Epidemiology of African swine fever in Estonia and characterization of one virus strain

Abstract

African swine fever (ASF) is an OIE-listed, dangerous viral disease of pigs, which has a devastating impact on animal health and the pig industry in many countries worldwide. During the period 2014–2020, ASF has been the main animal health issue in Estonia. This dissertation consists of three independent studies, which have been conducted with the aim to analyse the epidemiology of ASF and the course of the epidemic in the Estonian wild boar population, as well as in domestic pigs. In Study I, wild boar surveillance data (n = 7,015) collected from two distinct study areas from September 2014 to September 2016 were analysed. A statistically significant difference between the two areas in the temporal course of ASF seroprevalence was found. These findings indicate that ASF might have been introduced to the north-east of Estonia earlier then to the south of the country. The probability of detecting an ASF-positive animal was higher in young animals (< 1 year). Within wild boar found dead, there was a higher probability of finding an ASF-positive result compared to hunted animals. In Study II, the biological characteristics of the ASF virus strain (Est 14/WB) circulating in the wild boar population of north-east Estonia in 2014 were evaluated. Oronasal inoculation of ten young wild boar led to an acute and severe course of the disease in all infected animals. Nine animals died and one animal recovered completely from the disease. In conclusion, the ASFV strain was still found to be highly virulent. In Study III, the epidemiology of ASF in all 26 domestic pig outbreak herds that occurred in Estonia during the period 2015–2017 was retrospectively analysed. On most of the farms, the first clinical signs were mild and not ASF-specific despite the high virulence of the circulating virus. The highest mortality (29.7%) was seen on backyard farms (1–9 pigs) and the lowest (0.7%) on large commercial farms (> 1000 pigs). The spread of the virus within farms was slow and the contagiousness of the virus was relatively low. Farms of all sizes and types have been at risk. The results suggest that the increase in ASF cases in local wild boar populations is the main risk factor leading to the infection of farms; 88% of domestic outbreaks occurred in areas where ASF virus was detected in wild boar prior to the outbreak, within a radius of 15 km from the outbreak farm.

Sigade Aafrika katk (SAK) on ohtlik sigade viirushaigus, mis põhjustab tõsiseid tagajärgi nii loomade tervisele kui ka majanduslikku kahju sektorile. Ajavahemikul 2014-2020 on SAK olnud peamine loomatervise probleem Eestis. Käesolev väitekiri koosneb kolmest eraldiseisvast uuringust, mis on viidud läbi eesmärgiga analüüsida SAK-i epidemioloogiat ning epideemia arengut Eesti metssigade populatsioonis ja kodusigadel. Esimeses uuringus analüüsiti kahest eraldiasuvast metssigade populatsioonist ajavahemikul septembrist 2014 kuni septembrini 2016 kogutud metssigade SAK andmeid (n=7015). Ilmnes, et SAK-i epidemioloogia oli neis kahes populatsioonis erinev. Seroloogiliste tulemuste ajaline analüüs näitas, et mediaan-aja mõju levimusele oli uuritavates piirkondades oluliselt erinev. Sellest tulenevalt võib eeldada, et SAK-i viirus ringles Kirde-Eestis enne, kui see Eestis ja piirkonnas ametlikult diagnoositi. Veel selgus, et tõenäosus leida SAK-positiivne metssiga oli kõrgem noorte loomade rühmas (vanus < 1 aasta) ning surnuna leitud loomade seas. Teises uuringus selgitati Kirde-Eestis 2014. aastal ringelnud SAK-i viirustüve (Est 14/WB) bioloogilisi omadusi. Katses nakatati oronasaalselt kümme umbes nelja kuu vanust metssiga. Kõigil loomadel tekkis äge haigestumine, neist üheksa looma surid ning üks tervenes haigusest täielikult. Katse tulemusel ei tuvastatud uuritud viirustüve virulentsuse langust. Kolmandas uuringus selgitati SAK-i epidemioloogiat kodusigadel. Selleks viidi epidemioloogiline uuring läbi kõigis (n = 26) seafarmides, kus ajavahemikul 2015–2017 diagnoositi SAK-i puhangud. Enamikus nakatunud karjades olid SAK-i esimesed kliinilised tunnused vaatamata haigustekitaja kõrgele virulentsusele leebed ning ebatüüpilised. Kõige kõrgemat suremust (29,7%) võis täheldada kodumajapidamistes (1-9 siga) ning kõige madalamat (0,7%) suurtes tootmisfarmides (>1000 siga). Viiruse levik nakatunud farmides oli aeglane ning nakkavus väike. Risk nakatuda tuvastati kõigis farmide suurusgruppides ning pidamisviiside juures. Uuringu tulemusel ilmnes, et SAKV-i levik kodusigade farmide ümbruskonna metssigade populatsioonis oli peamine riskitegur kodusigade nakatumiseks; 88% juhtudest leiti SAK-positiivseid metssigu puhangufarmist kuni 15 km kaugusel.