Estimated Transmission Rate Research Articles

Spatial dynamics of an epidemic of severe acute respiratory syndrome in an urban area

To map risk of exposure to severe acute respiratory syndrome (SARS) in an urban area and assess the ability of traditional interventions to control dispersion of the disease. Data on the Beijing SARS epidemic were used to map spatial clusters of identified contacts and to estimate transmission of SARS using a model with a time-dependent transmission rate. The estimated transmission rate decreased dramatically from 20 to 30 April 2003. The total number of cases in the epidemic in Beijing was estimated to be 2521. Hierarchical clustering revealed that risk-exposures were widespread, but clustered in a pattern that is distinctly related to the Beijing urban ring roads. Traditional control measures can be very effective at reducing transmission of SARS. Spatial patterns of risk-exposures can inform disease surveillance, prediction and control by identifying spatial target areas on which interventions should be focused.

The Potential of <I>Aedes triseriatus</I> (Diptera: Culicidae) as an Enzootic Vector of West Nile Virus

The susceptibility of Aedes triseriatus (Say) (Diptera: Culicidae) to low levels of West Nile virus (family Flaviviridae, genus Flavivirus, WNV) was determined and compared with that of Culex pipiens L. to assess the likelihood of its participation in an enzootic cycle involving mammals. Ae. triseriatus and Cx. pipiens were exposed to WNV by feeding on baby chickens with WNV serum titers ranging from 10(4.1 +/- 0.1) to 10(8.6 +/- 0.1) plaque-forming units (PFU)/ml and from 10(4.1 +/- 0.1) to 10(7.0) PFU/ml, respectively. Infection rates and 95% confidence intervals (CIs) of 8% (4, 14) and 25% (15, 38) occurred in Ae. triseriatus and Cx. pipiens after feeding on chickens with WNV titers of 10(4.1 +/- 0.1) PFU/ml and increased to 65% (49, 79) and 100% (72, 100) in Ae. triseriatus and Cx. pipiens after feeding on chickens with titers of 10(7.1 +/- 0.1) PFU/ml. The mean infection rate of Ae. triseriatus ranged from 97% (84, 100) to 100% (79, 100) after feeding on chickens with WNV titers of > or = 10(8.2) PFU/ml. The infectious dose (ID)50 values for Ae. triseriatus and Cx. pipiens were 10(6.5) (6.4, 6.7) and 10(4.9) (4.6, 5.1) PFU/ml, respectively. The combined estimated transmission rate of Ae. triseriatus at 14 and 18 d after feeding on chickens with a mean WNV titer of 10(8.6 +/- 0.1) PFU/ml was 55%. Although Ae. triseriatus is significantly less susceptible to WNV than Cx. pipiens, the susceptibility of Ae. triseriatus to WNV titers < 10(5.0) PFU/ml and its ability to transmit WNV suggest that Ae. triseriatus has the potential to be an enzootic vector among mammalian populations.

Quantifying Transmission of Campylobacter spp. among Broilers

Campylobacter species are frequently identified as a cause of human gastroenteritis, often from eating or mishandling contaminated poultry products. Quantitative knowledge of transmission of Campylobacter in broiler flocks is necessary, as this may help to determine the moment of introduction of Campylobacter in broiler flocks more precisely. The aim of this study was to determine the transmission rate parameter in broiler flocks. Four experiments were performed, each with four Campylobacter-inoculated chicks housed with 396 contact chicks per group. Colonization was monitored by regularly testing fecal samples for Campylobacter. A mathematical model was used to quantify the transmission rate, which was determined to be 1.04 new cases per colonized chick per day. This would imply that, for example, in a flock of 20,000 broilers, the prevalence of Campylobacter would increase from 5% to 95% within 6 days after Campylobacter introduction. The model and the estimated transmission rate parameter can be used to develop a suitable sampling scheme to determine transmission in commercial broiler flocks, to estimate whether control measures can reduce the transmission rate, or to estimate when Campylobacter was introduced into a colonized broiler flock on the basis of the time course of transmission in the flock.

