51
|
Tseng M. Interactions between the parasite's previous and current environment mediate the outcome of parasite infection. Am Nat 2006; 168:565-71. [PMID: 17004228 DOI: 10.1086/507997] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/03/2006] [Accepted: 06/16/2006] [Indexed: 11/03/2022]
Abstract
The study of parasite virulence has generally focused on the conditions under which virulence is expected to increase or decrease over time and how the interactions between hosts and their environments may mediate the outcome of infection. Recently, parasite traits such as transmission, offspring production, and development have also been shown to be influenced by environmental variation. What is unclear is how variation in the parasite's environment may impact virulence. Recent theory demonstrates that plasticity can promote the evolution of decreased virulence; thus, understanding whether the parasite's environment can mediate virulence can improve predictions regarding the outcome of parasite infection. Here, an obligate mosquito parasite was reared in hosts fed high or low levels of food. Parasite oocysts (offspring) produced in these two host environments were subsequently fed to uninfected hosts. Parasites originating from well-fed hosts were found to be more virulent to these subsequent hosts compared to parasites originating from poorly fed hosts. Additionally, this effect was apparent only when current hosts were food deprived. These results demonstrate that parasite virulence was mediated by a cross-generational effect of the environment and that the overall outcome of infection was modified by variation in both the parasite's and host's environments.
Collapse
Affiliation(s)
- Michelle Tseng
- Department of Biology, Indiana University, Bloomington, Indiana 47405, USA. mtsengzzoology.ubc.ca
| |
Collapse
|
52
|
Bell AS, Roode JC, Sim D, Read AF. WITHIN-HOST COMPETITION IN GENETICALLY DIVERSE MALARIA INFECTIONS: PARASITE VIRULENCE AND COMPETITIVE SUCCESS. Evolution 2006. [DOI: 10.1111/j.0014-3820.2006.tb01215.x] [Citation(s) in RCA: 185] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
|
53
|
Perkins SL, Sarkar IN, Carter R. The phylogeny of rodent malaria parasites: simultaneous analysis across three genomes. INFECTION GENETICS AND EVOLUTION 2006; 7:74-83. [PMID: 16765106 DOI: 10.1016/j.meegid.2006.04.005] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/28/2005] [Revised: 04/26/2006] [Accepted: 04/29/2006] [Indexed: 11/18/2022]
Abstract
Species of Plasmodium that naturally infect wild rodents but can also be maintained in laboratory mice have long been used as model systems in which to study the biology of malaria parasites. Several of these rodent parasites are now providing useful genomic comparisons to those species that cause malaria in humans. Here we examined the phylogenetic relationships of 19 strains of rodent malaria parasites including four species native to African thicket rats (Plasmodium berghei, Plasmodium chabaudi, Plasmodium vinckei, and Plasmodium yoelii) and one from a porcupine (Plasmodium atheruri) using DNA sequence data collected from seven genes from each of the three parasite genomes. These included the nuclear dihydrofolate reductase gene and a cysteine protease gene, mitochondrial cytochrome b and cytochrome oxidase I genes, and the elongation factor tufA, caseinolytic protease C, and "open reading frame 470" genes from the apicoplast genome, for a combined total of 5049 nucleotides. Using simultaneous analysis, a method of combining each of the gene partitions into a super-matrix, two equally parsimonious trees were recovered. Bayesian analysis of the dataset produced the same topology. The basic species groups were well supported, with the exception of the placement of P. atheruri within the P. vinckei clade. Named subspecies showed a wide array of genetic differentiation, but fell into monophyletic groups.
Collapse
Affiliation(s)
- Susan L Perkins
- Division of Invertebrates, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA.
