Newcastle disease (ND) is a highly contagious viral disease of birds that poses a general threat to global poultry industry resulting from significant economic losses [1].

The ND pathogen is the RNA-containing virus of Avian orthoavulavirus javaense species, which belongs to the Paramyxoviridae family, Avulavirinae subfamily, Orthoavulavirus genus, previously classified as Avian paramyxovirus 1, or Newcastle disease virus (NDV) [2]. Dimitrov K. M. et al. in 2019, offered a classification based on the genetic properties of NDV [3]. Out of the two distinguished NDV classes, class I included genotype 1 only. Class II consisted of at least 20 different genotypes, which were divided into subgenotypes. In recent decades, genotype V, circulating in Americas, and genotype VII, circulating in other countries of the world, have become the most crucial for poultry industry. In the Russian Federation, NDV isolates are represented by different genetic and biological groups, including virulent and avirulent viruses, as well as vaccine strains [4].

Newcastle disease virus is capable of infecting over 200 avian species. Domestic Galliformes demonstrate the highest susceptibility to NDV. Infected birds have pathological lesions in the respiratory and digestive tracts, as well as the central nervous system. Mortality rates in non-vaccinated poultry flocks can be up to 100% [5]. Disease cases are also registered in geese, pheasants and guinea fowls [6]. Quails are susceptible to NDV infection: experimental infection with NDV virulent strains resulted in clinical signs that occur on days 3 to 14 post infection and mortality rates lower than in chickens [7][8]. At the same time, specific antibodies are recorded on day 14 post infection. In quails vaccinated against ND (La Sota strain), the antibody peak is observed on day 40 post vaccination, with further decrease after day 46.

Synanthropic (magpies, pigeons, sparrows, etc.) and wild birds are natural NDV carriers [9][10]. Wild waterfowl and migratory birds serve as the primary natural reservoirs for NDV. The disease exhibits seasonal patterns, largely driven by the annual migrations of wild birds. Waterfowl (domestic ducks and geese) are considered NDV reservoirs, since these birds are resistant to strains that are highly virulent for chickens.

However, since the 1990s there have been reports of ND outbreaks in domestic waterfowl in the Asian countries, including Korea, Japan and China. In the infected duck flocks, a drop in egg production was observed, with a morbidity rate of approximately 80% and a mortality rate ranging from 30 to 50%. Affected birds exhibited both diarrheal and neurological symptoms. Similar ND outbreaks were reported in geese flocks in China [6][11]. Virulent NDV strains detected in geese across Asia during the 2000s were classified as genotype VIId. Xu Q. et al. [11] demonstrated that experimental infection of geese with virulent genotype VIId NDV strain induced a robust early cellular immune response, a finding associated with the unique characteristics of ND pathogenesis in geese [11]. Wan H. et al. described virus transmission from infected geese to chickens under contact housing conditions [6].

Newcastle disease is reported in many countries worldwide and there is a mandatory requirement to send corresponding notifications on disease cases triggered by highly virulent isolates of the Avian ortho­avulavirus javaense to the World Organization for Animal Health (WOAH).

Newcastle disease outbreaks, notified to the WOAH over the past four years, were registered in more than 50 countries around the world (in Asia, Europe, America and Africa), including Russia. According to the WOAH data, ND outbreaks in poultry were reported over a four-year period in backyards of 16 subjects included into to the Central, Volga, Ural, Siberian, Far Eastern and Southern Federal Districts of the Russian Federation. Recent ND epizootics in numerous Asian and European countries have been caused by viruses from various subtypes of genotype VII [12][13]. In the Russian Federation, this genotype was first isolated from chickens in 2006, during an ND outbreak on a poultry farm in the Amur Oblast. Subsequently, NDV genotype VII caused sporadic outbreaks among backyard poultry in different regions of the country [14][15][16][17]. ND cases in pigeons are recorded annually in Russia [9][15][16].

In order to prevent ND, some countries, including the Russian Federation, vaccinate poultry with various vaccines [18][19][20]. Vaccination efficacy is assessed by measuring specific antibody titers against NDV before immunization and at various time points thereafter, using hemagglutination inhibition (HI) or enzyme-linked immunosorbent assay (ELISA) methods [21][22]. Serological tests are of limited value for ND surveillance and diagnosis owing to the widespread implementation of poultry vaccination programs [23][24].

This paper provides results of serological monitoring program for ND in poultry, implemented in 2023–2024. The surveillance was carried out under the state mandate of the Russian Federal Service for Veterinary and Phytosanitary Supervision (Rosselkhoznadzor) for the oversight of highly dangerous infectious diseases.

