Search PubMed⌕ Search

Biomedical subjects

R M Anderson

Publications and source records attributed to R M Anderson.

At least 307 records · Page 17Linked to original sources
PubMed1984

Experimental studies of age-prevalence curves for Schistosoma mansoni infections in populations of Biomphalaria glabrata.

We report the results of experimental studies of the generation of age-prevalence curves for Schistosoma mansoni infections in laboratory populations of Biomphalaria glabrata. Within snail populations of varying sizes and age structures, the net force of infection is shown to be linearly dependent on the rate at which miracidia are introduced into the aquatic habitat of the host. For individual snails, the per capita force of infection is shown to be related to snail age and size, and the death rate of shedding snails is demonstrated to be dependent on the period of time during which a snail has been releasing cercariae. Both factors are important determinants of the proportion of infected snails within populations of hosts and may generate convex age-prevalence curves. Comparisons of snail abundance in populations either exposed, or not exposed, to infection suggest that S. mansoni can act to significantly depress the population growth of its intermediate host. Mathematical models are developed, incorporating the age structure of snail populations, to aid in the interpretation of experimental results.

Age Factors↗
PubMed1984

The population dynamics of Gyrodactylus bullatarudis (Monogenea) within laboratory populations of the fish host Poecilia reticulata.

Experiments were undertaken to investigate the factors which influence the transmission dynamics of Gyrodactylus bullatarudis within populations of laboratory guppies. The parasites possess a number of biological attributes which are almost unique amongst helminth parasites of vertebrates. These include the ability to reproduce viviparously and directly on the surface of the host, the ability to survive death of the host (for a short period of time) and the ability to transfer between hosts (despite the absence of a specialized transmission stage in the parasite's life-cycle). Long-term laboratory experiments demonstrated the inability of the parasite to persist within populations of the host in the absence of the continual introduction of naive susceptible fish. With regular addition of susceptible fish, the parasite population exhibited recurrent epidemic behaviour. The magnitudes of the epidemics and the time interval between them, were dependent on the rate at which fish were added to the populations. The parasite was over-dispersed in its distribution within the experimental fish populations and was a significant cause of host mortality (in a manner related to parasite burden). The experimental results suggest that acquired resistance to infection is an important factor determining epidemic behaviour.

Animals↗
PubMed1984

Systematic temporal changes in host susceptibility to infection: demographic mechanisms.

Simple mathematical models are developed to examine the influence of variability in host susceptibility to infection, on the dynamics of host-parasite population interactions. When hosts differ in their innate susceptibility (at birth), to infection by a specific parasite, the average susceptibility of the host population as a whole may show systematic changes through time. Such patterns may arise as a result of demographic factors associated with the interaction between host and parasite populations, in the absence of inheritance mechanisms (a genetic component) or acquired resistance (an immunological component). The general significance of this observation is discussed in terms of the coevolution of host-parasite associations.

Animals↗
PubMed1984

Spatial, temporal, and genetic heterogeneity in host populations and the design of immunization programmes.

The relevance of heterogeneity in infectious disease transmission to the design of immunization programmes is examined by the use of simple deterministic models of recurrent epidemic behaviour. In the context of spatial heterogeneity it is shown that an optimal vaccination programme, which concentrates on groups with relatively high density, can lead to eradication with overall coverage levels less than those estimated by a model which assumes homogeneity in host spatial distribution. Age-related heterogeneity in transmission can result in higher or lower coverage levels than those predicted by models which assume age-independent transmission, depending on the quantitative detail of how transmission changes with age. Genetic heterogeneity in host susceptibility to infection is shown to produce effects that are similar to those induced by age-related changes in transmission. It is concluded that much greater attention should be focused on transmission heterogeneity in future work on models of recurrent epidemic behaviour and the design of immunization programmes.

Biometry↗
PubMed1983

Epidemiology and genetics in the coevolution of parasites and hosts.

Recent studies suggest that parasites (interpreted broadly to include viruses, bacteria, protozoans and helminths) may influence the numerical magnitude or geographical distribution of their host populations; most of such studies focus on the population biology and epidemiology of the host-parasite association, taking no explicit account of the genetics. Other researchers have explored the possibility that the coevolution of hosts and parasites may be responsible for much of the genetic diversity found in natural populations, and may even be the main reason for sexual reproduction; such genetic studies rarely take accurate account of the density- and frequency-dependent effects associated with the transmission and maintenance of parasitic infections. This paper aims to combine epidemiology and genetics, reviewing the way in which earlier studies fit into a wider scheme and offering some new ideas about host-parasite coevolution. One central conclusion is that 'successful' parasites need not necessarily evolve to be harmless: both theory and some empirical evidence (particularly from the myxoma-rabbit system) indicate that many coevolutionary paths are possible, depending on the relation between virulence and transmissibility of the parasite or pathogen.

Animals↗
PubMed1983

The brain in experimental portal-systemic encephalopathy. I. Morphological changes in three animal models.

Morphological features of three models of portal-systemic encephalopathy in the rat were studied and compared with plasma ammonia levels and clinical observations. Carbon tetrachloride-induced cirrhosis with terminal coma produced a wide variety of structural changes in the brain whose severity was related to plasma ammonia levels at the time of death. These changes included diffuse gliosis, Alzheimer cells and focal neuronal necrosis but did not include spongiform changes in cerebral or cerebellar cortex. Porta-caval anastomosis (PCA) did not appear to produce any significant neurological symptoms. Rats with PCA of durations 1-30 weeks were studied and over this time the structural changes included astrocytic nuclear swelling, swelling of perivascular astrocytic foot-processes and spongiform change in the molecular layer of the cerebellum. No evidence of Alzheimer cells or gliosis was seen and plasma ammonia levels at no stage exceed twice the normal levels. Porta-caval anastomosis followed by gavage feeding with ammoniated cationic exchange resin produced severe neurological symptoms and marked hyperammonaemia. In these animals not only astrocytes but oligodendrocytes and neurons showed nuclear and cytoplasmic swelling and numerous Alzheimer type II cells were seen, together with a diffuse gliosis, but no evidence of spongiform change in the cerebral or cerebellar cortex was seen. It is concluded that ammonium ions are important in the genesis of morphological changes in the brain in rat models of portal-systemic encephalopathy, but the relevance of these changes to neurological dysfunction is uncertain.

