Virulence and local adaptation of a horizontally transmitted parasite

Emerging and reemerging infectious diseases: a multidisciplinary perspective

Predictions that infectious diseases would be eliminated as a major threat to human health have been shattered by emerging and reemerging infections, among them acquired immunodeficiency syndrome (AIDS), hemorrhagic fevers, marked increases in infections caused by antimicrobial-resistant bacteria, and the resurgence of tuberculosis and malaria. Understanding the dynamics of emerging and reemerging infections is critical to efforts to reduce the morbidity and mortality of such infections, to establish policy related to preparedness for infectious threats, and for decisions on where to use limited resources in the fight against infections. In order to offer a multidisciplinary perspective, 23 infectious disease specialists, epidemiologists, geneticists, microbiologists, and population biologists participated in an open forum at Emory University on emerging and reemerging infectious diseases. As summarized below, the group addressed questions about the definition, the identification, the factors responsible for, and multidisciplinary approaches to emerging and reemerging infections.

Development, life cycle, ultrastructure and phylogenetic position of Pasteuria ramosa Metchnikoff 1888: rediscovery of an obligate endoparasite of Daphnia magna Straus

The development, life cycle, ultrastructure and phylogenetic position of an obligate, spore-forming endoparasite of Daphnia magna Straus is described. The microparasite was found in the body cavity of three Daphnia species (D. magna , D. pulex and D. longispa )collected in England and Russia during 1992-1994 and maintained in artificial culture by co-cultivation with D. magna . Transmission of the endoparasite occurred horizontally through waterborne spores released from the remains of dead infected hosts. Progeny of infected hosts were never infected, indicating that vertical transmission does not occur. Egg production by infected mothers ceased soon after infection and death ensued after 46 days ( ± 7 standard error) at 20 °C. Phase contrast light microscopy and transmission electron microscopy of the infection process showed the endoparasite to have a polymorphic life cycle beginning with the appearance of branched ‘cauliflower-like’ rosettes and ended with the development of single, oval endospores, nippled at one end and with complex internal structure. Endospore formation resembled that found in endosporeforming bacteria. Morphologically the parasite has strong resemblance to the Pasteuria ramosa that Metchnikoff isolated from D. magna and D. pulex in Ukraine and described in 1888. Identification of this parasite has been an enduring puzzle since Metchnikoff. The previously confused phylogenetic position of P. ramosa (it has been classified as bacterium, yeast and protozoa) was resolved by sequencing the 16SrDNA molecule. Fluorescent in situ hybridizations confirmed that the 16S rDNA sequence obtained from the spores within the D. magna body cavity originated from the endoparasite. Maximum likelihood and maximum parsimony analysis showed that P. ramosa belongs to the low G + C Gram positive branch of the eubacteria and resides within a clade containing Bacillus tusciae , Alicyclobacillus cycloheptanicus and A. acidocaldarius as its nearest neighbours. These results confirm suggestions that this parasite is a bacterium and refute its previous tentative placement based on its morphological complexity among the Actinomycetales.

The curse of the pharaoh: the evolution of virulence in pathogens with long living propagules

We used a mathematical model to evaluate the hypothesis that parasites and pathogens with long living propagules should evolve high levels of virulence, i.e. high rates of pathogen-induced host mortality. Our model shows that the optimal level of virulence is independent of the longevity of the propagules either: (i) if the density or the prevalence of infected hosts is in (or fluctuates around) equilibrium; or (ii) if the death rate of the infected host population is high compared with that of the propagules. The level of virulence that maximizes the parasite's fitness (Multhusian parameter) increases with increasing longevity of its propagules only if the host-parasite system has not reached equilibrium and the death rate of the propagules is high relative to that of the infected hosts. Therefore, for parasites that have recently invaded a susceptible host population, greater propagule longevity may initially favour higher virulence; but once the equilibrium is reached the optimal virulence is independent of propagule longevity.

