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Biomedical subjects

D J Kok

Publications and source records attributed to D J Kok.

At least 73 records · Page 4Linked to original sources
PubMed1995

The efficacy of Diflubenzuron against the body louse (Damalinia limbata) of Angora goats.

The efficacy of an insect growth regulator, Diflubenzuron, to control biting lice (Damalinia limbata) on Angora goats was investigated. Lice on goats which were dipped in a concentration of 625 g Diflubenzuron to 1,000 1 water and then kept in quarantine was eradicated after a single treatment. Nymphs remained present on these goats up to 4 weeks and adult lice up to 8 weeks post-treatment. A single treatment provided greater than 90% reduction in nymphal stages from Week 6 to Week 16 post-treatment in treated goats continuously exposed to reinfestation. The treatment reduced adult louse infestations on these goats by 78-94% through Weeks 6-16 post-treatment. At the end of the trial (24 weeks post-treatment), these goats had 88% fewer nymphs and 84% fewer adult lice compared with untreated control goats.

Animals↗
PubMed1995

Urinary Tamm-Horsfall protein increased after potassium citrate therapy in calcium stone formers.

OBJECTIVES: To evaluate the effect of oral potassium citrate therapy on urinary excretion rates of citrate. Tamm-Horsfall protein (THP), and on calcium oxalate monohydrate crystal agglomeration inhibition [tm], in patients with recurrent calcium stone formation. METHODS: To evaluate the effect of oral therapy with potassium citrate on urinary citrate, THP, and [tm], 24-hour urine samples were collected before and at least 2 months after initiation of oral potassium citrate therapy in 33 calcium stone-forming patients who had no dietary restrictions. The citrate concentration was measured by an adaptation of a citrate lyase method. Urinary disaggregated THP concentration was determined with a quantitative enzyme-linked immunosorbent assay. The [tm] was determined by observing the effects of patients' urine, before and after oral potassium citrate therapy, on the uptake of 45Ca2+ onto the surfaces of added preformed calcium oxalate crystals in a supersaturated solution of calcium oxalate, using the in vitro kinetic method described by other investigators. RESULTS: We observed an increased urinary excretion rate of citrate from a mean of 1.9 mmol/24 h prealkali to 2.6 mmol/24 h postalkali (P < 0.0004) and of THP from a mean of 94.0 mg/24 h prealkali to 199.3 mg/24 h postalkali (P < 0.0016). A corresponding increase in [tm] from a mean of 177.1 minutes prealkali to 221.0 minutes postalkali (P < 0.024) was also observed. CONCLUSIONS: To our knowledge this is the first report correlating increased urinary citrate with THP excretion rate following oral alkalinization with potassium citrate in calcium stone formers. Of clinical importance is the corresponding increase in [tm], which was previously shown to be inversely related to stone-forming activity. Moreover, urinary citrate and THP are known to have a synergistic effect on [tm]. Our data suggest that the effectiveness of potassium citrate therapy in calcium stone-forming patients may, at least in part, be due to increased levels of THP.

Adult↗
PubMed1995

The role of Hyalomma ticks in foot infestations and temporary lameness of sheep in a semi-arid region of South Africa.

