Assessment of risk from long term exposure to waterborne pathogens

Disease due to waterborne pathogens, whether in outbreak or endemic form, continues to be a problem in both the developing and the developed world. Control of waterborne disease requires accurate assessment of the pathogen dose-response relation and of likely patterns of exposure. Heretofore, risk assessment of pathogen exposure has been done on the basis of several standard biologically plausible dose-response models. In this paper, the problem of estimating the long-term risk from waterborne pathogens is put into a rigorous mathematical and statistical framework. The implications of the biologic assumptions embedded in the dose-response models (e.g., heterogeneity in susceptibility) are fully considered, as are the likely patterns of long-term exposure (e.g., temporal correlations within individuals and heterogeneity of mean exposures). Two types of long-term risk are described, risk per person-time and risk per individual where the latter is the risk of infection at least once. The effects on these risks of heterogeneity in individuals susceptibilities and mean exposures and of temporal correlations of exposures are described, both theoretically and empirically using a sample of experimental data sets. Because different models with equal plausibility may give very different results in the low-dose range but fit the experimental data equally well, we apply the model uncertainty algorithm of Buckland et al. (1997) on example data sets. Finally, the computational aspects of the general problem, which are often challenging, are discussed along with the conditions under which simplifying approximations may be utilized.     

Beta-normal distribution in dose–response modeling and risk assessment for quantitative responses

To establish allowable daily intakes for humans from animal bioassay experiments, benchmark doses corresponding to low levels of risk have been proposed to replace the no-observed-adverse-effect level for non-cancer endpoints. When the experimental outcomes are quantal, each animal can be classified with or without the disease. The proportion of affected animals is observed as a function of dose and calculation of the benchmark dose is relatively simple. For quantitative responses, on the other hand, one method is to convert the continuous data to quantal data and proceed with benchmark dose estimation. Another method which has found more popularity (Crump, Risk Anal 15:79–89; 1995) is to fit an appropriate dose–response model to the continuous data, and directly estimate the risk and benchmark doses. The normal distribution has often been used in the past as a dose–response model. However, for non-symmetric data, the normal distribution can lead to erroneous results. Here, we propose the use of the class of beta-normal distribution and demonstrate its application in risk assessment for quantitative responses. The most important feature of this class of distributions is its generality, encompassing a wide range of distributional shapes including the normal distribution as a special case. The properties of the model are briefly discussed and risk estimates are derived based on the asymptotic properties of the maximum likelihood estimates. An example is used for illustration.

Growth,reproduction and mortality of the sea hare Dolabella auricularia (Gastropoda: Aplysiidae) in the Central Visayas,Philippines

The growth parameters W and K of the von Bertalanffy growth equation were estimated from size-frequency data of the sea hare Dolabella auricularia using an objective, computer-based method. The results obtained from two locations in the central Philippines were comparable, although based on only a few individuals; the means for both sites were W=493 g and K=0.9 (yearly basis). A value of Z=3.66, corresponding to an annual survival rate of 2.6%, was estimated for the juveniles and adults from a length-converted catch curve. Spawning and recruitment were found to occur throughout the year with peaks in May to July and September to October.ICLARM Contribution No. 170     

Simulation of herbicide impacts on a plant community: comparing model predictions of the plant community model IBC-grass to empirical data

Background

Semi-natural plant communities such as field boundaries play an important ecological role in agricultural landscapes, e.g., provision of refuge for plant and other species, food web support or habitat connectivity. To prevent undesired effects of herbicide applications on these communities and their structure, the registration and application are regulated by risk assessment schemes in many industrialized countries. Standardized individual-level greenhouse experiments are conducted on a selection of crop and wild plant species to characterize the effects of herbicide loads potentially reaching off-field areas on non-target plants. Uncertainties regarding the protectiveness of such approaches to risk assessment might be addressed by assessment factors that are often under discussion. As an alternative approach, plant community models can be used to predict potential effects on plant communities of interest based on extrapolation of the individual-level effects measured in the standardized greenhouse experiments. In this study, we analyzed the reliability and adequacy of the plant community model IBC-grass (individual-based plant community model for grasslands) by comparing model predictions with empirically measured effects at the plant community level.

Results

We showed that the effects predicted by the model IBC-grass were in accordance with the empirical data. Based on the species-specific dose responses (calculated from empirical effects in monocultures measured 4 weeks after application), the model was able to realistically predict short-term herbicide impacts on communities when compared to empirical data.