| | | | | |
Collapse
|
54
|
Lohse K, Gutierrez A, Kaltz O. EXPERIMENTAL EVOLUTION OF RESISTANCE IN PARAMECIUM CAUDATUM AGAINST THE BACTERIAL PARASITE HOLOSPORA UNDULATA. Evolution 2006. [DOI: 10.1111/j.0014-3820.2006.tb01196.x] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
|
55
|
Miller MR, White A, Boots M. THE EVOLUTION OF PARASITES IN RESPONSE TO TOLERANCE IN THEIR HOSTS: THE GOOD, THE BAD, AND APPARENT COMMENSALISM. Evolution 2006. [DOI: 10.1111/j.0014-3820.2006.tb01173.x] [Citation(s) in RCA: 139] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
|
56
|
Bell AS, de Roode JC, Sim D, Read AF. WITHIN-HOST COMPETITION IN GENETICALLY DIVERSE MALARIA INFECTIONS: PARASITE VIRULENCE AND COMPETITIVE SUCCESS. Evolution 2006. [DOI: 10.1554/05-611.1] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
|
57
|
Miller MR, White A, Boots M. THE EVOLUTION OF PARASITES IN RESPONSE TO TOLERANCE IN THEIR HOSTS: THE GOOD, THE BAD, AND APPARENT COMMENSALISM. Evolution 2006. [DOI: 10.1554/05-654.1] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
|
58
|
Lohse K, Gutierrez A, Kaltz O. EXPERIMENTAL EVOLUTION OF RESISTANCE IN PARAMECIUM CAUDATUM AGAINST THE BACTERIAL PARASITE HOLOSPORA UNDULATA. Evolution 2006. [DOI: 10.1554/05-656.1] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
|
59
|
Brunner JL, Richards K, Collins JP. Dose and host characteristics influence virulence of ranavirus infections. Oecologia 2005; 144:399-406. [PMID: 15891818 DOI: 10.1007/s00442-005-0093-5] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2004] [Accepted: 03/21/2005] [Indexed: 11/27/2022]
Abstract
Parasites play a prominent role in the ecology, evolution, and more recently, conservation of many organisms. For example, emerging infectious diseases, including a group of lethal ranaviruses, are associated with the declines and extinctions of amphibians around the world. An increasingly important basic and applied question is: what controls parasite virulence? We used a dose-response experiment with three laboratory-bred clutches of tiger salamander larvae (Ambystoma tigrinum) to test how the size of inoculum and host genetic factors influence the dynamics and outcome of ranavirus infections. We found that infection rates increased with dose and were strongly affected by clutch identity and host life history stage. Case mortality increased with dose of inoculum, but was unaffected by host characteristics. Average survival time decreased with dose and differed among clutches, but this was largely due to differences in the time to onset of symptoms. Overall, our results suggest that dose of inoculum and host characteristics (life history stage and genetic background) influence the establishment and early virus replication, and therefore the virulence of ranavirus infections.
Collapse
Affiliation(s)
- Jesse L Brunner
- School of Life Sciences, Arizona State University, Tempe, AZ, 85287-4501, USA.
| | | | | |
Collapse
|
60
|
Alizon S, van Baalen M. Emergence of a convex trade-off between transmission and virulence. Am Nat 2005; 165:E155-67. [PMID: 15937740 DOI: 10.1086/430053] [Citation(s) in RCA: 126] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2004] [Accepted: 01/11/2005] [Indexed: 11/03/2022]
Abstract
Most models of virulence evolution assume that a parasite cannot raise its transmission rate without causing more harm to its host. However, the existence of such trade-off relationships has recently been challenged. Here, we study how a trade-off can emerge from a model that explicitly incorporates within-host dynamics. We find that the existence and the convexity of the trade-off are robust, which implies a definite level of evolutionarily stable virulence (ESV) for the parasite. However, we also show that the dependence of the ESV on parameter values may be very strong. One possible consequence of this sensitivity is that relationships between transmission and virulence observed across populations need not conform to the patterns expected on the basis of a common (fixed) trade-off. We discuss possible experiments and implications of our results for the development of virulence management strategies.
Collapse
Affiliation(s)
- Samuel Alizon
- Laboratoire d'Ecologie, Centre Nationale de la Recherche Scientifique-Unité Mixte de Recherche (CNRS-UMR) 7625, Ecole Normale Supérieure, 75005 Paris, France.
