Dr. Christian Bottomley

Christian focused his PhD on the dynamics of African river-blindness (Onchocerclasis volvulus). His work deals with vertebrate hosts, which are frequently infected with multiple helminth species. There is a body of experimental evidence to suggest that infection with one parasite species can have either an antagonistic or synergistic effect on another species; such interactions may occur through parasite establishment, survival and fecundity. The extent to which such interactions are involved in the organization of helminth communities is largely unknown. Mathematical models based on Markov processes are used to explore two themes:

1) The effect of interspecific interactions on the joint distribution of helminth parasites in a population of hosts, and

2) conditions under which interacting species can coexist. To explore the former, Christian formulated models  that describe the process by which helminths of two species are acquired and lost in a cohort of ageing hosts.

In these models, the interspecific interaction occurred at the point of parasite establishment within the host such that the rate of establishment depended on the current worm burdens of the two species. The results were used to highlight some of the difficulties associated with inferring interspecific interactions from ecological data. He investigated the relationship between competition and species coexistence using models of the long-term dynamics of interacting species. Christian developed models in which there is a free-living larval stage whose population size is dependent on the size of the adult worm population. The models were analyzed using ëhybridí and ëmoment-closureí approximations; the former involving the replacement of stochastic components of the model with deterministic approximations, and the latter assuming a functional relationship between higher and lower order moments based on a specified distribution.

Christian derived the Lotka-Volterra model of competition for the case where hosts are equally exposed to parasites of the same species. Coexistence of two competing species was promoted by heterogeneous host exposure to each parasite species, provided that the rates of exposure to the two parasite species are not perfectly, positively correlated, and provided that the degree of heterogeneity in host exposure is similar for both species. In addition, he showed that the conditions required for coexistence are the same regardless of whether competition occurs at the point of parasite establishment within the host or via parasite fecundity. These results were discussed within the context of helminth community ecology. Since obtaining his PhD, Cristian worked as a medical statistician in the Department of Primary Care and Population Sciences at UCL, before joining the Tropical Epidemiology Group in June 2008. Currently, Christian is a Research Fellow in Medical Statistics at London School of Hygiene and Tropical Medicine.

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