Biological material tested. The Rosselkhoznadzor Territorial Administrations sampled biological material (avian sera) in 2023–2024.

Test methods. The collected sera samples were tested using commercial diagnostic kits produced by the Federal Centre for Animal Health (Russia), i.e. HI test-kit for detection of antibodies against Newcastle disease virus and single-dilution ELISA test-kit for detection of antibodies against Newcastle disease virus in compliance with the instructions for use.

Treatment of the tested samples. Before the test procedure, all sera samples received for the tests were inactivated at 56 °C for 30 minutes in a serum inactivator (or in a water bath).

This work was conducted under the state assignment for epizootiological monitoring, as stipulated by Decrees No. 1915 (dated 20 December 2022) and No. 1630 (dated 22 December 2023) of the Russian Federal Service for Veterinary and Phytosanitary Surveillance (Rosselkhoznadzor). More than 66,700 samples were tested for antibodies to NDV. Avian sera were delivered from 69 and 74 regions of the Russian Federation in 2023 (35,005 samples) and in 2024 (31,766 samples), respectively.

Currently, poultry farming in the Russian Federation is represented by large commercial poultry farms using intensive farming systems, small family-operated farms and backyards.

26,983 and 26,004 sera samples were collected from commercial poultry in 2023 and 2024, respectively. Table 1 gives information on antibodies against NDV detected in poultry sera (from chickens, turkeys, ducks, geese, and quails) taken on commercial poultry farms in the Russian Federation.

In 2023, monitoring tests included samples from 237 commercial farms (establishments) of 60 RF Subjects, in 2024 – from 280 farms of 74 RF Subjects. In 2024, more positive samples were detected in the Russian Federation as a whole than in the previous year (81 and 74%).

Results of detecting antibodies to NDV in chicken sera (collected on commercial poultry farms in 2023–2024 and tested in HI and ELISA) are illustrated in Figures 1 and 2.

In 2023, 25,490 samples from chicken collected on 222 commercial poultry farms in 8 RF Federal Districts were tested, antibodies to NDV were detected in 19,451 samples (76%). In 2024, antibodies to NDV were detected in 19,699 (85%) out of 23,208 samples from 250 poultry farms (Fig. 1).

ND serological tests conducted in 2023 revealed the minimum percentage of positive samples (58%) in the North Caucasian Federal District (NCFD). Slightly more positive samples, between 62 and 70%, were detected in chickens from the Northwestern (NWFD), Ural (UFD) and Volga (VFD) Federal Districts. In the Siberian (Siberian FD), Southern (Southern FD) and Central (CFD) Federal Districts, number of positive samples ranged between 82 and 88%, and the maximum number (95%) was detected in the Far Eastern (FEFD) Federal District. In 2024, in 7 out of 8 Federal Districts, with the exception for the NWFD (77%), the proportion of seropositive chickens ranged from 80 to 91% (Fig. 2).

Results of detecting antibodies to NDV in sera from other poultry species collected on commercial poultry farms are illustrated in Figures 3 and 4. Turkey sera were tested using two methods: ELISA and HI test; only HI was used for geese, ducks and quails.

989 turkey sera samples collected in 2023 on 8 poultry farms in 4 RF Federal Districts (CFD, VFD, Siberian FD and Southern FD) were tested and antibodies were detected in 529 (54%). In 2024, antibodies to NDV were detected in 1,271 (82%) out of 1,542 sera samples from 17 poultry farms in 7 Federal Districts (excluding the FEFD). Samples from turkeys that are not vaccinated against ND from the NWFD showed negative results (Fig. 3).

As illustrated by the accompanying documents from the farms, the detected specific antibodies were produced in response to live or inactivated ND vaccines. Number of seropositive poultry on farms depended on a number of factors, including the vaccine type and the vaccination schedule. Use of new, more effective vaccines, including those inactivated and produced from virulent NDV strains, increased the proportion of immunized birds in the flock and enhanced the level of the immune response [25][26][27].

Sera from ducks were delivered from the CFD, the Siberian FD and the Southern FD: 290 samples from 4 poultry farms in 2023 and 591 samples from 4 poultry farms in 2024. Antibodies specific to NDV were detected in only 2 samples (0.7%) on one poultry farm in the Siberian FD in 2023 (Fig. 4).

In 2023, 133 samples from geese were collected on the UFD poultry farms and tested; no antibodies to NDV were detected. In 2024, 373 samples taken from 6 poultry farms in the VFD and UFD were tested. Antibodies to NDV were detected in 86 samples (23%) from geese not vaccinated against ND (from the Republics of Bashkortostan and Tatarstan). Domestic waterfowl (ducks and geese) are rarely vaccinated against ND on commercial farms, since these avian species are less susceptible to ND than chickens, but at the same time they can be the disease reservoir on poultry farms [11][28].

