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Biomedical subjects

D J Kok

Publications and source records attributed to D J Kok.

At least 55 records · Page 3Linked to original sources
PubMed1998

Individual host variations in tick infestations of cattle in a resource-poor community.

Relative resistance levels of cattle against tick infestations in the communal grazing area of Botshabelo in the south-eastern Free State were determined. The objective was to establish whether differences in resistance can be exploited to contribute to tick control methods used by small-scale farmers in resource-poor environments. Ten cows (Bos taurus crosses) between the ages of 18 months and four years were used and tick counts were conducted once a month over a period of 12 months to compare their total tick burdens. Tick burdens of the various animals were compared mutually as well as with the mean tick burden of the group as a whole. Tick numbers varied throughout the year on all individuals but some animals consistently tended to have either higher or lower numbers than the mean of the group. Tick burdens on cattle classified as having a relatively low resistance to tick infestations increased eleven-fold from January to June 1996 compared to a six-fold increase on cattle categorized as belonging to the high resistance group. Twenty-eight percent of the cattle in the total study group carried 50% of the ticks collected (60,079). It is recommended that farmers in the region visually assess B. decoloratus burdens, the most abundant tick species, and sell or cull the most susceptible animals first in their normal program of utilization of the animals. This should eventually result in the direct improvement of the overall tick resistance of their cattle herds.

Analysis of Variance↗
PubMed1998

Tick diversity, abundance and seasonal dynamics in a resource-poor urban environment in the Free State Province.

The objectives of this study were to determine the diversity, seasonal dynamics and abundance of ticks infesting cattle in urban, small-scale farming communities in and around Botshabelo and Thaba Nchu in the eastern Free State Province, South Africa. A total of ten cattle, ear-tagged for individual identification, were investigated monthly at each of five localities. Adult ticks were removed from the right hand side of each animal and placed in containers filled with 70% ethanol. They were subsequently identified and their numbers quantified. Immature Otobius megnini were counted but not removed. A total of 244,538 adult ticks of ten different species were collected over the 12-month study period. The tick species, in decreasing order of relative abundance, were: Boophilus decoloratus (87.26%), Rhipicephalus evertsi evertsi (6.86%), Hyalomma marginatum rufipes (2.42%), Otobius megnini (1.85%) Rhipicephalus follis (0.76%), Rhipicephalus gertrudae (0.54%), Rhipicephalus sp. (0.21%), Ixodes rubicundus (0.08%), Hyalomma truncatum (0.01%) and Margaropus winthemi (0.004%). The three most abundant species, namely B. decoloratus, R. evertsi evertsi and H. marginatum rufipes, occurred at all localities but with significant differences in abundance. M. winthemi ticks occurred only in the Thaba Nchu area and were not found at any of the there localities in Botshabelo. Significant differences in tick burdens between the six warm months (September to February) and the six cooler months (March to August) were found for most of the species recorded. Boophilus decoloratus occurred in significantly higher numbers in autumn (March to May) and winter (June to August) compared to spring (September to November) and summer (December to February), with 76.8% of the total B. decoloratus burden occurring during the cooler months.

Altitude↗
PubMed1997

Intratubular crystallization events.

Can urolithiasis start as an intratubular event? Under severe hyperoxaluric conditions in animal models at least crystal formation can. Recently models have been presented that assess the chances of crystal formation under more normal conditions. These models describe changes in fluid composition as this passes through the nephron, these conditions being simulated in in vitro experiments. It appears that under naturally occurring intratubular conditions calcium-salt crystallization takes place within the time tubular fluid normally spends in the nephron. Precipitation starts with a calcium-phosphate phase under conditions found in the thin lambs. This crystalline phase then (partly) dissolves when collecting duct conditions are used, thereby inducing formation of calcium oxalates. Under these conditions the latter increase in size by way of crystal growth and agglomeration. Large particle formation and cell adhesion can eventually result in particle retention and subsequent stone formation. Viewing urolithiasis as originally an intratubular event has consequences for in vitro experiments and treatments, which are discussed in this paper.

Animals↗
PubMed1997

Supporting experimental evidence of host specificity among southern African polystomes (Polystomatidae:Monogenea).