Use of Strain Typing Data to Estimate Bacterial Transmission Rates in Healthcare Settings

To create an affordable and accurate method for continuously monitoring bacterial transmission rates in healthcare settings. We present a discrete simulation model that relies on the relationship between in-hospital transmission rates and strain diversity. We also present a proof of concept application of this model to a prospective molecular epidemiology data set to estimate transmission rates for Pseudomonas aeruginosa and Staphylococcus aureus. Inpatient units of an academic referral center. All inpatients with nosocomial infections. Mathematical model to estimate transmission rates. Maximum likelihood estimates for transmission rates of these two species on different hospital units ranged from 0 to 0.36 transmission event per colonized patient per day. This approach is feasible, although estimates of transmission rates based solely on strain typed clinical cultures may be too imprecise for routine use in infection control. A modest level of surveillance sampling substantially improves the estimation accuracy.

Utilizing HIV transmission rates to assist in prioritizing HIV prevention services

The overall, annual HIV transmission rate in the US has been about 4% since the early 1990s. In this paper, we find that the transmission rate for persons unaware of their HIV serostatus is approximately 10.79%, and for persons aware of their HIV seropositivity is about 1.73% overall. However, the latter statistic can be further disaggregated. For persons who receive HIV counselling and testing, and for whom these services are effective, the transmission rate is near 0%. For persons who do not receive counselling services as well as testing, or for whom counselling and testing is insufficient to induce behavioural change, the HIV transmission rate is between approximately 2% and 4%. These estimated transmission rates, as well as the raw number of transmissions and the specific population sizes examined here, suggest some clear recommendations (provided here) for prioritizing HIV prevention services for persons living with HIV.

SRAMT: a hybrid sender and receiver-based adaptation scheme for TCP friendly multicast transmission

In this paper, we describe a hybrid sender and receiver-based adaptation scheme for multicast transmission of multimedia data, which we call SRAMT (Sender–Receiver based Adaptation scheme for Multicast Transmission). The most prominent features of SRAMT are its distributed (to sender and receivers) transmission rate estimation algorithm and its innovative RTT (Round Trip Time) estimation algorithm based on one-way delay measurements. With the use of SRAMT, we ensure that sender will transmit TCP friendly traffic and receivers with different capabilities (in terms of available bandwidth) are able to receive the multimedia information. We evaluate SRAMT through a number of simulations and compare it with other schemes available to the literature. Main target of the simulations was the examination of SRAMT behavior to a heterogeneous group of receivers and its behavior against TCP connections.

Deoxynivalenol and zearalenone residues in eggs of laying hens fed with a naturally contaminated diet: effects on egg production and estimation of transmission rates from feed to eggs.

The potential for the Fusarium mycotoxins 4-deoxynivalenol (DON) and zearalenone (ZON) to enter the human food chain through contaminated eggs was assessed using a controlled feed study. Four groups of laying hens (eight in each group) were fed a diet that included differing amounts of naturally contaminated wheat containing DON ( approximately 20 mg kg(-1)) and ZON (0.5 mg kg(-1)). Eggs were collected and pooled from each group on a daily basis. Pooled samples were analyzed by liquid chromatography with mass spectrometry detection (LC-MS/MS). The method allowed DON, other type B trichothecenes, ZON, and its metabolites to be determined in a single multi-residue analysis. The selectivity of the MS/MS procedure allowed cleanup to be minimized (for DON, cleanup by immunoaffinity column was used) or eliminated (for ZON). The limits of detection of 0.01 microg kg(-1) for DON and 0.1 microg kg(-1) for ZON in eggs were lower than previously published methods. None of the samples analyzed had detectable levels of ZON or its metabolites. Although maximum levels of DON contamination (10 mg kg(-1) feed) were relatively high, no adverse effects were observed on egg production. On the basis of the determined DON levels in the hen's diet and the determined levels of DON in the corresponding eggs, transmission rates of 15 000:1, 18 000:1, and 29 000:1 for treatment levels 5, 7.5, and 10 mg DON kg(-1) feed, respectively, were found. These results show that, although eggs could be a human exposure route for DON, the levels are insignificant compared to the other sources, although the presence of metabolites of DON was not studied.

QUANTIFICATION AND ANALYSIS OF TRANSMISSION RATES FOR SOILBORNE EPIDEMICS

The rates of transmission of infection from inoculum or infecteds to susceptible hosts are critical determinants of epidemics, yet no formal experimental methods have been described for their quantification and analysis in spatially explicit epidemics. Replicated microcosms of >400 radish seedlings and with tight control of environmental conditions were exposed to known amounts of inoculum of the fungal plant pathogen Rhizoctonia solani. Spatiotemporal maps of disease progress were used to distinguish between primary and secondary infections and to count changes with time in the number of infected plants and the number of contacts between susceptible and neighboring infected plants. Transmission rates were defined within a compartmental S–I (susceptible–infected) model for plant epidemics and estimated empirically using counts from spatial maps. The transmission rate for primary infection declined with time; the transmission rate for secondary infection rose initially and then declined. We discuss the mechanisms contributing to the changes in transmission rates with time and show that spatial mapping combined with an epidemiological analysis provides accurate empirical estimates of transmission rates.

