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Biomedical subjects

J R Lawson

Publications and source records attributed to J R Lawson.

At least 19 recordsLinked to original sources
PubMed2006

Cardiovascular consequences of life-long exposure to dietary isoflavones in the rat.

Dietary soy intake in man is proposed to provide cardiovascular protection, but it is not established whether this property is attributable to the soy protein per se or to associated dietary isoflavones. This investigation aimed to establish whether the dietary isoflavones in soy protein affect cardiovascular function. Ten days prior to mating, male and female Wistar rats were habituated to either a soy based isoflavone rich diet (plasma concentration 1.87 micromol l(-1) isoflavones) or the same diet after isoflavone elution (plasma isoflavone not detectable). Offspring were weaned onto and maintained on the same diet as their dam and sire for 6 months. Blood pressure, and constrictor and dilator responses in the aorta and mesenteric resistance arteries were assessed at 3 and 6 months of age. There was no effect of isoflavone removal from the diet on blood pressure, heart rate, aortic function or mesenteric artery contractile function, at either 3 or 6 months of age. Resistance mesenteric arteries from 6-month-old female rats fed the isoflavone rich diet demonstrated a modest increase in arterial distensibility compared with those fed the depleted diet, and mesenteric arteries from male and female rats fed the isoflavone rich diet showed increased sensitivity to acetylcholine. In summary, the isoflavone content of soy protein has no influence on blood pressure in healthy rats fed a diet based on soy protein, but influences small artery function.

Animals↗
PubMed1991

Alternative alloys for resin-bonded retainers.

Traditionally, resin-bonded fixed partial dentures have been made with nickel-chrome-beryllium alloys and cemented with conventional resin luting cements. However, alternative alloys for resin-bonded retainers offer improved physical and biocompatible properties, and resin-metal bond strengths twice that of traditional methods can be achieved. The superior bonds obtained with etched base metals bonded with adhesive resins and silica-coated alloys bonded with silane-coupling agents make these the most desirable techniques available.

Dental Alloys↗
PubMed1990

Transmission of taeniid tapeworm eggs via blowflies to intermediate hosts.

Experiments showed that dead blowflies containing eggs of Taenia hydatigena can transmit infection if ingested by lambs during grazing. Rabbits became infected with T. pisiformis after grazing grass exposed to blowflies that had been in contact with dog faeces containing the eggs of this parasite. When captive blowflies were exposed first to dog faeces containing proglottids of T. hydatigena and then to cooked meat, 100% of pigs fed on this meat became infected. The experiments are intended to model the transmission dynamics that result in human larval tapeworm infections. The implications of the results to the epidemiology of human hydatid diseases caused by Echinococcus granulosus and E. multilocularis as well as of T. solium cysticercosis are discussed.

Animals↗
PubMed1990

Population dynamics in echinococcosis and cysticercosis: regulation of Taenia hydatigena and T. ovis in lambs through passively transferred immunity.

A comparison has been made of the interactions between passively transferred and actively acquired immunity in regulating populations of Taenia hydatigena and T. ovis. When ewes were grazed prior to parturition under a high infection pressure, immunity was transferred to their offspring for up to 8 weeks. A qualitative difference between the species was the destruction of larval T. ovis prior to their establishment ('pre-encystment immunity') and that of T. hydatigena after they had become established ('post-encystment immunity') in the challenged lambs. The major difference in terms of population regulation between the two parasites was that infection occurred with T. hydatigena but not with T. ovis in those lambs reared from birth for 16 weeks under high infection pressure. Passive, like active immunity, is a density-dependent constraint. It plays an important role in the population regulation of T. ovis, but not of T. hydatigena. This is discussed in terms of transmission in the natural environment, an hypothesis on humoral protection and the need to elucidate pathways of protection when immunization schedules are being evaluated for controlling the taeniid zoonoses.

Animals↗
PubMed1989

The ovine cysticercosis as models for research into the epidemiology and control of the human and porcine cysticercosis Taenia solium: I. Epidemiological considerations.

A brief review is given of the transmission dynamics of the ovine cysticercoses Taenia hydatigena and T. ovis. The importance of the basic reproductive rate (Ro) and the parasite, host and socio-ecological factors determining the epidemiological steady states of Taeniidae are described. The transmission dynamics of these ovine cysticercoses have been used as models to assist in identifying the biological and epidemiological research needed for developing strategies for the control of porcine cysticercosis caused by T. solium.

