Dynamics of bed bug infestations and control under disclosure policies

Bed bugs have reemerged in the United States and worldwide over recent decades, presenting a major challenge to both public health practitioners and housing authorities. A number of municipalities have proposed or initiated policies to stem the bed bug epidemic, but little guidance is available to evaluate them. One contentious policy is disclosure, whereby landlords are obligated to notify potential tenants of current or prior bed bug infestations. Aimed to protect tenants from leasing an infested rental unit, disclosure also creates a kind of quarantine, partially and temporarily removing infested units from the market. Here, we develop a mathematical model for the spread of bed bugs in a generalized rental market, calibrate it to parameters of bed bug dispersion and housing turnover, and use it to evaluate the costs and benefits of disclosure policies to landlords. We find disclosure to be an effective control policy to curb infestation prevalence. Over the short term (within 5 years), disclosure policies result in modest increases in cost to landlords, while over the long term, reductions of infestation prevalence lead, on average, to savings. These results are insensitive to different assumptions regarding the prevalence of infestation, rate of introduction of bed bugs from other municipalities, and the strength of the quarantine effect created by disclosure. Beyond its application to bed bugs, our model offers a framework to evaluate policies to curtail the spread of household pests and is appropriate for systems in which spillover effects result in highly nonlinear cost–benefit relationships

Rapid distortion theory for differential diffusion

Rapid distortion theory (RDT) is used to examine differential diffusion of active and passive scalars in unsheared, initially isotropic turbulence. RDT is well suited to study differential diffusion because it applies to strongly stratified flows with weak turbulence—that is, the conditions under which differential diffusion occurs. The theory reproduces several key features of the evolution of scalar fluxes and scalar flux spectra observed in direct numerical simulations (DNS). Predictions of the diffusivity ratio match laboratory results well when a parameter of the theory is related to a parameter of the experiments. RDT also allows parameters such as molecular diffusivities to be varied over a wider range than DNS can currently reach. RDT may prove to be a useful tool for computing mixing in weakly turbulent parts of the stratified ocean interior and possibly for parameterizing subgrid scale mixing in general circulation models

Systematic microstructure variability in double-diffusively stable coastal waters of nonuniform density gradient

Author Posting. © American Geophysical Union, 2002. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 107, C10 (2002): 3144, doi:10.1029/2001JC000844.Conductivity microstructure, water velocity, and stratification were measured during a tow-yo transect near the New England shelf/slope front in early August 1997. Velocity data were collected with an acoustic Doppler profiler on the ship. The other data were collected with a towed platform. Estimates of χ, the rate of dissipation of temperature variance, were computed from the conductivity data with vertical resolution of 0.3 m. Relationships between χ and shear, temperature gradient, buoyancy frequency (N), and gradient Richardson number (Ri) were explored, with special focus on measurements taken in waters stable to double-diffusive processes (to avoid ambiguity of interpretation) and exhibiting variable density gradient (N ranging from 5 to 40 cph). For this subset of data, χ computed from data grouped into classes of local mean temperature gradient (inline equation/dz) was proportional to inline equation/dz to the 0.7 power, which is consistent with diapycnal thermal eddy diffusivity K being proportional to (inline equation/dz)−1.3 within the framework of the Osborn-Cox model that relates χ and the mean temperature gradient to the heat flux. No correlation between K and Ri was observed, with Ri computed at 4 m vertical scale, so that systematic inhomogeneous large-scale forcing is not responsible for a false correlation of K and inline equation/dz. Water mass salinity characteristics in the area caused N2 to be proportional to (inline equation/dz)4/5, rather than to inline equation/dz as in the isohaline case, giving rise to the steep inverse relation K = 10−10N−3.3, with N in radians/s and diffusivity in m2/s. The fit K = 2 × 10−9N−2.5 results if one questionable data ensemble is disregarded. These relations are comparable to results obtained previously from the near-bottom tow data. They are not intended to be universal formulae but are meant to describe the conditions we encountered. They are not expected to hold at high and low values of N outside of our measurement range. An interpretation is that under these conditions the less strongly stratified (lower N) layers in this shelf area are more prone to instability of the larger-scale shear than the intervening interfaces, with the subsequent greater energy dissipation in the layers leading to higher buoyancy flux KN2 in the layers than in the interfaces.This work was supported by the U.S. Office of Naval Research (Grants N-00014-95-1-0633 and N00014-95-1-1064) and by a WHOI postdoctoral scholarship award to C.R.R

Resuspension of E. coli from Direct Fecal Deposits in Stream

Direct fecal deposits from cattle provide a significant source of E. coli to streams and therefore pose a threat to human health in agricultural watersheds. Experiments were conducted in a flume (9.1 m long, 0.6 m wide, and 0.6 m deep) with flow of 0.0106 m3 s-1 , an average velocity of 11.4 cm s-1 ,and water depth of 15.24 cm to measure the resuspension and deposition of E. coli from an undisturbed standard cowpat. Water samples were collected 1.22 m and 3.66 m downstream of the deposited cowpat, and at each downstream cross-section nine samples were collected to characterize the bacterial movement. E. coli in water samples were separated into the attached and unattached phases by filtration to assess the mechanism of transport. The cumulative load contribution from a single deposited cowpat after one hour was 2.49×10 9 cfu 3.66 m downstream. The composite E. coli concentrations at all sampling points and times exceeded the federal standards for primary contact in the United States of 126 cfu/100 ml. Between 77.2 and 99.5% of all E. coli downstream of the direct deposit were associated with particulates. The resuspension rate was 5.91×107 and 9.52×104 cfu m-2 s-1 0.5 min and 60 minutes after deposition, respectively, 1.22 m downstream of the deposit and 2.19×106 and 3.14×103 cfu m-2 s-1 0.5 min and 60 min after deposition, respectively, 3.66 m downstream of the deposit. Results from this study are useful to improve modeling techniques to predict in-stream E. coli concentrations from direct fecal deposits and emphasize the need to implement management practices to reduce livestock access to streams