Seasonal and ontogenetic dynamics in trophic transmission of parasites

Transmission rates from the intermediate (amphipods) to the definitive hosts (fish) were quantified for two helminth species (Cyathocephalus truncatus, Cestoda, and Cystidicola farionis, Nematoda) both seasonally and through the ontogeny of the final hosts (arctic charr, Salvelinus alpinus, and brown trout, Salmo trutta). Amphipods (Gammarus lacustris) were important prey for both fish species, especially in the autumn. Both parasite species had low infection levels in amphipods compared to high abundance in fish. The seasonal variations in transmission rate of C. truncatus procercoids from amphipods to fish were in accordance with the observed abundance in fish hosts, being highest in the autumn and lowest during late winter and early summer. During summer, however, the estimated monthly transmission rates of C. truncatus were higher than the observed infection levels in the fish, suggesting restricted establishment success and shorter development time and longevity of the cestode in fish at higher temperatures. The accumulated transmission of C. farionis over the ontogeny of arctic charr was similar to the observed age‐specific infection levels, reflecting a high establishment success and longevity of this parasite in charr. In contrast, brown trout exhibited an infection level that was much lower than the estimated transmission rates, suggesting a high resistance against C. farionis in these fish. The magnitudes of the estimated transmission rates were sufficient to explain the paradoxical contrast between low infection levels in the intermediate and high in the final hosts.

Parasite transmission: reconciling theory and reality

Summary Arguably the most important and elusive component of host–parasite models is the transmission function. Considerable empirical and theoretical work has focused on determining the correct formulation of this function although, to date, there has been little attempt to combine these studies to develop general insights into how observed transmission rates affect host–parasite dynamics. Here, estimates of transmission rates from a range of host–parasite systems in the literature are described using a phenomenological function which takes into account how transmission varies with host and parasite densities. This function is placed in the appropriate model framework to determine the consequences of the observed transmission rates for each system. All of the parasites had decreasing per capita transmission rates with increasing parasite densities suggesting that parasites tend to saturate at high densities, either as hosts become limiting or due to heterogeneities amongst the host population. In terms of the responses to host density, the parasites fell into two groups: those with increasing or decreasing transmission rates. This dichotomy was due to the biology of the organisms; the former group infect through cannibalism, which increased at high densities as the individuals became stressed, whereas the latter group infected through free‐living stages, resulting in a form of spatial structuring reducing the number of hosts available for infection. A metapopulation model was developed where hosts and parasites interacted in discrete patches according to the appropriate transmission function, with neighbouring patches linked by dispersal. The model suggested that small‐scale, localized transmission events can drive large‐scale epizootics at the metapopulation level. This emphasizes the importance of correctly describing and quantifying the transmission function at the individual level. Traditionally, the formulation of the transmission function has depended on the scale of observation. This work shows that transmission should be considered from the viewpoint of the organisms concerned. Observed transmission rates are a consequence of the biology of the individuals meaning it should be possible to develop a priori hypotheses concerning the nature of the transmission function from a basic understanding of the life history of the organisms concerned.

Vector competence of Culex tarsalis from Orange County, California, for West Nile virus.

To evaluate the vector competence of Culex tarsalis Coquillett for West Nile virus (WN), females reared from larvae collected in Huntington Beach, Orange County, CA, were fed on 2-3-day-old chickens previously inoculated with a New York strain (Crow 397-99) of WN. The Cx. tarsalis mosquitoes were efficient laboratory vectors of WN, with estimated transmission rates of 81% and 91% for mosquitoes that ingested 10(6.5) or 10(7.3) plaque-forming units of WN/mL of blood, respectively. Based on efficiency of viral transmission and the role of this species in the transmission of the closely related St. Louis encephalitis virus, Cx. tarsalis should be considered a potentially important vector of WN in the western United States.

Laboratory transmission of La Crosse virus by Ochlerotatus j. japonicus (Diptera: Culicidae).