Animals↗
PubMed1989

The ovine cysticercosis as models for research into the epidemiology and control of the human and porcine cysticercosis Taenia solium: II. The application of control.

Field trials and control programmes demonstrate that ovine echinococcosis (Echinococcus granulosus) and the cysticercoses (Taenia hydatigena and T. ovis) have different stabilities and may not respond in the same way to control. The first-named, which is usually in the endemic state, is readily transformed by a dog-dosing programme to extinction status. In contrast, the cysticercoses, usually in the hyperendemic state, may only be transformed to the endemic state. The consequence of this includes an increase in the larval population by superinfection due to a loss of immunity following this change in epidemiological status. These field trials and control programmes have been used as models for porcine cysticercosis caused by T. solium. It is concluded that future research should model the life cycle of this parasite mathematically and determine the most cost-effective control strategies. The impact of these on human prevalence can then be addressed by pilot field trials.

Animals↗
PubMed1988

Population dynamics in echinococcosis and cysticercosis: economic assessment of control strategies for Echinococcus granulosus, Taenia ovis and T. hydatigena.

An official control programme against Echinococcus granulosus and Taenia hydatigena has been in operation in New Zealand for more than 28 years and against Taenia ovis for more than 18 years. This unique effort to control three metazoan parasites at the same time has led to a change from endemic to extinction status for E. granulosus but only a change from hyperendemic to endemic status for T. hydatigena and T. ovis. This has presented problems in determining the most cost-effective future control strategies. To facilitate this, a benefit/cost analysis of 20 options for the combined control of E. granulosus, T. hydatigena and T. ovis in New Zealand was undertaken. This showed that for E. granulosus a future change from the current non-targeted to a targeted approach is strongly indicated. For T. ovis 6 options were cost-effective using a discount rate of 10%. These were (1) a targeted control package using a vaccine in the non-targeted attack phase; (2) a targeted control package using a larvicide in the attack phase; (3) the transfer of all losses due to and responsibility for the control of T. ovis to the producer who administers a larvicide to sheep to be killed for dog food; (4) the transfer of all losses due to and responsibility for the control of T. ovis to the producer who administers praziquantel every 6 weeks to dogs; (5) and (6) two options involving the discontinuation of control. Control of T. hydatigena was assumed to be an incidental outcome of the policies for the other two parasites.

Animals↗
PubMed1987

Towards global control of cystic and alveolar hydatid diseases.

Control programmes against Echinococcus granulosus in its dog-sheep transmission cycle (Fig. 1) have been successful in many parts of the world'. In contrast, the related E. multilocularis presents a much more complex problem for control authorities. Unlike E. granulosus, the life cycle of E. multilocularis predominantly involves sylvatic hosts (e.g. rodents and foxes) (Fig. 2) and the control of wild life echinococcosis presents a formidable challenge to ecologists and epidemiologists. This review contrasts the two parasites, explaining why E. granulosus in its domestic dog-sheep life cycle has been so responsive to control, and examines the prospects for control of E. multilocularis.

Journal Article↗
PubMed1987

Population dynamics in echinococcosis and cysticercosis: evaluation of the biological parameters of Taenia hydatigena and T. ovis and comparison with those of Echinococcus granulosus.

An evaluation has been made of the biological and epidemiological parameters that determine the basic reproductive rates of Taenia hydatigena and T. ovis. These host-parasite systems are characterized by (i) no overcrowding in either host; (ii) no parasite-induced mortality of either host; (iii) no density-dependent constraint in the definitive host, but a strong, rapidly mobilized, short-acting immunity in the intermediate host and (iv) egg production which, in the natural environment, is high enough to prevent superinfection. It is considered that tapeworms with these characteristics are more stable to fluctuations in environmental conditions and to control measures such as dog dosing, than species which have a low egg production and infectivity such as Echinococcus granulosus. Reciprocal immunity exists between T. hydatigena and T. ovis in sheep. Exposure to T. hydatigena suppresses infection by T. ovis, but not by E. granulosus. This has important epidemiological consequences where these parasites co-exist.

Animals↗
PubMed1987

Population dynamics in echinococcosis and cysticercosis: mathematical model of the life-cycles of Taenia hydatigena and T. ovis.

