The evolution of covert, silent infection as a parasite strategy

The evolution of covert, silent infection as a parasite strategy

The vector potential of the rabbit flea (Cediopsylla simplex) and a mosquito (Aedes triseriatus) was investigated for Herpesvirus sylvilagus transmission among cottontail rabbits (Sylvilagus floridanus). The flu is more than just a bad cold and can occasionally lead to serious complications, including death. Examples include adenovirus, coxsackievirus, Epstein-Barr virus, HIV, varicella (chickenpox) and human herpes virus 6. 1). Virus acquisition and persistence in the insects was evaluated by attempting to recover the virus from 19 pools of mosquitoes engorged on viremic blood and 36 pools of engorged fleas or those living on viremic hosts for 1-21 days. Descriptive statistics and logistic regression were used in the analysis of the answers to evaluate the relationships between the use of traditional methods and other risk factors. We tested the correlation between the superparasitism phenotype and PCR amplification of the putative viral marker using several experimental conditions (including horizontal transfers) and several parasitoid genotypes.

frugiperda larvae confirmed the expression of the SfMNPV ie-0 and Sf29 genes, indicating that the insect colony harbors a covert SfMNPV infection. It has been associated with all four clinical presentations of KS (the classic, endemic, AIDS-related, and iatrogenic forms) (6, 15). Pri-miRNA stem–loops, which consist of an ∼32-base-pair (bp) imperfect stem and a ≥10-nt terminal loop, are cleaved by the RNase III enzyme Drosha, acting together with its cofactor DGCR8, ∼22 bp from the stem/loop junction (Han et al. melanogaster. In the realm of vaccine development, insertion of subunits of the targeted disease (e.g., arboviruses like dengue fever) into a baculovirus genome allows for rapid production in insect cell lines. 1998; Tompkins & Begon 1999; Tompkins et al. As of January 2005, the CDC ArboNET has recorded avian or animal WNV infections in every state except Alaska, Hawaii, and Washington.

The long-term association between Hz-1 virus and host cells is generally referred to as persistent viral infection. Latent viral infections are difficult to observe and study; consequently, these phenomena are usually neglected and rarely reported (3, 18, 30). The lab’s ability to sequence and identify never-before-seen viruses has recently shed new light in a series of recently published papers on insects – including whiteflies and dragonflies – carrying a diverse range of viruses. Many types of fungi exist and cause problems in humans, animals and plants. When a mosquito feeds on the infected bird, the virus is passed to the insect along with the blood meal. Discuss airborne transmission of disease, distinguishing between droplet spread versus droplet nuclei; give examples of each of these two types of airborne spread. Cell lines derived from patients with PELs and individuals chronically infected with HHV-8, which mainly express latent and low levels of lytic antigens, have been used for latent or lytic antgen IFAs.

Cooper, D., J. In the nineteenth century, the definitive cause of baculovirus infections was identified when occlusion bodies were first seen under a light microscope [13]. In particular, there is an increasing awareness that parasites may produce silent or ‘covert’ infections in addition to the more obvious pathological ‘overt’ infection. Here, when an individual becomes infected, the infection can either become overt causing obvious disease symptoms, and leading to horizontal transmission events, or it may become silent/covert. In the medical community, these infections tend to be referred to as ‘silent’ or ‘dormant’, while in the environmental/virology literature they are referred to as covert. This is an apparently asymptomatic infection that is not horizontally transmitted, where the parasites may, for example, remain within a cell or integrate within the host’s chromosomes (Cheung 1991; Delecluse et al. 1993; Ahmed et al.