Ammonia↗
PubMed1983

The brain in experimental portal-systemic encephalopathy. II. Water and electrolyte changes.

The water content and amounts of sodium, potassium and chloride were measured in the brains of normal rats, rats with PCA, normal rats fed ammoniated cationic exchange resin, and rats with PCA fed the resin. Plasma electrolytes and ammonia levels were also measured, and sodium and chloride spaces were calculated. Rats with PCA showed increased water content, sodium space and chloride space in the brainstem compared to controls. Rats with PCA fed ammoniated resin showed increased chloride content and Na+:K+ ratio in the brainstem, and an increased chloride space in the brainstem. In these rats the chloride spaces in the cerebrum and cerebellum exceeded the sodium spaces. It is concluded that high circulating ammonia levels can in vivo produce ionic shifts which may interfere with nervous function. It is also concluded that increased cytoplasmic osmolarity produced by ammonium ion-induced stimulation of (Na+ + Ka+) ATPase may result in the appearance of swollen astrocytes in conventional electron micrographs.

Ammonia↗
PubMed1983

The interrelationship of factors controlling cardiac output.

Our understanding of the mechanics of circulation may be broadened by testing old and new concepts on a new hydraulic model made possible by a unique pump which resulted from the evolution of open heart surgical equipment. Findings in human physiology which have corollaries in the model can be analyzed more easily in the model, with the resulting conclusions transferable with reasonable validity. Factors thought to control cardiac output were tested. The results indicate that there are two separate and distinct sets of control factors, with only one set being operative at a time. It is the set that is potentially limiting cardiac output the most that is the determinant at any time. Past paradoxes in our perception of human circulatory physiology are accommodated by the resulting concept.

Cardiac Output↗
PubMed1983

Vaccination against rubella and measles: quantitative investigations of different policies.

This paper uses relatively simple and deterministic mathematical models to examine the impact that different immunization policies have on the age-specific incidence of rubella and measles. Following earlier work by Knox (1980) and others, we show that immunization programmes can, under some circumstances, increase the total number of cases among older age groups; the implications for the overall incidence of measles encephalitis and of congenital rubella syndrome are examined, paying attention both to the eventual equilibrium and to the short-term effect in the first few decades after immunization is initiated. Throughout, we use data (from the U.K., and U.S.A. and other countries) both in the estimation of the epidemiological parameters in our models, and in comparison between theoretical predictions and observed facts. The conclusions defy brief summary and are set out at the end of the paper.

Adolescent↗
PubMed1983

Basilar artery dissection.

Dissection of the basilar artery caused sudden coma and death in a 40-year-old man. Atypical clinical features were explained at necropsy. A ventral dissection of the artery within its outer layers resulted in destruction of the pontine tegmentum with sparing of the basis pontis. An unsuspected defect in the internal elastic lamina in the left internal carotid artery was also found suggesting a more generalised disorder of arterial walls. Basilar artery dissection should be considered in the diagnosis of coma in young people.

Adult↗
PubMed1983

Crystal-controlled telemetry system for long-term acquisition of physiological data.

A crystal-controlled telemetry system was developed for long-term high-reliability recording of blood pressure in unrestrained animals. The miniaturized transducer-transmitter system is powered by two lithium batteries that allow for more than 200 h of continuous recording of blood pressure before battery replacement. The total system has a flat frequency response of 0-25 Hz and a time stability of +/- 1 mmHg/day. Linearity is better than 0.5%. The high degree of dependability of the system permits us to perform continuous, unattended, around-the-clock recordings of pressure and heart rate in unrestrained animals. Very high dependability is achieved by the use of a novel piezoresistive blood pressure transducer and by the inherent stability of the crystal-controlled radio-frequency oscillator of the transmitter. The receiver is also crystal tuned to the measured transmitter frequency. This transmitter-receiver frequency locking ensures very reliable reception of the telemetered signal.

Animals↗
PubMed1983

Visual perceptions and observations of an aphakic surgeon.

A surgeon analyzes his perception following removal of cataracts and repair of a retinal detachment. His conclusions are: aphakic eyes perceive ultraviolet light, the optic disc is perceived as a point rather than a space the size of the disc, a certain minimal similarity of light intensity on the two retinas is necessary for fusion of images, reduction telescopes can be made by a combination of contact lenses and trifocals which balance the image size in unilateral aphakia, the visual angle of cataract glasses may be only 56 degrees but is perceived as 90 degrees with a 22 degrees blind sector, a patient can observe the progress of his own cataract by looking through a pinhole, and the optimal prescription for contact lenses in aphakia is with acute vision provided at 75 cm. rather than at 5 or 6 m.

Aphakia, Postcataract↗
PubMed1982

Population dynamics of human helminth infections: control by chemotherapy.

An analysis is presented of the population dynamics of the major helminth parasites of man with the aim of understanding observed patterns in the age-specific prevalence and intensity of infection. Mathematical models are used to investigate the possibility of controlling helminth diseases by mass chemotherapy, and to explore the advantages of selective treatment of the most heavily infected individuals in a community.

Adolescent↗