It is shown that under the conditions that prevailed in New Zealand in the late 1950s, Taenia hydatigena was hyperendemic, the life-cycle being regulated by a density-dependent constraint in the form of acquired immunity, and T. ovis was rare. The control measures that caused Echinococcus granulosus, which was endemic at the time, to decline towards extinction reduced T. hydatigena and T. ovis to endemic status only. A non-linear integrodifferential equation model, which was previously linearized to describe the life-cycle of E. granulosus in dogs and sheep in New Zealand, is used to describe the life-cycles of T. hydatigena and T. ovis. The model is then used to compare and contrast the population dynamics of these three species. The model is used to demonstrate that the endemic steady state is structurally unstable, and may be asymptotically unstable to small perturbations. It is also shown that despite the lower infection pressure experienced by the intermediate host in the endemic state, the numbers of larvae in sheep may be higher than in the hyperendemic state. Finally it is shown that the partial success of the control measures against T. hydatigena may have caused an increase in the numbers and prevalence of T. ovis larvae in sheep due to the reciprocal immunity between the two species.

Age Factors↗
PubMed1986

Population dynamics in echinococcosis and cysticercosis: comparison of the response of Echinococcus granulosus, Taenia hydatigena and T. ovis to control.

A comparative study has been made of the progress in the control of ovine echinococcosis caused by Echinococcus granulosus and the ovine cysticercoses caused by Taenia hydatigena and T. ovis in New Zealand. The methods of control included an educational and a dog-dosing programme. In the Styx field trial, the Otago/Southland Surveillance Programme and the national control programme, E. granulosus declined towards extinction. In contrast, this same control effort transformed the cysticercoses from hyper-endemic to endemic status with focal epidemics. In this unstable state, the density-dependent constraint was negligible and superinfection occurred.

Animals↗
PubMed1986

Population dynamics in echinococcosis and cysticercosis: biological parameters of Echinococcus granulosus in dogs and sheep.

The numerical distributions of Echinococcus granulosus in an experimental dog population are described. At all dose rates of protoscoleces from 10 to 175000 the distribution of worms was over-dispersed. Host age had no effect. There was a direct proportionality between the infective-stage density and rate of infection, and between the latter and the index of clumping. The worm burdens were significantly higher in the proximal than distal portions of the small intestine. Lengths of the 3- and 4-segmented worms increased from 4 to 10 and 4 to 8 weeks of age, respectively. Thereafter apolysis was asynchronous and could not be determined. Eggs were first detected in the faeces at 6 weeks and the mean age at oogenesis was 7.26 weeks. Retarded growth of the whole population of worms was observed in some dogs. For the first few infections, worm burdens varied widely in the same dog, but by the 6th infection 50% of the dog population had developed a relative insusceptibility to infection. Growth or oogenesis of the worms were not affected. A short-acting immune response was artificially induced in some dogs following the parenteral injection of activated embryos of E. granulosus, Taenia hydatigena, T. ovis, T. multiceps, T. pisiformis and T. serialis. The response affected either the number of worms established, growth or oogenesis or all three parameters. There was a strong positive correlation between numbers and lengths of worms in dogs with acquired and induced immunity, indicating that no 'crowding' effects were involved. In sheep populations the mean number of cysts which established was directly proportional to the number of eggs given, implying that there was no negative feedback mechanism operating at this stage of the life-cycle. The distribution of the larval population in sheep was over-dispersed and the index of clumping increased with the size of the egg dose from 25 to 2500 eggs. Protoscoleces were first observed in cysts at 2 years and the proportion producing them increased with age, with an estimate of 50% of cysts containing protoscoleces at 6.29 years. No deaths were observed in dogs or sheep even when high parasite burdens were present, implying that E. granulosus does not regulate the population of its hosts.

Age Factors↗
PubMed1986

Population dynamics in echinococcosis and cysticercosis: mathematical model of the life-cycle of Echinococcus granulosus.

A mathematical model of the life-cycle of Echinococcus granulosus in dogs and sheep in New Zealand is constructed and used to discuss previously published experimental and survey data. The model is then used to describe the dynamics of transmission of the parasite, and the means by which it may be destabilized. It is found that under the conditions that prevailed in New Zealand during the late 1950s, at the time of surveys of this parasite, the dog-sheep life-cycle was not regulated by any effective density-dependent constraint. In contrast there was evidence for an effective acquisition of immunity to reinfection by cattle. The long time to maturity of the cyst in sheep, together with the practice of feeding aged sheep to dogs, provides a time delay in the intermediate host. By comparison, the time to maturity of the adult stage in dogs is short, but it is of sufficient magnitude to be a key factor in the destabilization of the system by a regular dog-dosing programme. The model used to describe the life-cycle is a linear integrodifferential equation of the Volterra type. Such equations are intrinsically unstable in that a small perturbation in parameters can drive a previous equilibrium solution to zero. At the time of the surveys, the value of the basic reproductive rate, R0, was close to 1, and it has since been reduced below 1 by control measures.

