The molecular epidemiology of parasites

Summary The explosion of new techniques, made available by the rapid advance in molecular biology, has provided a battery of novel approaches and technology which can be applied to more practical issues such as the epidemiology of parasites. In this review, we discuss the ways in which this new field of molecular epidemiology has contributed to and corroborated our existing knowledge of parasite epidemiology. Similar epidemiological questions can be asked about many different types of parasites and, using detailed examples such as the African trypanosomes and theLeishmania parasites, we discuss the techniques and the methodologies that have been or could be employed to solve many of these epidemiological problems.     

Generalized Lévy walks and the role of chemokines in migration of effector CD8+ T cells

Chemokines have a central role in regulating processes essential to the immune function of T cells, such as their migration within lymphoid tissues and targeting of pathogens in sites of inflammation. Here we track T cells using multi-photon microscopy to demonstrate that the chemokine CXCL10 enhances the ability of CD8+ T cells to control the pathogen Toxoplasma gondii in the brains of chronically infected mice. This chemokine boosts T-cell function in two different ways: it maintains the effector T-cell population in the brain and speeds up the average migration speed without changing the nature of the walk statistics. Notably, these statistics are not Brownian; rather, CD8+ T-cell motility in the brain is well described by a generalized Lévy walk. According to our model, this unexpected feature enables T cells to find rare targets with more than an order of magnitude more efficiency than Brownian random walkers. Thus, CD8+ T-cell behaviour is similar to Lévy strategies reported in organisms ranging from mussels to marine predators and monkeys, and CXCL10 aids T cells in shortening the average time taken to find rare targets.     

A phenomenological model for precursor volcanic eruptions

Intense explosions of relatively short duration frequently precede large explosive and effusive volcanic eruptions-by as much as weeks to months in the case of very viscous magmas. In some cases, such pre-eruption activity has served as a sufficient warning to those living in the vicinity to evacuate and avoid calamity. Precursor events seem to be related to the formation of a magma pathway to the surface, but their precise interpretation is a long-standing puzzle. It has been inferred from theoretical studies that exsolution of volatiles might create an almost separate gas pocket at the tip of a propagating dyke. Here we explain the role that such a process may have, using a laboratory study of the transient propagation of a liquid-filled crack with a gas pocket at its tip that grows with time. We show that once the gas pocket acquires sufficient buoyancy to overcome the fracture resistance of the host solid the dynamics of the gas pocket, rather than those of the liquid, determine the velocity of the crack tip. Furthermore, we find that the gas can ultimately separate from the liquid. We propose that fast-moving, gas-rich pockets reaching the surface ahead of the main liquid-filled fissure could be the origin of many precursor eruptions.     

Hybrid formation between African trypanosomes during cyclical transmission

Trypanosomes of the species Trypanosoma brucei reproduce primarily by binary fission, but the frequency of enzyme electrophoretic variants in natural populations of T. brucei has provided indirect evidence for the existence of a sexual cycle. These studies, coupled with studies of restriction fragment length polymorphisms of genes encoding glycolytic enzymes, have also provided evidence for T. brucei being diploid. Here we report direct evidence of gene exchange between two different clones of trypanosomes after mixed infection and full cyclical development in the tsetse fly vector.