Small area disease mapping of cancer incidence in British Columbia using Bayesian spatial models and the smallareamapp R Package.

Introduction: There is an increasing interest in small area analyses in cancer surveillance; however, technical capacity is limited and accessible analytical approaches remain to be determined. This study demonstrates an accessible approach for small area cancer risk estimation using Bayesian hierarchical models and data visualization through the smallareamapp R package. Materials and methods: Incident lung (N = 26,448), female breast (N = 28,466), cervical (N = 1,478), and colorectal (N = 25,457) cancers diagnosed among British Columbia (BC) residents between 2011 and 2018 were obtained from the BC Cancer Registry. Indirect age-standardization was used to derive age-adjusted expected counts and standardized incidence ratios (SIRs) relative to provincial rates. Moran's I was used to assess the strength and direction of spatial autocorrelation. A modified Besag, York and Mollie model (BYM2) was used for model incidence counts to calculate posterior median relative risks (RR) by Community Health Service Areas (CHSA; N = 218), adjusting for spatial dependencies. Integrated Nested Laplace Approximation (INLA) was used for Bayesian model implementation. Areas with exceedance probabilities (above a threshold RR = 1.1) greater or equal to 80% were considered to have an elevated risk. The posterior median and 95% credible intervals (CrI) for the spatially structured effect were reported. Predictive posterior checks were conducted through predictive integral transformation values and observed versus fitted values. Results: The proportion of variance in the RR explained by a spatial effect ranged from 4.4% (male colorectal) to 19.2% (female breast). Lung cancer showed the greatest number of CHSAs with elevated risk (Nwomen = 50/218, Nmen = 44/218), representing 2357 total excess cases. The largest lung cancer RRs were 1.67 (95% CrI = 1.06-2.50; exceedance probability = 96%; cases = 13) among women and 2.49 (95% CrI = 2.14-2.88; exceedance probability = 100%; cases = 174) among men. Areas with small population sizes and extreme SIRs were generally smoothed towards the null (RR = 1.0). Discussion: We present a ready-to-use approach for small area cancer risk estimation and disease mapping using BYM2 and exceedance probabilities. We developed the smallareamapp R package, which provides a user-friendly interface through an R-Shiny application, for epidemiologists and surveillance experts to examine geographic variation in risk. These methods and tools can be used to estimate risk, generate hypotheses, and examine ecologic associations while adjusting for spatial dependency.

Social Contacts and Transmission of COVID-19 in British Columbia, Canada.

Background: Close-contact rates are thought to be a driving force behind the transmission of many infectious respiratory diseases. Yet, contact rates and their relation to transmission and the impact of control measures, are seldom quantified. We quantify the response of contact rates, reported cases and transmission of COVID-19, to public health contact-restriction orders, and examine the associations among these three variables in the province of British Columbia, Canada. Methods: We derived time series data for contact rates, daily cases and transmission of COVID-19 from a social contacts survey, reported case counts and by fitting a transmission model to reported cases, respectively. We used segmented regression to investigate impacts of public health orders; Pearson correlation to determine associations between contact rates and transmission; and vector autoregressive modeling to quantify lagged associations between contacts rates, daily cases, and transmission. Results: Declines in contact rates and transmission occurred concurrently with the announcement of public health orders, whereas declines in cases showed a reporting delay of about 2 weeks. Contact rates were a significant driver of COVID-19 and explained roughly 19 and 20% of the variation in new cases and transmission, respectively. Interestingly, increases in COVID-19 transmission and cases were followed by reduced contact rates: overall, daily cases explained about 10% of the variation in subsequent contact rates. Conclusion: We showed that close-contact rates were a significant time-series driver of transmission and ultimately of reported cases of COVID-19 in British Columbia, Canada and that they varied in response to public health orders. Our results also suggest possible behavioral feedback, by which increased reported cases lead to reduced subsequent contact rates. Our findings help to explain and validate the commonly assumed, but rarely measured, response of close contact rates to public health guidelines and their impact on the dynamics of infectious diseases.

Quantifying transmissibility of SARS-CoV-2 and impact of intervention within long-term healthcare facilities.

