Mixed inoculations of a microsporidian parasite with horizontal and vertical infections

Vizoso, Dita B.; Ebert, Dieter

doi:10.1007/s00442-004-1771-4

Cited by 57 publications

(30 citation statements)

References 47 publications

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“…Theory predicts that the optimal level of virulence depends on the relative frequencies of horizontal and vertical transmission (Ebert & Herre 1996). Although it can be complicated by spatial population structure ( Ferdy 2009), phenotypic plasticity ( Vizoso & Ebert 2005b) or multiplicity of infection ( Vizoso & Ebert 2005a), the general prediction is that vertical transmission of pathogens selects for lower virulence. When viruses are primarily maintained by vertical transmission we would expect selection to favour reduced virulence in order to maximize the virus's basic reproductive number, which is directly dependent on the progeny of the host they are currently infecting and, thus, host fitness.…”

Section: Discussionmentioning

confidence: 99%

Mode of transmission and the evolution of arbovirus virulence in mosquito vectors

2009

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The traditional assumption that vector-borne pathogens should evolve towards a benign relationship with their arthropod vectors has been challenged on theoretical grounds and empirical evidence. However, in the case of arboviruses (arthropod-borne viruses), although a number of investigators have reported experimental evidence for virus-induced vector mortality, others have failed to detect any significant impact. Whether this variation in the observed level of arbovirus virulence depends on biological traits or experimental design is unclear. Here, we perform a meta-analysis of studies across a range of mosquitovirus systems to show that, overall, arboviruses do reduce the survival of their mosquito vectors, but that the magnitude of the effect depends on the vector/virus taxonomic groups and the mode of virus transmission. Alphaviruses were associated with highest virulence levels in mosquitoes. Horizontal transmission (intrathoracic inoculation or oral infection) was correlated with significant virus-induced mortality, whereas a lack of adverse effect was found for Aedes mosquitoes infected transovarially by bunyaviruses-a group of viruses characterized by high natural rates of vertical transmission in their enzootic vectors. Our findings are consistent with the general prediction that vertically transmitted pathogens should be less virulent than those transmitted horizontally. We conclude that varying degrees of virulence observed among vector-virus systems probably reflect different selective pressures imposed on arboviruses that are primarily transmitted horizontally versus vertically.

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Section: Discussionmentioning

confidence: 99%

Mode of transmission and the evolution of arbovirus virulence in mosquito vectors

2009

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“…Since there is genetic variation in D. magna-O. bayeri interactions (Mucklow et al 2004;Vizoso & Ebert 2005a, b) and local adaptation of parasites has been shown for other D. magna parasites (Ebert 1994;Ebert et al 1998), some of these new genotypes may be more resistant to O. bayeri infection than their mothers, and thus uninfected. Since the genetic composition of host populations has been shown to influence prevalence dynamics (e.g.…”

Section: Discussion (A) Diapause Drives Prevalence Cyclesmentioning

confidence: 99%

Apparent seasonality of parasite dynamics: analysis of cyclic prevalence patterns

Lass

Ebert

2005

Proc. R. Soc. B.

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Seasonal disease dynamics are common in nature, but their causes are often unknown. Our case study provides insight into the cyclic prevalence pattern of the horizontally and vertically transmitted microsporidium Octosporea bayeri in its Daphnia magna host. Data from several populations over a four year period revealed a regular prevalence increase during summer and a decrease over winter when hosts underwent diapause. Prevalence also decreased after summer diapause indicating that the decline is causally linked to diapause rather than to winter conditions. Experiments showed that host diapause itself can explain a certain proportion of the decline. The decline further depends on the environmental conditions during diapause: infected resting eggs suffered from higher mortality under experimental winter than under experimental summer diapause conditions. Investigating the mechanisms of prevalence increase after diapause, the parasite was found to survive winter outside its host, enabling horizontal infection of susceptible hosts in the following growing season. Allowing for horizontal transmission in experimental host populations resulted in a steep prevalence increase, while excluding it led to a pronounced decline. Thus, the apparent seasonality in O. bayeri prevalence is characterized by a decline during host diapause followed by horizontal spread of the parasite during the host's asexual growth phase.

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“…Thus with more frequent warmer and longer summers virulent strains are able to invade and transmit in a dense fish population better than the nonvirulent strains. High growth rates have been previously shown to correlate with virulence in malaria parasites (de Roode et al 2005a,b;Bell et al 2006) and in microsporidian parasites of Daphnia (Ebert & Mangin 1997;Vizoso & Ebert 2005).…”

Section: Discussionmentioning

confidence: 99%

Intensive fish farming and the evolution of pathogen virulence: the case of columnaris disease in Finland

et al. 2009

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Ecological changes affect pathogen epidemiology and evolution and may trigger the emergence of novel diseases. Aquaculture radically alters the ecology of fish and their pathogens. Here we show an increase in the occurrence of the bacterial fish disease Flavobacterium columnare in salmon fingerlings at a fish farm in northern Finland over 23 years. We hypothesize that this emergence was owing to evolutionary changes in bacterial virulence. We base this argument on several observations. First, the emergence was associated with increased severity of symptoms. Second, F. columnare strains vary in virulence, with more lethal strains inducing more severe symptoms prior to death. Third, more virulent strains have greater infectivity, higher tissue-degrading capacity and higher growth rates. Fourth, pathogen strains co-occur, so that strains compete. Fifth, F. columnare can transmit efficiently from dead fish, and maintain infectivity in sterilized water for months, strongly reducing the fitness cost of host death likely experienced by the pathogen in nature. Moreover, this saprophytic infectiousness means that chemotherapy strongly select for strains that rapidly kill their hosts: dead fish remain infectious; treated fish do not. Finally, high stocking densities of homogeneous subsets of fish greatly enhance transmission opportunities. We suggest that fish farms provide an environment that promotes the circulation of more virulent strains of F. columnare. This effect is intensified by the recent increases in summer water temperature. More generally, we predict that intensive fish farming will lead to the evolution of more virulent pathogens.

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Mixed inoculations of a microsporidian parasite with horizontal and vertical infections

Cited by 57 publications

References 47 publications

Mode of transmission and the evolution of arbovirus virulence in mosquito vectors

Mode of transmission and the evolution of arbovirus virulence in mosquito vectors

Apparent seasonality of parasite dynamics: analysis of cyclic prevalence patterns

Intensive fish farming and the evolution of pathogen virulence: the case of columnaris disease in Finland

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