Conclusion

The results presented in this study demonstrate an approach how the current standard greenhouse experiments—measuring herbicide impacts on individual-level—can be coupled with the model IBC-grass to estimate effects on plant community level. In this way, it can be used as a tool in ecological risk assessment.

     

Grazing and ingestion rates of nauplii,copepodids and adults of the marine planktonic copepod Calanus helgolandicus

The average grazing and ingestion rates of all stages of the marine planktonic copepod Calanus helgolandicus (Calanoida) from nauplius stage IV to adults were measured experimentally at 15°C in agitated cultures. The chain-forming diatom Lauderia borealis and the unarmoured dinoflagellate Gymnodinium splendens were offered as food. The food concentrations were close to natural conditions and ranged from 36 to 101 g of organic carbon per liter. The medium body weights expressed in g of organic carbon of almost all larval stages raised at 49 g C/1 were identical with the weight of the same stages caught in the Pacific Ocean off La Jolla, California, USA. In a log-log system, grazing and ingestion rates increased almost linearly with increasing body weight. Grazing rates ranged from 4 to 21 ml/day/nauplius stage IV to 286 ml to 773 ml/day/female. Ingestion rates increased from 0.2 g to 0.8 g C/day/nauplius stage IV to 18 g to 69 g C/day/female. Grazing and ingestion rates per unit body weight decreased gradually with increasing body weight. The daily ingested amount of food decreased from 292 to 481% of the body weight (g C) of nauplius stage V to 28–85% of the body weight of adult females. Grazing and ingestion performances of all stages increased with increasing particle size. Grazing rates decreased and ingestion rates increased with increasing food concentrations. The published data on food intake of the different age groups of C. helgolandicus show that the young stages of herbivorous planktonic copepods can play a major part in the consumption of phytoplankton in the sea due to their high grazing and ingestion rates.     

Do kin always make better neighbours?: The effects of territory quality

Summary The effects of territory quality on kin-biased territorial defence behaviour and fitness of juvenile rainbow trout (Oncorhynchus mykiss) were examined by manipulating food and predation risk levels in an artificial stream channel. Groups of related (full sibling) or unrelated fry were observed in the channel under one of four treatments: (1) high food-low predator (high territory quality); (2) high food-high predator; (3) low food-low predator; and (4) low food-high predator (low territory quality). Fish within kin groups always initiated fewer aggressive interactions, defended smaller territories, exhibited higher proportions of threat type territorial defence behaviours (as opposed to overtly aggressive behaviours) and had a higher mean weight increase than non-kin groups. Within both kin and non-kin groups, decreasing territory quality significantly increased the frequency of aggressive interactions, the size of territories and decreased growth. It is argued that while kin-biased territorial defence behaviours are always present, decreased availability of resources and increased predation risk serve to reduce the magnitude of this kin bias. However, even in low-quality territories, the benefit of associating with kin may increase the probability of overwintering survival (increased fitness) of the young fish by increasing body size.     

The effect of male calling on female locomotor activity of Teleogryllus commodus

Summary The calling song of the -cricket, Teleogryllus commodus, attracts for mating. Both calling and -locomotor activity are under circadian control and the behaviors occur approximately at the same time. Daily playback of calling songs for 3–12 h in constant light failed to entrain -running. Instead, calling acts as an external stimulus which can release -locomotion at any time of the 24-h period, while the clock-induced activity continues to run freely. The stimulatory effect of the calling song causes flexible extension of -locomotor activity, but a subsequent restorative period is necessary. The ecological consequences of acoustical stimulation are discussed.     

13C/12C ratios and the trophic importance of algae in Florida Syringodium filiforme seagrass meadows

Over 380 stable carbon isotope (¹³C) analyses made during 1981–82 showed that Syringodium filiforme Kutz seagrass meadows in the Indian River lagoon of eastern Florida have food webs based on algal rather than seagrass carbon. Seagrasses averaging approximately-8 were isotopically distinct from algae epiphytic on seagrass blades (X=-19.3) and particulate organic matter in the water column X=-21.6. ¹³C values of most fauna ranged between-16 and-22, as would be expected if food web carbon were derived solely from algal sources. These results counter the idea that seagrass detritus is the dominant carbon source in seagrass ecosystems. Two factors that may contribute to the low apparent importance of seagrass in the study area are high algal productivities that equal or exceed S. filiforme productivity and the high rates of seagrass leaf export from meadows.     