Estimates of the basic reproduction number (R 0) for COVID-19 are particularly variable in the context of transmission within locations such as long-term healthcare (LTHC) facilities. We sought to characterize the heterogeneity of R 0 across known outbreaks within these facilities. We used a unique comprehensive dataset of all outbreaks that occurred within LTHC facilities in British Columbia, Canada as of 21 September 2020. We estimated R 0 in 18 LTHC outbreaks with a novel Bayesian hierarchical dynamic model of susceptible, exposed, infected and recovered individuals, incorporating heterogeneity of R 0 between facilities. We further compared these estimates to those obtained with standard methods that use the exponential growth rate and maximum likelihood. The total size of outbreaks varied dramatically, with range of attack rates 2%-86%. The Bayesian analysis provided an overall estimate of R 0 = 2.51 (90% credible interval 0.47-9.0), with individual facility estimates ranging between 0.56 and 9.17. Uncertainty in these estimates was more constrained than standard methods, particularly for smaller outbreaks informed by the population-level model. We further estimated that intervention led to 61% (52%-69%) of all potential cases being averted within the LTHC facilities, or 75% (68%-79%) when using a model with multi-level intervention effect. Understanding of transmission risks and impact of intervention are essential in planning during the ongoing global pandemic, particularly in high-risk environments such as LTHC facilities.

How much leeway is there to relax COVID-19 control measures?

Following successful non-pharmaceutical interventions (NPI) aiming to control COVID-19, many jurisdictions reopened their economies and borders. As little immunity had developed in most populations, re-establishing higher contact carried substantial risks, and therefore many locations began to see resurgence in COVID-19 cases. We present a Bayesian method to estimate the leeway to reopen, or alternatively the strength of change required to re-establish COVID-19 control, in a range of jurisdictions experiencing different COVID-19 epidemics. We estimated the timing and strength of initial control measures such as widespread distancing and compared the leeway jurisdictions had to reopen immediately after NPI measures to later estimates of leeway. Finally, we quantified risks associated with reopening and the likely burden of new cases due to introductions from other jurisdictions. We found widely varying leeway to reopen. After initial NPI measures took effect, some jurisdictions had substantial leeway (e.g., Japan, New Zealand, Germany) with > 0.99 probability that contact rates were below 80% of the threshold for epidemic growth. Others had little leeway (e.g., the United Kingdom, Washington State) and some had none (e.g., Sweden, California). For most such regions, increases in contact rate of 1.5-2 fold would have had high (> 0.7) probability of exceeding past peak sizes. Most jurisdictions experienced June-August trajectories consistent with our projections of contact rate increases of 1-2-fold. Under such relaxation scenarios for some regions, we projected up to ∼100 additional cases if just one case were imported per week over six weeks, even between jurisdictions with comparable COVID-19 risk. We provide an R package covidseir to enable jurisdictions to estimate leeway and forecast cases under different future contact patterns. Estimates of leeway can establish a quantitative basis for decisions about reopening. We recommend a cautious approach to reopening economies and borders, coupled with strong monitoring for changes in transmission.

Evaluation of a multisectoral intervention to mitigate the risk of severe acute respiratory coronavirus virus 2 (SARS-CoV-2) transmission in long-term care facilities.

OBJECTIVE: A Canadian health authority implemented a multisectoral intervention designed to control severe acute respiratory coronavirus virus 2 (SARS-CoV-2) transmission during long-term care facility (LTCF) outbreaks. The primary objective was to evaluate the effectiveness of the intervention 14 days after implementation. DESIGN: Quasi-experimental, segmented regression analysis. INTERVENTION: A series of outbreak measures classified into 4 categories: case and contact management, proactive case detection, rigorous infection control practices and resource prioritization and stewardship. METHODS: A mixed-effects segmented Poisson regression model was fitted to the incidence rate of coronavirus disease 2019 (COVID-19), calculated every 2 days, within each facility and case type (staff vs residents). For each facility, the outbreak time period was segmented into an early outbreak period (within 14 days of the intervention) and postintervention period (beyond 14 days following the intervention). Model outputs quantified COVID-19 incidence trend and rate changes between these 2 periods. A secondary model was constructed to identify effect modification by case type. RESULTS: The significant upward trend in COVID-19 incidence rate during the early outbreak period (rate ratio [RR], 1.07; 95% confidence interval [CI], 1.03-1.11; P < .001) reversed during the postintervention period (RR, 0.73; 95% CI, 0.67-0.80; P < .001). The average trend did not differ by case type during the early outbreak period (P > .05) or the postintervention period (P > .05). However, staff had a 70% larger decrease in the average rate of COVID-19 during the postintervention period than residents (RR, 0.30; 95% CI, 0.10-0.88; P < .05). CONCLUSIONS: Our study provides evidence for the effectiveness of this intervention to reduce the transmission of COVID-19 in LTCFs. This intervention can be adapted and utilized by other jurisdictions to protect the vulnerable individuals in LTCFs.