| | | |
Collapse
|
61
|
Mackinnon MJ, Bell A, Read AF. The effects of mosquito transmission and population bottlenecking on virulence, multiplication rate and rosetting in rodent malaria. Int J Parasitol 2005; 35:145-53. [PMID: 15710435 DOI: 10.1016/j.ijpara.2004.11.011] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2004] [Revised: 11/08/2004] [Accepted: 11/11/2004] [Indexed: 10/26/2022]
Abstract
Malaria parasites vary in virulence, but the effects of mosquito transmission on virulence phenotypes have not been systematically analysed. Using six lines of malaria parasite that varied widely in virulence, three of which had been serially blood-stage passaged many times, we found that mosquito transmission led to a general reduction in malaria virulence. Despite that, the between-line variation in virulence remained. Forcing serially passaged lines through extreme population bottlenecks (<5 parasites) reduced virulence in only one of two lines. That reduction was to a level intermediate between that of the virulent parental and avirulent ancestral line. Mosquito transmission did not reverse the increased parasite replication rates that had accrued during serial passage, but it did increase rosetting frequencies. Re-setting of asexual stage genes during the sexual stages of the life cycle, coupled with stochastic sampling of parasites with variable virulence during population bottlenecks, could account for the virulence reductions and increased rosetting induced by mosquito transmission.
Collapse
Affiliation(s)
- M J Mackinnon
- School of Biological Sciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK.
| | | | | |
Collapse
|
62
|
HASTINGS ALAN, ARZBERGER PETER, BOLKER BEN, COLLINS SCOTT, IVES ANTHONYR, JOHNSON NORMANA, PALMER MARGARETA. Quantitative Bioscience for the 21st Century. Bioscience 2005. [DOI: 10.1641/0006-3568(2005)055[0511:qbftsc]2.0.co;2] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
|
63
|
Mackinnon MJ, Read AF. Virulence in malaria: an evolutionary viewpoint. Philos Trans R Soc Lond B Biol Sci 2004; 359:965-86. [PMID: 15306410 PMCID: PMC1693375 DOI: 10.1098/rstb.2003.1414] [Citation(s) in RCA: 197] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Malaria parasites cause much morbidity and mortality to their human hosts. From our evolutionary perspective, this is because virulence is positively associated with parasite transmission rate. Natural selection therefore drives virulence upwards, but only to the point where the cost to transmission caused by host death begins to outweigh the transmission benefits. In this review, we summarize data from the laboratory rodent malaria model, Plasmodium chabaudi, and field data on the human malaria parasite, P. falciparum, in relation to this virulence trade-off hypothesis. The data from both species show strong positive correlations between asexual multiplication, transmission rate, infection length, morbidity and mortality, and therefore support the underlying assumptions of the hypothesis. Moreover, the P. falciparum data show that expected total lifetime transmission of the parasite is maximized in young children in whom the fitness cost of host mortality balances the fitness benefits of higher transmission rates and slower clearance rates, thus exhibiting the hypothesized virulence trade-off. This evolutionary explanation of virulence appears to accord well with the clinical and molecular explanations of pathogenesis that involve cytoadherence, red cell invasion and immune evasion, although direct evidence of the fitness advantages of these mechanisms is scarce. One implication of this evolutionary view of virulence is that parasite populations are expected to evolve new levels of virulence in response to medical interventions such as vaccines and drugs.
Collapse
Affiliation(s)
- Margaret J Mackinnon
- School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JT, Scotland, UK.
| | | |
Collapse
|
64
|
Paul REL, Lafond T, Müller-Graf CDM, Nithiuthai S, Brey PT, Koella JC. Experimental evaluation of the relationship between lethal or non-lethal virulence and transmission success in malaria parasite infections. BMC Evol Biol 2004; 4:30. [PMID: 15355551 PMCID: PMC520815 DOI: 10.1186/1471-2148-4-30] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2003] [Accepted: 09/08/2004] [Indexed: 12/02/2022] Open
Abstract
Background Evolutionary theory suggests that the selection pressure on parasites to maximize their transmission determines their optimal host exploitation strategies and thus their virulence. Establishing the adaptive basis to parasite life history traits has important consequences for predicting parasite responses to public health interventions. In this study we examine the extent to which malaria parasites conform to the predicted adaptive trade-off between transmission and virulence, as defined by mortality. The majority of natural infections, however, result in sub-lethal virulent effects (e.g. anaemia) and are often composed of many strains. Both sub-lethal effects and pathogen population structure have been theoretically shown to have important consequences for virulence evolution. Thus, we additionally examine the relationship between anaemia and transmission in single and mixed clone infections. Results Whereas there was a trade-off between transmission success and virulence as defined by host mortality, contradictory clone-specific patterns occurred when defining virulence by anaemia. A negative relationship between anaemia and transmission success was found for one of the parasite clones, whereas there was no relationship for the other. Notably the two parasite clones also differed in a transmission phenotype (gametocyte sex ratio) that has previously been shown to respond adaptively to a changing blood environment. In addition, as predicted by evolutionary theory, mixed infections resulted in increased anaemia. The increased anaemia was, however, not correlated with any discernable parasite trait (e.g. parasite density) or with increased transmission. Conclusions We found some evidence supporting the hypothesis that there is an adaptive basis correlating virulence (as defined by host mortality) and transmission success in malaria parasites. This confirms the validity of applying evolutionary virulence theory to biomedical research and adds support to the prediction that partially effective vaccines may select for increasingly virulent malaria parasite strains. By contrast, there was no consistent correlation between transmission and sub-lethal anaemia, a more common outcome of malaria infection. However, overall, the data are not inconsistent with the recent proposal that sub-lethal effects may impose an upper limit on virulence. Moreover, clone specific differences in transmission phenotypes linked to anaemia do suggest that there is considerable adaptive potential relating anaemia and transmission that may lead to uncertain consequences following intervention strategies.