An outbreak of lameness amongst Merino lambs, associated with the presence of Hyalomma ticks, was investigated on a farm in the south-western Free State, South Africa. The purpose was to follow the progress of the condition and to determine the extent of involvement of the two Hyalomma species which occur in the region. The flock of experimental sheep (n = 460) ranged free in natural veld under extensive farming conditions. During September and October 1993, adult ewes and lambs in this flock were examined at weekly intervals to determine tick identity, abundance and attachment-site preferences on lame and unaffected animals. Lameness occurred only among lambs, of which 68 were affected during the 8-week period. Hyalomma ticks tended to aggregate and mean numbers of ticks/aggregation were significantly higher on lame lambs (mean = 11.3) than on either the unaffected lambs (mean = 6.9) or the ewes (mean = 7.1). Most tick aggregations (72.4%) on the lame lambs occurred on the lower legs and feet, 34 out of 55 of these on the fetlocks or interdigital clefts. Hyalomma truncatum dominated (> 97%) on all animals examined. Only 15.8% of the Hyalomma marginatum rufipes recovered from the lame animals were attached to the legs. At two other localities at which H. marginatum rufipes was more abundant, or even dominant, few ticks attached to the lower legs or feet. Those that did were mostly H. truncatum (> 90%). Both H. marginatum rufipes and H. truncatum may attach to the same ventral and anogenital body regions, but H. truncatum has a tendency to attach also to the feet and lower legs. Although attachment of one or a few ticks in the axillar region or upper legs may cause lameness in sheep, the attachment of ticks to the interdigital clefts and fetlocks almost always causes lameness. The latter condition is more likely to occur in regions where H. truncatum dominates.

Animals↗
PubMed1995

Attachment preferences of Hyalomma truncatum and Hyalomma marginatum rufipes ticks (Acari: Ixodidae) on two sheep breeds.

Hyalomma ticks were collected from Merino and black-headed Dorper sheep and their attachment preferences determined. The preferred site of attachment for Hyalomma marginatum rufipes on both Dorper and Merino sheep was the anogenital and inguinal areas (75-76%). On Dorper sheep, H. truncatum attached predominantly to the anogenital and inguinal areas (67.7%). On Merino sheep, most adults of this species attached to the feet (26%). Almost equal percentages also attached to the anogenital/inguinal and brisket areas (21.6 and 22.9%, respectively). These differences should be considered when pour-on acaricides are applied to different breeds of sheep.

Animals↗
PubMed1994

Vertical migration of adult Ixodes rubicundus, the Karoo paralysis tick (Acari: Ixodidae).

Mortality due to Karoo paralysis amongst livestock is of considerable economic importance in South Africa. Indications are that the extent of vertical migration of adult ticks, to reach questing positions on grasses, is influenced by sudden climatic changes. Such influences as well as selection of questing substrates were investigated under controlled conditions. Vertical migration of adult Ixodes rubicundus on copper rods was determined in a square metal arena within a closed glass container in a room in which temperature and light could be controlled. Significantly more female and male ticks migrated vertically on 1.5 mm diameter rods than on 4.0 mm rods and on 450 mm long rods than on 100 mm rods. More ticks of both sexes migrated vertically at RH > 80% than at RH < 50% and at 12 degrees C than at 21 degrees C. More female than male and more older than younger ticks migrated vertically in all experiments. The results of this study contribute towards an understanding of the epidemiology of Karoo paralysis.

Animals↗
PubMed1994

Calcium oxalate stone agglomeration reflects stone-forming activity: citrate inhibition depends on macromolecules larger than 30 kilodalton.

To evaluate the clinical utility of in vitro calcium oxalate monohydrate (COM) crystallization kinetics measurements and to determine the effect of quantitative removal of urinary Tamm-Horsfall glycoprotein on such measurements, we examined 24-hour, room temperature urine collections of patients from our Stone Clinic and of normal subjects from our research laboratories at Ochsner Medical Institutions in New Orleans, LA, and compared their COM kinetic parameters in vitro before and after urine ultrafiltration (30 kd). Data from 53 calcium oxalate stone-forming patients (26% women; mean age, 47 years) who demonstrated radiographic or other evidence of forming at least one stone were compared with data from 22 healthy volunteers (25% women; mean age, 40 years). Hypercalciuria (> 7.5 mm/24 hr), hyperoxaluria (> 0.5 mm/24 hr), and hypocitraturia (< 2.0 mm/24 hr) were present in 38%, 26%, and 26% of the patient population, respectively. Urinary creatinine, urate, calcium, citrate, phosphate, oxalate, pH, volume, total immunoreactive-disaggregated Tamm-Horsfall glycoprotein, and the urine's effects on COM solubility, percent crystal growth inhibition, and crystal agglomeration inhibition [tm] were determined. Calcium oxalate monohydrate agglomeration inhibition, [tm], was reduced in stone-forming patients. It decreased with increasing stone frequency, making [tm] a useful tool for measuring the risk of stone recurrence. Urinary Tamm-Horsfall glycoprotein and citrate concentrations were linearly related to COM agglomeration inhibition. Their effects were synergistic. Tamm-Horsfall glycoprotein removal from urine reduced COM agglomeration inhibition dramatically. Alkali therapy increased urinary citrate concentration and increased [tm].(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