Quantifying the impact of COVID-19 control measures using a Bayesian model of physical distancing.

Extensive non-pharmaceutical and physical distancing measures are currently the primary interventions against coronavirus disease 2019 (COVID-19) worldwide. It is therefore urgent to estimate the impact such measures are having. We introduce a Bayesian epidemiological model in which a proportion of individuals are willing and able to participate in distancing, with the timing of distancing measures informed by survey data on attitudes to distancing and COVID-19. We fit our model to reported COVID-19 cases in British Columbia (BC), Canada, and five other jurisdictions, using an observation model that accounts for both underestimation and the delay between symptom onset and reporting. We estimated the impact that physical distancing (social distancing) has had on the contact rate and examined the projected impact of relaxing distancing measures. We found that, as of April 11 2020, distancing had a strong impact in BC, consistent with declines in reported cases and in hospitalization and intensive care unit numbers; individuals practising physical distancing experienced approximately 0.22 (0.11-0.34 90% CI [credible interval]) of their normal contact rate. The threshold above which prevalence was expected to grow was 0.55. We define the "contact ratio" to be the ratio of the estimated contact rate to the threshold rate at which cases are expected to grow; we estimated this contact ratio to be 0.40 (0.19-0.60) in BC. We developed an R package 'covidseir' to make our model available, and used it to quantify the impact of distancing in five additional jurisdictions. As of May 7, 2020, we estimated that New Zealand was well below its threshold value (contact ratio of 0.22 [0.11-0.34]), New York (0.60 [0.43-0.74]), Washington (0.84 [0.79-0.90]) and Florida (0.86 [0.76-0.96]) were progressively closer to theirs yet still below, but California (1.15 [1.07-1.23]) was above its threshold overall, with cases still rising. Accordingly, we found that BC, New Zealand, and New York may have had more room to relax distancing measures than the other jurisdictions, though this would need to be done cautiously and with total case volumes in mind. Our projections indicate that intermittent distancing measures-if sufficiently strong and robustly followed-could control COVID-19 transmission. This approach provides a useful tool for jurisdictions to monitor and assess current levels of distancing relative to their threshold, which will continue to be essential through subsequent waves of this pandemic.

Current State of Geospatial Methodologic Approaches in Canadian Population Oncology Research.

Geospatial analyses are increasingly used in population oncology. We provide a first review of geospatial analysis in Canadian population oncology research, compare to international peers, and identify future directions. Geospatial-focused peer-reviewed publications from 1992-2020 were compiled using PubMed, MEDLINE, Web of Science, and Google Scholar. Abstracts were screened for data derived from a Canadian cancer registry and use of geographic information systems. Studies were classified by geospatial methodology, geospatial unit, location, cancer site, and study year. Common limitations were documented from article discussion sections. Our search identified 71 publications using data from all provincial and national cancer registries. Thirty-nine percent (N = 28) were published in the most recent 5-year period (2016-2020). Geospatial methodologies included exposure assessment (32.4%), identifying spatial associations (21.1%), proximity analysis (16.9%), cluster detection (15.5%), and descriptive mapping (14.1%). Common limitations included confounding, ecologic fallacy, not accounting for residential mobility, and small case/population sizes. Geospatial analyses are increasingly used in Canadian population oncology; however, efforts are concentrated among a few provinces and common cancer sites, and data are over a decade old. Limitations were similar to those documented internationally, and more work is needed to address them. Organized efforts are needed to identify common challenges, develop leading practices, and identify shared priorities.