Collapse
Affiliation(s)
- REL Paul
- Unité de Biochimie et Biologie Moléculaire des Insectes, Institut Pasteur, 25 rue du Dr. Roux, 75724 Paris Cedex 15, France
- Laboratoire d'Entomologie Médicale, Institut Pasteur de Dakar, 36, Avenue Pasteur BP 220, Dakar, Sénégal
| | - T Lafond
- Laboratoire de Parasitologie Evolutive, CC237, CNRS UMR 7103, Université P. & M. Curie, 7 quai Saint Bernard, 75252 Paris Cedex 05, France
| | - CDM Müller-Graf
- Laboratoire de Parasitologie Evolutive, CC237, CNRS UMR 7103, Université P. & M. Curie, 7 quai Saint Bernard, 75252 Paris Cedex 05, France
| | - S Nithiuthai
- Veterinary Parasitology, Faculty of Veterinary Science, Chulalongkorn University, Bangkok 10330, Thailand
| | - PT Brey
- Unité de Biochimie et Biologie Moléculaire des Insectes, Institut Pasteur, 25 rue du Dr. Roux, 75724 Paris Cedex 15, France
| | - JC Koella
- Laboratoire de Parasitologie Evolutive, CC237, CNRS UMR 7103, Université P. & M. Curie, 7 quai Saint Bernard, 75252 Paris Cedex 05, France
| |
Collapse
|
65
|
Mackinnon MJ, Read AF. Immunity promotes virulence evolution in a malaria model. PLoS Biol 2004; 2:E230. [PMID: 15221031 PMCID: PMC434153 DOI: 10.1371/journal.pbio.0020230] [Citation(s) in RCA: 136] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2003] [Accepted: 05/25/2004] [Indexed: 11/19/2022] Open
Abstract
Evolutionary models predict that host immunity will shape the evolution of parasite virulence. While some assumptions of these models have been tested, the actual evolutionary outcome of immune selection on virulence has not. Using the mouse malaria model, Plasmodium chabaudi, we experimentally tested whether immune pressure promotes the evolution of more virulent pathogens by evolving parasite lines in immunized and nonimmunized ("naïve") mice using serial passage. We found that parasite lines evolved in immunized mice became more virulent to both naïve and immune mice than lines evolved in naïve mice. When these evolved lines were transmitted through mosquitoes, there was a general reduction in virulence across all lines. However, the immune-selected lines remained more virulent to naïve mice than the naïve-selected lines, though not to immunized mice. Thus, immune selection accelerated the rate of virulence evolution, rendering parasites more dangerous to naïve hosts. These results argue for further consideration of the evolutionary consequences for pathogen virulence of vaccination.