Although monogeneans of anurans are generally regarded as host-specific, there is a lack of conclusive experimental evidence. Infection and cross-infection experiments were conducted with oncomiracidia of Polystoma australis and P. marmorati. In a series of experiments, oncomiracidia were given a choice between natural and substitute host tadpoles. Oncomiracidia of P. australis became established in substitute hosts but showed a preference for the natural host, whereas the oncomiracidia of P. marmorati showed a strong and statistically significant preference for the natural host. The results indicated that although the oncomiracidia of southern African polystomes showed a strong preference for their natural hosts, not all parasites exercised the same degree of host specificity.

Animals↗
PubMed1997

Ultrastructural osteopontin localization in papillary stones induced in rats.

OBJECTIVES: To detect in situ the precise osteopontin (OPN) localization in papillary stones. METHODS: Immunocytochemical labelling procedures are applied to detect OPN localizations in crystalline material of renal papillary stones. The tissue-processing procedure for electron microscopy, which includes OsO4 postfixation, preserves both immunocytochemical OPN reactivity and cellular membrane contrast up to the ultrathin section. Reflection-contrast light microscopical images are correlated with high resolution transmission-electron microscopical observations from consecutive ultrathin epon sections. RESULTS: Preserved crystalline material in interstitial and peripheral papillary stones is recognized as calcium oxalate monohydrate. After section incubation with markers conjugated to an antibody against OPN (alpha OPN) the crystals are converted into ghosts. In the ghosts, alpha OPN markers are present around microcrystals. The size of these microcrystals ranges from several nanometers to micrometers. It is observed (due to the OsO4-preserved membranes) that interstitial cells are separated from the stone surfaces by unidentified extracellular material, also present in the center as a stone matrix. CONCLUSION: The microcrystal-growth inhibitor OPN is detected in situ in interstitial stones induced in the rat's papilla and at the surface of the papilla.

Animals↗
PubMed1997

Predation of livestock ticks by chickens as a tick-control method in a resource-poor urban environment.

The possible use of chickens as predators of livestock ticks was studied. Chickens were allowed to scavenge for 3 h among tick-infested cattle in a typical township backyard during the milking period. Boophilus decoloratus, Hyalomma marginatum rufipes, Rhipicephalus evertsi evertsi and Otobius megnini were recovered from the crops and gizzards at necropsy. The numbers of ticks ingested ranged from 0-128, with an average of 28.81 (+/- 8.42) per chicken. This study has confirmed that chickens are natural predators of livestock ticks and that chickens can be used as part of an integrated tick control plan in urban cattle-management systems in resource-poor communities in South Africa.

Animals↗
PubMed1996

Sex ratio characteristics in Ixodes rubicundus (Acari:Ixodidae), the Karoo paralysis tick.

The sex ratio is an important parameter which characterizes the state and dynamics of natural populations of animals. Although ixodid ticks are specialized ectoparasites, most species are bisexual and are characterized by a 1:1 sex ratio for their progeny. In natural populations and even in laboratory colonies, biased sex ratios are often observed. Ixodes rubicundus, the Karoo paralysis tick, parasitizes domestic stock and wild ungulates in South Africa. Adults quest from vegetation, can mate off or on the host and males are seldom parasitic. We hypothesized that the sex ratio for I. rubicundus would be 1:1 when observed directly in the progeny but that it would be strongly biased towards females in samples of parasitic adults. The results mostly supported the hypothesis but it was also shown that unexplained and unpredictable variations can occur. On hosts, females dominated strongly, except on adult angora goats where the sex ratio was biased in favour of the males. This disparity may be related to a greater retention of males in the coarse, curly hair of angora goats compared to the other hosts. Monthly variations in the sex ratios of the tick on hosts are believed to be related to the large fluctuations in sex ratios of questing ticks.

Animals↗
PubMed1996

Interbreed differences in the efficacy of 1% deltamethrin pour-on to protect small livestock against infestation with Ixodes rubicundus (Acari:Ixodidae).

