Characterization and immunology of inflenza virus type a isolated from duck in Kerala

Abstract

Ducks are generally regarded as resistant to influenza virus infection. During the early halves of 1985 and 1987 influenza A viruses were isolated from cases of respiratory disease causing 15 to 20 percent mortality in two to six week-old ducklings at Government Duck Farm, Niranam. Four strains of the virus (A/duck/India /1/85 (H9N2)-CDN, A/duck/India /2/87 (H3N?) –DT3; A/duck/India /7/87 (H3N2)-T19 and A/duck/India /14/87 (H9N?-lgl isolated from these out breaks were studied in detail with particular reference to their characteristics, Pathogenicity and immunogenicity. The strains were propagated in nine day embryonated chicken eggs by allantoic route of inoculation. All the four strains multiplied well killing the embryos in two to four days time. Though characteristic lesions were not present, the embryos were slightly congested. The chorioallantoic membranes showed moderate congestion and oedema but no pock lesions. Infectivity titers were 108.5 EID 50/0.2ml; 108.25 EID 50/0.2ml 105.75 EID 50/0.2ml and 108.5 EID 50/0.2ml respectively for CDN, DT3, T19 and lgl. The corresponding HA titres were 1:64 , 1:128, 1:64 and 1:256 The strains also multiplied well in duck embryos but no specific lesions were seen either in the embryo or on the chorioallantoic membranes. The infectivity titers were low compared to the titers obtained in chick embryos. All the strains were inactivated at 560C in 30 minutes. They lost their infectivity at PH 3.2 while HA property was not considerably reduced except for lgl. At PH 7.2 both infectivity and HA property were not affected. But infectivity was slightly affected and HA property was markedly reduced at PH 9.0. The viruses were chloroform sensitive and agglutinated red cells from cattle, sheep, goat, guinea pigs, horse, rabbit, rat mouse, chicken and man (0, A and B Groups). Chicken embryo fibroblastic infected with strain CDN produced cytopathic effects characterized by rounding of cells affecting the whole monolayer in 96 hours. The remaining three strains did not produce any CPE. In all the four cases virus infection was evidenced as the cell culture fluid gave haemagglutination. The infectivity titers of these fluids were 106.5 EID 50/0.2ml and 105.5 EID 50/0.2 ml 104.0 EID 50/0.2 ml and 105.5 EID 50/0.2 ml respectively for CDN, DT3, T19 and lgl. Two common antigens possibly the type specific MP and NP antigens were detected in the CAM extracts of embryos infected with these strains by agar gel precipitation and immunoelectrophoresis. Mean death time for these strains calculated according to the method adopted for NDV were 78 hours, 76.8 hours, 72 hours and 76 hours respectively for CDN, DT3, T19 and lgl, while the ICPI in day-old chicks were 0.325, 0.66, 0.00 and 0.16 and IVPI 0.00 in all cases. Pathogenicity and immunogenicity studies were carried out in day-old, one week-old and two week-old ducklings with strain CDN and in the week-old ducklings with strain lgl. Strain CDN produced hundred percent mortality in day old ducklings that received the virus by oral , occulonasal and subcutaneous routes. They died with no marked clinical signs except droopiness, discharges from the eyes. Slight diarhoea and ruffled feathers in some of them. The pathogenicity indices were 1.19, 1.15 and 1.51 respectively for group A,B and C. The ducklings that were kept to study contact infection revealed cloacal excretion of the virus till 12th day, through tracheal excretion stopped by 10th day. None of the birds in all the four groups that survived seven day of infection did reveal any specific seven day of infection did reveal any specific HI antibodies. In one week-old ducklings no clinical symptoms were observed. Virus could be isolated from cloacal and throat swabs before death and from tissues of dead birds. The pathogenicity indices were 0.36, 0.00 and 0.20 respectively for group A, B and C. In contact ducklings picked up infection as indicated from virus isolation and antibody titration. Both infected and incontact birds showed HI antibodies from 7th day onwards. By 14th day the titres reached peak level ranging between of 1:16 to 1:64 followed by a decline and remained steady through out the observation period. The two week-old ducklings did not show any clinical symptoms or death. But they revealed tracheal and cloacal shedding of the virus. The HI antibody titres never increased beyond 1:16. None of the incontent birds either showed clinical symptoms or death. Positive virus isolation could be made till the 7th day. Antibody response was also very poor and HI antibody titers never increased beyond 1:4. One week-old ducklings infected with strain lgl remained apparently healthy throughout the period of observation. However, virus isolations were made from cloacal and tracheal samples from 3rd to 7th day. In birds that received the virus by oral route, HI antibody titres were 1:4, 1:8 and 1:4 respectively on 7th, 14th and 21st day and nil by 28th day. In order two groups the birds did not reveal HI antibodies till 28th day. None of the incontact birds in all three groups showed clinical symptoms, death or any other indication of infection by the virus. In addition the sera from these birds were also negative for HI antibodies.