Collapse
Affiliation(s)
- Margaret J Mackinnon
- School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom.
| | | |
Collapse
|
66
|
Day T, Proulx SR. A general theory for the evolutionary dynamics of virulence. Am Nat 2004; 163:E40-63. [PMID: 15122509 DOI: 10.1086/382548] [Citation(s) in RCA: 173] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2003] [Accepted: 10/06/2003] [Indexed: 11/04/2022]
Abstract
Most theory on the evolution of virulence is based on a game-theoretic approach. One potential shortcoming of this approach is that it does not allow the prediction of the evolutionary dynamics of virulence. Such dynamics are of interest for several reasons: for experimental tests of theory, for the development of useful virulence management protocols, and for understanding virulence evolution in situations where the epidemiological dynamics never reach equilibrium and/or when evolutionary change occurs on a timescale comparable to that of the epidemiological dynamics. Here we present a general theory similar to that of quantitative genetics in evolutionary biology that allows for the easy construction of models that include both within-host mutation as well as superinfection and that is capable of predicting both the short- and long-term evolution of virulence. We illustrate the generality and intuitive appeal of the theory through a series of examples showing how it can lead to transparent interpretations of the selective forces governing virulence evolution. It also leads to novel predictions that are not possible using the game-theoretic approach. The general theory can be used to model the evolution of other pathogen traits as well.
Collapse
Affiliation(s)
- Troy Day
- Department of Mathematics, Queen's University, Kingston, Ontario, K7L 3N6, Canada.
| | | |
Collapse
|
67
|
Gouagna LC, Ferguson HM, Okech BA, Killeen GF, Kabiru EW, Beier JC, Githure JI, Yan G. Plasmodium falciparummalaria disease manifestations in humans and transmission toAnopheles gambiae: a field study in Western Kenya. Parasitology 2004; 128:235-43. [PMID: 15074873 DOI: 10.1017/s003118200300444x] [Citation(s) in RCA: 58] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Transmission of the malaria parasitePlasmodiumis influenced by many different host, vector and parasite factors. Here we conducted a field study at Mbita, an area of endemic malaria in Western Kenya, to test whether parasite transmission to mosquitoes is influenced by the severity of malaria infection in its human host at the time when gametocytes, the transmission forms, are present in the peripheral blood. We examined the infectivity of 81Plasmodium falciparumgametocyte carriers to mosquitoes. Of these, 21 were patients with fever and other malaria-related symptoms, and 60 were recruited among apparently healthy volunteers. Laboratory-rearedAnopheles gambiaes.s. (local strain) were experimentally infected with blood from these gametocyte carriers by membrane-feeding. The severity of the clinical symptoms was greater in febrile patients. These symptomatic patients had higher asexual parasitaemia and lower gametocyte densities (P=0·05) than healthy volunteers. Ookinete development occurred in only 6 out of the 21 symptomatic patients, of which only 33·3% successfully yielded oocysts. The oocyst prevalence was only 0·6% in the 546 mosquitoes that were fed on blood from this symptomatic group, with mean oocyst intensity of 0·2 (range 0–2) oocysts per mosquito. In contrast, a higher proportion (76·7%) of healthy gametocyte carriers yielded ookinetes, generating an oocyst rate of 12% in the 1332 mosquitoes that fed on them (mean intensity of 6·3, range: 1–105 oocysts per mosquito). Statistical analysis indicated that the increased infectivity of asymptomatic gametocyte carriers was not simply due to their greater gametocyte abundance, but also to the higher level of infectivity of their gametocytes, possibly due to lower parasite mortality within mosquitoes fed on blood from healthy hosts. These results suggest that blood factors and/or conditions correlated with illness reduceP. falciparumgametocyte infectivity.
Collapse
Affiliation(s)
- L C Gouagna
- Human Health Division, Mbita Point Research and Training Station, International Centre of Insect Physiology an2d Ecology, P.O. Box 30772, Nairobi, Kenya.
| | | | | | | | | | | | | | | |
Collapse
|
68
|
|
69
|
Ferguson HM, Mackinnon MJ, Chan BH, Read AF. MOSQUITO MORTALITY AND THE EVOLUTION OF MALARIA VIRULENCE. Evolution 2003. [DOI: 10.1554/03-211] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
|
70
|
Ebert D, Bull JJ. Challenging the trade-off model for the evolution of virulence: is virulence management feasible? Trends Microbiol 2003; 11:15-20. [PMID: 12526850 DOI: 10.1016/s0966-842x(02)00003-3] [Citation(s) in RCA: 211] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Progress in understanding the evolution of infectious diseases has inspired proposals to manage the evolution of pathogen (including parasite) virulence. A common view is that social interventions that lower pathogen transmission will indirectly select lower virulence because of a trade-off between transmission and virulence. Here, we argue that there is little theoretical justification and no empirical evidence for this plan. Although a trade-off model might apply to some pathogens, the mechanism appears too weak for rapid selection of substantial changes in virulence. Direct selection against virulence itself might be a more rewarding approach to managing the evolution of virulence.