Calcium oxalate nephrolithiasis, a free or fixed particle disease.

The chances of stone formation occurring through a free particle mechanism were calculated using the approach of Finlayson and Reid [1]. For these calculations we used new data on nephron dimensions, supersaturation and crystal growth rates in urine, and also incorporated the size increasing effect of crystal agglomeration. The calculations were performed assuming different levels of oxalate excretion, simulating the diurnal variation and acute hyperoxaluria following a dietary load. In addition urinary flow conditions were varied according to changes in daily urinary volume. It is shown that during the normal transit time of urine through the nephron, particles can obtain a size big enough to be retained in the nephron. This is mainly due to the size-increasing effect of the agglomeration process. The precipitable amount of oxalate present is not limiting for the maximum attainable particle size. However, acute increases in oxalate excretion do pose a risk because supersaturation is reached earlier in the nephron and consequently the crystal particles are allowed more time to increase in size. In conclusion, the present calculations demonstrate that during the normal transit time through the kidney, crystalline particles can be formed which are large enough to be retained because of their size and thus form the nidus of a stone. The highest risk is encountered at the end of those collecting ducts where crystals formed in nephrons with a long loop of Henle meet and agglomerate.

Animals↗
PubMed1994

A method for the assessment of blackfly (Diptera: Simuliidae) attraction to and engorgement on sheep in South Africa.

A suspended-net technique was used to capture blackflies attracted to and engorging on Dorper and Merino sheep on irrigated pastures. Two bait animals of each breed were restrained individually in wire-mesh enclosures located within four larger pens, each containing nine more animals of the same breed. Nets were suspended partly over the restrained animals and blackflies were collected at the end of 2-hour or longer periods. Mean numbers of blackflies captured in association with Merino (73.3/h) and Dorper sheep (89.1/h) did not differ significantly. Numbers of captured blackflies correlated (rho = 0.8361) with levels of irritation shown by sheep. Engorgement on Merinos was significantly (p = 0.009) higher than on Dorpers. Percentage engorgement was low, mostly below 10% on Merinos and below 5% on Dorpers. When high wind speeds and high saturation deficits (associated with temperatures in the mid-thirties) occurred, the success of capture was impaired. The technique is useful for quantifying the abundance of irritating blackflies in the immediate proximity of smallstock under experimental conditions.

Animals↗
PubMed1993

The appetence behaviour of two South African paralysis-inducing ixodid ticks.

The appetence responses of the ixodid ticks Ixodes rubicundus and Rhipicephalus punctatus to various stimuli were quantified under laboratory conditions. A greater percentage of I. rubicundus generally responded to the different stimuli at lower than at higher temperatures while the converse was usually true for R. punctatus. Radiation heat and shadowing evoked a response in the greatest percentage of I. rubicundus, whereas CO2 and host odour were responsible for a similar reaction in R. punctatus. A single perturbation of the substrate caused more than 50% of I. rubicundus to respond while constant perturbation resulted in a progressive decrease in the proportion of ticks reacting. Significantly more R. punctatus than I. rubicundus were attracted to a CO2 trap and the R. punctatus were also attracted to the trap over a greater distance. Significantly more older than younger ticks of both species responded to the various stimuli.

Animals↗
PubMed1993

Physicochemical considerations in the development and prevention of calcium oxalate urolithiasis.

