Combining surveillance and expert evidence of viral hemorrhagic septicemia freedom: a decision science approach.

The ability to combine evidence streams to establish disease freedom or prioritize surveillance is important for the evaluation of emerging diseases, such as viral hemorrhagic septicemia virus (VHSV) IVb in freshwater systems of the United States and Canada. Waterways provide a relatively unconstrained pathway for the spread of VHSV; and structured surveillance for emerging disease in open systems has many challenges. We introduce a decision framework for estimating VHSV infection probability that draws from multiple evidence streams and addresses challenges associated with the assessment of emerging disease. Using this approach, historical and risk-based evidence, whether empirical or expert-derived, supplement surveillance data to estimate disease probability. Surveillance-based estimates of VHSV prevalence were described using beta distributions. Subjective likelihood ratios (LRs), representing contextual risk, were elicited by asking experts to estimate the predicted occurrence of risk factors among VHSV-affected vs. VHSV-unaffected watersheds. We used the odds form of Bayes' theorem to aggregate expert and surveillance evidence to predict the risk-adjusted posterior probability of VHSV-infection for given watersheds. We also used LRs representing contextual risk to quantify the time value of past surveillance data. This evidence aggregation model predicts disease probability from the combined assessment of multiple sources of information. The method also provides a flexible framework for iterative revision of disease freedom status as knowledge and data evolve.

Differences in Lepeophtheirus salmonis abundance levels on Atlantic salmon farms in the Broughton Archipelago, British Columbia, Canada.

Sea lice data collected from Atlantic salmon farms in the Broughton Archipelago between 2003 and 2005 were examined for inter-regional differences in mobile Lepeophtheirus salmonis (Krøyer) abundance using the generalized linear model procedure. Factors such as age of the salmon populations, location of farms and time of year had a significant effect on the abundance of the mobile stages of L. salmonis whereas water temperature and salinity did not. Separate evaluation of SLICE treatment data found no significant difference in treatment frequency among the areas but did show that there were significantly lower numbers of farm treatments during the summer months when compared with other seasons. The role of environment and wild fish in influencing sea lice abundance on the farmed salmon is discussed. The findings suggest that effective management programmes for sea lice should not only be based on geographical location but should take into account other factors which could influence lice abundance levels.

Rest potential--cooperative selectivity.

It is often necessary to determine the selectivity and cooperativity constants of ions and other substances which are selectively and cooperatively adsorbed at surface sites of biological cells. A computer program based on the Levenberg-Marquardt method is given to determine the selectivity and cooperativity constants from the non-linear equation for the resting potential.

Heart muscle contraction oscillation.

van der Pol developed a mathematical model for self-sustained radio oscillations described by his non-linear differential equation D2X + epsilon(X2-1)DX + X = 0 in which X is a function of time T and D/DT the differential operator to T. For epsilon = 0, this is the differential equation for the harmonic oscillator which has sinusoidal solutions. For epsilon not equal to 0 the equation is non-linear. If epsilon > 1 van der Pol coined the name relaxation oscillations for its solutions. These are non-linear and quite different from simple sinusoidal oscillations. They are mathematical models for many physical and biological phenomena. van der Pol suggested that his equation is also a model for the heartbeat. However, biomedical oscillations, including the heartbeat, have a threshold which the mathematical model described by van der Pol's equation does not possess. It has, in addition to an unstable origin, only a stable limit cycle of Poincaré. In this paper, van der Pol's equation is extended in such a way that it has in addition to a stable origin and a stable limit cycle, an unstable limit cycle. Because it possesses such an unstable limit cycle, the extension obtained is a mathematical model for a threshold oscillation. It is also shown that an asymmetric analogy of the extended equation is a mathematical model for an isometric contraction of the mammalian cardiac muscle.

Baroreceptor control of pressure-flow relationships during hypoxemia.

In the absence of peripheral chemoreceptors, the effects of graded hypoxemia on the carotid sinus control of central and regional hemodynamics were studied in anesthetized mongrel dogs. Baroreceptor stimulation was effected by carotid sinus isolation and perfusion under controlled pressure. Blood flows were measured in the aorta and the celiac, mesenteric, left renal, and right iliac arteries. Carotid sinus reflex set-point pressures were well maintained until hypoxemia was severe. Carotid sinus reflex set-point gain was maximal during mild hypoxemia. Reflex operating point regional flows were unaffected by hypoxemia. A factorial analysis of overall reflex increases in mean aortic pressure, flow, and power during reduced baroreceptor stimulation showed potentiation by increasing hypoxemia. Corresponding effects of baroreceptor stimulation and hypoxemia on aortic resistance and heart rate were additive. Celiac, renal, and iliac blood flows increased during both hypoxemia and reduced baroreceptor stimulation. Only in the celiac bed were blood flow changes independent of concomitant changes in cardiac output. Thus, at maximum sympathetic stimulation (low carotid sinus pressure) during hypoxemia, the cardiovascular system maintained both central and regional blood flows at high systemic blood pressures independent of the peripheral chemoreceptors.

Tremor and somatosensory studies during chamber He-O2 compressions to 13.1, 25.2, 37.3, and 49.4 ATA.

Comparison was made between intentional and postural tremor amplitudes with a 2-to 50-Hz bandwidth in four men rapidly compressed from surface to 13.1, 25.2, and 37.3 ATA in 4 h while breathing helium-oxygen mixtures (Predictive Studies IV, University of Pennsylvania). Excursions with compression rates of 20 and 40 ft/min were then made to 49.4 ATA on exposure days 2 and 3, returning to saturation at 37.3 ATA for 6 additional days. During compression from 25.2 to 37.3 ATA, there were increases in tremor amplitudes for both intentional and postural tremor in the 3-to 7-Hz and 8- to 12-Hz ranges. Both types of tremor showed increases in amplitudes at 49.4 ATA on exposure days 2 and 3. Amplitude changes in postural tremor, which occurred only on the fastest compression to 49.4 ATA, were less evident than those in intentional tremor during the remaining dive profile, and adaptation to pressure exposure could be defined by day 8. Intentional tremors did not show adaptation at pressure.

Rate of potassium-sodium exchange by human lymphocytes: prediction of the cooperative adsorption model.

The equilibrium parameters of potassium-sodium distribution in human lymphocytes, determined experimentally in the preceding study (Negendank and Shaller, '79), were incorporated into a stochastic treatment of the cooperative adsorption model in order to predict the kinetics of "active" potassium-sodium exchange. The rate of uptake of potassium, in potassium-depleted, sodium-loaded cells, is complex and deviates markedly from simple exponential functions. The sigmoid form of the exchange data closely followed the predicted curve. This result enhances one's confidence in the usefulness and applicability of the cooperative adsorption model, and adds further support to the association-induction hypothesis as a coherent theory of cell physiology.

[14 C]2-Deoxyglucose uptake marks systems activated by rewarding brain stimulation.

Autoradiography of sections from the brains of anesthetized rats injected with [14C] 2-doexyglucose revealed neural systems unilaterally activated by rewarding electrical stimulation of the medial forebrain bundle. Since this new technique marks neural systems on the basis of their activity, it has great potential in functional neuroanatomy. The potential is easily realized in brain-stimulation studies, where systems may be driven to exceptionally high levels of activity.