Synthetic pyrethroid pour-on compounds have been developed for use on cattle but many are now also registered for the control of ectoparasites on small ruminants. The breed of small livestock has been known to influence the efficacy of various pour-ons and we hypothesized that differences in the duration of protection afforded by a synthetic pyrethroid pour-on against ticks on small livestock, would be greatest between hairy and woolly breeds. Angora goats with mohair, Dorper sheep with hairy wool, and Merino sheep with wool were treated with 1% m/v deltamethrin pour-on with an oil base at a dosage rate of 1 ml 5 kg-1 body mass. The duration of effective protection against Ixodes rubicundus, the Karoo paralysis tick, was determined under field conditions. Angora goats proved to be the highest risk group, but were afforded the least protection. Efficacy never reached 100% in this group and within 3 weeks the goats were already at risk of paralysis. Dorper sheep were afforded above 90% protection for 3 weeks and 85% after 4 weeks. In the case of the Merino sheep efficacy of protection remained above 90% up to 7 weeks post-treatment and was still 87% during Week 8. Differences in the amount of sebum and suint in the epidermis of the different breeds may be responsible for differences in the rate of spread and the rate of dilution of the active compound in the epidermis.

Administration, Topical↗
PubMed1996

Control of Karoo paralysis ticks through vegetation management.

Karoo paralysis, caused by feeding Ixodes rubicundus females, is a major disease of small stock in South Africa. Control methods currently practised are almost exclusively chemical based. To limit overdependance on chemicals, vegetation management was investigated as a possible method for control, to be incorporated in an integrated tick management system. Laboratory and field experiments were conducted to determine, firstly, the extent of vertical migration and survival of ticks on long and short copper rods which simulated grasses as questing substrates; secondly, the infestation burdens of sheep exposed to similar tick challenges in pens with long and short grass; and thirdly, the effect of trimming the lower crown line of wild olive trees, simulating the browsing effect of goats, on tick density in the immediate environment of the trees. When ticks were exposed to optimal ( > 45 cm) and sub-optimal ( < 10 cm) length rods on which to quest, the extent of vertical migration over extended periods of time (up to 87 days) was significantly higher (P < 0.001) for the ticks exposed to long rods. Also, almost 3 times as many ticks exposed to long rods survived compared to those exposed to short rods. Sheep exposed to long grass were infested by twice as many ticks compared to those exposed to short grass. Tick density at modified wild olive trees (Olea europaea africana) (0.027 ticks/m2) differed significantly (P < 0.05) from that at control trees (0.088 ticks/m2). It is recommended that coarse grazers such as cattle and horses should be used to graze down long grasses before sheep are introduced into camps known to be infested with I. rubicundus. Similarly, domestic goats can, through their browsing effect on shrubs and trees, modify the vegetation and as such play an important role in an integrated tick management system.

Animals↗
PubMed1996

Adult ixodid ticks on two cattle breeds in the south-western Free State, and their seasonal dynamics.

A study was conducted to compare the species of adult ixodid ticks on Friesian and Bonsmara cattle and their seasonal dynamics. Between five and ten animals of each breed were kept on natural False Upper Karoo vegetation, typical of the south-western Free State. Between May 1990 and October 1991, the animals were examined once or twice a month for the presence of adult ticks which were removed, counted and identified. Six species of ixodid ticks were recorded. The species composition on the two cattle breeds was remarkably similar. Ixodes rubicundus was dominant (55-57.7% of all ticks collected), followed by Hyalomma marginatum rufipes (28.9-31.5%) and Rhipicephalus punctatus *6.2-6.7%). Only small numbers of Hyalomma truncatum, Rhipicephalus evertsi evertsi and Rhipicephalus gertrudae were recorded. Almost twice as many ticks were collected from Friesian as from Bonsmara cattle. Ixodes rubicundus showed a typical winter-activity period. The other species were either active mostly during the warmer months, or their numbers were to low to determine any seasonal pattern. Apart from information on the diversity of ticks which infest cattle in the region, the study has also shown a greater resistance by Bonsmara cattle (a mixed Bos taurus/Bos indicus breed) to tick infestation. Bos indicus cattle can play an increasingly important role in integrated tick-management practices in southern Africa.

Animals↗
PubMed1996

Seasonal dynamics of the Karoo paralysis tick (Ixodes rubicundus): a comparative study on Merino and Dorper sheep.

