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Biomedical subjects

M G Roberts

Publications and source records attributed to M G Roberts.

52 records · Page 3Linked to original sources
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

An accurate demand feeder for fish, suitable for microcomputer control.

This paper describes an easily constructed and inexpensive demand feeder. The feeder is driven by an AC synchronous motor and gearbox and is suitable for microcomputer control. It will operate with inexpensive commercially available pelleted fish food, and will consistently deliver a single pellet for each operation of the motor. The components and materials can be purchased for approximately 23 pounds.

Animal Husbandry↗
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

Stability in cyclic epidemic models.

A general formulation for a family of cyclic epidemic models with density-dependent feedback mechanisms and removed classes is presented. A parameter, lambda, related to the basic reproductive rate determines the asymptotic behaviour of solutions of the model. It is shown that if lambda less than 1 the trivial solution is globally stable, and if lambda greater than 1 it is conditionally stable. The results are applied to a set of differential equations that has been used to model the life cycle of a parasite that has two hosts.

Animals↗
PubMed1982

Vaccination of sheep against Taenia ovis. The response to various dose rates of antigens obtained by incubation of oncospheres in vitro.

Activated oncospheres of Taenia ovis were incubated in vitro for 14 days and the antigens produced were collected and concentrated. Tenfold serial dilutions of the antigens were emulsified with Freund's incomplete adjuvant and injected intramuscularly into sheep. When subsequently challenged with T ovis eggs, these sheep displayed a level of immunity that was related to the amount of antigen injected. The calculated dose-response curve indicated that for the vaccine to be 90 per cent effective, the secretion from approximately 4600 T ovis oncospheres need to be injected into each sheep.

Animals↗
PubMed1978

Effects of hypothalamic lesions on the food intake of the goldfish (Carassius auratus).

Goldfish were trained to perform an operant in response in order to obtain food, thereby allowing the food intake to be accurately determined. The normal daily food intake was established for each fish before it was given a sham operation or bilateral hypothalamic lesions. It was then observed over a period of up to 60 days. Lesions of the lateral areas of the hypothalamus were found to cause cessations of operant feeding of up to 60 days duration, and cessations of feeding on manually presented food of up to 35 days. Lesions of the anterior-medial areas were followed by cessationsof operant feeding of up to 26 days. The post-lesioning food intake of fish lesioned in the posterior-medial area was similar to that of the control fish. No increases in operant feeding were observed after lesioning. The results are considered to be consistent with a genuine aphagia induced by lesions of the lateral hypothalamus.

Animals↗
PubMed1975

Behavioural effects of electrical stimulation of the hypothalamus of the goldfish (Carassius auratus).

Food-satiated goldfish were stimulated in tanks containing food on the surface of the water and on the bottom of the tank. Various components of the feeding response were seen, and complete feeding was observed, which was elicited from sites in the postero-medial hypothalamus. Evidence is put forward to support the supposition that the full response was due to a change in feeding motivation, rather than to changes in sensory or motor factors. Other types of behaviour, such as fin movements and swimming, could not be identified as part of any normal ethological sequence. However, vertical movements and gas-spitting responses indicated a possible hypothalamic control of buoyancy. None of the behaviours observed was seen to change in a social context.

Animals↗