The erythrocytes and plasma of a British and Brahman crossbred steer were labelled with 51Cr and 125I respectively. The radioactivity levels were subsequently maintained as constant as feasible by injecting the steers with calculated amounts of the appropriate labelled material on 3 consecutive days. The steers had previously been heavily infested with Boophilus microplus to ensure that all stages in the parasite's life-cycle would be present during the 4-day period, when the steers were being treated with isotopes.
Various stages of B. microplus larvae, nymphs and adults were collected and the uptake of red cells and plasma at each stage assessed by radioassay. In certain calculations, corrections were made for the uptake of blood fractions before the animals were made radioactive.
A relationship between the weight of the tick and its dietary intake was established. At all the stages of larval and nymphal feeding the plasma content of the diet was greater than that of the host blood. However, erythrocytes were detectable even in the earliest larval stages examined. Dropped fully engorged adult females contained more red cells per individual, and generally also more plasma, than engorged ticks removed from the host.
Fully engorged adult females took up as much as twice their own weight of blood components, but in none of the earlier stages did the tick concentrate its blood meal.
No obvious differences could be demonstrated statistically between the behaviour of the parasites on the two hosts. However, indications are that recently attached larvae took up more erythrocytes from the British animal.
We wish to thank Messrs A. K. Duffield, A. J. Short, B. Wilson, and Miss S. J. Shepherd, for skilful technical assistance.