No antibodies to NDV were detected in quail sera collected in two Federal Districts (CFD and FEFD) and tested within two years, including those from the vaccinated birds (Fig. 4). The absence of specific antibodies in the vaccinated quails observed in our monitoring tests may result from incorrect sampling time after vaccination or an incorrect vaccination schedule.

In 2023 and 2024, 7,796 and 5,484 poultry sera samples were collected in backyards and on family-operated farms, respectively, in 39 RF Subjects of 8 Federal Districts and tested for antibodies to NDV (Tables 2 and 3).

Tests of poultry sera collected in backyards and on family-operated farms detected antibodies specific to NDV in chickens, turkeys, ducks, geese and guinea fowls in 2023 and in chickens, turkeys, geese and pheasants in 2024.

As for chickens and turkeys, antibodies to NDV were detected in 35 and 53% of samples tested in 2023 and 2024, respectively.

As indicated in the accompanying documents, ND vaccination was not universally implemented across all backyards and family-operated poultry farms. Poultry for backyards were mainly purchased from poultry farms, where ND vaccination was implemented. Vaccine was typically administered at a single dose. Live vaccines, which were more frequently utilized, often induced a short-term immunity. The increase in vaccination coverage against ND in backyard flocks is evidenced by the rise in positive test results ranging from 35% in 2023 to 53% in 2024.

528 and 314 samples from domestic geese were delivered for tests in 2023 and 2024, respectively. Antibodies to NDV were detected in 67 (2023) and 24 (2024) samples from non-vaccinated geese brought from the CFD and the VFD. Antibodies were detected in sera from domestic ducks in 10 samples out of the 270 tes­ted ones (VFD). In 2024, no specific antibodies to NDV were detected in duck sera. Presence of antibodies in domestic waterfowl not vaccinated against ND may be associated with circulation of avirulent NDV strains, since samples for the test were taken from clinically healthy birds [11][28]. Antibodies to NDV were detec­ted in all samples from the vaccinated guinea fowl and pheasants.

Many species of wild birds are natural reservoirs and carriers of infectious disease pathogens [4][17], and therefore monitoring in wild fauna makes it possible to control occurrence and spread of dangerous avian infections, including ND.

Table 4 shows HI results for sera from wild birds collected during 2023–2024.

Testing sera samples from wild birds collected in seven RF Subjects revealed antibodies to NDV in 74 out of 504 submitted samples. Samples from birds from the NWFD, the VFD and the Siberian FD were positive: 59 samples from synanthropic birds (pigeons) and 15 samples from wild birds (ducks, geese and seagulls). Two-year monitoring in the Vologda Oblast revealed specific antibodies to NDV in samples from wild birds. Positive samples from pigeons were obtained from the Republic of Tatarstan, the Krasnoyarsk Krai and the Omsk Oblast. In most cases, they were collected next to large commercial poultry farms, thus suggesting a threat of infection spread to poultry.

In 2023–2024, ND situation in the Russian Federation was assessed using field sera samples from various poultry species and wild birds. Due to routine vaccination with live and inactivated vaccines throughout the poultry rearing cycle, commercial poultry demonstra­ted a high level of seropositivity to NDV. Due to insufficient protection against ND, backyard poultry pose a persistent threat as a potential source of a primary disease outbreak driven by virulent strains of the virus. Antibodies to NDV were detected in wild and synanthropic birds in 4 regions of the Russian Federation.

Contribution of the authors: Volkova M. A. – comprehensive literature review on the topic, processing and analysis of laboratory results, designing tables and figures, preparing a draft of the scientific manuscript; Chvala Ir. A. – conducting serological tests; Yaroslavtseva P. S. – conducting serological tests; Kulagina M. A. – conducting serological tests; Osipova O. S. – conducting serological tests; Guseva N. A. – conducting serological tests; Andreychuk D. B. – planning monitoring tests in regions, text editing.

Вклад авторов: Волкова М. А. – поиск и анализ литературы по теме, обработка и анализ результатов лабораторных исследований, составление таблиц и диаграмм, подготовка текста статьи; Чвала Ир. А. – проведение серологических исследований; Ярославцева П. С. –проведение серологических исследований; Кулагина М. А. – проведение серологических исследований; Осипова О. С. – проведение серологических исследований; Гусева Н. А. – проведение серологических исследований; Андрейчук Д. Б. – планирование мониторинговых исследований по регионам, редактирование текста.