Patterns of health care utilization among people who overdosed from illegal drugs: a descriptive analysis using the BC Provincial Overdose Cohort. / Habitudes d'utilisation des soins de santé chez les personnes ayant fait une surdose de drogues illicites : analyse descriptive fondée sur la Cohorte provinciale des victimes de surdoses de la Colombie-Britannique.

INTRODUCTION: British Columbia (BC) declared a public health emergency in April 2016 in response to a rapid rise in overdose deaths. Further understanding of health care utilization is needed to inform prevention strategies for individuals who overdose from illegal drugs. METHODS: The Provincial Overdose Cohort includes linked administrative data on health care utilization by individuals who experienced an illegal drug overdose event in BC between 1 January 2015 and 30 November 2016. Overdose cases were identified using data from ambulance services, coroners' investigations, poison control centre calls and hospital, emergency department and physician administrative records. In total, 10 455 overdose cases were identified and compared with 52 275 controls matched on age, sex and area of residence for a descriptive analysis of health care utilization. RESULTS: Two-thirds (66%) of overdose cases were male and about half (49%) were 20-39 years old. Over half of the cases (54%) visited the emergency department and about one-quarter (26%) were admitted to hospital in the year before the overdose event, compared with 17% and 9% of controls, respectively. Nevertheless, nearly onefifth (19%) of cases were recorded leaving the emergency department without being seen or against medical advice. High proportions of both cases (75%) and controls (72%) visited community-based physicians. Substance use and mental health-related concerns were the most common diagnoses among people who went on to overdose. CONCLUSION: People who overdosed frequently accessed the health care system in the year before the overdose event. In light of the high rates of health care use, there may be opportunities to identify at-risk individuals before they overdose and connect them with targeted programs and evidence-based interventions. Further work using the BC Provincial Overdose Cohort will focus on identifying risk factors for overdose events and death by overdose.

INTRODUCTION: La Colombie-Britannique (C.-B.) a déclaré un état d'urgence en santé publique en avril 2016 en réaction à une augmentation rapide du nombre de décès par surdose. Une meilleure compréhension de l'utilisation des soins de santé est nécessaire pour guider les stratégies de prévention pour les personnes qui font une surdose de drogues illicites. MÉTHODOLOGIE: La cohorte provinciale des victimes de surdoses comprend des données administratives couplées sur l'utilisation des soins de santé par les personnes qui ont été victimes d'une surdose de drogues illicites en Colombie-Britannique entre le 1er janvier 2015 et le 30 novembre 2016. Les cas de surdose ont été relevés à l'aide de données provenant des services ambulanciers, des enquêtes des coroners, des appels aux centres antipoison et des dossiers administratifs des hôpitaux, des services d'urgence et des médecins. Au total, 10 455 cas de surdose ont été recensés et comparés à 52 275 témoins appariés selon l'âge, le sexe et la zone de résidence en vue d'une analyse descriptive de l'utilisation des soins de santé. RÉSULTATS: Les deux tiers (66 %) des cas de surdose concernaient des hommes, et environ la moitié (49 %) les 20 à 39 ans. Plus de la moitié des cas (54 %) se sont rendus au service d'urgence, et environ le quart (26 %) ont été admis à l'hôpital au cours de l'année précédant la surdose, comparativement à respectivement 17 % et 9 % des témoins. Cependant, près d'un cinquième (19 %) des cas ont été enregistrés comme ayant quitté le service d'urgence sans avoir été vus par le médecin ou contre son avis. Des proportions élevées de cas (75 %) et de témoins (72 %) ont consulté un médecin en milieu communautaire. La consommation de substances et des problèmes en santé mentale ont été les diagnostics les plus courants chez les personnes qui ont fait une surdose. CONCLUSION: Les personnes qui ont fait une surdose ont souvent eu accès au système de soins de santé au cours de l'année précédant la surdose. Compte tenu de ces taux élevés d'utilisation des soins de santé, on pourrait peut-être repérer les personnes à risque avant qu'elles ne fassent de surdose et les aiguiller vers des programmes ciblés et des interventions fondées sur des données probantes. Il est prévu d'utiliser la cohorte provinciale des victimes de surdoses de la Colombie-Britannique pour déterminer les facteurs de risque relatifs aux surdoses et aux décès par surdose.

