Rekenen aan Afrikaanse paardenpest

In de computer van Aline de Koeijer bij het CVI is Nederland regelmatig een rampgebied. Ze analyseert de uitbraken van verschillende dierziektes. In opdracht van LNV probeert ze onder andere het verloop van een uitbraak van drie opkomende dierziekten te voorspellen: riftdalkoorts, de Afrikaanse paardenpest en Krim-Congo hemorragische koort

Review on the epidemiology and dynamics of BSE epidemics

The paper describes how the comprehensive surveillance of bovine spongiform encephalopathy (BSE) and studies carried out on these data has enhanced our knowledge of the epidemiology of BSE. Around 7 000 BSE cases were detected through the screening of about 50 million cattle with rapid tests in Europe. It confirmed that the clinical surveillance had a poor capacity to detect cases, and also showed the discrepancy of this passive surveillance efficiency between regions and production types (dairy/beef). Other risk factors for BSE were being in a dairy herd (three times more than beef), having a young age at first calving (for dairy cattle), being autumn-born (dairy and beef), and being in a herd with a very high milk yield. These findings focus the risk on the feeding regimen of calves/heifers. Several epidemiological studies across countries suggest that the feedborne source related to meat and bone meal (MBM) is the only substantiated route of infection ¿ even after the feed ban ¿, while it is not possible to exclude maternal transmission or milk replacers as a source of some infections. In most European countries, the average age of the cases is increasing over time and the prevalence decreasing, which reflects the effectiveness of control measures. Consistent results on the trend of the epidemic were obtained using back-calculation modelling, the R0 approach and Age-Period-Cohort models. Furthermore, active surveillance also resulted in the finding of atypical cases. These are distinct from previously found BSE and classified in two different forms based on biochemical characteristics; their prevalence is very low (36 cases up to 1st September 2007), affected animals were old and some of them displayed clinical signs. The origin and possibility of natural transmission is unknown

Infectious diseases among animals : combining models with data

To eradicate or control the spread of infectious diseases, knowledge on the spread of the infection between (groups of) animals is necessary. Models can include such information and can subsequently be used to observe the efficacy of various control measures in fighting the infection. However, the availability of information and data to build and quantify these models is essential for applying such models in real life. In this thesis, models on the spread of infectious diseases in animals are always combined with data concerning the host, the infectious agent, their interactions and often also case data from epidemic or endemic disease situations. To do so, various infections that are present in the Netherlands are discussed. We show that for Phocine Distemper Virus (PDV) the data of the epidemic in 1988 show that clustering of animals has a strong influence on the transmission and survival of the animals. A previously applied model to analyse the seal situation did not fit the data very well. A model that incorporates the clustering of the animals on sand banks gave a better fit. Due to their clustering, the death rate was higher than could be expected from the first model. To control and eradicate Infectious Bovine Rhinotracheitis (IBR) it is known that the persistence of the infection in previously infected animals may cause a delay in eradication. We have quantified the probability for such a virus to reactivate in the field, and combined that with a model that calculates the expected time to extinction. Thus, control measures in the eradication process can easily be compared for efficacy and a time frame can be defined. The control of BSE was an important issue in the last decade, but due to limited data, exact advice was difficult to find. By quantifying the transmission parameters, the various control measures can be compared for efficacy. Now that more information is available concerning risks for humans and cattle, optimisation of the surveillance and control can be introduced based on such models. The age distribution of BSE cases offers information on the efficacy of BSE control in the past, but also concerning the prevalence of the infection in the future. Risk assessment and modelling are especially important for countries that would like to prove the absence of BSE in their country. Monitoring of an animal disease situation may have many purposes, for instance to prove freedom from infection for a certain farm or country. In the last chapter of the thesis, we give an opening for extended modelling to quantify the risk of missing a new outbreak in a country that has the Free from disease status. An integrated approach of transmission models that specifically includes the frequency of sampling over time enables us to calculate the probability that an epidemic escapes from detectio

Spread and Control of Rift Valley Fever virus after accidental introduction in the Netherlands: a modelling study.

Rift Valley Fever (RVF) is a zoonotic vector-borne infection and causes a potentially severe disease in both humans and young animals. The Ministry of Economic Affairs, Agriculture and Innovation (EL&I) is interested in the risk of an outbreak of Rift Valley Fever virus (RVFV) for the Netherlands, and more knowledge is needed about the risk of introduction of the virus, the risk of spread (transmission) of the virus in the country once introduced, and the methods for control and surveillance. For this purpose, a mathematical model was developed to study (1) the probability of a RVF outbreak at different days of introduction during the year, (2) the probability of persistence of the infection during the entire year, and (3) outbreak size and duration at different days of introduction during the year

Kosten-baten analyse Bluetongue : schade epidemieën 2006 en 2007 en evaluatie vaccinatiestrategieën 2008

In dit rapport worden de economische gevolgen van Bluetongue (BT) in Nederland voor de BT epidemie van 2006 en 2007 geschat. Met behulp van een kostenbaten analyse worden tevens de mogelijke BT vaccinatieprogramma's voor 2008 geëvalueer

A mechanistic model to describe the spread of phocid distemper virus

1. The 1988 epizootic among seals in N.W. Europe led to the death of more than half of the population. Several researchers have fitted data from the epidemic with the Kermack and McKendrick model for disease spread. 2. We argue that for animals living in herds or colonies, like seals, the mutual contact behaviour is such that this model ought to be applied with special care for the distinction between numbers and densities. This is shown by a mechanistic description of the contacts among seals, which leads us to a different formulation of the standard model. 3. This formulation is useful for the description of epidemics among all kinds of animals living in herds. 4. Further analysis shows that the survival of infected animals has a disproportionately great influence on the intensity of the epidemic. 5. Marine pollution may not only have contributed to the high death rates, but may have intensified the epizootic as well

Quantification of within- and between-pen transmission of Fouth-and-Mouth disease virus in pigs

Quantified transmission parameters of Foot-and-Mouth Disease Virus (FMDV) are needed for epidemic models used for control and surveillance. In this study, we quantified the within- and between-pen transmission of FMDV in groups of pigs by estimating the daily transmission rate , i.e. the number of secondary infections caused by one infectious pig during one day, using an SIR (susceptible-infectious-removed) model. Within-pen transmission was studied in four groups of ten pigs in which 5 infected and 5 susceptible pigs had direct contact; between-pen transmission was studied in one group of ten pigs in which 5 infected and 5 susceptible pigs had indirect contact. Daily results of virus isolation of oropharyngeal fluid were used to quantify the transmission rate , using Generalised Linear Modelling (GLM) and a maximum likelihood method. In addition, we estimated the expected time to infection of the first pig within a pen Tw and in the indirect-contact pen Tb. The between-pen transmission rate b was estimated to be 0.59 (0.083-4.18) per day, which was significantly lower than the within-pen transmission rate w of 6.14 (3.75-10.06). Tw was 1.6 h, and Tb was 16 h. Our results show that the transmission rate is influenced by contact structure between pigs