The main features of avian influenza viruses (AIV) and Newcastle disease virus (APMV-1), the possibilities for isolation and identification in laboratory conditions, methods of diagnostics, main hosts, clinical signs and virus shedding are reviewed in chronological order. The other part of the review explains the mechanisms and interactions in cases of co-infection of AIV and APMV-1, either between them or with other pathogens in various indicator systemscell cultures, chick embryos or birds. The emphasis is placed on quantitative data on the virus present mainly in the first ten days following experimental infection of birds, the periods of virus carrier ship and shedding, clinical signs, pathological changes, diagnostic challenges.

Thirty four oropharyngeal swabs were collected from guinea fowl infected with a low-pathogenicity avian influenza A virus Н6N2 (LPAIV H6N2) and vaccinated with a lentogenic NDV strain La Sota. All samples were examined in HI test after attempts for isolation of viruses in 9-day-old chick embryos (CЕ) and by means of AIV-NDV rapid Ag kit (RapiGEN, South Korea). The results demonstrated that the rapid test could be used for guinea fowl despite its lower sensitivity of 91.67 % as compared to the HI test after isolation in CE. The test specificity was 100 % indicating that it could distinguish both viruses in co-infections.

The place, antigenic diversity and nomenclature of APMV-2 are successively described. The methods of virus isolation, significance for avian pathology and global distribution of infection through serological, virological surveys and experiments are reviewed. The first investigations with avian blood sera in Bulgaria (n=253) originating from 8 farms from different parts of the country to detect antibodies against APMV-2 are outlined. The data showed spread of infection among hens and chickens with 14.53 % positive samples, and presence of infection in all surveyed farms.

Eighteen 2-month-old guinea fowl (Numididae meleagris) birds were intravenously infected with 100 L H6N2 virus, while three formed a control group -non-infected. The birds were clinically examined daily throughout the entire experimental period and no clinical symptoms of disease were observed. On days 7, 14 and 21, six infected and 1 control birds were slaughtered for pathological investigations. All visceral organs were macroscopically analysed and samples from lungs, heart, spleen, liver, kidneys, pancreas, thymus, bursa and duodenum were immediately removed and fixed in 10% buffered formalin for at least 2 days. Slices of 5 μm thickness were prepared, embedded in paraffin and stained with haematoxylin and eosin (H/Е) by standard procedures. The preparations were examined on Leitz light microscope. From the conducted pathoanatomical examinations, notable findings included the smaller size of the spleen, thymus, and the bursa of Fabricius in all examined birds, compared to the control group. Microscopically, however, as a constant find in all infected birds, we observed reactions of different type and extent within the lymphoid tissue of the central and peripheral immunocompetent organs, which could be summarised in two primary groups -lymphoid-proliferative and degenerative. The changes observed in the birds' euthanised 21 days p.i. were considerably more pronounced. Within the organs of the central immune system (thymus and spleen) lesions of the atrophic-degenerative type were found out. The organs of the peripheral immune system (the spleen and the entire mucosa-associated lymphoid tissue, including the respiratory tract, the alimentary tract with Peyer's patches, separate follicles and caecal tonsils) exhibited simultaneously atrophic regressive changes in the spleen and varying degrees of lymphoid activity in the other areas. In two of the euthanised 14 days p.i. and three 21 days p.i. birds, a lymphoid proliferation of the nodular type within the mucosa-associated lymphoid tissue of the lungs was discovered.