Evolutionary dynamics and spatial genetic structure of epizootic hemorrhagic disease virus in the eastern United States

A simple model explains the dynamics of preferential host switching among mammal RNA viruses

A growing number of studies support a tendency toward preferential host switching, by parasites and pathogens, over relatively short phylogenetic distances. This suggests that a host switch is more probable if a potential host is closely related to the original host than if it is a more distant relative. However, despite its importance for the health of humans, livestock, and wildlife, the detailed dynamics of preferential host switching have, so far, been little studied. We present an empirical test of two theoretical models of preferential host switching, using observed phylogenetic distributions of host species for RNA viruses of three mammal orders (primates, carnivores, and ungulates). The analysis focuses on multihost RNA virus species, because their presence on multiple hosts and their estimated ages of origin indicate recent host switching. Approximate Bayesian computation was used to compare observed phylogenetic distances between hosts with those simulated under the theoretical models. The results support a decreasing sigmoidal model of preferential host switching, with a strong effect from increasing phylogenetic distance, on all three studied host phylogenies. This suggests that the dynamics of host switching are fundamentally similar for RNA viruses of different mammal orders and, potentially, a wider range of coevolutionary systems.

Epizootic heamorragic disease

Epizootic haemorrhagic disease (EHD) is an infectious non-contagious viral disease transmitted by insects of the genus Culicoides which affects wild and domestic ruminants. The causative agent, the epizootic haemorrhagic disease virus (EHDV), belongs to the family Reoviridae, genus Orbivirus and shares many morphological and structural characteristics with the other members of the genus such as bluetongue, African horse sickness and equine encephalosis viruses. In recent years EHD outbreaks have been reported in countries bordering the European Union. They caused disease in cattle and severe repercussion on the livestock industry of the affected countries. In the light of recent European bluetongue epizootic these events pose an increasing threat to the European Union. This review includes the most recent information regarding the virus and the disease as well as tools for its diagnosis and control. It is our conviction that more attention should be drawn to both EHDV and the disease itself in order to fulfil all these gaps and not to be unprepared in case future possible incursions.

Viral phylodynamics and the search for an 'effective number of infections'

Information on the dynamics of the effective population size over time can be obtained from the analysis of phylogenies, through the application of time-varying coalescent models. This approach has been used to study the dynamics of many different viruses, and has demonstrated a wide variety of patterns, which have been interpreted in the context of changes over time in the ‘effective number of infections’, a quantity proportional to the number of infected individuals. However, for infectious diseases, the rate of coalescence is driven primarily by new transmissions i.e. the incidence, and only indirectly by the number of infected individuals through sampling effects. Using commonly used epidemiological models, we show that the coalescence rate may indeed reflect the number of infected individuals during the initial phase of exponential growth when time is scaled by infectivity, but in general, a single change in time scale cannot be used to estimate the number of infected individuals. This has important implications when integrating phylogenetic data in the context of other epidemiological data.

An improved real-time polymerase chain reaction for the simultaneous detection of all serotypes of Epizootic hemorrhagic disease virus

Epizootic hemorrhagic disease virus (EHDV) is a significant pathogen of wild and sometimes domestic ungulates worldwide. Rapid and reliable methods for virus detection and identification play an essential part in the control of epizootic hemorrhagic disease (EHD). In the present study, a 1-step real-time polymerase chain reaction (PCR) group-specific assay was developed. The assay detects genome segment 5 (NS1) from all of the 8 serotypes of EHDV. Assay sensitivity was evaluated relative to a conventional gel-based nested PCR using cell culture-derived virus and diagnostic samples from clinically affected white-tailed deer (Odocoileus virginianus). The assay reliably amplified the NS1 gene from any of the EHDV strains tested, including isolates from each of the 8 EHDV serotypes. No cross-reactions were detected when all 24 serotypes of Bluetongue virus, a closely related member of the genus Orbivirus, were tested. A panel of 76 known EHDV-positive clinical samples was used to compare the performance of the assay relative to a previously reported real-time PCR assay. Results indicated that there was no statistically significant difference between the threshold cycle values obtained with both assays. A collection of 178 diagnostic samples submitted for EHD diagnosis was also used for test evaluation. The assay could be applied for rapid detection of EHDV in clinical samples from susceptible ruminants during an outbreak of the disease. In addition, this PCR assay has the benefits of being reliable and simple and could provide a valuable tool for studying the epidemiology of EHDV infection in susceptible ruminants by facilitating the detection of EHDV, regardless of the serotype.

Population dynamics and rates of molecular evolution of a recently emerged paramyxovirus, avian metapneumovirus subtype C

We report the existence of two distinct sublineages of avian metapneumovirus (MPV) subtype C, a virus which has caused serious economic loss in commercial turkey farms in the United States. This subtype is closely related to human MPV, infects multiple avian species, and is globally distributed. The evolutionary rates of this virus are estimated to be 1.3 x 10(-3) to 7 x 10(-3) substitutions per site per year, and coalescent estimates place its emergence between 1991 and 1996. The four genes examined show a concordant demographic pattern which is characterized by a rapid increase in population size followed by stable population grown until the present.