RNA genome stability of Toscana virus during serial transovarial transmission in the sandfly Phlebotomus perniciosus

Transmission of Heartland Virus (Bunyaviridae: Phlebovirus) by Experimentally Infected Amblyomma americanum (Acari: Ixodidae)

Heartland virus (HRTV; Bunyaviridae: Phlebovirus) is a recently described cause of human illness in the United States. After field studies conducted in 2012 implicated Amblyomma americanum (L.) as a vector of HRTV, we initiated experiments to assess the vector competence of A. americanum. Larval and nymphal ticks were immersed in high-titered suspensions of HRTV, and then tested for virus at various intervals postimmersion. In a later trial larval ticks were immersed in HRTV, followed by engorgement on a rabbit. A subset of postmolt nymphs was tested for HRTV to document transstadial transmission. Putatively infected nymphs were cofed with uninfected colony larvae to assess nonviremic transmission. In another trial, nymphs were fed on a rabbit and allowed to molt to the adult stage. Male and female ticks fed and mated upon a rabbit, and females were allowed to oviposit. Male and spent female ticks were tested for HRTV, and offspring of infected females were tested to assess vertical transmission. Infection rates of ≤50% were observed in immersed larvae and nymphs tested at intervals following immersion. Transstadial transmission from larvae to nymphs and then to adults was documented. HRTV was detected in a pool of nymphs molted from uninfected larvae cofed with infected nymphs. Vertical transmission of HRTV was observed in progeny of infected females. Infected females took longer to oviposit and produced fewer offspring. Serologic conversions (without viremia) in rabbits fed upon by immersed larvae or transstadially infected ticks indicate horizontal transmission of HRTV.

Selectively maintained paleoviruses in Holarctic water fleas reveal an ancient origin for phleboviruses

The ecological model, Daphnia pulex (Cladocera: Daphniidae), is broadly distributed in Holarctic freshwater habitats and has been the subject of multidisciplinary study for over half a century, but never has a natural RNA virus infection been reported in daphnids. Here we report on a group of paleoviruses related to RNA dependent RNA polymerase in the genome of D. pulex. Phylogenetic analysis suggests that these paleoviruses are derived from a viral lineage within the genus Phlebovirus. Comparison of the genomic sequences flanking individual paleoviruses reveal that some are orthologous viral insertions having been present in the common ancestor of the D. pulex species complex, which is millions of years old. Still, we detected some sites that have the signature of purifying selection. In contrast, other paleoviruses in this group seem to be unique to specific host lineages and even contain undisrupted open reading frames, suggesting either more recent acquisition, or selective maintenance.

The diversity of viruses infecting humans

Most human viruses have been discovered through the diseases they cause in animals, plants, bacteria or fungi. Recent finds include human bocaviruses, which now seem to have a global distribution, and cause respiratory tract disease in infants, and several new pathogenic human coronaviruses. The SARS coronavirus, genetically distinct from all previously known coronaviruses, caused a disease which was highly transmissible and very severe, eventually leading to 8000 cases worldwide with over 800 deaths. Many viruses which are transmitted to humans by invertebrates, such as insects or ticks, have the ability to infect and replicate in cells of both vertebrate and invertebrate origin. However human virology is a rapidly expanding field and recent technologies such as the polymerase chain reaction (PCR) amplification system have made it possible to look for previously unrecognized viruses which may or may not be involved in pathogenesis. For example viruses in the genus Anellovirus are found in 80% of human blood samples yet do not seem to cause any disease. This paper overviews known human vertebrate viruses, more recent discoveries, and recommends a systematic search for viruses which may already infect the human population but have so far remained undetected.

Genome-scale phylogenetic analyses of chikungunya virus reveal independent emergences of recent epidemics and various evolutionary rates

