This experiment was designed to measure the effects of infestation by B. microplus on cattle and to separate the effects of reduced food intake ("anorectic effect") from those due to the remaining factors of tick infestation ("specific effect"). Hereford cattle kept on a high-quality diet were studied over a treatment period of 11 weeks with the tick-infested animals being infested regularly with equal larval doses for each animal.The anorectic effect accounted for approximately 65% of the depression of body weight due to tick infestation. Body weights were not related to the numbers of maturing female ticks counted on the infested animals. However, the body weights were related to food intake, the large variation in which was considered a reflection of the variable effect of the toxic principle of the tick on the appetite of the cattle.After treatment, tick-infested cattle were kept clear of tick and run with the two control groups of cattle. Pasture was supplemented with extra rations. The compensatory gain made by the infested group was less than that of the group which had been matched with it for food intake and kept tick-free. This indicates a severe effect on the metabolism of the tick-infested animals, with prolonged after-effects.

No abstract

Tritiated water (THO) has been used in the study of exchangeable H atoms in proteins to provide information on molecular structure. On account of the high sensitivity of detection methods for tritium, it need be used only in tracer amounts and this leads to several advantages over deuterium : (i) there is less risk of conformational changes occurring in the labelled protein, (ii) lower protein concentrations may be used (0.2 to 2.0%), and (iii) rates of exchange are slower. The method has been used to study the T-H exchange of ribonuclease dissolved in THO and the H-T exchange of the N,O-tritiated protein dissolved in H2O between 0 and 70 �C. The extent of exchange was measured in the former case by T-assay on aliquots of dried protein and in the latter case by T-assay on water samples collected by sublimation. Optimal conditions for sampling, drying, and T-assay are described under which further exchange was minimized. In particular, it was found unnecessary to dry at temperatures above 40 �C. The number of exchangeable H atoms in the ribonuclease molecule was determined as a function of temperature. As reported by Schildkraut and Scheraga (1960) there appears to be a critical temperature below which certain H atoms will not exchange within 24 hr. However, the number of such atoms (50-60) is greater, and the critical temperature (42 �C) lower than previously reported. This discrepancy is discussed as well as the significance of slow and rapid H exchange in relation to protein conformation.

Twenty-four steers, comprising British (Hereford and Hereford x Shorthorn), Zebu (Africander), and Zebu cross (British x Brahman or Africander) breeds, were either maintained on pasture, or yarded and fed on diets of a low and a high nutritional value. Tritiated water was injected into the animals on five occasions at intervals of 3 months. The body water content and the water turnover rate were calculated, and some of the sources of variation defined. Observed differences in the water content are attributable to nutritional factors rather than to breed differences. The mean body water content ranged from 615 to 809 ml/kg fasting body weight, where the higher values were associated with a poor diet. The mean half-life of tritiated water was lower in summer (as low as 58 hr) than in winter (up to 128 hr) in grazing and well-fed yarded steers. On a poor diet, however, the half-life in yarded cattle remained high and almost constant throughout the year, dropping to below 100 hr on only a single occasion. Occasionally the half-life was breed dependent, but generally no significant differences between breeds could be found. While mean turnover rates of up to 7.1 ml kg-1 hr-1 were found in better-fed cattle in summer, the value in poorly fed animals was almost constant throughout the year at about 3.3 ml kg-1 hr-1. There was, however, a winter minimum in the well-fed yarded and grazing groups. The turnover rate was also influenced by breed only to a limited extent. The results are interpreted in the light of their possible significance in the adaptation to a tropical environment, and in relation to their value in predicting the body composition.

The erythrocytes and plasma of a British and Brahman crossbred steer were labelled with51Cr and125I 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 withBoophilus microplusto 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 ofB. micropluslarvae, 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.