Estimation of seroconversion rates for infectious diseases: Effects of age and noise.

The presence of serum antibodies is a biomarker of past infection. Instead of seroclassification aimed at measuring seroprevalence a population sample of serum antibody levels may be used to estimate the incidence of seroconversion. This article expands an earlier study into seroincidence estimation, employing models of the seroresponse that include probability of escaping infection, as well as nonexponential decay kinetics and different sources of noise. As previously, a constant force of infection is assumed. When the seroconversion rate is low, a substantial fraction of the population may not be old enough to have experienced any seroconversions, causing underestimation of seroconversion rates that may be substantial at young ages. A correction is given that can be shown to remove such age dependent bias. Simulation studies show that the updated models provide accurate estimates of seroconversion rates, but also that the presence of noise, when unaccounted for, may introduce considerable bias, especially at low (< 0.1/yr) seroconversion rates and young ages. The revised serocalculator scripts can be used to update the R package "seroincidence."

Kinetics of serum antibodies in response to infection with Yersinia enterocolitica.

Information on the kinetics of the serum antibody response to infection with Yersinia enterocolitica is essential to allow the estimation and comparison of seroconversion rates in a diversity of pools of cross-sectional serum antibody measurements. Data from 94 patients with acute enteritis caused by Yersinia infection were used. The follow-up period for the longitudinal study was 36 months, addressed by questionnaire. An indirect enzyme-linked immunosorbent assay method was adapted to determine the concentration of antibodies against Y. enterocolitica in human sera. A mathematical within-host model was used to describe the interaction between pathogen and immune system and the waning of immunity after clearing of the pathogen. All observed antibodies (IgG, IgM, IgA) reached peak levels shortly after infection and then decayed slowly indicating that the median levels decreased only little during the observation period. Estimated maximum peak antibody levels were highest in IgG. Seroresponse curves of all antibodies showed large individual variation between patients. There was no apparent pattern of variation with age, nor any notable difference between genders. Estimated half-times were very long for all antibodies, and their posterior distributions were highly skewed. IgA appeared to have the most persistent antibody response, compared with IgG and IgM. Median peak levels of all three antibodies were similar. There was no significance found between peak antibody levels and severity of symptoms of gastrointestinal infection and severity of joint pain. Our findings allow the use of cross-sectional serum antibody measurements as biomarkers, to estimate seroconversion rates. Such seroincidence estimates include asymptomatic seroconversions, thereby avoiding under-reporting, and allows the comparison of infection pressures among countries, independent of their healthcare and surveillance systems.

Waning and boosting: on the dynamics of immune status.

The aim is to describe the distribution of immune status (as captured by antibody level) on the basis of a within-host submodel for continuous waning and occasional boosting. Inspired by Feller's fundamental work and the more recent delay equation formulation of models for the dynamics of physiologically structured populations, we derive, for given force of infection, a linear renewal equation. The solution is obtained by generation expansion, with the generation number corresponding to the number of times the individual became infected. Our main result provides a precise characterization of the stable distribution of immune status.

Use of a Dose-Response Model to Study Temporal Trends in Spatial Exposure to Coxiella burnetii: Analysis of a Multiyear Outbreak of Q Fever.

The Netherlands underwent a large Q fever outbreak between 2007 and 2009. In this paper, we study spatial and temporal Coxiella burnetii exposure trends during this large outbreak as well as validate outcomes against other published studies and provide evidence to support hypotheses on the causes of the outbreak. To achieve this, we develop a framework using a dose-response model to translate acute Q fever case incidence into exposure estimates. More specifically, we incorporate a geostatistical model that accounts for spatial and temporal correlation of exposure estimates from a human Q fever dose-response model to quantify exposure trends during the outbreak. The 2051 cases, with the corresponding age, gender and residential addresses, reside in the region with the highest attack rates during the outbreak in the Netherlands between 2006 and 2009. We conclude that the multiyear outbreak in the Netherlands is caused by sustained release of infectious bacteria from the same sources, which suggests that earlier implementation of interventions may have prevented many of the cases. The model predicts the risk of infection and acute symptomatic Q fever from multiple exposure sources during a multiple-year outbreak providing a robust, evidence-based methodology to support decision-making and intervention design.

Linking the seroresponse to infection to within-host heterogeneity in antibody production.

