The virulence of low pathogenicity (LP) type A H7N2 avian influenza virus (AIV) isolates recovered from chickens in Delaware and the eastern shore of Maryland in 2004 was evaluated. Three-week-old leghorn- and broiler-type chickens and turkeys were inoculated via the conjunctival sac with 103.5–104.0 50% embryo infectious dose (EID50) of virus per bird with A/chicken/Delaware/Viva/04, A/chicken/Delaware/Hobo/04, and A/chicken/Maryland/Minh Ma/04. In broilers, the viruses produced respiratory signs, airsacculitis, and microscopic lesions in the trachea and lung. In contrast, signs and lesions were less severe in turkeys, and they were rarely observed in specific-pathogen-free (SPF) leghorns. In broilers and SPF leghorns, AIV peaked on day 3 postinoculation (PI), based on virus isolation and real-time reverse transcription-polymerase chain reaction, and antigen capture testing. Infection in turkeys peaked on day 7 PI. Serum antibodies generally were detected earlier in broilers (day 7 PI) than in turkeys or SPF leghorns (day 14 PI) using agar gel immunodiffusion, hemagglutination-inhibition, and the enzyme-linked immunosorbent assay. A second trial was performed to further examine the disease susceptibility of the leghorn chicken given the comparatively mild responses noted in the first trial. A 10-fold higher dose of 104.5–105.0EID50 per chick given via the conjunctival sac was used. In addition, commercial-type leghorns were tested as were chicks from the SPF leghorn source. The higher AIV dose resulted in more rapid and consistent rates of infection and higher serum antibody responses in both types of leghorn chickens. However, as observed in the first trial, clinical signs and microscopic lesions in both types of leghorns were infrequent and very mild. These findings indicate leghorn-type chickens, which are commonly used for pathogenicity assessments because of their availability, may not be the most suitable host for evaluating the virulence potential of LP AIV.
Abbreviations: AGID = agar gel immunodiffusion; AI = avian influenza; AIV = avian influenza virus; CAS = chorioallantoic sac; Ct = cycle threshold; EID50 = embryo infectious dose50; ELD50 = embryo lethal dose50; ELISA = enzyme-linked immunosorbent assay; HA = hemagglutination; HI = hemagglutination-inhibition; HP = high pathogenicity; LP = low pathogenicity; NVSL = National Veterinary Service Laboratory; PBS = phosphate-buffered saline; PI = postinoculation; RRT-PCR = real-time reverse transcription-polymerase chain reaction; SPF = specific-pathogen-free
Virulencia en pollos de engorde, aves leghorn y pavos, de aislados de virus H7N2 de influenza aviar de baja patogenicidad provenientes de la península de Delmarva.
Se evaluó la virulencia de aislamientos del virus de influenza aviar tipo A H7N2 aislados en el año 2004 a partir de aves domésticas en los estados de Delaware y la costa este de Maryland. Aves de la línea de postura (leghorn), pollos de engorde y pavos de tres semanas de edad fueron inoculados por vía ocular con una dosis infecciosa 50% para embrión de pollo de 103.5–104.0 de los siguientes virus: A/Pollo/Delaware/Viva/04, A/Pollo/Delaware/Hobo/04 y A/Pollo/Maryland/Minh Ma/04. En los pollos de engorde los virus produjeron signos respiratorios, aerosaculitis y lesiones microscópicas en la tráquea y pulmón. En contraste, las lesiones fueron menos severas en los pavos y fueron escasamente observadas en las aves leghorn libres de patógenos específicos. Mediante aislamiento viral, la prueba de reacción en cadena por la polimerasa transcriptasa reversa en tiempo real y pruebas de captura de antígenos, se determinó que en los pollos de engorde y en las aves libres de patógenos específicos los virus de influenza aviar alcanzaron un título máximo al terc