Sometimes, the damage to the heart muscle is permanent and heart failure persists after the inflammation has resolved. The sexual transmission rate of symptomatically infected people (transmission probability × contact rate), β, is assumed to range from 0.001 to 0.10, which means the transmission probability per sex act is between 0.007 (mild infectivity) and 0.70 (severe infectivity). All of these different types of silent/covert infections may or may not have a fitness cost to the host, and in some cases there may be vertical transmission from the infected individual to their offspring. A key characteristic of this strategy is that at some point in the future, these covert infections can become overt, leading to the potential for disease and horizontal transmission. To date, LbFV has not been molecularly characterized because it is difficult to purify viral particles, which probably are present in insects at a low density. Successive passages in vivo.OBs obtained from virus DNA-inoculated larvae were designated passage zero (P0) OBs. The ethics committee of the Institutional Review Board at the University of Nebraska approved the study.

2000). In all these cases, the endosymbionts are obligatory mutualists—they are essential for the survival of their host species. A long-term dynamic model for the gypsy moth derived from the equations shown [7] does a reasonably good job of replicating the pattern of forest acres defoliated in Maine from 1940 to 1996. Within human parasites, for example, there is evidence of covert infection in human herpes viruses (Kondo et al. Health providers must use thorough laboratory testing to differentiate WNV antibodies from those of other arboviruses. Our results clearly show that some of the viral genomes do insert into the host chromosome, and this suggests that persistent Hz-1 viral infection should be more accurately referred to as latent viral infection. Cells and virus.Spodopter frugiperda IPLB-Sf-21 was incubated in TC-100 insect cell culture medium, which contained 10% fetal bovine serum (FBS) at 26°C (Gibco BRL, Gaithersburg, MD) (23, 31).

Most notably, it is estimated that one-third of the human population is infected with Mycobacterium tuberculosis (Dye et al. This means that antibiotics are not needed for minor infections (for example, an ear or throat infection in an otherwise fit person). The majority of WNV infections will manifest asymptomatically. Temperate phages that commonly undergo the lytic cycle (infecting their bacterial hosts horizontally causing host lysis) also rarely lysogenize the host, persisting in a covert state that transmits vertically upon bacterial division (Brown et al. Baculovirus-infected Sf9 cells expressing GST alone were used as a negative control to detect background and nonspecific fluorescence. Possee, P. In turn, these experiments can be designed to address specific hypotheses about transmission dynamics or to develop mathematical models of disease transmission [5], [20], [21].

1999; Burden et al. 2002, 2003, 2006). The development and application of novel detection techniques are likely to find even more examples of these covert infections in wildlife and human pathogens. Although the implications of covert infection to the population dynamics of their hosts have been examined theoretically (Boots et al. 2003; Bonsall et al. 2005), the question remains: under what circumstances is covert infection favoured as a parasite strategy? The horizontal dashed line is .

1996; Geritz et al. 1998) that determines individual parasite strain invasion fitness within explicit ecological dynamics. In order to reduce the number of false-positive clones in the SSH-generated library, the SSH technology was combined with a mirror orientation selection procedure (24). Reactions were stopped in the linear phase of amplification (25 cycles) determined in preliminary assays. All slides were warmed to room temperature, individual serum samples were applied to each well, and the slides were incubated at 37°C for 30 min in a humidified chamber. 2008), which tend to be expressed in a developmental or tissue-specific manner (Landgraf et al. Toll was first discovered to be important in the resistance of Drosophila to fungi [33], later Toll-like receptors were shown to have fundamental functions in mammalian innate immunity [34].

This means that multiple replicates of a single experiment can be carried out in either the laboratory or, maybe more importantly, in nature. Persistence may clearly be favoured by vertical transmission, free-living stages that persist in the environment or via a reservoir host, but covert infection has also been hypothesized as a mechanism for persistence in a number of parasites (Burden et al. Montgomery, A. For these experiments all of the persistently infected cells were assayed at passages 200 to 230, except SFP4 cells were used at passage 30. The results were plotted as average luciferase activity versus time of infection from triplicate assays of three independent experiments. The host’s natural birth and death rates in the absence of disease are denoted a and b, respectively. The transmission rate of strain i is denoted βi.
The evolution of covert, silent infection as a parasite strategy

Huhn G’, J. The remaining fraction pi of exposed individuals are infected covertly. . Hughes, P. This comes with an important caveat: to date, little attention has been paid to evolution of host resistance or virus virulence when developing baculoviruses for biocontrol. The remaining (1−vi) are born to the susceptible class. Covert infections can become overt at rate ci.

