II.9 Ecological Epidemiology
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II.9 Ecological Epidemiology Michael Begon OUTLINE 1. Parasites, pathogens, and other definitions 2. The importance of ecological epidemiology 3. The dynamics of parasites within populations: Transmission 4. The population dynamics of infection 5. Parasites and the dynamics of hosts 6. Shared parasites—zoonoses Strictly speaking, epidemiology is the study of the dynamics of disease in a population of humans. In ecology, however, the term takes on a slightly different meaning. Ecologists tend to expand the usage to cover populations of any species , animal or plant, but they then restrict it to infectious diseases (as opposed to, say, cancers or heart disease). Studies of human epidemiology usually treat the host (human ) population as fixed in size and focus on the dynamics of disease within this population. What distinguishes ‘‘ecological’’ epidemiology is an acknowledgment that the dynamics of the parasite and the host populations may interact . Hence, we are interested in the dynamics of parasites in host populations, that may themselves vary substantially in size, and also in the effects of the parasites on the dynamics of the hosts. GLOSSARY basic reproductive number. Usually denoted R0, for microparasites, the average number of new infections that would arise from a single infectious host introduced into a population of susceptible hosts; for macroparasites, the average number of established , reproductively mature offspring produced by a mature parasite throughout its life in a population of uninfected hosts critical population size. The population size of susceptible hosts for which R0 ¼ 1, where R0 is the basic reproductive number, and which must therefore be exceeded if an infection is to spread in a population density-dependent transmission. Parasite transmission in which the rate of contact between susceptible hosts and the source of new infections increases with host density frequency-dependent transmission. Parasite transmission in which the rate of contact between susceptible hosts and the source of new infections is independent of host density herd immunity. Where a population contains too few susceptible hosts (either because of natural infection or immunization) for infection to be able to establish and spread within a population macroparasite. A parasite that grows but does not multiply in its host, producing infective stages that are released to infect new hosts; the macroparasites of animals mostly live on the body or in the body cavities (e.g., the gut); in plants, they are generally intercellular microparasite. A small, often intracellular parasite that multiplies directly within its host transmission threshold. The condition R0 ¼ 1, where R0 is the basic reproductive number, which must be crossed if an infection is to spread in a population vector. An organism carrying parasites from one host individual to another, within which there may or may not be parasite multiplication zoonosis. An infection that occurs naturally and can be sustained in a wildlife species but can also infect and cause disease in humans 1. PARASITES, PATHOGENS, AND OTHER DEFINITIONS A parasite is an organism that obtains its nutrients from one or a very few host individuals, normally causing harm but not causing death immediately. This distinguishes parasites from predators, which kill and consume many prey in their lifetime, and from grazers, which take small parts from many different prey. If a parasite infection gives rise to symptoms that are clearly harmful, the host is said to have a disease. Pathogen, then, is a term that may be applied to any parasite that gives rise to a disease (i.e., is pathogenic). The language used by plant pathologists and animal parasitologists is often very different, but for the ecologist , these differences are less striking than the resemblances. One distinction that is useful is that between microparasites and macroparasites. Microparasites are small, often intracellular, and they multiply directly within their host where they are often extremely numerous. Hence, it is usually impossible to count the number of microparasites in a host: ecologists normally study the number of infected hosts in a population. Examples include bacteria and viruses (e.g., the typhoid bacterium and the yellow net viruses of beet and tomato), protozoa infecting animals (e.g., the Plasmodium species that cause malaria), and some of the simpler fungi that infect plants. Macroparasites grow but do not multiply in their host. They produce infective stages that are released to infect new hosts. The macroparasites of animals mostly live on the body or in the body cavities (e.g., the gut) of their hosts. In plants, they are generally intercellular. It is often possible to count or at least to estimate the numbers...


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