Case description and epidemiological situation
The carcass of an adult female was found in a ditch between two harvested maize fields at a dry location exposed to direct sunlight, about 200 m off a country road in a community of the Spree-Neiße district, Brandenburg, Germany ca. 6 km from the border with Poland (Figure 1) by a person walking the dog. Foreleg bones of the largely decomposed carcass were submitted by the responsible hunter for ASFV testing and the carcass was buried on site on 7th September 2020. At the time of detection, the hunter noticed large numbers of maggots on and around the carcass. The mean temperature was 15°C (1st -7th September 2020) or 19°C (6th August – 7th September 2020), respectively. From 1st August – 9th September 2020, the average rainfall was 6 mm. The carcass was found in an advanced stage of decomposition and the minimum PMI was estimated at two weeks. At that time, the adjacent region in Poland was classified as a part I-region according to the implementing decision EU 2014/709, i.e. with a certain risk regarding ASF due to proximity of the infection in the feral population (Figure 1).
Since the occurrence of ASF in wild boar in Western Poland, the risk of introduction of ASF into Germany by migrating infected wild boar had substantially increased. Preventive measures such as building a mobile fence along the Polish border and increased surveillance in wild boar and domestic pigs were introduced. The direct distance from the first detected case in Brandenburg to the nearest ASF case in Poland, which was confirmed within the preceding four weeks (13th August to 9th September 2020), was 33.9 km. The ASF case in Poland nearest to Germany was detected at a distance of 10.4 km from the German border, but south of the now affected area. Due to lack of information on the surveillance intensity between the national border and the ASF focus in Western Poland, direct evidence for a westward expansion of the infected area in Western Poland is missing. Yet, it seems likely that the disease has been introduced by migrating wild boar. (A new case, confirmed on 30th September 2020, located approx. 60 km north of the focus of the first introduction into Germany, was also detected less than 2 km away from the border with Poland). However, it cannot be excluded that human activity (e.g. unsafe disposal of contaminated material) may have caused the initial infection. ASFV remains infectious for a long time in pig products, especially if blood is contained (Fischer, Hühr, Blome, Conraths, & Probst, 2020; Kolbasov, Tsybanov, Malogolovkin, Gazaev, & Mikolaxchuk, 2011; Mebus et al., 1997).
Wild boar and domestic pigs have been intensively monitored for ASF in Germany in recent years. Especially in areas close to the German-Polish-border, the efforts were intensified after the emergence of the disease in Western Poland. Figure 2 shows the results of surveillance within the affected region close to the German-Polish border for the period from 1st November 2019 to 9th September 2020. A total of 7.125 samples of wild boar (6.081 apparently healthy shot wild boar, 312 wild boar found dead, 74 animals shot sick and 658 animals killed in road traffic accidents) tested negative for ASF. Most wild boar are examined during winter, when hunting is at its maximum. Nevertheless, in the four months before the detection of the first case of ASF in Germany, 50 wild boar found dead, 12 animals shot sick and 165 animals killed in road traffic accidents also tested negative for ASFV.
During the following days of intensified carcass search, four more carcasses and a hunted wild boar with clinical signs were found 6 km away from the first site. These carcasses were fresh with an estimated minimum PMI of only a few days. Until 24th September 2020, a total of 32 cases of ASF were confirmed, of which nine were found close to the first site and 23 in a small area further north (Figure 3). In both locations, fresh and old carcasses of young and old wild boar were found (14 piglets, 6 subadults, 6 adult animals; the age of 6 animals could not be assessed) (Table 1). At the place, where most carcasses were found, the carcasses with the longest minimum PMI of 80 days were detected (Table 1).
PMI estimates were based on the macroscopically visible stage of decay, the presence of insects and the microclimatic conditions at the sites. However, when decomposition had reached an advanced stage and only single skeletal remains were found, dating the PMI became difficult, in particular when carcasses had obviously been used by large vertebrate scavengers (red foxes and wolves are present in the area).
Based on the available data, the virus was probably introduced into Germany in early July 2020 or even earlier. Since most of the detected ASFV-infected wild boar died in September 2020 (Table 1), the situation is still evolving.
From all these animals, dry blood swabs or bones (carcasses) or serum (shot wild boar) were sent for laboratory diagnosis to the LLBB and for confirmation to the NRL at the FLI.