Impact of a COPD comprehensive case management program on hospital length of stay and readmission rates.

BACKGROUND: COPD accounts for the highest rate of hospital admissions among major chronic diseases. COPD hospitalizations are associated with impaired quality of life, high health care utilization, and poor prognosis and result in an economic and a social burden that is both substantial and increasing. AIM: The aim of this study is to determine the efficacy of a comprehensive case management program (CCMP) in reducing length of stay (LOS) and risk of hospital admissions and readmissions in patients with COPD. MATERIALS AND METHODOLOGY: We retrospectively compared outcomes across five large hospitals in Vancouver, BC, Canada, following the implementation of a systems approach to the management of COPD patients who were identified in the hospital and followed up in the community for 90 days. We compared numbers, rates, and intervals of readmission and LOS during 2 years of active program delivery compared to 1 year prior to program implementation. RESULTS: A total of 1,564 patients with a clinical diagnosis of COPD were identified from 2,719 hospital admissions during the 3 years of study. The disease management program reduced COPD-related hospitalizations by 30% and hospitalizations for all causes by 13.6%. Similarly, the rate of readmission for all causes showed a significant decline, with hazard ratios (HRs) of 0.55 (year 1) and 0.51 (year 2) of intervention (P<0.001). In addition, patients' mean LOS (days) for COPD-related admissions declined significantly from 10.8 to 6.8 (P<0.05). CONCLUSION: A comprehensive disease management program for COPD patients, including education, case management, and follow-up, was associated with significant reduction in hospital admissions and LOS.

Illicit drug overdose deaths resulting from income assistance payments: Analysis of the 'check effect' using daily mortality data.

BACKGROUND: Although income assistance is an important source of support for low income individuals, there is evidence that adverse outcomes may increase when payments are disbursed on the same day for all recipients. The objective of this study was to assess the temporal patterns and causal relation between population-level illicit drug overdose deaths and income assistance payments using daily mortality data for British Columbia over a period of five years. METHODS: Retrospective data on daily mortality due to illicit drug overdose, 2009-2013, were provided by the BC Coroners Service. These data were analyzed using regression models and time series tests for causality in relation to dates of income assistance payments. RESULTS: 1343 deaths due to illicit drug overdose were reported in BC during 2009-2013; 394 occurred during cheque weeks (n=60) and 949 occurred during non-cheque weeks (n=202). Average weekly mortality due to illicit drug overdose was 40% higher during weeks of income assistance payments compared to weeks without payments (P<0.001). Consistent increases in mortality appeared the day after cheque disbursement and were significantly higher for two days, and marginally higher after 3 days, even when controlling for other temporal trends. Granger causality testing suggests the timing of cheque issue was causally linked to increased drug overdose mortality (P<0.001). CONCLUSIONS: Our findings clarify the temporal relation and causal impact of income assistance payments on illicit drug deaths. We estimate 77 avoidable deaths were attributable to the synchronized disbursement of income assistance cheques over the five year period. An important consideration is whether varying the timing of payments among recipients could reduce this excess mortality and the related demands on health and social services.

Trends in incidence, mortality, and survival for kidney cancer in Canada, 1986-2007.