Chikungunya virus (CHIKV), a mosquito-borne alphavirus, has traditionally circulated in Africa and Asia, causing human febrile illness accompanied by severe, chronic joint pain. In Africa, epidemic emergence of CHIKV involves the transition from an enzootic, sylvatic cycle involving arboreal mosquito vectors and nonhuman primates, into an urban cycle where peridomestic mosquitoes transmit among humans. In Asia, however, CHIKV appears to circulate only in the endemic, urban cycle. Recently, CHIKV emerged into the Indian Ocean and the Indian subcontinent to cause major epidemics. To examine patterns of CHIKV evolution and the origins of these outbreaks, as well as to examine whether evolutionary rates that vary between enzootic and epidemic transmission, we sequenced the genomes of 40 CHIKV strains and performed a phylogenetic analysis representing the most comprehensive study of its kind to date. We inferred that extant CHIKV strains evolved from an ancestor that existed within the last 500 years and that some geographic overlap exists between two main enzootic lineages previously thought to be geographically separated within Africa. We estimated that CHIKV was introduced from Africa into Asia 70 to 90 years ago. The recent Indian Ocean and Indian subcontinent epidemics appear to have emerged independently from the mainland of East Africa. This finding underscores the importance of surveillance to rapidly detect and control African outbreaks before exportation can occur. Significantly higher rates of nucleotide substitution appear to occur during urban than during enzootic transmission. These results suggest fundamental differences in transmission modes and/or dynamics in these two transmission cycles.

Transmission as a predictor of ecological host specificity with a focus on vertical transmission of microsporidia

Consideration of vertical transmission is particularly important for understanding the life cycles of entomopathogens that are naturally occurring in invertebrate populations, are a problem in beneficial insect colonies, or are under consideration as classical biological control agents. Empirical studies generally corroborate the evolutionary hypothesis that virulence should be relatively low for pathogen species that utilize vertical transmission as one mechanism for maintenance in the host population. Nevertheless, many entomopathogens with significant effects on host populations are vertically as well as horizontally transmitted. In addition to gaining a better understanding of pathogen-host interactions and population dynamics, studies of the host range and specificity of putative biological control agents can benefit by using transmission studies to better predict ecological host specificity from physiological data. Horizontal transmission requires a tightly organized host-pathogen relationship to succeed, but still involves, albeit restricted by host behavior and pathogen dosage, the physiological susceptibility of the nontarget host. Vertical transmission studies can provide increased stringency for determining the ecological host specificity of a species and may be one very accurate predictor of the ability of a pathogen to successfully host-switch when introduced into a naïve population.

Evolutionary influences in arboviral disease

Arthropod-borne viruses (arboviruses) generally require horizontal transmission by arthropod vectors among vertebrate hosts for their natural maintenance. This requirement for alternate replication in disparate hosts places unusual evolutionary constraints on these viruses, which have probably limited the evolution of arboviruses to only a few families of RNA viruses (Togaviridae, Flaviviridae, Bunyaviridae, Rhabdoviridae, Reoviridae, and Orthomyxoviridae) and a single DNA virus. Phylogenetic studies have suggested the dominance of purifying selection in the evolution of arboviruses, consistent with constraints imposed by differing replication environments and requirements in arthropod and vertebrate hosts. Molecular genetic studies of alphaviruses and flaviviruses have also identified several mutations that effect differentially the replication in vertebrate and mosquito cells, consistent with the view that arboviruses must adopt compromise fitness characteristics for each host. More recently, evidence of positive selection has also been obtained from these studies. However, experimental model systems employing arthropod and vertebrate cell cultures have yielded conflicting conclusions on the effect of alternating host infections, with host specialization inconsistently resulting in fitness gains or losses in the bypassed host cells. Further studies using in vivo systems to study experimental arbovirus evolution are critical to understanding and predicting disease emergence, which often results from virus adaptation to new vectors or amplification hosts. Reverse genetic technologies that are now available for most arbovirus groups should be exploited to test assumptions and hypotheses derived from retrospective phylogenetic approaches.

Effect of alternating passage on adaptation of sindbis virus to vertebrate and invertebrate cells

Mosquito-borne alphaviruses, which replicate alternately and obligately in mosquitoes and vertebrates, appear to experience lower rates of evolution than do many RNA viruses that replicate solely in vertebrates. This genetic stability is hypothesized to result from the alternating host cycle, which constrains evolution by imposing compromise fitness solutions in each host. To test this hypothesis, Sindbis virus was passaged serially, either in one cell type to eliminate host alteration or alternately between vertebrate (BHK) and mosquito (C6/36) cells. Following 20 to 50 serial passages, mutations were identified and changes in fitness were assessed using competition assays against genetically marked, surrogate parent viruses. Specialized viruses passaged in a single cell exhibited more mutations and amino acid changes per passage than those passaged alternately. Single host-adapted viruses exhibited fitness gains in the cells in which they specialized but fitness losses in the bypassed cell type. Most but not all viruses passaged alternately experienced lesser fitness gains than specialized viruses, with fewer mutations per passage. Clonal populations derived from alternately passaged viruses also exhibited adaptation to both cell lines, indicating that polymorphic populations are not required for simultaneous fitness gains in vertebrate and mosquito cells. Nearly all passaged viruses acquired Arg or Lys substitutions in the E2 envelope glycoprotein, but enhanced binding was only detected for BHK cells. These results support the hypothesis that arbovirus evolution may be constrained by alternating host transmission cycles, but they indicate a surprising ability for simultaneous adaptation to highly divergent cell types by combinations of mutations in single genomes.