Age Factors↗
PubMed1986

Control of echinococcosis/hydatidosis: present status of worldwide progress.

Progress in national and provincial control programmes directed against Echinococcus granulosus is reviewed. The results show that this parasite, because of the low basic reproductive rate in its domestic life-cycle, is unstable and is amenable to control or eradication by currently available methods. The difference between control and eradication is defined and some suggestions for extension of control are discussed.

Echinococcosis↗
PubMed1985

The survival in sheep and infectivity to dogs of Taenia hydatigena and T. ovis in sheep.

Lambs were reared from birth to 3 months of age on pasture contaminated with the eggs of either Taenia hydatigena or Taenia ovis. They were necropsied at 3, 6, 9 or 12 months. Almost all larvae that were viable at 3 months continued to survive throughout the experiment. Larvae of T. hydatigena were infective to dogs at 3 months, whereas those of T. ovis required more than 3 months to reach the infective stage.

Animals↗
PubMed1985

The potential role of blowflies in the transmission of taeniid tapeworm eggs.

In laboratory studies with 3 species of Calliphora more than half were shown to ingest eggs of Taenia hydatigena from the surface of dog faeces with a maximum of over 5000 eggs in 1 fly. Most eggs were voided within 48 h and between 38 and 48% of these eggs had lost their embryophores. In field studies carried out in the South Island of New Zealand, traps baited with dog faeces captured the following blowfly species in decreasing order of abundance: Hybopygia varia, Calliphora quadrimaculata, C. hortona and C. stygia. Peak numbers were trapped in January, February and March. Almost 25% of wild flies, caught after feeding for up to 3 min on dog faeces naturally contaminated with taeniid eggs, had eggs in their intestine. When administered to lambs all 4 species of fly transferred infection. Lambs grazed in winter on plots near kennels that had housed dogs with patent infections of T. hydatigena acquired higher worm burdens than those grazed further away. The burdens were greater downwind of the prevailing wind. After removal of the dogs, blowflies caught in the vicinity contained taeniid eggs but the contents of pitfall traps did not. Blowflies, and to a much lesser degree the insects and dust caught on sticky traps during the presence of the dogs, transferred infection when administered to naive lambs.

Animals↗
PubMed1983

Hydatidosis and cysticercosis: the dynamics of transmission.

The infective pattern of taeniids in their intermediate hosts is determined by the complex interaction of parasite- and host-related factors. Many of these have been examined in this review, but particular emphasis has been placed on environmental factors that affect the free-living egg. While number, infectivity and distribution of eggs are among the important factors determining the infective pattern, the dynamics of this aspect of transmission have been previously neglected. The biotic potential of the large taeniid tapeworms is enormous. Eggs deposited on pasture are subjected to the microclimatic effects of the environment. They seem to tolerate a relatively wide temperature range. Heat down to -30 degrees C. However, they are rapidly killed by low humidity at all temperatures. Under more favourable conditions, they have finite life spans largely determined by the environmental temperature. At deposition, the egg population seems to be at various stages of maturity. Immature eggs appear to be able to mature under suitable environmental conditions and to become infective. They then age, and their infectivity to the intermediate host declines. At the population level, the changes in infectivity with time are determined by the relative stages of maturity within the egg population and the temperatures that it experiences. Under certain circumstances the infectivity of a given population may increase temporarily. The movements and defaecation habits of the definitive host determine the primary site of egg deposition. However, evidence is accumulating that considerable dispersion occurs almost immediately afterwards. Eggs have been shown to disperse up to 80 m within 10 days and there are indications that small numbers travel much further. Intermediate hosts, such as cattle and sheep, generally avoid grazing areas contaminated with faeces. Thus, dispersal enhances the chance of the eggs being ingested. The combination of high biotic potential and long-range dispersal means that individual infected definitive hosts can be responsible for infecting intermediate hosts over a very wide area. Although various possible agents have been suggested, the mechanisms responsible for disseminating eggs are still uncertain.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