Viral hemorrhagic septicemia IVb status in the United States: inferences from surveillance activities and regional context.

The United States (U.S.) response to viral hemorrhagic septicemia virus (VHSV) IVb emergence in the Laurentian Great Lakes (GL) included risk-based surveillance for cost-effective decision support regarding the health of fish populations in open systems. All U.S. VHSV IVb isolations to date derive from free-ranging fish from GL States. Most originate in the region designated by US Geological Survey hydrologic unit code (HUC) 04, with the exception of two detections in neighboring Upper Mississippi (HUC 05) and Ohio (HUC 07) regions. For States outside the GL system, disease probability was assessed using multiple evidence sources. None substantiated VHSV IVb absence using surveillance alone, in part due to the limited temporal relevance of data in open systems. However, Bayesian odds risk-based analysis of surveillance and population context, coupled with exclusions where water temperatures likely preclude viral replication, achieved VHSV IVb freedom assurance for 14 non-GL States by the end of 2012, with partial evidence obtained for another 17 States. The non-GL region (defined as the aggregate of 4-digit HUCs located outside of GL States) met disease freedom targets for 2012 and is projected to maintain this status through 2016 without additional active surveillance. Projections hinge on continued basic biosecurity conditions such as movement restrictions and passive surveillance. Areas with navigable waterway connections to VHSV IVb-affected HUCs (and conducive water temperatures) should receive priority for resources in future surveillance or capacity building efforts. However, 6 years of absence of detections in non-GL States suggests that existing controls limit pathogen spread, and that even spread via natural pathways (e.g., water movement or migratory fish) appears contained to the Great Lakes system. This report exemplifies the cost-effective use of risk-based surveillance in decision support to assess and manage aquatic animal population health in open systems.

Multiple forms of short-term plasticity at excitatory synapses in rat medial prefrontal cortex.

Short-term synaptic plasticity, in particular short-term depression and facilitation, strongly influences neuronal activity in cerebral cortical circuits. We investigated short-term plasticity at excitatory synapses onto layer V pyramidal cells in the rat medial prefrontal cortex, a region whose synaptic dynamic properties have not been systematically examined. Using intracellular and extracellular recordings of synaptic responses evoked by stimulation in layers II/III in vitro, we found that short-term depression and short-term facilitation are similar to those described previously in other regions of the cortex. In addition, synapses in the prefrontal cortex prominently express augmentation, a longer lasting form of short-term synaptic enhancement. This consists of a 40-60% enhancement of synaptic transmission which lasts seconds to minutes and which can be induced by stimulus trains of moderate duration and frequency. Synapses onto layer III neurons in the primary visual cortex express substantially less augmentation, indicating that this is a synapse-specific property. Intracellular recordings from connected pairs of layer V pyramidal cells in the prefrontal cortex suggest that augmentation is a property of individual synapses that does not require activation of multiple synaptic inputs or neuromodulatory fibers. We propose that synaptic augmentation could function to enhance the ability of a neuronal circuit to sustain persistent activity after a transient stimulus. This idea is explored using a computer simulation of a simplified recurrent cortical network.