PURPOSE: Kidney cancer is one of the fastest rising cancers worldwide. We aimed to examine the trends in incidence, mortality, and survival for this cancer in Canada. METHODS: Incidence data for kidney cancer for 1986-2010 were from the Canadian Cancer Registry and the National Cancer Incidence Reporting System. These data were only available up to 2007 for the province of Quebec and consequently for the same year nationally, for Canada. Mortality data for 1986-2009 were from the Canadian Vital Statistics Death Database. Changes in age-standardized rates were analyzed by Joinpoint regression. Incidence rates were projected to 2025 using a Nordpred age-period-cohort model. Five-year relative survival ratios (RSR) were analyzed for 2004-2008 and earlier periods. RESULTS: Between 1986 and 2007, the age-standardized incidence rate (ASIR) per 100,000 rose from 13.4 to 17.9 in males and 7.7 to 10.3 in females. Annual increases in ASIR were greatest for age groups <65 years (males) and ≥65 years (females). The ASIRs increased significantly over time in both sexes for renal cell carcinoma (RCC) but not for other kidney cancer types. RCC rates are projected to increase until at least 2025. Mortality rates decreased only slightly in each sex since 1986 (0.4%/year in males; 0.8%/year in females). The 5-year RSR for kidney cancer was 68% but differed largely by morphology and age, and has increased slightly over time. CONCLUSIONS: The incidence rate of kidney cancer in Canada has risen since at least 1986, led largely by RCC. Increasing detection of incidental tumors, and growing obesity and hypertension rates are possible factors associated with this increase. Greater prevention of modifiable risk factors for kidney cancer is needed.

Esophageal cancer in Canada: trends according to morphology and anatomical location.

BACKGROUND: Esophageal adenocarcinoma has one of the fastest rising incidence rates and one of the lowest survival rates of any cancer type in the Western world. However, in many countries, trends in esophageal cancer differ according to tumour morphology and anatomical location. In Canada, incidence and survival trends for esophageal cancer subtypes are poorly known. METHODS: Cancer incidence and mortality rates were obtained from the Canadian Cancer Registry, the National Cancer Incidence Reporting System and the Canadian Vital Statistics Death databases for the period from 1986 to 2006. Observed trends (annual per cent change) and five-year relative survival ratios were estimated separately for esophageal adenocarcinoma and squamous cell carcinoma, and according to location (upper, middle, or lower one-third of the esophagus). Incidence rates were projected up to the year 2026. RESULTS: Annual age-standardized incidence rates for esophageal cancer in 2004 to 2006 were 6.1 and 1.7 per 100,000 for males and females, respectively. Esophageal adenocarcinoma incidence rose by 3.9% (males) and 3.6% (females) per year for the period 1986 to 2006, with the steepest increase in the lower one-third of the esophagus (4.8% and 5.0% per year among males and females, respectively). In contrast, squamous cell carcinoma incidence declined by 3.3% (males) and 3.2% (females) per year since the early 1990s. The five-year relative survival ratio for esophageal cancer was 13% between 2004 and 2006, approximately a 3% increase since the period from 1992 to 1994. Projected incidence rates showed increases of 40% to 50% for esophageal adenocarcinoma and decreases of 30% for squamous cell carcinoma by 2026. DISCUSSION: Although esophageal cancer is rare in Canada, the incidence of esophageal adenocarcinoma has doubled in the past 20 years, which may reflect the increasing prevalence of obesity and gastroesophageal reflux disease. Declines in squamous cell carcinoma may be the result of the decreases in the prevalence of smoking in Canada. Given the low survival rates and the potential for further increases in incidence, esophageal adenocarcinoma warrants close attention.

Consumption of a nectar alkaloid reduces pathogen load in bumble bees.

Diet has a significant effect on pathogen infections in animals and the consumption of secondary metabolites can either enhance or mitigate infection intensity. Secondary metabolites, which are commonly associated with herbivore defense, are also frequently found in floral nectar. One hypothesized function of this so-called toxic nectar is that it has antimicrobial properties, which may benefit insect pollinators by reducing the intensity of pathogen infections. We tested whether gelsemine, a nectar alkaloid of the bee-pollinated plant Gelsemium sempervirens, could reduce pathogen loads in bumble bees infected with the gut protozoan Crithidia bombi. In our first laboratory experiment, artificially infected bees consumed a daily diet of gelsemine post-infection to simulate continuous ingestion of alkaloid-rich nectar. In the second experiment, bees were inoculated with C. bombi cells that were pre-exposed to gelsemine, simulating the direct effects of nectar alkaloids on pathogen cells that are transmitted at flowers. Gelsemine significantly reduced the fecal intensity of C. bombi 7 days after infection when it was consumed continuously by infected bees, whereas direct exposure of the pathogen to gelsemine showed a non-significant trend toward reduced infection. Lighter pathogen loads may relieve bees from the behavioral impairments associated with the infection, thereby improving their foraging efficiency. If the collection of nectar secondary metabolites by pollinators is done as a means of self-medication, pollinators may selectively maintain secondary metabolites in the nectar of plants in natural populations.