Collapse
Affiliation(s)
- Dieter Ebert
- Ecologie et évolution, Département de biologie, Université de Fribourg, Chemin du Musée 10, Switzerland
| | | |
Collapse
|
71
|
Clay K, Schardl C. Evolutionary Origins and Ecological Consequences of Endophyte Symbiosis with Grasses. Am Nat 2002; 160 Suppl 4:S99-S127. [DOI: 10.1086/342161] [Citation(s) in RCA: 705] [Impact Index Per Article: 32.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
|
72
|
Gandon S, Agnew P, Michalakis Y. Coevolution between Parasite Virulence and Host Life‐History Traits. Am Nat 2002; 160:374-88. [DOI: 10.1086/341525] [Citation(s) in RCA: 91] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/04/2022]
|
73
|
Mackinnon MJ, Gaffney DJ, Read AF. Virulence in rodent malaria: host genotype by parasite genotype interactions. INFECTION, GENETICS AND EVOLUTION : JOURNAL OF MOLECULAR EPIDEMIOLOGY AND EVOLUTIONARY GENETICS IN INFECTIOUS DISEASES 2002; 1:287-96. [PMID: 12798007 DOI: 10.1016/s1567-1348(02)00039-4] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
In an effort to understand what limits the virulence of malaria parasites, we infected inbred mice of three genotypes (C57Bl/6J, CBA/Ca and DBA/2) with one of two parasite lines of the rodent malaria Plasmodium chabaudi. One of these parasite lines had been serially passaged through C57Bl/6J mice and had evolved higher asexual growth rate, virulence and transmission in the process. The other parasite line was the unadapted ancestral line which had low virulence. In all three host genotypes, the C57Bl/6J-adapted parasite line was more virulent than the ancestral line thus indicating that trade-offs in virulence between alternative host genotypes had not placed strong constraints on the evolution of high virulence in this system. By examining the infection dynamics for fitness-related components-asexual parasite population growth, transmission and virulence-we revealed alternative possible explanations for what sets the upper limit to virulence in nature. The total number of transmission forms (gametocytes) produced during the infection, a measure of parasite Darwinian fitness, was four-fold higher in mice that survived the infection than those which died. Among mice that survived, total gametocyte production was greatest in the host genotype that suffered intermediate levels of morbidity (anaemia and weight loss). Thus, there were transmission costs of high virulence that were partly due to host death (as most theoretical models of virulence evolution assume), but perhaps partly due to some factor related to high morbidity. Both mortality and morbidity-related factors might therefore influence the upper limit on virulence of malaria parasites.
Collapse
Affiliation(s)
- M J Mackinnon
- Institute of Cell, Animal and Population Biology, University of Edinburgh, West Mains Road, Edinburgh, Scotland EH9 3JT, UK.
| | | | | |
Collapse
|
74
|
Ferguson HM, Read AF. Genetic and environmental determinants of malaria parasite virulence in mosquitoes. Proc Biol Sci 2002; 269:1217-24. [PMID: 12065037 PMCID: PMC1691016 DOI: 10.1098/rspb.2002.2023] [Citation(s) in RCA: 104] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Models of malaria epidemiology and evolution are frequently based on the assumption that vector-parasitic associations are benign. Implicit in this assumption is the supposition that all Plasmodium parasites have an equal and neutral effect on vector survival, and thus that there is no parasite genetic variation for vector virulence. While some data support the assumption of avirulence, there has been no examination of the impact of parasite genetic diversity. We conducted a laboratory study with the rodent malaria parasite, Plasmodium chabaudi and the vector, Anopheles stephensi, to determine whether mosquito mortality varied with parasite genotype (CR and ER clones), infection diversity (single versus mixed genotype) and nutrient availability. Vector mortality varied significantly between parasite genotypes, but the rank order of virulence depended on environmental conditions. In standard conditions, mixed genotype infections were the most virulent but when glucose water was limited, mortality was highest in mosquitoes infected with CR. These genotype-by-environment interactions were repeatable across two experiments and could not be explained by variation in anaemia, gametocytaemia, blood meal size, mosquito body size, infection rate or oocyst burden. Variation in the genetic and environmental determinants of virulence may explain conflicting accounts of Plasmodium pathogenicity to mosquitoes in the malaria literature.