Ochlerotatus j. japonicus, a recent introduction to the United States, was studied to determine its capability to serve as a vector of La Crosse (LAC) virus. A field-collectedpopulation of Ochlerotatus triseriatus, the primary vector of LAC virus, was similarly tested for comparison. After Oc. j. japonicus ingested virus fromhamsters with viremias of 10(3.6-5.4) plaque-forming units (PFU)/ml of blood, its estimated transmission rates were 35-88%. These rates were slightly lower than, though similar to, those for Oc. triseriatus, 75-100%. Viral titers in Oc. j. japonicus peaked at approximately 10(5.5) PFU/ mosquito about 7 d after ingesting blood meal in which the concentration of LAC virus was PFU/ml of blood; virus had disseminated from the midgut in 100% (8/8) of these specimens. These data, combined with the close association between the habitats of Oc. j. japonicus and Oc. triseriatus and the reported expansion of the range of this newly discovered species in the eastern United States, indicate that Oc. j. japonicus could function as an additional vector of LAC virus.

Experimental Transmission of Eastern Equine Encephalitis Virus byOchlerotatus j. japonicus(Diptera: Culicidae)

We evaluated the potential for Ochlerotatus j. japonicus (Theobald), a newly recognized invasive mosquito species in the United States, to transmit eastern equine encephalitis (EEE) virus. Aedes albopictus (Skuse) and Culex pipiens (L.) were similarly tested for comparison. Ochlerotatus j. japonicus and Ae. albopictus became infected and transmitted EEE virus by bite after feeding on young chickens 1 d after they had been inoculated with EEE virus (viremias ranging from 10(7.0-8.7) plaque-forming units [PFU]/ml of blood). No Cx. pipiens (n = 20) had detectable levels of virus 14 d after feeding on an EEE-virus infected chicken with a viremia of 10(8.1) PFU per ml of blood. Depending on the viral titer in the donor chicken, infection rates ranged from 55-100% for Oc. j. japonicus and 93-100% for Ae. albopictus. In these two species, dissemination rates were identical to or nearly identical to infection rates. Depending on the viral titer in the blood meal, estimated transmission rates ranged from 15 to 25% for Oc. j. japonicus and 59-63% for Ae. albopictus. Studies of replication of EEE virus in Oc. j. japonicus showed that there was an "eclipse phase" in the first 4 d after an infectious blood meal, that viral titers peak by day 7 at around 10(5.7) per mosquito, and that virus escaped the mid-gut as soon as 3 d after the infectious blood meal. These data, combined with the opportunistic feeding behavior of Oc. j. japonicus in Asia and the reported expansion of its range in the eastern United States, indicate that it could function as a bridge vector for EEE virus between the enzootic Culiseta melanura (Coquillett)-avian cycle and susceptible mammalian hosts.

Transmission of infection by gastrointestional endoscopy: May 2001

Transmission of infection by gastrointestional endoscopy: May 2001

Maternal plasma viral RNA levels determine marked differences in mother-to-child transmission rates ofHIV-1 and HIV-2 in The Gambia

To determine the rates of, and risk factors for, mother-to-child transmission (MCT) of HIV-1 and HIV-2 infection in The Gambia. A blinded, prospective, community-based cohort study of 29.549 pregnant women attending the eight largest antenatal clinics in The Gambia. Women were tested for HIV-1 and HIV-2 infection. Infected subjects and a group of HIV-seronegative women were followed with their babies until 18 months after delivery. Maternal CD4 cell count percentages were measured before and 18 months after delivery, and the antenatal plasma viral load was determined. Babies were tested for HIV by the polymerase chain reaction and/or serology at 2, 9 and 18 months of age. The study enrolled 144 women positive for HIV-1 and 294 for HIV-2 plus 565 seronegative pregnant women: the mean antenatal percentage CD4 cell counts of 96 HIV-1-positive, 223 HIV-2-positive and 125 HIV-seronegative mothers were 31% [95% confidence interval (CI) 28-33], 41% (95% CI 39-42) and 47% (95% CI 45-49), respectively. The geometric mean antenatal plasma viral load of 94 HIV-1-infected women was 15,100 copies x 10(3) ml (95% CI 10,400-19,000) which was much higher than that of 60 randomly selected HIV-2-infected women, which was 410 copies x 10(3) ml (95% CI 150-910) (P < 0.001). The estimated transmission rate of HIV-1 was 24.4% (95% CI 14.6-33.9) and that of HIV-2 was 4.0% (95% CI 1.9-7.4). Five of 17 HIV-1-positive and three of eight HIV-2-positive babies were infected after 2 months of age. Birth in the rainy season [odds ratio (OR) 2.9; 95% CI 1.2-7.2], a low postnatal CD4 cell percentage (OR for a 10% fall 2.4; 95% CI 1.1-5.1) and a high maternal plasma viral load (OR for a 10-fold increase 2.9; 95% CI 1.1-7.8) were risk factors for transmission that applied equally to both viruses. Low maternal HIV-2 RNA levels, which on average are 37-fold less than in HIV-1 infection, relate to the low MCT rate of HIV-2.

