain (T) was infective for the tick, and the other (NT) lost infectivity for the tick because of repeated blood passaging in cattle. Parasites of the 2 strains in gut contents of adult female ticks were similar during the first 16 h post-repletion (PR), but thereafter their structure differed. From 16-64 h PR, the majority of T strain parasites were spherical and without processes. During the next 32 h elongate forms and vermicules developed. Fission bodies were seen within epithelial cells of the gut by 96 h PR. T-strain parasites in gut contents decreased in number from 96 h and were difficult to find at 144 h, the time of the final observation. NT strain parasites were plentiful throughout the period of observation. They were predominantly spherical, with a diameter of 1.5-15 .mu.m. Forms with obvious processes measuring up to 81 .mu.m in length were seen in large numbers at seemingly regular intervals of 16-144 h PR, suggesting that a process of development and divisions was being repeated. No vermicules or fission bodies were seen. T-strain, but not NT strain parasites, were seen in hemolymph and ova of the ticks and in their larval progeny. Continuous blood passaging of the NT strain apparently resulted in selection of parasites incapable of penetrating gut epithelial cells of the tick.