A recently published model for the serum antibody response to infection appeared well suited for use in statistical analyses of longitudinal serological data. The published model assumed exponential decay with fixed rates for pathogen and serum antibody kinetics, ignoring any within-host heterogeneity in the seroresponse. A bi-exponential model shows that there is rapid initial decay followed by a prolonged period of persistent low serum antibody concentrations. We propose a small modification of the decay model that greatly increases its flexibility by allowing for non-exponential antibody decay. The modified model produces power functions that may be interpreted as a mixture of exponential decay curves, with a mixing distribution representing the relative contribution of many centres of antibody production to the serum antibody concentration. Fitting the power function decay model to observed longitudinal data for pertussis shows improved goodness of fit compared to the exponential decay model, with estimates for the shape parameter (r=2.2; 95% CI (1.7-2.8)) that differ from exponential shape (r=1). The power function decay model predicts more persistent antibody concentrations in the long term (symptomatic threshold reached >30 years after infection) which, when used in biomarker studies, will lead to lower estimates of seroconversion rates compared to exponential antibody decay.

Kinetics of antibody response to Coxiella burnetii infection (Q fever): Estimation of the seroresponse onset from antibody levels.

BACKGROUND: From 2007 to 2009, the Netherlands experienced a major Q fever epidemic. Long-term serological follow-up of acute Q fever patients enabled the investigation of longitudinal antibody responses and estimating the onset of the seroresponse in individual patients. METHODS: All available IgG and IgM phase I and II antibody measurements determined by immunofluorescence assay at month 3, 6, 12, and 48 from 2321 acute Q fever patients were retrospectively analyzed. Characteristic features of the antibody response were calculated. To model the seroresponse onset, serological data from patients diagnosed with a positive C. burnetii PCR test (n=364), and therefore with a known time of infection, were used as reference. RESULTS: In 9083 IgG samples and 3260 IgM samples large heterogeneity in shape and magnitude of antibody responses was observed. Phase II reached higher levels than phase I, and IgG antibodies were more persistent than IgM. The estimated seroresponse latency allowed for determining the time since start of the seroresponse from the concentrations of the different antibodies against C. burnetii. CONCLUSIONS: The extraordinary large serological dataset provides new insight into the kinetics of the immunoglobulins against C. burnetii antigens. This knowledge is useful for seroprevalence studies and helps to better understand infection dynamics.

Entero- and parechovirus distributions in surface water and probabilities of exposure to these viruses during water recreation.

Numerous studies have reported quantitative data on viruses in surface waters generated using different methodologies. In the current study, the impact of the use of either cell culture-based or molecular-based methods in quantitative microbial risk assessment was assessed. Previously and newly generated data on the presence of infectious human enteroviruses (HEV) and enterovirus and parechovirus RNA were used to estimate distributions of virus concentrations in surface waters. Because techniques for the detection of infectious human parechoviruses (HPeV) in surface waters were not available, a 'Parallelogram Approach' was used to estimate their concentrations based on the ratio infectious HEV/HEV RNA. The obtained virus concentrations were then used to estimate the probability of exposure for children during recreation in such virus contaminated surface waters. Human enterovirus cell culture/PCR ratios ranged from 2.3 × 10(-3) to 0.28. This broad range of ratios indicates that care should be taken in assuming a fixed ratio for assessing the risk with PCR based virus concentrations. The probabilities of exposure to both enteroviruses and parechoviruses were calculated, using our Parallelogram Approach for the calculation of infectious parechoviruses. For both viruses it was observed that the detection method significantly influenced the probability of exposure. Based on the calculated culture data, PCR data, and the ingestion volume, it was estimated that the mean probabilities of exposure, of recreating children, to surface water containing viruses were 0.087 (infectious enteroviruses), 0.71 (enterovirus particles), 0.28 (parechovirus particles) and 0.025 (calculated infectious parechoviruses) per recreation event. The mean probabilities of exposure of children recreating in surface water from which drinking water is produced to infectious enteroviruses were estimated for nine locations and varied between 1.5 × 10(-4) - 0.09 per recreation event. In this study, the use of the rotavirus dose response relationship as a surrogate was avoided. Instead, the probabilities of exposure were derived as a function of the distributions of the calculated doses. Our 'Parallelogram Approach' was used to estimate the unavailable infectious parechovirus concentrations using Monte Carlo simulations, and the exposure assessment carried out showed that virus concentrations present in surface waters could pose a health risk for children and other vulnerable populations.

Shedding of norovirus in symptomatic and asymptomatic infections.

