Virulence

Abstract

Abstract. -Why do parasites harm their hosts? Intuition suggests that parasites should evolve to be benign whenever the host is needed for transmission. Yet a growing theoretical literature offers several models to explain why natural selection may favor virulent parasites over avirulent ones. This perspective first organizes these models into a simple framework and then evaluates the empirical evidence for and against the models. There is relatively scant evidence to support any of the models rigorously, and indeed, there are only a few unequivocal observations of virulence actually evolving in parasite populations. These shortcomings are surmountable, however, and empirical models of host-parasite interactions have been developed for many kinds of pathogens so that the relevant data could be acquired in the near future.Aside from academic interest, the evolution of virulence has potential medical and agricultural ramifications that may provide evolutionary biology with opportunities for contributions to human welfare. For example, understanding the evolution of parasite virulence may help us design better vaccines, prevent the emergence of highly virulent strains in the future, and diminish the virulence of present pathogens. These potential applications notwithstanding, the usefulness of an evolutionary theory of virulence to social problems has not been demonstrated and is even doubtful in some cases. One promising area for contributions from evolutionary theory is in designing live, attenuated vaccines.Key words.-Evolution, microparasite, population biology, virulence, virus.Received April 10, 1994. Accepted April 12, 1994 Evolutionary biologists are becoming increasingly enamored of the idea that evolutionary thinking may contribute substantially to the alleviation of human health problems. Much of the fanfare has centered on applications of natural selection to disease (the Darwinian Medicine school; Profet 1991Profet , 1993Williams and Nesse 1991; Ewald 1994) but these applications of evolutionary biology are so far without obvious success in penetrating the fields of medicine. In contrast, a far more visible impact from evolutionary biology is the use of phylogenetic reconstruction to trace routes and sources of in-I

Exposure of leopard frogs to a pesticide mixture affects life history characteristics of the lungworm Rhabdias ranae

“…However, research using other host-parasite system has shown that life history characteristics related to parasite transmission are intimately linked with virulence; those parasites that are transmitted faster presumably become more virulent (Garnick 1992;Bull 1994;Ebert and Herre 1996;Frank 1996;Day 2001). In fact, while most scientists define virulence as the extent of damage or death caused to a host (Ewald 1994), others have observed that virulence is a property of both host and parasite (Bull 1994;Poulin and Combes 1999). Thus, Garnick (1992) referred to virulence as the ability to develop and reproduce in a host, and Poulin and Combes (1999) remarked that studies of virulence must emphasize the ability of the parasite to multiply.…”

Ecological impacts of invasive alien species on bees

“…Therefore, in spite of quarantine controls, infested commercial colonies have been introduced to some countries (Goka et al, 2001). Following interspecific transmission of parasites, virulence on new hosts could be higher or lower, although more noticeable in the former case (Bull, 1994). Furthermore, it is postulated that the sudden decline of five North American bumblebee species might be related to N. bombi outbreak in bumblebee facilities that decimated the commercial colonies of B. occidentalis (Thorp and Shepherd, 2005;Winter et al, 2006;Williams and Osborne, 2009).…”

The Ecology and Evolution of Fungal Viruses

“…However, the vast majority of fungal viruses have little or no detectable effect on host phenotype or fitness (Buck, 1986(Buck, , 1998Ghabrial, 1998;Ghabrial and Suzuki, 2009;McCabe et al, 1999;Pearson et al, 2009). The "conventional wisdom" about virulence is that associations persisting for a long time should evolve to being benign or even beneficial, and particularly so for pathogens that depend primarily on vertical transmission to host offspring (Bull, 1994;Levin, 1996). A pathogen that depends on vertical transmission and adversely affects the fitness of its host (i.e., virulence) will, on average, reduce its own fitness because vertical transmission will be reduced; in this case, therefore, virulence will be selected against.…”