Functional reference in an alarm signal given during nest defence: seet calls of yellow warblers denote brood-parasitic brown-headed cowbirds

Field observations and model-presentation experiments have shown that yellow warblers (Dendroica petechia) produce seet calls preferentially in response to brood-parasitic brown-headed cowbirds (Molothrus ater). In this study, we investigated whether seet calls are functionally referential alarm calls denoting cowbirds by determining whether female warblers responded appropriately to seet calls in the absence of a cowbird, whether alarm calling by warblers varied with response urgency, and how warblers in a population allopatric with cowbirds responded to cowbird and avian predator models and seet playbacks. As a control, we presented chip calls, which are elicited by nest predators as well as by non-threatening intruders, but are not strongly associated with cowbirds. Yellow warblers responded differently to playbacks of seet than chip calls. To seet playbacks, almost 60% of females gave seet calls and rushed to sit in their nests, responses typically elicited by cowbirds, whereas these responses were given infrequently in response to chip calls. Yellow warblers seet called equally in situations that simulated low, medium and high risk of parasitism, which suggests that these calls did not vary with response urgency. In a population allopatric with cowbirds, seet calls were rarely produced in response to cowbird or avian nest predator models and never to seet playbacks. These results suggest that seet calls are functionally referential signals denoting cowbirds and that cowbird parasitism was a strong selective pressure in the evolution of functional referentiality in the seet call of yellow warblers.Communicated by W.A. Searcy     

Social foraging in honey bees: how nectar foragers assess their colony's nutritional status

Summary A honey bee colony operates as a tightly integrated unit of behavioral action. One manifestation of this in the context of foraging is a colony's ability to adjust its selectivity among nectar sources in relation to its nutritional status. When a colony's food situation is good, it exploits only highly profitable patches of flowers, but when its situation is poor, a colony's foragers will exploit both highly profitable and less profitable flower patches. The nectar foragers in a colony acquire information about their colony's nutritional status by noting the difficulty of finding food storer bees to receive their nectar, rather than by evaluating directly the variables determining their colony's food situation: rate of nectar intake and amount of empty storage comb. (The food storer bees in a colony are the bees that collect nectar from returning foragers and store it in the honey combs. They are the age group (generally 12–18 day old bees) that is older than the nurse bees but younger than the foragers. Food storers make up approximately 20% of a colony members.) The mathematical theory for the behavior of queues indicates that the waiting time experienced by nectar foragers before unloading to food storers (queue length) is a reliable and sensitive indicator of a colony's nutritional status. Queue length is automatically determined by the ratio of two rates which are directly related to a colony's nutritional condition: the rate of arrival of loaded nectar foragers at the hive (arrival rate) and the rate of arrival of empty food storers at the nectar delivery area (service rate). These two rates are a function of the colony's nectar intake rate and its empty comb area, respectively. Although waiting time conveys crucial information about the colony's nutritional status, it has not been molded by natural selection to serve this purpose. Unlike signals, which are evolved specifically to convey information, this cue conveys information as an automatic by-product. Such cues may prove more important than signals in colony integration.     

Particle capture in the musselMytilus edulis: The role of latero-frontal cirri

Microscope video graphs of particle paths near one-filament-thick mussel gill preparations, stimulated with a nerve transmitter (10^–6 M serotonin which restores normal ciliary activity), were used to disclose the capture of 6 m algal cells. Suspended algal cells carried with the water were stopped for a while at the entrance to the interfilament gap by the action of the latero-frontal cirri (Ifc), and transferred to the frontal side of the filament to be transported towards the marginal food groove. The event of transfer took place during approximately a time interval of 1150 to 1/25 s. To gain a better understanding of the capture mechanism and retention efficiency versus particle size, the flow through and around the Ifc was theoretically estimated. Normally beating Ifc create periodic, unsteady, three-dimensional flows at the entrance to the interfilament canal. During the active beat most of the water is deflected to flow around the branching cilia of the Ifc while some of the water is strained by these. Large particles (> 4 m) are stopped and transferred to the frontal current, whereas smaller particles either follow the flow around the Ifc and escape or they are stopped by the branching cilia.     

Comparing model averaging with other model selection strategies for benchmark dose estimation

Model averaging (MA) has been proposed as a method of accommodating model uncertainty when estimating risk. Although the use of MA is inherently appealing, little is known about its performance using general modeling conditions. We investigate the use of MA for estimating excess risk using a Monte Carlo simulation. Dichotomous response data are simulated under various assumed underlying dose–response curves, and nine dose–response models (from the USEPA Benchmark dose model suite) are fit to obtain both model specific and MA risk estimates. The benchmark dose estimates (BMDs) from the MA method, as well as estimates from other commonly selected models, e.g., best fitting model or the model resulting in the smallest BMD, are compared to the true benchmark dose value to better understand both bias and coverage behavior in the estimation procedure. The MA method has a small bias when estimating the BMD that is similar to the bias of BMD estimates derived from the assumed model. Further, when a broader range of models are included in the family of models considered in the MA process, the lower bound estimate provided coverage close to the nominal level, which is superior to the other strategies considered. This approach provides an alternative method for risk managers to estimate risk while incorporating model uncertainty.