Norovirus is the most frequent cause of acute infectious gastroenteritis and it is difficult to control in crowded environments like hospitals and nursing homes. Transmission depends on oral intake of virus deposited in the environment by infectious subjects. Data from volunteer studies indicate that virus concentrations in stool are highly variable, but systematic studies of the time-course of shedding and its individual variation are lacking. This paper quantifies norovirus shedding in a large population of 102 subjects, including asymptomatic shedders, and uses a longitudinal model to generalize shedding patterns. Enhanced surveillance for studies of transmission of norovirus in hospital outbreaks has yielded a considerable number of faecal samples from symptomatic and asymptomatic shedders, both from patients and staff. Norovirus concentrations were determined by real-time PCR. A quantitative dynamic model was fitted to the shedding data, in a multilevel Bayesian framework, to study the time-course of shedding and its variation. The results indicate that shedding in asymptomatic subjects is similar to that in symptomatic infections, both showing considerable variation in peak levels (average 105-109 /g faeces) as well as duration of virus shedding (average 8-60 days). Patients appear to shed higher numbers of virus than staff, for slightly longer durations, but the differences are too small to be significant. Given equal shedding, the greater contribution of symptomatic cases to transmission must be caused by their higher efficiency in spreading these viruses. The results of this study will be helpful for risk studies that need to quantify the deposition of virus in the environment.

A two-phase within-host model for immune response and its application to serological profiles of pertussis.

We present a simple phenomenological within-host model describing both the interaction between a pathogen and the immune system and the waning of immunity after clearing of the pathogen. We implement the model into a Bayesian hierarchical framework to estimate its parameters for pertussis using Markov chain Monte Carlo methods. We show that the model captures some essential features of the kinetics of titers of IgG against pertussis toxin. We identify a threshold antibody level that separates a large increase in antibody level upon infection from a small increase and accordingly might be interpreted as a threshold separating clinical from subclinical infections. We contrast predictions of the model with observations reported in the literature and based on independent data and find a remarkable correspondence.

Decline of IgG pertussis toxin measured in umbilical cord blood, and neonatal and early infant serum.

Maternal pertussis-specific antibodies are passively acquired by infants during pregnancy. An IgG pertussis toxin (IgG-PT) concentration of >20 U/ml is considered to protect neonates against pertussis. To evaluate the IgG concentration at birth and during the first two months of life, we examined the IgG-PT concentration in the umbilical cord blood and three times during the neonatal and early infant period. IgG-PT was measured by validated IgG-specific enzyme-linked immunosorbent assays (ELISA) in umbilical cord blood and in Guthrie card blood samples of umbilical cord blood in 2,790 children, born between 1 August 2006 and 1 December 2008. These measurements were comparable. All children with concentrations of IgG-PT >30 U/ml were included. IgG-PT was also measured in Guthrie card blood samples, when the neonates or early infants were 5 days, 1 month and 2 months old. The mean concentrations of IgG-PT were calculated. The mean concentration of IgG-PT in umbilical cord blood was 60.1 U/ml (LN 4.1; 0.6 SD; n = 103). At the age of 5 days, 1 month and 2 months, the mean concentration of IgG-PT was 40.6 U/ml (LN 3.7; 0.5 SD; n = 103), 20.7 U/ml (LN 3.0; 0.7 SD; n = 62) and 16.7 U/ml (LN 2.8; 0.9 SD; n = 61), respectively. Four percent of the neonates had a concentration of IgG-PT >30 U/ml in umbilical cord blood, which declined to levels around the concentration needed for protection against pertussis (>20 U/ml) in the first two months of life. Hence, it is of great importance to further investigate the safety of maternal immunisation during pregnancy to prevent life-threatening pertussis in newborns.

A novel method for evaluating natural and vaccine induced serological responses to Bordetella pertussis antigens.

We studied the time course of serum IgG antibodies against 3 different pertussis vaccine antigens: PT (pertussis toxin), FHA (filamentous hemagglutinin), Prn (pertactin) in sera from individuals vaccinated with four different pertussis vaccines at 4 years of age: (N=44, 44, 23 and 23, respectively,) and compared the responses to/after natural infection with Bordetella pertussis (N=44, age 1-8 years). These longitudinal data were analyzed with a novel method, using a mathematical model to describe the observed responses, and their variation among subjects. This allowed us to estimate biologically meaningful characteristics of the serum antibody response, like peak level and decay rate, and to compare these among natural infections and vaccine responses. Compared to natural infection, responses to PT after vaccination with the tested vaccines are smaller in magnitude and tend to decay slightly faster. When present in vaccines, FHA and Prn tend to produce high peak levels, higher than those in naturally infected patients, but these decay faster. As expected, the Dutch whole cell vaccine produced lower antibody responses than the acellular vaccines. This model allows a better comparison of the kinetics of vaccine induced antibody responses and after natural infection over a long follow up period.

Time-course of antibody responses against Coxiella burnetii following acute Q fever.