Collapse
Affiliation(s)
- H M Ferguson
- Institute of Cell, Animal and Population Biology, University of Edinburgh, Edinburgh EH9 3JT, UK.
| | | |
Collapse
|
75
|
Abstract
Rosetting is a property of many malaria parasite species that has been linked to virulence in the major species infecting humans, Plasmodium falciparum. Here, the basic properties of rosettes in the rodent malaria laboratory model, P. chabaudi, were studied with a view to future studies on the role of rosetting in malaria parasite virulence and transmission. Rosetting occurred in 14 out of the 15 P. chabaudi clones studied, varied consistently between clones, and ranged between 9 and 37% at full parasite maturity. Rosetting frequency markedly declined after the mouse reached peak parasitemia, possibly due to host immunity. Consistent with P. falciparum and P. vivax, rosettes in P. chabaudi were disrupted by treatment with trypsin and EDTA. However, P. chabaudi rosettes were insensitive to sulfated glycoconjugates (heparin, heparan sulfate and fucoidan). The molecular basis of rosetting in P. chabaudi is unknown at present, but the results suggest that the molecules involved may differ from those in human-infecting species.
Collapse
Affiliation(s)
- Margaret J Mackinnon
- Institute of Cell, Animal and Population Biology, University of Edinburgh, West Mains Rd., Edinburgh, East Lothian, UK.
| | | | | |
Collapse
|
76
|
Taylor LH, Welburn SC, Woolhouse MEJ. Theileria annulata: virulence and transmission from single and mixed clone infections in cattle. Exp Parasitol 2002; 100:186-95. [PMID: 12173404 DOI: 10.1016/s0014-4894(02)00017-6] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Theoretically, parasite virulence should be higher for faster growing parasites, and higher in mixed infections compared to single-clone infections. Virulence should also be positively correlated to transmission rates. Theileria annulata provides a good model system for studying such hypotheses, as parasite replication causes harm to the host, and there is evidence suggesting that the genetic complexity of an infection might affect its virulence. Two clones of T. annulata were chosen, one fast growing and one slow growing in vitro and these were used to establish cattle infections, either alone, or in a mixed infection. Virulence was measured using lymph node expansion, temperature, and blood parameters as correlates. As predicted, the faster growing clone was found to produce higher virulence. Mixed infections did not show higher virulence than single-clone infections, but interactions within mixed infections resulted in more transmission stage production than seen in either of the single-clone infections. Index Descriptors and Abbreviations. Theileria annulata, Apicomplexa, mixed infections, virulence, growth rates, red blood cell, RBC; packed cell volume, PCV.
Collapse
Affiliation(s)
- Louise H Taylor
- Centre for Tropical Veterinary Medicine, University of Edinburgh, Roslin, Midlothian, EH25 9RG Scotland, UK.
| | | | | |
Collapse
|
77
|
Eisen RJ, DeNardo DF. Life history of a malaria parasite (Plasmodium mexicanum) in its host, the western fence lizard (Sceloporus occidentalis): host testosterone as a source of seasonal and among-host variation? J Parasitol 2000; 86:1041-5. [PMID: 11128477 DOI: 10.1645/0022-3395(2000)086[1041:lhoamp]2.0.co;2] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022] Open
Abstract
The course of infection of a malaria parasite (Plasmodium mexicanum) is highly variable in its host, the fence lizard (Sceloporus occidentalis). However, a seasonal trend is superimposed on this variation such that gametocyte production is intensified during mid- to late summer. Host testosterone levels follow a similar seasonal fluctuation and are variable among individual lizards. We sought to determine if testosterone levels affect seasonal and among-host variation in 11 P. mexicanum life history traits: rate of increase in level of infection (3 measures), peak parasitemia (3 measures), duration of increase (3 measures), time to detectable infection, and timing of production of gametocytes. We followed the course of infection in 125 male S. occidentalis, each randomly assigned to 1 of 4 treatment groups: castrated, castrated and implanted with exogenous testosterone, sham implanted, and unmanipulated controls. Median values for the 11 life history traits did not differ among treatment groups, and variances were homogeneous among the treatment groups for 10/11 traits. However, elevated testosterone significantly reduced the variation in timing of the onset of gametocyte production. Therefore, testosterone does not appear to be a primary regulator of P. mexicanum life history, yet testosterone may have some effect on when gametocytes first become detectable.