Note there is no sterilization when fi=0, no vertical transmission when vi=0 and that when pi=1 the model behaves as a classic SEI system. The terms RYW−RZM in equations (2.2a)–(2.2d) represent the manner in which superinfection (subsequent infection of an already infected individual) is represented and is described in more detail below. For a wide range of parameters, the population densities of the single-strain model tend to a stable equilibrium with coexistence of the susceptible, overt and covert classes (Boots et al. 2003). Of course, the estimate of was obtained by assuming random mixing and a constant vector population over the course of the epidemic. In §2c,d, we add seasonality to the model and look at populations that can exhibit cyclic or chaotic dynamics. are the resident equilibrium solutions.

on 10 first-instar D. Larvae were reared at 25°C, and virus mortality was recorded every 12 h until the insects had either died or pupated. By using the strategy described in Fig. 2007). Preliminary analysis showed that the P-element was not responsible for virus resistance (unpublished data). However, broad range chemical or biological insecticides may be undesirable because beneficial insects may also be affected. In insect baculovirus systems, it has been shown that asymptomatic, avirulent (covert) infection can be induced into becoming symptomatic and virulent (overt) when the host contracts a rival infection (Burden et al.

For certain viruses the RNA is replicated by a viral enzyme (transcriptase) contained in the virion, or produced by the host cell using the viral RNA as a messenger. After two washings in TNM-FH medium, Colcemid (2.5 μg/ml; Sigma) was added 1 h before harvesting. Mapping of the hhi1 transcript of HzNV-1 virus. A simple way to model this is to have the proportion of infections of the incoming strain becoming covert, determining the winner of the competition. Suitable superinfection terms can be defined for all these cases. Some viruses have only a few genes coding for capsid proteins. Therefore, under a wide range of plausible assumptions about how superinfection occurs, covert infection will never evolve as a parasite strategy.

When the mIFAs were performed with the ORF65, K8.1, and ORF73 antigens separately, 74% (23/31) of the Sf9 three-antigen mIFA-positive and BC3 mIFA-positive samples in group A reacted to both the latent (ORF73) and the lytic (ORF65 or K8.1) antigens. 2004. The ESS is that all infections are covert (figure 1c,d) and therefore, all overt infections come from conversions of covert infections. As such, in all infections, there is a period of protective covert infection followed by overt infection. This process is equivalent to the SEI epidemiology seen in many disease interactions. When we examine the evolution of the rate at which infections convert from covert to overt (denoted by c), we find that the fastest conversion rate will always outcompete the others unless infection is protective and a(1−f)v>b (see equation (A 11)). Therefore, SEI-type interactions are never favoured unless the covert infection is protective during the exposed (E) period.

The analysis so far has considered the evolution of covert infection under equilibrium population dynamics. One possibility is that covert infection is a parasite response to variation in host population density. Many organisms show distinct seasonality in their reproduction, leading to population variation. We therefore examine the evolutionary outcome for non-equilibrium population dynamics and include population variability through forcing the birth rate, by substituting a with an oscillating term a(1+δ sin(2πt(ω)) in equations (2.1a)–(2.1e). We use equations (2.2a)–(2.2d) for the superinfection terms in this and the next sections; these terms describe an infection that does not provide protection. Emerging F1 females were fertilized by NSref males, transferred to unparasitized Drosophila hosts for 24 h, and screened for putative viral infection by PCR. Mean concentrations of DNA (ng/ml) extracted from samples of 5 × 108 OBs of SfWT, Sfbac, and Sf29null bacmids at each passage.