Large outbreaks of Q fever in The Netherlands have provided a unique opportunity for studying longitudinal serum antibody responses in patients. Results are presented of a cohort of 344 patients with acute symptoms of Q fever with three or more serum samples per patient. In all these serum samples IgM and IgG against phase 1 and 2 Coxiella burnetii were measured by an immunofluorescence assay. A mathematical model of the dynamic interaction of serum antibodies and pathogens was used in a mixed model framework to quantitatively analyse responses to C. burnetii infection. Responses show strong heterogeneity, with individual serum antibody responses widely different in magnitude and shape. Features of the response, peak titre and decay rate, are used to characterize the diversity of the observed responses. Binary mixture analysis of IgG peak levels (phases 1 and 2) reveals a class of patients with high IgG peak titres that decay slowly and may represent potential chronic cases. When combining the results of mixture analysis into an odds score, it is concluded that not only high IgG phase 1 may be predictive for chronic Q fever, but also that high IgG phase 2 may aid in detecting such putative chronic cases.

Campylobacter seroconversion rates in selected countries in the European Union.

As a major foodborne pathogen, Campylobacter is frequently isolated from food sources of animal origin. In contrast, human Campylobacter illness is relatively rare, but has a considerable health burden due to acute enteric illness as well as severe sequelae. To study silent transmission, serum antibodies can be used as biomarkers to estimate seroconversion rates, as a proxy for infection pressure. This novel approach to serology shows that infections are much more common than disease, possibly because most infections remain asymptomatic. This study used antibody titres measured in serum samples collected from healthy subjects selected randomly in the general population from several countries in the European Union (EU). Estimates of seroconversion rates to Campylobacter were calculated for seven countries: Romania, Poland, Italy, France, Finland, Denmark and The Netherlands. Results indicate high infection pressures in all these countries, slightly increasing in Eastern EU countries. Of these countries, the differences in rates of notified illnesses are much greater, with low numbers in France and Poland, possibly indicating lower probability of detection due to differences in the notification systems, but in the latter case it cannot be excluded that more frequent exposure confers better protection due to acquired immunity.

Biomarker dynamics: estimating infection rates from serological data.

The marginal distribution of serum antibody titres in a cross-sectional population sample can be expressed as a function of the infection rate, taking into account heterogeneity in peak levels and decay rates. This marginal model allows estimation of incidences, as well as simple tests for homogeneity across age, gender or geographic strata, using likelihood ratio tests. An example is given using Campylobacter serum antibody data. Using a hierarchical dynamic model to analyse data from a follow-up study in patients with symptomatic Campylobacter infection, we show that the serum antibody response consists of a rapid increase to peak levels followed by a slow decline with a geometric mean halftime of 1.4, 0.6 and 0.3 years for IgG, IgM and IgA, respectively. Antibody peak levels and decay rates were highly variable among subjects. Incidence estimates are consistent among different antibody classes (IgG, IgM and IgA). High seroconversion rates indicate that Campylobacter infection is a frequent event, occurring approximately once every year in any adult person, in the Netherlands, supporting the conclusion that a small fraction of infections leads to symptoms severe enough for notification.

Antibody levels against B. pertussis in neonates measured in dried blood spots.

AIM: We designed this study to investigate if immunoglobuline G-Pertussis toxin (IgG-PT) against Bordetella pertussis in umbilical cord blood can reliably be determined in dried blood spots on filter paper (Guthrie) cards. PATIENTS AND METHODS: We prospectively included 129 mothers and their newborns born in a general hospital in the Netherlands. The relation between IgG-PT against B. pertussis from the umbilical cord measured in dried blood spots (Guthrie card) and in serum samples was studied by means of a Bland-Altman graph, using regression analysis to evaluate the level of agreement of both measurement methods. RESULTS: IgG-PT in Guthrie cards show a high coefficient of correlation with IgG-PT in serum samples from the umbilical cord when calibrated against blood spot calibrators (p<0.05). CONCLUSION: Maternal IgG-PT against B. pertussis measured in cord blood applied to Guthrie cards and calibrated against blood spot calibrators show good agreement with measurement of IgG-PT in cord serum. This offers new perspectives for future studies concerning B. pertussis antibodies.

Combining risk assessment and epidemiological risk factors to elucidate the sources of human E. coli O157 infection.

E. coli O157 can be transmitted to humans by three primary (foodborne, environmental, waterborne) and one secondary (person-to-person transmission) pathways. A regression model and quantitative microbiological risk assessments (QMRAs) were applied to determine the relative importance of the primary transmission pathways in NE Scotland. Both approaches indicated that waterborne infection was the least important but it was unclear whether food or the environment was the main source of infection. The QMRAs over-predicted the number of cases by a factor of 30 and this could be because all E. coli O157 strains may not be equally infective and/or the level of infectivity in the dose-response model was too high. The efficacy of potential risk mitigation strategies to reduce human exposure to E. coli O157 using QMRAs was simulated. Risk mitigation strategies focusing on food and environment are likely to have the biggest impact on infection figures.

