Michael Begon

Plague: Past, Present, and Future

The authors argue that plague should be taken much more seriously by the international health community.

Trade-Offs with Resistance to a Granulosis Virus in the Indian Meal Moth, Examined by a Laboratory Evolution Experiment

When microbial agents are used as pest-control agents, resistance in the host may be selected for. If resistance occurs there are potentially fitness costs due to trade-offs between resistance and other life-history traits. Genotypic trade-offs with resistance to a virus in a lepidopteran host are examined by a micro-evolutionary selection experiment. Six populations of the Indian meal moth, Plodia interpunctella, were established, three of which supported a granulosis virus infection (selected insects) while the remaining three acted as virus-free controls. After a period of 2 years, bioassays with the virus showed that selected moths were 1.96-fold more resistant to infection (LD 50 s) than those derived from the virus-free control populations

Cowpox: reservoir hosts and geographic range.

It is generally accepted that the reservoir hosts of cowpox virus are wild rodents, although direct evidence for this is lacking for much of the viruss geographic range. Here, through a combination of serology and PCR, we demonstrate conclusively that the main hosts in Great Britain are bank voles, wood mice and short-tailed field voles. However, we also suggest that wood mice may not be able to maintain infection alone, explaining the absence of cowpox from Ireland where voles are generally not found. Infection in wild rodents varies seasonally, and this variation probably underlies the marked seasonal incidence of infection in accidental hosts such as humans and domestic cats.

Transmission dynamics of a zoonotic pathogen within and between wildlife host species

The transmission dynamics of the cowpox virus infection have been quantified in two mixed populations of bank voles (Clethrionomys glareolus) and wood mice (Apodemus sylvaticus), through analyses of detailed time-series of the numbers of susceptible, infectious and newly infected individuals. The cowpox virus is a zoonosis which circulates in these rodent hosts and has been shown to have an adverse effect on reproductive output. The transmission dynamics within species is best described as frequency dependent rather than density dependent, contrary to the ‘mass action’ assumption of most previous studies, both theoretical and empirical. Estimation of a transmission coefficient for each species in each population also allows annual and seasonal variations in transmission dynamics to be investigated through an analysis of regression residuals. Transmission between host species is found to be negligible despite their close co–habitation. The consequences of this for the combining ability of hosts as zoonotic reservoirs, and for apparent competition between hosts, are discussed.

Seasonal dynamics of Anaplasma phagocytophila in a rodent-tick (Ixodes trianguliceps) system, United Kingdom.

We investigated the reservoir role of European wild rodents for Anaplasma phagocytophila using polymerase chain reaction (PCR) analysis of blood collected from individually tagged rodents captured monthly over 2 years. The only tick species observed in the woodland study site was Ixodes trianguliceps, and ruminant reservoir hosts were not known to occur. A. phagocytophila infections were detected in both bank voles and wood mice but were restricted to periods of peak nymphal and adult tick activity. Most PCR-positive rodents were positive only once, suggesting that rodent infections are generally short-lived and that ticks rather than rodents may maintain the infection over winter. Bank voles were more likely to be PCR positive than wood mice, possibly because detectable infections are longer lived in bank voles. This study confirms that woodland rodents can maintain A. phagocytophila in Great Britain in the absence of other reservoir hosts and suggests that I. trianguliceps is a competent vector.

Plague dynamics are driven by climate variation

The bacterium Yersinia pestis causes bubonic plague. In Central Asia, where human plague is still reported regularly, the bacterium is common in natural populations of great gerbils. By using field data from 1949–1995 and previously undescribed statistical techniques, we show that Y. pestis prevalence in gerbils increases with warmer springs and wetter summers: A 1°C increase in spring is predicted to lead to a >50% increase in prevalence. Climatic conditions favoring plague apparently existed in this region at the onset of the Black Death as well as when the most recent plague pandemic arose in the same region, and they are expected to continue or become more favorable as a result of climate change. Threats of outbreaks may thus be increasing where humans live in close contact with rodents and fleas (or other wildlife) harboring endemic plague.

Antibiotic resistance found in wild rodents

Resistance to antibiotics is an increasingly common problem in both veterinary and human medicine, and its management is the subject of urgent debate. Efforts to reduce this resistance are based on the assumption that it is maintained in bacterial populations as a result of exposure to antibiotics, and that restricting the use of antibiotics should therefore restrain the spread of resistance. But we have found that antibiotic resistance is prevalent in populations of wild rodents that have not been exposed to antibiotics, indicating that approaches to control it based on this assumption may be overoptimistic.

Should egg size and clutch size decrease with age

We analyse a model in which females gather resources for eggs in a pre-reproductive phase, and are then, in the reproductive phase, subject to mortality during the production of successive clutches of offspring. We find that if clutch size is constrained, egg size should decline with maternal age. Where egg fitness shows diminishing returns with egg size, and maternal mortality is age-independent, the decline in egg size will be linear, and the rate of decline will be proportional to maternal mortality. All females with the same survival prospects follow the same trajectory of decline, but larger females enter the trajectory at a larger egg size. Similarly, if egg size is constrained, clutch size declines with maternal age, the rate of decline increases with maternal mortality, and larger females lay larger clutches initially. Support for the models is found in a wide variety of species, and in particular in a number of recent studies of butterflies.

Enterobacteria - Antibiotic resistance found in wild rodents

Resistance to antibiotics is an increasingly common problem in both veterinary and human medicine, and its management is the subject of urgent debate. Efforts to reduce this resistance are based on the assumption that it is maintained in bacterial populations as a result of exposure to antibiotics, and that restricting the use of antibiotics should therefore restrain the spread of resistance. But we have found that antibiotic resistance is prevalent in populations of wild rodents that have not been exposed to antibiotics, indicating that approaches to control it based on this assumption may be overoptimistic.

The abundance threshold for plague as a critical percolation phenomenon.

Percolation theory is most commonly associated with the slow flow of liquid through a porous medium, with applications to the physical sciences. Epidemiological applications have been anticipated for disease systems where the host is a plant or volume of soil, and hence is fixed in space. However, no natural examples have been reported. The central question of interest in percolation theory, the possibility of an infinite connected cluster, corresponds in infectious disease to a positive probability of an epidemic. Archived records of plague (infection with Yersinia pestis) in populations of great gerbils (Rhombomys opimus) in Kazakhstan have been used to show that epizootics only occur when more than about 0.33 of the burrow systems built by the host are occupied by family groups. The underlying mechanism for this abundance threshold is unknown. Here we present evidence that it is a percolation threshold, which arises from the difference in scale between the movements that transport infectious fleas between family groups and the vast size of contiguous landscapes colonized by gerbils. Conventional theory predicts that abundance thresholds for the spread of infectious disease arise when transmission between hosts is density dependent such that the basic reproduction number (R0) increases with abundance, attaining 1 at the threshold. Percolation thresholds, however, are separate, spatially explicit thresholds that indicate long-range connectivity in a system and do not coincide with R0 = 1. Abundance thresholds are the theoretical basis for attempts to manage infectious disease by reducing the abundance of susceptibles, including vaccination and the culling of wildlife. This first natural example of a percolation threshold in a disease system invites a re-appraisal of other invasion thresholds, such as those for epidemic viral infections in African lions (Panthera leo), and of other disease systems such as bovine tuberculosis (caused by Mycobacterium bovis) in badgers (Meles meles).