Karoo paralysis in South Africa is induced in livestock by feeding female Ixodes rubicundus ticks when infestation densities on hosts exceed certain critical levels. It has been shown previously that Angora goats are at a higher risk of being paralysed than Merino sheep, and such differences have been related to differences in feeding behaviour and spatial distribution of the two small-stock breeds. We hypothesized that differences in infestation densities with Karoo paralysis ticks would also occur between Merino and Dorper sheep breeds. A study was conducted under natural conditions in the south-western Free State, to compare infestation burdens of the two sheep breeds and also to investigate seasonal patterns and annual variations in variations in terms of rainfall and temperature. Ten animals of each breed ran free in an area with a known history of Karoo paralysis and were examined on an approximately fortnightly basis, from March 1992 to December 1995, to determine tick abundance. Differences between the two breeds were significant (P < 0.05) during 1992 and 1993, but not during 1994. During the first two years, peak abundance of ticks was reached earlier in Dorper than in Merino sheep, and it also reached higher levels in Dorper than in Merino sheep (mean = 17.9 and 7.3, respectively). In 1993, two Dorper, but no Merino sheep, were paralysed. Dorper sheep are clearly at a higher risk of being paralysed than are Merino sheep, and as such, they can serve as indicators of adult tick activity and hence of the time to commence prophylactic treatment. Differences between the two breeds are probably related to differences in grazing patterns. Marked variation in abundance and the time of onset of peak activity of I. rubicundus occurred over the years. Tick numbers were high in 1993 and 1995, but very low in 1992. In 1993, peak activity occurred earlier (April) than during the other years (June or July). These differences are related to differences in prevailing environmental conditions that influence tick activity in a complex manner. Heightened humidity and lower temperatures during the early stages of seasonal activity of the tick (April or May), normally result in peak abundance of ticks on hosts at that time.

Animals↗
PubMed1996

Spatial and temporal variations in the commencement of seasonal activity in the Karoo paralysis tick, Ixodes rubicundis.

Successful prophylaxis of paralysis, induced in small stock by feeding female Ixodes rubicundus, is dependent on the accurate determination of the commencement of seasonal activity by the tick. The commencement of this activity was recorded for 11 consecutive years on a farm in the south-western Free State, South Africa, and for shorter periods on other farms, some of these in regions with markedly colder climates. The colder the mean minimum atmospheric temperatures during the 2 months preceding the start to tick activity, the earlier it commenced. This could differ by 4 weeks from year to year on the same farm. In a region with a low effective temperature activity commenced between 3-8 weeks earlier than in a region with a higher effective temperature.

Animals↗
PubMed1996

Crystallization and stone formation inside the nephron.

A model is presented visualizing the events leading to calcium-salt, crystal- and stone-formation inside the nephron. For each nephron segment, handling of urine components relevant to stone formation is considered and urine composition determined. This information was applied to nucleation experiments simulating passage of urine through a nephron. The model and in vitro experiments suggest that within normal transit times for the respective nephron segments, particles of a hydroxyapatite-like material first form near the bend in the Loop of Henle of juxtamedullary nephrons. From there on, calcium oxalate particles start to appear: first dihydrate, then monohydrate. In the collecting duct system, particle size increases primarily due to crystal agglomeration. Several conclusions with clinical and experimental relevance can be drawn. An increase in urinary volume does not decrease the chance of crystal formation in the Loop of Henle, but does decrease passage time through the collecting ducts, and thus, the time allowed for large particle formation. A calcium load does not increase the risk for nucleation up to the distal tubule, but does increase the risk of large particle formation in the collecting ducts. An oxalate load increases the chance for nucleation throughout the nephron. For experiments simulating crystallization processes occurring inside the nephron, diluted urines should be used. They should be diluted 16 to 50 times for testing nucleation, 2 to 30 times for testing crystal growth, and 2 to 20 times for testing crystal agglomeration. Undiluted urines may be used to mimic conditions in the pelvis and the bladder.

Calcium Oxalate↗
PubMed1995

Physicochemical effects of a new slow-release potassium phosphate preparation (UroPhos-K) in absorptive hypercalciuria.

