The flock was stressed as it had resorted to flapping flight to return to its migratory route, and a number of birds died. A virulent strain of West Nile virus was isolated from the brains of eleven dead juveniles. Other white storks subsequently tested in Israel have shown anti-WNV antibodies. In 2008 three juvenile white storks from a Polish wildlife refuge yielded seropositive results indicating exposure to the virus, but the context or existence of the virus in Poland is unclear. The white stork's decline due to industrialisation and agricultural changes (principally the draining of wetlands and conversion of meadows to crops such as maize) began in the 19th century: the last wild individual in Belgium was seen in 1895, in Sweden in 1955, in Switzerland in 1950 and in the Netherlands in 1991. However, the species has since been reintroduced to many regions. It has been rated as least concern by the IUCN since 1994, after being evaluated as near threatened in 1988. The white stork is one of the species to which the "Agreement on the Conservation of African-Eurasian Migratory Waterbirds" (AEWA) applies. Parties to the agreement are required to engage in a wide range of conservation strategies described in a detailed action plan. The plan is intended to address key issues such as species and habitat conservation, management of human activities, research, education, and implementation. Threats include the continued loss of wetlands, collisions with overhead power lines, use of persistent pesticides (such as DDT) to combat locusts in Africa, and largely illegal hunting on migration routes and wintering grounds. A large population of white storks breeds in central (Poland, Ukraine and Germany) and southern Europe (Spain and Turkey). In a 2004/05 census, there were 52,500 pairs in Poland, 30,000 pairs in Ukraine, 20,000 pairs in Belarus, 13,000 pairs in Lithuania (the highest known density of this species in the world), 10,700 pairs in Latvia, and 10,200 in Russia. There were around 5,500 pairs in Romania, 5,300 in Hungary and an estimated 4,956 breeding pairs in Bulgaria. In Germany, the majority of the total 4,482 pairs were in the eastern region, especially in the states of Brandenburg and Mecklenburg-Vorpommern (1296 and 863 pairs in 2008 respectively). Apart from Spain and Portugal (33,217 and 7,684 pairs in 2004/05 respectively), populations are generally much less stable.