Calcium oxalate urolithiasis is very common in western societies. In recent years significant progress has been made in identifying and quantitating physico-chemical processes involved in calcium oxalate urinary stone formation. The ability of urine to inhibit the agglomeration of calcium oxalate crystals is an important protective mechanism against stone formation. The process of crystal agglomeration is modulated to a large extent by citrate and it is disturbed in patients with hypocitraturia. Dietary factors, in particular high animal protein intake appear to affect adversely the ability of urine to inhibit calcium oxalate crystal agglomeration. Effective prevention of calcium oxalate urolithiasis should aim at restoring the urine's ability to inhibit the agglomeration of crystals and more emphasis should be given to dietary measures.

Calcium Oxalate↗
PubMed1993

Calculation of titratable acidity from urinary stone risk factors.

Using urine samples and standard solutions, this study demonstrates that the existing procedure for measuring titratable acidity in the urine is not reliable and may result in overestimates of up to 25%. The accuracy is affected by loss of CO2, the presence of uric acid crystals, and the precipitation of calciumphosphate phases during the titration. A method is presented for calculating titratable acidity, using a number of routinely-measured urine components and a computer program for calculating complex equilibria in the urine. The calculated titratable acidity is shown to be more reliable then the measured one. The results are compiled in a nomogram from which the titratable acidity can be directly read. When the parameters of urine pH, PO4 content and pCO2 are used, the accuracy of the nomogram is > 96% for urine samples with a pH value above 6.0 and > 89% for urine samples with a pH value below 6.0. For all samples, the accuracy is improved to > 97% when the nomogram using uric acid and citrate content is used in additionally.

Acids↗
PubMed1991

Diagnosis of liver tapeworm, Stilesia hepatica, infection in sheep.

A practical method of diagnosing Stilesia hepatica in live sheep is described. Intestinal contents and faeces were sieved through a sieve with apertures of 1.0 mm onto a sieve with apertures of 100 microns to reduce volume and turbidity. The residue on the fine sieve was examined microscopically. Gravid proglottids passed through the intestine without being digested. Intact proglottids were recovered from the intestine and faeces of infected sheep. Used purely on faeces, the technique is simple and reliable as a diagnostic method. Its application could facilitate basic research into the biology and epidemiology of the parasite.

Animals↗
PubMed1990

Crystal agglomeration is a major element in calcium oxalate urinary stone formation.

The effects of urines from 36 healthy subjects and 86 calcium oxalate renal stone formers on calcium oxalate monohydrate crystallization kinetics were studied using a seeded crystal growth method in which the solubility, the growth and the agglomeration of the crystals are measured as three separate and system-independent parameters. The urines of healthy subjects were found to increase the solubility and to strongly inhibit the growth and the agglomeration of calcium oxalate monohydrate crystals. The urines of stone formers had a similar effect on the solubility, but a significantly lower ability to inhibit the crystal growth and the crystal agglomeration. Of these two kinetic processes the inhibition of crystal agglomeration was more clearly affected, with 55% of the stone formers having abnormally low values, while the changes in crystal growth inhibition occurred within the normal range. The defect in crystal agglomeration inhibition was related to stone frequency, and urines from patients with very high stone frequency rates had also the most severely impaired ability to inhibit the agglomeration of the calcium oxalate monohydrate crystals. The inhibitory effect of urines on crystal agglomeration was found to be related to its citrate content (r = 0.68, P less than 0.001). All patients with hypocitraturia, except two, had also abnormally low values for crystal agglomeration inhibition. In a group of 15 hypocitraturic stone formers, alkali treatment for a mean period of 18 months resulted in a parallel increase in urinary citrate excretion and in the ability of urines to inhibit crystal agglomeration (r = 0.77, P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium Oxalate↗
PubMed1990

The effects of dietary excesses in animal protein and in sodium on the composition and the crystallization kinetics of calcium oxalate monohydrate in urines of healthy men.

