The biological economy of water and cholesterol in the cattle tick Boophilus microplus (Canestrini)

Abstract

The cattle tick Boophilus microplus (Canestrini) is a parasite of economic importance in tropical and subtropical coastal areas of Australia. Aspects of its biology have been studied with the hope of providing data which may help in the eventual development of a means of specific and lasting control. Results of these studies can be summarised as follows:1. Changes in water content and cuticle weight during adult female parasitic life were measured, utilization of water and dry matter for egg production by the engorged tick was investigated. The data suggest that capacity for cuticle growth, and thus for distension, determines water content in relation to body weight, and this in turn the fecundity of the female. Figures for Australian B. microplus are compared with those for this species in Japan, where higher average engorged weight, cuticle weight, percentage water content, and numbers of eggs laid have been reported.2. Eggs laid over a known period of time were incubated under standard conditions until hatching commenced. Changes in weight, water content, total lipid, and total and free cholesterol were measured. Water content fell until approximately half way through the incubation period, then rose to equal the quantity originally present. Lipid was utilized for a short period only and was not a major food reserve. Total cholesterol remained constant, but the percentage of free cholesterol rose considerably during incubation. This rise and the utilization of lipid coincided in time with the increase in water content. It seems probable that the egg possesses, or develops, a means of controlling water loss, and that at a certain stage of embryonic development a major, mobilization of food reserves takes place.3. The increase in cholesterol content of the engorging female was measured. The rate of accumulation of sterol as body weight increased was not constant, but followed a pattern for which the best fitting mathematical model was a two-step function quadratic in terms of weight. The ratio of free to total cholesterol also varied, being highest when total cholesterol was increasing most rapidly, and lowest when little cholesterol was being added. The percentage of cholesterol in the dry weight added by the tick as body weight increased bore no relation to the figure calculated for this in host blood. It seems likely that the tick is able to control the retention of ingested cholesterol, rather than passively accumulating it. This control may be exercised by way of salivary regurgitation of excess sterol.4. Cholesterol and its esters appear to be the only steroid materials present in the tissues of female B. microplus and their eggs. The total cholesterol content of newly dropped engorged females shows a direct relationship with body weight. The percentage remaining in females after the completion of oviposition varies with the weight of the newly engorged tick. Eggs contain a virtually constant quantity. A balance sheet constructed from figures for ticks and their eggs accounts for only 37% to 51% of the cholesterol present in newly engorged ticks weighing from 100 to 300 mg. It appears possible that an adjustment of the relationship between cholesterol content and either dry weight or water content may take place prior to detachment, and that the cholesterol not accounted for in the balance sheet may be required by the ovipositing tick for purposes other than maintenance and incorporation into eggs. This cholesterol or its metabolites may be located in the egg wax.5. Egg wax examined by thin-layer chromatography showed the presence of substances which gave reactions typical of steroids, and a preliminary analysis of the wax was carried out. Three fractions, designated hexane-soluble, benzene-soluble and were separated. The hexane-soluble fraction contained the substances reacting like steroids, and this was examined in more detail by column, thin-layer, and gas-liquid chromatography, and ultraviolet spectrophotometry. Hydrocarbons formed a small proportion of this fraction, the major constituents being saturated alkyl esters. The presence of a hydrocarbon with the UV spectrum of 2, 4, 6-cholestatriene was confirmed, and a sterol with a similar spectrum was isolated. Cholesterol and cholesteryl esters appeared to be absent. A preliminary separation of other steroid-like substances was made, and the UV spectra of the fractions recorded, as a basis for further chemical investigation. A tentative scheme for the separation of the components of the benzene-insoluble fraction of the wax was also recorded.