INTRODUCTION & METHODS

1. Understand to Control ! The interplay of wild boar ecology and CSF epidemiology lit up by a simulation model Stephanie Kramer-Schadt 1, Néstor Fernández 1,2, Hans-Hermann Thulke 1 INTRODUCTION & METHODS THE WILD BOAR SUBMODEL INDIVIDUAL LEVEL • PROBLEM: • Classical swine fever (CSF) in wild boar is thought to be the main factor for CSF outbreaks in domestic pigs • Several attempts to eradicate CSF in wild boar failed to yield convincing results • AIM: • to achieve a better understanding of CSF cycling in wild boar populations • to evaluate different control strategies (vaccination) • TOOL: • spatially-explicit individual-based model • allows to include individual behaviour in a given landscape • allows to understand mechanisms at population level based on individuals • disease processes are linked to individual boars • GENERAL METHODS: • cell represents home range • weekly steps (incubation period) • Females give birth to piglets once a year • each time step age dependent mortality probabilities are applied reproduction spatial process: female group splits mortality probability J F M A M J J A S O N D adult piglet subadult POPULATION LEVEL • Spatial process: • Carrying capacity of home range is set to adult females • herd splits when carrying capacity is exceeded (depending on free neighbouring cells), • or regulation of birth probability females with offspring J F M A M J J A S O N D THE VIRUS SUBMODEL I THE VIRUS SUBMODEL II DISEASE COURSE VIRULENCE • virulence is expressed as the survival probability of infected animals • It is a function of theprobability of being transiently infected PTRANS (recovery after 3 weeks) and different survival times TS for lethally infected ones (depending on a maximum survival time TMAX and an exponent X), representing different probabilities of acute and chronic infection • e.g., high virulence has a low proportion of transiently infected and of chronic lethal animals. postnatal infection prenatal infection Pinf prob. of effective infection effect of low maternal antibody titre (weeks) TMA Effect of maternal antibodies If mother immune: piglet protected (12 weeks) mother infected transmission rate ß: ß = - N * ln(1- pSI); pSI = 1-(1- pINF)I after TMA weeks fully susceptible Pinf partially susceptible for TMA weeks susceptible N number of individuals in herd pSI probability that a susceptible individual S gets infected I Pinf acute Pinf Pinf chronic Pinf Pinf transiently infected lethally infected transiently infected lethally infected acute chronic immune removed removed immune • disease transmission between boars is modelled with an effective infection probabilityPINF (including contact and infection probability) • within-herd and between-herd infection probability is distinguished Psurvival= BINOM(PTRANS)+ ( (1 + TS / TMAX)X * (1 – BINOM(PTRANS))) RESULTS DISCUSSION • Persistence of the disease for more than 5 years was mostly determined by virulence (GLM with logit link). With high virulence, the disease always went extinct, whereas low virulence caused cycling of the disease. • Other significant effects were effective infection probability between members of the same herd PINF , season of the virus introduction into the population and duration of the effect of maternal antibodies TMA (p < 0.0001). • Both preventive and post-outbreak vaccination schemes reaching 50% of the population under a low virulence scenario favored disease persistence. This is due to newborn susceptibles. When they get infected, they can survive for a long time carrying the disease. Under a high virulence scenario, the disease was always self-limiting. • VIRULENCE, KEY TO PERSISTENCE !? • The key importance of low virulence is related to the higher number both of chronic infected animals shedding the virus for a long time and of transiently infected boars. Besides other parameters usually considered in epidemiological models (R0, β), CSF field investigations should focus on estimations of virulence. • The longer an effect of maternal antibodies TMA, the lower is the persistence of the disease. This is because newborn susceptible piglets can get immune after an infection and thus the number of susceptibles is lower. Introducing the virus in April, just before the peak in seasonal reproduction, has a strong positive effect on disease persistence because the number of susceptibles is high due to newborn piglets. Low infection probability positively influences disease persistence, as it slows down the process. • Vaccination can even have negative results when we deal with a virus of low virulence, because the disease can persist longer. The role of virulence should be better understood before applying vaccination schemes. low high low high 1 UFZ Centre for Environmental Research, Dept. of Ecological Modeling, Leipzig, Germany; Stephanie.Kramer@ufz.de 2 Estación Biológica de Doñana, Consejo Superior de Investigaciones Cientificas, Dept. of Applied Biology, Sevilla, Spain @ 2004