A recombinant Toscana virus nucleoprotein in a diagnostic immunoblot test system

Sandfly fever, a vector-borne disease endemic in the Mediterranean region, is caused by Toscana virus (TOS). The disease is increasingly important as a travel-related infection. Serological diagnosis is currently dependent on viral antigens derived from TOS-infected cell cultures. In this study, we report the cloning and expression of the TOS nucleoprotein (N) in Escherichia coli and evaluation of the recombinant (r) TOS N protein as an antigen for immunoblot assays. The TOS N gene was amplified by reverse-transcriptase polymerase chain reaction and cloned into the bacterial expression vector pTrcHis-A. Sera with known TOS antibody status were used to evaluate the immunoblot assay. The expressed rTOS N protein was purified and used as antigen for immunoblots. By recombinant immunoblot, the TOS antibody status (IgM and/or IgG) of the test panel was correctly identified. No cross-reactivity was detected. The rTOS N protein is useful as an antigen for immunoblot assays, and will enable more laboratories to perform TOS antibody diagnosis.

Nested RT-PCR for detection of sandfly fever virus, serotype Toscana, in clinical specimens, with confirmation by nucleotide sequence analysis

A single tube, reverse transcriptase/polymerase chain reaction (RT-PCR) was developed and evaluated for detecting a 400-bp product of the small RNA of sandfly fever virus, serotype Toscana (TOS). For more sensitive detection of genomic TOS RNA, a nested PCR amplifying a 243-bp cDNA within the RT-PCR product was established. Nucleotide sequence analysis of first- and second-round PCR products using the dideoxy cycle sequencing technique confirmed a previously published sequence of the TOS reference strain (ISS. Phl.3). By nested PCR, genomic TOS RNA was amplified from two consecutive sera taken 3 and 7 weeks after the onset of illness in one patient, and from CSF of a second patient obtained at the onset of meningitis. Authenticity of amplified PCR products was confirmed by nucleotide sequence analysis, revealing a sequence identical to the TOS reference strain. RT-PCR and nested PCR are useful for laboratory diagnosis and studies of the molecular epidemiology of TOS infection.

Genetic heterogeneity of the RNA genome population of the plant virus U5-TMV

The genetic heterogeneity in a population of the U5 strain of tobacco mosaic virus (U5-TMV) was studied. The T1 fingerprint characterizing a cloned population did not vary after a new cloning step in the local lesion host Nicotiana tabacum Xanthi-nc, nor during four series of 20 passages in the systemic host N. tabacum Samsum. No heterogeneity was observed among 10 clones derived from the cloned populations, while 1 of 18 clones derived from a 20-fold passaged population differed from the rest in 1 of 55 oligonucleotides. A higher heterogeneity was found in an uncloned field isolate in which 2 of 10 clones differed in type in 1 and 2 oligonucleotides, respectively. These data agree with those reported for bacterial and animal RNA viruses and are compatible with the quasi-species model for RNA populations. On the other hand, the intrapopulational heterogeneities found for U5-TMV are considerably smaller than those reported for other RNA viruses, our data showing a high genetic stability for U5-TMV.

Genomic stability of La Crosse virus during vertical and horizontal transmission

We have used ribonuclease T1 oligonucleotide fingerprint analysis to study genomic stability of La Crosse virus (Bunyaviridae) during vertical and horizontal transmission in the laboratory. No RNA genomic changes were detected in vertebrate cell culture-propagated virus isolated (following ingestion and replication) from the natural host, Aedes triseriatus. Genomic changes were not detected during transovarial passage of the virus through two generations of mosquitoes, nor were changes detected in the genomes of virus isolated from suckling mice that had been fed upon by second generation transovarially-infected mosquitoes. These results demonstrate that despite the well-documented phenomena of rapid nucleotide change in RNA virus genomes under various conditions, the La Crosse virus genome can remain stable during transovarial transmission in the insect host and during transfer between the insect and vertebrate hosts. The evolutionary implications of these results are discussed.