A new slow-release, neutral potassium phosphate salt (UroPhos-K) has been formulated in order to minimize gastrointestinal side effects and avoid sodium-induced calciuria. It was tested in a prospective randomized, double-blind trial in a group of 21 kidney stone patients with absorptive hypercalciuria type I (AH). Twelve patients allocated to the UroPhos-K group received four tablets twice daily with breakfast and an evening snack providing 1240 mg of phosphorus and 63.5 mEq of potassium daily. Nine patients assigned to the placebo group received placebo tablets of the same appearance containing excipient only. Subjects were studied during a 3-day period in the hospital while consuming a constant metabolic diet containing 400 mg Ca, 100 mEq Na, and 800 mg P per day before and after 3 months of treatment. Treatment with UroPhos-K did not cause any significant gastrointestinal side effects; nor did it raise fasting serum K or phosphorus, or reduce hemoglobin or creatinine clearance. It was associated with a rise in urinary K from 46 +/- 7 to 98 +/- 9 mEq per day and phosphorus from 744 +/- 185 to 1535 +/- 112 mg per day (p < 0.001 each). UroPhos-K treatment reduced urinary Ca from 288 +/- 63 to 171 +/- 49 mg/day (p < 0.001), without altering oxalate excretion. It reduced the urinary saturation of calcium oxalate without altering that of brushite. Moreover, by increasing urinary excretion of inhibitors (citrate and pyrophosphate), it reduced the propensity for spontaneous nucleation of brushite (increased formation product of brushite) and inhibited crystal agglomeration of calcium oxalate.(ABSTRACT TRUNCATED AT 250 WORDS)

Administration, Oral↗
PubMed1995

An evaluation of strategic and threshold control measures against the Karoo paralysis tick, Ixodes rubicundus (Acari: Ixodidae) in South Africa.

Paralysis caused by feeding female Ixodes rubicundus ticks is a major problem in large areas of South Africa. As the life cycle of the tick extends over a period of 2 years, it was hypothesized that strategic treatment of sheep with an acaricide over a 2 year period, timed to kill most engorging females, should markedly lower the biotic potential of the tick. Two flocks of sheep grazing in separate paddocks known to be infested with I. rubicundus were treated either strategically or on a threshold basis (i.e. only when tick challenge exceeded a predetermined critical level in terms of paralysis) for a 2 year period. The tick burdens of untreated control sheep running with the two flocks were monitored over a 4 year period and their seasonal dynamics determined. The times at which peak infestations occurred were similar for both flocks of sheep, but significant differences in mean tick burdens between the two flocks were recorded. Tick numbers on sheep in the strategically treated flock did not decrease during the third and fourth years of the trial as was expected. Possible reasons for this were low stocking densities, especially during times of peak abundance of adults and the presence of wild hosts which maintained tick populations.

Animals↗
PubMed1995

Improvement of field sampling methods for adult Karoo paralysis ticks, Ixodes rubicundus (Acari: Ixodidae), through addition of host odour.

The responsiveness of Ixodes rubicundus ticks on questing substrates and the success of their attachment to non-living substrates were investigated. The purpose of this study was to relate responsiveness to conditions of temperature and humidity, to compare the efficacy of flagging and dragging methods to estimate the size of populations of adult I. rubicundus in the field, and to determine the possible influence of host odour on the efficacy of these methods. Responsiveness was tested under varying conditions of temperature and humidity under field conditions, and the same ticks were used to determine the duration of attachment to a flannel cloth, either impregnated with host odour from sheep wool or without it. Flagging and dragging methods were compared under laboratory conditions with cloths either treated with host odour or not. Within the range of ambient temperatures recorded during this study (7-25 degrees C) most of the ticks (86%) were responsive. No obvious relationship between temperature, relative humidity and responsiveness of ticks was evident. Ticks remained on average 32.9 seconds (n = 64) on cloth treated with host odour compared to 9.1 seconds (n = 54) on untreated cloth. Flagging was 1.5-1.7 times as effective as dragging and treatment of the cloths with host odour increased the efficacy 2.4 (dragging) to 2.8 (flagging) times. The reactions of ticks to external stimuli should, therefore, be taken into account to maximize field sampling success.

Animals↗
PubMed1995

A comparison of Ixodes rubicundus (Acari: Ixodidae) infestations on Friesian and Bonsmara cattle in South Africa.

This study was conducted to compare the relative resistance of crossbred Bos indicus X B. taurus Bonsmara and B. taurus Friesian cattle to Ixodes rubicundus (Karoo paralysis tick) infestations. During periods of peak abundance of the ticks, Friesian oxen harboured almost twice or more than twice as many ticks as either Bonsmara oxen or cows. During periods of low tick abundance tick burdens on both cattle breeds were closely similar. It is envisaged that cattle can play an important role in an integrated control strategy against the Karoo paralysis tick.

Animals↗