Collapse
Affiliation(s)
- R J Eisen
- Department of Biology, University of Vermont, Burlington 05405, USA
| | | |
Collapse
|
78
|
Eisen RJ. Variation in life-history traits of Plasmodium mexicanum, a malaria parasite infecting western fence lizards: a longitudinal study. CAN J ZOOL 2000. [DOI: 10.1139/z00-057] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The life history of malaria parasites (Plasmodium spp.) is directly related to their transmission, virulence, and population dynamics. I followed the life history of Plasmodium mexicanum in naturally infected western fence lizards (Sceloporus occidentalis) over a 4-year period, using a mark-recapture technique. The life-history traits measured included peak parasitemias and population growth rates of asexual forms, gametocytes, and total parasites. Among malaria infections, variation was high for all measured traits. Growth rates varied up to 11-fold, and among stable infections, average asexual parasitemias ranged from 0.2 to 13.2 and gametocytemias from 0.5 to 66.2 parasites per 1000 erythrocytes. This variation was not related to infection prevalences, which were similar among years and between male and female hosts. Host age and gender were not related to peak parasitemia or average growth rate of asexual forms. However, the growth rate of gametoctyes was higher in older lizards. Gametocytemia and parasitemia were significantly higher late in the warm season, when sand-fly vectors are active. These data reveal that life-history traits of P. mexicanum are highly variable within an infected host population, and that the variation is partially related to the age of the infected host or the time of year the host was examined.
Collapse
|
79
|
Gandon S, Michalakis Y. Evolution of parasite virulence against qualitative or quantitative host resistance. Proc Biol Sci 2000; 267:985-90. [PMID: 10874747 PMCID: PMC1690632 DOI: 10.1098/rspb.2000.1100] [Citation(s) in RCA: 125] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
We analysed the effects of two different modes of host resistance on the evolution of parasite virulence. Hosts can either adopt an all-or-nothing qualitative response (i.e. resistant hosts cannot be infected) or a quantitative form of resistance (i.e. which reduces the within-host growth rate of the parasite). We show that the mode of host resistance greatly affects the evolutionary outcome. Specifically, a qualitative form of resistance reduces parasite virulence, while a quantitative form of resistance generally selects for higher virulence.
Collapse
Affiliation(s)
- S Gandon
- Laboratoire d'Ecologie, CNRS-UMR 7625 Case 237, Université Pierre et Marie Curie, Paris, France.
| | | |
Collapse
|
80
|
Eisen RJ, Schall JJ. Life history of a malaria parasite (Plasmodium mexicanum): independent traits and basis for variation. Proc Biol Sci 2000; 267:793-9. [PMID: 10819149 PMCID: PMC1690602 DOI: 10.1098/rspb.2000.1073] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Plasmodium mexicanum, a malaria parasite of lizards, exhibits substantial variation among infections in the life-history traits which define its blood-dwelling stages. Such variation in life histories among infections is common in Plasmodium and may influence the ecology and evolution of the parasite's transmission success and virulence. Insight into these issues requires identification of independent traits (some traits may be bound by developmental trade-offs) and the importance of genetic versus host effects producing the variation. We studied 11 life-history traits in 120 induced infections of P. mexicanum in its natural lizard host (20 each from six donor infections). The traits varied among infections and fell into three clusters: rate/peak (rate of increase and peak parasitaemia of asexuals and gametocytes), time (duration of pre-patent period and the infection's growth) and maturity (timing of first gametocytes). Thus, few life-history traits define an infection in the lizard's blood. Donor effects were significant for ten traits and two trait clusters (maturity was the exception) suggesting genetic differences among infections may influence the rate of increase and peak parasitaemia, but not the timing of the first production of gametocytes.
Collapse
Affiliation(s)
- R J Eisen
- Department of Biology, University of Vermont, Burlington 05405, USA
| | | |
Collapse
|