Lead in a residential environment in Jamaica

The background levels of lead in Jamaica in soils and sediments, estimated at 37 mg kg^–1, are relatively high compared with world averages. Several areas have values in excess of this due to mineralisation and pollution. One such is the residential Hope Flats/Kintyre area in which levels of lead up to 2.5% are found in the soils and up to 8 g kg^–1 in the water of the nearby Hope River. The blood lead levels of a sample of children were in the range 5.7–57 g dl^–1. The high lead levels suggest a potential health risk, particularly for the children. This can be minimised by programmes which include community education, case management and abatement to reduce the lead exposure.     

In-situ studies on the life cycle of Diastylis rathkei (Cumacea: Crustacea)

Aspects of the life cycle of the cumacean Diastylis rathkei Kröyer, the most important food item of demersal fishes in the Western Baltic Sea, were investigated from the Underwater Laboratory (UWL) Helgoland in Lübeck Bay. In August the population density was 10.1±3.6 individuals 100 cm^-2 in fine-medium sand (Md=292 m), and 3.6±2.4 individuals in coarser medium sand (Md=470 m). In both substrata, abundance values decreased until October, at each station to a different extent. Particular attention was paid to vertical migrations of D. rathkei. Plankton catches and numerous direct observations provided evidence that the pelagic phase, observed at night, is always combined with ecdysis. During short periods of high swarming and moulting activities, the population structure changed considerably: juveniles of the 3rd stage dominated in early August; thereafter they were replaced by prematures, the last juvenile stage. The first mature individuals were observed in October. Some data are given on growth rate.     

Selective particle ingestion by oyster larvae (Crassostrea virginica) feeding on natural seston and cultured algae

I investigated selective particle ingestion by oyster larvae (Crassostrea virginica) feeding on natural seston from Chesapeake Bay and laboratory-cultured algae of different sizes or chemical content. In 15 of 16 experiments with complex natural suspensions as food, small(<150 m) and large (>150 m) larvae selected most strongly for small (2 to 4 m) food particles, but in the presence of a large (>10 m)-cell dinoflagellate bloom, large larvae strongly selected much larger (22 to 30 m) food material (presumably dinoflagellates). When fed simplified mixtures of four cultured algal species (Synechococcus bacillaris, Isochrysis sp., Dunaliella tertiolecta, and Prorocentrum minimum) ranging in size from 1 to 11 m, small larvae preferred 1 m algae while large larvae preferred 11 m algae. In experiments with algal mixtures, and with suspensions of natural particles and added algae, large larvae preferred algal species harvested from exponential-phase cultures over other species from stationary-phase cultures. Larval ingestion rates of the cultured alga Thalassiosira pseudonana were about three times higher for cells with a low carbon:nitrogen ratio (7.2:1) than for high C:N ratio (16.2:1) cells when these cells were offered separately in suspensions of equal concentration. As a result, more algal cells, algal C, and algal N was ingested by larvae fed low C:N cells. However, larvae did not show a significant preference for either type of cell when they were offered in a 1:1 cell mixture. Feeding patterns of C. virginica larvae in natural food suspensions can vary with the composition of these complex suspensions, and ingestion seems dependent not only on the size, but on the growth rate and chemical quality of food particles.     

Efficient size-selective bacterivory by phagotrophic nanoflagellates in aquatic ecosystems

Size-selective grazing on bacterioplankton by phagotrophic nanoflagellates was analyzed and modelled. The proposed model resembles a Monod equation (Michaelis-Menten-like expression) in which clearance rates by heterotrophic nanoflagellates depend on bacterioplnnkton biovolume. Larger bacteria were ingested faster than smaller bacteria. The proposed model was in agreement with experimental data from different authors, both from cultures and natural assemblages. Size-selective grazing efficiency was analyzed as the ability of phagotrophic nanoflagellates to discriminate between cells differing in volume by a factor of two, e.g., corresponding to dividing and nondividing bacteria. Unlike previously published models, the proposed model suggests that phagotrophic nanoflagellates could be highly effective size-selective grazers for small bacteria (<0.1 m³), which are the most common bacterial sizes in planktonic systems. However, phagotrophic nanoflagellates were unable to size-discriminate dividing and nondividing bacteria for volumes >0.1 m³. These results strongly support the hypothesis that heterotrophic nanoflagellate grazing, by discriminating between different sizes of dividing and nondividing bacteria, may actually be regulating bacterial size and growth rate in natural aquatic ecosystems.     