The sensitivity of the screening strategy was then evaluated by using group A (positive group) samples and was calculated as described in the Materials and Methods section. Similar results were also obtained in parallel experiments using O’nyong-nyong virus, a mosquito-borne arbovirus that uses primates as a reservoir (Myles et al. We extended this analysis by quantifying, in cell culture, the viral titer in these flies after DCV infection (Figure 1F). The results indicate that the system with population variability has one ESS that is an evolutionary attractor, p* (and qualitatively similar PIPs occur for all parameter values chosen in this study). We confirm the results by conducting simulations of the evolutionary process (see the electronic supplementary material for details). Other viral proteins, such as some of the human immunodeficiency virus, appear to be actively toxic, but those are the exception, not the rule. With use of the membrane-binding fluorescent dye CellTracker CM-DiI system to label and observe the persistently infected cells, the centers of all plaques contained a fluorescence-labeled cell that originated from the input of the persistently infected cell (Fig.

Northern blot analysis showed that the level of hhi1 transcripts had decreased by up to 95% by 12 hpi (Fig. These occur between the parameter ranges δ=0.4 and 0.55 (figure 3b), where there are two stable, non-equilibrium, population dynamic attractors. The outcome depends upon initial conditions with either a positive level of covert infection as shown, or zero covert infection (details of how the PIPs are produced and the position of the ESSs is determined are contained in the electronic supplementary material). acyclovir is an antiviral that requires viral proteins to become active. The extreme oscillations in the overtly infected population are always present when a non-zero covert infection strategy is favoured. ORF73 is the major latent protein and encodes the latency-associated nuclear antigen. The position of the ESS is dependent on the characteristics of both the host and the parasite.

Higher proportions of vertical transmission v in births from covertly infected hosts lead to higher values of covert infection (figure 4a). It is important to note, however; that even with no vertical transmission covert infection can evolve. Vertical infection is beneficial to covert infection as the population declines, but even without vertical infection, covert individuals have lower death rates so still benefit during declines. The presence of vertical infection will make the evolution of covert infection more likely, but is not necessary. The dependency on the level of sterilization acts in the same way as vertical transmission. Covert infection evolves with or without any effects on fecundity, but is more likely when effects are smaller. In figure 4, as in figure 3b, there are parameter ranges with multiple, stable, population dynamic attractors and the ESS value depends on which attractor the solutions converge.

Plots of the ESS of proportion of covert infection against (a) proportion of vertical infection, (b) disease-induced mortality due to overt infection, (c) mean host birth rate and (d) strength of transmission variation, where (a–c) are for variable birth rate and (d) is for variable transmission rate. The squares and circles show the result of evolutionary simulations starting from a low and high proportion of covert infections, respectively. The sudden ‘jumps’ in the value of the ESS are caused by transformations in the underlying population dynamics (such as a period doubling bifurcation). heterotoma and D. In addition, the number of colonies produced by Sf29null DNA samples on chloramphenicol-kanamycin plates and the relative proportions of colonies grown in chloramphenicol relative to colonies grown in chloramphenicol-kanamycin also decreased during serial passage, from a ratio of 1:1 at P0 to ratios of 1:0.71, 1:0.31, 1:0.17, 1:0.09, and 1:0.05 at P1, P2, P3, P4, and P5, respectively. While this strategy increases the specificity of detection of HHV-8-specific antibodies, it may decrease the sensitivity of detection. Whether or not viral infection of mammalian cells ever induces an effective antiviral RNAi response remains to be fully determined.

PCR amplification with mt 12S rRNA primers was done as a DNA extraction control. The sudden ‘jumps’ in value of the ESS occur due to changes in the underlying oscillatory population dynamics, which are influenced by the interplay between the mortality parameter, α, and the forcing term. The general trend is that increasing disease-induced mortality results in a slight decrease in the evolved value of the proportion of covert infection.

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