Parasite-host coevolution and geographic patterns of parasite infectivity and host susceptibility

Ebert (1994) has proposed the rule that parasites are, with few exceptions, more infective to sympatric hosts than to allopatric hosts. We test this rule using field data for schistosome infections of planorbid snails and find that, although sympatric parasite-host combinations do tend to be more compatible, there are exceptions where particular allopatric parasite-host populations are significantly more compatible. We develop a mathematical model of the dynamics of the parasite-host interaction where parasite infectivity and host susceptibility are defined by the matching of genotypes in a diploid system, The model predicts dynamic polymorphisms where parasite allele frequencies track host allele frequencies but with a lag. Because of this lag, it is possible for allopatric combinations to be more compatible than sympatric combinations. Any 'rule' that precludes this possibility is unlikely to prove robust.

The evolution and maintenance of virulence in microparasites

In recent years, population and evolutionary biologists have questioned the traditional view that parasite-mediated morbidity and mortality¿virulence¿is a primitive character and an artifact of recent associations between parasites and their hosts. A number of hypotheses have been proposed that favor virulence and suggest that it will be maintained by natural selection. According to some of these hypotheses, the pathogenicity of HIV, Vibrio cholerae, Mycobacterium tuberculosis,theShigella,as well as Plasmodium falciparum,and many other microparasites, are not only maintained by natural selection, but their virulence increases or decreases as an evolutionary response to changes in environmental conditions or the density and/or behavior of the human population. Other hypotheses propose that the virulence of microparasites is not directly favored by natural selection; rather, microparasite-mediated morbidity and mortality are either coincidental to parasite-expressed characters (virulence determinants that evolved for other functions) or the product of short-sighted evolution in infected hosts. These hypotheses for the evolution and maintenance of microparasite virulence are critically reviewed, and suggestions are made for testing them experimentally.

[A modern view of the evolution of virulence]

According to the prevailing, traditional view parasites should develop reduced virulence towards their hosts, because more virulent pathogens are more likely to drive the hosts, and thus themselves to extinction. Virulence is considered to be a primitive stage of a parasitive-host association. However the usefulness and validity of this view have been questioned. Recent studies suggest that parasites need not necessarily evolve towards reduced virulence. The points of view of Darwinian medicine in the direction of the evolution of virulence there may be many possible coevolutionary trajectories, depending on the details of the parasite's life-history, the host's behavior and the transmissibility of the parasite. Theoretical and epidemiological evidences indicate that pathogens transmitted by arthropod vectors are significantly more lethal to humans than those transmitted by personal contact. Water borne enteric pathogens are less virulent after purification of water supplies. Recent experiments also support the emerging theory that parasitism can evolve to be either more or less virulent in a long-term host, depending on the way the parasite is transmitted to the host and on the environment in which they live.

Coinfection and the evolution of parasite virulence

Analyses of the selection pressures acting on parasite virulence are made more complicated when individual hosts can simultaneously harbour many different strains or genotypes of a parasite. Here we explore the evolutionary dynamics of host-parasite associations in which individual hosts can be coinfected with many different parasite strains. (We take coinfection to mean that each strain transmits at a rate unaffected by the presence of others in the same host.) This study thus represents the opposite extreme to our earlier work on superinfection in which there is a dominance hierarchy such that only the most virulent strain present in a host is transmitted. For highly diverse populations of parasite strains, we find that such coinfection leads to selection for strains whose virulence-levels lie in a relatively narrow band close to the maximum consistent with the parasite's basic preproductive ratio, R0, exceeding unity.

Evolutionary pressures in the spread and persistence of infectious agents in vertebrate populations

Infectious agents have considerable potential to regulate or constrain the population growth of vertebrate hosts in natural habitats. A broad theoretical framework provides many insights into how the biology of the parasite and the demography of the host interact to determine this impact. It may manifest itself as a steady influence over time via stable endemic infection or in a recurrent epidemic fashion, sometimes with unpredictable intervals between epidemics depending on the generation time of the pathogen (time from infection to recovery or host death), its ability to induce lasting immunity and the population growth rate of the host species. Building on these notions, the paper focuses on recent work on the population dynamics of genetically variable pathogen populations and examines the factors that determine the evolution of virulence and the maintenance of genetic diversity in both host and pathogen. Recent research extends conventional theoretical templates to include population genetic elements and the within-host dynamics of the parasite and its interaction with the vertebrate immune system.