Dietary excesses in animal protein and/or salt have been implicated as risk factors in calcium oxalate urolithiasis. The underlying physicochemical mechanism is, however, not known. Eight healthy men were given four different diets varying in animal protein and in sodium content for 1 week each. On a high protein intake (2 g/kg.day) significant changes in urinary calcium, uric acid, and citrate excretion rates were found. Similar changes in calcium and citrate were induced by a high sodium intake (310 mmol/day). The changes were more pronounced when a high protein was combined with a high sodium diet. Urinary calcium increased from 3.79 +/- 0.31 to 6.42 +/- 0.61 mmol/24 h and urinary uric acid from 4.69 +/- 0.26 to 8.0 +/- 0.47, whereas urinary citrate decreased from 3.93 +/- 0.53 to 2.79 +/- 0.34 mmol/24 h. All three dietary regimens induced a significant decrease in the ability of urines to inhibit calcium oxalate monohydrate crystal agglomeration, which was most marked during the combined diet (from 345 +/- 39 to 205 +/- 28 min). The ability of urines to inhibit crystal agglomeration was related to their citrate content (r = 0.69, P less than 0.0001). These results show that high animal protein and/or sodium intake decrease the ability of urines to inhibit the agglomeration of calcium oxalate crystals and provide a possible physicochemical explanation for the adverse effects of dietary aberrations on renal stone formation.

Adult↗
PubMed1988

Modulation of calcium oxalate monohydrate crystallization kinetics in vitro.

The effects of several low and high molecular weight (mol wt) compounds on the kinetics of calcium oxalate crystallization were examined using a seeded crystal growth method in which the solubility, the growth and the agglomeration of calcium oxalate crystals were measured as three separate and system-independent parameters. Citrate, magnesium, phosphate, pyrophosphate, chondroitinsulphate, pentosanpolysulphate and heparin were tested in a wide range of concentrations. The solubility of calcium oxalate crystals was increased only by citrate and magnesium. The crystal growth was inhibited by all compounds tested, but those with the high mol wt had the greatest effect at low concentrations. In contrast, inhibition of crystal agglomeration was achieved only by the low mol wt compounds; citrate was found to be the most potent inhibitor at concentrations likely to be present in normal urine. The high mol wt substances, despite their potent crystal growth inhibitory activity, had no effect on agglomeration. The results show that growth and agglomeration of calcium oxalate crystals are separate processes which are differently modulated by various compounds. They further provide a possible explanation for the pathogenetic role of citrate in hypocitraturic renal stone disease.

Calcium Oxalate↗
PubMed1986

Polysaccharide from coccoliths (CaCO3 biomineral). Influence on crystallization of calcium oxalate monohydrate.

A polysaccharide associated with coccoliths of the marine alga Emiliania huxleyi (coccoliths are elaborately shaped calcite biominerals) was isolated and its influence on the crystallization of calcium oxalate monohydrate crystals was studied. Crystallization was monitored in a carefully controlled system by measuring the incorporation of 45Ca tracer from a supersaturated solution into seed crystals of calcium oxalate monohydrate in the absence and in the presence of polysaccharide. The method allowed differentiation between effects on solubility, growth and agglomeration of crystals. At the very low concentrations used in this study, the polysaccharide had no significant effect on the solubility product; it strongly inhibited the growth and strongly stimulated the agglomeration of the crystals. Thus, the two processes of growth and agglomeration, being both crystal-surface-related processes, may react in opposite directions upon surface adhesion of the additive. This finding opens new insights on how a mineralization process may be controlled. The inhibitory effect on growth is shown to proceed through a monolayer type of adsorption of the polysaccharide onto the crystal surface. The portion of the polysaccharide used for the stimulatory effect on agglomeration shows a different type of adsorption, whereby less crystal surface is covered per molecule of polysaccharide. This strongly suggests, that the mechanism whereby agglomeration is stimulated operates through 'viscous binding', with the polysaccharide bridging the gap between two crystal surfaces. In the discussion these findings are related to some possible biological functions of the polysaccharide.

Calcification, Physiologic↗