In Vivo Fitness Associated with High Virulence in a Vertebrate Virus Is a Complex Trait Regulated by Host Entry, Replication, and Shedding

Abstract

The relationship between pathogen fitness and virulence is typically examined by quantifying only one or two pathogen fitness traits. More specifically, it is regularly assumed that within-host replication, as a precursor to transmission, is the driving force behind virulence. In reality, many traits contribute to pathogen fitness, and each trait could drive the evolution of virulence in different ways. Here, we independently quantified four viral infection cycle traits, namely, host entry, within-host replication, within-host coinfection fitness, and shedding, in vivo, in the vertebrate virus Infectious hematopoietic necrosis virus (IHNV). We examined how each of these stages of the viral infection cycle contributes to the fitness of IHNV genotypes that differ in virulence in rainbow trout. This enabled us to determine how infection cycle fitness traits are independently associated with virulence. We found that viral fitness was independently regulated by each of the traits examined, with the largest impact on fitness being provided by within-host replication. Furthermore, the more virulent of the two genotypes of IHNV we used had advantages in all of the traits quantified. Our results are thus congruent with the assumption that virulence and within-host replication are correlated but suggest that infection cycle fitness is complex and that replication is not the only trait associated with virulence.Despite a recent surge in research, the evolution of viral virulence remains a controversial issue (1,7,10,18,22,37,50,60). Some studies have suggested a positive link between viral fitness and virulence (6, 13, 17, 39, 42, 49, 52-54, 58, 59), whereas others found no evidence of such an association (20,24,27) or even indicated that the converse, that virulence is negatively associated with viral fitness, is true (36, 43). Despite the fact that these studies span a wide range of virus and host taxa, a limitation of many of these studies is that viral fitness was typically estimated from only one or two virus traits, at one stage of the viral infection cycle. In reality, viral fitness is likely shaped by multiple traits at each of the viral infection stages, i.e., entry into the host, replication in the host, and shedding from the host, all of which could differentially impact selection for virulence (5).Most estimates of viral fitness come from examinations of the within-host replication stage of the viral infection cycle. For a few systems, researchers have made an effort to provide a detailed assessment of the importance of within-host dynamics on fitness, by separately quantifying replication in single-genotype infections and the relative abilities of virus genotypes to produce infectious progeny in a coinfection environment, herein referred to as coinfection fitness (15,39,59). However, the connection between within-host fitness and transmission remains elusive (20, 24, 42, 52), partly due to an incomplete understanding of virus investment into shedding and entry (8,45). These limitations are compounded in verteb...

First field evidence of the evolution from a non-virulent HPR0 to a virulent HPR-deleted infectious salmon anaemia virus

“…Sequencing and functional characterization further suggest both HPR-deletion and a Q 266 L substitution, or insertion adjacent to the cleavage site in the F protein influences ISAV virulence [22], by promoting viral fusion and the activation of proteolytic cleavage [23,24]. However, other viral functions, for example virus receptor binding [25], virus uptake, replication rate, shedding of new virions [26,27], modulation of the host immune response [28,29] and the ability to spread to new hosts [30], can also influence virulence.…”

The Role of Virulence in In Vivo Superinfection Fitness of the Vertebrate RNA Virus Infectious Hematopoietic Necrosis Virus

“…For example, competition experiments have been performed using many species of arbovirus in their mosquito vectors and have determined correlations between viral fitness and epidemiologic observations or recorded disease severity in humans (11,14). Through such studies, a variety of viral traits have been associated with viral fitness, including replication rate, fidelity of genome replication, rate of viral assembly, entry, shedding, and virulence (7,20,(22)(23)(24)(25). However, such studies exploring fitness of vertebrate viruses in vivo are limited in number.…”

“…These experiments are useful to directly compare two variants and can demonstrate subtle fitness differences between two variants that may not have been evident by comparing single-infection kinetics. Recently, there have been a number of coinfection fitness studies using in vivo models for vertebrate viruses (2,(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21). For example, competition experiments have been performed using many species of arbovirus in their mosquito vectors and have determined correlations between viral fitness and epidemiologic observations or recorded disease severity in humans (11,14).…”

“…Infection with infectious hematopoietic necrosis virus (IHNV) of the natural hosts Oncorhynchus mykiss (rainbow trout) and Oncorhynchus nerka (sockeye salmon) has been used as a tractable model system for studying viral fitness in coinfection in vivo (16,17,19,20,36). IHNV is a significant pathogen of salmonid species which originated in the North American Pacific Northwest and has since spread to Europe and Asia (37).…”

A Single Mutation in the Glycophorin A Binding Site of Hepatitis A Virus Enhances Virus Clearance from the Blood and Results in a Lower Fitness Variant

“…ness compared to the parental virus have been isolated naturally from patients, while natural mutants with variations in the glycophorin A binding site and similar in vitro fitness to that of the parental strain have never been described. This is probably related to the incomplete picture offered by in vitro assays to evaluate the actual in vivo fitness of a virus in its host, as reported very recently (39). Additionally, it has been documented that viral pathogenesis is determined in part by the spread of the virus in the organism to reach the target tissues (28,38).…”