Newcastle disease is a notifiable disease causing severe production losses and trade restriction with a significant economic impact on the poultry industry worldwide. The reservoirs of NDV are wild birds and domestic poultry and occasional asymptomatic zoonotic infections have also been reported in humans. NDV has a very wide host range of about 250 bird species [2; 6; 7; 8]. Migratory birds are the main reservoirs for NDV and play a key role in transmitting the virus in a transboundary fashion amongst regions and countries. Turkey is home to flyways of migratory birds which connect Europe and Asia, and thus could provide an early warning base for circulating NDV strains between the Eurasian countries. Therefore, we determine the dominance of NDV strains prevailing in both migratory and nonmigratory birds and ducks in the Marmara region of Turkey that could potentially pose disease risks to local commerial and a poultry as well as transnational dissemination risks to other coutries via wild bird migration [6; 9; 16; 7]. Results of virus isolation has shown that AMPV–1 was prevalent between 0.5 and 2.5% in waterfowl including ducks [17; 18]. However, serological prevalence was reported up to 60% [18; 19]. At present, there is no report on monitoring wild birds for NDV in Turkey. However, in Turkey, 4 and 81 domestic avian cases were reported to OIE in 2016 and 2017, respectively. There has been no report in 2018.
In a similiar study in Sanjiang natural reserve of Heilongjiang Province of China, migratory waterfowls were monitored for NDV. NDVs were isolated from waterfowls (mallard, goose, common teal and mandarin duck) [20]. In the North Sea, 543 passerine birds were investgated and the lentogenic strain of AMPV–1 was detected in 1.1% of birds [21]. In the USA, virulent strains of NDV have been found in wild birds but more frequently in pigeons, doves and doublecrested Cormorants. Research on NDV in wild ducks, gulls, and shorebirds found novel viral diversity, but no fusion gene sequences associated with high pathogenicity in poultry [7; 8]. However, it has been reported that most prevalent virulent genotype VII causing the endemics in Asia are co circulating into the ducks and chicken [22; 23]. Different genotypes of NDV viurses are prevalent in both poultry and wild birds. For example, F gene of 47 NDV isolates analysed from poultry outbreaks in Bulgaria were belong to genotypes II, IV, V and VIIb [24]. The subgenotype VIIb was also found in the Middle East [25]. Later sstudy revealed that genotype VIId is circulating in Bulgaria and Ukraine [16]. This subgenotype from Bulgaria and Ukraine may have been part of a broader epizootic process in Eastern Europe rather than separate introductions from Asia or Africa. Similarly, analysis of 2 velogenic strains of NDV from ducks in China showed closer identity with genotype VII [22]. In the last few decades, genotype VI and genotype VII of NDV have been causing sporadic disease outbreaks in many countries in Asia and Europe including Denmark, Sweden, Switzerland, Austria, Hungary, Greece Germany, Belgium, Netherlands, Spain, Italy, Middle East, the Indian subcontinent and Indonesia [26]. Isolates of velogenic NDVs from domestic and synanthropic birds (pigeons, crows, and jackdaws) in Kazakhstan, Kirghizia, Ukraine, and Russia in 1993 to 2007 was sequenced and they were clustered in genotype VII comprising VIIa, VIIb, VIId [27].
In the past, NDV-II, VI and VII lineageas were found in domestic poultry. This is the first study indicating the NDV lineage VIIi is circulating amongst wild birds and can spread virus in and amongst countries. Lately virulent strains of NDV belonging to genotype VII have been causing severe diseases outbreaks in poultry in many neigbouring countries of Turkey. Genotype VIId has been isolated from Bulgaria and Ukraine between 2002 and 2013. Iran has reported poultry outbreaks with genotype VIIb [25] and VIId [27] and VIIi [28; 29]. These studies conclude that genotype VII is a domiant strain in poultry and wild migratory birds and gradually undergoing adaptive changes, retaining fitness to suvive in both immune and naturally exposed birds and having the ability to spread via migatory birds.
Our study validates these findings, 4 out of the 5 F gene sequences (MK210596.1, MK210597.1, MK210598.1, MK210600.1) of NDV had a greater similarity with NDV strains (KT585631.1, KP271974.1, KP271975.1, KP271976.1, KP271979.1) that were previously isolated in Turkey, Bulgaria (KP271973) and Georgia (KP271972.1) suggesting that this genotype remains endemic. However, they were slightly different than previously isolated Turkish strains (KT585617.1 and KT585629.1). Our data indicate that recent Turkish isolates of this study showed some degree of molecular evolution when compared to the earlier isolates [15]. Interestingly, one (waterfowl) of the 5 NDV-F gene sequences (MK210599.1) obtained in this study was slightly different than previously isolated Turkish NDV strains (KT585631.1, KP271974.1, KP271975.1, KP271976.1, KP271979.1) and strains isolated from Bulgaria (KP271973) and Georgia (KP271972.1). This indicate, multiple variants of genotype VIIi are co-circulating in birds. The data may also predict an introduction of isolates from neighbouring countries. Importantly, although the strains found in this study are closely related, there is a relatively small degree of molecular divergence within F gene of the Turkish NDV isolate (KT585617.1 and KT585629.1) and strains from Iran (MG871466.1) Pakistan (KP776462.1), Israel (MH432252.1) and Belgium (MH432252.2). The currently used modified live viruses, LaSota and Hitchner-B1vaccine strains are clustering on a different branch of the phylogenetic tree than the NDV isolates obtained in this study. As suggested recently by Dimitrov and others [2017], efficacy of the above mentioned modified live vaccines against the presently circulating NDV strains needs to be taken into consideration. Therefore, new vaccines and vaccination strategies may be required after searching the effcacy of current vaccines and application failures for NDV in chickens in the field.