Search PubMed⌕ Search

Biomedical subjects

R G Bowers

Publications and source records attributed to R G Bowers.

8 recordsLinked to original sources
PubMed1998

A model of disease and vaccination for infections with acute and chronic phases.

A general model is presented of a disease in which both recovered and vaccinated individuals are protected from acute disease, but are still susceptible to chronic infection. The special threshold conditions for the establishment and persistence of such a disease are derived and explained in full. The efficacies of alternative vaccination strategies are detailed and a specific example of such a disease is given by examining feline calicivirus (FCV), a cause of upper respiratory tract disease in cats.

Acute Disease↗
PubMed1997

Community structure and the interplay between interspecific infection and competition.

Our motivation is the need to understand how two different interactions between species-shared infection and interspecific competition-combine to determine community structure. We introduce a proto-typical model of two hosts sharing a pathogen and also competing directly. We discuss forces of infection, forces of competition and invasion criteria and their relevance to long-term outcomes and community structure. To understand their interplay, we consider first purely competitive and second purely infective interactions. We then investigate our full model to establish how the two forces combine and how the combination and related invasion criteria determine community structure. The forces of infection and competition do not merely add; there is a synergetic resistance to invasion. Using generalised invasion criteria and subsidiary conditions for the feasibility and stability of uninfected coexistence, we classify long-term outcomes. We distinguish two main routes to three-species coexistence. In the first, two host species, each of which would not alone support the pathogen, support it jointly if interspecific competition is relatively weak, interspecific infection strong. In the second, at least one host species would alone support the pathogen and both are invadable by the other, but subsidiary conditions yield two cases. In one, infected coexistence results when the two hosts would coexist stably purely competitively and at sufficiently high densities to support the pathogen jointly. Thus coexistence is promoted by weak interspecific competition but there is a tension between weak interspecific infection favouring invadability and strong interspecific infection promoting pathogen survival. In the other, infected coexistence results when the two hosts would not coexist in the absence of the pathogen. This pathogen-mediated host coexistence is expected where there is strong intraspecific infection (lowering densities) and weak interspecific infection (favouring invadibility) as necessary. Results are compared with previous work and apparent competition and resource- and transmission-mediated coexistence are discussed.

Animals↗
PubMed1996

Host-pathogen systems in a spatially patchy environment.

A discrete model for a host-pathogen system is developed and is used to represent the dynamics in each patch within a landscape of n x n patches. These patches are linked by between-generation dispersal to neighbouring patches. Important results (compared to similar 'coupled map lattice' studies) include an increase in the likelihood of metapopulation extinction if the natural loss of pathogen particles is low, and the observation of a radial wave pattern (not previously reported) where the wavefront propagates uniformly from a central focus. This result has additional significance in that it permits the system to exhibit 'intermittency' between two quasi-stable spatial patterns: spirals and radial waves. With intermittent behaviour, the dynamics may look consistent when viewed at one time scale, but over a longer time scale they can alter dramatically and repeatedly between the two patterns. There is also evidence of clear links between spatial structure and temporal metapopulation behaviour in both the intermittent and 'pure' regions, verified by results from an algorithmic complexity measure and a spectral analysis of the temporal dynamics.

Animals↗
PubMed1994

Life-history trade-offs and the evolution of pathogen resistance: competition between host strains.

The dynamics of a 'resistant' and a 'susceptible' strain of a self-regulated host species, in the presence of a directly transmitted pathogen, is investigated. The two strains trade off differences in pathogen transmissibility (as an aspect of pathogen resistance) against differences in birth rate and/or resistance to crowding. Depending on parameter values, either strain may be eliminated, or the two may coexist (along with the pathogen). Coexistence (polymorphism), unsurprisingly, requires an appropriate balance between the different advantages possessed by the two strains. The probability of coexistence through such a balance, however, varies nonlinearly with the degree of difference between the strains: coexistence is least likely between two very similar strains. Resistance is most likely to evolve in hosts with the characteristics of many insect pests. Moreover, with highly pathogenic pathogens, a 'susceptible' strain may exclude a 'resistant' strain because its higher growth rate is more effective against the pathogen than reduced transmissibility. 'Resistance' can reside in parameters other than those directly associated with the pathogen. Although no cycles arise and no chaotic behaviour is found, an oscillatory approach to equilibrium is commonly observed, signalling the possibility of observable oscillations in strain frequency in the (more variable) real world.

Animals↗
PubMed1994

Host-host-pathogen models and microbial pest control: the effect of host self regulation.

A model has been investigated of the dynamics of the interaction between two hosts that are both attacked by a common pathogen with free-living infective stages, where the hosts are also subject to self-regulation. If either host interacted with the pathogen alone, two types of dynamics would be possible: an uninfected state where the host settles at its carrying capacity, and an infected state where the host settles at, or cycles around, a density lower than the carrying capacity. The three possible combination of two hosts have been investigated: uninfected-uninfected (both hosts uninfected if alone with the pathogen), infected-uninfected and infected-infected. A range of dynamics is generated, depending on parameter values, including infected co-existence of the two hosts (arrived at by a variety of routes), uninfected co-existence of the two hosts, exclusion of one host by the other which remains in an infected state, and a number of outcomes contingent on the initial densities in the system. Free-living infective stages make uninfected co-existence more likely and introduce additional contingency into the dynamics. The implications for microbial pest control are into the dynamics. The implications for microbial pest control are markedly different from those derived from related models without host self-regulation. There appears to be little chance of a non-target host undermining pest control, relatively little chance of the non-target enhancing pest control and a small but non-negligible threat to non-targets when parameter values are appropriate. The application of the results is commended but great caution is urged.

Animals↗
PubMed1994

The population dynamics of microparasites and vertebrate hosts: the importance of immunity and recovery.

The models of Anderson and May on the dynamics of vertebrate (1979, Nature 280, 361-367) and invertebrate (1981, Philos, Trans. R. Soc. 291, 451-524) populations and their microparasites have been extended and elaborated. Hence, in a series of models the effects of a range of biological factors have been considered. These models taken together clarify in particular the effects of recovery from the disease back to a state of susceptibility and the additional effects of recovery to a state of immunity. In general recovery increases both the threshold density and the equilibrium density but does not alter the prevalence of infection or the region in parameter space in which the host is regulated. Immunity causes a further increase in the equilibrium density, does not alter either the prevalence of infection or the threshold density, but reduces the region in which there is regulation. In both cases exceptions tend to occur when there is density dependence.

Animals↗
PubMed1991

A host-host-pathogen model with free-living infective stages, applicable to microbial pest control.

A model has been investigated of the dynamics of the interaction between two hosts which are both attacked by a common pathogen, where the pathogen has free-living infective stages the population size of which must itself be modelled explicitly, and where the host species do not interact with one another except through their shared pathogen. If either host interacted with the pathogen alone, three broad classes of dynamics would be possible: host regulation, pathogen persistence and pathogen extinction. Here, all possible types of combinations of hosts are examined: regulation-regulation (both hosts would be regulated if they interacted with the pathogen alone), regulation-persistence, regulation-extinction, persistence-persistence persistence-extinction and extinction-extinction. A wide range of dynamics is generated, including a number of patterns quite unlike those found in the one-host pathogen case (e.g. persistence in one host, elimination of the other host) and behaviour contingent on initial densities in the system. For clarity and pertinence, attention is focused on the case where one host is a pest, the pathogen is a potential microbial control agent, and the other host is a non-target species which it is undesirable to harm. The model suggests, broadly, that non-targets are unlikely to be seriously threatened in such cases, and also that non-targets, far from undermining pest control, are quite likely to contribute to its efficacy.

Animals↗