Optic-fiber communications: Nonlinear-transformation group

A new method for finding solutions of the nonlinear Schrodinger equation is proposed. A comutative multiplicative group of nonlinear transformations, which operate on stationary localized solutions, enables a consideration of fractal subspaces in the solution space, stability, and deterministic chaos. An increase of the transmission rate in the optic-fiber communications can be based on new forms of localized stationary solutions, without significant change of input power. The estimated transmission rate is 50 Gbit/s, for certain available soliton transmission systems.

HIV Vertical Transmission Rate Determinations Are Subject to Differing Definitions and Therefore Different Rates

The HIV infection status of a cohort of 600 prospectively followed children born to HIV infected mothers was determined using HIV peripheral blood culture tests at 0, 3, and 6 months of age, HIV serology at ≥15 months, and CDC AIDS criteria. We estimated transmission rates using five methods which differed in how HIV indeterminates are handled. These methods were applied at two points in time to illustrate effects of length of follow-up of the cohort on results. In January 1997, 30 months after the last birth, transmission rate estimates ranged from 15.5% (known positives/known positives × known negatives) to 18.1% (known positives × those with one positive culture × deaths/entire cohort minus those lacking negative cultures at age ≥5 months). Estimates ranged from 14.8% to 20.7% using the subcohort of 284 children followed ≥12 months as of May 1993. These results indicate that methods for assigning HIV infection status and for handling HIV indeterminates should be carefully defined when estimating transmission rates.

Two-part survival models applied to administrative data for determining rate of and predictors for maternal-child transmission of HIV.

In analysing maternal-child HIV transmission from Medicaid claims data, we must deal with follow-up that is sometimes so short that we cannot claim that an apparently uninfected infant is actually uninfected as opposed to not yet exhibiting HIV-associated symptoms. To overcome this, we have been using analyses of 'time-to-diagnosis' of HIV infection to estimate transmission rates and predictors of transmission. Such analyses mix the event of transmission with that of our ability to diagnose HIV infection from coded claims data. We would like to separate these two pieces. Also, due to incomplete follow-up, Kaplan-Meier analyses will underestimate transmission rates. In econometrics and biostatistics there are two-part (mixture) models that can serve the goal of separating transmission from the process of diagnosing HIV infection in the newborn. Farewell describes a model that combines a logistic regression for the yes/no event (in our case, HIV transmission) and a Weibull regression model for the survival analysis portion (in our case, time-to-diagnosis). We use this approach to fit models that have potentially separate covariates for transmission and for time-to-diagnosis. The results allow us to identify predictors of transmission and estimate transmission rates with reduced concern for adequacy of follow-up.

Transmission and population dynamics of HTLV-1 infection

We present a modelling approach for transmission and population dynamics of human T-cell lymphotropic-virus-type-I (HTLV-I) infection. A steady-state analysis of the model was used to estimate transmission rates for males and females. The rate of heterosexual transmission was 2.7 times higher if the carrier was male. A dynamic transmission model was developed and used to study the impact of different intervention measures in the Jamaican population. Interventions concerning horizontal (heterosexual) transmission exceeds the effect of breaking vertical (mother-to-child) transmission, although breast feeding also has an important impact on the total number of infected individuals.

Allometry and simple epidemic models for microparasites.

Simple mathematical models for microparasites offer a useful way to examine the population dynamics of different viral and bacterial pathogens. One constraint in applying these models in free-living host populations is the paucity of data with which to estimate transmission rates. Here we recast a standard epidemiological model by setting the birth and death rates of the host population and its density as simple allometric functions of host body weight. We then use standard threshold theorems for the model in order to estimate the minimum rate of transmission for the parasite to establish itself in a mammalian host population. Transmission rates that produce different comparable values of the parasites' basic reproductive number, RO, are themselves allometric functions of host body size. We have extended the model to show that hosts having different body sizes suffer epidemic outbreaks whose frequency scales with body size. The expected epidemic periods for pathogens in different mammalian populations correspond to cycles observed in free-living populations.