Does pathogen spillover from commercially reared bumble bees threaten wild pollinators?

The conservation of insect pollinators is drawing attention because of reported declines in bee species and the 'ecosystem services' they provide. This issue has been brought to a head by recent devastating losses of honey bees throughout North America (so called, 'Colony Collapse Disorder'); yet, we still have little understanding of the cause(s) of bee declines. Wild bumble bees (Bombus spp.) have also suffered serious declines and circumstantial evidence suggests that pathogen 'spillover' from commercially reared bumble bees, which are used extensively to pollinate greenhouse crops, is a possible cause. We constructed a spatially explicit model of pathogen spillover in bumble bees and, using laboratory experiments and the literature, estimated parameter values for the spillover of Crithidia bombi, a destructive pathogen commonly found in commercial Bombus. We also monitored wild bumble bee populations near greenhouses for evidence of pathogen spillover, and compared the fit of our model to patterns of C. bombi infection observed in the field. Our model predicts that, during the first three months of spillover, transmission from commercial hives would infect up to 20% of wild bumble bees within 2 km of the greenhouse. However, a travelling wave of disease is predicted to form suddenly, infecting up to 35-100% of wild Bombus, and spread away from the greenhouse at a rate of 2 km/wk. In the field, although we did not observe a large epizootic wave of infection, the prevalences of C. bombi near greenhouses were consistent with our model. Indeed, we found that spillover has allowed C. bombi to invade several wild bumble bee species near greenhouses. Given the available evidence, it is likely that pathogen spillover from commercial bees is contributing to the ongoing decline of wild Bombus in North America. Improved management of domestic bees, for example by reducing their parasite loads and their overlap with wild congeners, could diminish or even eliminate pathogen spillover.

Contact networks and transmission of an intestinal pathogen in bumble bee (Bombus impatiens) colonies.

In socially living animals, individuals interact through complex networks of contact that may influence the spread of disease. Whereas traditional epidemiological models typically assume no social structure, network theory suggests that an individual's location in the network determines its risk of infection. Empirical, especially experimental, studies of disease spread on networks are lacking, however, largely due to a shortage of amenable study systems. We used automated video-tracking to quantify networks of physical contact among individuals within colonies of the social bumble bee Bombus impatiens. We explored the effects of network structure on pathogen transmission in naturally and artificially infected hives. We show for the first time that contact network structure determines the spread of a contagious pathogen (Crithidia bombi) in social insect colonies. Differences in rates of infection among colonies resulted largely from differences in network density among hives. Within colonies, a bee's rate of contact with infected nestmates emerged as the only significant predictor of infection risk. The activity of bees, in terms of their movement rates and division of labour (e.g., brood care, nest care, foraging), did not influence risk of infection. Our results suggest that contact networks may have an important influence on the transmission of pathogens in social insects and, possibly, other social animals.

Bumble-bee foragers infected by a gut parasite have an impaired ability to utilize floral information.

Parasitic infection can influence a variety of behavioural mechanisms in animals, but little is known about the effects of infection on the cognitive processes underlying ecologically relevant behaviours. Here, we examined whether parasitic infection alters cognitive aspects of foraging in a social insect, the bumble-bee (Bombus impatiens). In controlled experiments, we assessed the ability of foraging bees to discriminate rewarding from non-rewarding flowers on the basis of colour and odour. We found that natural and experimental infection by a protozoan parasite (Crithidia bombi, which lives exclusively within the gut tract), impaired the ability of foragers to learn the colour of rewarding flowers. Parasitic infection can thus disrupt central nervous system pathways that mediate cognitive processes in bumble-bees and as a consequence, can reduce their ability to monitor floral resources and make economic foraging decisions. It is postulated that this infection-induced change to cognitive function in bumble-bees is the result of communication between immune and nervous systems. Parasitized animals, including invertebrates, can therefore show subtle behavioural changes that are nonetheless ecologically significant and reflect complex mechanisms.