Human beings are highly susceptible to low doses of Trichinella spp.

Trichinella is an important foodborne pathogen causing considerable morbidity and mortality. To prevent human trichinellosis, meat inspection for Trichinella spp. at slaughter is a key instrument. Current testing is based on minimal infectious dose in humans, but a scientific basis for this approach is lacking. To this end, a dose-response model must be developed, allowing translation of exposure into disease burden at the population level. We developed novel methods for dose-response assessment using outbreak data incorporating sexual reproduction of the parasite. A selection of suitable outbreak studies, reporting numbers exposed and infected, as well as estimated doses, was collated from a literature study. Humans appear to be highly susceptible: exposure to low doses (few larvae) is associated with a considerable risk of infection. As a consequence, levels of Trichinella in meat must be low to maintain acceptable health risks.

Seroepidemiological studies indicate frequent and repeated exposure to Campylobacter spp. during childhood.

The annual number of episodes of clinical gastroenteritis caused by Campylobacter spp. in The Netherlands is estimated to be 75 000, i.e. once per 200 person life-years. This number is based on extrapolation of culture results from population-based studies. The number of culture-confirmed cases of Campylobacter infection peaks in the first 3 years of life and again between the ages of 20 and 25 years. The seroepidemiology of Campylobacter describes the relationship between age and exposure to Campylobacter and reflects both symptomatic and asymptomatic infections. Using a validated ELISA system, antibodies to Campylobacter were measured in an age-stratified sample (n=456) of the PIENTER serum collection of the Dutch general population. The seroprevalence of Campylobacter IgG antibodies increased with age, reaching almost 100% at age 20 years. Antibody levels steadily increased with age until young adulthood, suggesting repeated exposure to Campylobacter. In conclusion, seroepidemiological data demonstrated repeated exposures to Campylobacter throughout life, most of which do not lead to clinical symptoms. From young adulthood, >95% of the population in The Netherlands had serological evidence for exposure to Campylobacter.

Enteric virus infection risk from intrusion of sewage into a drinking water distribution network.

Contaminants from the soil surrounding drinking water distribution systems are thought to not enter the drinking water when sufficient internal pressure is maintained. Pressure transients may cause short intervals of negative pressure, and the soil near drinking water pipes often contains fecal material due to the proximity of sewage lines, so that a pressure event may cause intrusion of pathogens. This paper presents a risk model for predicting intrusion and dilution of viruses and their transport to consumers. Random entry and dilution of virus was simulated by embedding the hydraulic model into a Monte Carlo simulation. Special attention was given to adjusting for the coincidence of virus presence and use of tap water, as independently occurring short-term events within the longer interval that the virus is predicted to travel in any branch of the distribution system. The probability that a consumer drinks water contaminated with virus is small, but when this happens the virus concentration tends to be high and the risk of infection may be considerable. The spatial distribution of infection risk is highly heterogeneous. The presence of a chlorine residual reduces the infection risk.

Differences in clinical presentation between norovirus genotypes in nursing homes.

BACKGROUND: In healthcare settings, norovirus (NoV) outbreaks are predominated by genotype II.4 (GII.4) strains. Periodically, new variants of GII.4 emerge, causing a temporary increase of outbreaks. OBJECTIVES: To study the relationship between symptoms and NoV genotype. STUDY DESIGN: Data of 49 nursing homes which were monitored for NoV outbreaks in the winter seasons of 2005/2006 and/or 2006/2007 were used. RESULTS: Data on symptoms and duration of illness were available for 465 residents and 174 staff members from 28 NoV outbreaks. Genotype GII.4 was responsible for 21 outbreaks. Attack rates for residents seemed to be higher in GII.4 outbreaks compared to other genotypes. In outbreaks caused by GII.4, residents vomited more often than in outbreaks with other genotypes. They also had more often complaints of nausea, abdominal cramps, fever, and mucus in stool. The GII.4 2004 variant outbreaks showed higher percentages of nausea, stomach ache, and fever than outbreaks with the GII.4 2006a variant. Differences in duration of illness were not found. In nursing home staff, no clear differences were found between outbreaks caused by GII.4 and non-GII.4 NoVs. CONCLUSIONS: Genotype GII.4 was found to be related to more symptomatic disease, including more residents vomiting, and to a lesser extent, higher attack rates among residents.