Long-distance transport of 14C-labelled assimilates in the Fucales: directionality,pathway and velocity

A directed translocation of ¹⁴C-labelled photoassimilates was shown to occur in the thalli of six species of brown seaweeds belonging to the Fucales. The apical growing regions of these algae act as strong sinks in the source-tosink relationship of the long-distance transport. Physical interruptions of the thalli showed that the midrib is the main pathway of the assimilates in Fucus vesiculosus and F. serratus. ¹⁴C-histoautoradiography has shown that translocation takes place through the filamentous cells of the medulla. The translocation velocity of the labelled assimilates, calculated from time source profiles of the tracer, was estimated to be 2 to 4 cm · h^-1. It is suggested that the sieve elements of the Fucales act in a way similar to the conducting sieve elements of the Laminariales.     

Exposure assessment of heavy metals (Cd,Hg, and Pb) by the intake of local foods from Zhejiang,China

Considering the environmental pollution, food safety is of great concern to the consumers. The present study was conducted to assess the health risk of cadmium (Cd), mercury (Hg), and lead (Pb) through the dietary intake in Zhejiang, China. Eight hundred and sixty two food samples including aquatic products, meat, vegetables, milk and dairy products, and cereal grains were analyzed. Only 2.44 % (Cd), 1.39 % (Hg), and 1.51 % (Pb) of the samples exceeded the maximum allowable concentration set by Chinese Ministry of Health. The average dietary intakes of Cd, Hg, and Pb were estimated to be 0.26, 0.14, and 0.55 μg/kg bw/day, respectively. Compared with the reference doses, the mean exposure of Cd, Hg, and Pb was all less than the tolerable intake value. Only at the 95th percentile level, Cd and Hg exposure exceeded the values of tolerable intakes by 40 and 277 %, respectively. It indicates that there is low health risk to the dietary exposure of Cd, Hg, and Pb for general people in Zhejiang province, China.     

Population structure,food relations and ecological role of marine oligochaetes,with special reference to meiobenthic species

Data on abundance, biomass and biovolume demonstrate the significant ecological role of oligochaetes in the littoral marine benthos. Their numerical and productive importance is comparable to that of many other common meio-and macrofauna groups from various littoral areas. Oligochaetes often exhibit nutritional specialization (e.g. bacteria or diatoms attached to detritus or sand grains). Consequently, food supply can control their population structure and distribution. Few oligochaetes are, apparently, consumed by predators. Hence, only a small portion of their biomass is transferred to higher trophic levels, while the main part is decomposed directly. Most oligochaetes seem to represent final links of rather short food chains. Ecologically, marine oligochaetes attain major importance only in littoral areas.This investigation was supported by the Sonderforschungsbereich 94 (Meeresforschung) at the University of Hamburg.     

A single-chain-based multidimensional Markov chain model for subsurface characterization

Multidimensional Markov chain models in geosciences were often built on multiple chains, one in each direction, and assumed these 1-D chains to be independent of each other. Thus, unwanted transitions (i.e., transitions of multiple chains to the same location with unequal states) inevitably occur and have to be excluded in estimating the states at unobserved locations. This consequently may result in unreliable estimates, such as underestimation of small classes (i.e., classes with smaller than average areas) in simulated realizations. This paper presents a single-chain-based multidimensional Markov chain model for estimation (i.e., prediction and conditional stochastic simulation) of spatial distribution of subsurface formations with borehole data. The model assumes that a single Markov chain moves in a lattice space, interacting with its nearest known neighbors through different transition probability rules in different cardinal directions. The conditional probability distribution of the Markov chain at the location to be estimated is formulated in an explicit form by following the Bayes’ Theorem and the conditional independence of sparse data in cardinal directions. Since no unwanted transitions are involved, the model can estimate all classes fairly. Transiogram models (i.e., 1-D continuous Markov transition probability diagrams) are used to provide transition probability input with needed lags to generalize the model. Therefore, conditional simulation can be conducted directly and efficiently. The model provides an alternative for heterogeneity characterization of subsurface formations.