Metabolic changes in cattle due to the specific effect of the tick, Boophilus microplus.

1. An experiment was designed to provide information on the alterations in body metabolism which would account for the loss of body-weight in cattle due to the specific effect (factors other than reduced food intake) of the tick Boophilus microplus. 2. Two groups of British (Shorthorn x Hereford) and Africander x British calves, each approximately months olds, were used: one group (treated) of each breed was tick-infested and the other (control) was tick-free. Within breeds, calves in the control group were pair-fed to calves in the treated group. 3. In both breeds, the effect of ticks: (a) depressed packed cell volume, serum alkaline phosphatase (EC 3.1.3.1) and amylase (EC 3.2.1.1.) activities, plasma cholesterol and phospholipid levels, serum iron and albumin levels, (b) increased the plasma levels of urea-nitrogen and gamma-globulin (c) increased rectal temperature, water intake, urine volume, urinary and faecal total N, urinary urea-N and alpha-amino acids, the excretion of water, sodium and potassium in the faeces and (d) reduced N balance, N and dry-matter digestibilities. 4. In the British breed, ticks increased the excretion of K with a corresponding decrease in the excretion of Na in the urine and increased the plasma clearance of bromsulphthalein. 5. A second experiment showed that the specific effect of tick infestation increased the flow of organic matter (OM) from the abomasum and the fractional turnover of rumen fluid of Hereford steers. It was also shown that the decrease in OM digestibility in the gastrointestinal tract was largely due to a decrease in OM digestibility in the rumen and that the increased urinary urea excretion and plasma urea concentration was caused by higher production rates of urea despite a tendency for lowered urea degradation in the gastrointestinal tract.